Maciej Stasiak, Mariusz Głąbowski Arkadiusz Wiśniewski, Piotr Zwierzykowski
Modeling and Dimensioning of Mobile Networks: from GSM to LTE
Maciej Stasiak, Mariusz Głąbowski Arkadiusz Wiśniewski, Piotr Zwierzykowski
GSM
Modeling and Dimensioning of Mobile Networks: from GSM to LTE
GSM system – introduction 1/3
• GSM – Global System for Mobile Communication
• Operates in 900 and 1800 MHz
• Access to the radio link is based on frequency division
multiple access (FDMA) and time division multiple
access (TDMA)
• Each band available for the system is divided into
channels with bandwidth of 200 kHz
• For the GSM 900 system there are 124 available
channels (separate for the uplink and downlink
direction), and for the GSM 1800 374 channels
GSM system – introduction 2/3
Feature/Bandwidth GSM 900 GSM 1800
Uplink (MHz) 890-915 1710-1785
Downlink (MHz) 935-960 1805-1880
Number of available channels
124 374
GSM system – introduction 3/3
GSM system – architecture 1/6
• In the GSM system three basic subsystems can be
distinguished: o base station subsystem (BSS)
o core network (CN)
o user equipment (UE)
• Between particular elements of the system the
interfaces are defined
GSM system – architecture 2/6
• Base station subsystem – includes system of base stations and their controllers
• Base station provide optimum radio coverage of a given area and communicates with user equipment over air interface
• The operation of the base station subsystem is controlled by the base station controller (BSC)
• This manages radio resources allocation, controlls the setting-up of calls, gathers results of measurements carried out by base station and mobile station
• The BSC is also responisble for power controll and handover controll
• Interface A enables the BSS system to be connected to mobile switching center (MSC)
• Interface Gb connects BSS with packet switching element
GSM system – architecture 3/6
• The main elements of the core network are: o mobile switching center (MSC)
o visitor’s location register (VLR)
o home location register (HLR)
o authentication ceter (AUC)
o equipment identification register (EIR)
o serving GPRS support node (SGSN)
o gateway GPRS support node (GGSN)
GSM system – architecture 4/6
GSM system – architecture 5/6
• MSC basic task is to control and regulate services
provided by the system, circuit switching, and
gathering billing information
• VLR registry keeps information concerning mobile
stations available in the area of one, or several, MSC
switching centers
• SGSN is the equivalent of the MSC switching center of
packet switching
• GGSN is an interface between the mobile packet
network and external packet networks
GSM system – architecture 6/6
• HLR is a central database that contains details of each
mobile phone subscriber authorized to use the GSM
core network and includes authorization data
• AUC generates sets of keys used in encryption of
transmission, identifies the mobile station and the
network, and controls and regulates the integrity of
transmitted data
• EIR is a data base that keeps a list of numbers
identifying a given mobile station – IMEI (International
Mobile Equipment Identity)
GSM system – time structure 1/3
• In the GSM system each carrier frequency is divided
into eight time slots
• Packet transmission is commenced every 4.615 ms and
single bit lasts 3.69 µs
• Typical packet, except access packet, has 148 bits,
thus its duration is about 546 µs
• The duration of a single time slot is 577 µs, wich allows
for maintaining a steady interval between successively
transmitted packets
GSM system – time structure 2/3
GSM system – time structure 3/3
GSM system – logical channels 1/4
• Logical channels can be divided into two categories: o control channels – are used to set up a connection in the radio network for
transmission of control data
o traffic channels – are used to transmit user data
• In the GSM system, speech signals are transmitted with
traffic channels (TCHs). Speech can be transmitted at
full rate, 13 kbps, or at half rate, 6.5 kbps
GSM system – logical channels 2/4
GSM system – logical channels 3/4
• Frequency correction channel (FCCH) – used by the mobile station to tune to the carrier frequency, a frequency correction burst is transmitted on the channel by generating unmodulated sine waves
• Synchronization channel (SCH) – transmits base station identity code (BSIC), which allows the mobile station to identify the base station and to convey synchronization information
• Broadcast control channel (BCCH) – used for transmission of control information such as: radio channel frequency used by a given cell, neighbor cell list, information on the paging channel, configuration of logical channels in the base station
• Paging channel (PCH) – with the PCH the base station initiates a connection with the mobile station
GSM system – logical channels 4/4
• Random access channel (RACH) – used by mobile station for initial
access to a system (with the RACH the mobile station initiates a
connection with base station)
• Access grant channel (AGCH) – used by the base station to assign resources to a mobile station requesting access to the network
• Stand alone dedicated control channel (SDCCH) – used to
provide a reliable connection for signaling and SMS messages, for
authentication, and to provide information on location update
• Slow associated control channel (SACCH) – supports the SDCCH
channel, used for sending network measurement reports and
information related to power control procedures
• Fast associated control channel (FACCH) – coupled with the speech channel, used for immediate transmission of information
related to, for example, cell handover
High Speed Circuit Switched Data 1/4
• High Speed Circuit Switched Data is an additional feature of the GSM network and was introduced in phase 2
• HSCSD technology enables a simultaneous application of several speech channels for a single data transmission link
• A connection can be set up that makes use simultaneously of n channels (time slots) in the radio interface, where n takes on the values n = 1; 2; ...; 8
• A HSCSD connection can be set up only when the mobile station is capable of using several radio channels simultaneously
• Additional modifications are needed in the BSS system that involve multiplexing of a component data stream in one 64 kbps channel of A interface
High Speed Circuit Switched Data 2/4
• A HSCSD connection can have a symmetrical configuration – the same number of speech channels is allocated for the uplink and the downlink direction – or a non-symmetrical configuration
• A non-symmetrical configuration is chosen when the subscriber requirements cannot be accomplished in a symmetrical configuration
• The maximum link transmission speed that can be achieved by the HSCSD technology depends on the number of channels used in the radio interface and on the applied coding
• Due to a necessity of transmitting all channels included in a HSCSD connection in one link 64 kbps in interface A, the transmission speed is limited to 57.6 kbps
High Speed Circuit Switched Data 3/4
Data rate performance in radio interface [kbps]
TCH/F4.8 TCH/F9.6 TCH/F14.4
4.8 kbps 1 N/A N/A
9.6 kbps 2 1 N/A
14.4 kbps 3 N/A 1
19.2 kbps 4 2 N/A
28.8 kbps N/A 3 2
38.4 kbps N/A 4 N/A
43.2 kbps N/A N/A 3
57.6 kbps N/A N/A 4
High Speed Circuit Switched Data 4/4
GPRS Packet Transmission 1/6
• The implementation of packet transmission in the GSM
system requires changes in the structure of the system
• New elements are included in the network – SGSN and
GGSN nodes
• According to the assumptions given in 3GPP
specification, GPRS should allow: o pulse data transmission in which time interval between individual moments of
transmission is considerably higher than the average transmission delay
o frequent transmission (several times per minute) of small amount of data (bursty
data transfer up to 500 octets)
o occasional transmission of large volumes of data
GPRS Packet Transmission 2/6
• The GPRS technology makes it possible to transmit data
in several channels. Within one packet connection, the
mobile station as well as the base station can make
simultaneous use of 8 time slots in a frame
• All users of a packet service can share resources
available for data transmission that are allocated, due
to the asymmetry of traffic, separately for the uplink
and for the downlink direction
• The radio interface resources can be dynamically
shared by speech service and packet data transmission
depending on the configuration of the network
GPRS Packet Transmission 3/6
• Following logical channels, responsible for transmission
of data and signalling information are introduced: o Packet Common Control Channel (PCCCH)
• Packet Random Access Channel (PRACH) – used by the mobile station to
initiate uplink transfer of user data or signalling information
• Packet Paging Channel (PPCH) – used to page a mobile station preceding
downlink direction packet transfer. The channel can also be used for
establishing speech connections
• Packet Access Grant Channel (PAGCH) – used in the packet transfer
establishment phase to send resource assignment to a mobile station
preceding packet transfer
• Packet Notification Channel (PNCH) – used to send point to multipoint
multicast notification information to a group of mobile stations preceding
multicast packet transfer
GPRS Packet Transmission 4/6
• Following logical channels, responsible for transmission
of data and signalling information are introduced: o Packet Broadcast Control Channel (PBCCH) – used to broadcast packet
system information
o Packet Data Traffic Channel (PDTCH) – allocated for user data transfer. Several
PDTCH channels can be allocated to a given mobile station. They can be
allocated temporarily to one or more mobile stations
o Packet Associated Control Channel (PACCH) – used to transmit signalling
information related to a given mobile station, such as those related to power
control or packet reception acknowledgement messages
o Packet Timing Advance Control Channel, Uplink (PTCCH/U) – used to ensure
that the correct timing advance is maintained for each mobile station. In the
uplink direction, the channel is used by a mobile station to send an access burst
o Packet Timing Advance Control Channel, Downlink (PTCCH/D) – used to send
packets in the downlink direction to assess the needed timing advance in order
to achieve frame synchronization
GPRS Packet Transmission 5/6
• Four coding schemes (CS) have been defined for the
GPRS transmission: CS-1, CS-2, CS-3 and CS-4
• Particular schemes are characterized by different user
data transmission speed and by a various degree of
error protection procedures
• CS-1 coding, due to its highest level of protection (error
correction), is used for channels with highest
interference and for signalling channels
• CS-4 coding enables the fastest data transmission
speed – 21,4 kbps for one channel, but has no
protection
GPRS Packet Transmission 6/6
Coding scheme Coding efficiency Data rate [kbps]
CS-1 1/2 9.05
CS-2 2/3 13.4
CS-3 3/4 15.6
CS-4 1 21.4
EDGE Packet Trasnmission 1/2
• Voice transmission in the GSM system and the GPRS packet
transmission uses the Gaussian minimum shift keying (GMSK)
modulation. This is a binary modulation with speed of 270.833
kbps
• EDGE technology achives higher speed value with the same
bandwidth of the radio channel thanks multivalue, eight-
level phase shift keying modulation (8PSK)
• EDGE uses nine coding schemes: MCS-1 – MCS-9
• Each coding scheme is characterized by a different data
transmission speed and data protection
• Transmission speed for one PDTCH channel with MCS-1
coding is 8.8 kbps, for MCS-9 coding scheme is 59.2 kbps
EDGE Packet Trasnmission 2/2
Coding scheme Coding efficiency Modulation Data rate [kbps]
MCS-9 1.0 8PSK 59.2
MCS-8 0.92 8PSK 54.4
MCS-7 0.76 8PSK 44.8
MCS-6 0.49 8PSK 29.6 27.2
MCS-5 0.37 8PSK 22.4
MCS-4 1.0 GMSK 17.6
MCS-3 0.85 GMSK 14.8 13.6
MCS-2 0.66 GMSK 11.2
MCS-1 0.53 GMSK 8.8