GSM and CDMA Report

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)

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

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

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

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

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

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 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

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

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