gsm and cdma report
TRANSCRIPT
CHAPTER-I
INTRODUCTION TO GSM
11 Introduction
GSM (GLOBAL SYSTEM FOR MOBILE COMMUNICATION) is the worldrsquos most widely
deployed and fastest growing digital cellular standard Currently here are over 200 million
GSM subscribers world-wide - two-thirds of the worldrsquos digital mobile population - and this
figure is increasing by nearly four new users per second GSM covers every continent being
the technology of choice for over 360 operators in more than 137 countries
But this is only the beginning of the wireless revolution The industry predicts that there will
be nearly 600 million GSM customers by 2003 Internet use has grown at an almost parallel
exponential rate to GSM subscriptions fuelling the growth of GSM and its development
towards 3rd Generation multimedia services GSM is leading the way in this evolution
moving far beyond the traditional mobile telephone offering to embrace a whole range of
services that provide unrivalled benefits to users operators and investors alike
GSMrsquos been able to grow develop and adapt to meet changing user demands across a wide
range of markets around the world This is due to the unique co-operation of manufacturers
operators and standards bodies world-wide working together to develop and market GSM
As a result since its first commercial rollout GSM has led to the development of a global
multi-vendor market stimulating competition cost reduction and technological advances
based on open standards International roaming is a key feature unique to GSM which allows
users to stay in touch wherever they travel Users can also access data services such as the
unique SMS (Short Message Service) fax transmission and e-mail access on the move GSM
has also enabled regulators to successfully introduce competition in mobile services world-
wide
GSM offers voice and data services to users on the move world-wide Pre-paid services have
also been a key driver behind GSMrsquos success opening up mobile services to new subscriber
groups in established and developing markets alike In many cases pre-paid subscriptions
have overtaken traditional subscriptions as users recognize the flexibility and control that
pre-paid services can offer Mobile users are already heavily exploiting the data services
1
available on GSM Business users are sending faxes and accessing e-mail facilities via their
GSM terminals as they travel Business and private users are exploiting SMS for quick
communication and to receive information such as stock quotes sports headlines and news
bulletins As GSM develops towards 3rd Generation services new opportunities will
continue to make GSM the most attractive choice
SIM cards - the thumbnail or credit card-sized module that contains subscription related data
as well as security management and personal telephone books - allow users to change
handsets to get one with the latest design or one with additional features The SIM has
proven to be a driver of quality and innovation from both manufacturers and operators which
have benefited customers and the entire industry SIM cards have also proved to be
invaluable for future evolution into 3rd Generation services where special protocols and tool
kits offer possibilities to download and run applications andor features on the handset or
card with embedded personal data for management and security In addition operators and
service providers can easily and cost-effectively upgrade feature-sets or capabilities by
issuing new cards to users without affecting handsets
12 GSM Standards
The system specifications for GSM networks are
Frequency Band Uplink 890MHz-915Mhz
Downlink 935MHZ-960MHz
The GSM system is originally specified to operate in the 900MHz band so even before a
commercially viable system could be in place governments of various countries were told to
reserve the frequency band for GSM The frequencies are arranged into pairs so that unique
sets can be defined There are 125 channels in GSM 900 however only 124 are used the first
pair are not used as it is employed as a Guard Band Shown below is the GSM frequency
band although this is not the only band over which GSM operates it also operate in the
1800Mhz and 1900Mhz band also
2
Figure 11 GSM Uplink and Downlink
At this moment it would be important to mention here the need to use a lower frequency for
uplink The reason is since this carries the information from the MS to the BTS over the Air
Interface using a higher frequency means higher attenuation Secondly to compensate for the
attenuation we need to send the signal at a higher power which consumes more battery
power and leading to a smaller talk time
Duplex Distance 45MHz
This is the standard distance between the uplink and the downlink frequencies This is not
constant for all versions of GSM however the separation between the uplink amp downlink
bands is constant to 20kHz
Carrier Separation 200kHz
Figure 12 Carrier Separation
In GSM we have uplink and downlink carriers These individual carriers are separated
200kHz apart therefore we get 125 uplink amp downlink carriers These carriers are then so
arranged so that we get 124 ARFCNrsquos (absolute radio frequency carrier numbers) for GSM
3
900 they start form 1-124 Henceforth any mention of channels will be done using their
ARFCN The figure below shows the carriers
Each carrier frequency is then divided according to time using a TDMA scheme Each of the
carrier frequencies is divided into a 120ms multiframe A multiframe is made up of 26
frames Two of these frames are used for control purposes while the remaining 24 frames are
used for traffic as shown
Figure 13 TDMA Multiframe
Modulation Gaussian Minimum Phase shift Keying(GMSK)
The modulation method used in GSM had to be very specific according to the needs of
communication and also to cater for the anomalies in the radio interface However this would
be taken in detail in a later section of this report
Transmission Rate 270kbps
Access Method Time Division Multiple Access(TDMA)
TDMA is used in GSM in conjunction with FDMA to allow voice communication The
200Khz channel is divided into 8 slots and each slot represents a call However the whole
channel is available to the caller were a caller gets particular time duration in a round robin
fashion to proceed with his call as described in the figure below
Figure 14 TDMA Frame
Speech Coder Rapid Pulse Excitation linear Predictive Coder coding at 13kbps
In modern landline telephone systems digital coding is used The electrical variations
induced into the microphone are sampled and each sample is then converted into a digital
4
code The voice waveform is then sampled at a rate of 8 kHz Each sample is then converted
into an 8 bit binary number representing 256 distinct values Since we sample 8000 times per
second and each sample is 8 binary bits we have a bit-rate of
8kHz 8 bits = 64kbps
This bitrate is unrealistic to transmit across a radio network since interference will likely
ruin the transmitted waveform In GSM speech encoding works to compress the speech
waveform into a sample that results in a lower bitrate using RPE-LPC The actual process
will be discussed later in the section where the journey from speech to radio waves is
considered
Over The Channel Bit Rate 228kbps
Slow Frequency Hopping 217hopssecond
GSM can use slow frequency hopping where the mobile station and the base station transmit
each TDMA frame on a different carrier frequency A form of slow frequency hopping is
used by GSM to help combat the multipath burst errors characteristic of cellular
environments Each base station has its own pattern for hopping from one carrier frequency
to another from slot to slot with mobiles using that base station following suit This
frequency hopping also reduces the incidence of co-channel interference between clusters of
cells The frequency-hopping algorithm is broadcast on the Broadcast Control Channel Since
multipath fading is dependent on carrier frequency slow frequency hopping help mitigate the
problem Frequency hopping is an option for each individual cell and a base station is not
required to support this feature
Synchronisation Compensation 223sec
Equalisation Limit Up to 16sec time dispersion
Typical Base Station Transmit Power 320W
Frame Duration 4615ms
Channel Coding Half-Rate Convolutional Coder
13 GSM Bands
5
E-GSM This represents an extension of the lower end of the two sub blocks by 10MHz
adding 50 More ARFCNrsquos to the Primary GSM (P-GSM)
Uplink Frequency 880MHz-915MHz
Downlink Frequency 925MHz-960MHz
DCS 1800 At a late stage in GSM development the existing technology was modified to
meet the need for PCN networks This involves changes to the radio interface which
moves spectrum allocation up to around 18Ghz More spectrum is available in this
frequency range for two sub-blocks of 75Mhz with duplex spacing of 95Mhz giving a
total of 374 carriers
Uplink Frequency 1710Mhz-1785Mhz
Downlink Frequency 1805Mhz-1880Mhz
PCS 1900 Used in the USA The FCC has split the designated spectrum into six duplex
blocks The USA has been divided into 51 Major Trading Areas and 493 Basic Trading
Areas An MTA is broadly equivalent in size to a state whilst a BTA approximates to a
large city Each MTA has access to 3x15MHz block and each BTA has access to 3x5MHz
blocks
14 Features of GSM
The GSM services are grouped into three categories
1 Teleservices (TS)
2 Bearer services (BS)
3 Supplementary services (SS)
141 Teleservices
Regular telephony emergency calls and voice messaging are within TS Telephony the old
bidirectional speech calls is certainly the most popular of all services An emergency call is a
feature that allows the mobile subscriber to contact a nearby emergency service such as
police by dialing a unique number Voice messaging permits a message to be stored within
the voice mailbox of the called party either because the called party is not reachable or
because the calling party chooses to do so
142 Bearer Services
6
Data services short message service (SMS) cell broadcast and local features are within BS
Rates up to 96 kbits are supported With a suitable data terminal or computer connected
directly to the mobile apparatus data may be sent through circuit-switched or packet-
switched networks Short messages containing as many as 160 alphanumeric characters can
be transmitted to or from a mobile phone In this case a message center is necessary The
broadcast mode (to all subscribers) in a given geographic area may also be used for short
messages of up to 93 alphanumeric characters Some local features of the mobile terminal
may be used These may include for example abbreviated dialing edition of short messages
repetition of failed calls and others
143 Supplementary Services
Some of the SS are as follows
Advice of charge This SS details the cost of a call in progress
Barring of all outgoing calls This SS blocks outgoing calls
Barring of international calls This SS blocks incoming or outgoing international calls as
a whole or only those associated with a specific basic service as desired
Barring of roaming calls This SS blocks all the incoming roaming calls or only those
associated with a specific service
Call forwarding This SS forwards all incoming calls or only those associated with a
specific basic service to another directory number The forwarding may be unconditional
or may be performed when the mobile subscriber is busy when there is no reply when
the mobile subscriber is not reachable or when there is radio congestion
Call hold This SS allows interruption of a communication on an existing call
Subsequent reestablishment of the call is permitted
Call waiting This SS permits the notification of an incoming call when the mobile
subscriber is busy
Call transfer This SS permits the transference of an established incoming or outgoing
call to a third party
Completion of calls to busy subscribers This SS allows notification of when a busy
called subscriber becomes free At this time if desired the call is reinitiated
Closed user group This SS allows a group of subscribers to communicate only among
themselves
7
Calling number identification presentationrestriction This SS permits the presentation or
restricts the presentation of the calling partyrsquos identification number (or additional
address information)
Connected number identification presentation This SS indicates the phone number that
has been reached
Free phone service This SS allocates a number to a mobile subscriber and all calls to
that number are free of charge for the calling party
Malicious call identification This SS permits the registration of malicious nuisance and
obscene incoming calls
Three-party service This SS permits the establishment of conference calls
CHAPTER-II
GSM Network Architecture and GSM Channels
8
21 GSM Network Architecture
GSM network architecture is divided into following three categories
Mobile Station (MS) This consists of the mobile telephone fax machine etc This is the
part of the network that the subscriber will see
Base Station Subsystem (BSS) This is the part of the network which provides the radio
interconnection from the MS to the land-based switching equipment
Network Switching Subsystem (NSS) This consists of the Mobile services Switching
Centre (MSC) and its associated system-control databases and processors together with the
required interfaces This is the part which provides for interconnection between the GSM
network and the Public Switched Telephone Network (PSTN)
Network Management System This enables the network provider to configure and
maintain the network from a central location
211 Mobile Station (MS)
In GSM there is a difference between the physical equipment and the subscription The
mobile station is piece of equipment which can be vehicle installed portable or hand-held
In GSM there is a small unit called the Subscriber Identity Module (SIM) which is a separate
physical entity eg an IC-card also called a smart card SIM and the mobile equipment
together make up the mobile station Without SIM the MS cannot get access to the GSM
network except for emergency traffic While the SIM-card is connected to the subscription
and not to the MS the subscriber can use another MS as well as his own This then raises the
problem of stolen MSrsquo since it is no use barring the subscription if the equipment is stolen
We need a database that contains the unique hardware identity of the equipment the
Equipment Identity Register (EIR) The EIR is connected to the MSC over a signalling link
This enables the MSC to check the validity of the equipment An non-type-approved MS can
also be barred in this way The authentication of the subscription is done by parameters from
AUC
9
Fig 21 GSM Network Architecture
10
212 Base Station Subsystem(BSS)
The GSM Base Station System is the equipment located at a cell site It comprises combination
of digital and RF equipment The BSS provides the link between the MS and the MSC BSS
communicates with the MS over the digital air interface and with the MSC via 2 Mbits links
Fig 22 Base Station Subsystem
The BSS consists of three major hardware components
Base Transceiver Station (BTS) The BTS contains the RF components that provide the
air interface for a particular cell This is the part of the GSM network which communicates
with the MS The antenna is included as part of the BTS
11
Base Station Controller (BSC) The BSC as its name implies provides the control for the
BSS The BSC communicates directly with the MSC The BSC may control single or
multiple BTSs
Transcoder (TC) The Transcoder is used to compact the signals from the MS so that they
are more efficiently sent over the terrestrial interfaces Although the transcoder is
considered to be a part of the BSS it is very often located closer to the MSC The
transcoder is used to reduce the rate at which the traffic (voicedata) is transmitted over the
air interface Although the transcoder is part of the BSS it is often found physically closer
to the NSS to allow more efficient use of the terrestrial links
BSS Configurations
Fig 23 BSS Configurations
12
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM
BTSs and BSC may be either located at the same cell site ldquoco-locatedrdquo or located at different
sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs than BSCs
in a network Another BSS configuration is the ldquoDaisy Chainrdquo A BTS need not to
communicate directly with the BSC which controls it it can be connected to the BTS rather
than all the way to BSC Problems may arise when chaining BTSs due to the transmission
delay through the chain the length of the chain therefore should be kept sufficiently short to
prevent the round trip speech delay becoming too long
2121 Base Transceiver Station
BTS is the hardware component in the GSM architecture It provides an interface between
Mobile Station (MS) and Base Station Subsystem (BSS) The BTS provides radio channels
(RF carriers) for a specific RF coverage area The radio channel is the communication link
between the MSs within an RF coverage area and the BSS The BTS also has a limited amount
of control functionality which reduces the amount of traffic between the BTS and BSC
2122 Base Station Controller
Any operational information required by the BTS will be received via the BSC Likewise any
information required about the BTS (by the OMC for example) will be obtained by the BSC
BSC incorporates a digital switching matrix which it uses to connect the radio channels on the
air interface with the terrestrial circuits from the MSC The BSC switching matrix also allows
the BSC to perform ldquohandoversrdquo between radio channels on BTSs under its control without
involving the MSC The purpose of the BSC is to perform a variety of functions Some of them
are listed below
Controls the BTS components
Performs Call Processing
Performs Operations and Maintenance
Provides the OampM link between the BSS and the OMC
Provides the A Interface between the BSS and the MSC
Manages the radio channels
Transfers signaling information to and from different mobile stations (MS)
13
2123 Transcoder
Transcoder is required to convert the speech or data output from MSC(64 kbits PCM) into
the form specified by GSM specifications for the transmission over the air interface that is
between BSS and MS (64 kbits to 16 kbits and vice versa)The 64 kbits Pulse Code
Modulation (PCM) circuits from MSC if transmitted on the air interface without
modification would occupy an excessive amount of radio bandwidth This would use the
available radio spectrum inefficiently The required is therefore reduced by processing the 64
kbits circuits so that the amount of information required to transmit digitized voice falls to a
gross rate of 16 kbits The transcoding function may be located at the MSC BSC or BTS
213 Network Switching Subsystem
The Network Switching System includes the main switching functions of the GSM network It
also contains the databases required for subscriber data and mobility management Its main
function is to manage communications between the GSM network and other
telecommunications networks Various components of the Network Switching System are
listed below
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Centre (AUC)
Inter Working Function (IWF)
Echo Canceller (EC)
In addition to the more traditional elements of a cellular telephone system GSM has Location
Register network entities These entities are the Home Location Register (HLR) Visitor
Location Register (VLR) and the Equipment Identity Register (EIR) The location registers
are database-oriented processing nodes which address the problems of managing subscriber
data and keeping track of a MS location as it roams around the network Functionally the
Interworking Function and the Echo Cancellers may be considered as parts of the MSC since
their activities are inextricably linked with those of the switch as it connects speech and data
calls to and from the MSs
2131 Mobile Switching Centre
14
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
available on GSM Business users are sending faxes and accessing e-mail facilities via their
GSM terminals as they travel Business and private users are exploiting SMS for quick
communication and to receive information such as stock quotes sports headlines and news
bulletins As GSM develops towards 3rd Generation services new opportunities will
continue to make GSM the most attractive choice
SIM cards - the thumbnail or credit card-sized module that contains subscription related data
as well as security management and personal telephone books - allow users to change
handsets to get one with the latest design or one with additional features The SIM has
proven to be a driver of quality and innovation from both manufacturers and operators which
have benefited customers and the entire industry SIM cards have also proved to be
invaluable for future evolution into 3rd Generation services where special protocols and tool
kits offer possibilities to download and run applications andor features on the handset or
card with embedded personal data for management and security In addition operators and
service providers can easily and cost-effectively upgrade feature-sets or capabilities by
issuing new cards to users without affecting handsets
12 GSM Standards
The system specifications for GSM networks are
Frequency Band Uplink 890MHz-915Mhz
Downlink 935MHZ-960MHz
The GSM system is originally specified to operate in the 900MHz band so even before a
commercially viable system could be in place governments of various countries were told to
reserve the frequency band for GSM The frequencies are arranged into pairs so that unique
sets can be defined There are 125 channels in GSM 900 however only 124 are used the first
pair are not used as it is employed as a Guard Band Shown below is the GSM frequency
band although this is not the only band over which GSM operates it also operate in the
1800Mhz and 1900Mhz band also
2
Figure 11 GSM Uplink and Downlink
At this moment it would be important to mention here the need to use a lower frequency for
uplink The reason is since this carries the information from the MS to the BTS over the Air
Interface using a higher frequency means higher attenuation Secondly to compensate for the
attenuation we need to send the signal at a higher power which consumes more battery
power and leading to a smaller talk time
Duplex Distance 45MHz
This is the standard distance between the uplink and the downlink frequencies This is not
constant for all versions of GSM however the separation between the uplink amp downlink
bands is constant to 20kHz
Carrier Separation 200kHz
Figure 12 Carrier Separation
In GSM we have uplink and downlink carriers These individual carriers are separated
200kHz apart therefore we get 125 uplink amp downlink carriers These carriers are then so
arranged so that we get 124 ARFCNrsquos (absolute radio frequency carrier numbers) for GSM
3
900 they start form 1-124 Henceforth any mention of channels will be done using their
ARFCN The figure below shows the carriers
Each carrier frequency is then divided according to time using a TDMA scheme Each of the
carrier frequencies is divided into a 120ms multiframe A multiframe is made up of 26
frames Two of these frames are used for control purposes while the remaining 24 frames are
used for traffic as shown
Figure 13 TDMA Multiframe
Modulation Gaussian Minimum Phase shift Keying(GMSK)
The modulation method used in GSM had to be very specific according to the needs of
communication and also to cater for the anomalies in the radio interface However this would
be taken in detail in a later section of this report
Transmission Rate 270kbps
Access Method Time Division Multiple Access(TDMA)
TDMA is used in GSM in conjunction with FDMA to allow voice communication The
200Khz channel is divided into 8 slots and each slot represents a call However the whole
channel is available to the caller were a caller gets particular time duration in a round robin
fashion to proceed with his call as described in the figure below
Figure 14 TDMA Frame
Speech Coder Rapid Pulse Excitation linear Predictive Coder coding at 13kbps
In modern landline telephone systems digital coding is used The electrical variations
induced into the microphone are sampled and each sample is then converted into a digital
4
code The voice waveform is then sampled at a rate of 8 kHz Each sample is then converted
into an 8 bit binary number representing 256 distinct values Since we sample 8000 times per
second and each sample is 8 binary bits we have a bit-rate of
8kHz 8 bits = 64kbps
This bitrate is unrealistic to transmit across a radio network since interference will likely
ruin the transmitted waveform In GSM speech encoding works to compress the speech
waveform into a sample that results in a lower bitrate using RPE-LPC The actual process
will be discussed later in the section where the journey from speech to radio waves is
considered
Over The Channel Bit Rate 228kbps
Slow Frequency Hopping 217hopssecond
GSM can use slow frequency hopping where the mobile station and the base station transmit
each TDMA frame on a different carrier frequency A form of slow frequency hopping is
used by GSM to help combat the multipath burst errors characteristic of cellular
environments Each base station has its own pattern for hopping from one carrier frequency
to another from slot to slot with mobiles using that base station following suit This
frequency hopping also reduces the incidence of co-channel interference between clusters of
cells The frequency-hopping algorithm is broadcast on the Broadcast Control Channel Since
multipath fading is dependent on carrier frequency slow frequency hopping help mitigate the
problem Frequency hopping is an option for each individual cell and a base station is not
required to support this feature
Synchronisation Compensation 223sec
Equalisation Limit Up to 16sec time dispersion
Typical Base Station Transmit Power 320W
Frame Duration 4615ms
Channel Coding Half-Rate Convolutional Coder
13 GSM Bands
5
E-GSM This represents an extension of the lower end of the two sub blocks by 10MHz
adding 50 More ARFCNrsquos to the Primary GSM (P-GSM)
Uplink Frequency 880MHz-915MHz
Downlink Frequency 925MHz-960MHz
DCS 1800 At a late stage in GSM development the existing technology was modified to
meet the need for PCN networks This involves changes to the radio interface which
moves spectrum allocation up to around 18Ghz More spectrum is available in this
frequency range for two sub-blocks of 75Mhz with duplex spacing of 95Mhz giving a
total of 374 carriers
Uplink Frequency 1710Mhz-1785Mhz
Downlink Frequency 1805Mhz-1880Mhz
PCS 1900 Used in the USA The FCC has split the designated spectrum into six duplex
blocks The USA has been divided into 51 Major Trading Areas and 493 Basic Trading
Areas An MTA is broadly equivalent in size to a state whilst a BTA approximates to a
large city Each MTA has access to 3x15MHz block and each BTA has access to 3x5MHz
blocks
14 Features of GSM
The GSM services are grouped into three categories
1 Teleservices (TS)
2 Bearer services (BS)
3 Supplementary services (SS)
141 Teleservices
Regular telephony emergency calls and voice messaging are within TS Telephony the old
bidirectional speech calls is certainly the most popular of all services An emergency call is a
feature that allows the mobile subscriber to contact a nearby emergency service such as
police by dialing a unique number Voice messaging permits a message to be stored within
the voice mailbox of the called party either because the called party is not reachable or
because the calling party chooses to do so
142 Bearer Services
6
Data services short message service (SMS) cell broadcast and local features are within BS
Rates up to 96 kbits are supported With a suitable data terminal or computer connected
directly to the mobile apparatus data may be sent through circuit-switched or packet-
switched networks Short messages containing as many as 160 alphanumeric characters can
be transmitted to or from a mobile phone In this case a message center is necessary The
broadcast mode (to all subscribers) in a given geographic area may also be used for short
messages of up to 93 alphanumeric characters Some local features of the mobile terminal
may be used These may include for example abbreviated dialing edition of short messages
repetition of failed calls and others
143 Supplementary Services
Some of the SS are as follows
Advice of charge This SS details the cost of a call in progress
Barring of all outgoing calls This SS blocks outgoing calls
Barring of international calls This SS blocks incoming or outgoing international calls as
a whole or only those associated with a specific basic service as desired
Barring of roaming calls This SS blocks all the incoming roaming calls or only those
associated with a specific service
Call forwarding This SS forwards all incoming calls or only those associated with a
specific basic service to another directory number The forwarding may be unconditional
or may be performed when the mobile subscriber is busy when there is no reply when
the mobile subscriber is not reachable or when there is radio congestion
Call hold This SS allows interruption of a communication on an existing call
Subsequent reestablishment of the call is permitted
Call waiting This SS permits the notification of an incoming call when the mobile
subscriber is busy
Call transfer This SS permits the transference of an established incoming or outgoing
call to a third party
Completion of calls to busy subscribers This SS allows notification of when a busy
called subscriber becomes free At this time if desired the call is reinitiated
Closed user group This SS allows a group of subscribers to communicate only among
themselves
7
Calling number identification presentationrestriction This SS permits the presentation or
restricts the presentation of the calling partyrsquos identification number (or additional
address information)
Connected number identification presentation This SS indicates the phone number that
has been reached
Free phone service This SS allocates a number to a mobile subscriber and all calls to
that number are free of charge for the calling party
Malicious call identification This SS permits the registration of malicious nuisance and
obscene incoming calls
Three-party service This SS permits the establishment of conference calls
CHAPTER-II
GSM Network Architecture and GSM Channels
8
21 GSM Network Architecture
GSM network architecture is divided into following three categories
Mobile Station (MS) This consists of the mobile telephone fax machine etc This is the
part of the network that the subscriber will see
Base Station Subsystem (BSS) This is the part of the network which provides the radio
interconnection from the MS to the land-based switching equipment
Network Switching Subsystem (NSS) This consists of the Mobile services Switching
Centre (MSC) and its associated system-control databases and processors together with the
required interfaces This is the part which provides for interconnection between the GSM
network and the Public Switched Telephone Network (PSTN)
Network Management System This enables the network provider to configure and
maintain the network from a central location
211 Mobile Station (MS)
In GSM there is a difference between the physical equipment and the subscription The
mobile station is piece of equipment which can be vehicle installed portable or hand-held
In GSM there is a small unit called the Subscriber Identity Module (SIM) which is a separate
physical entity eg an IC-card also called a smart card SIM and the mobile equipment
together make up the mobile station Without SIM the MS cannot get access to the GSM
network except for emergency traffic While the SIM-card is connected to the subscription
and not to the MS the subscriber can use another MS as well as his own This then raises the
problem of stolen MSrsquo since it is no use barring the subscription if the equipment is stolen
We need a database that contains the unique hardware identity of the equipment the
Equipment Identity Register (EIR) The EIR is connected to the MSC over a signalling link
This enables the MSC to check the validity of the equipment An non-type-approved MS can
also be barred in this way The authentication of the subscription is done by parameters from
AUC
9
Fig 21 GSM Network Architecture
10
212 Base Station Subsystem(BSS)
The GSM Base Station System is the equipment located at a cell site It comprises combination
of digital and RF equipment The BSS provides the link between the MS and the MSC BSS
communicates with the MS over the digital air interface and with the MSC via 2 Mbits links
Fig 22 Base Station Subsystem
The BSS consists of three major hardware components
Base Transceiver Station (BTS) The BTS contains the RF components that provide the
air interface for a particular cell This is the part of the GSM network which communicates
with the MS The antenna is included as part of the BTS
11
Base Station Controller (BSC) The BSC as its name implies provides the control for the
BSS The BSC communicates directly with the MSC The BSC may control single or
multiple BTSs
Transcoder (TC) The Transcoder is used to compact the signals from the MS so that they
are more efficiently sent over the terrestrial interfaces Although the transcoder is
considered to be a part of the BSS it is very often located closer to the MSC The
transcoder is used to reduce the rate at which the traffic (voicedata) is transmitted over the
air interface Although the transcoder is part of the BSS it is often found physically closer
to the NSS to allow more efficient use of the terrestrial links
BSS Configurations
Fig 23 BSS Configurations
12
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM
BTSs and BSC may be either located at the same cell site ldquoco-locatedrdquo or located at different
sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs than BSCs
in a network Another BSS configuration is the ldquoDaisy Chainrdquo A BTS need not to
communicate directly with the BSC which controls it it can be connected to the BTS rather
than all the way to BSC Problems may arise when chaining BTSs due to the transmission
delay through the chain the length of the chain therefore should be kept sufficiently short to
prevent the round trip speech delay becoming too long
2121 Base Transceiver Station
BTS is the hardware component in the GSM architecture It provides an interface between
Mobile Station (MS) and Base Station Subsystem (BSS) The BTS provides radio channels
(RF carriers) for a specific RF coverage area The radio channel is the communication link
between the MSs within an RF coverage area and the BSS The BTS also has a limited amount
of control functionality which reduces the amount of traffic between the BTS and BSC
2122 Base Station Controller
Any operational information required by the BTS will be received via the BSC Likewise any
information required about the BTS (by the OMC for example) will be obtained by the BSC
BSC incorporates a digital switching matrix which it uses to connect the radio channels on the
air interface with the terrestrial circuits from the MSC The BSC switching matrix also allows
the BSC to perform ldquohandoversrdquo between radio channels on BTSs under its control without
involving the MSC The purpose of the BSC is to perform a variety of functions Some of them
are listed below
Controls the BTS components
Performs Call Processing
Performs Operations and Maintenance
Provides the OampM link between the BSS and the OMC
Provides the A Interface between the BSS and the MSC
Manages the radio channels
Transfers signaling information to and from different mobile stations (MS)
13
2123 Transcoder
Transcoder is required to convert the speech or data output from MSC(64 kbits PCM) into
the form specified by GSM specifications for the transmission over the air interface that is
between BSS and MS (64 kbits to 16 kbits and vice versa)The 64 kbits Pulse Code
Modulation (PCM) circuits from MSC if transmitted on the air interface without
modification would occupy an excessive amount of radio bandwidth This would use the
available radio spectrum inefficiently The required is therefore reduced by processing the 64
kbits circuits so that the amount of information required to transmit digitized voice falls to a
gross rate of 16 kbits The transcoding function may be located at the MSC BSC or BTS
213 Network Switching Subsystem
The Network Switching System includes the main switching functions of the GSM network It
also contains the databases required for subscriber data and mobility management Its main
function is to manage communications between the GSM network and other
telecommunications networks Various components of the Network Switching System are
listed below
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Centre (AUC)
Inter Working Function (IWF)
Echo Canceller (EC)
In addition to the more traditional elements of a cellular telephone system GSM has Location
Register network entities These entities are the Home Location Register (HLR) Visitor
Location Register (VLR) and the Equipment Identity Register (EIR) The location registers
are database-oriented processing nodes which address the problems of managing subscriber
data and keeping track of a MS location as it roams around the network Functionally the
Interworking Function and the Echo Cancellers may be considered as parts of the MSC since
their activities are inextricably linked with those of the switch as it connects speech and data
calls to and from the MSs
2131 Mobile Switching Centre
14
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Figure 11 GSM Uplink and Downlink
At this moment it would be important to mention here the need to use a lower frequency for
uplink The reason is since this carries the information from the MS to the BTS over the Air
Interface using a higher frequency means higher attenuation Secondly to compensate for the
attenuation we need to send the signal at a higher power which consumes more battery
power and leading to a smaller talk time
Duplex Distance 45MHz
This is the standard distance between the uplink and the downlink frequencies This is not
constant for all versions of GSM however the separation between the uplink amp downlink
bands is constant to 20kHz
Carrier Separation 200kHz
Figure 12 Carrier Separation
In GSM we have uplink and downlink carriers These individual carriers are separated
200kHz apart therefore we get 125 uplink amp downlink carriers These carriers are then so
arranged so that we get 124 ARFCNrsquos (absolute radio frequency carrier numbers) for GSM
3
900 they start form 1-124 Henceforth any mention of channels will be done using their
ARFCN The figure below shows the carriers
Each carrier frequency is then divided according to time using a TDMA scheme Each of the
carrier frequencies is divided into a 120ms multiframe A multiframe is made up of 26
frames Two of these frames are used for control purposes while the remaining 24 frames are
used for traffic as shown
Figure 13 TDMA Multiframe
Modulation Gaussian Minimum Phase shift Keying(GMSK)
The modulation method used in GSM had to be very specific according to the needs of
communication and also to cater for the anomalies in the radio interface However this would
be taken in detail in a later section of this report
Transmission Rate 270kbps
Access Method Time Division Multiple Access(TDMA)
TDMA is used in GSM in conjunction with FDMA to allow voice communication The
200Khz channel is divided into 8 slots and each slot represents a call However the whole
channel is available to the caller were a caller gets particular time duration in a round robin
fashion to proceed with his call as described in the figure below
Figure 14 TDMA Frame
Speech Coder Rapid Pulse Excitation linear Predictive Coder coding at 13kbps
In modern landline telephone systems digital coding is used The electrical variations
induced into the microphone are sampled and each sample is then converted into a digital
4
code The voice waveform is then sampled at a rate of 8 kHz Each sample is then converted
into an 8 bit binary number representing 256 distinct values Since we sample 8000 times per
second and each sample is 8 binary bits we have a bit-rate of
8kHz 8 bits = 64kbps
This bitrate is unrealistic to transmit across a radio network since interference will likely
ruin the transmitted waveform In GSM speech encoding works to compress the speech
waveform into a sample that results in a lower bitrate using RPE-LPC The actual process
will be discussed later in the section where the journey from speech to radio waves is
considered
Over The Channel Bit Rate 228kbps
Slow Frequency Hopping 217hopssecond
GSM can use slow frequency hopping where the mobile station and the base station transmit
each TDMA frame on a different carrier frequency A form of slow frequency hopping is
used by GSM to help combat the multipath burst errors characteristic of cellular
environments Each base station has its own pattern for hopping from one carrier frequency
to another from slot to slot with mobiles using that base station following suit This
frequency hopping also reduces the incidence of co-channel interference between clusters of
cells The frequency-hopping algorithm is broadcast on the Broadcast Control Channel Since
multipath fading is dependent on carrier frequency slow frequency hopping help mitigate the
problem Frequency hopping is an option for each individual cell and a base station is not
required to support this feature
Synchronisation Compensation 223sec
Equalisation Limit Up to 16sec time dispersion
Typical Base Station Transmit Power 320W
Frame Duration 4615ms
Channel Coding Half-Rate Convolutional Coder
13 GSM Bands
5
E-GSM This represents an extension of the lower end of the two sub blocks by 10MHz
adding 50 More ARFCNrsquos to the Primary GSM (P-GSM)
Uplink Frequency 880MHz-915MHz
Downlink Frequency 925MHz-960MHz
DCS 1800 At a late stage in GSM development the existing technology was modified to
meet the need for PCN networks This involves changes to the radio interface which
moves spectrum allocation up to around 18Ghz More spectrum is available in this
frequency range for two sub-blocks of 75Mhz with duplex spacing of 95Mhz giving a
total of 374 carriers
Uplink Frequency 1710Mhz-1785Mhz
Downlink Frequency 1805Mhz-1880Mhz
PCS 1900 Used in the USA The FCC has split the designated spectrum into six duplex
blocks The USA has been divided into 51 Major Trading Areas and 493 Basic Trading
Areas An MTA is broadly equivalent in size to a state whilst a BTA approximates to a
large city Each MTA has access to 3x15MHz block and each BTA has access to 3x5MHz
blocks
14 Features of GSM
The GSM services are grouped into three categories
1 Teleservices (TS)
2 Bearer services (BS)
3 Supplementary services (SS)
141 Teleservices
Regular telephony emergency calls and voice messaging are within TS Telephony the old
bidirectional speech calls is certainly the most popular of all services An emergency call is a
feature that allows the mobile subscriber to contact a nearby emergency service such as
police by dialing a unique number Voice messaging permits a message to be stored within
the voice mailbox of the called party either because the called party is not reachable or
because the calling party chooses to do so
142 Bearer Services
6
Data services short message service (SMS) cell broadcast and local features are within BS
Rates up to 96 kbits are supported With a suitable data terminal or computer connected
directly to the mobile apparatus data may be sent through circuit-switched or packet-
switched networks Short messages containing as many as 160 alphanumeric characters can
be transmitted to or from a mobile phone In this case a message center is necessary The
broadcast mode (to all subscribers) in a given geographic area may also be used for short
messages of up to 93 alphanumeric characters Some local features of the mobile terminal
may be used These may include for example abbreviated dialing edition of short messages
repetition of failed calls and others
143 Supplementary Services
Some of the SS are as follows
Advice of charge This SS details the cost of a call in progress
Barring of all outgoing calls This SS blocks outgoing calls
Barring of international calls This SS blocks incoming or outgoing international calls as
a whole or only those associated with a specific basic service as desired
Barring of roaming calls This SS blocks all the incoming roaming calls or only those
associated with a specific service
Call forwarding This SS forwards all incoming calls or only those associated with a
specific basic service to another directory number The forwarding may be unconditional
or may be performed when the mobile subscriber is busy when there is no reply when
the mobile subscriber is not reachable or when there is radio congestion
Call hold This SS allows interruption of a communication on an existing call
Subsequent reestablishment of the call is permitted
Call waiting This SS permits the notification of an incoming call when the mobile
subscriber is busy
Call transfer This SS permits the transference of an established incoming or outgoing
call to a third party
Completion of calls to busy subscribers This SS allows notification of when a busy
called subscriber becomes free At this time if desired the call is reinitiated
Closed user group This SS allows a group of subscribers to communicate only among
themselves
7
Calling number identification presentationrestriction This SS permits the presentation or
restricts the presentation of the calling partyrsquos identification number (or additional
address information)
Connected number identification presentation This SS indicates the phone number that
has been reached
Free phone service This SS allocates a number to a mobile subscriber and all calls to
that number are free of charge for the calling party
Malicious call identification This SS permits the registration of malicious nuisance and
obscene incoming calls
Three-party service This SS permits the establishment of conference calls
CHAPTER-II
GSM Network Architecture and GSM Channels
8
21 GSM Network Architecture
GSM network architecture is divided into following three categories
Mobile Station (MS) This consists of the mobile telephone fax machine etc This is the
part of the network that the subscriber will see
Base Station Subsystem (BSS) This is the part of the network which provides the radio
interconnection from the MS to the land-based switching equipment
Network Switching Subsystem (NSS) This consists of the Mobile services Switching
Centre (MSC) and its associated system-control databases and processors together with the
required interfaces This is the part which provides for interconnection between the GSM
network and the Public Switched Telephone Network (PSTN)
Network Management System This enables the network provider to configure and
maintain the network from a central location
211 Mobile Station (MS)
In GSM there is a difference between the physical equipment and the subscription The
mobile station is piece of equipment which can be vehicle installed portable or hand-held
In GSM there is a small unit called the Subscriber Identity Module (SIM) which is a separate
physical entity eg an IC-card also called a smart card SIM and the mobile equipment
together make up the mobile station Without SIM the MS cannot get access to the GSM
network except for emergency traffic While the SIM-card is connected to the subscription
and not to the MS the subscriber can use another MS as well as his own This then raises the
problem of stolen MSrsquo since it is no use barring the subscription if the equipment is stolen
We need a database that contains the unique hardware identity of the equipment the
Equipment Identity Register (EIR) The EIR is connected to the MSC over a signalling link
This enables the MSC to check the validity of the equipment An non-type-approved MS can
also be barred in this way The authentication of the subscription is done by parameters from
AUC
9
Fig 21 GSM Network Architecture
10
212 Base Station Subsystem(BSS)
The GSM Base Station System is the equipment located at a cell site It comprises combination
of digital and RF equipment The BSS provides the link between the MS and the MSC BSS
communicates with the MS over the digital air interface and with the MSC via 2 Mbits links
Fig 22 Base Station Subsystem
The BSS consists of three major hardware components
Base Transceiver Station (BTS) The BTS contains the RF components that provide the
air interface for a particular cell This is the part of the GSM network which communicates
with the MS The antenna is included as part of the BTS
11
Base Station Controller (BSC) The BSC as its name implies provides the control for the
BSS The BSC communicates directly with the MSC The BSC may control single or
multiple BTSs
Transcoder (TC) The Transcoder is used to compact the signals from the MS so that they
are more efficiently sent over the terrestrial interfaces Although the transcoder is
considered to be a part of the BSS it is very often located closer to the MSC The
transcoder is used to reduce the rate at which the traffic (voicedata) is transmitted over the
air interface Although the transcoder is part of the BSS it is often found physically closer
to the NSS to allow more efficient use of the terrestrial links
BSS Configurations
Fig 23 BSS Configurations
12
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM
BTSs and BSC may be either located at the same cell site ldquoco-locatedrdquo or located at different
sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs than BSCs
in a network Another BSS configuration is the ldquoDaisy Chainrdquo A BTS need not to
communicate directly with the BSC which controls it it can be connected to the BTS rather
than all the way to BSC Problems may arise when chaining BTSs due to the transmission
delay through the chain the length of the chain therefore should be kept sufficiently short to
prevent the round trip speech delay becoming too long
2121 Base Transceiver Station
BTS is the hardware component in the GSM architecture It provides an interface between
Mobile Station (MS) and Base Station Subsystem (BSS) The BTS provides radio channels
(RF carriers) for a specific RF coverage area The radio channel is the communication link
between the MSs within an RF coverage area and the BSS The BTS also has a limited amount
of control functionality which reduces the amount of traffic between the BTS and BSC
2122 Base Station Controller
Any operational information required by the BTS will be received via the BSC Likewise any
information required about the BTS (by the OMC for example) will be obtained by the BSC
BSC incorporates a digital switching matrix which it uses to connect the radio channels on the
air interface with the terrestrial circuits from the MSC The BSC switching matrix also allows
the BSC to perform ldquohandoversrdquo between radio channels on BTSs under its control without
involving the MSC The purpose of the BSC is to perform a variety of functions Some of them
are listed below
Controls the BTS components
Performs Call Processing
Performs Operations and Maintenance
Provides the OampM link between the BSS and the OMC
Provides the A Interface between the BSS and the MSC
Manages the radio channels
Transfers signaling information to and from different mobile stations (MS)
13
2123 Transcoder
Transcoder is required to convert the speech or data output from MSC(64 kbits PCM) into
the form specified by GSM specifications for the transmission over the air interface that is
between BSS and MS (64 kbits to 16 kbits and vice versa)The 64 kbits Pulse Code
Modulation (PCM) circuits from MSC if transmitted on the air interface without
modification would occupy an excessive amount of radio bandwidth This would use the
available radio spectrum inefficiently The required is therefore reduced by processing the 64
kbits circuits so that the amount of information required to transmit digitized voice falls to a
gross rate of 16 kbits The transcoding function may be located at the MSC BSC or BTS
213 Network Switching Subsystem
The Network Switching System includes the main switching functions of the GSM network It
also contains the databases required for subscriber data and mobility management Its main
function is to manage communications between the GSM network and other
telecommunications networks Various components of the Network Switching System are
listed below
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Centre (AUC)
Inter Working Function (IWF)
Echo Canceller (EC)
In addition to the more traditional elements of a cellular telephone system GSM has Location
Register network entities These entities are the Home Location Register (HLR) Visitor
Location Register (VLR) and the Equipment Identity Register (EIR) The location registers
are database-oriented processing nodes which address the problems of managing subscriber
data and keeping track of a MS location as it roams around the network Functionally the
Interworking Function and the Echo Cancellers may be considered as parts of the MSC since
their activities are inextricably linked with those of the switch as it connects speech and data
calls to and from the MSs
2131 Mobile Switching Centre
14
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
900 they start form 1-124 Henceforth any mention of channels will be done using their
ARFCN The figure below shows the carriers
Each carrier frequency is then divided according to time using a TDMA scheme Each of the
carrier frequencies is divided into a 120ms multiframe A multiframe is made up of 26
frames Two of these frames are used for control purposes while the remaining 24 frames are
used for traffic as shown
Figure 13 TDMA Multiframe
Modulation Gaussian Minimum Phase shift Keying(GMSK)
The modulation method used in GSM had to be very specific according to the needs of
communication and also to cater for the anomalies in the radio interface However this would
be taken in detail in a later section of this report
Transmission Rate 270kbps
Access Method Time Division Multiple Access(TDMA)
TDMA is used in GSM in conjunction with FDMA to allow voice communication The
200Khz channel is divided into 8 slots and each slot represents a call However the whole
channel is available to the caller were a caller gets particular time duration in a round robin
fashion to proceed with his call as described in the figure below
Figure 14 TDMA Frame
Speech Coder Rapid Pulse Excitation linear Predictive Coder coding at 13kbps
In modern landline telephone systems digital coding is used The electrical variations
induced into the microphone are sampled and each sample is then converted into a digital
4
code The voice waveform is then sampled at a rate of 8 kHz Each sample is then converted
into an 8 bit binary number representing 256 distinct values Since we sample 8000 times per
second and each sample is 8 binary bits we have a bit-rate of
8kHz 8 bits = 64kbps
This bitrate is unrealistic to transmit across a radio network since interference will likely
ruin the transmitted waveform In GSM speech encoding works to compress the speech
waveform into a sample that results in a lower bitrate using RPE-LPC The actual process
will be discussed later in the section where the journey from speech to radio waves is
considered
Over The Channel Bit Rate 228kbps
Slow Frequency Hopping 217hopssecond
GSM can use slow frequency hopping where the mobile station and the base station transmit
each TDMA frame on a different carrier frequency A form of slow frequency hopping is
used by GSM to help combat the multipath burst errors characteristic of cellular
environments Each base station has its own pattern for hopping from one carrier frequency
to another from slot to slot with mobiles using that base station following suit This
frequency hopping also reduces the incidence of co-channel interference between clusters of
cells The frequency-hopping algorithm is broadcast on the Broadcast Control Channel Since
multipath fading is dependent on carrier frequency slow frequency hopping help mitigate the
problem Frequency hopping is an option for each individual cell and a base station is not
required to support this feature
Synchronisation Compensation 223sec
Equalisation Limit Up to 16sec time dispersion
Typical Base Station Transmit Power 320W
Frame Duration 4615ms
Channel Coding Half-Rate Convolutional Coder
13 GSM Bands
5
E-GSM This represents an extension of the lower end of the two sub blocks by 10MHz
adding 50 More ARFCNrsquos to the Primary GSM (P-GSM)
Uplink Frequency 880MHz-915MHz
Downlink Frequency 925MHz-960MHz
DCS 1800 At a late stage in GSM development the existing technology was modified to
meet the need for PCN networks This involves changes to the radio interface which
moves spectrum allocation up to around 18Ghz More spectrum is available in this
frequency range for two sub-blocks of 75Mhz with duplex spacing of 95Mhz giving a
total of 374 carriers
Uplink Frequency 1710Mhz-1785Mhz
Downlink Frequency 1805Mhz-1880Mhz
PCS 1900 Used in the USA The FCC has split the designated spectrum into six duplex
blocks The USA has been divided into 51 Major Trading Areas and 493 Basic Trading
Areas An MTA is broadly equivalent in size to a state whilst a BTA approximates to a
large city Each MTA has access to 3x15MHz block and each BTA has access to 3x5MHz
blocks
14 Features of GSM
The GSM services are grouped into three categories
1 Teleservices (TS)
2 Bearer services (BS)
3 Supplementary services (SS)
141 Teleservices
Regular telephony emergency calls and voice messaging are within TS Telephony the old
bidirectional speech calls is certainly the most popular of all services An emergency call is a
feature that allows the mobile subscriber to contact a nearby emergency service such as
police by dialing a unique number Voice messaging permits a message to be stored within
the voice mailbox of the called party either because the called party is not reachable or
because the calling party chooses to do so
142 Bearer Services
6
Data services short message service (SMS) cell broadcast and local features are within BS
Rates up to 96 kbits are supported With a suitable data terminal or computer connected
directly to the mobile apparatus data may be sent through circuit-switched or packet-
switched networks Short messages containing as many as 160 alphanumeric characters can
be transmitted to or from a mobile phone In this case a message center is necessary The
broadcast mode (to all subscribers) in a given geographic area may also be used for short
messages of up to 93 alphanumeric characters Some local features of the mobile terminal
may be used These may include for example abbreviated dialing edition of short messages
repetition of failed calls and others
143 Supplementary Services
Some of the SS are as follows
Advice of charge This SS details the cost of a call in progress
Barring of all outgoing calls This SS blocks outgoing calls
Barring of international calls This SS blocks incoming or outgoing international calls as
a whole or only those associated with a specific basic service as desired
Barring of roaming calls This SS blocks all the incoming roaming calls or only those
associated with a specific service
Call forwarding This SS forwards all incoming calls or only those associated with a
specific basic service to another directory number The forwarding may be unconditional
or may be performed when the mobile subscriber is busy when there is no reply when
the mobile subscriber is not reachable or when there is radio congestion
Call hold This SS allows interruption of a communication on an existing call
Subsequent reestablishment of the call is permitted
Call waiting This SS permits the notification of an incoming call when the mobile
subscriber is busy
Call transfer This SS permits the transference of an established incoming or outgoing
call to a third party
Completion of calls to busy subscribers This SS allows notification of when a busy
called subscriber becomes free At this time if desired the call is reinitiated
Closed user group This SS allows a group of subscribers to communicate only among
themselves
7
Calling number identification presentationrestriction This SS permits the presentation or
restricts the presentation of the calling partyrsquos identification number (or additional
address information)
Connected number identification presentation This SS indicates the phone number that
has been reached
Free phone service This SS allocates a number to a mobile subscriber and all calls to
that number are free of charge for the calling party
Malicious call identification This SS permits the registration of malicious nuisance and
obscene incoming calls
Three-party service This SS permits the establishment of conference calls
CHAPTER-II
GSM Network Architecture and GSM Channels
8
21 GSM Network Architecture
GSM network architecture is divided into following three categories
Mobile Station (MS) This consists of the mobile telephone fax machine etc This is the
part of the network that the subscriber will see
Base Station Subsystem (BSS) This is the part of the network which provides the radio
interconnection from the MS to the land-based switching equipment
Network Switching Subsystem (NSS) This consists of the Mobile services Switching
Centre (MSC) and its associated system-control databases and processors together with the
required interfaces This is the part which provides for interconnection between the GSM
network and the Public Switched Telephone Network (PSTN)
Network Management System This enables the network provider to configure and
maintain the network from a central location
211 Mobile Station (MS)
In GSM there is a difference between the physical equipment and the subscription The
mobile station is piece of equipment which can be vehicle installed portable or hand-held
In GSM there is a small unit called the Subscriber Identity Module (SIM) which is a separate
physical entity eg an IC-card also called a smart card SIM and the mobile equipment
together make up the mobile station Without SIM the MS cannot get access to the GSM
network except for emergency traffic While the SIM-card is connected to the subscription
and not to the MS the subscriber can use another MS as well as his own This then raises the
problem of stolen MSrsquo since it is no use barring the subscription if the equipment is stolen
We need a database that contains the unique hardware identity of the equipment the
Equipment Identity Register (EIR) The EIR is connected to the MSC over a signalling link
This enables the MSC to check the validity of the equipment An non-type-approved MS can
also be barred in this way The authentication of the subscription is done by parameters from
AUC
9
Fig 21 GSM Network Architecture
10
212 Base Station Subsystem(BSS)
The GSM Base Station System is the equipment located at a cell site It comprises combination
of digital and RF equipment The BSS provides the link between the MS and the MSC BSS
communicates with the MS over the digital air interface and with the MSC via 2 Mbits links
Fig 22 Base Station Subsystem
The BSS consists of three major hardware components
Base Transceiver Station (BTS) The BTS contains the RF components that provide the
air interface for a particular cell This is the part of the GSM network which communicates
with the MS The antenna is included as part of the BTS
11
Base Station Controller (BSC) The BSC as its name implies provides the control for the
BSS The BSC communicates directly with the MSC The BSC may control single or
multiple BTSs
Transcoder (TC) The Transcoder is used to compact the signals from the MS so that they
are more efficiently sent over the terrestrial interfaces Although the transcoder is
considered to be a part of the BSS it is very often located closer to the MSC The
transcoder is used to reduce the rate at which the traffic (voicedata) is transmitted over the
air interface Although the transcoder is part of the BSS it is often found physically closer
to the NSS to allow more efficient use of the terrestrial links
BSS Configurations
Fig 23 BSS Configurations
12
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM
BTSs and BSC may be either located at the same cell site ldquoco-locatedrdquo or located at different
sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs than BSCs
in a network Another BSS configuration is the ldquoDaisy Chainrdquo A BTS need not to
communicate directly with the BSC which controls it it can be connected to the BTS rather
than all the way to BSC Problems may arise when chaining BTSs due to the transmission
delay through the chain the length of the chain therefore should be kept sufficiently short to
prevent the round trip speech delay becoming too long
2121 Base Transceiver Station
BTS is the hardware component in the GSM architecture It provides an interface between
Mobile Station (MS) and Base Station Subsystem (BSS) The BTS provides radio channels
(RF carriers) for a specific RF coverage area The radio channel is the communication link
between the MSs within an RF coverage area and the BSS The BTS also has a limited amount
of control functionality which reduces the amount of traffic between the BTS and BSC
2122 Base Station Controller
Any operational information required by the BTS will be received via the BSC Likewise any
information required about the BTS (by the OMC for example) will be obtained by the BSC
BSC incorporates a digital switching matrix which it uses to connect the radio channels on the
air interface with the terrestrial circuits from the MSC The BSC switching matrix also allows
the BSC to perform ldquohandoversrdquo between radio channels on BTSs under its control without
involving the MSC The purpose of the BSC is to perform a variety of functions Some of them
are listed below
Controls the BTS components
Performs Call Processing
Performs Operations and Maintenance
Provides the OampM link between the BSS and the OMC
Provides the A Interface between the BSS and the MSC
Manages the radio channels
Transfers signaling information to and from different mobile stations (MS)
13
2123 Transcoder
Transcoder is required to convert the speech or data output from MSC(64 kbits PCM) into
the form specified by GSM specifications for the transmission over the air interface that is
between BSS and MS (64 kbits to 16 kbits and vice versa)The 64 kbits Pulse Code
Modulation (PCM) circuits from MSC if transmitted on the air interface without
modification would occupy an excessive amount of radio bandwidth This would use the
available radio spectrum inefficiently The required is therefore reduced by processing the 64
kbits circuits so that the amount of information required to transmit digitized voice falls to a
gross rate of 16 kbits The transcoding function may be located at the MSC BSC or BTS
213 Network Switching Subsystem
The Network Switching System includes the main switching functions of the GSM network It
also contains the databases required for subscriber data and mobility management Its main
function is to manage communications between the GSM network and other
telecommunications networks Various components of the Network Switching System are
listed below
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Centre (AUC)
Inter Working Function (IWF)
Echo Canceller (EC)
In addition to the more traditional elements of a cellular telephone system GSM has Location
Register network entities These entities are the Home Location Register (HLR) Visitor
Location Register (VLR) and the Equipment Identity Register (EIR) The location registers
are database-oriented processing nodes which address the problems of managing subscriber
data and keeping track of a MS location as it roams around the network Functionally the
Interworking Function and the Echo Cancellers may be considered as parts of the MSC since
their activities are inextricably linked with those of the switch as it connects speech and data
calls to and from the MSs
2131 Mobile Switching Centre
14
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
code The voice waveform is then sampled at a rate of 8 kHz Each sample is then converted
into an 8 bit binary number representing 256 distinct values Since we sample 8000 times per
second and each sample is 8 binary bits we have a bit-rate of
8kHz 8 bits = 64kbps
This bitrate is unrealistic to transmit across a radio network since interference will likely
ruin the transmitted waveform In GSM speech encoding works to compress the speech
waveform into a sample that results in a lower bitrate using RPE-LPC The actual process
will be discussed later in the section where the journey from speech to radio waves is
considered
Over The Channel Bit Rate 228kbps
Slow Frequency Hopping 217hopssecond
GSM can use slow frequency hopping where the mobile station and the base station transmit
each TDMA frame on a different carrier frequency A form of slow frequency hopping is
used by GSM to help combat the multipath burst errors characteristic of cellular
environments Each base station has its own pattern for hopping from one carrier frequency
to another from slot to slot with mobiles using that base station following suit This
frequency hopping also reduces the incidence of co-channel interference between clusters of
cells The frequency-hopping algorithm is broadcast on the Broadcast Control Channel Since
multipath fading is dependent on carrier frequency slow frequency hopping help mitigate the
problem Frequency hopping is an option for each individual cell and a base station is not
required to support this feature
Synchronisation Compensation 223sec
Equalisation Limit Up to 16sec time dispersion
Typical Base Station Transmit Power 320W
Frame Duration 4615ms
Channel Coding Half-Rate Convolutional Coder
13 GSM Bands
5
E-GSM This represents an extension of the lower end of the two sub blocks by 10MHz
adding 50 More ARFCNrsquos to the Primary GSM (P-GSM)
Uplink Frequency 880MHz-915MHz
Downlink Frequency 925MHz-960MHz
DCS 1800 At a late stage in GSM development the existing technology was modified to
meet the need for PCN networks This involves changes to the radio interface which
moves spectrum allocation up to around 18Ghz More spectrum is available in this
frequency range for two sub-blocks of 75Mhz with duplex spacing of 95Mhz giving a
total of 374 carriers
Uplink Frequency 1710Mhz-1785Mhz
Downlink Frequency 1805Mhz-1880Mhz
PCS 1900 Used in the USA The FCC has split the designated spectrum into six duplex
blocks The USA has been divided into 51 Major Trading Areas and 493 Basic Trading
Areas An MTA is broadly equivalent in size to a state whilst a BTA approximates to a
large city Each MTA has access to 3x15MHz block and each BTA has access to 3x5MHz
blocks
14 Features of GSM
The GSM services are grouped into three categories
1 Teleservices (TS)
2 Bearer services (BS)
3 Supplementary services (SS)
141 Teleservices
Regular telephony emergency calls and voice messaging are within TS Telephony the old
bidirectional speech calls is certainly the most popular of all services An emergency call is a
feature that allows the mobile subscriber to contact a nearby emergency service such as
police by dialing a unique number Voice messaging permits a message to be stored within
the voice mailbox of the called party either because the called party is not reachable or
because the calling party chooses to do so
142 Bearer Services
6
Data services short message service (SMS) cell broadcast and local features are within BS
Rates up to 96 kbits are supported With a suitable data terminal or computer connected
directly to the mobile apparatus data may be sent through circuit-switched or packet-
switched networks Short messages containing as many as 160 alphanumeric characters can
be transmitted to or from a mobile phone In this case a message center is necessary The
broadcast mode (to all subscribers) in a given geographic area may also be used for short
messages of up to 93 alphanumeric characters Some local features of the mobile terminal
may be used These may include for example abbreviated dialing edition of short messages
repetition of failed calls and others
143 Supplementary Services
Some of the SS are as follows
Advice of charge This SS details the cost of a call in progress
Barring of all outgoing calls This SS blocks outgoing calls
Barring of international calls This SS blocks incoming or outgoing international calls as
a whole or only those associated with a specific basic service as desired
Barring of roaming calls This SS blocks all the incoming roaming calls or only those
associated with a specific service
Call forwarding This SS forwards all incoming calls or only those associated with a
specific basic service to another directory number The forwarding may be unconditional
or may be performed when the mobile subscriber is busy when there is no reply when
the mobile subscriber is not reachable or when there is radio congestion
Call hold This SS allows interruption of a communication on an existing call
Subsequent reestablishment of the call is permitted
Call waiting This SS permits the notification of an incoming call when the mobile
subscriber is busy
Call transfer This SS permits the transference of an established incoming or outgoing
call to a third party
Completion of calls to busy subscribers This SS allows notification of when a busy
called subscriber becomes free At this time if desired the call is reinitiated
Closed user group This SS allows a group of subscribers to communicate only among
themselves
7
Calling number identification presentationrestriction This SS permits the presentation or
restricts the presentation of the calling partyrsquos identification number (or additional
address information)
Connected number identification presentation This SS indicates the phone number that
has been reached
Free phone service This SS allocates a number to a mobile subscriber and all calls to
that number are free of charge for the calling party
Malicious call identification This SS permits the registration of malicious nuisance and
obscene incoming calls
Three-party service This SS permits the establishment of conference calls
CHAPTER-II
GSM Network Architecture and GSM Channels
8
21 GSM Network Architecture
GSM network architecture is divided into following three categories
Mobile Station (MS) This consists of the mobile telephone fax machine etc This is the
part of the network that the subscriber will see
Base Station Subsystem (BSS) This is the part of the network which provides the radio
interconnection from the MS to the land-based switching equipment
Network Switching Subsystem (NSS) This consists of the Mobile services Switching
Centre (MSC) and its associated system-control databases and processors together with the
required interfaces This is the part which provides for interconnection between the GSM
network and the Public Switched Telephone Network (PSTN)
Network Management System This enables the network provider to configure and
maintain the network from a central location
211 Mobile Station (MS)
In GSM there is a difference between the physical equipment and the subscription The
mobile station is piece of equipment which can be vehicle installed portable or hand-held
In GSM there is a small unit called the Subscriber Identity Module (SIM) which is a separate
physical entity eg an IC-card also called a smart card SIM and the mobile equipment
together make up the mobile station Without SIM the MS cannot get access to the GSM
network except for emergency traffic While the SIM-card is connected to the subscription
and not to the MS the subscriber can use another MS as well as his own This then raises the
problem of stolen MSrsquo since it is no use barring the subscription if the equipment is stolen
We need a database that contains the unique hardware identity of the equipment the
Equipment Identity Register (EIR) The EIR is connected to the MSC over a signalling link
This enables the MSC to check the validity of the equipment An non-type-approved MS can
also be barred in this way The authentication of the subscription is done by parameters from
AUC
9
Fig 21 GSM Network Architecture
10
212 Base Station Subsystem(BSS)
The GSM Base Station System is the equipment located at a cell site It comprises combination
of digital and RF equipment The BSS provides the link between the MS and the MSC BSS
communicates with the MS over the digital air interface and with the MSC via 2 Mbits links
Fig 22 Base Station Subsystem
The BSS consists of three major hardware components
Base Transceiver Station (BTS) The BTS contains the RF components that provide the
air interface for a particular cell This is the part of the GSM network which communicates
with the MS The antenna is included as part of the BTS
11
Base Station Controller (BSC) The BSC as its name implies provides the control for the
BSS The BSC communicates directly with the MSC The BSC may control single or
multiple BTSs
Transcoder (TC) The Transcoder is used to compact the signals from the MS so that they
are more efficiently sent over the terrestrial interfaces Although the transcoder is
considered to be a part of the BSS it is very often located closer to the MSC The
transcoder is used to reduce the rate at which the traffic (voicedata) is transmitted over the
air interface Although the transcoder is part of the BSS it is often found physically closer
to the NSS to allow more efficient use of the terrestrial links
BSS Configurations
Fig 23 BSS Configurations
12
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM
BTSs and BSC may be either located at the same cell site ldquoco-locatedrdquo or located at different
sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs than BSCs
in a network Another BSS configuration is the ldquoDaisy Chainrdquo A BTS need not to
communicate directly with the BSC which controls it it can be connected to the BTS rather
than all the way to BSC Problems may arise when chaining BTSs due to the transmission
delay through the chain the length of the chain therefore should be kept sufficiently short to
prevent the round trip speech delay becoming too long
2121 Base Transceiver Station
BTS is the hardware component in the GSM architecture It provides an interface between
Mobile Station (MS) and Base Station Subsystem (BSS) The BTS provides radio channels
(RF carriers) for a specific RF coverage area The radio channel is the communication link
between the MSs within an RF coverage area and the BSS The BTS also has a limited amount
of control functionality which reduces the amount of traffic between the BTS and BSC
2122 Base Station Controller
Any operational information required by the BTS will be received via the BSC Likewise any
information required about the BTS (by the OMC for example) will be obtained by the BSC
BSC incorporates a digital switching matrix which it uses to connect the radio channels on the
air interface with the terrestrial circuits from the MSC The BSC switching matrix also allows
the BSC to perform ldquohandoversrdquo between radio channels on BTSs under its control without
involving the MSC The purpose of the BSC is to perform a variety of functions Some of them
are listed below
Controls the BTS components
Performs Call Processing
Performs Operations and Maintenance
Provides the OampM link between the BSS and the OMC
Provides the A Interface between the BSS and the MSC
Manages the radio channels
Transfers signaling information to and from different mobile stations (MS)
13
2123 Transcoder
Transcoder is required to convert the speech or data output from MSC(64 kbits PCM) into
the form specified by GSM specifications for the transmission over the air interface that is
between BSS and MS (64 kbits to 16 kbits and vice versa)The 64 kbits Pulse Code
Modulation (PCM) circuits from MSC if transmitted on the air interface without
modification would occupy an excessive amount of radio bandwidth This would use the
available radio spectrum inefficiently The required is therefore reduced by processing the 64
kbits circuits so that the amount of information required to transmit digitized voice falls to a
gross rate of 16 kbits The transcoding function may be located at the MSC BSC or BTS
213 Network Switching Subsystem
The Network Switching System includes the main switching functions of the GSM network It
also contains the databases required for subscriber data and mobility management Its main
function is to manage communications between the GSM network and other
telecommunications networks Various components of the Network Switching System are
listed below
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Centre (AUC)
Inter Working Function (IWF)
Echo Canceller (EC)
In addition to the more traditional elements of a cellular telephone system GSM has Location
Register network entities These entities are the Home Location Register (HLR) Visitor
Location Register (VLR) and the Equipment Identity Register (EIR) The location registers
are database-oriented processing nodes which address the problems of managing subscriber
data and keeping track of a MS location as it roams around the network Functionally the
Interworking Function and the Echo Cancellers may be considered as parts of the MSC since
their activities are inextricably linked with those of the switch as it connects speech and data
calls to and from the MSs
2131 Mobile Switching Centre
14
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
E-GSM This represents an extension of the lower end of the two sub blocks by 10MHz
adding 50 More ARFCNrsquos to the Primary GSM (P-GSM)
Uplink Frequency 880MHz-915MHz
Downlink Frequency 925MHz-960MHz
DCS 1800 At a late stage in GSM development the existing technology was modified to
meet the need for PCN networks This involves changes to the radio interface which
moves spectrum allocation up to around 18Ghz More spectrum is available in this
frequency range for two sub-blocks of 75Mhz with duplex spacing of 95Mhz giving a
total of 374 carriers
Uplink Frequency 1710Mhz-1785Mhz
Downlink Frequency 1805Mhz-1880Mhz
PCS 1900 Used in the USA The FCC has split the designated spectrum into six duplex
blocks The USA has been divided into 51 Major Trading Areas and 493 Basic Trading
Areas An MTA is broadly equivalent in size to a state whilst a BTA approximates to a
large city Each MTA has access to 3x15MHz block and each BTA has access to 3x5MHz
blocks
14 Features of GSM
The GSM services are grouped into three categories
1 Teleservices (TS)
2 Bearer services (BS)
3 Supplementary services (SS)
141 Teleservices
Regular telephony emergency calls and voice messaging are within TS Telephony the old
bidirectional speech calls is certainly the most popular of all services An emergency call is a
feature that allows the mobile subscriber to contact a nearby emergency service such as
police by dialing a unique number Voice messaging permits a message to be stored within
the voice mailbox of the called party either because the called party is not reachable or
because the calling party chooses to do so
142 Bearer Services
6
Data services short message service (SMS) cell broadcast and local features are within BS
Rates up to 96 kbits are supported With a suitable data terminal or computer connected
directly to the mobile apparatus data may be sent through circuit-switched or packet-
switched networks Short messages containing as many as 160 alphanumeric characters can
be transmitted to or from a mobile phone In this case a message center is necessary The
broadcast mode (to all subscribers) in a given geographic area may also be used for short
messages of up to 93 alphanumeric characters Some local features of the mobile terminal
may be used These may include for example abbreviated dialing edition of short messages
repetition of failed calls and others
143 Supplementary Services
Some of the SS are as follows
Advice of charge This SS details the cost of a call in progress
Barring of all outgoing calls This SS blocks outgoing calls
Barring of international calls This SS blocks incoming or outgoing international calls as
a whole or only those associated with a specific basic service as desired
Barring of roaming calls This SS blocks all the incoming roaming calls or only those
associated with a specific service
Call forwarding This SS forwards all incoming calls or only those associated with a
specific basic service to another directory number The forwarding may be unconditional
or may be performed when the mobile subscriber is busy when there is no reply when
the mobile subscriber is not reachable or when there is radio congestion
Call hold This SS allows interruption of a communication on an existing call
Subsequent reestablishment of the call is permitted
Call waiting This SS permits the notification of an incoming call when the mobile
subscriber is busy
Call transfer This SS permits the transference of an established incoming or outgoing
call to a third party
Completion of calls to busy subscribers This SS allows notification of when a busy
called subscriber becomes free At this time if desired the call is reinitiated
Closed user group This SS allows a group of subscribers to communicate only among
themselves
7
Calling number identification presentationrestriction This SS permits the presentation or
restricts the presentation of the calling partyrsquos identification number (or additional
address information)
Connected number identification presentation This SS indicates the phone number that
has been reached
Free phone service This SS allocates a number to a mobile subscriber and all calls to
that number are free of charge for the calling party
Malicious call identification This SS permits the registration of malicious nuisance and
obscene incoming calls
Three-party service This SS permits the establishment of conference calls
CHAPTER-II
GSM Network Architecture and GSM Channels
8
21 GSM Network Architecture
GSM network architecture is divided into following three categories
Mobile Station (MS) This consists of the mobile telephone fax machine etc This is the
part of the network that the subscriber will see
Base Station Subsystem (BSS) This is the part of the network which provides the radio
interconnection from the MS to the land-based switching equipment
Network Switching Subsystem (NSS) This consists of the Mobile services Switching
Centre (MSC) and its associated system-control databases and processors together with the
required interfaces This is the part which provides for interconnection between the GSM
network and the Public Switched Telephone Network (PSTN)
Network Management System This enables the network provider to configure and
maintain the network from a central location
211 Mobile Station (MS)
In GSM there is a difference between the physical equipment and the subscription The
mobile station is piece of equipment which can be vehicle installed portable or hand-held
In GSM there is a small unit called the Subscriber Identity Module (SIM) which is a separate
physical entity eg an IC-card also called a smart card SIM and the mobile equipment
together make up the mobile station Without SIM the MS cannot get access to the GSM
network except for emergency traffic While the SIM-card is connected to the subscription
and not to the MS the subscriber can use another MS as well as his own This then raises the
problem of stolen MSrsquo since it is no use barring the subscription if the equipment is stolen
We need a database that contains the unique hardware identity of the equipment the
Equipment Identity Register (EIR) The EIR is connected to the MSC over a signalling link
This enables the MSC to check the validity of the equipment An non-type-approved MS can
also be barred in this way The authentication of the subscription is done by parameters from
AUC
9
Fig 21 GSM Network Architecture
10
212 Base Station Subsystem(BSS)
The GSM Base Station System is the equipment located at a cell site It comprises combination
of digital and RF equipment The BSS provides the link between the MS and the MSC BSS
communicates with the MS over the digital air interface and with the MSC via 2 Mbits links
Fig 22 Base Station Subsystem
The BSS consists of three major hardware components
Base Transceiver Station (BTS) The BTS contains the RF components that provide the
air interface for a particular cell This is the part of the GSM network which communicates
with the MS The antenna is included as part of the BTS
11
Base Station Controller (BSC) The BSC as its name implies provides the control for the
BSS The BSC communicates directly with the MSC The BSC may control single or
multiple BTSs
Transcoder (TC) The Transcoder is used to compact the signals from the MS so that they
are more efficiently sent over the terrestrial interfaces Although the transcoder is
considered to be a part of the BSS it is very often located closer to the MSC The
transcoder is used to reduce the rate at which the traffic (voicedata) is transmitted over the
air interface Although the transcoder is part of the BSS it is often found physically closer
to the NSS to allow more efficient use of the terrestrial links
BSS Configurations
Fig 23 BSS Configurations
12
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM
BTSs and BSC may be either located at the same cell site ldquoco-locatedrdquo or located at different
sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs than BSCs
in a network Another BSS configuration is the ldquoDaisy Chainrdquo A BTS need not to
communicate directly with the BSC which controls it it can be connected to the BTS rather
than all the way to BSC Problems may arise when chaining BTSs due to the transmission
delay through the chain the length of the chain therefore should be kept sufficiently short to
prevent the round trip speech delay becoming too long
2121 Base Transceiver Station
BTS is the hardware component in the GSM architecture It provides an interface between
Mobile Station (MS) and Base Station Subsystem (BSS) The BTS provides radio channels
(RF carriers) for a specific RF coverage area The radio channel is the communication link
between the MSs within an RF coverage area and the BSS The BTS also has a limited amount
of control functionality which reduces the amount of traffic between the BTS and BSC
2122 Base Station Controller
Any operational information required by the BTS will be received via the BSC Likewise any
information required about the BTS (by the OMC for example) will be obtained by the BSC
BSC incorporates a digital switching matrix which it uses to connect the radio channels on the
air interface with the terrestrial circuits from the MSC The BSC switching matrix also allows
the BSC to perform ldquohandoversrdquo between radio channels on BTSs under its control without
involving the MSC The purpose of the BSC is to perform a variety of functions Some of them
are listed below
Controls the BTS components
Performs Call Processing
Performs Operations and Maintenance
Provides the OampM link between the BSS and the OMC
Provides the A Interface between the BSS and the MSC
Manages the radio channels
Transfers signaling information to and from different mobile stations (MS)
13
2123 Transcoder
Transcoder is required to convert the speech or data output from MSC(64 kbits PCM) into
the form specified by GSM specifications for the transmission over the air interface that is
between BSS and MS (64 kbits to 16 kbits and vice versa)The 64 kbits Pulse Code
Modulation (PCM) circuits from MSC if transmitted on the air interface without
modification would occupy an excessive amount of radio bandwidth This would use the
available radio spectrum inefficiently The required is therefore reduced by processing the 64
kbits circuits so that the amount of information required to transmit digitized voice falls to a
gross rate of 16 kbits The transcoding function may be located at the MSC BSC or BTS
213 Network Switching Subsystem
The Network Switching System includes the main switching functions of the GSM network It
also contains the databases required for subscriber data and mobility management Its main
function is to manage communications between the GSM network and other
telecommunications networks Various components of the Network Switching System are
listed below
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Centre (AUC)
Inter Working Function (IWF)
Echo Canceller (EC)
In addition to the more traditional elements of a cellular telephone system GSM has Location
Register network entities These entities are the Home Location Register (HLR) Visitor
Location Register (VLR) and the Equipment Identity Register (EIR) The location registers
are database-oriented processing nodes which address the problems of managing subscriber
data and keeping track of a MS location as it roams around the network Functionally the
Interworking Function and the Echo Cancellers may be considered as parts of the MSC since
their activities are inextricably linked with those of the switch as it connects speech and data
calls to and from the MSs
2131 Mobile Switching Centre
14
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Data services short message service (SMS) cell broadcast and local features are within BS
Rates up to 96 kbits are supported With a suitable data terminal or computer connected
directly to the mobile apparatus data may be sent through circuit-switched or packet-
switched networks Short messages containing as many as 160 alphanumeric characters can
be transmitted to or from a mobile phone In this case a message center is necessary The
broadcast mode (to all subscribers) in a given geographic area may also be used for short
messages of up to 93 alphanumeric characters Some local features of the mobile terminal
may be used These may include for example abbreviated dialing edition of short messages
repetition of failed calls and others
143 Supplementary Services
Some of the SS are as follows
Advice of charge This SS details the cost of a call in progress
Barring of all outgoing calls This SS blocks outgoing calls
Barring of international calls This SS blocks incoming or outgoing international calls as
a whole or only those associated with a specific basic service as desired
Barring of roaming calls This SS blocks all the incoming roaming calls or only those
associated with a specific service
Call forwarding This SS forwards all incoming calls or only those associated with a
specific basic service to another directory number The forwarding may be unconditional
or may be performed when the mobile subscriber is busy when there is no reply when
the mobile subscriber is not reachable or when there is radio congestion
Call hold This SS allows interruption of a communication on an existing call
Subsequent reestablishment of the call is permitted
Call waiting This SS permits the notification of an incoming call when the mobile
subscriber is busy
Call transfer This SS permits the transference of an established incoming or outgoing
call to a third party
Completion of calls to busy subscribers This SS allows notification of when a busy
called subscriber becomes free At this time if desired the call is reinitiated
Closed user group This SS allows a group of subscribers to communicate only among
themselves
7
Calling number identification presentationrestriction This SS permits the presentation or
restricts the presentation of the calling partyrsquos identification number (or additional
address information)
Connected number identification presentation This SS indicates the phone number that
has been reached
Free phone service This SS allocates a number to a mobile subscriber and all calls to
that number are free of charge for the calling party
Malicious call identification This SS permits the registration of malicious nuisance and
obscene incoming calls
Three-party service This SS permits the establishment of conference calls
CHAPTER-II
GSM Network Architecture and GSM Channels
8
21 GSM Network Architecture
GSM network architecture is divided into following three categories
Mobile Station (MS) This consists of the mobile telephone fax machine etc This is the
part of the network that the subscriber will see
Base Station Subsystem (BSS) This is the part of the network which provides the radio
interconnection from the MS to the land-based switching equipment
Network Switching Subsystem (NSS) This consists of the Mobile services Switching
Centre (MSC) and its associated system-control databases and processors together with the
required interfaces This is the part which provides for interconnection between the GSM
network and the Public Switched Telephone Network (PSTN)
Network Management System This enables the network provider to configure and
maintain the network from a central location
211 Mobile Station (MS)
In GSM there is a difference between the physical equipment and the subscription The
mobile station is piece of equipment which can be vehicle installed portable or hand-held
In GSM there is a small unit called the Subscriber Identity Module (SIM) which is a separate
physical entity eg an IC-card also called a smart card SIM and the mobile equipment
together make up the mobile station Without SIM the MS cannot get access to the GSM
network except for emergency traffic While the SIM-card is connected to the subscription
and not to the MS the subscriber can use another MS as well as his own This then raises the
problem of stolen MSrsquo since it is no use barring the subscription if the equipment is stolen
We need a database that contains the unique hardware identity of the equipment the
Equipment Identity Register (EIR) The EIR is connected to the MSC over a signalling link
This enables the MSC to check the validity of the equipment An non-type-approved MS can
also be barred in this way The authentication of the subscription is done by parameters from
AUC
9
Fig 21 GSM Network Architecture
10
212 Base Station Subsystem(BSS)
The GSM Base Station System is the equipment located at a cell site It comprises combination
of digital and RF equipment The BSS provides the link between the MS and the MSC BSS
communicates with the MS over the digital air interface and with the MSC via 2 Mbits links
Fig 22 Base Station Subsystem
The BSS consists of three major hardware components
Base Transceiver Station (BTS) The BTS contains the RF components that provide the
air interface for a particular cell This is the part of the GSM network which communicates
with the MS The antenna is included as part of the BTS
11
Base Station Controller (BSC) The BSC as its name implies provides the control for the
BSS The BSC communicates directly with the MSC The BSC may control single or
multiple BTSs
Transcoder (TC) The Transcoder is used to compact the signals from the MS so that they
are more efficiently sent over the terrestrial interfaces Although the transcoder is
considered to be a part of the BSS it is very often located closer to the MSC The
transcoder is used to reduce the rate at which the traffic (voicedata) is transmitted over the
air interface Although the transcoder is part of the BSS it is often found physically closer
to the NSS to allow more efficient use of the terrestrial links
BSS Configurations
Fig 23 BSS Configurations
12
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM
BTSs and BSC may be either located at the same cell site ldquoco-locatedrdquo or located at different
sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs than BSCs
in a network Another BSS configuration is the ldquoDaisy Chainrdquo A BTS need not to
communicate directly with the BSC which controls it it can be connected to the BTS rather
than all the way to BSC Problems may arise when chaining BTSs due to the transmission
delay through the chain the length of the chain therefore should be kept sufficiently short to
prevent the round trip speech delay becoming too long
2121 Base Transceiver Station
BTS is the hardware component in the GSM architecture It provides an interface between
Mobile Station (MS) and Base Station Subsystem (BSS) The BTS provides radio channels
(RF carriers) for a specific RF coverage area The radio channel is the communication link
between the MSs within an RF coverage area and the BSS The BTS also has a limited amount
of control functionality which reduces the amount of traffic between the BTS and BSC
2122 Base Station Controller
Any operational information required by the BTS will be received via the BSC Likewise any
information required about the BTS (by the OMC for example) will be obtained by the BSC
BSC incorporates a digital switching matrix which it uses to connect the radio channels on the
air interface with the terrestrial circuits from the MSC The BSC switching matrix also allows
the BSC to perform ldquohandoversrdquo between radio channels on BTSs under its control without
involving the MSC The purpose of the BSC is to perform a variety of functions Some of them
are listed below
Controls the BTS components
Performs Call Processing
Performs Operations and Maintenance
Provides the OampM link between the BSS and the OMC
Provides the A Interface between the BSS and the MSC
Manages the radio channels
Transfers signaling information to and from different mobile stations (MS)
13
2123 Transcoder
Transcoder is required to convert the speech or data output from MSC(64 kbits PCM) into
the form specified by GSM specifications for the transmission over the air interface that is
between BSS and MS (64 kbits to 16 kbits and vice versa)The 64 kbits Pulse Code
Modulation (PCM) circuits from MSC if transmitted on the air interface without
modification would occupy an excessive amount of radio bandwidth This would use the
available radio spectrum inefficiently The required is therefore reduced by processing the 64
kbits circuits so that the amount of information required to transmit digitized voice falls to a
gross rate of 16 kbits The transcoding function may be located at the MSC BSC or BTS
213 Network Switching Subsystem
The Network Switching System includes the main switching functions of the GSM network It
also contains the databases required for subscriber data and mobility management Its main
function is to manage communications between the GSM network and other
telecommunications networks Various components of the Network Switching System are
listed below
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Centre (AUC)
Inter Working Function (IWF)
Echo Canceller (EC)
In addition to the more traditional elements of a cellular telephone system GSM has Location
Register network entities These entities are the Home Location Register (HLR) Visitor
Location Register (VLR) and the Equipment Identity Register (EIR) The location registers
are database-oriented processing nodes which address the problems of managing subscriber
data and keeping track of a MS location as it roams around the network Functionally the
Interworking Function and the Echo Cancellers may be considered as parts of the MSC since
their activities are inextricably linked with those of the switch as it connects speech and data
calls to and from the MSs
2131 Mobile Switching Centre
14
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Calling number identification presentationrestriction This SS permits the presentation or
restricts the presentation of the calling partyrsquos identification number (or additional
address information)
Connected number identification presentation This SS indicates the phone number that
has been reached
Free phone service This SS allocates a number to a mobile subscriber and all calls to
that number are free of charge for the calling party
Malicious call identification This SS permits the registration of malicious nuisance and
obscene incoming calls
Three-party service This SS permits the establishment of conference calls
CHAPTER-II
GSM Network Architecture and GSM Channels
8
21 GSM Network Architecture
GSM network architecture is divided into following three categories
Mobile Station (MS) This consists of the mobile telephone fax machine etc This is the
part of the network that the subscriber will see
Base Station Subsystem (BSS) This is the part of the network which provides the radio
interconnection from the MS to the land-based switching equipment
Network Switching Subsystem (NSS) This consists of the Mobile services Switching
Centre (MSC) and its associated system-control databases and processors together with the
required interfaces This is the part which provides for interconnection between the GSM
network and the Public Switched Telephone Network (PSTN)
Network Management System This enables the network provider to configure and
maintain the network from a central location
211 Mobile Station (MS)
In GSM there is a difference between the physical equipment and the subscription The
mobile station is piece of equipment which can be vehicle installed portable or hand-held
In GSM there is a small unit called the Subscriber Identity Module (SIM) which is a separate
physical entity eg an IC-card also called a smart card SIM and the mobile equipment
together make up the mobile station Without SIM the MS cannot get access to the GSM
network except for emergency traffic While the SIM-card is connected to the subscription
and not to the MS the subscriber can use another MS as well as his own This then raises the
problem of stolen MSrsquo since it is no use barring the subscription if the equipment is stolen
We need a database that contains the unique hardware identity of the equipment the
Equipment Identity Register (EIR) The EIR is connected to the MSC over a signalling link
This enables the MSC to check the validity of the equipment An non-type-approved MS can
also be barred in this way The authentication of the subscription is done by parameters from
AUC
9
Fig 21 GSM Network Architecture
10
212 Base Station Subsystem(BSS)
The GSM Base Station System is the equipment located at a cell site It comprises combination
of digital and RF equipment The BSS provides the link between the MS and the MSC BSS
communicates with the MS over the digital air interface and with the MSC via 2 Mbits links
Fig 22 Base Station Subsystem
The BSS consists of three major hardware components
Base Transceiver Station (BTS) The BTS contains the RF components that provide the
air interface for a particular cell This is the part of the GSM network which communicates
with the MS The antenna is included as part of the BTS
11
Base Station Controller (BSC) The BSC as its name implies provides the control for the
BSS The BSC communicates directly with the MSC The BSC may control single or
multiple BTSs
Transcoder (TC) The Transcoder is used to compact the signals from the MS so that they
are more efficiently sent over the terrestrial interfaces Although the transcoder is
considered to be a part of the BSS it is very often located closer to the MSC The
transcoder is used to reduce the rate at which the traffic (voicedata) is transmitted over the
air interface Although the transcoder is part of the BSS it is often found physically closer
to the NSS to allow more efficient use of the terrestrial links
BSS Configurations
Fig 23 BSS Configurations
12
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM
BTSs and BSC may be either located at the same cell site ldquoco-locatedrdquo or located at different
sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs than BSCs
in a network Another BSS configuration is the ldquoDaisy Chainrdquo A BTS need not to
communicate directly with the BSC which controls it it can be connected to the BTS rather
than all the way to BSC Problems may arise when chaining BTSs due to the transmission
delay through the chain the length of the chain therefore should be kept sufficiently short to
prevent the round trip speech delay becoming too long
2121 Base Transceiver Station
BTS is the hardware component in the GSM architecture It provides an interface between
Mobile Station (MS) and Base Station Subsystem (BSS) The BTS provides radio channels
(RF carriers) for a specific RF coverage area The radio channel is the communication link
between the MSs within an RF coverage area and the BSS The BTS also has a limited amount
of control functionality which reduces the amount of traffic between the BTS and BSC
2122 Base Station Controller
Any operational information required by the BTS will be received via the BSC Likewise any
information required about the BTS (by the OMC for example) will be obtained by the BSC
BSC incorporates a digital switching matrix which it uses to connect the radio channels on the
air interface with the terrestrial circuits from the MSC The BSC switching matrix also allows
the BSC to perform ldquohandoversrdquo between radio channels on BTSs under its control without
involving the MSC The purpose of the BSC is to perform a variety of functions Some of them
are listed below
Controls the BTS components
Performs Call Processing
Performs Operations and Maintenance
Provides the OampM link between the BSS and the OMC
Provides the A Interface between the BSS and the MSC
Manages the radio channels
Transfers signaling information to and from different mobile stations (MS)
13
2123 Transcoder
Transcoder is required to convert the speech or data output from MSC(64 kbits PCM) into
the form specified by GSM specifications for the transmission over the air interface that is
between BSS and MS (64 kbits to 16 kbits and vice versa)The 64 kbits Pulse Code
Modulation (PCM) circuits from MSC if transmitted on the air interface without
modification would occupy an excessive amount of radio bandwidth This would use the
available radio spectrum inefficiently The required is therefore reduced by processing the 64
kbits circuits so that the amount of information required to transmit digitized voice falls to a
gross rate of 16 kbits The transcoding function may be located at the MSC BSC or BTS
213 Network Switching Subsystem
The Network Switching System includes the main switching functions of the GSM network It
also contains the databases required for subscriber data and mobility management Its main
function is to manage communications between the GSM network and other
telecommunications networks Various components of the Network Switching System are
listed below
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Centre (AUC)
Inter Working Function (IWF)
Echo Canceller (EC)
In addition to the more traditional elements of a cellular telephone system GSM has Location
Register network entities These entities are the Home Location Register (HLR) Visitor
Location Register (VLR) and the Equipment Identity Register (EIR) The location registers
are database-oriented processing nodes which address the problems of managing subscriber
data and keeping track of a MS location as it roams around the network Functionally the
Interworking Function and the Echo Cancellers may be considered as parts of the MSC since
their activities are inextricably linked with those of the switch as it connects speech and data
calls to and from the MSs
2131 Mobile Switching Centre
14
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
21 GSM Network Architecture
GSM network architecture is divided into following three categories
Mobile Station (MS) This consists of the mobile telephone fax machine etc This is the
part of the network that the subscriber will see
Base Station Subsystem (BSS) This is the part of the network which provides the radio
interconnection from the MS to the land-based switching equipment
Network Switching Subsystem (NSS) This consists of the Mobile services Switching
Centre (MSC) and its associated system-control databases and processors together with the
required interfaces This is the part which provides for interconnection between the GSM
network and the Public Switched Telephone Network (PSTN)
Network Management System This enables the network provider to configure and
maintain the network from a central location
211 Mobile Station (MS)
In GSM there is a difference between the physical equipment and the subscription The
mobile station is piece of equipment which can be vehicle installed portable or hand-held
In GSM there is a small unit called the Subscriber Identity Module (SIM) which is a separate
physical entity eg an IC-card also called a smart card SIM and the mobile equipment
together make up the mobile station Without SIM the MS cannot get access to the GSM
network except for emergency traffic While the SIM-card is connected to the subscription
and not to the MS the subscriber can use another MS as well as his own This then raises the
problem of stolen MSrsquo since it is no use barring the subscription if the equipment is stolen
We need a database that contains the unique hardware identity of the equipment the
Equipment Identity Register (EIR) The EIR is connected to the MSC over a signalling link
This enables the MSC to check the validity of the equipment An non-type-approved MS can
also be barred in this way The authentication of the subscription is done by parameters from
AUC
9
Fig 21 GSM Network Architecture
10
212 Base Station Subsystem(BSS)
The GSM Base Station System is the equipment located at a cell site It comprises combination
of digital and RF equipment The BSS provides the link between the MS and the MSC BSS
communicates with the MS over the digital air interface and with the MSC via 2 Mbits links
Fig 22 Base Station Subsystem
The BSS consists of three major hardware components
Base Transceiver Station (BTS) The BTS contains the RF components that provide the
air interface for a particular cell This is the part of the GSM network which communicates
with the MS The antenna is included as part of the BTS
11
Base Station Controller (BSC) The BSC as its name implies provides the control for the
BSS The BSC communicates directly with the MSC The BSC may control single or
multiple BTSs
Transcoder (TC) The Transcoder is used to compact the signals from the MS so that they
are more efficiently sent over the terrestrial interfaces Although the transcoder is
considered to be a part of the BSS it is very often located closer to the MSC The
transcoder is used to reduce the rate at which the traffic (voicedata) is transmitted over the
air interface Although the transcoder is part of the BSS it is often found physically closer
to the NSS to allow more efficient use of the terrestrial links
BSS Configurations
Fig 23 BSS Configurations
12
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM
BTSs and BSC may be either located at the same cell site ldquoco-locatedrdquo or located at different
sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs than BSCs
in a network Another BSS configuration is the ldquoDaisy Chainrdquo A BTS need not to
communicate directly with the BSC which controls it it can be connected to the BTS rather
than all the way to BSC Problems may arise when chaining BTSs due to the transmission
delay through the chain the length of the chain therefore should be kept sufficiently short to
prevent the round trip speech delay becoming too long
2121 Base Transceiver Station
BTS is the hardware component in the GSM architecture It provides an interface between
Mobile Station (MS) and Base Station Subsystem (BSS) The BTS provides radio channels
(RF carriers) for a specific RF coverage area The radio channel is the communication link
between the MSs within an RF coverage area and the BSS The BTS also has a limited amount
of control functionality which reduces the amount of traffic between the BTS and BSC
2122 Base Station Controller
Any operational information required by the BTS will be received via the BSC Likewise any
information required about the BTS (by the OMC for example) will be obtained by the BSC
BSC incorporates a digital switching matrix which it uses to connect the radio channels on the
air interface with the terrestrial circuits from the MSC The BSC switching matrix also allows
the BSC to perform ldquohandoversrdquo between radio channels on BTSs under its control without
involving the MSC The purpose of the BSC is to perform a variety of functions Some of them
are listed below
Controls the BTS components
Performs Call Processing
Performs Operations and Maintenance
Provides the OampM link between the BSS and the OMC
Provides the A Interface between the BSS and the MSC
Manages the radio channels
Transfers signaling information to and from different mobile stations (MS)
13
2123 Transcoder
Transcoder is required to convert the speech or data output from MSC(64 kbits PCM) into
the form specified by GSM specifications for the transmission over the air interface that is
between BSS and MS (64 kbits to 16 kbits and vice versa)The 64 kbits Pulse Code
Modulation (PCM) circuits from MSC if transmitted on the air interface without
modification would occupy an excessive amount of radio bandwidth This would use the
available radio spectrum inefficiently The required is therefore reduced by processing the 64
kbits circuits so that the amount of information required to transmit digitized voice falls to a
gross rate of 16 kbits The transcoding function may be located at the MSC BSC or BTS
213 Network Switching Subsystem
The Network Switching System includes the main switching functions of the GSM network It
also contains the databases required for subscriber data and mobility management Its main
function is to manage communications between the GSM network and other
telecommunications networks Various components of the Network Switching System are
listed below
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Centre (AUC)
Inter Working Function (IWF)
Echo Canceller (EC)
In addition to the more traditional elements of a cellular telephone system GSM has Location
Register network entities These entities are the Home Location Register (HLR) Visitor
Location Register (VLR) and the Equipment Identity Register (EIR) The location registers
are database-oriented processing nodes which address the problems of managing subscriber
data and keeping track of a MS location as it roams around the network Functionally the
Interworking Function and the Echo Cancellers may be considered as parts of the MSC since
their activities are inextricably linked with those of the switch as it connects speech and data
calls to and from the MSs
2131 Mobile Switching Centre
14
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Fig 21 GSM Network Architecture
10
212 Base Station Subsystem(BSS)
The GSM Base Station System is the equipment located at a cell site It comprises combination
of digital and RF equipment The BSS provides the link between the MS and the MSC BSS
communicates with the MS over the digital air interface and with the MSC via 2 Mbits links
Fig 22 Base Station Subsystem
The BSS consists of three major hardware components
Base Transceiver Station (BTS) The BTS contains the RF components that provide the
air interface for a particular cell This is the part of the GSM network which communicates
with the MS The antenna is included as part of the BTS
11
Base Station Controller (BSC) The BSC as its name implies provides the control for the
BSS The BSC communicates directly with the MSC The BSC may control single or
multiple BTSs
Transcoder (TC) The Transcoder is used to compact the signals from the MS so that they
are more efficiently sent over the terrestrial interfaces Although the transcoder is
considered to be a part of the BSS it is very often located closer to the MSC The
transcoder is used to reduce the rate at which the traffic (voicedata) is transmitted over the
air interface Although the transcoder is part of the BSS it is often found physically closer
to the NSS to allow more efficient use of the terrestrial links
BSS Configurations
Fig 23 BSS Configurations
12
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM
BTSs and BSC may be either located at the same cell site ldquoco-locatedrdquo or located at different
sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs than BSCs
in a network Another BSS configuration is the ldquoDaisy Chainrdquo A BTS need not to
communicate directly with the BSC which controls it it can be connected to the BTS rather
than all the way to BSC Problems may arise when chaining BTSs due to the transmission
delay through the chain the length of the chain therefore should be kept sufficiently short to
prevent the round trip speech delay becoming too long
2121 Base Transceiver Station
BTS is the hardware component in the GSM architecture It provides an interface between
Mobile Station (MS) and Base Station Subsystem (BSS) The BTS provides radio channels
(RF carriers) for a specific RF coverage area The radio channel is the communication link
between the MSs within an RF coverage area and the BSS The BTS also has a limited amount
of control functionality which reduces the amount of traffic between the BTS and BSC
2122 Base Station Controller
Any operational information required by the BTS will be received via the BSC Likewise any
information required about the BTS (by the OMC for example) will be obtained by the BSC
BSC incorporates a digital switching matrix which it uses to connect the radio channels on the
air interface with the terrestrial circuits from the MSC The BSC switching matrix also allows
the BSC to perform ldquohandoversrdquo between radio channels on BTSs under its control without
involving the MSC The purpose of the BSC is to perform a variety of functions Some of them
are listed below
Controls the BTS components
Performs Call Processing
Performs Operations and Maintenance
Provides the OampM link between the BSS and the OMC
Provides the A Interface between the BSS and the MSC
Manages the radio channels
Transfers signaling information to and from different mobile stations (MS)
13
2123 Transcoder
Transcoder is required to convert the speech or data output from MSC(64 kbits PCM) into
the form specified by GSM specifications for the transmission over the air interface that is
between BSS and MS (64 kbits to 16 kbits and vice versa)The 64 kbits Pulse Code
Modulation (PCM) circuits from MSC if transmitted on the air interface without
modification would occupy an excessive amount of radio bandwidth This would use the
available radio spectrum inefficiently The required is therefore reduced by processing the 64
kbits circuits so that the amount of information required to transmit digitized voice falls to a
gross rate of 16 kbits The transcoding function may be located at the MSC BSC or BTS
213 Network Switching Subsystem
The Network Switching System includes the main switching functions of the GSM network It
also contains the databases required for subscriber data and mobility management Its main
function is to manage communications between the GSM network and other
telecommunications networks Various components of the Network Switching System are
listed below
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Centre (AUC)
Inter Working Function (IWF)
Echo Canceller (EC)
In addition to the more traditional elements of a cellular telephone system GSM has Location
Register network entities These entities are the Home Location Register (HLR) Visitor
Location Register (VLR) and the Equipment Identity Register (EIR) The location registers
are database-oriented processing nodes which address the problems of managing subscriber
data and keeping track of a MS location as it roams around the network Functionally the
Interworking Function and the Echo Cancellers may be considered as parts of the MSC since
their activities are inextricably linked with those of the switch as it connects speech and data
calls to and from the MSs
2131 Mobile Switching Centre
14
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
212 Base Station Subsystem(BSS)
The GSM Base Station System is the equipment located at a cell site It comprises combination
of digital and RF equipment The BSS provides the link between the MS and the MSC BSS
communicates with the MS over the digital air interface and with the MSC via 2 Mbits links
Fig 22 Base Station Subsystem
The BSS consists of three major hardware components
Base Transceiver Station (BTS) The BTS contains the RF components that provide the
air interface for a particular cell This is the part of the GSM network which communicates
with the MS The antenna is included as part of the BTS
11
Base Station Controller (BSC) The BSC as its name implies provides the control for the
BSS The BSC communicates directly with the MSC The BSC may control single or
multiple BTSs
Transcoder (TC) The Transcoder is used to compact the signals from the MS so that they
are more efficiently sent over the terrestrial interfaces Although the transcoder is
considered to be a part of the BSS it is very often located closer to the MSC The
transcoder is used to reduce the rate at which the traffic (voicedata) is transmitted over the
air interface Although the transcoder is part of the BSS it is often found physically closer
to the NSS to allow more efficient use of the terrestrial links
BSS Configurations
Fig 23 BSS Configurations
12
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM
BTSs and BSC may be either located at the same cell site ldquoco-locatedrdquo or located at different
sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs than BSCs
in a network Another BSS configuration is the ldquoDaisy Chainrdquo A BTS need not to
communicate directly with the BSC which controls it it can be connected to the BTS rather
than all the way to BSC Problems may arise when chaining BTSs due to the transmission
delay through the chain the length of the chain therefore should be kept sufficiently short to
prevent the round trip speech delay becoming too long
2121 Base Transceiver Station
BTS is the hardware component in the GSM architecture It provides an interface between
Mobile Station (MS) and Base Station Subsystem (BSS) The BTS provides radio channels
(RF carriers) for a specific RF coverage area The radio channel is the communication link
between the MSs within an RF coverage area and the BSS The BTS also has a limited amount
of control functionality which reduces the amount of traffic between the BTS and BSC
2122 Base Station Controller
Any operational information required by the BTS will be received via the BSC Likewise any
information required about the BTS (by the OMC for example) will be obtained by the BSC
BSC incorporates a digital switching matrix which it uses to connect the radio channels on the
air interface with the terrestrial circuits from the MSC The BSC switching matrix also allows
the BSC to perform ldquohandoversrdquo between radio channels on BTSs under its control without
involving the MSC The purpose of the BSC is to perform a variety of functions Some of them
are listed below
Controls the BTS components
Performs Call Processing
Performs Operations and Maintenance
Provides the OampM link between the BSS and the OMC
Provides the A Interface between the BSS and the MSC
Manages the radio channels
Transfers signaling information to and from different mobile stations (MS)
13
2123 Transcoder
Transcoder is required to convert the speech or data output from MSC(64 kbits PCM) into
the form specified by GSM specifications for the transmission over the air interface that is
between BSS and MS (64 kbits to 16 kbits and vice versa)The 64 kbits Pulse Code
Modulation (PCM) circuits from MSC if transmitted on the air interface without
modification would occupy an excessive amount of radio bandwidth This would use the
available radio spectrum inefficiently The required is therefore reduced by processing the 64
kbits circuits so that the amount of information required to transmit digitized voice falls to a
gross rate of 16 kbits The transcoding function may be located at the MSC BSC or BTS
213 Network Switching Subsystem
The Network Switching System includes the main switching functions of the GSM network It
also contains the databases required for subscriber data and mobility management Its main
function is to manage communications between the GSM network and other
telecommunications networks Various components of the Network Switching System are
listed below
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Centre (AUC)
Inter Working Function (IWF)
Echo Canceller (EC)
In addition to the more traditional elements of a cellular telephone system GSM has Location
Register network entities These entities are the Home Location Register (HLR) Visitor
Location Register (VLR) and the Equipment Identity Register (EIR) The location registers
are database-oriented processing nodes which address the problems of managing subscriber
data and keeping track of a MS location as it roams around the network Functionally the
Interworking Function and the Echo Cancellers may be considered as parts of the MSC since
their activities are inextricably linked with those of the switch as it connects speech and data
calls to and from the MSs
2131 Mobile Switching Centre
14
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Base Station Controller (BSC) The BSC as its name implies provides the control for the
BSS The BSC communicates directly with the MSC The BSC may control single or
multiple BTSs
Transcoder (TC) The Transcoder is used to compact the signals from the MS so that they
are more efficiently sent over the terrestrial interfaces Although the transcoder is
considered to be a part of the BSS it is very often located closer to the MSC The
transcoder is used to reduce the rate at which the traffic (voicedata) is transmitted over the
air interface Although the transcoder is part of the BSS it is often found physically closer
to the NSS to allow more efficient use of the terrestrial links
BSS Configurations
Fig 23 BSS Configurations
12
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM
BTSs and BSC may be either located at the same cell site ldquoco-locatedrdquo or located at different
sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs than BSCs
in a network Another BSS configuration is the ldquoDaisy Chainrdquo A BTS need not to
communicate directly with the BSC which controls it it can be connected to the BTS rather
than all the way to BSC Problems may arise when chaining BTSs due to the transmission
delay through the chain the length of the chain therefore should be kept sufficiently short to
prevent the round trip speech delay becoming too long
2121 Base Transceiver Station
BTS is the hardware component in the GSM architecture It provides an interface between
Mobile Station (MS) and Base Station Subsystem (BSS) The BTS provides radio channels
(RF carriers) for a specific RF coverage area The radio channel is the communication link
between the MSs within an RF coverage area and the BSS The BTS also has a limited amount
of control functionality which reduces the amount of traffic between the BTS and BSC
2122 Base Station Controller
Any operational information required by the BTS will be received via the BSC Likewise any
information required about the BTS (by the OMC for example) will be obtained by the BSC
BSC incorporates a digital switching matrix which it uses to connect the radio channels on the
air interface with the terrestrial circuits from the MSC The BSC switching matrix also allows
the BSC to perform ldquohandoversrdquo between radio channels on BTSs under its control without
involving the MSC The purpose of the BSC is to perform a variety of functions Some of them
are listed below
Controls the BTS components
Performs Call Processing
Performs Operations and Maintenance
Provides the OampM link between the BSS and the OMC
Provides the A Interface between the BSS and the MSC
Manages the radio channels
Transfers signaling information to and from different mobile stations (MS)
13
2123 Transcoder
Transcoder is required to convert the speech or data output from MSC(64 kbits PCM) into
the form specified by GSM specifications for the transmission over the air interface that is
between BSS and MS (64 kbits to 16 kbits and vice versa)The 64 kbits Pulse Code
Modulation (PCM) circuits from MSC if transmitted on the air interface without
modification would occupy an excessive amount of radio bandwidth This would use the
available radio spectrum inefficiently The required is therefore reduced by processing the 64
kbits circuits so that the amount of information required to transmit digitized voice falls to a
gross rate of 16 kbits The transcoding function may be located at the MSC BSC or BTS
213 Network Switching Subsystem
The Network Switching System includes the main switching functions of the GSM network It
also contains the databases required for subscriber data and mobility management Its main
function is to manage communications between the GSM network and other
telecommunications networks Various components of the Network Switching System are
listed below
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Centre (AUC)
Inter Working Function (IWF)
Echo Canceller (EC)
In addition to the more traditional elements of a cellular telephone system GSM has Location
Register network entities These entities are the Home Location Register (HLR) Visitor
Location Register (VLR) and the Equipment Identity Register (EIR) The location registers
are database-oriented processing nodes which address the problems of managing subscriber
data and keeping track of a MS location as it roams around the network Functionally the
Interworking Function and the Echo Cancellers may be considered as parts of the MSC since
their activities are inextricably linked with those of the switch as it connects speech and data
calls to and from the MSs
2131 Mobile Switching Centre
14
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM
BTSs and BSC may be either located at the same cell site ldquoco-locatedrdquo or located at different
sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs than BSCs
in a network Another BSS configuration is the ldquoDaisy Chainrdquo A BTS need not to
communicate directly with the BSC which controls it it can be connected to the BTS rather
than all the way to BSC Problems may arise when chaining BTSs due to the transmission
delay through the chain the length of the chain therefore should be kept sufficiently short to
prevent the round trip speech delay becoming too long
2121 Base Transceiver Station
BTS is the hardware component in the GSM architecture It provides an interface between
Mobile Station (MS) and Base Station Subsystem (BSS) The BTS provides radio channels
(RF carriers) for a specific RF coverage area The radio channel is the communication link
between the MSs within an RF coverage area and the BSS The BTS also has a limited amount
of control functionality which reduces the amount of traffic between the BTS and BSC
2122 Base Station Controller
Any operational information required by the BTS will be received via the BSC Likewise any
information required about the BTS (by the OMC for example) will be obtained by the BSC
BSC incorporates a digital switching matrix which it uses to connect the radio channels on the
air interface with the terrestrial circuits from the MSC The BSC switching matrix also allows
the BSC to perform ldquohandoversrdquo between radio channels on BTSs under its control without
involving the MSC The purpose of the BSC is to perform a variety of functions Some of them
are listed below
Controls the BTS components
Performs Call Processing
Performs Operations and Maintenance
Provides the OampM link between the BSS and the OMC
Provides the A Interface between the BSS and the MSC
Manages the radio channels
Transfers signaling information to and from different mobile stations (MS)
13
2123 Transcoder
Transcoder is required to convert the speech or data output from MSC(64 kbits PCM) into
the form specified by GSM specifications for the transmission over the air interface that is
between BSS and MS (64 kbits to 16 kbits and vice versa)The 64 kbits Pulse Code
Modulation (PCM) circuits from MSC if transmitted on the air interface without
modification would occupy an excessive amount of radio bandwidth This would use the
available radio spectrum inefficiently The required is therefore reduced by processing the 64
kbits circuits so that the amount of information required to transmit digitized voice falls to a
gross rate of 16 kbits The transcoding function may be located at the MSC BSC or BTS
213 Network Switching Subsystem
The Network Switching System includes the main switching functions of the GSM network It
also contains the databases required for subscriber data and mobility management Its main
function is to manage communications between the GSM network and other
telecommunications networks Various components of the Network Switching System are
listed below
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Centre (AUC)
Inter Working Function (IWF)
Echo Canceller (EC)
In addition to the more traditional elements of a cellular telephone system GSM has Location
Register network entities These entities are the Home Location Register (HLR) Visitor
Location Register (VLR) and the Equipment Identity Register (EIR) The location registers
are database-oriented processing nodes which address the problems of managing subscriber
data and keeping track of a MS location as it roams around the network Functionally the
Interworking Function and the Echo Cancellers may be considered as parts of the MSC since
their activities are inextricably linked with those of the switch as it connects speech and data
calls to and from the MSs
2131 Mobile Switching Centre
14
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
2123 Transcoder
Transcoder is required to convert the speech or data output from MSC(64 kbits PCM) into
the form specified by GSM specifications for the transmission over the air interface that is
between BSS and MS (64 kbits to 16 kbits and vice versa)The 64 kbits Pulse Code
Modulation (PCM) circuits from MSC if transmitted on the air interface without
modification would occupy an excessive amount of radio bandwidth This would use the
available radio spectrum inefficiently The required is therefore reduced by processing the 64
kbits circuits so that the amount of information required to transmit digitized voice falls to a
gross rate of 16 kbits The transcoding function may be located at the MSC BSC or BTS
213 Network Switching Subsystem
The Network Switching System includes the main switching functions of the GSM network It
also contains the databases required for subscriber data and mobility management Its main
function is to manage communications between the GSM network and other
telecommunications networks Various components of the Network Switching System are
listed below
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Centre (AUC)
Inter Working Function (IWF)
Echo Canceller (EC)
In addition to the more traditional elements of a cellular telephone system GSM has Location
Register network entities These entities are the Home Location Register (HLR) Visitor
Location Register (VLR) and the Equipment Identity Register (EIR) The location registers
are database-oriented processing nodes which address the problems of managing subscriber
data and keeping track of a MS location as it roams around the network Functionally the
Interworking Function and the Echo Cancellers may be considered as parts of the MSC since
their activities are inextricably linked with those of the switch as it connects speech and data
calls to and from the MSs
2131 Mobile Switching Centre
14
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
MSC is included in the GSM operation for call-switching Its overall purpose is similar to
that of any telephone exchange MSC carries out several different functions depending upon
its position in the network When MSC provides interface between the PSTN and the BSSs
in the GSM network it will be known as a Gateway MSC In this position it will provide the
switching required for all MS originated or terminated traffic
Functions carried out by MSC are written below
Call ProcessingIncludes control of datavoice call setup inter-BSS and inter-MSC handovers and control of
mobility management (subscriber validation and location)
Operations and Maintenance Support
Includes database management traffic metering and measurement and a manndashmachine
interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Billing
Collects call billing data
2132 Home Location RegisterHLR maintains a permanent register of all the subscribers for instance subscriber identity
numbers and the subscribed services that is the services the subscriber is allowed to use In
addition to the fixed data HLR also keeps track of current location of its customers Also
MSC asks for routing information from HLR if a call is to be set up to a mobile station
(mobile terminated call) HLR further consists of two more network elements as listed below
Authentication Centre (AuC)
Equipment Identity Register (EIR)
2133 Authentication Centre
The Authentication Centre provides security information to the network so that we can verify
the SIM cards (authentication between the mobile station and VLR and cipher the information
transmitted in the air interface between the mobile station and Base Transceiver Station
15
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
(BTS)) The Authentication Centre supports the VLRrsquos work by issuing so-called
authentication triplets upon request It will normally be co-located with the Home Location
Register (HLR) as it will be required to continuously access and update as necessary the
system subscriber records The AUCHLR centre can be co-located with the MSC or located
remote from the MSC The authentication process will usually take place each time the
subscriber ldquoinitializesrdquo on the system
2134 Equipment Identity Register
Similar to Authentication Control the Equipment Identity Register is used for security
reasons But while the Authentication Control provides information for verifying the SIM
cards the EIR is responsible for checking IMEI (checking the validity of the mobile
equipment) When performed the mobile station is requested to provide the International
Mobile Equipment Identity (IMEI) number This number consists of type approval code
final assembly code and serial number of the mobile station
The EIR consists of three following lists
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for
some other reason
Grey List
Contains IMEIs of MS which have problems (for example faulty software) These are not
however sufficiently significant to warrant a lsquolsquoblack listingrdquo The EIR database is remotely
accessed by the MSCs in the network and can also be accessed by an MSC in a different
PLMN As in the case of the HLR a network may well contain more than one EIR with each
EIR controlling certain blocks of IMEI numbers The MSC contains a translation facility
which when given an IMEI returns the address of the EIR controlling the appropriate section
of the equipment database
2135 Visitor Location Register (VLR)
VLR is a database which contains information about the subscribers currently being in the
service area of the MSCVLR such as
Identification numbers of the subscribers
16
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Security information for authentication of the SIM card and for pipelining
Services that the subscriber can use
The VLR carries out location registrations and updates It means that when a mobile station
comes to a new MSCVLR serving area it must register itself in the VLR in other words
perform a location update A mobile station must always be registered in a VLR in order to
use the services of the network Also the mobile stations located in the own network is
always registered in a VLR The VLR database is temporary in the sense that the data is held
as long as the subscriber is within the service area It also contains the address to every
subscriberrsquos home location register (HLR)
This function eliminates the need for excessive and time-consuming references to the ldquohomerdquo
HLR database The additional data stored in the VLR is listed below
Mobile status (busyfreeno answer etc)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number (MSRN)
2136 Interworking Function (IWF)
The IWF provides the function to enable the GSM system to interface with the various forms
of public and private data networks currently available The basic features of the IWF are
listed below
Data rate adaption
Protocol conversion
Some systems require more IWF capability than others and this depends upon the network to
which it is being connected
The IWF also incorporates a lsquolsquomodem bankrdquo which may be used when for example the GSM
Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an
analog modem
2137 Echo Canceller (EC)An EC is used on the PSTN side of the MSC for all voice circuits Echo control is required at
the switch because the inherent GSM system delay can cause an unacceptable echo condition
even on short distance PSTN circuit connections The total round trip delay introduced by the
GSM system (the cumulative delay caused by call processing speech encoding and decoding
17
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
etc) is approximately 180 ms This might not be apparent to the MS subscriber but for the
inclusion of a 2-wire to 4-wire hybrid transformer in the circuit This is required at the land
partyrsquos local switch because the standard telephone connection is 2-wire The transformer
causes the echo This does not affect the land subscriber
214 Network Management Subsystem
The NMC offers the ability to provide a hierarchical network management of a complete
GSM system It is responsible for operations and maintenance at the network level supported
by the OMCs which are responsible for regional network management The NMC is
therefore a single logical facility at the top of the network management hierarchy
The NMC has high level view of network as a series of network nodes and interconnecting
communications facilities the OMC on the other hand is used to filter the information from
the network equipment for forwarding to the NMC thus allowing it to focus on the issues
requiring national co-ordination
The functions of NMS are listed below
Monitors nodes of the network
Monitors GSM network element statistics
Monitors OMC regions amp provides information to OMC staff
Passes on statistical information from one OMC region to another to improve problem
solving strategies
Enables long term planning for the entire network
2141 Network Management Center
18
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Fig 24 Network Management Subsystem
2142 Operations and Maintenance Center (OMC)
The OMC provides a central point from which to control and monitor the other network
entities (ie base stations switches database etc) as well as monitor the quality of service
being provided by the network
There are two types of OMC as follows
OMC (R)
OMC controls specifically the Base Station System
OMC (S)
OMC controls specifically the Network Switching System
The OMC should support the following functions as per ITS-TS recommendations
19
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Fault management
Configuration management
Performance management
These functions cover the whole of the GSM network elements from the level of individual
BTSs up to MSCs and HLRs
Fault Management
The purpose of fault management is to ensure smooth operation of the network and rapid
correction of any kind of problems that are detected Fault management provides network
operator with the information about the current status of alarm events and maintains a history
database of alarms The alarms are stored in the NMS database and their database can be
deleted according to the criteria specified by the network operator
Configuration Management
The purpose of configuration management is to maintain up-to-date information about the
operation and configuration status of all the network elements Specific configuration
functions include the management of radio network software and hardware management of
network elements time synchronization and the security operations
Performance Management
In performance management the NMS collects measurement data from the individual
network elements and stores in a database On the basis of these data the network operator is
able to compare the actual performance of the network with the planned performance and
detect both good and bad performance areas within network
22 Interfaces used in GSM
Following are the interfaces used in GSM network
Um The air interface is used for exchanges between an MS and a BSS LAPDm a
modified version of the ISDN LAPD is used for signalling
Abis This is the internal interface linking the BSC and a BTS and it has not been
standardised The Abis interface allows control of the radio equipment and radio
frequency allocation in the BTS
20
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
A The A interface is between the BSS and the MSC The A interface manages the
allocation of suitable radio resources to the MSrsquos and mobility management
B The B interface between the MSC and the VLR uses the MAPB protocol Most
MSCrsquos are associated with a VLR making the B interface internal Whenever the
MSC needs access to data regarding an MS located in its area it interrogates the VLR
using the MAPB protocol over the B interface
C The C interface is between the HLR and a GMSC or an SMS-G Each call
originating outside of GSM (ie a MS terminating call from the PSTN) has to go
through a Gateway to obtain the routing information required to complete the call
and the MAPC protocol over the C interface is used for this purpose Also the MSC
may optionally forward billing information to the HLR after call clearing
D The D interface is between the VLR and HLR and uses the MAPD protocol to
exchange the data related to the location of the MS and to the management of the
subscriber
E The E interface interconnects two MSCs The E interface exchanges data related
to handover between the anchor and relay MSCs using the MAPE protocol
F The F interface connects the MSC to the EIR and uses the MAPF protocol to
verify the status of the IMEI that the MSC has retrieved from the MS
G The G interface interconnects two VLRrsquos of different MSCs and uses the MAPG
protocol to transfer subscriber information during eg a location update procedure
H The H interface is between the MSC and the SMS-G and uses the MAPH
protocol to support the transfer of short messages
21
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Fig 25 Interfaces used in GSM
I The I interface (not shown in Figure) is the interface between the MSC and the MS
Messages exchanged over the I interface are relayed transparently through the BSS
23 GSM Channels
There are two types of channels used in GSM network namely physical channels and logical
channels The physical channel is the medium over which the information is carried in the
case of a terrestrial interface this would be a cable The logical channels consist of the
information carried over the physical channel
231 Physical Channels
22
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
A single GSM RF carrier can support up to eight MS subscribers simultaneously The diagram
opposite shows how this is accomplished Each channel occupies the carrier for one eighth of
the time This is a technique called Time Division Multiple Access Time is divided into
discrete periods called ldquotimeslotsrdquo The timeslots are arranged in sequence and are
conventionally numbered 0 to 7 Each repetition of this sequence is called a ldquoTDMA framerdquo
Each MS telephone call occupies one timeslot (0ndash7) within the frame until the call is
terminated or a handover occurs The TDMA frames are then built into further frame
structures according to the type of channel For such a system to work correctly the timing of
the transmissions to and from the mobiles is critical The MS or Base Station must transmit the
information related to one call at exactly the right moment or the timeslot will be missed The
information carried in one timeslot is called a ldquoburstrdquo Each data burst occupying its allocated
timeslot within successive TDMA frames provides a single GSM physical channel carrying a
varying number of logical channels between the MS and BTS
232 Logical Channels
Logical channels are further classified into two main groups namely traffic channels and
control channels
2321 Traffic Channels (TCH)
The traffic channel carries speech or data information GSM traffic channels can be half-rate or
full-rate and may carry either digitized speech or user data When transmitted as full-rate user
data is contained within one TS per frame When transmitted as half-rate user data is mapped
onto the same time slot but is sent in alternate frames That is two half-rate channel users
would share the same time slot but would alternately transmit every other frame
Full-Rate TCH
Full-Rate Speech Channel (TCHFS)
The full-rate speech channel carries user speech which is digitized at a raw data rate of 13
kbps With GSM channel coding added to the digitized speech the full-rate speech channel
carries 228 kbps
Full-Rate Data Channel for 9600 bps (TCHF96)
23
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
The full-rate traffic data channel carries user data which is sent at 9600 bpsWith
additional forward error correction coding applied by the GSM standard the 9600 bps
data is sent at 228 kbps
Full-Rate Data Channel for 4800 bps (TCHF48)
The full-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 228 kbps
Full-Rate Data Channel for 2400 bps (TCHF24)
The full-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 228 kbps
Half-Rate TCH
Half-Rate Speech Channel (TCHHS)
The half-rate speech channel carries user speech which is digitized at a raw data rate of
65 kbps With GSM channel coding added to the digitized speech the half-rate speech
channel carries 114 kbps
Half-Rate Data Channel for 4800 bps (TCHH48)
The half-rate traffic data channel carries user data which is sent at 4800 bpsWith
additional forward error correction coding applied by the GSM standard the 4800 bps
data is sent at 114 kbps
Half-Rate Data Channel for 2400 bps (TCHH24)
The half-rate traffic data channel carries user data which is sent at 2400 bpsWith
additional forward error correction coding applied by the GSM standard the 2400 bps
data is sent at 114 kbps
2322 Control Channels (CCH)
Control channels are further subdivided into three categories
Broadcast Channels (BCH)
Common Control Channel (CCCH)
Dedicated Control Channel (DCCH)
23221 Broadcast Channels (BCH)
24
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
The broadcast channel operates on the forward link of ARCN within the cell and transmits
data only in the first time slot (TS 0) of certain GSM frames There are three types of BCH as
shown below
Broadcast Control Channel (BCCH)
The BCCH is a forward control channel that is used to broadcast information such as cell
and network identity and the operating characteristics of the cell The BCCH also
broadcasts a list of channels that are currently in use within the cell
Frequency Correction Channel (FCCH)
The FCCH is a special data burst which occupies TS 0 for every first GSM frame and is
repeated every ten frames within a control channel multiframe The FCCH allows each
subscriber unit to synchronize its internal frequency standard to the exact frequency of
the base station
Synchronization Channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is
used to identify the serving base station while allowing each mobile to frame synchronize
with the base station The frame number (FN) is sent with the base station identity code
(BSIC) during SCH burst
23222 Common Control Channels (CCCH)
Paging Channel (PCH)
The PCH provides paging signals from the base station to all the mobiles of the cell and
notifies a specific mobile of an incoming call which originates from the PSTN The PCH
transmits IMSI number of the target subscriber along with a request for the
acknowledgement from the mobile unit on the RACH
Random Access Channel (RACH)
The RACH is a reverse link channel used by the subscriber unit to acknowledge a page
from the PCH and is also used by the mobiles to originate a call The RACH uses slotted
ALOHA access scheme
Access Grant Channel (AGCH)
25
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
The AGCH is used by the base station to provide forward link communication to the
mobile to operate in a particular physical channel with a dedicated control channel The
AGCH is used by the base station to respond to a RACH sent by a mobile station in a
previous CCCH frame
23223 Dedicated Control Channels (DCCH)
There are three different types of dedicated control channels as shown below
Stand-alone Dedicated Control Channels (SDCCH)
Slow-associated Control Channels (SACCH)
Fast-associated Control Channels (FACCH)
The stand-alone dedicated control channels (SDCCH) are used for providing signaling
services required by the users The slow and fast associated control channels are used for
supervisory data transmissions between the mobile station and the base station during a call
Stand-alone Dedicated Control Channels(SDCCH)
The SDCCH carries the signaling data following the connection of the mobile station
with the base station and just before the assignment of TCH is issued by the base station
The SDCCH ensures that the mobile station and the base station remain connected while
the base station and MSC verify the subscriber unit and allocate resources for the mobile
Slow Associated Control Channel (SACCH)
The SACCH is always associated with a traffic channel or a SDCCH On the forward
link the SACCH is used to send slow but regularly changing control information to the
mobile such as transmit power level instructions and specific timing advance instructions
for each user on the ARFCN The reverse SACCH carries information about the received
signal strength and quality of the TCH as well as BCH measurement results from the
neighboring cells
Fast Associated Control Channel (FACCH)
FACCH carries urgent messages and contains essentially the same type of information as
the SDCCH A FCCH is assigned whenever a SDCCH has not been dedicated for a
particular user and there is an urgent message
24 GSM Burst Concept
26
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Fig 26 GSM Burst Concept
241 Multiframes The 26-frame Traffic Channel MultiframeThe 12th frame (no 13) in the 26-frame traffic channel multiframe is used by the Slow
Associated Control Channel (SACCH) which carries link control information to and from the
MSndashBTS Each timeslot in a cell allocated to traffic channel usage will follow this format that
is 12 bursts of traffic 1 burst of SACCH 12 bursts of traffic and 1 idle The duration of a 26-
27
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
frame traffic channel multiframe is 120 ms (26 TDMA frames) When half rate is used each
frame of the 26-frame traffic channel multiframe allocated for traffic will now carry two MS
subscriber calls (the data rate for each MS is halved over the air interface) Although the data
rate for traffic is halved each MS still requires the
same amount of SACCH information to be transmitted therefore frame 12 WILL BE USED as
SACCH for one half of the MSs and the others will use it as their IDLE frame and the same
applies for frame 25 this will be used by the MSs for SACCH (those who used frame 12 as
IDLE) and the other half will use it as their IDLE frame
The 51-frame Control Channel MultiframeThe BCCHCCCH 51-frame structure illustrated on the opposite page will apply to timeslot 0
of each TDMA frame on the lsquoBCCH carrierrsquo (the RF carrier frequency to which BCCH is
assigned on a per cell basis) In the diagram each vertical step represents one repetition of the
timeslot (= one TDMA frame) with the first repetition (numbered 0) at the bottom
Looking at the uplink (MSndashBSS) direction all timeslot 0s are allocated to RACH This is
fairly obvious because RACH is the only control channel in the BCCHCCCH group which
works in the uplink direction In the downlink direction (BSSndashMS) the arrangement is more
interesting Starting at frame 0 of the 51-frame structure the first timeslot 0 is occupied by a
frequency burst (lsquoFrsquo in the diagram) the second by a synchronizing burst (lsquoSrsquo) and then the
following four repetitions of timeslot 0 by BCCH data (B) in frames 2ndash5 The following four
repetitions of timeslot 0 in frames 6ndash9 are allocated to CCCH traffic (C) that is to either PCH
(mobile paging channel) or AGCH (access grant channel)
Then follows a timeslot 0 of frames 10 and 11 a repeat of the frequency and synchronising
bursts (F and S) four further CCCH bursts (C) and so on Note that the last timeslot 0 in the
sequence (the fifty-first frame ndash frame 50) is idle
242 Superframes and HyperframesThis number of TDMA frames is termed a ldquosuperframerdquo and it takes 612 s to transmit This
arrangement means that the timing of the traffic channel multiframe is always moving in relation
to that of the control channel multiframe and this enables a MS to receive and decode BCCH
information from surrounding cells If the two multiframes were exact multiples of each other
then control channel timeslots would be permanently lsquomaskedrsquo by traffic channel timeslot
activity This changing relationship between the two multiframes is particularly important for
28
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
example to a MS which needs to be able to monitor and report the RSSIs of neighbour cells (it
needs to be able to lsquoseersquo all the BCCHs of those cells in order to do this)
The ldquohyperframerdquo consists of 2048 superframes this is used in connection with ciphering and
frequency hopping The hyperframe lasts for over three hours after this time the ciphering and
frequency hopping algorithms are restarted
Fig 27 GSM Frames
29
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
25 GSM BurstsThe burst is the sequence of bits transmitted by the BTS or the MS the timeslot is the discrete
period of real time within which it must arrive in order to be correctly decoded by the receiver
Fig 28 GSM Bursts
30
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
There are various types of bursts as shown below Normal Burst
The normal burst carries traffic channels and all types of control channels apart from those
mentioned specifically below (Bi-directional)
Frequency Correction Burst
This burst carries FCCH downlink to correct the frequency of the MSrsquos local oscillator
effectively locking it to that of the BTS
Synchronization Burst
So called because its function is to carry SCH downlink synchronizing the timing of the
MS to that of the BTS
Dummy Burst
Used when there is no information to be carried on the unused timeslots of the BCCH
Carrier (downlink only)
Access Burst
This burst is of much shorter duration than the other types The increased guard period is
necessary because the timing of its transmission is unknown When this burst is
transmitted the BTS does not know the location of the MS and therefore the timing of the
message from the MS cannot be accurately accounted for (The Access Burst is uplink
only)
26 Error Protection and DetectionTo protect the logical channels from transmission errors introduced by the radio path many
different coding schemes are used The diagram overleaf illustrates the coding process for speech
control and data channels the sequence is very complex The coding and interleaving schemes
depend on the type of logical channel to be encoded All logical channels require some form of
convolutional encoding but since protection needs are different the code rates may also differ
Three coding protection schemes
Speech Channel EncodingThe speech information for one 20 ms speech block is divided over eight GSM bursts This
ensures that if bursts are lost due to interference over the air interface the speech can still be
accurately reproduced
31
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Common Control Channel Encoding20 ms of information over the air will carry four bursts of control information for example
BCCH This enables the bursts to be inserted into one TDMA multiframe
Data Channel EncodingThe data information is spread over 22 bursts This is because every bit of data information is
very important Therefore when the data is reconstructed at the receiver if a burst is lost only
a very small proportion of the 20 ms block of data will be lost The error encoding mechanisms
should then enable the missing data to be reconstructed
261 Speech Channel CodingThe BTS receives transcoded speech over the A-bis interface from the BSC At this point the
speech is organized into its individual logical channels by the BTS These logical channels of
information are then channel coded before being transmitted over the air interface
The transcoded speech information is received in frames each containing 260 bits The speech
bits are grouped into three classes of sensitivity to errors depending on their importance to the
intelligibility of speech
Class 1aThree parity bits are derived from the 50 class 1a bits Transmission errors within these bits are
catastrophic to speech intelligibility therefore the speech decoder is able to detect
uncorrectable errors within the class 1a bits If there are class 1a bit errors the whole block is
usually ignored
Class 1bThe 132 class 1b bits are not parity checked but are fed together with the class 1a and parity
bits to a convolutional encoder Four tail bits are added which set the registers in the receiver
to a known state for decoding purposes
Class 2The 78 least sensitive bits are not protected at all The resulting 456 bit block is then
interleaved before being sent over the air interface
32
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Fig 29 Speech Channel Coding
262 Control Channel EncodingBTS it is received as a block of 184 bits These bits are first protected with a cyclic block code of
a class known as a Fire Code This is particularly suitable for the detection and correction of burst
errors as it uses 40 parity bits Before the convolutional encoding four tail bits are added which
set the registers in the receiver to a known state for decoding purposes The output from the
encoding process for each block of 184 bits of signalling data is 456 bits exactly the same as for
speech The resulting 456 bit block is then interleaved before being sent over the air interface
Fig 210 Control Channel Encoding
33
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
263 Data Channel EncodingData channels are encoded using a convolutional code only With the 96 kbits data some coded
bits need to be removed (punctuated) before interleaving so that like the speech and control
channels they contain 456 bits every 20 msThe data traffic channels require a higher net rate
(lsquonet ratersquo means the bit rate before coding bits have been added) than their actual transmission
rate For example the 96 kbits service will require 12 kbits because status signals (such as the
RS-232 DTR (Data Terminal Ready)) have to be transmitted as well The output from the
encoding process for each block of 240 bits of data traffic is 456 bits exactly the same as for
speech and control The resulting 456 bit block is then interleaved before being sent over the air
interface
Fig 211 Data Channel Encoding
27 Mapping Logical Channels onto the TDMA Frame Structure InterleavingBitstream into bursts can be transmitted within the TDMA frame structure It is at this stage that
the process of interleaving is carried out Interleaving spreads the content of one traffic block
across several TDMA timeslots The following interleaving depths are used
34
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Speech ndash 8 blocks
Control ndash 4 blocks
Data ndash 22 blocks
This process is an important one for it safeguards the data in the harsh air interface radio
environment Because of interference noise or physical interruption of the radio path bursts may
be destroyed or corrupted as they travel between MS and BTS a figure of 10ndash20 is quite
normal The purpose of interleaving is to ensure that only some of the data from each traffic
block is contained within each burst By this means when a burst is not correctly received the
loss does not affect overall transmission quality because the error correction techniques are able
to interpolate for the missing data If the system worked by simply having one traffic block per
burst then it would be unable to do this and transmission quality would suffer It is interleaving
that is largely responsible for the robustness of the GSM air interface thus enabling it to
withstand significant noise and interference and maintain the quality of service presented to the
subscriber
28 Transmission TimingTo simplify the design of the MS the GSM specifications specify an offset of three timeslots
between the BSS and MS timing thus avoiding the necessity for the MS to transmit and receive
simultaneously The diagram opposite illustrates this The synchronization of a TDMA system is
critical because bursts have to be transmitted and received within the ldquoreal timerdquo timeslots
allotted to them The further the MS is from the base station then obviously the longer it will
take for the bursts to travel the distance between them The GSM BTS caters for this problem by
instructing the MS to advance its timing ((that is transmit earlier) to compensate for the increased
propagation delay This advance is then superimposed upon the three timeslot nominal offset The
timing advance information is sent to the MS twice every second using the SACCH The
maximum timing advance is approximately 233 1048576s This caters for a maximum cell radius of
approximately 35 km
29 Multipath FadingMultipath Fading results from a signal travelling from a transmitter to a receiver by a number of
routes This is caused by the signal being reflected from objects or being influenced by
atmospheric effects as it passes for example through layers of air of varying temperatures and
humidity Received signals will therefore arrive at different times and not be in phase with each
35
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
other they will have experienced time dispersion On arrival at the receiver the signals combine
either constructively or destructively the overall effect being to add together or to cancel each
other out If the latter applies there may be hardly any usable signal at all The frequency band
used for GSM transmission means that a lsquolsquogoodrdquo location may be only 15 cm from a lsquolsquobadrdquo
location
GSM offers five techniques which combat multipath fading effects
Equalization
Diversity
Frequency hopping
Interleaving
Channel coding
The equalizer must be able to cope with a dispersion of up to 17 microseconds
Equalization
Due to the signal dispersion caused by multipath signals the receiver cannot be sure exactly
when a burst will arrive and how distorted it will be To help the receiver identify and
synchronize to the burst a Training Sequence is sent at the centre of the burst This is a set
sequence of bits which is known by both the transmitter and receiver When a burst of
information is received the equalizer searches for the training sequence code When it has
been found the equaliser measures and then mimics the distortion which the signal has been
subjected to The equalizer then compares the received data with the distorted possible
transmitted sequences and chooses the most likely one There are eight different Training
Sequence codes numbered 0ndash7 Nearby cells operating with the same RF carrier frequency will
use different Training Sequence Codes to enable the receiver the discern the correct signal
DiversityWhen diversity is implemented two antennas are situated at the receiver These antennas are
placed several wavelengths apart to ensure minimum correlation between the two receive
paths The two signals are then combined and the signal strength improved
36
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Fig 212 Multipath Fading
Frequency HoppingFrequency hopping allows the RF channel used for carrying signalling channel timeslots or
traffic channel (TCH) timeslots to change frequency every frame (or 4615 msec) This
capability provides a high degree of immunity to interference due to the effect of interference
averaging as well as providing protection against signal fading The effective ldquoradio channel
interference averagingrdquo assumes that radio channel interference does not exist on every
allocated channel and the RF channel carrying TCH timeslots changes to a new allocated RF
channel every frame Therefore the overall received data communication experiences
interference only part of the time All mobile subscribers are capable of frequency hopping
under the control of the BSS To implement this feature the BSS software must include the
frequency hopping option Cyclic or pseudo random frequency hopping patterns are possible
by network provider selection
37
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
CHAPTER-III
GSM BASIC CALL SEQUENCE amp ANTENNA
31 In the MS to Land direction
The BTS receives a data message from the MS which it passes it to the BSC The BSC relays
the message to the MSC via C7 signaling links and the MSC then sets up the call to the land
subscriber via the PSTN The MSC connects the PSTN to the GSM network and allocates a
terrestrial circuit to the BSS serving the MSrsquos location The BSC of that BSS sets up the air
interface channel to the MS and then connects that channel to the allocated terrestrial circuit
completing the connection between the two subscribers
Fig 31 MS to Land direction
38
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Step-wise details of Mobile to Land sequence are shown below
Subscriber presses the ldquosendrdquo key initiating a ldquoChannel Requestrdquo message from MS to
BSS This is followed by the assignment of DCCH by the BSS and the establishment of
signaling link between the MS and BSS
The message ldquoRequest for Servicerdquo is passed to MSC which relays it to VLR The VLR
will carry out the process of authentication if the MS has been previously registered on this
VLR and if not then the VLR will have to obtain authentication parameters from HLR
If authentication is successful call will continue If ciphering is to be used this is initiated
at this time as a setup message contains sensitive information
The message ldquoSet-Uprdquo is sent by the MS to MSC accompanied by the call information
The message is forwarded from the MSC to the VLR
The MSC may initiate the MS IMEI check This check may occur later in the message
sequence
In response to the message ldquoSet-Uprdquo (sent at step 4 above) the VLR sends the message
ldquoComplete Callrdquo to the MSC which notifies the MS with ldquoCall Proceedingrdquo
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air interface traffic channel The MS responds to the BSS with ldquoAssignment
Completerdquo
An ldquoInitial and Final Address Message (IFAM)rdquo is sent to the PSTN Ring tone is applied
at the MS in response to ldquoAlertingrdquo which the MSC sends to the MS when the PSTN
responds with an ldquoAddress Complete Message (ACM)
When answered (ldquoAnswer (ANS)rdquo from PSTN) the message ldquoContactrdquo is forwarded to the
MS by the MSC stopping the MS ring tone The MSC then connects the GSM traffic
channel to the PSTN circuit thus completing the end to end traffic connection
Conversation takes place for the duration of call
32 In the Land to MS direction
The MSC receives its initial data message from the PSTN (via C7) and then establishes the
location of the MS by referencing the HLR It then knows which other MSC to contact to
establish the call and that MSC then sets up the call via the BSS serving the MSrsquos location
39
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Fig 32 (A) Land to MS direction
A C7 ldquoInitial and Final Address Message (IFAM)rdquo arrives the ldquogatewayrdquo MSC (GMSC)
The MS to be called is identified by its MS-ISDN
Using the message ldquoSend Routing Infordquo still tagged by the MSrsquos MS-ISDN the GMSC
requests routing information from the HLR This forwards the message now retagged
with the MSrsquos IMSI to the VLR serving the LAI in which the MS is currently located
The requested information will enable the GMSC to identify the MSC to which the IFAM
must be directed
The VLR responds with the message ldquoRouting Information Acknowledgementrdquo now
tagged with an MSRN which is either newly drawn from its pool of MSRNs or already
40
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
associated with the MS being called the GMSC now sends an IFAM to the MSC serving
the MSs location tagged with the MSRN
The lsquovisitorrsquo MSC then requests call-setup information from the VLR (ldquoSend Info for
Call Setuprdquo)
The VLR response is the ldquoPagerdquo message back to the MSC containing the required
information The MSC then sends ldquoPaging Requestrdquo to the MS via the appropriate BSS
The MS responds and requests a dedicated control channel from the BSS (ldquoChannel
Requestrdquo) and the air interface signaling link is established Once established this
dedicated control channel carries ldquoPaging Responserdquo to the BSS which relays it to the VLR
via the MSC
Fig 32 (B) Land to MS direction (Contd)
41
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
The MS subscriber is authenticated and cipher mode is set (opt) The ldquoComplete Callrdquo
message is then sent to the MSC from the VLR This is relayed to the MS via the BSS as
the message ldquoSetuprdquo
The MS sends the message ldquoCall Confirmationrdquo to the MSC This indicates that the MS
is capable of receiving a call and the MSC sends an ldquoAddress Complete Message
(ACM)rdquo to the GMSC which relays it to the PSTN The landline subscriber will now hear
the ringtone
The MSC then assigns a traffic channel to the BSS (ldquoAssignment Commandrdquo) which in
turn assigns an air-interface traffic channel The MS responds to the BSS (which
Responds in turn to the MSC) with ldquoAssignment Completerdquo The MS now rings sending
the message ldquoAlertrdquo to the MSC as a confirmation
When the GSM subscriber answers the MS sends the message ldquoConnectrdquo to the MSC
The MSC acknowledges this (ldquoConnect Ackrdquo) and sends ldquoAnswer (ANS)rdquo to the GMSC
and PSTN The landline subscriberrsquos ring tone stops and the GMSC and MSC connect
the GSM traffic channel and the PSTN circuit together
Conversation takes place for the duration of call
33 MS initiated Call Clearing Sequence
The MS initiates the clearing of the call by sending the ldquoDisconnectrdquo message to the
MSC The MSC will then send a ldquoReleaserdquo message to the PSTN which will then start
to release the fixed network circuits associated with the call The MSC will also send a
ldquoReleaserdquo message to the MS to indicate that it may clear down the call
When the MS receives the message it will release the call and respond with the
ldquoRelease Completerdquo message The PSTN will also respond with a ldquoRelease Completerdquo
message
The MSC now initiates the freeing up of air interface radio resources and the A-
interface terrestrial resources related to the call The MSC will then send a ldquoClear
Commandrdquo to the BSS The BSS in turn will send a ldquoChannel Releaserdquo on to the MS
and this will start the release of radio resources used for the call The BSS will then
respond to the MSC with the ldquoClear Completerdquo message indicating that it has released
the radio and terrestrial resources
42
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
The BSS will now complete the release of the radio resources by sending the ldquoDISCrdquo
message to the MS The MS will respond with an ldquoUnnumbered Acknowledgement
(UA)rdquo message
The MSC will now initiate the release of the signaling connection related to the call
The MSC will send the ldquoReleasedrdquo message to the BSS which will respond with the
ldquoRelease Completerdquo message
The call is now cleared and all the resources are available for another subscriber
Fig 33 MS initiated Call Clearing Sequence
43
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
34 Inter-BSS Handover Sequence
Fig 34 Inter-BSS Handover Sequence
The MS is in the conversation state and is continuously compiling measurements both of
the current transmission and the broadcast control channels of up to thirty surrounding
cells The measurements from the six best cells are reported back to BSS after every 480
ms
44
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
When a handover is requires due to low Received Signal Strength Indicator (RSSI) or
poor signal quality the existing ldquooriginatingrdquo BSS (oBSS) notified the MSC (ldquoHandover
Requiredrdquo)
The target or lsquonewrsquo BSS (nBSS) is alerted with the message ldquoHandover Requestrdquo tagged
with the TMSI
The new BSS allocates a Handover Reference Number which it uses to determine
whether the correct MS gains across to the air interface channel which it allocates and
acknowledges the MSCrsquos request with ldquoHandover Request Ackrdquo This is tagged with the
HO reference number The nBSS assigns a traffic channel
The MSC via the oBSS orders the MS to change to the new channel with the message
ldquoHandover Commandrdquo on FACCH
There is an information interchange between nBSS and MS This uses the FACCH
channel but an access burst is used The messages and the information carried depend
upon the type of handover being performed
Once all the necessary information has been transferred the message ldquoHandover
Completerdquo is sent to the MSC
The MSC now sends a ldquoClear Commandrdquo to the oBSS due to which the radio resources
for another MS are freed The channel is not cleared until this point in case the new BSS
cannot accommodate the MS being handed over
The MS still in conversation mode then continues to prepare periodic measurement
reports and sends them to the new BSS
35 Location Area Update
A location update is initiated by the MS when it detects that it has entered into a new
location area The location area is transmitted on the BCCH as the LAI The MS will be
assigned an SDCCH by the BSS the location updating procedure will be carried out
using this channel
One the SDCCH has been assigned the MS transmits a ldquoLocation Update Requestrdquo
message This message is received by the MSC which then sends the new LAI and the
current MS TMSI number to the VLR The information will be sent to the HLR if the
MS has not previously been updated on the network
45
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Authentication and ciphering may now take place if required
The VLR will now assign a new TMSI for the MS this number will be sent to the MSC
using ldquoForward New TMSIrdquo message The VLR will now initiate the ldquoLocation Update
Requestrdquo message which will transmit the new TMSI and LAI to the MS
Once the MS has been stored both the TMSI and the LAI on its SIM card will be send
the ldquoTMSI Relocate Completerdquo message to the MSC The MSC will then send the
ldquoTMSI ACKrdquo message to the VLR to confirm that the location update has been
completed
The SDCCH will then be released by the MS
Fig 35 Location Area Update
46
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
36 Authentication and Ciphering
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples are
sent in groups of six and stored in the VLR This ensures that the VLR can carry out the
authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
Fig 36 Authentication and Ciphering
47
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Authentication may be executed during call setup location updating and supplementary
services The HLRAUC produce the authentication parameters (SRESRANDKc) are
called ldquotriplesrdquo Triples are sent to the VLR where the MS is registered These triples
are sent in groups of six and stored in the VLR This ensures that the VLR can carry out
the authentication and that it will not have to contact the HLR
The VLR initiates the authentication by sending the message ldquoAuthenticaterdquo to the
MSC The MSC will repackage this message and send it on to the MS The message is
an ldquoAuthentication Requestrdquo and contains the random number RAND
The MS responds with the ldquoAuthentication Responserdquo message which contains the
signed response (SRES)
If the authentication is successful the VLR will request that the MSC start ciphering
procedures using the ldquoStart Cipheringrdquo message This message contains information
indicating whether ciphering is required
The MSC will start ciphering procedures by sending the ldquoCipher Mode Commandrdquo
message to the BSS This message contains the encryption information required by the
BSS The BSS will respond with the ldquoCipher Mode Completerdquo message
37 Antenna
371 Introduction
Antennas form an essential part of radio communication systems An antenna is a structure
capable of receiving and transmitting electromagnetic waves It is generally a metallic object
used to convert high frequency current into electromagnetic waves and vice versa
Functioning of an antenna can be said to be analogous to open circuited transmission line and
when electrical energy travels through open circuited transmission line electrical and
magnetic fields will set-up A part of this energy will be radiated depending upon impedance
matching of free space and the line An antenna can be viewed as a transitional structure
between the free space and the transmission line (such as a co-axial cable) An important
property of an antenna is its ability to focus and shape the radiated power in space like it
enhances the power in some wanted directions and suppresses the power in the other
48
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
directions Many different types of antennas exist Each type is specifically designed for
special purposes
372 Antenna Types
In mobile communication two main categories of antenna are used
Omni-directional Antenna
Directional Antenna
Omni-directional Antenna
Omnidirectional antennas used mostly in rural areas In all the horizontal direction these
antennas radiate with equal power In the vertical plane these antennas radiate uniformly
across all azimuth angles and have a main beam with upper and lower side lobes
Directional Antennas
Directional antennas are most widely used in the mobile cellular systems to get higher gain
compared to omnidirectional antennas and also to minimize the interference effects in the
network In the vertical plane these antennas radiate uniformly across all the azimuth angles
and have a main beam with upper and lower side lobes In these type of antennas the
radiation is directed at a specific angle instead of uniformly across all the azimuth angles as
in case of omnidirectional antennas
The unit for the measurement of antenna is ldquodecibelrdquo (db) It is ten times logarithmic of
relative power that is
Db=10 log(P1P2)
Where
P1=Power to be measured
P2=Relative power with respect to which P1 is to be measured
373 Antenna Characteristics
There are various antenna characteristics Some of them are listed below
Radiation pattern
Antenna gain
Front-to-back Ratio
First Null Beamwidth
Polarization
Antenna Impedance
49
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Side lobes
Back Lobe
Main Lobe
Radiation Pattern
This is one of the main characteristics of an antenna The antenna pattern is a graphical
representation in three dimensions of the radiation of an antenna as a function of angular
deviation Antenna radiation performance is usually measured and recorded in two
orthogonal principal planes ie E-plane and H-plane or vertical and horizontal planes The
radiation pattern of an antenna consists of many lobes out of which there is only one main
lobe and several minor lobes also termed as side lobes as shown in fig 331 below A side
lobe which occurs in a direction opposite to that of main lobe is called as a back lobe and this
back lobe is responsible for reduction in power radiated by the main lobe
Fig 37 Radiation Pattern of an antenna
Antenna GainAntenna gain is the ratio of power radiated by the test antenna in a particular direction to the
reference antenna at same input power
Half Power Beamwidth (HPBW)
It is the angular span during which the antenna radiates half to that of the maximum power or
3 db less than the power radiated in the main lobe axis
Front-to-back Ratio (FBR)
It is ratio of maximum directivity of an antenna in the forward direction to its directivity in
rearward or backward direction The power taken in the numerator as well as the
denominator is in db
Antenna Impedance
When potential is applied at the two ends of an array structure of antennas then the ratio
current produced at both the ends is known as the antenna impedance When antenna
50
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
impedance matches the cable impedance then maximum power is coupled into the antennas
Typically its value is 50 ohms
Polarization
Polarization is the orientation of electric field vector in a specified direction It is of four
types namely horizontal vertical circular (elliptical) and cross polarization But in mobile
cellular communication the most widely used types of polarization are vertical polarization
and cross polarization
Frequency Bandwith
Frequency bandwith is the range of frequencies within which the performance of an antenna
with respect to some characteristics conforms to the specified standard
374 Antenna Downtilting
Network planners often have the problem that the base station provides an over coverage If
an overlapping area between the two cells is too large then switching occurring between the
base station (handover) increases There may even be an interference of a neighbouring cell
with the same frequency In general the vertical pattern of an antenna radiates the main
energy towards the horizon Only that part of energy which is radiated below the horizon can
be used for the coverage of sector Antenna downtilting limits the area covered by an antenna
by reducing the field strength in the horizon Antenna downtilting is the downward tilt of the
vertical pattern towards the ground by a fixed angle measured with respect to the horizon
Downtilting of an antenna changes the position of half power beamwidth and the first null
relative to the horizon Normally the maximum gain is at 0 degrees and never intersects the
horizon A small downtilt places the beam maximum at the cell edge With appropriate
downtilt the received signal strength within the cell improves due to the placement of the
main lobe within the cell radius and falls off in the regions approaching the cell boundary and
towards the reuse cell There are two methods of downtilting
Mechanical downtilting
Electrical downtilting
3741 Mechanical Downtilting
Mechanical downtilting consists of physically rotating an antenna downward about an axis
from it vertical position In mechanical downtilt as the front lobe moves downward the back
lobe moves upwards This is one of its drawback as compared to the electrical downtilt
51
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
because coverage behind the antenna can be negatively affected as the back lobe rises above
the horizon Its advantage is that the power of the side lobes and the back lobe gets added
into the main lobe thus increasing the power radiated in the main lobe
3742 Electrical Downtilting
Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards
This allows the antenna to be mounted vertically Electrical downtilt is the only practical way
to achieve pattern downtilting with omnidirectional antennas Electrical downtitlt affects both
front and back lobes If the front lobe is downtilted the back lobe is also downtilted by an
equal amount It also reduces the gain equally at all the angles on the horizon Variable
electrical downtilt antennas are very costly
CHAPTER-IV
52
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
BTS INSTALLATION
41 Introduction
Cell site consist of a BTS and antenna system to transmit and receive in a particular geographical location ranging from 200m to 25 kmCell Site consists of Tower Antennas Shelter
BTS Transmission Rack PIU SMPS Battery Bank Alarm Sensors Earthing Cables
Connectors Generator
42 Requirements Information Material Tool
421 Information RequiredBasic information Site ID Site address Customer no Ownerrsquos no and address Key info Activity to be performed Site layout
ACTIVITY i Microwave antenna Frequency(near end far end) Power RSL Even Capacity Height Degree Microwave hop size Radio type
53
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Polarizationii GSM
Height Degree Electrical tilt Mechanical tilt Number of sectors
422 Tools Required
SPANNER SET ADJUSTABLE SPANNER
MALLET HAMMER
KNIFE
LINER MULTI BIT SET SMALL HEXA KRONE TOOL
TESTER RATCHET SET SOLDERING IRON L-KEYSCREW DRIVER SET
BNC TOOL ROUGH FILE RJ-45 TOOL
HAND CRIMPING TOOL
ELPRESS HD PLIER NOSE PLIER
CABLE CUTTER TIE CUTTER PUNCH BIG HEXA
MULTIMETER DRILL MACHINE ROUND FILE SITE ANALYZER
43 Base Transceiver Station (BTS)
The BTS network element consists of the hardware components such as radios interface modules and antenna systems that provide the Air Interface between the BSS and the MSs
The BTS provides radio channels (RF carriers) for a specific RF coverage area The radio channel is the communication link between the MSs within an RF coverage
area and the BSS The BTS also has a limited amount of control functionality which reduces the amount of
traffic between the BTS and BSC
Functions of BTS are as below Channel Allocation Radio Channel Management Channel configurations Handover Control Frequency Management Traffic Channel Management Encryption Encoding Timing Advance Paging
54
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
BTS Parts
431 BTS Configurations
BSC may control several BTSs the maximum number of BTSs which may be controlled by
one BSC is not specified by GSM Individual manufacturerrsquos specifications may vary greatly
The BTSs and BSC may either be located at the same cell site ldquoco-locatedrdquo or located at
different sites ldquoRemoterdquo In reality most BTSs will be remote as there are many more BTSs
than BSCs in a network Another BSS configuration is the daisy chain A BTS need not
communicate directly with the BSC which controls it it can be connected to the BSC via a
chain of BTSs Daisy chaining reduces the amount of cabling required to set up a network as
a BTS can be connected to its nearest BTS rather than all the way to the BSC Problems may
arise when chaining BTSs due to the transmission delay through the chain The length of the
chain must therefore be kept sufficiently short to prevent the round trip speech delay
becoming too long Other topologies are also permitted including stars and loops Loops are
55
BASEBAND UNIT
RF UNIT
COMMON CONTROL UNIT
POWER UNIT
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
used to introduce redundancy into the network for example if a BTS connection was lost the
BTS may still be able to communicate with the BSC if a second connection is available
Fig 41 BTS Configurations
432 RBS 2216
56
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Fig 42 Dual Radio Unit
57
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
The DRU consist of two GSM TRXs hybrid combiner two TX filters two duplex filters
and two bias injectors Up to six DRUs can be installed in one RBS 2216 enabling up to 12
TRXs per cabinet Various versions of the DRU exist for different frequency bands All
DRUs support all time slots for General Packet Radio Services (GPRS) and Enhanced Data
Rates for Global Evolution (EDGE) The integrated hybrid combiner is used to increase the
number of TRXs per antenna but it can be bypassed (to get higher output power) when
setting the configuration The DRU also supports Transmitter Coherent Combining (TCC)
which provides an increased cell radius for the downlink TCC mode means that the same
signal is transmitted through both TRXs in the DRU inside which the signals are added
coherently in a hybrid combiner The result is 6 dB more signal output power compared with
the combined version To compensate for the improved downlink and fully balance the link
budget when TCC is used it is also possible to configure 4- Way Receiver Diversity
(4WRD) and combining uplink reception from four separate RX antenna branches This
gives improvements both from increased RX antenna
areas (~3 dB) and from better compensation of fast fading (~13 dB) The duplex filters allow
both receiver and transmitter path connections to a common antenna The duplex
configurations also minimize the number of feeders and antennas required
433 RX Splitter
Fig 43 RX Splitter
RX splitters enable RX signals to be shared thereby minimizing the number of TMAs required in configurations with six or more carriers in a sector RX splitters are also used for RX sharing between the RBS 2216 and other RBSs
58
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
434 Tower-Mounted Amplifier Control Module
Fig 44 Tower-Mounted Amplifier Control Module
The TMA-CM monitors and controls the TMAs to which it also supplies power through the
bias injectors in the DRUs The TMA-CM
Supplies power to up to six TMAs through the bias injectors Monitors the TMAs Controls the TMAs Supervises indicators Provides short circuit protection Supervises cables
Number of units 0-3
59
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
435Distribution Switch Unit
Fig 45 Distribution Switch Unit
The DXU is the system interface for 2 Mbps (E1) or 15 Mbps (T1) transmission links It also
cross-connects individual time slots to certain TRXs and extracts the synchronization
information from the PCM link to generate a timing reference for the RBS
The DXU supports Link-Access Procedures on D-Channel (LAPD) multiplexing LAPD concentration Multi-drop Four transmission ports (E1T1) Synchronized radio network through an external GPS receiver Transceiver Group (TG) synchronization Extended RBS holdover time
Number of units 0-2
60
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
436AC and DC Connection Units
Fig 46 AC and DC Connection Units
An ACCU or DCCU is the single connection point for power It distributes the power to the
PSUs
The ACCU is used for 200250 V AC
The DCCU is used for minus48 V DC
+24 V configurations do not require PSUs nor is there any need for an ACCUDCCU +24 V
supply is connected directly to the IDM
ACCU AC Control UnitDCCU DC Control Unit
61
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
437Power Supply Unit
Fig 47 Power Supply Unit
PSUs rectify or convert incoming power to the regulated +24 V DC system power and are
available in two versions AC (200250 V) and DC (minus48 V) This means the RBS can be
powered directly from the mains or from an existing minus48 V DC power supply No PSU is
required when the RBS is powered from a +24 V DC power supply
PSUs are connected in parallel at the secondary side and can be configured with N+1
redundancy
When using a battery backup system an additional PSU is recommended to reduce battery-
charging times An unused PSU can of course also be used for this purpose
Number of units 0-3
Four configuration types are available to choose from depending on the way the transmitter
signals are combined combined uncombined mixed (combined and uncombined) and
TCC4WRD
Combined mode means that the GSM carriers are subject to hybrid combining inside
the DRU The output power is 35 dB lower than in uncombined mode The advantage is
that combined mode reduces the number of feeders and antennas for large
configurations This is why it is normally used for high-capacity coverage in dense
urban areas Combined capacity is added in steps of two carriers per sector
Configurations based on combined mode are sometimes referred to as capacity mode
62
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Uncombined mode means that the GSM carriers are not subject to any combining The
output power is 35 dB higher than in combined mode The drawback is that
uncombined mode requires more feeders and antennas than combined mode which is
why it is normally used for coverage in rural road and suburban environments
Uncombined capacity is added in steps of two carriers that can be in either one or two
sectors Configurations based on uncombined mode are sometimes referred to as
coverage mode
TCC4WRD mode means that two GSM carriers are coherently combined in one GSM
carrier that has almost double the output power of uncombined mode To achieve a
balanced link budget 4WRD can be used on the uplink together with TMAs resulting
in uplink improvements of 36 dB Configurations based on TCC4WRD are sometimes
referred to as supreme coverage mode
To achieve a balanced link budget bear in mind the following
DRUs provide only one GSM carrier instead of two
The minimum configuration is two DRUs per cell because four RX paths are needed
using only TCC requires only one DRU
The minimum antenna system is four feeders and four antenna ports per sector (TCC
can be used with 2WRD and only two antennas per sector)
Using 4WRD requires using TMAs
Mixed mode uses a combination of combined and uncombined modes as described
above One DRU can eg be used for two sectors resulting in one uncombined carrier
in each of the sectors The advantage of smart range is that it allows smaller capacity
expansions in steps of two carriers per sector than in combined mode alone A sector
using mixed mode has at least three TRXs
PROCEDURE 1
ACTIVITY Site survey for BTS MW amp GSM Installation
63
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
STEPS
Draw a layout for shelter with present status with labeled space measurement
Check BTS space availability
Check MW Antenna height availability
Roxtec entry space for new feeders
Power availability ie (MCB free or not) if free note it rating
MMU space is available at transmission rack or not
Feeder Routing space at inner and outer cable ladder
Measure feeder cable if cable and power cable length
Space at EGB IGB
Owner issue
In case of MBC check type of antenna present
Pole mount is present at given GSM antenna height or not
Make a report and shelter layout
PROCEDURE 2
64
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
ACTIVITY Installation of a site
STEPS
Install indoor cable tray put four l angles to the four edges and fix cable tray with wall
mounted channels at the both sides Connect both trays at both end and door side and
rear wall
Distance 100m from sidewall and 100m below roxtec
Install Tx rack and BTS cabinet
Tx rack ndashn 150m from sidewall and 30m-50m from backside wall
Mark holes with marker drill put fastener place rack and tight nuts
Put two l supports on the top of Tx rack similarly place BTS
Install DDF as per customer plan
Connect DDF cable to BTS equipment
Install the battery bank as per layout plan Make sure all inter connected strips
are tightend
Place SMPS on top of battery bank and tight it with nut bolts
Install cable cut for battery bank and SMPS cabling Connect the RBS power supply
cable to the RBs and route it to the SMPS and connect ot to the load port and neutral
bar Then connect the battery ip cable tray of battery end
Connect the ac cable from PIU to the SMPS in the MCB
Route alarm cable from SMPS to DDU and connect according to the need Route power
cable from SMPS to Tx rack
Do the earthing of each equipments as per points All to be labeled with printed
sticker
Place krone block 4 inch downward DDU and AMM at 15 foot top from Tx rack
Install fan unit attach with AMM and connect power cable from DDU Insert MMU at
AMM a lot and give+24v dc
Place pulley and rope at tower at given GSM height and degree
Tide the GSM antenna with rope make a large angle with tower and then drag the
rope Place the antenna at given height and tight the nut bolts at clamp
Set the degree with the help of compass and set tilt
65
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
Cut the feeder cable and make feeder connectors at both end tape them properly and
route the feeder on VCT and hct from antenna to BTS with the help of clamps
Provide a proper drip loop near the Roxtec and there should not be any bend less than
120 degree
Do the feeder earthing near the 1m above the VCT and near the Roxtec
Similarly place the mw antenna at near and far end and check its polarization degree
height
Route the IF cable from mw to MMU Make a loop of 3m if cable and tie it near the
mw for further extension
Do if earthing same as feeder cable
Now go for alignment
Turn on the MMU at both ends and feed the frequency power RSl at both ends
Place the multimeter at mw both ends
At far end go for horizontal alignment and then for vertical alignment and lock the
position with max value
Repeat the same at near end till the desired value is achieved
Check for all punch points if clear go for commissioning
66
REFERENCES
67
- 12 GSM Standards
-
REFERENCES
67
- 12 GSM Standards
-