chapter 2 gsm system principles and call flow

8/6/2019 Chapter 2 GSM System Principles and Call Flow

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GSM Radio Network Planning and Optimization

2GSM

Table of Contents

Chapter 1 GSM Principles and Call Flow ....................................................................................3

1.1 GSM Frequency Band Allocation .....................................................................................3

1.2 Multiple Access Technology and Logical Channel............................................................. 4

1.2.1 GSM Multiple Access Technology ...........................................................................4

1.2.2 TDMA Frame ..........................................................................................................5

1.2.3 Burst....................................................................................................................... 7

1.2.4 Logical Channel...................................................................................................... 9

1.3 Data Transmission ..........................................................................................................12

1.3.1 Voice Coding ........................................................................................................13

1.3.2 Channel Coding ....................................................................................................14

1.3.3 Interleaving ...........................................................................................................15

1.3.4 Encryption ............................................................................................................17

1.3.5 Modulation and Demodulation ..............................................................................17

1.4 Timing advance ...............................................................................................................18

1.5 System Information .........................................................................................................19

1.6 Cell Selection and Re-Selection ......................................................................................21

1.6.1 Cell Selection ........................................................................................................21

1.6.2 Cell Selection Process ..........................................................................................22

1.6.3 Down Link Failure ................................................................................................23

1.6.4 Cell Re-Selection Process ....................................................................................23

1.7 Frequency Hopping ........................................................................................................24

1.7.1 Types of Frequency Hopping ................................................................................25

1.7.2 Frequency Hopping Algorithm ..............................................................................27

1.7.3 Benefits of Frequency Hopping .............................................................................30

1.8 Discontinuous Reception and Discontinuous Transmission ............................................32

1.8.1 Discontinuous Reception and Paging Channel..................................................... 32

1.8.2 DTX ......................................................................................................................34

1.9 Power Control................................................................................................................. 36

1.9.1 Power Control Overview ......................................................................................36

1.9.2 MS Power Control................................................................................................. 36

1.9.3 BTS Power Control............................................................................................... 38

1.9.4 Power Control Processing ....................................................................................39

1.10 Immediate Assignment Procedure ................................................................................41

1.10.1 Network Access License and Random Access Request..................................... 41

1.10.2 Initial Immediate Assignment.............................................................................. 42

1.10.3 Initial Message ....................................................................................................43

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1.10.4 Immediate Assignment Failure ............................................................................44

1.11 Authentication and Encryption ......................................................................................45

1.11.1 Authentication ....................................................................................................45

1.11.2 Encryption ..........................................................................................................48

1.11.3 TMSI Reallocation ..............................................................................................49

1.11.4 Exceptional Situations .........................................................................................50

1.12 Location Update ............................................................................................................51

1.12.1 Generic Location Update (Inter-LA Location Update) .........................................51

1.12.2 Periodic Location updating .................................................................................53

1.12.3 IMSI Attach and Detach ......................................................................................54

1.12.4 Exceptional Situations ........................................................................................55

1.13 MS Originating Call Flow ...............................................................................................57

1.13.1 Called Number Analysis .....................................................................................58

1.13.2 Voice Channel Assignment (Follow-up Assignment) ...........................................58

1.13.3 Call Connection .................................................................................................62

1.13.4 Call Release .......................................................................................................62


1.14 MS Originated Call Flow ...............................................................................................66

1.14.1 Enquiry ...............................................................................................................66

1.14.2 Paging ...............................................................................................................67

1.14.3 Call Establishment for the Called Party ..............................................................68

1.14.4 The Influence of Call Transfer to Routing ............................................................69

1.14.5 Exceptional Situations ........................................................................................701.15 HO .................................................................................................................................72

1.15.1 HO Preparation ...................................................................................................73

1.15.2 HO Types ............................................................................................................76

1.15.3 HO Process Analysis ..........................................................................................78


1.16 Call Re-Establishment .................................................................................................88

1.16.1 Introduction .........................................................................................................88

1.16.2 Call Re-Establishment Procedure .......................................................................89


1.16.4 SM Procedure .....................................................................................................91

1.16.5 Short Message Procedure on SDCCH When MS is calling ...............................91

1.16.6 Short Message Procedure on SDCCH When MS is called ................................92

1.16.7 Short Message Procedure on SACCH When MS is calling ................................93

1.16.8 Short Message Procedure on SACCH when MS is called ..................................94

1.17 CBS ...............................................................................................................................94

1.17.1 CBS Mechanism ................................................................................................95

1.17.2 BSC-BTS Message Transmission Mode .............................................................96

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Chapter 1 GSM Principles and Call Flow

1.1 GSM Frequency Band Allocation

GSM cellular system can be divided into GSM900M and DCS1800M according to

frequency band, with carrier frequency interval of 200 KHz and up and down

frequencies as follows:

Table 1.1 GSM frequency allocation

Frequencyband(MHz)

Bandwidth(MHz)

Frequencynumber

Carrierfrequencynumber

(pair)

GSM900 Up 890915

Down 935960

25 1124 124

DCS1800 Up 17101785

Down 18051880

75 512885 374

Up and down are classified according to base station. Base station transmitting -

mobile station receiving is down; mobile station transmitting - base station receivingis up.

With the expanding services, GSM protocol adds EGSM(expanded GSM frequency

band) and RGSM (expanded GSM frequency band including railway service) to the

original GSM900 frequency band. The frequency band allocation is as follows:

Table 1.2 EGSM/RGSM frequency allocation

Frequencyband(MHz)

Bandwidth(MHz)

Frequencynumber

Carrierfrequencynumber

(pair)

EGSM Up 880915

Down 925960

35 0124

9751023

174

RGSM Up 876915

Down 921960

40 0124

9551023

199

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1.2 Multiple Access Technology and Logical Channel

1.2.1 GSM Multiple Access Technology

In cellular mobile communications system, since many mobiles stations communicate

with other mobiles stations through one base station, it is necessary to distinguish the

signals from different mobile stations and base stations for them to identify their own

signals. The way to this problem is called multiple access technology. There are now

five kinds of Multiple access technology, namely: Frequency Division Multiple Access

(FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access

(CDMA), Space Division Multiple Access (SDMA), and polar division multiple access

(PDMA).

GSM multiple access technology focuses on TDMA, and takes FDMA as

complement. The following only introduces FDMA and TDMA technologies.

I. FDMA

FDMA divides the whole frequency band into many single radio channels (transmitting

and receiving carrier frequency pairs). Each channel transmits one path of speech or

control information. Any subscriber has access to one of these channels under the

control of the system.

Analog cellular system is a typical example of FDMA application. Digital cellular

system also uses FDMA, but not the pure frequency allocation. For example, GSM

takes FDMA technology.

II. TDMA

TDMA divides a broadband radio carrier into several time division channels according

to time (or timeslot). Each subscriber takes one timeslot and sends or receives

signals only in the specified timeslot. TDMA is applied in digital cellular system and

GSM.

GSM adopts a technology combined with FDMA and TDMA.

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1.2.2 TDMA Frame

The basic conception of GSM in terms of radio path is burst. Burst is a transmissionunit consists of over one hundred of modulation bits. It has a duration limit and takes

a limited radio frequency. They are exported in time and frequency window which is

called slot. To be specific, in system frequency band, central frequency of slot is set in

every 200 KHz (in FDMA). Slot occurs periodically in each 15/26 ms, which is about

0.577 ms (in TDMA).The interval between two slots is called timeslot. Its duration is

used as time unit, called burst period (BP).

Time/frequency map illustrates the concept of slot. Each slot is expressed as one little

rectangle with 15/26ms length and 200 KHz width. See 1.2.2. Similarly, the 200 KHz

bandwidth in GSM is called frequency slot, equal to radio frequency channel in GSMprotocol.

Burst represents different meaning in different situation. Sometimes it concerns time

frequency rectangle unit, and sometimes not. Similarly, timeslot sometimes

concerns time value, and sometimes means using one of every eight slots

periodically.

Using a given channel means transmitting burst with a particular frequency at

particular time, that is, a particular slot. Generally, the slot of a channel is not

continuous in time.

Figure 1.2 Timeslot

5

Frequency

200kHz

BP

15/26ms Slot

Time


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Physical channel combines frequency division multiple access and time division

multiple access together. It consists of timeslot flow that connects base station (BS)

and mobile station (MS).The position of these timeslots in TDMA frame is fixed. 1.2.2

shows the complete structure of TDMA frame, including timeslot and burst. TDMA

frame is a repetitive physical frame in radio link.

One TDMA frame consists of eight basic timeslots, about 60/134.615ms in total.

Each timeslot is a basic physical channel with 156.25 elements, coving

15/260.557ms.

There are two kinds of multiframes, consisting of 26 and 51 continuous TDMA frames

respectively. Multiframes are applied when different logical channels are multiple used

in one physical channel.

The 26 multiframe, with a period of 120 ms, is used in traffic channel and associatedcontrol channel. Among the 26 bursts, 24 are used in traffic and 2 are used in

signaling.

The 51 multiframe, with a period of 3060/13235.385 ms, is specially used in control

channel.

Many multiframes together form a super frame. Super frame is a continuous

5126TDMA frame, that is to say, a super frame consists of fifty-one 26 TDMA

multiframes or twenty-six 51 TOMA multiframes. The period of super frame is 1,326

TDMA frames, or 6.12 s.

Many super frames together form a hyper frame.

A hyper frame consists of 2,048 super frames with a period of 12,533.7s, or 3 hours

and 28 53 760. It is used in encrypted voice and data. Each period of hyper frame

consists of 2,715,648 TDMA frames numbered from 0 to 2,715,648. The frame

number is transmitted in sync channel.

The structure of GSM frame is shown in 1.2.2.

6

0 1 2 3 2044 2045 2046 2047

0 1 2 3 48 49 5047

0 1 24 25

0 1 24 25 1 49 500

0 1 4 5 762 3

TB3

TB3

GP8.25 TB tail bits

TB3

TB3

GP8.25

GP guard perioTB3

TB3

GP8.25

TB3

TB3

GP 68.25

58 information bits26 training sequency58 information bits

constant bits 142

information bits 39extended training sequency64information bits 39

synchronization sequence 41information bits 36

Normal burst NB

Frequency correction burst FB

synchronized burst SB

Access burst AB

1 Hyper frame =2018 Super frames =2715648 TDMA frames (3 28 53 760 )

1 Super frame =1326 TDMA frames 6.12 s

1 Multiframe =26TDMA frames 120 ms 1 Multiframe =51 TDMA frames 3060/13ms

1 TDMA frame =8 time slots 120/26=4.615ms

1 time slot =156.25 bits duration 15/26=0.557ms 1bit duration 48/13=3.68us

BCCHCCCHSDCCH

TCHSACCH/TFACCH


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Figure 1.3 Structure of TDMA frame

1.2.3 Burst

Burst is the message layout of a timeslot in TDMA channel, which means each burst

is sent to a timeslot of TDMA frame.

Different message in the burst determines its layout.

There are five kinds of bursts:

Normal burst: used to carry messages in TCH, FACCH, SACCH, SDCCH,

BCCH, PCH and AGCH channels

Access burst: used to carry message in RACH channel

Frequency correction burst: used to carry message in FCCH channel

Synchronization burst: used to carry message in SCH channel

Dummy burst: transmitted when no specific message transmission request from

system (In cells, standard frequency sends message continuously)

Each kind of burst includes the following elements:

Tail bits: Its value is always 0 to help equalizer judge start bit and stop bit to

avoid lost synchronization.

Information bits: It is used to describe traffic and signaling information, except

idle burst and frequency correction burst.

Training sequence: It is a known sequence, used for equalizer to generate

channel model (a way to eliminate dispersion). Training sequence is known by

both transmitter and receiver. It can be used to identify the location of other bits

from the same burst and roughly estimate the interference situation of

transmission channel when the receiver gets this sequence. Training sequence

can be divided into eight categories in normal burst. It usually has the same BCC

setting with cells, but when accessed to burst and synchronization bust, training

sequence is fixed and does not change with cells. For example, in access burst,

training sequence is fixed (occupying 41 bits). The 36-bit message digit of the

random access burst includes BSIC information of the cell. BSIC settings of the

same BCCH should be different, in order to avoid mis-decoding of random

access burst from neighboring cells into local access.

Guard period: It is a blank space. Since each carrier frequency can carry a

maximum of eight subscribers, it is necessary to guarantee the non-overlapping

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of each timeslot in transmission. Although timing advance technology (introduced

later) is used, bursts from different mobile stations still show little slips; therefore,

protection interval is adopted to allow transmitter to fluctuate in a proper range in

GSM. On the other hand, GSM requires protection bits to keep constant

transmission amplitude of the effective burst (except protection bits) and properly

attenuate the transmission amplitude of mobile station. The amplitude

attenuation of two sequential bursts as well as proper modulation bit stream can

reduce the interference to other RF channels.

The following is a detailed introduction to the structure and content of burst:

Access burst

It is used for random access (channel request from network and switchover access).

It is the first burst that the base station needs in uplink modulation.

Access burst includes a 41-bit training sequence, 36-information bit, and its protection

interval is 68.25 bits. There is only one kind of training sequence in access burst.

Since the possibility of interference is rather little, it is unnecessary to add extra kinds

of training sequences. Both training sequence and protection interval are longer than

normal bursts in order to offset the bug of timing advance ignorance in the first access

of mobile station (or switch over to another BTS) and improve demodulation ability of

the system.

Frequency correction burst

It is used for frequency synchronization in mobile station, equal to an unmodulated

carrier. This sequence has 142 constant bits for frequency synchronization. Its

structure is pretty simple with all constant bits being 0. After modulated, it becomes a

pure sine wave. It is used in FCCH channel for mobile station to find and modulate

synchronization burst of the same cell. When mobile station gets the frequency

through this burst, it can read the information of following bursts (such as SCH and

BCCH) in the same physical channel. Protection interval and tail bit are the same with

that of normal burst.

Synchronization burst

With a 64-bit training sequence and two 39-bit information fields, synchronization

burst is used for time synchronization of mobile station in SCH channel. It belongs to

downlink. Since it is the first burst required to be modulated by mobile station, its

training sequence is relatively long and easy to be detected.

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Normal burst

It has two 58-bit groups used in message field. To be more specific, two 58-bit groups

are used to transmit subscriber data or voice together with two stealing flags. Normal

burst is used to describe whether the transmitted is traffic information or signaling

information. For example, to distinguish TCH and FACCH (when TCH channel is used

as FACCH channel to transmit signaling, the stealing flag of the 8 half bursts should

be set to 1. It has no other use in channels except in TCH channel, but can be

regarded as the extension of training sequence and always set to 1.Normal burst also

includes two 3-bit tails and a protection interval of 8.25 bits. The only bug is that the

receiver has to store the preceding part of burst before modulation. Normal burst has

a total of 26 bits, 16 of which are information bits. In order to get 26 bits, it copies the

first five bits to the end of the training sequence and the last five bits to the head of

the training sequence. There are eight kinds of such training sequence (these eight

sequences have the least relevancy with each other). They correspond to different

base station color code (BCC, 3 bits) respectively to distinguish the two cells using

the same frequency.

Dummy burst

This kind of bust is sometimes sent by BTS without carrying any information. Its

format is the same with normal burst. The encrypted bits are changed into mixed bits

with certain bit model.

1.2.4 Logical Channel

In real networking, each cell has several carrier frequencies and each frequency has

eight timeslots, proving eight basic physical channels. Logical channel carries out

time multiplexing in one physical channel. It is classified according to the type of

information in physical channel. Different logical channel transmits different type of

information between BS and MS, such as signaling and data service. GSM defines

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different burst type for different logical channel.

In GSM, logical channel is divided into dedicated channel (DCH) and common

channel (CCH), or traffic channel (TCH) and control channel (CCH) sometimes.

I. TCH

TCH carries coded voice or subscriber data. It is divided into full rate TCH (TCH/F)

and half rate TCH (TCH/H) with 22.8 bit/s information and 11.4 Kbit/s information

respectively. Using half of the timeslots in TCH/F can get TCH/H. A carrier frequency

can provide eight kinds of TCH/F or sixteen kinds of TCH/H. Voice channel types are

as follows:

Enhanced full rate speech TCH (TCH/EFS)

Full rate speech TCH (TCH/EFS) Full rate 9.6 Kbit/s TCH (TCH/F9.6)

Full rate 4.8 Kbit/s TCH (TCH/F4.8)

Full rate 2.4 Kbit/s TCH (TCH/F2.4)

II. CCH

CCH is used to transmit signaling or synchronous data. It mainly consists of

broadcast channel (BCCH), common control channel (CCCH), and dedicated control

channel (DCCH).

III. BCCH

Frequency Correction Channel (FCCH)

It carries the information for frequency correction in mobile station. Through FCCH,

mobile station can locate a cell and demodulate other information in the same cell,

and recognize whether this carrier frequency is BCCH or not.

Sync Channel (SCH)

After FCCH decoding, mobile station has to decode SCH information. This

information contains mobile station frame synchronization and base station

identification. Base station identification code (BSIC) occupies six bits, three of which

are PLMN color codes ranging from zero to seven, and the other three are base

station color codes (BCCs) ranging from zero to seven.

Reduced TDMA frame (RFN) occupies 22 bits.

BCCH

Generally, each BTS has a transceiver containing BCCH in order to broadcast system

information to mobile station. System information enables mobile station to work

efficiently in null state.

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IV. CCCH

Paging Channel (PCH)

PCH is a downlink channel used to page mobile station. When the network wants tocommunicate with a certain mobile station, it sends paging information marked as

TMSI or IMSI through PCH to all the cells in LAC area according to the current LAC

registered in mobile station.

Access Grant Channel (AGCH)

AGCH is a downlink channel used for base station to respond the network access

request of mobile station, that is, to allocate a SDCCH or TCH directly. AGCH and

PCH share the same radio resource. Keep a fixed number of blocks for AGCH or just

borrow PCH when AGCH requires without keeping special AGCH block (AGB).

Random Access Channel (RACH)

RACH is an uplink channel used for mobile station to request SDCCH allocation in

random network access application. The request includes the reason to build 3-bit

(call request, paging response, location update request and short message request)

and 5-bit reference random number for mobile station to identify its own access grant

message.

V. DCCH

Stand-alone Dedicated Control Channel (SDCCH)

SDCCH is a bi-directional dedicated channel used to transmit information of signaling,

location update, short message, authentication, encrypted command, channel

allocation, and complementary services. It can be divided into SD/8 and SD/4.

Slow Associated Control Channel (SACCH)

SACCH works with traffic channel or SDCCH to transmit subscriber information and

some specific information at the same time. Uplink mainly transmits radio

measurement report and the first layer head information; downlink mainly transmits

part system information and the first layer head information. The information includes

quality of communications, LAI, CELL ID, BCCH signal strength in neighboring cells,

NCC limit, cell options, TA, and power control level.

Fast Associated Control Channel (FACCH)

FACCH works with TCH to provide signaling information with a rate and timeliness

much higher than that provided by SACCH.

There is another control channel called cell broadcast channel (CBCH) besides the

three control channels mentioned above. It is used in downlink and carries short

message service cell broadcast (SMSCB) information. CBCH uses a physical channel

same as SDCCH.

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VI. Channel Combination

Logical channel is mapped to physical channel according to certain rules. The channe

l combinations specified in GSM protocol are as follows: TCH/F + FACCH/F + SACCH/TF

TCH/H(0,1) + FACCH/H(0,1) + SACCH/TH(0,1)

TCH/H(0,0) + FACCH/H(0,1) + SACCH/TH(0,1) + TCH/H(1,1)

FCCH + SCH + BCCH + CCCH (main BCCH)

FCCH + SCH + BCCH + CCCH + SDCCH/4(0..3) + SACCH/C4(0..3)(BCCH

combination)

BCCH + CCCH(BCCH extension)

SDCCH/8(0. .7) + SACCH/C8(0. .7)

VII. Uncombined BCCH/SDCCH and Combined BCCH/SDCCH

Paging information transmits in the timeslot 0 of BCCH. Timeslot 0 has the following s

ub channels:

Broadcast channel (BCH): FCCH, SCH, BCCH

CCCH: PCH, AGCH

DCCH (combined BCCH/SDCCH): SDCCH, SACCH, CBCH ( if using cell

broadcast)

Physical channel timeslot 0 is made of multiframes logically. Each multiframe is 235.4

ms in length. Multiframe has different channel configurations, such as combinedBCCH/SDCCH and uncombined BCCH/SDCCH. Different configuration has different

paging capacity.

Uncombined BCCH/SDCCH

Each frame of Uncombined BCCH/SDCCH can have nine paging blocks. The timeslot

0 of BCCH carrier frequency does not have SDCCH channel or CBCH channel.

Combined BCCH/SDCCH

Each multiframe of combined BCCH/SDCCH can have three paging blocks. The

timeslot 0 of BCCH carrier frequency contains four SDCCH subchannels (no CBCH)

or three SDCCH and one CBCH subchannel.

The configuration of combined BCCH/SDCCH has a great influence on paging

capacity. Each multiframe has only three paging blocks instead of nine in uncombined

BCCH/SDCCH, which means the paging capacity of cells with combined

BCCH/SDCCH is only one third of that of cells with uncombined BCCH/SDCCH.

1.3 Data Transmission

Radio channel has totally different characteristics from wired channel. Radio channel

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has a strong time-varying characteristic. It has a high error rate when the signal is

influenced by interferences, multipath fading, or shadow fading. In order to solve

these problems, it is necessary to protect the signals through a series of

transformation and inverse transformation from original subscriber data or signaling

data to the information carried by radio wave and then to subscriber data or signaling

data. These transformations include channel coding and decoding, interleaving and

de-interleaving, burst formatting, encryption and decryption, modulation and

demodulation. See 1.3

Figure 1.4 Forward and reverse data transmission process

1.3.1 Voice Coding

Modern digital communication system usually uses voice compression technology.

GSM takes tone and noise from human throat as well as the mouth and tongue filter

effect of acoustics as voice encoder to establish a model. The model parameters

transmit through TCH channel.

Voice encoder is based on residual excited linear prediction encoder (REIP) and its

compression effect is strengthened through long term predictor (LTP). LTP improves

residual data encoding by removing the vowel part of voice.

Voice encoder divides voice into several 20 ms voice blocks and samples each block

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with 8 kHz, so each block has 160 samples. Each sample is quantified through

frequency A 13 bits (frequency 14 bits). Since the compression rates of frequency A

and frequency are different, add three and two 0 bits to the quantification values

respectively, and then each sample gets 16 bits quantification value. Therefore, 128

Kbit/s data flow is obtained after digitizing but before encoding. This data flow is too

fast to transmit in radio path and has to be compressed in encoder. With full speed

encoder, each voice block is encoded into 260 bits to form a 13 Kbit/s source coding

rate. Next is channel coding. With 20 ms as a unit, 260 bits are output after

compression encoding, so the encoding rate is 13Kbit /s.

Compared with the direct coding transmission of voice in traditional PCM channel, the

13kbps voice rate of GSM is much lower. More advance voice encoder can reduce

the rate to 6.5kbps (half rate encoding).

1.3.2 Channel Coding

Channel coding is used to improve transmission quality and remove the influence of

interferential factors on signals at the price of increasing bits and information. The

basic way of coding is adding some redundant information to the original data. The

added data is calculated on the basis of original data with certain rules. The decoding

process of receiving end is judging and correcting errors with this redundant bit. If the

redundant bit of received data calculated with the same way is different from the

received redundant bit, errors must have occurred in transmission. Different code is

used in different transmission mode. In practice, several coding schemes are always

combined together. Common coding schemes include block convolutional code, error

correcting cyclic code and parity code.

In GSM, each logical channel has its own coding and interleaving mode, but the

principle is trying to form a unified coding structure.

Encode information bit into a unified block code consisting of information bits and

parity check bits.

Encode block code into convolutional code and form coding bits (usually 456

bits). Reassemble and interleave coding bits and add a stealing flag to form

interleaving bits.

All these operations are based on block. The block size depends on channel type.

After channel coding, all channels (except RACH and SCH) are made of 464-bit

block, that is, 456 coded information bits plus 8-bit header (header is used to

distinguish TCH and FACCH). Then these blocks are reinterleaved (concerning

channel).

In TCH/F voice service; this block carries one speech frame of information. In control

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coding only works for detection and correction of signal error or short error string.

Therefore, it is hoped to find a way to separate the continuous bits in a message, that

is, to transmit the continuous bits in a discontinuous mode so as to change the error

channel into discrete channel. Therefore, even if an error occurs, it is only about a

single or very short bit stream and will not interrupt the decoding of the entire burst or

even the entire information block. Channel coding will correct the error bit under such

circumstances. This method is called interleaving technology. Interleaving technology

is the most effective code grouping method to separate error codes.

The essence of interleaving is to disperse the b bits into n bursts in order to change

the adjacent relationship between bits. Greater n value leads to better transmission

performance but longer transmission delay. Therefore, these two factors must be

considered in interleaving. Interleaving is always related to the use of channel. GSM

adopts secondary interleaving method.

After channel coding, The 456 bits are divided into eight groups; each group contains

57 bits. This is the first interleaving, also called internal interleaving. After first

interleaving, the continuity of information in a group is broken. As one burst contains

two groups of 57-bit voice information, if the two-group 57 bits of a 20 ms voice block

after first interleaving are inserted to the same burst, the loss of this burst will lead to

25% loss of bits for this 20 ms voice block. Channel coding cannot restore so much

loss. Therefore, a secondary interleaving, also called inter-block interleaving, is

required between two voice blocks. The entire interleaving process is shown in 1.3.3.

Figure 1.5 Interleaving process

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After internal interleaving, the 456 bits of a voice block B are divided into eight

groups. Interleave the first four groups of voice block B (B0, B1, B2, and B3) with the

last four groups of voice block A (A4, A5, A6, and A6), and then (BO, A4), (B1, A5),

(B2, A6), and (B3, A7) form four bursts. In order to break the consistency of bits, put

block A at even position and block B at odd position of bursts, that is, to put B0 at odd

position and A4 at even position. Similarly, interleave the last four groups of block B

with the first four groups of block C.

Therefore, a 20 ms speech frame is inserted into eight normal bursts after secondary

interleaving. Theses eight bursts are transmitted one by one, so the loss of one burst

only affects 12.5% voice bits. In addition, as these bursts have no relations with each

other, they can be corrected by channel coding.

The secondary interleaving of control channel (SACCH, FACCH, SDCCH, BCCH,

PCH, or AGCH) is different from voice interleaving which requires three voice blocks.

The 456-bit voice block is divided into eight groups after internal interleaving (the

same as that of voice block), and then the first four groups are interleaved with the

last four groups (the same interleaving method as that of voice block) to get four

bursts.

Interleaving is an effective way to avoid interference, but it has a long delay. In the

transmission of a 20 ms voice block, the delay period is (9*8)-7=65 bursts (SACCH

occupying one burst), which is 37.5 ms. Therefore, MS and trunk circuit have echo

cancellers added to remove the echo due to delay.

1.3.4 Encryption

Security is a very important feature in digital transmission system. GSM provides high

security through transmission encryption. This kind of encryption can be used in

voice, user data, and signaling. It is used for normal burst only and has nothing to do

with data type.

Encryption is achieved by XOR operation of poison random sequence (generated

through A5 algorithm of encryption key Kc and frame number) and the 114

information bits of normal burst.

The same poison random sequence generated at receiving end and the received

encryption sequence together produce the required data after XOR operation

1.3.5 Modulation and Demodulation

Modulation and demodulation is the last step of signal processing. GSM modulation

adopts GMSK technology with BT being 0.3 at the speed of 270.833 Kbit/s and Viterbi

algorithm. The function of modulation is to add a certain feature to electromagnetic

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wave according to the rules. This feature is the data to transmit. In GSM, the phase of

electromagnetic field bears the information.

The function of demodulation is to receive signals and restore the data in a modulated

electromagnetic wave. A binary numeral has to be changed into a low-frequency

modulated signal first, and then into an electromagnetic wave. Demodulation is the

reverse process of modulation.

1.4 Timing advance

Signal transmission has a delay. If the MS moves away from BTS during calling, the

signal from BTS to MS will be delayed, so will the signal from MS to BTS. If the delay

is too long, the signal in one timeslot from MS cannot be correctly decoded, and this

timeslot may even overlap with the timeslot of the next signal from other MS, leading

to inter-timeslot interference. Therefore, the report header carries the delay value

measured by MS. BTS monitors the arrive time of call and send command to MS with

the frequency of 480 ms, prompting MS the timing advance (TA) value. The range of

this value is 063(0233 us), and the maximum coverage area is 35km. The

calculation is as follows:

1/23.7us/bit63bit*c=35km

3.7us/bit is the duration per bit (156/577); 63bit is the maximum bit for time

coordination; c is light velocity (transmission rate of signal); 1/2 is related to theround-trip of signal.

According to the preceding description, 1bit to 554 m, due to the influence of multi-

path transmission and the accuracy of MS synchronization, TA error may be about 3

bits (1.6km).

Sometimes a greater coverage area is required, such as in coastal areas. Therefore,

the number of channels that each TRX contains must be reduced. The method is to

bind odd and even timeslots, so there are only four channels (0/1, 2/3, 4/5, and 6/7)

for each TDMA frame in extended cell. Allocate channels 0, 2, 4, and 6 to MS. Within

35 KM around BTS, the TA value of MS is in the normal range 0-63; for the area

beyond 35 KM, TA value stays at 63. This technology is called extended cell

technology. The maximum value of TA in BTS measurement report is

63+156.25=219.25 bit, so the maximum radius of coverage area is:

1/23.7us (63+156.25) 3108m/s=120km

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Figure 1.6 Principle of dual timeslot extended cell

The principle of dual timeslot extended cell is shown in 1.4. In real scheme, in order to

improve the utilization of TRX, both common TRXs and dual timeslot TRXs can beincluded. BCCH must be in dual timeslot TRX to receive random access from any

area. The calls within 35 km are allocated to common TRX; the calls within 35 km

120 km and the switched in calls are allocated to dual timeslot TRX. If the system

detects the switched in call is within 35km, it will switch over this call to common TRX.

If the MS in conversation goes beyond 35 km, an intra-cell switchover will be carried

out. Therefore, both the capacity requirement for remote areas and the coverage

requirement for local areas can be satisfied.

1.5 System Information

System information is sent to MS from network in broadcast form. It informs all the

MSs within the coverage area of location area, cell selection and re-selection,

neighbor cell information, channel allocation and random access control. By receiving

system information, MS can quickly and accurately locate network resources and

make full use of all kinds of services that network provides. There are 16 types of

system information: type1, 2, 2bis, 2ter, 3, 4, 5, 5bis, 5ter, 6, 7, 8, and 13.

System information is transmitted on BCCH or SACCH. MS receives system

information in different mode from different logic channel.

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In idle mode, system information 1 4, 7, and 8 are transmitted on BCCH ;

In communication mode, system information 5 and 6 are transmitted on SACCH;

The content of system information is as follows:

System information 1 cell channel description + RACH control parameter,

transmitted on BCCH

System information 2 frequency description of neighbor cell + RACH controlinformation + network color code (NCC) permitted, transmitted on BCCH, used

for cell re-selection

System information 2bis Extended neighbor cell BCCH frequency description+ RACH control information, transmitted on BCCH, used for cell re-selection.

System information 2ter Extended neighbor cell BCCH frequency description,transmitted on BCCH, used for cell re-selection.

System information 3 Cell identity + location area identity (LAI) + controlchannel description + cell option + cell selection parameter + RACH control

parameter, transmitted on BCCH.

System information 4 LAI + cell selection parameter + RACH controlparameter + CBCH channel description + CBCH mobile configuration,

transmitted on BCCH.

System information 5 Neighbor cell BCCH frequency description, transmittedon SACCH channel, used for cell handover.

System information 5bis Extended neighbor cell BCCH frequency description,transmitted on SACCH channel, used for cell handover.

System information 5ter Extended neighbor cell BCCH frequency description,transmitted on SACCH channel, used for cell handover.

System information 6 Cell Global Identification (CGI) + cell optionNCCPermitted, transmitted on SACCH.

System information 7 cell re-selection parameter System information 8 cell re-selection parameterBCCH is a low-capacity channel, every 51 multiframes ((235 ms) have only four

frames (one information block) to transmit a 23 byte LAPDm message.

Each information unit contains: Cell channel description contains all the frequencies used in this cell.

RACH control information contains parameters such as Max Retrans,

TX_integer, CBA, RE, EC, and AC CN.

Neighbor cell BCCH frequency description contains the BCCH frequency that the

neighbor cell uses.

Allowed PLMN is used to provide NCC Permitted that MS monitors on BCCH

TRX.

Control channel description contains parameters such as MS

ATTACH/DEATTACH allowed Indicator ATT, BS-AG-BLKS-RES, CCCH-CONF,

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BA-PA-MFRMS, and T3212.

Cell option contains parameters such as power control (PWRC) indication,

discontinuous Transmission (DTX) indication, and RADIO-LINK-TIMEOUT.

Cell selection parameter contains parameters such as cell re-selection

hysteresis, MS-TXPWR-MAX-CCH, and RXLEV-ACCESS-MIN.

CBCH channel description contains channel type and TDMA deviation (the

combination mode of dedicated channel), timeslot number (TN), training

sequence code (TSC), hopping frequency channel indication H, mobile allocation

index offset (MAIO), hopping frequency sequence number (HSN) and absolute

radio frequency channel number ( ARFCN).

CBCH mobile configuration contains the relationship between hopping channel

sequence and cell channel description.

Cell re-selection parameter contains CELLRESELIND, cell bar qualify (CBQ),

cell reselection offset (CRO), temporary offset (TO), and penalty time (PT).

1.6 Cell Selection and Re-Selection

1.6.1 Cell Selection

When a MS is switched on, it tries to contact GSM PLMN that the SIM permits and

select a proper cell to extract control channel parameters and other system

information. This process is called cell selection.

The priority levels of cells include normal, low, and barred. Low priority level cell is

selected when there is no proper normal cell.

A proper cell means:

The cell belongs to the selected network;

The cell is not barred;

The cell is not in the national prohibited roaming location area;

The path loss between MS and BTS is under the limit set by network.

The priority level of a cell is determined by CELL_BAR_QUALIFY (CBQ) and

CELL_BAR_ACCESS (CBA).

Table 6.1 Cell priority level

CBQ CBA Cell priority level Cell re-selection status

0 0 Normal Normal

1 1 Barred Barred

0 1 Low Normal

1 0 Low Normal

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1.6.2 Cell Selection Process

To perform cell selection and re-selection, MS requires all the frequencies monitored

to stay at the unweighted average value of Relev RLA_C.

I. Cell Selection When MS Storing No BCCH Information

MS searches all RF channels (at least 30 channels for 900 M, 40 for 1800 M, and 40

for PSC1900) in the system to obtain the Relev of each RF channel, and calculate the

RLA_C based on at least five samples in three to five seconds, and then arrange

these levels in descending order to select the proper BCCH. MS selects the cells with

normal priority first. If the proper cells have low priority, MS will select the cell with the

highest Relev. MS has already decoded and identified all these frequencies by now. If

there is no proper cell, MS will keep on searching. It takes a maximum of 0.5 s to

synchronize a BCCH TRX and 1.9 s to read the synchronized BCCH TRX data,

except that it takes n*1.9s(n>1)to obtain the system information.

II. Cell Selection When MS Storing BCCH Information

If MS stores the BCCH frequency list of the former selected networks, MS will perform

measurement sampling procedure (only for the stored BCCH TRX) according to this

list. If the cell selection within this list fails, common cell selection will be performed. If

all the cells have low priority level, MS will select the cell with the highest Relev. MS

has already decoded and identified all these frequencies by now. When a 900 M MS

enters the 900/1800 network, MS will probably choose 900 M network and ignore the

priority level, because the MS stores all the 900 M frequency information in BCCH

frequency list.

III. Cell Selection Criteria

Parameter C1 is the path loss criteria for cell selection, C1 of the service cell must

exceed 0, the formula is as follows:

C1= RLA_C - RXLEV_ACCESS_MIN- MAX ((MS_TXPWR_MAX_CCH- P), 0) (2-1)

For DCS 1800 cells:

C1 = RLA_C - RXLEV_ACCESS_MIN- MAX ((MS_TXPWR_MAX_CCH + POWER

OFFSET- P), 0)

In the formula:

RLA_C: Average value of Relev

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RXLEV_ACCESS_MIN: Minimum Relev that MS allows

MS_TXPWR_MAX_CCH: Maximum transmit power on control channel

P: Maximum transmit power of MS

POWER OFFSET Power offset related to MS_TXPWR_MAX_CCH used by

DCS1800 cells.

1.6.3 Down Link Failure

Downlink failure criteria are based on DSC. When a mobile phone stays in a cell,

DSC is initialized to an integer most close to 90/N ( N is BS_PA_MFRMS, range

value: 29). Each time when mobile phone successfully decodes a message on its

paging subchannel, DSC increases by 1, but DSC cannot exceed the initial value;

when decoding fails, DSC decreases by 4. When DSC


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includes several factors, such as RLA_C, cell restriction (decided by cell_bar and

cell_bar_qualify), and access state of the neighbor cell.

Cell re-selection adopts C2 algorithm. The calculation formula is as follows:

When PENALTY TIME is not 11111

C2=C1+CELL_RESELECT_OFFSETTEMPORARY_OFFSET*H (PENALTY_TIME

T);

When PENALTY_TIME is 11111

C2=C1-CELL_RESELECT_OFFSET.

When X>0, function H(x) =0; when XO, function H(x) =1.

T is a timer; its initial value is 0. When a cell is included in the six neighbor cells with

strongest signals by MS, the timer T of this cell begins to time; when a cell is excluded

from the six neighbor cells with strongest signals by MS, T will be reset.

CELL_RESELECT_OFFSET adjusts the value of C2.

After T starts, TEMPORARY_OFFSET will modify the C2 algorithm according to the

defined value before the penalty time in order to avoid a micro cell or a cell with small

coverage area is selected by a fast moving MS. If the defined penalty time is out, the

temporary offset will be ignored. Penalty time can avoid the frequent cell re-selection

in those coverage areas like express highway.

These parameters in C2 algorithm works only when

CELL_RESELECTION_INDICATION is activated. Otherwise, MS will ignore the

setting of CELL_RESELECT_OFFSET, TEMPORARY_OFFSET, and

PENALTY_TIME, under such circumstances, C2=C1.

Cell re-selection will be triggered under the following conditions:

The C2 value of a certain cell (belonging to the same location area with the

current cell) exceeds that of the current cell by 5 seconds successively;

The C2 value of a certain cell (belonging to different location area from the

current cell) exceeds the sum of the C2 value of the current service cell and cell

selection hysteresis value by 5 seconds successively;

The current service cell is barred;

MS detects downlink failure;

The C1 value of the service cell is less than 0 for 5 seconds successively.

1.7 Frequency Hopping

With the ever growing traffic volume and the limited frequency resource, frequency

reuse is more and more aggressive. Therefore, the problem of how to reduce

frequency interference becomes more and more remarkable. The essence of anti-

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interference is to fully utilize the current spectrum, time domain, and space resources.

The key measures include frequency hopping, discontinuous transmission (DTX), and

power control. Frequency hopping also can effectively reduce the influence of fast

fading.

1.7.1 Types of Frequency Hopping

GSM radio interface uses slow frequency hopping (SFH) technology. The difference

between slow frequency hopping and fast frequency hopping is that the frequency of

latter changes faster than frequency modulation. In GSM, the frequency remains the

same during burst transmission. Therefore, GSM frequency hopping belongs to slow

frequency hopping.

In frequency hopping, the carrier frequency is controlled by a sequence and hops with

time. This sequence is frequency hopping sequence. Frequency hopping sequence is

a sequence of frequencies decided by hopping sequence number (HSN), mobile

allocation index offset (MAIO) and frame number (FN) through a certain algorithm in

the mobile allocation containing N frequencies. The N channels of different timeslots

can use the same hopping sequence. The different channels of the same timeslot in

the same cell adopt different MAIO.

Frequency hopping can be divided into frame hopping and timeslot hopping according

to time domain and RF hoping and baseband hopping according to implementation

mode.

Frame hopping: the hopping frequency changes once in each TDMA frame

period. Each TRX can be regarded as a channel. The TCH of BCCH TRX cannot

join in the frequency hopping in a cell. The hopping TRX should have a different

MAIO. Frame hopping is an exception of timeslot hopping.

Timeslot hopping: the timeslot frequency of each TDMA frame changes once.

The TCH of BCCH TRX can join in the frequency hopping, which happens in

baseband hopping.

RF hopping: both transmission and reception of TRX join in the frequency

hopping. The number hopping frequencies can exceed the number of TRXs inthe cell.

Baseband hopping: each transceiver works at a fixed frequency. TX does not join

in frequency hopping. Frequency hopping is performed through the handover of

banseband signal. Therefore, the number of hopping frequencies cannot exceed

the number of TRXs in the cell.

The two frequency hopping modes above are based on BTS. As for MS, since each

MS has only one TRX unit, RF hopping is the only mode.

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I. Baseband Hopping

The system has multiple baseband and TRX processing unit. Each TRX processing

unit has a fixed working frequency; each baseband processing unit processes oneline of service information and sends the processed information to the TRX unit with

bus topology in time sequence according to frequency hopping rule. This kind of

frequency hopping is called baseband hopping.

In baseband hopping, each transceiver works with a fixed frequency. The bursts on

the same speech path are sent to each transceiver. Baseband hopping is based on

the handover of baseband signals. Since the transceiver of each BTS has a fixed

working frequency, both broadband combiner and cavity combiner can be adopted.

The number of TRXs decides the maximum number of frequency hopping. The

problem for baseband hopping is that if one TRX board fails, the corresponding codeword will be lost, thus affecting all the calls under hopping mode in the cell.

Figure 1.7 Baseband hopping

II. RF Hopping

Under this mode, each line of service information is processed by fixed baseband unit

and frequency band unit. The working frequency of frequency band unit is provided

by frequency combiner. Under the control of control unit, frequency can be changed

according to certain rules. In RF hopping, the frequencies used by a TRX to handle all

the bursts of a call come from the frequency change of combiner, instead of the

handover of baseband signals. The number of TRXs is not limited by carrier

frequency. As the working frequency of TRX changes, which means the frequency of

the input port to combiner changes, only broadband combiner can be adopted. This

kind of broadband combiner leads to about 3dB insertion loss in two-in-one

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combination and the loss is greater in the link insertion of multi-combiner. GSM

protocol does not specify which kind of frequency hopping is used in GSM BTS. The

mode of frequency hopping can be decided by operators according to the

equipments.

Figure 1.8 RF hopping

1.7.2 Frequency Hopping Algorithm

The parameters related to frequency hopping algorithm are as follows:

CA: cell allocation, the collection of frequencies used by a cell FN: TDMA frame number, broadcasted on sync channel. FN (02715647)

synchronizes BTS with MS

MA: mobile allocation, the collection of radio frequencies used for MS frequency

hopping. It is a subset of CA. MA contains N frequencies, 1N64.

MAIO: mobile allocation index offset, (0N-1). During communication, the radio

frequency at air interface is an element of MA. Mobile allocation index (MAI, 0

N-1) is used to determine the element of MA. That is to say, the actual frequency

used is decided by MAI. MAIO is the initial offset of MAI and it is used to avoid

the contention of frequency by several channels at the same time.

HSN: hopping sequence number (063). It determines that the hopping

sequence with concentrated frequencies is adopted in frequency hopping. When

HSN=0, the hopping is cyclic hopping; when HSN0, the hopping is random

hopping.

The proper setting of parameters is based on the understanding of the use of each

parameter in hopping algorithm and the hopping theory. The proper setting ensures

the healthy working state of the system. 1.7.2 is the flow chart of frequency hopping

algorithm.

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FN

T2(025)

FN

T3(050)

MAI

(m0mN-1)

MAIO

(0N-1)

Represent

in 7 bits

T1R=

T1 MOD 64

Exclusive OR

FN

T1(02047)

HSN

(063)

Addition

Look-up table

Addition

M'=M mod 2^NBINT=T3 mod2^NBIN

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MAI, integer (0 ... N 1) : MAI = (FN + MAIO) modulo N (2-2)

Otherwise, see 1.7.2:

M, integer (0 ... 152) : M = T2 + RNTABLE((HSN xor T1R) + T3)

S, integer (0 ... N 1) : M' = M modulo (2 NBIN)

T' = T3 modulo (2 ^ NBIN)

If M' < N:

S = M'

Otherwise:

S = (M'+T') modulo N

MAI, integer (0 ... N 1) : MAI = (S + MAIO) modulo N (2-3)

Remarks: For the cyclic hopping in discontinuous transmission (DTX), the number of

hopping frequencies should avoid N mod 13 = 0, because under such condition, the

probability of transmission and measurement of SACCH frame at the same frequency

is rather high, and the harms are obvious. See the description of DTX in section 1.8

RNTABLE is a function with the parameters from integer 0 to 113, GSM protocol

defines its values as shown in 1.7.2:

Table 9.1 RNTABLE(X)

The following conclusion can be used in the rough estimate of whether inter-

frequency or intra-frequency collision exists:

MAI=(S+MAIO) MOD N

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RFCHN=MA (MAI);

When HSN=0, S equals the frame number, in other cases, S is only related to frame

number and frequency hopping number. When HSN is fixed and frame number is the

same, S must be the same. Therefore, as the TRXs of each sync cell have the same

frame number, different hopping groups in sync cells can adopt the same HSN. A

proper configuration of MAIO can avoid the inter-cell or intra-cell frequency collision

within the same BTS. The aggressive frequency reuse adopts this theory.

1.7.3 Benefits of Frequency Hopping

In GSM, frequency hopping has two benefits: frequency diversity and interference

averaging.

I. Frequency Diversity

Frequency hopping can reduce the influence of signal strength change due to

multipath transmission. This effect equals that of frequency diversity. In mobile

communications, Rayleigh fading leads to the great change of radio signal in a short

time. This kind of change is related to frequency: a more independent fading

accompanies a greater frequency difference. The 200 KHz interval generally ensures

the independence of inter-frequency fading, while the 1 MHz interval can fully

guarantee this kind of independence. Through frequency hopping, all the bursts

containing the code word of the same speech frame are protected from the damage

of Rayleigh fading in the same way. See I.

Figure 1.10 Fading

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Statistics shows that frequency hopping gain is related to environmental factors,

especially to the moving speed of MS. When the MS moves at a high speed, the

location difference between two bursts on the same channel is also affected by other

kinds of fading. The higher the speed is, the lower the gain will be. Frequency

diversity benefits a lot to a large number of MSs moving at low speed.

Frequency hopping gain is also related to the number of frequencies. When the

number of frequencies decreases, the hopping gain falls. The relationship between

the number of frequencies and hopping gain can be explained in this way: frequency

hopping is pseudo spectrum spread, and the hopping gain is the processing gain after

transmission frequency band spread. The basic way to test frequency hopping gain is

to calculate the differences between different C/I at different hopping frequencies

under the same FER. These C/I differences are the frequency hopping gain.

The relationship between the number of frequencies and frequency hopping gain is

shown in I. (The actual gain may be affected by environment)

Table 10.1 The relationship between the number of frequencies and frequency

hopping gain

Number of TRXs infrequency hopping

Gain of frequency diversity(dB)

=1 0

2 3

3 4

4 5

5 5.5

6 6

7 6.3

8 6.5

9 6.8

10 6.9

>=11 7

II. Interference Averaging

Frequency hopping provides the diversity of interference on transmission channel, so

that all the bursts containing the code word of the same speech frame are protected

from the damage of interference in the same way. Through error correction coding

and interleaving of the system, the original data can be restored from the rest part of

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the received flow. The hopping gain is obtained only when the interference is in

narrowband distribution. If the interference is in broadband distribution, all the bursts

will be destroyed and the original data cannot be restored. Therefore, no hopping gain

is obtained. The common interference after frequency hopping can be regarded in

narrowband distribution.

In frequency hopping, error rate tends to increase in the test, but we feel the

conversation quality improves. It is because although the error rate increases, the

influence of interference is homogenized in frequency hopping, the speech restoring

ability improves because of the interleaving and de-interleaving before. In GPRS data

services, frequency hopping can be harmful when the data rate is rather high (CS4).

1.8 Discontinuous Reception and Discontinuous

Transmission

1.8.1 Discontinuous Reception and Paging Channel

In idle mode, if MS selects a cell as its service cell, it begins to receive the paging

information from this cell. But in order to reduce power consumption, discontinuous

reception (DRX) is introduced in GSM. Each user (IMSI) belongs to a paging group

and each paging group corresponds to a paging subchannel. MS can calculate which

group it belongs to based on the last three digits of its IMSI and the configuration of

paging channel in this location area, and then locate the paging subchannel of this

paging group. In fact, in idle mode, MS just listens to the paging information from the

system on its subchannel (MS also monitors the Relev of BCCH carrier frequency in

non-service area during this period of time) and ignores the information on other

paging subchannels. Some of the hardware equipments are even switched off to save

the power of MS. But MS must complete the required task of network information

measurement within a specified time.

Through DRX, MS can receive the broadcast short messages that the users want to

know with less power consumption, thus extending the service time. BSC has to send

scheduling messages to support DRX at MS. One scheduling message contains lots

of broadcast short messages to be sent soon. The time that all broadcast short

messages of a scheduling information takes is a scheduling cycle. Scheduling

information contains the description of all short messages to be broadcast in order

and also indicates the position of the messages in scheduling cycle. Through

scheduling messages, MS can find the broadcast short messages it wants quickly so

as to reduce its power consumption.

The number of paging subchannels of each cell can be calculated based on the

configuration type of CCCH, BS_AG_BLKS_RES (the number of blocks belonging to

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AGCH in 51 multiframe), and BS_PA_MFRMS (the number of 51 multiframes used as

one paging subchannel cycle).

When there are three CCCHs in a 51 multiframe, the number of paging subchannels

is (3- BS_AG_BLKS_RES) BS_PA_MFRMS

When there are nine CCCHs in a 51 multiframe, the number of paging subchannels is

(9- BS_AG_BLKS_RES)BS_PA_MFRMS

In addition, the configuration of CCCH parameters has the following principles:

The greater the parameter BS_PA_MFRMS, the more the paging subchannels,

and the less the users of each paging subchannel, but the total capacity of the

system remains the same, because the average delay of the paging information

on radio channel increases. When the ratio of retransmission waiting is relatively

high, BS_PA_MFRMS should be improved to increase the paging subchannels;

when the ratio of retransmission waiting is relatively low, BS_PA_MFRMS should

be reduced to shorten the paging delay.

The capacities of paging subchannels of all cells in a location area should be the

same, because the paging message of a location area must be sent in all the

cells of this location area at the same time.

The longer the cycle of paging channel, the less power the MS in this service

area takes. For example, in cities, this cycle can be defined as 2, which means

MS listens to paging messages once for every 102 frames. In rural areas, this

cycle can be defined as 4 or 6. The MS with the paging channel cycle of 6consumes 18% less power than the MS with the paging channel cycle of 2. After

measuring the system information, MS enters the rest state and listens to the

paging information in the specified paging blocks only and measures the Relev

of BCCH of neighbor cells at the same time. After 30 s, MS will listen to system

information again to judge the cell re-selection process.

In GSM, CCCH mainly includes AGCH and PCH. Its primary function is to

transmit immediate assignment messages and paging messages. CCCH can be

one or several physical channels and it can also share a physical channel with

SDCCH. The combination mode of CCCH depends on the parameter

CCCH_CONF. The configuration of CCCH_CONF must be consistent with the

actual configuration. It is recommended that when there is only one TRX in a

cell, the configuration of CCCH can be a physical channel shared with SDCCH

(3 CCCH information blocks).

When the traffic volume is extremely large, in case one physical timeslot is not

enough, GSM specification allows the configuration of multiple CCCH channels

on the TRX besides BCCH, but these channels must be used in timeslot 0, 2, 4,

and 6.

When CCCH_CONF is confirmed, parameter BS_AG_BLKS_RES actually

decides the ratio of AGCH and PCH on CCCH. It is recommended that this

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parameter be configured as little as possible in order to reduce the response

time of MS to paging.

1.8.2 DTX

I. DTX Overview

During communication, only 40% time is used for conversation; no useful information

is transmitted during the rest 60% time. If all the information is transmitted to network,

many of the system resources will be wasted, in addition, the interference will

aggravate. In order to solve this problem, GSM adopts DTX technology to stop signal

transmission when there is no voice signal. Therefore, the interference level is

reduced and the system efficiency is improved.

There are two kinds of transmission modes in GSM: normal mode and discontinuous

transmission (DTX) mode. In normal mode, noise and voice have the same

transmission quality. In DTX mode, the transmission of unuseful messages is

prohibited. MS only sends man-made noise signals that are tolerable, which means

this noise will not annoy the listeners nor affect the conversation. This kind of noise is

called comfort noise. In DTX mode, 260-bit code is transmitted in every 480 ms; in

normal mode, 260-bit code is transmitted in every 20 ms.

Whether the downlink DTX is adopted or not is controlled by network operators of the

exchange part. This kind of control is based on BSC. The control information istransmitted to baseband processing part through dedicated signaling channel, and

then arrives at TC through the inband signaling of TRAU frame to indicate whether

downlink DTX is adopted. For some vendors, the downlink DTX can be configured on

the basis of cell.

Uplink DTX is configured by network operators of the radio part. The parameter DTX

in system information consists of 2 bits. Its coding scheme is shown in I:

Table 10.2 Value range of DTX

DTX Meaning

00 MS can use DTX

01 MS must use DTX

10 MS is not allowed to use DTX

11 Reserve

Parameter DTX is contained in cell option of information unit and transmitted

periodically in the system information of each cell broadcast. MS decides whether to

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start DTX function based on this information.

DTX can be used for voice signal transmission and nontransparent data transmission.

BCCH TRX does not use this technology. The benefits of DTX are listed below:

Uplink DTX can save MS batteries and reduce interference.

Downlink DTX can save BTS power consumption and reduce interference and

intra-BTS intermodulation.

Uplink DTX and downlink DTX used together can improve the intra-frequency

ratio of the system. This kind of improvement, when used in aggressive-

frequency-reuse cell planning, especially when used with frequency hopping, can

greatly expand the system capacity.

II. Voice Activity Detection

For voice activity detection (VAD), the source must indicate when the transmission is

required. When DTX mode is activated, the encoder must detect the signal is voice or

noise. Therefore, the VAD is required. VAD can differentiate voice from noise through

calculating some signal parameters and threshold values. This kind of differentiation

is based on an energy rule: the energy of noise is always lower than that of voice.

VAD generates a group of threshold value in every 20 ms to judge whether the next

20ms block is voice or noise. When the background noise is too loud, the noise signal

will be regarded as voice signal to transmit.

III. Silence Indicator

The coding procedure of noise is the same as that of voice. After sampling and

quantification, a noise block will be produce by encoder in every 20ms. Like voice

block, the coded noise block also contains 260 bits, which forms a SID frame. The

SID frame will go through channel coding, interleaving, encryption and modulation

and finally be sent by eight continuous bursts.

On TCH, a complete SACCH information block has four 26 muliframe cycles (480

ms). In order to differentiate voice frame and SID frame, these eight continuous

bursts are arranged at the beginning of the third multiframe. During other time of the

480 ms, no information is transmitted except SACCH timeslot. The SID frame made

from the 20 ms noise block is interleaved with the preceding frame and the following

frame; the first SID frame is interleaved with the preceding voice frame and the

following SID frame.

IV. Measurement

Uplink DTX and downlink DTX are two irrelevant procedures that are activated by

system parameters respectively. There are two kinds of measurement in GSM: full

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measurement and sub measurement.

Global measurement is the average of the level and quality of the 104 timeslots in a

measurement cycle (four 26 multiframes); local measurement is the average of level

and quality of 12 timeslots, including eight continuous TCH bursts (for TCH/F, 0-103

TDMA frames as a cycle. The frame numbers of these eight bursts are 52, 53, 54, 55,

56, 57, 58, and 59. when no voice or signaling is transmitted, the descriptor of comfort

noise they contain is called SID) and four SACCH bursts (0-103 TDMA frames as a

cycle, for timeslot 0, the frame numbers of these four bursts are 12, 38, 64, and 90;

for timeslot 1, the frame number is that of timeslot 0 plus 13. similarly, the frame

numbers that the eight timeslots correspond to can be obtained in this way). In order

to achieve uniformity, no matter the uplink DTX or downlink DTX is activated or not,

BTS and MS must complete these two kinds of measurement. Each SACCH

measurement report of BTS and MS indicates whether DTX is used in last

measurement report time. BSC choose one of the two kinds of measurement based

on this indication.

1.9 Power Control

1.9.1 Power Control Overview

Power control is to change the transmission power of MS or BTS (or both) in radio

mode within certain area. Power control can reduce the system interference and

improve the spectrum utilization and prolong the service time of MS battery. When

the Relev and quality is good, the transmission power of the peer end can be reduced

to lower the interference to other calls.

In GSM, power control can be used in uplink and downlink respectively. The power

control range for uplink MS is 20 dB30dB. Based on the power class of MS (most

MSs belongs to class 4, which means the maximum transmission power is 33 dbm),

each step can change 2 dB. The downlink power control range is decided by

equipment manufacturer. Although whether to adopt uplink or downlink power control

function is decided by network operators, all MSs and BTS equipments must support

this function. BSS manages the power control in the two directions.

To facilitate BCCH frequency pull-in and the measurement of Relev (including the

Relev of neighbor cell BCCH frequency), GSM protocol specifies that no power

control is allowed for the timeslots in the downlink of BCCH TRX.

1.9.2 MS Power Control

The power control of MS includes two adjustment stages: stable adjustment stage

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and initial adjustment stage. Stable adjustment is the common way to implement

power control algorithm. Initial adjustment is used at the beginning of call connection.

When a connection occurs, MS sends signals with nominal power (before receiving

power adjustment commend, the nominal transmission power of MS is the maximum

transmission power on BCCH of the cell. If MS does not support this power level, it

will adopt other power level most close to this level, such as the maximum power

level supported by the classmark of MS in indication message establishment).

Therefore, MS accesses to network through RACH with the maximum power

broadcast on BCCH. When MS power is lower than this value, it will transmit with its

maximum transmission power. The system specifies that the power level of the first

message that MS sends on DCH is also this value. The system control begins after

MS receives the power control command in SACCH information block from SDCCH

or TCH.

Since BTS can support multi-call at the same time, the Rxlev should be quickly

reduced in the new connection. Otherwise, other calls supported by this BTS will

deteriorate and the calls in other cells will also be affected. The purpose of initial

adjustment stage is to quickly reduce the transmission power of MS to get the stable

MR, so MS can be adjusted according to stable power control algorithm.

The required parameters in uplink power control, the expected uplink Rxlev, and the

uplink received quality can be adjusted according to the situation of the cell. After

receiving a certain number of uplink MRs, the system compares the actual uplink

Rxlev and received quality obtained by interpolation, filtering, and other methods with

the expected values and calculate the power level that the MS should be adjusted to

through power control algorithm. If the calculated power level differs from the output

power level of MS and meets certain limit conditions (such as step limit of power

adjustment and range limit of MS output power), the system will send power

adjustment command.

The command of changing MS power and the required time advance will be sent to

MS in the layer 1 header of each downlink SACCH information block. MS will

configure the power level it uses now in its uplink SACCH information block and send

it to BTS in measurement report. This level is the power level of the last burst in theprevious SACCH measurement cycle. When MS receives the power control

information in SACCH information block from DCH, it will transmit with this power

level. One power control message does not make the MS switch to the required level

immediately. The maximum change rate of MS power is 2 dB for every 60 ms. For 12

dB, before MS receives the next power control message, it will not end as one

SACCH measurement cycle takes 480 ms. In addition, it takes three measurement

cycles to send power control message and execute the command. Therefore, the

power control cycle should not be too short in order to ensure its accuracy. See 1.9.2.

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Figure 1.11 Execution of power control command

The purpose of uplink power control adjustment is to minimize the difference between

the actual uplink Rxlev and received quality and the expected uplink Rxlev and

received quality. The purpose of interpolation and filtering is to process the lost

measurement reports and remove temporary nature to ensure the stability of power

control algorithm.

The difference between initial adjustment and stable adjustment is that the expected

uplink Relev and received quality and the length of filter in initial adjustment are

different from that of stable adjustment, and the initial adjustment only has downlink

adjustment.

1.9.3 BTS Power Control

BTS power control is an optional function. It is similar to MS power control, but it only

uses stable power control algorithm. The required parameters are Rxlev threshold

(lower limit), and the maximum transmission level can be received (upper limit). The

Relev is divided into 64 levels ranging from 0 to 63. Level 0 is the lowest Rxlev; level

63 is the highest Rxlev.

BTS power control is divided into static power control and dynamic power control.

Dynamic power control is the fine tuning based on static power control. There are six

steps (2 dB/step) of static power control according to Protocol 0505. If the maximum

output power is 46 dBm (40W), the step 6 is 34 dBm.

Static power control step is defined in the cell distributes list of data management

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system, which specifies the maximum output power (suppose this value is Pn) of

static power control. For step 15 of dynamic power control, the corresponding value

range is Pn dBPn-30dB. When the maximum power control still cannot satisfy the

requirement, adjust static power control step to improve the maximum output power

of dynamic power control Pn.

1.9.4 Power Control Processing

I. Measurement Report Interpolation

Each measurement report has a sequence number. If network detects incontinuous

sequence numbers, it means some of the measurement reports are missing. The

network will complete the reports based on interpolation algorithm.

As shown in I, the network receives measurement reports n and n+4. It detects the

sequence numbers are not continuous, so it uses an algorithm to add n+1, n+2, and

n+3 (yellow) to complete the reports.

The purpose of measurement report interpolation is to avoid call loss when the power

is too low.

Figure 1.12 Measurement report interpolation

II. Measurement Report Filtering

Network will not judge the state of MS based on only one measurement result,

because that is too incomprehensive, in addition, the MS may be fluctuating.

Therefore, filtering is required. Filtering combines several continuous measurement

results together to determine the state of MS during this period of time. In II, the

network uses four measurement reports (yellow).

TA has filters for Rxlev and received quality of uplink and downlink

The purpose of measurement report filtering is to remove temporary nature and

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ensure the algorithm stability.

Figure 1.13 Measurement report filtering

III. Power Control Adjustment

Calculate the power adjustment value based on the difference between the Rxlev and

the expected value.

Power control adjustment based on Rxlev

Power control module compares the estimate value of Rxlev obtained through pre-

processing of measurement report with the expected value, and calculates the step

length of adjustment. In power control algorithm, variable step is often used for quick

power control.

Power control adjustment based on received quality

Power control module compares the estimate value of received quality obtained

t

chapter 2 gsm system principles and call flow

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