GO NA01 E1 0 GSM Basic Principle-63

7/28/2019 GO NA01 E1 0 GSM Basic Principle-63

http://slidepdf.com/reader/full/go-na01-e1-0-gsm-basic-principle-63 1/59

GSM Basic Principle

Course Objectives:

·Aware of the Development Background of GSM technology

·Grasp GSM Network structure and Features

·Describe GSM Voice Service Process Procedure

State GSM key technology

Grasp physical and logical channel in air interface



Contents

1 GSM Basic.....................................................................................................................................................1

1.1 2G Mobile Communication Technology Evolution.................................................................................1

1.2 Mobile Communication Technology Development Trend......................................................................5

1.3 GSM History............................................................................................................................................6

1.4 GSM Features...........................................................................................................................................7

1.5 GSM Specifications.................................................................................................................................8

1.6 GSM Network Structure..........................................................................................................................9

1.7 GSM Protocol Platform.........................................................................................................................12

1.8 Available GSM Services........................................................................................................................15

1.8.1 Telecommunications Services Provided by the GSM............................................................15

1.8.2 Supplementary Services of the GSM System........................................................................16

1.9 Operation Band......................................................................................................................................17

2 GSM Events.................................................................................................................................................21

2.1 Status of Mobile Subscriber...................................................................................................................21

2.1.1 Attach Flag upon MS Power-on.............................................................................................21

2.1.2 Detach upon MS Power-off....................................................................................................22

2.1.3 MS Busy.................................................................................................................................22

2.1.4 Periodical Registration...........................................................................................................22

2.2 Location Update.....................................................................................................................................22

2.2.1 Normal Location Update .......................................................................................................23

2.2.2 Periodical Location Update....................................................................................................23

2.2.3 IMSI Attach............................................................................................................................23

2.3 Handover................................................................................................................................................23

i



2.3.1 Purpose of Handover..............................................................................................................23

2.3.2 Classification of Handover.....................................................................................................24

2.4 Cell selection and Reselection...............................................................................................................24

2.4.1 Cell selection..........................................................................................................................24

2.4.2 Cell reselection.......................................................................................................................25

2.5 Authentication ......................................................................................................................................25

2.6 Encryption .............................................................................................................................................26

3 GSM Speech Processing.............................................................................................................................29

3.1 GSM Speech Processing........................................................................................................................29

3.2 Voice encoding.......................................................................................................................................29

3.3 Channel Encoding..................................................................................................................................30

3.4 Interlacing Technology...........................................................................................................................31

3.5 Encryption/Decryption...........................................................................................................................35

3.6 Modulation and Demodulation..............................................................................................................35

4 GSM Key Technologies...............................................................................................................................37

4.1 Diversity Reception................................................................................................................................37

4.2 Discontinuous Transmission..................................................................................................................38

4.3 Power Control........................................................................................................................................39

4.4 Timing Advance.....................................................................................................................................42

4.5 Frequency Hopping................................................................................................................................43

5 Frame Structure and Radio Channels......................................................................................................47

5.1 Radio Frame Structure...........................................................................................................................47

5.2 Physical Channel....................................................................................................................................48

5.3 Logical Channels....................................................................................................................................49

5.3.1 Common Channel...................................................................................................................50

5.3.2 Dedicated Channel..................................................................................................................51

ii



GSM Basic

5.3.3 Channel Combination.............................................................................................................51

5.4 Mapping between Logical and Physical Channels................................................................................53

iii



1 GSM Basic

1.1 2G Mobile Communication Technology Evolution

Brief History of Evolution

The outline of GSM history is shown below:

1979 - Europe wide frequency band reserved for cellular

1982 - Groupe Spécial Mobile (GSM) created within CEPT

1986 – Eight proposals put forward by European countries after extensive

research and experiments accepted in Paris

1988 - ETSI took over GSM Committee

1990 - The phase 1 GSM recommendations frozen

1991 - GSM Committee renamed Special Mobile Group and GSM renamed as

Global System for Mobile Communication

1992 - GSM launched for commercial operations

1993 – Major part of GSM phase 2 standard completed

1994 – A new research phase (Phase 2+) added to improve GSM for mobile data

services

Mobile Communication during 1920 ~ 1940

In 1920, mobile communication system was first used by military while in 1940’s; it

was put in use for civil purpose.

Mobile communication started flourishing in recent decade. Its development phases are

as follows:

First generation (1G) mobile communication system

Second generation (2G) mobile communication system

Third generation (3G) mobile communication system

1



1G during 1980’s

Since 1980's, 1G analog mobile communication system adopts cellular networkingtechnology. Till 1982 Cellular Systems were exclusively Analog Radio Technology.

At the end of 1980’s Analog System was unable to meet continuing demands due to:

Severely confined spectrum allocations

Interference in multipath fading environment

Incompatibility among various analog systems

Inability to substantially reduce the cost of mobile terminals and infrastructure

required

Easy to eavesdrop and misuse the subscriber’s account

Standards of First Generation

Different standards of first generation are shown in Table 1.1 -1.

Table 1.1-1 Different Standards of First Generation

Standard Origin Frequency Band

Advanced Mobile Phone System

(AMPS) North America 800 MHz

Nordic Mobile Telephone System-

450/900 (NMT-450/900)

North Europe

(Scandinavian)450 & 900 MHz

Total Access Communication System

(TACS)U.K. 900 MHz

2G during 1990’s

During 1990s, Digital mobile communication system characterized by digital

transmission, Time Division Multiple Access (TDMA), and narrowband Code DivisionMultiple Access (CDMA) were developed.

Standards of Second Generation

Different standards of second generation are:

GSM

CDMA IS95

Personal Digital Cellular (PDC)

2



2 GSM Events

Advantages of 2G

Compared with 1G mobile communication system, 2G mobile communication systemhas the following advantages:

Provides high spectrum utilization and large system capacity.

Provides diversified services (voice services and low-rate circuit-switched data

services).

Enables automatic roaming.

Provides better voice quality.

Provides good security.

Can be interconnected with ISDN and PSTN.

Basic structure of GSM network is shown in Fig 1.1 -1.

Fig 1.1-1 Basic Structure of GSM Network

Discrepancies of 2G

2G mobile communication system has the following discrepancies:

Provides low-rate data services only and cannot support multi-media service.

For example, Internet data access speed of GSM MS can reach 9.6 kbps

theoretically.

3



GSM Basic Principle

Different 2G mobile communication systems in the world use different

frequencies, therefore it is difficult to implement global roaming.

Internet, E-business, and multi-media communication is developing very

rapidly. Failing to provide strong support to data communication has already

constrained the development of 2G system. Demand for higher data rate and

more diversified services leads to evolution from 2G to 3G.

Fig 1.1 -2 shows the evolution process.

IS-95CDMA

PDC

GSM

IS-136

IS-95-B

HSCSDGPRS

IS-136+IS-136HS

IS-2000MC WCDMA

ARIBWCDMA

UTRAWCDMA

IMT-2000

2G 2.5G 3G

EDGEUWC-136

2.75G

Fig 1.1-2 Evolution from 2G to 3G

GSM 2.5G

GSM system (2.5G) Phase2 and Phase2+ were then developed, adopting high-rate

adaptive coding solution. GPRS provides the data rate up to 171 kbps. Two high-rate

data service options are:

High Speed Circuit Switched Data (HSCSD) based on high-speed data bit rate

and circuit switching

General Packet Radio Service (GPRS) based on packet switched data

GSM 2.75G

Enhanced Data Rates for GSM Evolution (EDGE) developed by the European

Telecommunications Standards Institute (ETSI) adopts 8-PSK (Phase Shift Keying)

modulation. It supports data rate up to 384 kbps theoretically. EDGE is more advanced

than GPRS. However, EDGE cannot provide rate up to 2 Mbps as 3G system does.

Therefore EDGE is often called 2.75G.

4



2 GSM Events

1.2 Mobile Communication Technology Development Trend

3G Research during 1980’s

3G research, development, and establishment started in mid 1980’s.

IMT-2000

International Mobile Telecommunication 2000 (IMT-2000) established by International

Telecommunications Union (ITU) introduces 3G.

IMT-2000 introduces:

Mobile data service and some fixed high-speed data services through one or

more radio channels

Fixed network platform

A global standard

IMT-2000 services, which are compatible with other fixed network services

High quality

Use of common band in the world

Small terminals used in the world

Global roaming

Multi-media services and terminals

Higher frequency utilization

Flexibility for development to the next generation

High-speed hierarchical data rate

Rate up to 2 Mbps while stationary

Rate up to 384 kbps during walking speed

Rate up to 144 kbps while in vehicle

Instead of having pure technology, communication system is currently

developing into a mode featuring the combination of services and technology.

Communication technology is estimated to undergo the largest change in future.

It is strategic transition from voice services to data services from the aspect of

market application and service demand. This change has deeply influenced the

5



GSM Basic Principle

development trend of communication technology.

4G Services

Some researchers and telecom operators describe fourth-generation (4G) mobile

communication system as a new world better than 3G, which can provide:

Many unimaginable applications

Over 100 Mbps data transmission rate, which is 10,000 times of current MSs

and 50 times of 3G MSs

High-performance multi-media contents

Service as a personal identification device through ID application

Service for high-resolution movies and TV programs, acting as bridge of

combined broadcast and new telecommunication infrastructure

Some services such as 4G wireless instant connections, are cheaper than 3G

services.

1.3 GSM History

Because analog mobile communication system had limited expansion capability,

Global System for Mobile Communication (GSM) was developed on demand for

capacity expansion which achieved global success. It operates at 900 MHz band within

European countries.

GSM Development process is as follows:

1982: Conference of European Posts and Telegraphs (CEPT) formed a study

group called the Group Special Mobile (GSM) to study and develop 2G mobile

communication system.

1986: Eight proposals put forward by European countries after massive research

and experiments were accepted in Paris, and on-site experiments were

performed.

1987: After on-site test, demonstration, and comparison, GSM member countries

have reached an agreement that digital system adopts narrowband Time Division

Multiple Access (TDMA), Regular Pulse Excitation-Long Term Prediction

(RPE-LTP), voice coding, and Gaussian Minimum Shift Keying (GMSK)

modulation.

6



2 GSM Events

1998: Eighteen European countries reached GSM Memorandum of

Understanding (MOU).

1989: GSM took effect.

1991: First GSM network was deployed in Europe.

1992: GSM standard was frozen.

1993: Major part of GSM phase II standard was completed.

1994: A new research phase (Phase 2+) was added to further improvement of

GSM as a platform of mobile data services.

1.4 GSM Features

GSM system has the following features:

High Spectrum efficiency

GSM system features high spectrum efficiency due to the high-efficient modulator,

channel coding, interleaving, balancing, and voice coding technologies adopted.

Large capacity

Volumetric efficiency (number of channels/cell/MHz) of GSM system is three to five

times higher than that of Total Access Communication System (TACS).

High voice quality

Digital transmission technologies and GSM specifications, voice quality is irrelevant

with radio transmission quality.

Open interfaces epic

GSM standard provides open air interface, also open interfaces between networks and

those between network entities, such as A interface and Abis interface.

High security

MS identification code encryption makes eavesdropper unable to determine the MS

number, ensuring subscriber’s location security. Voice encryption, signaling data, and

identification codes make the eavesdropper unable to receive the communication

contents.

Interconnection with Integrated Services Digital Network (ISDN) and PSTN.

7



GSM Basic Principle

GSM can interconnect with other networks through current standard interfaces, such as

Integrated Service User Part (ISUP) or Telephone User Part (TUP).

Roaming function

GSM supports roaming by introducing Subscriber Identity Module (SIM) card that

separates subscriber from the terminal equipment.

Diversified services

GSM provides diversified services, tele-services, bearer services, and supplementary

services.

Inter-cell handover

During conversation, MS continues to report the detailed radio environment of local

cell and neighboring cells to serving base station. If inter-cell handover is required, MS

sends a handover request to serving base station.

1.5 GSM Specifications

European Telecommunications Standards Institute (ETSI) initiated and made GSM

standard.

ETSI developed GSM in several phases and set up more Special Mobile Groups

(SMG) to make the related GSM standard.

GSM detailed specifications conform on functions and interfaces only, not on

hardware. Purpose is to reduce the restriction on designers, enabling the operators to

purchase equipment from different manufacturers.

GSM technical specifications consist of 12 fields:

Field 1: General

Field 2: Services

Field 3: Network Functions

Field 4: MS-BS Interfaces and Protocols

Field 5: Physical Layer on Radio Path

Field 6: Speech Coding

Field 7: MS Terminal Adaptor

8



2 GSM Events

Field 8: BS-MSC Interface

Field 9: Network Inter-working

Field 10: Service Inter-working

Field 11: Equipment and Model Acceptance Specification

Field 12: Operation and Maintenance

1.6 GSM Network Structure

Fig 1.6 -3 shows the basic GSM network structure.

BSC TRAU MSC/VLR

SMC

GMSC

AUC

IWF EIR

HLR

PSTNISDNPDN

BTS

MS

BTS

MS

Traffic & Signaling

Signaling

Fig 1.6-3 GSM Network Structure

GSM system consists of:

Network Subsystem (NSS)

Base Station Subsystem (BSS)

Operation and Maintenance Subsystem (OMS)

Mobile Station (MS)

Network Switching Subsystem

NSS is the core element of network switching which interfaces with subscriber services

for voice and data.

9



GSM Basic Principle

NSS Main components are:

Mobile Switching Centre (MSC)

Home Location Register (HLR)

Visitor Location Register (VLR)

Equipment Identification Register (EIR)

Authentication Centre (AUC)

Short Message Centre (SMC)

Home Location Register - HLR is a central database of a system. HLR stores all the

information related to subscribers, including the roaming authority, basic services,

supplementary services, and current location information. It provides routing

information for MSC for call setup. HLR may cover several MSC service areas or even

the whole PLMN.

Visitor Location Register - VLR stores all subscriber information in its coverage area

and provides call setup conditions for the registered mobile subscribers. As a dynamic

database, VLR must exchange large volume of data with HLR to ensure data validity.

When an MS leaves the controlling area of a VLR, it registers in another VLR. The

original VLR deletes the temporary records of that subscriber. VLR integrated within

MSC.

Equipment Identification Register - EIR stores the parameters related to MS. It can

identify, monitor, and block the MS. ERI preventing unauthorized MS from accessing

the network.

Authentication Centre - AUC is a strictly protected database that stores subscriber

authentication information and encryption parameters. AUC integrated with HLR

physically.

Base Station Subsystem BSS serves as a bridge between NSS and MS. It performs

radio channel management and wireless reception and transmission. Base Station

Controller (BSC) and Base Transceiver Station (BTS) are main components of BSS.

Base Station Controller - Located between MSC and BTS, it controls and manages

more than one BTS. It performs radio channel assignments. BTS and MS transmit

power control, and inter-cell handover. BSC is also small a switch that converge and

connects local network with the MSC through A interface. Abis interface connects BTS

to BSC.

10



2 GSM Events

Base Transceiver Station - BTS is wireless transceiving equipment controlled by the

BSC in BSS. BTS carries radio transmission. It performs wired-related wireless

conversion, radio diversity, radio channel encryption, and hopping. Um interface

connects BTS to MS.

Transcoding and Rate Adaptation Unit - TRAU Located between BSC and MSC,

TRAU transcodes between 16 kbps RPE-LTP codes and 64 kbps A law PCM codes.

Operation and Maintenance Subsystem OMS is operation & maintenance part of

GSM. Functional units in GSM are connected to OMS internal networks. OMS

monitors various functional units in GSM network, submits status report, and performs

fault diagnosis.

OMS consists of two parts: OMC – System (OMC-S) and OMC-Radio (OMC-R). The

OMC-S performs operation and NSS maintenance, while OMC-R performs operation

and BSS maintenance.

Mobile Station

MS is subscriber equipment in GSM, it can be vehicle installed or hand portable. MS

consists of mobile equipment and SIM.

Mobile equipment processes voice signals, receives and transmits radio signals.

SIM stores all information required for identifying a subscriber and security

information, preventing unauthorized subscribers. Mobile equipment cannot access

GSM network without a SIM card.

Network Service Area

GSM service area refers to the total area covered by networks of all GSM operators.

Network consists of several MSC service areas, each of which consists of several cells.

Logically, several cells form a location area (LA).

MSC Service Area - A Public Land Mobile Network (PLMN) includes multiple MSC

service areas. MSC service area refers to the MSC coverage area, that is, the total area

covered by BTS under control of BSC connected to MSC. All MSs in the service area

table register in local VLR. Therefore, in actual network, MSC is always integrated

with VLR as a node.

Location Area - Each MSC/VLR service area includes multiple of LAs. MS can move

freely without performing location update in LA. Hence, LA is the paging area of a

broadcast paging message. An LA belongs to one MSC/VLR only, that is, LA cannot

11



GSM Basic Principle

cross MSC/VLR. The system can identify different LA via LA Identity (LAI).

Cell - LA contains several cells. Each cell has a unique Cell Global Identification(CGI), which indicates a basic radio coverage area in a network.

Fig 1.6 -4shows the relationship among different coverage areas in a GSM network.

GSM service areaThe total network coverage provided by all GSM operators

PLMN service areaThe network coverage provided by a GSM operator

MSC service area

The area controlled by an MSC

Location areaAn area for location update and paging

CellA service area provided by a

specific BTS

Fig 1.6-4 Relationship among Coverage Areas in a GSM Network

1.7 GSM Protocol PlatformGSM technical specifications make clear and normative definition of interfaces and

protocols between subsystems and various functional entities. Interface refers to the

point where two adjacent entities are connected. Protocol defines the rules for

information exchange at the connection point.

GSM Interfaces

Fig 1.7 -5 shows the GSM interfaces.

MS BTS BSC MSC

VLR VLR

HLR

MSCEIR

Sm

Um

Abis A B

D

C

E F

G

Fig 1.7-5 shows the GSM interfaces.

12



2 GSM Events

Sm Interface: Man-machine interface implemented in MS. It is an interface between

subscribers and PLMN. MS consists of keyboard, LCD, and SIM card.

Um Interface: Radio interface between MS and BTS. It is an important interface in

PLMN. Digital mobile communication network has different radio interface as

compared to analogue mobile communication network.

A Interface: It is an interface between BSC and MSC. Base station management

information, call processing interface, mobility management information, and specific

communication information are transferred through A interface.

Abis Interface: It is an interface between BSC and BTS. Supports all services

provided to subscribers. Also supports the control of BTS radio equipment and

management of radio resources assigned.

B Interface: It is an interface between MSC and VLR. VLR is a database locating and

managing MS when MS roams in the related MSC control area. MSC can query the

current location of MS from VLR and update MS location. When subscriber uses a

special supplementary service or changes a relevant service, MSC notifies the VLR.

Sometime VLR also updates information in HLR.

C Interface: It is an interface between MSC and HLR. C interface transfers

management and route selection information. When a call finishes, MSC sends the

billing information to HLR. When PSTN cannot get location information of a mobile

subscriber, the related GMSC queries HLR of the subscriber to obtain the roaming

number of the called MS, and then transfers it to the PSTN.

D Interface: It is an interface between HLR and VLR. Exchanges MS location

information and subscriber management information. To enable a mobile subscriber to

originate or receive calls in the whole service area, data must be exchanged between

HLR and VLR. VLR notifies HLR about the current location of MS belonging to HLR,

and then provides MS roaming number. HLR sends VLR all the data required to

support the services of the MS. When an MS roams to the service area of another VLR,

HLR notifies the previous VLR to delete the relevant MS information. When MS uses

supplementary services, or some parameters are changed, D interface is also used to

exchange the related information.

E Interface: It is an Interface between MSCs. It exchanges the handover information

between two MSCs. When MS in a conversation moves from one MSC service area to

another MSC service area, inter-cell handover occurs to maintain the conversation. At

13



GSM Basic Principle

that time, related MSCs exchange the handover information through E interface.

F Interface: It is an interface between MSC and EIR. It exchanges the MSmanagement information, such as IMEI, between MSC and EIR.

G Interface: It is an interface between VLRs. When MS uses a Temporary Mobile

Subscriber Identity (TMSI) to register with a new VLR, the relevant information is

exchanged between VLRs through G interface. This interface also searches IMSI of the

subscriber from VLR that registers TMSI.

GSM Protocol Structure and OSI

2G cellular mobile network GSM adopts Open System Interconnection (OSI) model to

define its protocol structure. Fig 1.7 -6 shows GSM interface protocol model, which

defines the interfaces and protocols between MS and MSC.

Um interface Abis interface A interface

CM

MM

RRM

LAPDm

Radio

CM

MM

RRM

MTP

64

kbit/s

RRM

LAPDm

Radio

LAPD

64

kbit/s

RRM

LAPD

64

kbit/s

MTP

64

kbit/s

SCCP SCCP

MS BTS BSC MSC

Fig 1.7-6 GSM Interface Protocol Model

OSI reference model is a hierarchical structure. According to the hierarchy concept,

communication process can be divided into several logical layers from lowest to

highest layer. In different systems, the entities in the same layer that exchange

information for the same purpose are called peer entities. Entities in adjacent layers

interact with each other through the common layer. The lower layers provide services

to higher layers. The services provided by layer N is a combination of the services and

functions provided by the layers below it.

First layer of Um interface protocol is physical layer, which is marked as L1 and

it is a lowest layer. L1 provides basic radio channels for the information

transmission of higher layers.

14



2 GSM Events

Second layer L2 is data link layer, which is marked as LAPDm. It covers various data

transmission structures and controls data transmission.

Application layer is the third highest layer L3. It covers various messages and

programs, and controls services. L3 includes Radio Resource Management (RRM),

Mobility Management (MM) and Call connection Management (CM).

Abis interface protocol is slightly different from Um interface protocol. Its

physical layer is 64 kbps land line, and link layer is LAPD.

First layer of A interface protocol is 64 kbps land line, and second layer is the

Message Transfer Part (MTP), which is part of Common Channel Signalling7

(CCS7) network. MTP consists of many network protocols and centralizes all

link layer protocols. Signaling connection control part (SCCP) and MTP

together represent a network layer protocol on A interface.

In BSC both MM and CM are transparently transmitted

1.8 Available GSM Services

1.8.1 Telecommunications Services Provided by the GSM

1. Circuit Services

1) Voice Service

Full-rate voice service

Half-rate voice service

Enhanced full-rate voice service

2) Data service

14.4Kbit/s full-rate data service




2. SMS services(support Chinese short messages)

1) Point-to-point short message service

Point-to-point short message service with the mobile user serving as called

15



GSM Basic Principle

Point-to-point short message service with the mobile user serving as caller

2) Cell Broadcast Short Message

Cell broadcast service originated from the SMS center or the OMC-R.

3. Packet Services

1) GPRS service

2) EDGE service

At present, the point-to-point interactive telecom services are supported,

including:

Access to the database: Allocate service to users as needed, e.g. Internet, and

provide storing and forwarding, as well as information processing for user-to-

user communications.

Session service: Provide bi-directional user-to-user and port-to-port real time

information communication, e.g. Internet Telnet service.

Tele-action service: Applicable to small-volume data processing services, credit

card confirmations, lottery transactions, electronic monitoring, remote meter

reading (water, electricity and gas), monitoring systems, and so on.

1.8.2 Supplementary Services of the GSM System

GSM supplementary services are diversified, including:

Call Forwarding Unconditional: forward all incoming calls to the number specified by

the subscriber.

Barring: barring of outgoing/coming calls.

Call Waiting: When a call is connected for a subscriber, indication of a new coming

call is given to the subscriber. The subscriber can accept, reject or ignore the waiting

call.

Call Hold: A subscriber can suspend the connected call to do other things.

Multiparty Service: A simultaneous communication with up to six parties is allowed.

Closed User Group: The subscribers of CUG are restricted from outgoing and

incoming calls, but they can normally communicate with each other.

Hot Billing: The network generates an instant call billing message from the billing

16



2 GSM Events

manager. It is applicable to leased phone service, including all kinds of call modes.

Bills are generated and presented to the subscriber immediately after the call is ended.

1.9 Operation Band

1. Working band

Currently, the GSM communication system works at 900 MHz, extended 900

MHz and 1800 MHz, or 1900 MHz band in some countries.

1) 900 MHz band

Uplink (MS transmitting and BS receiving) frequency range: 890 MHz ~ 915MHz

Downlink (BS transmitting and MS receiving) frequency range: 935 MHz ~

960MHz

2) Extended 900 MHz band

Uplink (MS transmitting and BS receiving) frequency range: 880 MHz ~ 915

MHz

Downlink (BS transmitting and MS receiving) frequency range: 925 MHz ~ 960MHz

3) 1,800 MHz band

Uplink (MS transmitting and BS receiving) frequency range: 1,710 MHz ~

1,785 MHz

Downlink (BS transmitting and MS receiving) frequency range: 1,805 MHz ~

1,880 MHz

4) 1,900 MHz band

Uplink (MS transmitting and BS receiving) frequency range: 1,850 MHz ~

1,910 MHz

Downlink (BS transmitting and MS receiving) frequency range: 1,930 MHz ~

1,990 MHz

2. Channel interval

The interval between two adjacent channels in any band is 200 kHz.

3. Channel configuration

17



GSM Basic Principle

All channels are configured with the same interval.

1) 900 MHz band

The channel numbers are in the range of 1 ~ 124. There are 124 frequency bands

in all.

The relationship between a channel number and nominal central frequency of a

frequency band is illustrated as follows:

Fu (n) = 890 + 0.2 × n-512 (MHz), uplink

Fd (n) = Fu (n) + 45 (MHz), downlink

Where, 1 ≤ n ≤ 124, n is a channel number, or an Absolute Radio Frequency

Channel Number (ARFCN).

2) Extended 900MHz band

The channel numbers are in the range of 0 ~ 124 and 975 ~ 1023. There are 174

frequency bands in all.



Fu (n) = 890 + 0.2 × n (MHz), 0 ≤ n ≤ 124

Fu (n) = 890 + 0.2 × (n-1024) (MHz), 975 ≤ n ≤ 1023

Fd (n) = Fu (n) + 45 (MHz)

3) 1,800 MHz band

The channel numbers are in the range of 512 ~ 885. There are 374 frequency

bands in all.



Fu (n) = 1710.2 + 0.2 × (n-512) (MHz)

Fd (n) = Fu (n) + 95 (MHz)

512 ≤ n ≤ 885

4) 1,900 MHz band

The channel numbers are in the range of 512 ~ 811. There are 300 frequency

bands in all.

18



2 GSM Events



Fu (n) = 1850.2 + 0.2 × (n-512) (MHz)

Fd (n) = Fu (n) + 80 (MHz)

512 ≤ n ≤ 811

4. Duplex transceiving interval

1) 900 MHz band

The duplex transceiving interval is 45 MHz.

2) Extended 900 MHz band

The duplex transceiving interval is 45MHz.

3) 1,800 MHz band


4) 1,900 MHz band


19



2 GSM Events

2.1 Status of Mobile Subscriber

Mobile subscriber is generally in one of the following three states: MS power-on (idle),

MS power-off, and MS busy. Thus, the network needs to process these states

accordingly.

2.1.1 Attach Flag upon MS Power-on

IMSI attach is divided into three cases:

1. If the MS is powered on for the first time, The SIM card does not store the LAI.

MS sends a Location Update Request to the MSC, notifying the GSM system

that this is a new subscriber in this location area. MSC sends a Location Update

Request to the HLR according to the IMSI number sent by this subscriber. HLR

records the number of the MSC sending the request and the corresponding VLR

number, and returns a Location Update Accepted message to the MSC. By now,

MSC has been activated and it will add an Attach flag to the IMSI of the

subscriber in the VLR. Then it sends a Location Update Acknowledgement

message to the MS. The SIM card of the MS records the LAI.

2. If the MS is not powered on for the first time, instead the MS is powered off and

then powered on again, and if the LAI received by the MS is inconsistent with

that stored in the SIM card, the MS sends a Location Update Request to the

MSC. The VLR must judge whether the original LAI is in its own service area.

If yes, MSC only needs to replace the original LAI in the SIM card of thesubscriber with the new LAI.

If no, MSC sends a Location Update Request to the HLR according to the

information in the IMSI of the subscriber. HLR records the number of MSC

sending the request in the database and returns a Location Update Accepted

message. Then MSC adds an Attach flag to the IMSI of the subscriber and

returns the Location Update Acknowledgement message to the MS. MS replaces

the original LAI on the SIM card with the new LAI.

3. If the MS is powered on again, and the LAI received is consistent with the

21



original LAI stored in the SIM card. VLR only adds Attach flag to this

subscriber.

2.1.2 Detach upon MS Power-off

After the MS is powered off, the MS sends a Detach Request to the MSC. After the

MSC receives the request, it informs VLR to add the Detach flag to IMSI of this MS.

At this time, HLR does not receive the notice indicating that this subscriber is detached

from the network. After this subscriber is paged, the HLR requests the MSRN from the

MSC/VLR. At this time, the VLR informs the HLR that this MS is powered off.

2.1.3 MS Busy

In this case, the MS is allocated with a traffic channel to transmit the voice or data and

the IMSI of the subscriber is marked as Busy.

2.1.4 Periodical Registration

When the MS sends the IMSI Detach message to the network, it is possible that the

GSM system cannot decode properly due to the poor radio quality or other reasons and

still believes that MS is in Attach status. Or when the MS is powered on but has

roamed beyond the service coverage, i.e., a blind area, the GSM system does not know

it and still believes that the MS is in Attach status. In both cases, if the subscriber is

paged, the system will keep sending paging messages, wasting radio resources.

To solve the above problems, the measure of forced registration is taken in the GSM

system: The MS must make registration at a regular interval. This is called periodical

location update. If the GSM system does not receive the periodical registration

information of the MS, the VLR where the MS resides records the Implicit Detach

status of the MS. When the correct periodical registration information is received

again, the status is changed into Attach status.

2.2 Location Update

When the MS changes the location area, it finds out that the LAI in its SIM card is

inconsistent with the LAI received. Thus, it registers the location information. This

flow is called location update. Location update is originated by the MS. There are three

type of location update:

Normal Location Update.

22



2 GSM Events

Periodical Location Update

IMSI Attach.

2.2.1 Normal Location Update

When the MS roams to a new location area, it finds out that the LAI in its SIM card is

inconsistent with the LAI received. Thus, it originates a Location Update Request to

the current MSC/VLR If the new LAI and old LAI below to same MSC/VLR, Location

Update just renew LAI in VLR. If not, the new MSC/VLR should require MS data

from its HLR, HLR send back MS data to new MSC/VLR and inform old MSC/VLR

to delete MS record at the same time. MS register its LAI in new MSC/VLR, HLR

save the new MSC/VLR number.

2.2.2 Periodical Location Update

When MS make periodical register to MSC, periodical location update happens

2.2.3 IMSI Attach

When MS Power On, it will start a location update process to MSC/VLR, the location

update process is same as that in normal location update.

2.3 Handover

When a mobile subscriber who is engaged in a conversation moves from one BSS to

another, handover function ensures that the link set up for this mobile subscriber is not

interrupted. Whether to perform handover is determined by the BSS. When the BSS

finds out that the communication quality of the current radio link degrades, it performs

different types of handover according to the actual situation. MSS can also request the

handover according to the traffic information.

2.3.1 Purpose of Handover

1. Save the calls in progress（ bad quality)

2. Cell-boundary handing over to improve ongoing calls (weak signal)

3. Intra-cell hand-over reducing interference within a cell (severe interference)

4. Directed Retry increase call completion success rate

5. Compelled hand-over to balance traffic distribution of inter-cells.

23



GSM Basic Principle

2.3.2 Classification of Handover

According to the scope of handover , it can be divided into the following types

1. Intra-cell hand-over

2. Inter-cell hand-over

3. Inter-BSC hand-over of same MSC

4. Inter-MSCs hand-over

According to the synchronous relationship between MS and BTS when handover

happens, there are three type of handover:

1. Synchronous: MS use the same TA both in destination and target cell. This

usually applies to hand-over of same cell or different sectors within the same

cell. This is the hand-over with highest speed.

2. Asynchronous: MS don’t know the TA to be used in target cell. When either of

the two cells doesn’t synchronize with BSC, this mode should be used. The

hand-over speed is low.

3. Pseudo-synchronous: MS is able to calculate out the TA it should use in the

target cell. When both cells have synchronized with BSC, this mode may be

used. The hand-over speed is fast.

2.4 Cell selection and Reselection

2.4.1 Cell selection

After a MS is turned on, it will attempt to contact a common GSM PLMN, so the MS

will select an appropriate cell, and extract from it the parameters of the control channel

and the prerequisite system information. Such a selection process is referred to as cellselection. The quality of a radio channel is an important factor of cell selection. The

GSM specification defines the path loss criterion C1, and such appropriate cell must

ensure that C1>0. The C1 is calculated according to the following formula:

C1=RXLEV-RXLEV_ACCESS_MIN-MAX((MS_TXPWR_MAX_CCH-P), 0)

Where:

The RXLEV is the average reception level.

The RXLEV_ACCESS_MIN is the minimum level at which the MS is allowed to

24



2 GSM Events

access.

The MS_TXPWR_MAX_CCH is the maximum power level of the CCH.

The “P” is the maximum transmitted power of the MS.

MAX (X, Y) = X; If X≥Y.

MAX (X, Y) = Y; If Y≥ X.

After the MS selects a cell, it will stay in the selected cell if no major changes have

occurred to various conditions.

2.4.2 Cell reselection

After a MS selects a cell, the MS will stay in the selected cell as long as no major

changes occur to various conditions. At the same time, the MS starts to measure the

signal level of the BCCH carrier of the adjacent cells, records the six adjacent cells

with the highest signal levels, and extracts from them the various system messages and

control messages of each adjacent cell. When the appropriate conditions are met, the

MS will switch from the current cell to another cell, a process known as cell

reselection. Such appropriate conditions include multiple factors, including cell

priority, and whether the cell is prohibited from access. Among them, an important

factor is the quality of the radio channel. When the signal quality of the adjacent cell

exceeds that of the current cell, cell reselection is triggered. For cell reselection, the

channel quality criterion is determined by the C2 parameter, which is calculated

according to the following formula:

2.5 Authentication

Fig. 2.5 -1 shows the authentication process, where RAND is the question asked by

the network side and only the legal subscriber can give the correct answer SRES.

RAND is generated by the random number generator of the AUC on the network side.

It is 128 bits in length. The value of RAND is obtained in a random manner from the

range of 0~2128 –1.

SRES is called a signed response. It is obtained through the calculation of subscriber’s

unique key parameter Ki. It is 32 bits in length.

Ki is stored in the SIM card and AUC in a very confidential way. Even the subscribers

do not know their own Ki. Ki can be of any format and any length.

25



GSM Basic Principle

A3 algorithm is the authentication algorithm determined by the carrier. It is also

confidential. The only restriction of the A3 algorithm is the length of the input

parameter (RAND is 128 bits in length) and the size of the output parameter (SRES

must be 32 bits).

Mobile Terminal Network

A3 algorithm

Random number generator Ki RAND

SRES'

SRES

Ki

A3 algorithm

Fig. 2.5-1 Authentication Process

2.6 Encryption

In the GSM, the position of encryption and decryption over the transmission link

allows the transmitting data in all dedicated modes to use the same protection method.

The transmitting method can be the subscriber information (such as voice and data),

subscriber-specific signaling (such as message carrying the called number), or even the

system-specific signaling (such as the message carrying radio measurement result for

the handover).

Encryption and decryption are the exclusive or operation (this algorithm is called the

A5 algorithm) of 114 radio burst pulse code bits and one 114-bit encryption sequence

generated by a special algorithm. To obtain each burst encryption sequence, A5

calculates on two inputs: One is the frame number and the other is the key (Kc) agreed

upon by the MS and network, as shown in Fig. 2.6 -2. Two different sequences are

used over the uplink and downlink. For each burst, one sequence is used for the

encryption inside the MS and meanwhile used as the decryption sequence in BTS. The

other sequence is used for the encryption of BTS and meanwhile used as the decryption

sequence in MS.

26



2 GSM Events

A5

Frame No.

(22-bit)Kc (64-bit)

A5

S1

(114-bit)

S2 S1 S2

MS BTS

Frame No.

(22-bit)Kc (64-bit)

(114-bit) (114-bit) (114-bit)

Fig. 2.6-2 Encryption Algorithm

1. Frame number: Frame number is encoded into a serials of three values, which

are 22 bits in total.

Frame number of each burst varies with the type of radio channel. Each burst

dedicated for communication on the same direction uses different encryption

sequence.

2. A5 algorithm

A5 algorithm must be defined in the global range. This algorithm can be

describes into the two 114-bit sequence black boxes generated by a 22-bit

parameter (frame number) and a 64-bit parameter (Kc).

3. Kc

Before the encryption, Kc must be agreed upon by both the MS and network. In

the GSM, the Kc is calculated during the authentication and then stored in the

SIM card permanently. On the network side, this potential key is also stored in

the visited MSC/VLR and ready for use in the encryption.

The algorithm that uses the RAND (same with the one used for authentication)

and Ki to calculate the Kc is called A8 algorithm. Like the A3 algorithm that

calculate the SRES using RAND and Ki, the A8 algorithm also needs to be

determined by the carrier.

Fig. 2.6 -3 shows how the Kc is calculated.

27



GSM Basic Principle

Mobile Terminal Network

A8 algorithm

Random number

generator KiRAND

Kc

Ki

Kc

A8 algorithm

Fig. 2.6-3 Kc Calculation Method

28



3 GSM Speech Processing

3.1 GSM Speech Processing

In the GSM system, the MS processes voice signals on wireless interfaces as shown in

Fig 3.1 -7.

A/D

D/A

Voicecoding

Channel

coding

Interle

aving

EncryptionBurst pulse

forming

Modulation

Voice

decoding

Channel

decoding Deinterleaving DecryptionBurst pulse

disassembleDemodulation

260bit/20ms 456bit/20ms 33.8kbit/s 270kbit/s

Fig 3.1-7 Voice Processing in the GSM System

The process of sending voice signals is as follows: for analog voice signals, first make

A/D conversion before doing voice coding to output 13Kbit/s digital voice signals. Tocontrol errors in the process of transmission, channel coding and interlacing processing

shall be conducted on digital voice signals, which are then encrypted according to the

input/output bit stream of 1:1. These bits are grouped into 8 1/2 burst pulse sequences

(corresponding to voice signals/20ms segment) before they are transmitted at about

270Kbit/s in the appropriate timeslots.

The process of receiving voice signals is as follows: for the wireless signals sent by

BTS, first do demodulation before decomposing and decrypting burst pulses. After

every 8 1/2 burst pulse sequences are received, they are subjected to interlacing

processing and re-assembled into 456 bit information. After that, do channel decoding

and detect and correct the errors that occur in the middle of transmission before finally

conducting voice decoding of the bit stream generated by the decoder and converting it

analog voices.

3.2 Voice encoding

Given below is a brief introduction to the voice coding process of the GSM system

using full-rate voice coding as an example.

29



Currently, what the GSM system uses is 13kb/s voice coding scheme, known as RPE-

LTP (Rule Pulse Excitation-Long Term Prediction). The aim of this scheme is to produce

near-PSTN voice quality when no error occurs.

It first divides the voice into voice blocks by 20ms and samples it with 8kHz frequency

to get 160 sample values. Then each sample value is quantified to generate 16bit digital

voice signals. The 128Kbit/s data stream is obtained this way. As the rate is too high to

be transmitted on the wireless path, it needs to be compressed by a coder. If a full-rate

coder is used, each voice block will be compressed into 260bits to generate 13Kbit/s

source code rate in the end. The process of processing other signals such as channel

coding comes after that.

On the BTS side, BTS can recover 13Kbit/s source rate, but to generate 16Kbit/s rate so

that it can be transmitted on the Abis interface, it is necessary to add 3Kbit/s signaling so as

to control the operation of the remote TC. On the TC side, to accommodate 64Kbit/s

transmission rate of A interface, it is also necessary to conduct rate conversion between

13Kbit/s and 64Kbit/s.

3.3 Channel Encoding

Channel coding serves to improve transmission quality and overcome the negative

impact of interferences on signals.

Using specialized redundancy technology, channel coding inserts redundancy bits in

certain regularity at the transmitting end for coding while the receiving end in the

process of decoding detects error codes and corrects errors as many as possible using

these redundancy bits to recover the originally transmitted information.

The coding schemes as used in GSM are convolutional code and packet code which are

used in a combinational way in actual applications.

Convolutional code: compiles k information bits into n bits. Both k and n are very

small so that they are suitable for transmission in a serial port manner. Besides they

also show very little delay. The coded n code elements are not only related to k

information code elements of this packet, but also to information code elements in the

preceding (N-1), where N is called constraint length. The convolutional code is

generally represented as (n, k, N). The error tolerance of the convolutional codes

increases as N increases while its error rate decreases as N increases. The convolutional

code is mainly designed for error correction. When the demodulator uses the maximum

30



5 Frame Structure and Radio Channels

likelihood estimation method, it can generate very effective error correction results.

Convolution code can be expressed as (n, k, N). The error-correction capability in

convolution encoding grows stronger with the rise of N, while the error probability

decreases exponentially as N rises. The convolution code is used to correct errors, and

it is effective when the decoder works in the maximum likelihood estimate mode.

Packet code: This is a kind of shortened loop code, which gets the redundancy bits by

increasing the exclusive-or algorithm of information bits and maps k input information

bits to no output binary code elements (n>k) through exclusive-or algorithm. The

packet code is designed mainly to detect and correct error codes in groups and it is

used in a mixed way with the convolutional code. The packet code is used for detecting

and correcting errors in groups. It is generally used along with the convolution code.

3.4 Interlacing Technology

The occurrence of burst error codes in wireless communication is usually caused by

fading that lasts a long time. It is not enough to detect and correct errors in the above-

mentioned channel coding scheme. To better address the issue of error codes, the

interlacing technology is introduced to the system. The interleaving technology is

adopted in channels to better solve the error problems.

Interlacing is in fact to send separately the original continuous bits of a message block

in certain regularity. In other words, the original continuous block in the middle of

transmission becomes discontinuous and creates a group of interlaced transmission

message blocks. At the receiving end, this kind of interlacing message blocks is

restored (de-interlaced) to original message blocks. To control the operations and

sessions, the TCAP are classified into two layers, CSL and TSL. The CSL is used to

manage the operations and the TSL is used to manage the transactions (sessions), as

shown in Fig 3.4 -8.

31



GSM Basic Principle

Packet

Interleaving

Packet after

interleaving

Error code

11 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

11 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4

11 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

Fig 3.4-8 Interleaving Technology

After the interlacing technology is applied, if a message is lost in the middle of

transmission, it is in fact part of each message block that is lost, but the whole part of

it. The missing messages can be recovered easily with the coding technology.

In the GSM, different coding and interleaving modes are used in different types of

channels. See Table 3.4 -2 for details.

Table 3.4-2 Coding and Interweaving of Circuit Logical Channels

Channel Type

Input

Rate

(Kbit/s)

Input Code

Block bits

Code Output

Code

Block bits

Interleaving DepthCheck Bit Tail Bit

Convolutional

Code Rate

TCH/F

S

Ia 13 50Parity

check, 3 4 1/2456 On eight 1/2 bursts

Ib 13 132

II 13 78

TCH/

HS

Ia 5.6 22Parity

check, 3 6 1/3228 On four 1/2 bursts

Ib 5.6 73II 5.6 17

TCH/F9.6

TCH/H4.8

12

6240 4

1/2, one bit is

removed

every 15 bits.

456Combine on 22

unequal bursts

TCH/F4.8 6 120 32 1/3 456 Ditto

TCH/F2.4 3.6 72 4 1/6 456 On eight 1/2 bursts

TCH/H2.4 3.6 144 8 1/3 456Combine on 22

unequal bursts

SCH 25

Parity

check, 10 4 1/2 78

Combine on one SB

burst

32




Channel Type

Input

Rate

(Kbit/s)

Input Code

Block bits

Code Output

Code

Block bits

Interleaving DepthCheck Bit Tail Bit

Convolutional

Code Rate

RACH 8Parity

check, 64 1/2 36

Combine on one AB

burst

FACCH 184Packet

coding, 404 1/2 456 On eight 1/2 bursts

SACCH

BCCH

SDCCH

AGCH

PCH

184Packet

coding, 404 1/2 456 On four whole bursts

The voice input rate on TCH/FS is 13 Kbit/s, that is, each speech frame lasts 20 ms and

contains 260 bits. According to the interference of different bits on voice, the 260 bits

are divided into I category (182 bits in total) and II category (78 bits in total). The I

category is further divided into Ia and Ib. The Ia bits are very important bits. If any of

them is incorrect, the subscriber will hear a loud noise in 20 ms voice interval. There

are 50 Ia bits and 132 Ib bits. That is, the 260 bits in a speech frame (20 ms) is { d (0),

d (1),…, d (181), d (182), …, d (259)}. The part with a single line is I category, and

that with a double-line is II category. It is similar to the TCH/HS.

Table 3.4 -2 gives the coding and interleaving adopted in different types of

transmission. The first column lists the channels and the related transmission mode.

The Input Code Block column gives the size of the data block (bits) before channel

coding. The Output Code Block column gives the size of the data block (bits) after

channel coding. In Code, the parameters are listed in the same sequence as the coding

sequence. The tail bit is "0". The decoding is in the reverse order.

Following is description of channel coding and interweaving, taking voice

communication for example.

In the GSM, the voice input rate on TCH/FS is 13kb/s, that is, 260 bits are transmitted

every 20ms. The 260 bits are protected by means of segmented coding.

Among the 260 bits, 182 bits adopts 1/2 convolutional coding, and the remaining 78

bits are not protected. Among the 182 bits, 50 bits are performed with parity check and

then with 1/2 convolutional coding. Three information bits are added. Those 50 bits are

called Ia bits. The other 132 bits, called Ib bits, are performed with 1/2 convolutional

coding directly.

33



GSM Basic Principle

Fig 3.4 -9 shows the interleaving algorithm of voice signals on TCH/F. After channel

coding, 456 bits are carried in every 20ms. Those bits are divided into eight groups,

with the 57 bits in each group carried in different burst pulses (eight BPs in total). To

maximize irrelevancy between the bit sequences, the bits should be arranged as

described in Table 3.4 -3.

0 1 2 3 4 5 6 78 9 10 11 12 13 14 15

. . . . . . . .

. . . . . . . .

. . . . . . . .

1 2 3 4 5 6 7 8

456bits

0 1 2 3 4 5 6 78 9 10 11 12 13 14 15

. . . . . . . .

. . . . . . . .

. . . . . . . .

456bits

0 1 2 3 4 5 6 78 9 10 11 12 13 14 15

. . . . . . . .

. . . . . . . .

. . . . . . . .

456bits

0 1 2 3 4 5 6 78 9 10 11 12 13 14 15

. . . . . . . .

. . . . . . . .

. . . . . . . .

456bits

57 1 57 1 57 1 57 1 57 1 57 1 57 1 57 1

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

A block

Odd bits

B block

Even bits116 bit 116 bit 116 bit

Fig 3.4-9 Interleaving of Cells

Table 3.4-3 Full-rate speech interleaving algorithm

No. Item Note

1 0, 8, …, 448 Even bits (B block) in BP (N)

2 1, 9, …, 449 Even bits (B block) of BP (N 1)

3 2, 10, …, 450 Even bits (B block) of BP (N 2)

4 3, 11, …, 451 Even bits (B block) of BP (N + 3)

5 4, 12, …, 52 Odd bits (A block) v BP (N 4)



8 7, 15, …, 455 Odd bits (A block) v BP (N + 7)

456 bits are divided into eight groups (rows). Each group has 57 bits (columns),

occupying Block A or Block B of BP (N) to BP (N+7). After interleaving, a BP carries

114 bits of information plus 2 bits of stolen frame (116 bits in total). The 114 bits

contain 57 bits (odd bits) of information block A and 57 bits (even bits) of information

34




block B. The remaining two bits indicate respectively whether the first half BP (odd

bit) and the last half BP (even bit) are subscriber data or fast channel associated

signaling.

3.5 Encryption/Decryption

There are encryption measures available in the GSM system. They are applicable to

voice, data and signaling. They are independent of the data type and work for normal

bursts only. Encryption is accomplished by exclusive or operation of an encryption

sequence (computed by A5 encryption algorithm via key Kc and frame number) and

114 information bits on a normal burst.

The original transmission data can be obtained by using the same sequence at the

receiving end to conduct exclusive-or operation with the encryption sequence.

3.6 Modulation and Demodulation

Modulation and demodulation are the last step in signal processing. Using GMSK

modulation mode at a rate of 270.833 k Baud, GSM usually conducts demodulation

with Viterbi algorithm (with a balanced demodulation method). Demodulation is the

reverse process of modulation.

GMSK is a special digital FM modulation mode. The modulation rate is 270.833

kilobauds. The Frequency Shift Keying (FSK) modulation with bit rate four times of

frequency offset is called MSK (Minimum Shift-frequency Keying). In GSM, the

Gaussian demodulation filter is used to further reduce the modulation spectrum. It can

cut the frequency conversion speed.

The GMSK can be expressed by a I/Q diagram. If there is no Gaussian filter, when a

series of constant 1s are sent, the MSK signal will be kept in the state that is higher than the center frequency 67.708 kHz of the carrier. If the center frequency of the

carrier serves as the fixed phase reference, the signal 67.708 kHz will cause steady

increment of phase. The phase rotates 360° at 67,708 times per second. In a bit period

(1/270.833 kHz), the phase moves 1/4 a circle in the I/G diagram, that is, 90°. The data

1 can be looked as 90° plus the phase. Two 1s makes a phase increment by 180°, three

1s makes a increment by 270°, and so on. The data 0 indicates the same phase change

in the reverse direction.

The actual phase track is strictly controlled. In the GSM, digital filter and 1/Q or digital

35



GSM Basic Principle

FM modulator are used to generate correct phase track accurately. The Root Mean

Square (RMS) between the actual track and the ideal track allowed by GSM

specifications cannot exceed 5°, and the peak deviation cannot exceed 20°.

36



4 GSM Key Technologies

4.1 Diversity Reception

The diversity reception technology is usually introduced to the GSM system to receive

on several tributaries the signals with little relativity but carrying the same information

and then output the signals after they are combined. In this way, the impact of fading

on the stability of receiving signals can be played down.

There are ways of diversity as follows: space diversity, frequency diversity, time

diversity and polarization diversity.

1. Space Diversity

Two receiving antennas are set in the space to receive independently the same

signals before combining and outputting them. In this way, the degree of fading

can be dramatically lowered. This is the so-called space diversity. The space

diversity is based on the fact that the field strength varies randomly with the

space. The longer the distance, the more variant the multi-path transmission will be, and the less relevant the receiving filed strength will be. The relevancy refers

to the similarity between the signals. Therefore, the necessary distance must be

determined. According to the test and statistics, CCIR suggests the spacing

between two antennas should be larger than 0.6 wavelength, namely d>0.6λ, to

achieve a satisfactory diversity result and that it should be better to near the odd

number multiplication of λ/4. Even if the distance between antennas is shortened

to be λ/4, good diversity effect can be achieved.

2. Time Diversity

Time diversity is to send the same message with some delays or part of the

message in different time within the delay range tolerable by the system. In the

GSM system, time diversity is achieved by the interlacing technology. In the

GSM, interleaving technology is adopted to implement the time diversity.

3. Frequency Diversity

Frequency diversity means more than two frequencies send a signal

concurrently. The receiving end combines the signals of different frequencies

37



and reduces or eliminates the impact of fading with different paths of the

wireless carrier waves of varied frequencies. The frequency diversity is effective

and requires one set of antenna only. Frequency diversity in GSM is

implemented by frequency hopping technology.

4. Polarization Diversity

Polarization diversity is to receive signals by making two pairs of receiving

antennas with polarization direction into some angles against each other, which

can generate a good diversity result. The two sets of polarized antennae in

polarity diversity can be integrated in one set of antenna. Thus, only one

receiving antenna and one transmitter antenna are required in a cell. If duplexer is adopted, only one transceiving antenna is required. It saves antennas greatly.

4.2 Discontinuous Transmission

There are two voice transmission modes. One is continuous voice coding (one speech

frame every 20ms) no matter whether the subscriber is talking or not. Another is

discontinuous transmission (DTX) with 13kb/s coding in voice activation period and

500b/s coding in non voice activation period. In addition, a comfort noise frame (20ms

per frame) is transmitted every 480ms, as shown in Fig 4.2 -10.

There are two purposes of employing the DTX mode: one is to lower the general

interference level in the air; the second is to save the power of transmitters. However,

the DTX may slightly lower the transmission quality. Therefore, the DTX mode and

common mode are optional.

TRAU BTS

BTS MS

Comfort

noise frame

Speech frame

480 ms

Fig 4.2-10 Speech Frame Transmission in DTX Mode

38




4.3 Power Control

Power control means to control the actual transmitting power (keep it as low as

possible) of MS or BS in radio propagation, so as to reduce the power consumption of

MS/BS and the interference of the entire GSM network. Needless to say, the

prerequisite of power control is to ensure the good communication quality of the

ongoing calls. The power control process is simply illustrated in Fig 4.3 -11.

A B

Fig 4.3-11 Power Control

As shown in Fig. 1.5-16, the MS at point A is far from the BS antenna. Because the

propagation loss of electric wave in air is in direct proportion to n power of the

distance, the MS at A needs higher transmit power to ensure good communication

quality. Comparatively, point B is closer to the BS transmission antenna, hence smaller

transmission loss; therefore, to obtain similar communication quality, a mobile phone

at point B can use lower transmission power during communication. When a mobile

phone in communication is moving from point A towards point B, the power control

can reduce its transmitting power gradually. On the contrary, if it is moving from point

B towards point A, the power control can increase its transmitting power gradually.

The power control is classified as uplink power control and downlink power control,

they function separately. By uplink power control, it means to control the MS

transmitting power, while downlink power control means to control the BS transmitting

power. No matter uplink power control or downlink power control, the uplink or

downlink interference is suppressed as the transmit power is reduced. Meanwhile the

power consumption of the MS or base station is reduced. The most obvious benefits are

the average conversation quality of the whole GSM network is greatly increased, and

the MS standby time is prolonged.

1. Power control process

39



GSM Basic Principle

The original information used for decision making during a power control

process is obtained from the measurement data of the MS and BS and

corresponding control decision can be made after processing and analyzing of

the original data. Similar to the handover control process, the whole power

control process is shown in Fig 4.3 -12.

Measurement data saving

Average measurement data

processing

Power control decision

making

Power control command

sending

Measurement data correction

Fig 4.3-12 Power Control Process

1) Measurement data saving

The measurement data related to power control includes uplink signal level,

uplink signal quality, downlink signal level, and downlink signal quality.

2) Average measurement data processing

To reduce the influence of complex radio transmission on the measurement

values, the smooth processing of the measurement data usually adopts the

forward averaging method. That is, the average value of multiple measurement

values is used to make a power control decision. The parameter setting in

averaging calculation may vary with the types of the measurement data, i.e.,

quantity of the measurement data to be used may be different.

3) Power control decision making

In the decision making of power control, there are three parameters: a threshold,

an N value, and a P value. Among the latest N average values, if there are P

40




parameters exceed the threshold, the signal level is too high or the signal quality

is good; if there are P parameters are lower than the threshold, the signal level is

too low or the signal quality is poor.

According to the condition of the signal level or quality, the mobile phone or BS

can judge how to control the transmitting power, and the increase or decrease

amplitudes are determined by the pre-configured values.

4) Power control command sending

According to the power control decision, the corresponding control command is

sent to the BS, which will then execute the command or transfer it to MS.

5) Measurement data correction

After power control, the original measurement data and average values are

useless. If the useless information is still kept, it may cause incorrect power

control decision. Therefore, it is necessary to discard the outdated data or update

it for later use.

The fastest power control can be performed once every 480 ms, which is the

highest speed that the measurement data is reported. In other words, an entire

power control process is executed once in at least 480ms.

2. High-speed power control

The control extent of the power control process recommended by ETSI is fixed

as 2dB or 4dB normally. However, in most practical cases the fixed power

control extent is unable to achieve optimal effects, for a simple example:

When an MS initiates a call at a location very near to the BS antenna, its start

transmitting power is the max. transmitting power of the MS in the system

message broadcast in the cell BCCH (MS_TXPWR_MAX_CCH). It’s obvious

that at this time as the MS is quite close to the MS antenna, the power control

process is supposed to reduce its transmitting power as fast as possible.

However, it can hardly be achieved by the power control process recommended

by the ETSI specifications, because only 2dB or 4dB is decreased each time. In

addition, there is an interval between every two power control processes

(because enough new measurement data need be collected). Therefore, it takes a

long time to reduce the transmit power of the MS to a proper value. It is the

same in the downlink direction. Obviously this is disadvantageous in terms of

41



GSM Basic Principle

reducing interference to the whole GSM network. To improve this, the power

control extent each time should be increased, which is the core idea of the high-

speed power control.

The high-speed power control can, according to the actual signal strength and

quality, work out the power control extent to be realized, without the limitation

of the fixed extent, thus solving the power control problem without much effort

when the MS makes the initial access. Of course its functions are not limited to

this situation. It can work in many cases e.g. fast moving mobile phones,

sudden interference or obstacles. Whenever large extent power control is

required, the high-speed power control process is the ideal solution.

4.4 Timing Advance

In the GSM, because TDMA is adopted in the air interface, the MS must employ the

TSs allocated to it only, and remain inactive in other time. Otherwise, it may affect the

MSs using other TSs on the same carrier.

In the GSM, the MS requires three intervals between timeslots when receiving or

transmitting signals. See Fig 4.4 -13.

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1Offset

Downlink:

Uplink:

Sent by the BTS Sent by the MS

TDMA frame number

TDMA frame number

Fig 4.4-13 Uplink and Downlink Offset of TCH

Suppose an MS occupies TS2 and moves away from the base station, the messages sent

42





GSM Basic Principle

sequence. Finally, the signals are sent via the RF filter to the antenna for transmission.

The receiver determines the receiving frequency according to FH synchronization

signals and FH sequence, receives corresponding signals after FH for demodulation.

The basic structure of FH is illustrated in Fig 4.5 -14.

Synchronization

circuit

Frequency

modulation

sequence

generator

Variable

frequency

synthesizer

Message

modulation

Up

converter

Send

Message

demodulationDown converter

Receive

Fig 4.5-14 Basic Structure of FH

Features of frequency hopping technology: The frequency hopping technology can be

employed to increase the working band of the system so as to enhance the anti-

jamming and anti-jamming capability of the communication system. Frequency

hopping can help improve and protect the pulse of the effective information part from

the impact of Rayleigh fading in the communication environment. After frequency

hopping is done, the original data are recovered by means of channel decoding. The

times of frequency hopping are increased to boost frequency hopping gains so as to

enhance the anti-jamming and anti-fading capability of the system.

The frequency hopping technology is actually to avoid external interferences so that

they cannot follow the changes of frequencies, thus avoiding or markedly lowering

same-channel interference and frequency selective fading. The reason to increase the

number of hoppings is that the gain of frequency hopping system is equal to the ratio of

frequency hopping system bandwidth to N minimum frequency hopping intervals.

Usually, the FH number should be greater than three. If frequency diversity is also

available for the FH system, and a message is transmitted by several groups of

44




frequency hopping simultaneously and then judged by the law of large numbers, more

subscribers can use services at the same time with least mutual interference.

The frequency hopping comprises baseband hopping and RF hopping.

The baseband hopping enables the transmit and receive frequencies of each

carrier unit to remain unchanged. At different frame number (FN) moment, the

frame unit sends data to different carrier units.

RF FH is to control the frequency synthesizer of each transceiver, making it hop

according to different schemes in different timeslots.

45




GSM air interface uses TDMA based frame structure. Communication services are

obtained by transmission of information using logical channels on physical channels.

Mapping between the logical channel and physical channel is the process that arranges

the information to be sent to the suitable TDMA frames and timeslots.

5.1 Radio Frame Structure

Five levels of GSM radio frame structure are timeslot, TDMA frame, multiframe,

superframe and hyperframe.

Timeslot is the basic unit of a physical channel.

TDMA frame consists of eight timeslots. It is a basic unit occupying carrier

bandwidth. Each carrier has eight timeslots.

There are two types of multiframes:

One type of multiframe consists of 26 TDMA frames. This type of multiframe is

used in TCH, SACCH, and FACCH.

The other type of multiframe consists of 51 TDMA frames. This type of

multiframe is used in BCCH, CCCH, and SDCCH.

The superframe is a consecutive 51 x 26 TDMA frame. It consists of 51 26-

multiframes or 26 51-multiframes.

The hyperframe consists of 2,048 superframes.

Fig 5.1 -15 shows GSM frame structure.

47



0

TDMA frame

00 01

1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

02

0 1 2 3 4 5 6 7

0 1 2 3 4 22232425

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 4748 4950

0 1 2 3 47 48 49 50

0 1 2524

2042 2043 204420452046 20476543210

1 26-multiframe = 26 TDMA frames (120 ms) 1 51-multiframe = 51 TDMA frames (3036/13 ms)

1 superframe = 1326 TDMA frames (6.12s)= 51 26-multiframe or 26 51-multiframes

1 hyperframe = 2048 superframes = 2715648 TDMA frames

Fig 5.1-15 GSM Frame Structure

5.2 Physical Channel

GSM adopts mixed technology of Frequency Division Multiple Access (FDMA) and

Time Division Multiple Access (TDMA). GSM features high frequency utilization.

FDMA - enables 124 carrier frequencies (carriers for short) to be assigned to the

uplink (from the MS to the BTS) 890 MHz – 915 MHz or downlink (from the BTS to

the MS) 935 MHz – 960 MHz in GSM900 band. Interval between carriers is 200 kHz.

Carriers in the uplink and downlink are in pairs called duplex communication mode.

Interval between duplex receiving and transmitting carrier pair is 45 MHz.

TDMA - enables each carrier of GSM900 band to be divided into eight time segments.

Each time segment is called a timeslot. See Fig 5.2 -16.

This type of timeslot is called a channel or a physical channel. Eight consecutive

timeslots on a carrier constitute a TDMA frame, that is, a carrier of GSM provideseight physical channels.

48




16/25 ms

200 kHz

Timeslot

Time

Frequency

Fig 5.2-16 Time-Frequency Structure of Physical Channel

Eight timeslots in TDMA frame are called physical channels.

5.3 Logical Channels

Each physical channel is time multiplexed with different logical channels. Logical

channels carry various signaling or traffic information based on user and network

requirements. To provide signaling traffic control, logical channels map on physical

channels.

Logical channels are classified into Common Channel and Dedicated Channel.

EnhancedFull-ratechannel

LogicalChannels

Fast AssociatedControl Channel

(FACCH)

Slow AssociatedControl Channel

(SACCH)

Stand-aloneDedicated ControlChannel (SDCCH)

FrequencyCorrection

Channel (FCCH)

Common ControlChannel (CCCH)

DedicatedChannel

BroadcastChannel (BCH)

DedicatedControl Channel

(DCCH)

Traffic Channel(TCH)

Broadcast ControlChannel (BCCH)

Paging Channel(PCH)

Random AccessChannel (RACH)

Access GrantChannel (AGCH)

CommonChannel

Half-ratechannel(TCH/H)

Full-rateChannel(TCH/F)

Synchroniza-tion Channel

(SCH)

Fig 5.3-17 GSM Logical Channels

49



GSM Basic Principle

5.3.1 Common Channel

Common Channel is classified in two main types:

Broadcast Channel (BCH): BCH transmits broadcast messages from base station to

MS. Broadcast Channel is unidirectional channel from base station to MS. It is of three

types:

Frequency Correction Channel (FCCH): It carries the information used to correct

the MS frequency. MS receives frequency correction information through FCCH and

corrects its time base frequency.

Synchronization Channel (SCH): It carries frame synchronization (TDMA frame

number) information and Base Station Identity Code (BSIC) to MS.

Broadcast Control Channel (BCCH): It broadcasts general information of BTS. For

example, broadcasts the local cell and neighboring cell information, and

synchronization (time and frequency) information. MS listens to BCCH periodically to

obtain the information transmitted on it, such as the Local Area Identity, List of

Neighboring Cell, frequency table used in local cell, cell identity, power control

indication, intermittent transmission permission, access control, and CBCH

description. BCCH carrier is transmitted by base station at a fixed power, and its signal

strength is measured by all MSs.

Common Control Channel (CCCH): CCCH is point-to-multipoint bi-directional

channel. It carries signals required to set up a connection between base station and MS.

It is of three types:

Paging Channel (PCH): It broadcasts paging messages from base station to MS. It is a

downlink channel.

Random Access Channel (RACH): MS sends information to base station through this

channel when accessing the network at random. The information sent includes response

to the paging message of base station and access of mobile-originated call. MS also

applies for a Stand-alone Dedicated Control Channel (SDCCH) from base station

through this channel. RACH is an uplink channel.

Access Grant Channel: The base station sends the assigned SDCCH to the MS that

accesses the network successfully through this channel. The AGCH is a downlink

channel.

50




5.3.2 Dedicated Channel

Dedicated channel is a traffic channel which carries voice and data. Some types of

dedicated channel are used for the control purpose.

Dedicated Channel is classified in two main types:

Dedicated Control Channel (DCCH): DCCH is a point-to-point bi-directional

channel between base station and MS. It is of three types:

Stand-alone Dedicated Control Channel (SDCCH): It carries signaling and

channel information between base station and MS, such as the authentication

and registration signaling messages. During the establishment of a call, SDCCH

supports bi-directional data transmission and short messages transfer.

Slow Associated Control Channel (SACCH): Through this channel, base

station sends power control message and frame adjustment message to MS, and

receives signal strength report and link quality report from MS.

Fast Associated Control Channel (FACCH): It carries inter-cell handover

signaling messages between base station and MS.

Traffic Channel (TCH): TCH carries voice and data. According to switching mode,

TCH can be divided into circuit-switched channel and data-switched channel.

According to transmission rate, TCH can be divided into full-rate channels and half-

rate channels.

Rate of the GSM full-rate channel is 13 kbps, and that of the GSM half-rate channel is

6.5 kbps. In addition, the enhanced full-rate channel has same rate as the full-rate

channel, which is 13 kbps. However, it has better compressed coding scheme than full-

rate channel. That is why enhanced full-rate channel provides better voice quality.

5.3.3 Channel Combination

In actual application, different types of logical channels are mapped on the same

physical channel. This is called channel combination.

Following are nine GSM channel combinations:

Full-rate traffic channel (TCHFull): TCH/F + FACCH/F + SACCH/TF

Half-rate traffic channel (TCHHalf): TCH/H (0, 1) + FACCH/H(0, 1) +

SACCH/TH (0, 1)

Half-rate1 traffic channel (TCHHalf2): TCH/H (0, 0) + FACCH/H (0, 1)

51



GSM Basic Principle

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

SDCCH: SDCCH/8 (0,…

, 7) + SACCH/C8 (0,…

, 7)

Main broadcast control channel (MainBCCH): FCCH + SCH + BCCH + CCCH

Combined broadcast control channel (BCCHCombined): FCCH + SCH +

BCCH + CCCH + SDCCH/4 (0,…,3) + SACCH/C4 (0,…, 3)

Broadcast channel (BCH): FCCH + SCH + BCCH

Cell broadcast channel (BCCHwithCBCH): FCCH + SCH + BCCH + CCCH +

SDCCH/4 (0,…, 3) + SACCH/C4 (0,…, 3) + CBCH

Slow dedicated control channel (SDCCHwithCBCH): SDCCH + SACCH +

CBCH

Among the above channel combinations, CCCH = PCH + RACH + AGCH. As a

downlink channel, only CBCH carries cell broadcast information and shares the

physical channel with SDCCH.

Each cell broadcasts FCCH and SCH. The basic combination in the downlink direction

includes FCCH, SCH, BCCH and CCCH (PCH + AGCH). It is allocated to TN0 of

BCCH carrier configured for a cell, as shown in Fig 5.3 -18.

SF B C

R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R

51 frames

SF C C SF C C SF C C I

R R R R R R R R R R

D0 D1 D2D

3D4 D5 D6 D7 A0 A1 A2 A3

SF C C

R R R R R R R R R R

III

D0 D1 D2D

3D4 D5 D6 D7 A4 A5 A6 A7 III

A1 A2 A3 III

A5 A6 A7 III

D0 D1 D2 D3D

4D5 D6 D7 A0

D0 D1 D2 D3 D4 D5 D6 D7 A4

SF B C SF C C SF D0

D1

SF D2

D3

ISF A0

A1

SF B C SF C C SFD

0

D

1SF

D

2

D

3ISF

A

2

A

3

D3

D3

R R

R R

A2 A3

A0 A1

D

2D

2

SF

SF

D

0

D

1D

0

D

1

R R R R R R R R R R R R R R R R R R R R R R R

R R R R R R R R R R R R R R R R R R R R R R R

F: FCCH S: SCH

B: BCCH C: CCCH (CCCH=PCH+AGCH+RACH)

R: RACH D: SDCCH

A: SACCH/C I: Idle

BCCH+CCC

HDownlink

BCCH+CCC

HUplink

8 SDCCH/8

Downlink

8 SDCCH/8Uplink

BCCH+CCC

H+4SDCCH/4Downlink

BCCH+CCC

H+4SDCCH/4

Uplink

(a) FCCH+SCH+BCCH+CCCH

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

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

Fig 5.3-18 Frame Channel Structure

For half-rate voice channel combination, each timeslot has two half-rate sub-channels

52




and corresponding SACCH, with 26 TDMA frames as a multiframe.

Fig 5.3 -19 shows the frame structure.

H

0

H

0

S

1

S

0

H

1

H

0

H

0

H

0

H

0

H

0

H

0

H

0

H

0

H

0

H

0

H

1

H

1

H

1

H

1

H

1

H

1

H

1

H

1

H

1

H

1

H

1

26 frames

Fig 5.3-19 Half-Rate Voice Channel Frame Structure

5.4 Mapping between Logical and Physical Channels

Logical channels in GSM are much more than the eight physical channels that a GSM

carrier can provide. If each logical channel is configured with a physical channel, the

eight physical channels provided by a carrier are not enough.

In such case, extra carriers must be added. However, the communication in this way is

not highly effective. The way to solve this problem is to multiplex the CCCH, that is,

multiplex the CCCH on one or two physical channels.

Mapping between physical channels and logical channels in GSM is as follows:

Base station has N carriers, and each carrier has eight timeslots. Define the carriers asf 0, f 1, f 2 … Downlink starts from timeslot 0 (TS0) of f 0. TS0 is used to map with control

channel only. f 0 is also called broadcast control channel (BCCH).

Fig 5.4 -20 shows BCCH and CCCH on TS0 multiplexing.

012 7012 701

FS B C FS C C FS C C FS C C FS C C I

TDMA

frame

BCCH+CCCH

Downlink

F (FCCH): MS synchronizes its frequency through it.S (SYCH): MS reads TDMA frame number and Base Station Identity Code (BSIC)

through it.

B (BCCH): MS reads the general inforamtion of the cell through it.I (IDLE): Idle frame, containing no information. It serves as the end flag of the

multi-frame.

Fig 5.4-20 Multiplexing of BCCH and CCCH on TSO

BCCH and CCCH occupy total 51 TS0s. Although only the TS0 of each frame is

53



GSM Basic Principle

occupied, the total length is 51 TDMA frames in terms of time. Each time when an idle

frame appears, the multiframe ends. After that, a new multiframe starts from F and S.

Repeat like this, and TDMA multiframe is constructed.

When there is no paging or call connected, the base station always transmits on f 0. This

enables MS to detect the signal strength of the base station to determine the cell to be

used.

For the uplink, the TS0 on f 0 does not include the above channels. It is used for the MS

access only, that is, it is used as the RACH.

Fig 5.4 -21 shows the TS0 of 51 consecutive TDMA frames.

012 7012 701

RR

TDMA

frame

RACH

UplinkRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR

Fig 5.4-21 Multiplexing of RACH on TSO

BCCH, FCCH, SCH, PCH, AGCH, and RACH are all mapped on TS0. RACH is

mapped on uplink, and the rest are mapped on downlink.

TS1 on downlink f 0 is used to map DCCH to physical channel.

Fig 5.4 -22 shows the mapping relationship.

012 7012 701

D0 I

TDMAframe

SDCCH+SA CCCH

Downlink

D1 D2 D3 D4 D5 D6 D7 A0 A1 A2 A3 I I

D0 ID1 D2 D3 D4 D5 D6 D7 A4 A5 A6 A7 I I

Fig 5.4-22 Multiplexing of SDCCH and SACCH on TS1 (Downlink)

54




Since the bit rate in call setup and registration is quite low, eight dedicated control

channels can be placed on one timeslot to improve the multiplexing ratio of the

timeslot.

SDCCH and SACCH have 102 timeslots in total, that is, 102 time division

multiplexing (TDM) frames.

DX (D0, D1 …) of SDCCH is used in the early time when a call is set up. When the

MS transfers to the TCH, and the subscriber starts the conversation or the release is

triggered after registration, the DX is used by other MSs.

AX (A0, A1 …) of the SACCH transfers unimportant control information, such as

radio measurement data, that is TS0 of 51 consecutive TDMA frames.

TS1 on the uplink f 0 has the same structure with the TS1 on the downlink f 0. They have

an offset in time, which means bi-directional connection can be performed at the same

time for an MS.

Fig 5.4 -23 shows the multiplexing of the SDCCH and SACCH on TS1 of the uplink

f 0.

012 7012 701

A5

TDMAframe

SDCCH+ SACCCH

Uplink

A6 A7 D0 D1 D2 D3 D4 D5 D6 D7

A1 A2 A3 D0 D1 D2 D3 D4 D5 D6 D7

I I I

I I I

A0

A4

DX: same as uplink AX: Same as downlink

Fig 5.4-23 Multiplexing of SDCCH and SACCH on TS1 (Uplink)

Uplink and downlink TS0 and TS1 on f 0 are used by the logical control channel, while

other six physical channels (TS2 to TS7) are used by TCH.

Fig 5.4 -24 shows the mapping from TCH to physical channel.

55



GSM Basic Principle

T=TCH A=SACCH I=Idle

Fig 5.4-24 TCH Multiplexing

Fig 5.4 -24 shows TS2 time division multiplexing.

TCH carries voice or data. SACCH carries control commands such as the command to

change the output power.

Idle I does not contain any information but is used in measurement.

TDM is implemented on TS2 with 26 timeslots as a cycle.

The idle timeslot I serves as the beginning or end of the repeated sequence.

Uplink TCH is of the same structure with the downlink TCH. They only have a time

offset, which is three timeslots. That is, the TS2 of the uplink and that of the downlink

do not appear simultaneously, which means that the MS does not send or receive data

at the same time.

Fig 5.4 -25 shows the offset between the uplink and downlink of the TCH.

0

TDMA frame number

Uplink C0

00 01

From BTS to MS

From MS to BTS

Downlink C0

45MHz (GSM900)

95MHz (DCS1800)

Offset

1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0

TDMA frame number

00 01

1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Fig 5.4-25 Offset between Uplink and Downlink of the TCH

The conclusion is that on carrier f 0:

TS0: a logical control channel, with repeat cycle of 51 timeslots.

56




TS1: a logical control channel, with repeat cycle of 102 timeslots.

TS2: a logical traffic channel, with repeat cycle of 26 timeslots.

TS3 to TS7: logical traffic channels, with repeat cycle of 26 timeslots.

The TS0 to TS7 of other f 0 – f N are all traffic channels.

Documents

GO NA01 E1 0 GSM Basic Principle-63