gsm&gprs basic

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1 GSM Basics

Summary

This chapter examines briefly the evolutionary course of mobile communications and

the features of the GSM system and introduces the basic concepts and must-have

know-how so as to lay down a foundation for the further study of the ZXG10 BSS

product.

Training

Target

Through the study of this chapter, a learner is expected to

know the features and composition of the GSM system

know the interfaces of the GSM system

understand all the features of the wireless channels of the GSM system

understand the process of processing the voice signals of the wireless interface**

Exercises

1. How many features does the GSM system have? What components is the GSM

system generally comprised of?

Mobile communication is primarily aimed to establishing communication between any

communication parties anytime, anywhere.

Judging from the perspective of communication network, mobile network can be

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viewed as an extension of wired communication network. It consists of two parts:

wireless part and wired part. The wireless part offers access to the users terminal and

transmits voices and data reliably in the air using the limited frequency resources; the

wired part performs network functions, including switching, user management,

roaming and authentication, and makes up PLMN (Public Land Mobile Network) .

1.1 Evolution of GSM

Evolving from the 40s up to date, the mobile communication system can be divided

into three phases by its evolutionary course and orientation: the first generation analog

cellular communication system, the second generation digital cellular mobilecommunication system and the third generation mobile communication system.

Owing to the defects as found in the analog cellular communication system like low

voice quality, low security, susceptibility to tapping, easy creation of phony handsets,

etc., all the communication equipment manufacturers started the research of the digital

communication system. In the 1990s, the TDMA and narrow band CDMA were

developed as the main bases for mobile phone systems, and this is called the second

generation (2G) mobile phone systems. The products are of two categories:

TDMA system

The most remarkable feature of TDMA series is that it achieves the mobile

communication function using the time division multiple address technology as well as

the frequency division multiple address technology. Of the TDMA series, the mature

and representative systems are pan-Europe GSM, D-AMPS of the USA and PDC of

Japan. The above three kinds of products have the same features: digital, TDMA, better

voice quality than the frist generation, secure, able to transfer data, able to roam

automatically, etc. The three different systems have their own advantages: PDC system

features high frequency utilization, D-AMPS system has the largest capacity, and GSM

technology is the most mature technology, which, based on OSI, has an open technical

standard and can be developed on the largest scale.

N-CDMA system

Code Division Multiple Access (CDMA) radio technology is a new kind of digital

cellular technology following the GSM and other digital communication technologies.

N-CDMA series is the narrow band CDMA (N-CDMA) developed mainly by

Qualcomm on the basis of IS-95.

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By using the digital transmission method, and such key technologies as spread

spectrum communication, power control, soft capacity, soft handover, voice activation,

voice coding, multiple address, diversity receiving and RAKE receiving, the CDMA

system has remarkable advantages, which promoted the mobile communication

technology to a new phase of development.

However, as users grow in number and digital communications develop, the second

generation mobile phone system reveals gradually its shortcomings, which are mainly

as follows: scarcity of wireless frequency resources, inability to meet the demand for

fast data services, etc. Hence the third generation cellular mobile communication

system (3G) , also known as IMT-2000, emerges as the evolutionary orientation for

future communication. In this case, the GSM system will also migrate smoothly to 3G

as the technologies and products of all the manufacturers mature day by day.

ItemsMultiple access technology

As is known to all, in the electric wave coverage area of wireless communications environment, it is a

primary consideration for any transmission system to establish channels between intranet subscribers.

In fact, the nature of the consideration is multiple access mobile communication. The theorectical

base for multiple access connection is the signal segmentation technology, i.e., properly design

signals at the transmitting end, so that the signals transmited by all stations are different from each

other; the receiving end has the signal recognition capability to separate and select relevant signals

from the mixed signals.

Currently, the wireless multiple access modes are: FDMA in the analog system, TDMA and CDMA in

the digital system.

FDMA: frequency division multiple access. In a relatively narrow-band channel of the frequency

range, the signal powers are gathered before they are transmitted with varied signals allocated to

channels of different frequencies. The interruptions sent to or coming from the neighboring channels

are limited by the band-pass optical filter. In this way, only the energy of the useful signals can pass

through the specified narrow band with signals of other frequencies excluded.

TDMA: time division multiple address. One channel is made up of a series of periodic time slots. The

energy of varied signals is allocated to different timeslots. The interruptions are limited with time gate

so that only the energy of useful signals can pass through the specified timeslots.

CDMA: Code division multiple access. Each signal is allocated with a pseudo-random binary for

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frequency expansion. The energy of varied signals is allocated to different pseudo-random serials. In

the receiver, signals are separated with the correlator, which receives the selected binary sequence

only and compresses its spectrum. The bandwidth of the signals that do not comply with the binary

sequence of the user will not be compressed. As a result, only the information about the useful signals

is identified and extracted.

1.2 Features of the GSM System

The principal features of GSM can be summarized as:

Spectrum efficiency: the system shows high spectrum efficiency using the high-

efficiency modulator, channel coding, interleaving, balancing and voice coding

technologies.

Capacity: As the transmission bandwidth of each channel increases, the requirement

for the shared frequency multiplexing carrier-to-interference ratio drops to 9db. In this

case, the shared frequency multiplexing mode of the GSM system can be reduced to

4/12 or 3/9 or even smaller (the analog system is 7/21) . This plus the introduction of

half-rate voice coding and automatic traffic allocation to reduce the number of cross-

regional switchover makes the capacity efficiency (number of channels in each MHz

for each cell) of the GSM system 3~5 times higher than the TACS system.

Voice quality: In view of the features of the digital transmission technology and the

definitions of the air interface and voice coding in the GSM specifications, when it is

above the threshold value, the voice quality always reaches the same level, irrelevant to

the quality of wireless transmission.

Open interface: The open interfaces offered by GSM standard are not limited to air

interfaces. They also include the interfaces between networks and between equipment

entities on the network, such as A interface and Abis interface.

Security: Security is achieved through the use of authentication, encryption and TMSI

number. Authentication serves to verify a users network entry authority. Encryption is

designed for the air interface, determined by the SIM card and the secret key of

network AUC. TMSI is a temporary ID number assigned by the service network to a

user to guard against the leakage of his geographic location when he is being traced. It

is interconnected with ISDN, PSTN, etc. The existing interfaces such as ISUP or TUP

are usually used when it interconnects with other networks.

Roaming: Roaming is achieved on the basis of the SIM card. Roaming, an important

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feature of mobile communication, indicates a user can gain access automatically to

another network from one network. The GSM system offers the global roaming

function, which of course requires some protocols between network operators, such as

billing. In the GSM system, roaming is achieved on the basis of the ID number on the

SIM card and the IMSI called International Mobile Subscribers ID number. This

means subscribers can get into other countries with the SIM card rather than the

terminal device, which can be rented and the purpose of keeping the subscriber number

and the billing account number unchanged can still be fulfilled.

1.3 System compositions

The GSM communication system consists mainly of three parts: mobile switching sub-

system (MSS) , base-station sub-system (BSS) and mobile station (MS) , as shown in

Fig. 1.3-1.

IBM

IBM

BSS MSS

MS

MS

PSTN

Other

PLMNs

Um interface A interface

Fig. 1.3-1 Composition of GSM System

Mobile Switching Subsystem (MSS)

Performs such functions as information exchange, user information management, call

connection and number management.

Base Station Subsystem (BSS)

The BSS system is the system device that is controlled by BSC in a certain wireless

coverage area and communicates with MS. It performs such functions channel

allocation, subscriber access and paging and message transmission.

Mobile station

MS which is the mobile equipment of the GSM system consists of two parts: mobile

terminal and customer ID card (SIM card) . The mobile terminal is nothing but a

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handset, which performs such functions as voice coding, channel coding, information

encryption, information modulation and demodulation, information transmission and

receiving.

In addition, there should also be operation and maintenance sub-system on the GSM

network.

Also including the operation and maintenance subsystem (OMC) , the GSM system

manages and monitors the entire GSM network. With OMC, it implements the

functions like monitoring, status reporting and fault diagnosis of all the component

functions within the GSM network.

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1.4 GSM Interface

The position of ZXG10-MB in the GSM digital mobile communication network is as

shown in Fig. 1.4-2.

BSCn

BTS

MSC/VLR

SGSN

SMC

HLR/AUC

EIR

MS

Other PLMNs

GGSNGGSN

PDN TE

PSTNMSC/VLR

Abis interface

BTS

MS

Um

interface

Abis interface

BSC1

A

interface

Gb

interface

Fig. 1.4-2 Position of the System in the Network

Provides a bridge between the fixed part and the wireless part in the GSM network,

connects the mobile station for communications directly via the radio interface (Um)

and connects the mobile switch at the network side.

The primary interfaces that BSS has in the GSM system are as follows: A interface that

connects BSC to MSC; Gb interface that connects BSC to SGSN; Abis interface that

connects BSC to BTS; Ox interface that connects BSC to OMC-R and Um interface

that connects BTS to MS; Ater interface between BSC and TC when TC is remote,

using the sub-multiplexing unit.

1.4.1 A Interfaces

The interface between BSC and MSC is called A interface. Specifically speaking, A

interface is the interface between TC and MSC.

The code pattern converter TC in the GSM system is designed mainly to perform voice

conversion between voice code and 64kbit/s A law PCM code in addition to adaptation

processing of data rates in the circuit-type data services. TC can be either placed on the

BSC side or the MSC side. In a typical implementation scheme, TC is located between

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MSC and BSC.

Using the E1 interface, A interface establishes connection via two means: 75 coaxial-cable or 120 twisted pair line.

At the A interface, the data link layer uses MTP2 protocol, the network layer MTP3

and SCCP protocols and the application layer BSSMAP.

1.4.2 Abis interface

The interface between BSC and BTS is called Abis interface. BSC connects with BTS

via the Abis interface with its two sides equipped with BS interface equipment.

Abis interface, a self-defined interface inside BSS, uses the E1 interface and

establishes connection via two means: 75 or 120 twisted pair line.

At the Abis interface, the data link layer uses the LapD protocol. There are application

protocols like RR in the upper layer.

1.4.3 The Gb interface

The interface between BSC and SGSN is called Gb interface (frame relay) , with which

BSC connects with SGSN.

BSC connects to SGSN via the E1 line, at the access rate of N64kbit/s (1N32) or

2048kbit/s. The timeslot and bandwidth as used on the E1 line are specified by the

operator.

At Gb interface, BSC is designed mainly to implement RLC/MAC protocol, NS

protocol and BSSGP protocol.

1.4.4 QX interface:

The interface between BSC and OMC (background operation & maintenance center) is

called Qx interface, which allows for operation & maintenance command input and

system maintenance information output.

The Qx interface supports the following kinds of connections: connection via X.25

dedicated line, connection via the public packet switched network (PSPDN) , half-

permanent connection from BSC to OMC via A interface circuit and connection via the

Ethernet interface.

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1.4.5 Um interface

The Um interface, an air interface, is a communication interface between MS and BTS.

First, it achieves compatibility of the mobile stations of various manufacturers with the

networks of different operators, thus making possible the roaming of mobile stations.

Second, the creation of it addresses the spectrum efficiency of the cellular system with

the introduction of some anti-jamming technologies and measures to reduce

interference. The Um interface establishes the physical connection, namely the wireless

link, from MS to the fixed part of the GSM system. Beyond that, it is responsible for

transmitting information regarding wireless resource management, mobility

management and connection management.

ItemsInterface

So interfaces represent joints between two entities and protocols are rules for information exchange

joints.

1.5 Structure of the Regions Covered by GSM Wirelessly

In the GSM system, due to the mobility of users, the location information is a verycrucial parameter, which can be presented in the way as shown Fig. 1.5-3.

GSM service area (all member countries)

PLMN service area (one or several for each country)

MSC service area (area controlled by one MSC)

Location area (location and paging area)

Cell (area covered by a specific BTS)

Fig. 1.5-3 Relations between GSM Cells

The smallest indivisible area of the GSM network is a BS (omni antenna) or the area

covered by the fan antenna of a BS, also known as cell.

Several cells make up an LA, the division of which is set by the network operator. One

LA might have bearing to one or several BSCs, but it belongs to one MSC only. The

information of LA is stored in the MSC/VLR of the system, which identifies the LA

with the LAI.

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One MSC service area that is the common area covered by all the cells it controls can

be comprised of one or several LAIs.

The service area of PLMN (Public Land Mobile Network) consists of one or several

MSC service areas. There is one or several service areas for each country. For instance,

in our country, China Mobiles national GSM mobile communication network is

represented with the network number 00; China Unicoms national GSM mobile

communication network is represented with the network number 01.

The service area of GSM is made up of PLMNs of the countries in the world.

ItemsLA Location Area

To confirm the position of the mobile station, the region covered by each GSM PLMN is divided into

several LAs. One LA can contain one or several cells. The network will store the LA of each mobile

station as location information for paging the mobile station. The paging of the mobile station is made

by paging all the cells in the location area where the mobile station is located . In the planning of the

network, it is paramount to divide location areas. In the division of location areas, on the premise that

no excessively high call load occurs, try to minimize the number of location updates.

When the mobile station changes its location area, it conducts registration when finding there is

difference between the LAI stored in the memory and the LAI it receives. This process is called

location update, which is initiated by the mobile station.

PLMN

PLMN (public land mobile communication network) is defined as a network deployed and run by an

operator and delivering mobile communication services to the public. The global GSM system is

comprised of several PLMNs. Currently, there are two PLMNs in our country: China Mobiles PLMN

and China Unicoms PLMN.

1.6 Description of GSM Wireless Channel

In the GSM system, channels are categorized into logical channel and physical

channel.

Timeslot is the basic physical channel, one carrier frequency containing 8 physical

channels, which support logical channels.

Logical channels are divided by function into transaction channel (TCH) and control

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channel (CCH) .

1.6.1 Channel Allocation and Frequency Multiplexing

1.6.1.1 Channel Allocation

1. Working frequency band

Currently, the GSM communication system can run on 900MHz, extended

900MHz and 1800MHz frequency bands. Some countries use the 1900MHz

frequency band.

1) f = 900 MHz

Uplinking (transmitted by MS and received by BS) frequency range:

890MHz~915MHz

Downlining (transmitted by BS and received by MS) frequency range:

935MHz~960MHz

2) Extended 900MHz frequency band


880 MHz~915 MHz


925 MHz~960 MHz

3) f = 1800 MHz


1710MHz~1785MHz


1805MHz~1880MHz

4) f = 1900 MHz


1850MHz~1910MHz


1930MHz~1990MHz

2. Channel interval

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The interval of the two adjacent channels of all frequency bands is 200kHz.

3. Channel configuration

Based on the interval channel configuration approach.

1) f = 900 MHz

The channel serial numbers are 1~124, totaling 124 frequency points.

The relation between the channel serial number and the frequency point nominal

central frequency is:

Fun=8900.2nMHzupstream

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

Where 1n124, n is channel frequency number or absolute RF channel number

ARFCN.


The channel serial numbers are 0~124 and 975~1023, totaling 174 frequency

points.


central frequency is

Fun=8900.2nMHz0n124

Fun=8900.2n1024MHz975n1023

Fdn=Fun45MHz 3) f = 1800 MHz




Fu (n) =1710.2+0.2 (n-512) (MHz)

Fdn=Fun95MHz 512n885

4) f =1900 MHz


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Fun=1850.20.2n512MHz

Fdn=Fun80MHz 512n811

4. Interval between transmitting and receiving:

1) f = 900 MHz

The interval of duplex transceiving frequency is 45MHz.



3) f = 1800 MHz


4) f = 1900 MHz


1.6.1.2 Frequency multiplexing

Currently, the way the GSM achieves coverage is by the cell system, namely, the

cellular system, which divides the whole service area of GSM network into several

cells, each of which is equipped with a BS responsible for liaison and control of mobile

communication of the local cell. In addition, it establishes communication between

inter-cell mobile users and local call users under the unified control of MSC.

In the cellular system, in the situation where the capacity of the system is expanded by

means of frequency multiplexing, that is, the interval between cells is far enough (the

interference signal does not impact the receiving of useful signals) , the same

frequency can be used. Normally, the available N channels can be divided into F

groups, allocated sequentially to adjacent cells as shown in Fig. 1.6-4, where the

number of channels for each cell is N/F. If the omni antenna is used, a BS (O in the

diagram) , called Type O site, is usually set in the center of each cell. If the directional

fan antenna is used, a BS (S in the diagram) , called type S site, is usually set in the

cross point of three cells. This site covers three adjacent cells.

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B

G

A

C

D F

O

S

E

Fig. 1.6-4 Diagram of Frequency Multiplexing in Cellular Cell

4/12 and 3/9 frequency multiplexing modes are usually used in the ZXG10 system. The

4/12 multiplexing mode divides frequencies into 12 groups and allocates by turn them

into 4 sites (A, B, C and D) , each of which can use 3 frequency groups. The

composition of cells based on 4/12 multiplexing is shown in Fig. 1.6-5.

A 3

D 2B 1

C 3

B 2D 1

D 3

A 2C 1

B 3

C 2A 1

B 3

C 2A 1

A 3

A 1B 1

D 1

D 3D 2

C 3

B 2A 1

C 3D 2

C 3

C 1

D 2B 1C 2A 1

A 2C 1

D 3

A

B

C

D

Fig. 1.6-5 Diagram of Cells Multiplexed by 4/12

In the case of 3/9 multiplexing, that is, dividing limited frequencies into 9 groups and

allocating them by turn into 3 sites (A, B and C) , each of which can use 3 frequency

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groups, as shown in Fig. 1.6-6.

A 3

C 2B 1

B 3

A 2C 1

C 3

B 2A 1

A 3

C 2B 1

B 3

A 2C 1

B 3

A 1C 1

A 1

A 3A 2

C 3

B 2A 1

A 3A 3

C 3

C 1

B 2A 1B 2A 1

A 2C 1

B 3

A

B

C

Fig. 1.6-6 Diagram of Cells Multiplexed by 3/9

As shown from the above two frequency multiplexing modes, as frequency

multiplexing increases in density, that is to say, decreases in frequency groups, the

frequency utilization ratio and the number of users increase, but the interval of

frequency multiplexing decreases. In the meantime, there also arises the interference

between cells. For example, the carrier-to-interference ratio C/I (the interference of

another cell with the service area when different cells use the same frequency) , and

C/A (the interference of the adjacent channels with the channels used by the service

area in the frequency multiplexing mode) decrease.

At the time of frequency multiplexing, priority is given to C/I and C/A. In the GSM

system, it is generally required that C/I9dB and C/A-9dB.

After the frequency multiplexing relation is defined; in other words, the frequency

group N that needs to be divided is defined, for the 4/12 frequency multiplexing mode,

N=12; for the 3/9 multiplexing mode, N=9.

1.6.2 TS1 number

The multiple address technology is a kind of technology with which several users share

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a channel. There are several modes: frequency division multiple address (FDMA) ,

time division multiple address TDMA and code division multipleaddressCDMA.

In the GSM system, the air interface is based on the mixed mode of FDMA and

TDMA. The interval of the carrier wave channel is 200KHz. Each carrier wave is

divided by time into 8 timeslots, each of which is a channel, occupying 15/26ms (about

577s) , so one carrier wave can be used concurrently by a maximum of 8 mobile

users.

The description of one channel in the GSM system in time domain and frequency

domain is shown in Fig. 1.6-7

Frequency

15/26ms

200kHz

0 1 2 3 4 5 6 7 8Time

Timeslot

Fig. 1.6-7 Timeslots in Time Domain and Frequency Domain

1.6.3 TDMA Frame

In the GSM system, there are 8 timeslots for each carrier frequency. The eight adjacent

timeslots make up a basic unit, called TDMA frame. Several TDMA frames make up a

multi-frame, as shown in Fig. 1.6-8.

There are two kinds of multi-frame in the GSM circuit service: multi-frame of 26

frame and multi-frame of 51 frame.

Multi-frame of 26 frame: includes 26 TDMA frames with a period of 120ms, designed

forservice channel and associated control channel.

Multi-frame of 51 frame: includes 51 TDMA frames with a period of 3060/13ms

(approximately 235ms) , designed forcontrol channels.

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0

0

0

0

0

0

1

1

1

1

1

1

2

2

2

2046 2047

24 25

24 25

49

49

50

50

48

3 4 5 6 7

1Super high frame=2048Super frame=2715648 TDMA frame

TCH

SACCH/T

FACCH

BCCH

CCCH

SDCCH

1Super frame=1326 TDMA frame

1multi-frame=26TDMA

frame=120ms

1multi-frame=51TDMA

frame=(3060/13 ms)

1TDMA

frame=8BP=120/26ms

Fig. 1.6-8 Software structure

Several multi-frames make up a super frame, which is a continuous 5126 TDMA

frame, with a period of 1326 TDMA frames, namely, 6.12s.

Comprised of 2048 super frames, the super high frame has a period of 12533.76s. Each

period of the super high frame contains 2715648 TDMA frames, namely, TDMA frame

number (FN) ranging from 0 to 2715647.

Compared with the 26 multi-frame structure or 51 multi-frame structure of the circuit

service, GPRS introduces the multi-frame structure comprised of 52 TDMA frames.

The mapping of the logical channels on all the packet data channel (PDCH) is based on

this kind of multi-frame structure as shown in Fig. 1.6-9.

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B0 B1 B2 B3T TB4 IB5 B6 B7 B8 B9 B10 B11 I

52 multi-frame

B0~B11:BLOCK; T:For PTCCH frame; I:Idle frame

Fig. 1.6-9 Software structure

The multi-frame structure of PDCH contains 12 BLOCKs, each of which is composed

of 4 continuous TDMA frames. In addition, there are two idle frames and two TDMA

frames used for PTCCH (packet time advance control channel) , totaling 52 TDMA

frames.

In the packet service, except PRACH (packet random access channel) and PTCCH/U,

the basic composition unit of other packet logical channels is BLOCK.

In one 52 multi-frame, the sequence of 12 BLOCK occupation is defined in this way:

B0, B6, B3, B9, B1, B7, B4, B10, B2, B8, B5, B11.

1.6.4 Channel of Circuit Service

1.6.4.1 Traffic Channel

The service channel that bears code voice or user data distinguishes full-rate service

channel (TCH/F) and half-rate service channel (TCH/H) :

1. Voice Services

TCH/F: full rate voice service channel, with a total rate of22.8kb/s

TCH/H: half-rate voice service channel, with a rate of11.4kb/s

2. Data services

TCH/F9.6: 9.6 kbit/s full-rate data traffic channel

TCH/F4.8: 4.8kbit/s full-rate data service channel

TCH/H4.8: 4.8kbit/s half-rate data service channel

TCH/F2.4: 2.4 kbit/s full-rate data traffic channel

TCH/H2.4: 2.4kbit/s half-rate data service channel

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1.6.4.2 CCH

The control channel is designed to bear signaling or synchronous data. There are three

kinds of control channel: broadcast channel, common control channel and dedicated

control channel.

1. The broadcast channel

The broadcast channel is a one-point-to-multiple-point one-way downstream

control channel, which is a one-way transmission from BS to mobile station. It

is designed to broadcast all kinds of messages to MS. There are three kinds of

channel:

1) FCCH: frequency calibration channel, which bears the information for MS

frequency correction.

2) SCH: synchronous channel, bearing the ID information about MS frame

synchronization and BS transceiving station (BTS) .

3) BCCH: broadcast control channel, designed to send cell information.There is

always a transceiver in each BTS containing this channel to broadcast system

information to all the mobile stations of the cell.

2. Common Control Channel

The common control channel is a one-point-to-multiple-point two-way control

channel, shared by MS on the network. There are three kinds of common control

channel:

1) PCH: paging channel, designed to help BTS page MS (downstream channel) .

2) RACH: random access channel, designed to help MS submit network entry

request randomly; that is, to request the allocation of dedicated control channels

(upstream channels) .

3) AGCH: agreed access channel, designed to help BTS to respond to the random

access request; that is, to allocate a dedicated control channel or allocate directly

a TCH (downstream channel) .

3. Dedicated Control Channel

The dedicated control channel is a point-to-point two-way control channel. At

the time of service, BTS allocates it to MS for point-to-point transmission

between BTS and MS.

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1) SDCCH: standalone dedicated control channel, designed to transport

information like channel allocation. There are kinds of SDCCH as follows:

SDCCH/8:Specified Control Channel

SDCCH/4:standalone dedicated control channel combined with BCCH/CCCH.

2) SACCH: slow associated control channel, used together with a service channel

or a SDCCH and designed to transport some specific information in the middle

of user information transmission, such as power and frame adjustment control

information, measurement data, etc. There are kinds of channel as follows:

SACCH/TF: slow associated control channel associated with TCH/F

SACCH/TH: slow associated control channel associated with TCH/H

SACCH/C4: slow associated control channel associated with SDCCH/4

SACCH/C8: slow associated control channel associated with SDCCH/8

3) FACCH: fast associated control channel, used together with a service channel

and bearing the same signals as SDCCH,but it allocates FACCH and establishes

connection through the frames (called stolen frames) via the service channel

and transports commands like cross-cell switchover only when SDCCH is not

allocated. There are kinds of FACCH as follows:

FACCH/F: full-rate fast associated control channel

FACCH/H: half-rate fast associated control channel

Normally, TCH/F and SACCH are allocated in pairs. The combination of TCH/F

and SACCH is presented as TACH/F.

1.6.4.3 Channel Combination

In actual applications, the logical channels of different types are always mapped to the

same physical channel. This is called channel combination.

There are 9 kinds of channel combination types:

1. TCHFull TCH/F+FACCH/F+SACCH/TF full rate service channel

2. TCHHalf TCH/H+FACCH/H+SACCH/TH half-rate service channel

3. TCHHalf2 TCH/H+FACCH/H+SACCH/TH+TCH/H half-rate 1 service

channel

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4. Specified Control Channel

5. MainBCCH FCCH+SCH+BCCH+CCCH main broadcast control channel

6. BCCHCombined FCCH+SCH+BCCH+CCCH+SDCCH+SACCH combined

broadcast control channel

7. BCH FCCH+SCH+BCCH broadcast channel

8. BCCHwithCBCH FCCH+SCH+BCCH+CCCH+SDCCH+SACCH+CBCH cell

broadcast channel

9. Slow dedicated control channel

Where,

1) CCCH=PCH+RACH++AGCH.

2) CBCH: Only the downstream channel bears the cell broadcast information,

sharing the same physical channel with SDCCH.

Each honeycomb broadcasts one FCCH and one SCH. Its basic combination contains

one FCCH, one SCH, one BCCH and one CCCH (PCH+AGCH) in the downstream

direction and allocates them strictly to TN0 of BCCH carrier frequency configured for

the cell as shown in Fig. 1.6-10.

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 frame

SF C C SF C C SF C C I

R R R R R R R R R R

D0 D1 D2 D3 D4 D5 D6 D7 A0 A1 A2 A3

SF C C

R R R R R R R R R R

III

D0 D1 D2 D3 D4 D5 D6 D7 A4 A5 A6 A7 III

A1 A2 A3 III

A5 A6 A7 III

D0 D1 D2 D3 D4 D5 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 SF D0 D1 SF D2 D3 ISF A2 A3

D3

D3

R R

R R

A2 A3

A0 A1

D2

D2

SF

SF

D0 D1

D0 D1

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

FFCCH SSCH

BBCCH CCCCHCCCH=PCH+AGCH+RACH

RRACH DSDCCH

ASACCH/C Iidle

BCCH+CCCH

(Downlink)

BCCH+CCCH

8 SDCCH/8

8 SDCCH/8

BCCH+CCCH+4SD

CCH/4

BCCH+CCCH+4SD

CCH/4

(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)

(Upwnlink)

(Downlink)

(Upwnlink)

(Upwnlink)

(Downlink)

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Chapter

Fig. 1.6-10 Diagram of Channel Structure of 51 Frame

1.6.4.4 Arrangement of Channels in the Honeycomb

Given below are some examples of channel combination in a honeycomb (what is

shown in the brackets are sub-channels) .

1. The combination of channels for the small capacity honeycomb with only one

TRX

1) FCCH + SCH + BCCH + CCCH + SDCCH/4 (0.3) + SACCH/C4 (0.3)

2) TCH/F + FACCH/F + SACCH/TF

2. Mid-size honeycomb with 4 TRXs

1) 1 TN0 group: FCCH+SCH+BCCH+CCCH

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


3. High-capacity honeycomb with 12 TRXs

1) 1 TN0 group: FCCH+SCH+BCCH+CCCH

2) 1 TN2 group, one TN4 group and 1 TN6 group: BCCH+CCCH

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


1.6.5 Channels of Packet Service

The packet logical channels are divided by function into packet data transmission

channel (PDTCH) and packet control channel (PCCH).

1.6.5.1 Packet Data Transmission Channel

Unlike the circuit switching (CS) services, all the PDTCHs in the packet services are

one way. In other words, the upstream link is independent of the downstream link.

The packet data transmission channels include packet data transmission channel

PDTCH/U (upstream) and packet data transmission channel PDTCH/D (downstream) .

PDTCH bears user data. It is allocated temporarily to a specific MS or a group of MSs.

In a multi-timeslot mode, one MS can use as many as 8 PDTCHs at the same time.

PDTCH/U is designed to help MS to send packet data to the network; PDTCH/D is

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designed to help MS to receive packet data from the network.

1.6.5.2 Calling Control Channel

The packet control channel is shown in Table 1.6-1.

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Chapter

Table 1.6-1 Packet Control Channel

Name Classification

Calling Control

Channel

Reverse common control

channel

packet random access channel PRACH

(upstream)

packet paging channel PPCH (downstream)

packet access agreed channel PAGCH

(downstream)

packet notice channel PNCH (downstream)

packet broadcast control

channel PBCCH

packet broadcast control channel PBCCH

(downstream)

Dedicated Control Channel

Packet Associated Control Channel PACCH

packet time advance control upstream

channel PTCCH/U (upstream)

packet time advance control downstream

channel PTCCH/D (downstream)

1. Reverse common control channel

1) PRACH is designed to send packet access burst pulse and extended access burst

pulse or respond to the paging from BSS.

2) PPCH is designed either to page CS services or GPRS services, but the paging

of CS services is applicable only for MS Grade A and Grade B. PPCH that also

uses the paging group can support DRX.

3) Before MS sends the packet, PAGCH serves to allocate one or several PDTCHs

to MS so as to implement the transmission of the packet. When MS already

works in the packet transmission mode, the allocated resources can also be

transmitted in PACCH.

4) PNCH is designed to notify the mobile station the call from PTM-M. To monitor

PNCH, there must be DRX mode.

2. Packet Broadcast Control Channel PBCCH

PSI (packet system information) on PBCCH. The parameters borne by these

PSIs determine the mapping of all the channels in multi-frames.

If no PBCCH is allocated, these PSIs can also be sent on BCCH, which will

indicate clearly whether the local cell supports packet data services. If they are

supported and PBCCH exists, the combination configuration information on

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Chapter

PBCCH will be given.

3. Dedicated Control Channel

1) PACCH transmits signaling information regarding confirmation, power control,

etc. as well as bearing resources allocation and re-allocation information, which

can be used for the capacity allocation of PDTCH or newly-added PACCH in

the future. When an MS performs packet transmission, it can resume the circuit

switch mode by means of PACCH paging. PACCH is allocated dynamically to

the physical channel of PDTCH. It is a two-way channel.

2) PTCCH/U is designed to transport random access burst pulse and estimate the

time advance of an MS in a packet transmission mode.

The period of PTCCH/U is 8 52 multi-frames, including 16 PTCCH/U sub-

channels. The PTCCH/U sub-channel number that each MS has is determined

by the TAI (time advance index) that MS acquires from resources allocation.

The mapping of PTCCH/U on the physical channel is shown in Fig. 1-6.

3) PTCCH/D is designed to amend the time advance of several MSs.

One PTCCH/D corresponds to several PTCCH/Us.

PTCCH/D interweaves in 4 Bursts.

The mapping of PTCCH/D on the physical channel is shown in Fig. 1.6-11.

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Chapter

B 0 ~ B 1 1 = R a d i o b l o c k s

I d l e f r a m e s a r e n u m b e r e d f r o m 1 t o 3 1 [ o d d n u m b e r s ]

P T C C H f r a m e s a r e n u m b e r e d f r o m 0 t o 3 0 [ e v e n n u m b e r s ]

5 2 - m u l t i f r a m e n u m b e r n + 7 :

u p l i n k T A I = 1 4 T A I = 1 5

d o w n l i n k T A - m e s s a g e 4 T A - m e s s a g e 4

B 0 B 1 B 2 B 3 B 4 B 5 B 6 B 7 B 8 B 9 B 1 0 B 1 1 3 13 02 92 8

B 0 B 1 B 2 2 4 B 3 B 4 B 5 2 5 B 6 B 7 B 8 2 6 B 9 B 1 0 B 1 1 2 7


u p l i n k T A I = 1 2 T A I = 1 3


B 0 B 1 B 2 2 0 B 3 B 4 B 5 2 1 B 6 B 7 B 8 2 2 B 9 B 1 0 B 1 1 2 3


u p l i n k T A I = 1 0 T A I = 1 1


B 0 B 1 B 2 1 6 B 3 B 4 B 5 1 7 B 6 B 7 1 8 B 9 B 1 0 B 1 1 1 9


u p l i n k T A I = 8 T A I = 9


B 8


B 0 B 1 B 2 0 B 3 B 4 B 5 1 B 6 B 7 B 8 2 B 9 B 1 0 B 1 1 3

5 2 - m u l t i f r a m e n u m b e r n :

u p l i n k T A I = 0 T A I = 1

B 0 B 1 B 2 4 B 3 B 4 B 5 5 B 6 B 7 B 8 6 B 9 B 1 0 B 1 1 7


u p l i n k T A I = 2 T A I = 3



u p l i n k T A I = 4 T A I = 5


B 1B 0 B 2 8 B 3 B 4 B 69B 5 B 7 B 8 1 0 B 9 B 1 0 B 1 1 1 1

B 0 B 1 B 2 1 2 B 3 B 4 B 5 1 3 B 6 B 7 B 8 1 4 B 9 B 1 0 B 1 1 1 5


u p l i n k T A I = 6 T A I = 7


Fig. 1.6-11 Mapping of PTCCH on Physical Channel

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Chapter

1.6.5.3 Logic Channel

GPRS supports three newly added logical channel combinations:

1. PBCCH+PCCCH+PDTCH+PACCH+PTCCH

2. PCCCH+PDTCH+PACCH+PTCCH

3. PDTCH+PACCH+PTCCH

where PCCCH=PPCH+PRACH+PAGCH+PNCH.

All the packet logical channels are mapped to a physical channel (PDCH) .

The sharing of physical channels is done by Block. In other words, the type of logical

channel where each BLOCK belongs on one PDCH might change gradually. The

channel type is the message type ID that the BLOCK head contains, (except PRACH) .

For each PDCH allocated to MS, MS will be allocated with an upstream status flag

USF.

The network controls the multiplexing of the wireless blocks of several MSs on the

upstream PDCH with the upstream status flag USF.

USF is located in the header of each downstream wireless block, pointing to the next

upstream link wireless block.

When MS finds its own USF in the header of PDCHs downstream block, MS will be

able to use BX+1 (when X11) or B0 (when X=11) upstream block on the PDCH. If

permitted by the network, MS will also be able to use three closely following

BLOCKs.

In the direction of downstream, MS interprets each downstream BLOCK on the

allocated PDCH and determines whether this BLOCK belongs to itself depending on

TFI (which is an identifier allocated by the network to MS) .

1.7 Process of Processing Voice Signals of Wireless Interface **

In the GSM system, the process of processing voice signals of wireless interface by the

mobile station is shown in Fig. 1.7-12.

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Chapter

A /D

D /A

Voice

Coding

260b it/20m s 456b it/20m s 3 3 .8 kb i t/s 2 70 kb i t/s

Channel

Coding Interlacin g EncryptingBurst pulse

generationModulation

Voice

decodingChannel

decoding

De-

interlacingDecryption Burst pulse

decomposition Demodulation

Fig. 1.7-12 General Process of Processing Voices in GSM

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. To

control 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.

1.7.1 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.

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

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Chapter

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.

1.7.2 Discontinuous Transmission

There are two kinds of voice transmission: one is that the voice is being continuously

coded (one voice frame every 20ms) regardless of whether the user is talking or not;

the other is DTX (Discontinuous Transmission) . 13kb/s coding in voice-activated

period and 500b/s coding in voice-inactivated period. A comfort noise frame (every

frame 20ms) is transmitted every 480ms, as shown in Fig. 1.7-13.

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. The DTX

mode and general mode are optional because the DTX mode will cause a slight decline

in the transmission quality.

TRAU BTS

BTS MS

Comfort noise frame

480

ms

Voice frame

Fig. 1.7-13 Transmission of Voice Frames in DTX Mode

1.7.3 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

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Chapter

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 modes 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 codesincreases 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

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

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.

1.7.4 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 mode. To better address the issue of error codes, the

interlacing technology is introduced to the system.

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.

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, there are different coding and interlacing modes for varied channel types

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Chapter

as shown in Table 1-3 and Table 1-4.

1.7.5 Encryption/decryption.

There are encryption measures available in the GSM system. This kind of encryption

can be applied to voices, data and signaling, but it has no bearing to data types. It is

limited only to conventional burst pulses. Encryption can be done through an

encryption sequence (generated with encryption secret key Kc and frame number

through A5 encryption algorithm) and exclusive-or operation of 114 information bits in

the conventional burst pulse.

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.

1.7.6 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.

1.7.7 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. Space

diversity is achieved with field strength following the random changes of the

space. The farther the space, the more diversified the multi-path cast will be and

the smaller the relativity of the received field strength. By relativity it refers to

the degree of similarity between signals, so it is imperative to specify necessary

space range. According to the test and statistics, CCIR suggests the spacing

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Chapter

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

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

number multiplication of /4. If the spacing of antennas is reduced, it can also

produce a fairly good diversity result even if it is as small as /4.

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.

3. Frequency diversity

Frequency diversity means more than two frequencies send a signal

concurrently. The receiving end combines the signals of different frequencies

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

wireless carrier waves of varied frequencies. This approach generates quite good

efficiency, and only one pair of is required. In the GSM system, frequency

diversity is achieved with the 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. Polarization diversity can be achieved by

integrating two pairs of receiving antennas into one pair of antennas. In this

case, only one pair of transmitting antennas and one receiving antenna are

required in a cell. If the deplexer is used, only one pair of transceiving antennas

is required, which can greatly reduce the number of antennas.

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Chapter

1.7.8 Frequency Hopping Technology

In the digital mobile communication system, to enhance the anti-jamming capability of

the system, the spread spectrum technology is usually introduced. There are two

modes: direct spread mode and frequency hopping mode, which is used by the GSM

system.

There are two reasons for introducing frequency hopping. The first is based on the

principles of frequency diversity to fight Rayleigh fading. The mobile wireless

transmission will unavoidably undergoing a short-term amplitude change when

meeting with an obstacle. This change is called Rayleigh fading. Varied frequencies

differ in the fading they meet with. Additionally, as the frequency difference increases,

fading becomes more independent. Through frequency hopping, the burst pulse will

not be damaged by Rayleigh fading in the same way. The second is based on

interference source features. In the service intensive area, the cellular system is

susceptible to be limited by the interference generated by frequency multiplexing. The

relative carrier-to-interference ratio (C/I) shows the biggest change in calls. C depends

on the position of the mobile station relative to the BS. I determines whether the

frequency is used by the adjacent cell. The introduction of frequency hopping enables it

to disperse interference among many calls that might interfere with the cell rather than

concentrate it on a call.

Frequency hopping means hopping changes of the carrier wave frequency at a very

wide band range in a specific sequence. The control and information become baseband

signals after they are modulated and then sent to carrier wave modulation. The carrier

wave frequency changes the frequency under the control of pseudo-random code,

which is what we call frequency hopping sequence. In the end, the data are sent to the

antenna for transmission via the RF filter. The receiver determines the receiving

frequency according to the frequency hopping synchronous signals and frequency

hopping sequence and receives the pertinent signals which are subjected to frequency

hopping for demodulation. The basic ISDN architecture is shown in Fig. 1.7-14.

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Chapter

Send

Information

modulation

Upper

frequencyconversion

Synchronous

circuit

Frequency

hopping serial

generator

Changeable

frequency

combiner

ReceiveInformation

demodulation

Lower

frequency

conversion

Fig. 1.7-14 Basic Structural Diagram of Frequency Hopping

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 that the times of

frequency hopping are increased is that the gains of the frequency hopping system are

equal to the ratio between the band width of the frequency hopping system and N

minimum frequency hopping intervals. Therefore, more frequency hopping can

increase frequency hopping gains. The normal times of frequency hopping shall be less

than 3. If frequency diversity is added to the frequency hopping system and a message

is transmitted simultaneously with several groups of jump frequencies before it is

judged more effectively with the law of large numbers, more users will be able to work

concurrently while minimizing interference with each other.

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Chapter

2 Introduction to GPRS

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Chapter

2.1 Definition and Features of GPRS

2.1.1 Definition of GPRSGPRS: General Packet Radio Service.

GPRS is a packet data service introduced to GSM Phase 2+ to provide

users with end-to-end mobile data service based on packet switching and

transmission technologies. It can effectively utilize radio resources and

terrestrial resources of the network and is especially suitable for bursty data

services of long duration and small traffic.

2.1.2 Features of GPRS

High rate: The transmission rates of GPRS CS-1, CS-2, CS-3 and CS-

4 are 9.05 kb/s, 13.4 kb/s, 15.6 kb/s and21.4 kb/s in turn by virtual of

the multi-slot binding and high-speed coding scheme. Therefore, the

information transmission rate of GPRS can range from 9.05 kb/s of a

channel to 8*21.4 = 171.2 kb/s of eight channels. When adopting CS1

and CS2, it can provide the access rate as high as 8*13.4 = 115 Kbps;

and when adopting CS3 and CS4, its rate can be raised to 171 Kbps.

Always online and traffic charging. GPRS provides the performance of

Access anywhere and always on line and offers totally new means for

mobile subscribers to access the Internet and Intranets at high speed.

In addition, it provides the ideal time for mobile network operators to

proceed to the Internet access service and play the role of ISPs. Once

the GPRS terminal is powered on and connects the GPRS network, it

can always keep online for the subscriber to receive and send

information at any time without needing the dialup procedure as in the

CS mode. The GPRS terminal will not occupy network resources and

radio resources unless it is transmitting data and therefore the mobile

data subscriber can gain benefits from traffic charging: The subscriber

can be online for a long time without incurring high-premium bills and

the GPRS network is indeed an affordable high-speed packet network.

Mature technologies. GPRS provides data service solutions based on

the existing mature GSM technology and the network. The investment

is small but the effect is obvious.

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Chapter

2.1.3 Types of Services Borne by GPRS

The GPRS network provides bearer data services for mobile subscribers

including point-to-point (PTP) data service and point-to-multipoint (PTM)

data service. It also supports supplementary services and SMS.

I PTP ConnectionLess Network Service (PTP-CLNS)

The PTP-CLNS is a kind of datagram service. Data packets are

independent of one another, no end-to-end call setup procedure is

needed for information transmission between subscribers, each data

packet sent contains the destination address and the network selects

the route according to the destination address indicated in each packet

and the current network topology structure. There is no logic

connection for packet transmission and no acknowledgment protection

for packet delivery. The PTP-CLNS supports bursty non-interactive

application service and is a service supported by the IP protocol.

II PTP Connection-Oriented Network Service (PTP-CONS)

The PTP-CONS is a kind of virtual circuit service. It establishes logic

virtual channels (PVCs or SVCs) between two or more users to

transmit multiple channels of data packets. It requires the connection

establishment, data transmission and connection release procedures.

In the PLMN, when one mobile subscriber moves from one cell to

another cell, GPRS will keep the directional virtual circuit connection.

When the radio link fails, the virtual circuit must be cleared. Each

mobile subscriber can be allocated with a fixed address or dynamic

address and the data services use the gateway to indicate the address

of a data packet. In the home service area, each subscriber is

assigned a fixed address and can roam in its home service area. The

dynamic address is an address allocated by the gateway to the access

subscriber and is an address temporarily allocated during the

connection period to offer roaming service to the access subscriber.

The PTP-CONS supports emergency handling and interactive

application services and it is a service supported by the connection-

oriented network protocol CONP such as X.25. It improves the

reliability with the acknowledgement mode at the radio interface.

III Point-To-Multipoint data service (PTM)

The PTM service provided by GPRS can send information to multiple

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Chapter

subscribers according to the request of a certain service requester. It

can be further divided into PTM multi-channel broadcast service (PTM-

M), PTM group call service (PTM-G) and IP broadcast service (IP-M).

IV Other services

These are GPRS supplementary services, GSM SMS, anonymous

access service and GPRS telecommunication services.

In Phase I, GPRS supports the first two services if it does not adopt the

GS interface.

In addition, GPRS can provide numerous powerful network application

services based on the GPRS bearer service, for example:

1. Internet access

2. Wireless POS

3. Vehicle location (GPRS + GPS)

4. Stock and exchange rate information transmission

5. Telesensing, telemetry and telecontrol

6. Highway toll collection system

7. Enhanced SMS

8. Other data services

2.2 GPRS Network Structure

2.2.1 Composition of the GPRS System

The functions of GPRS are implemented by software upgrading. Only two

new network entities are added: The SGSN (Serving GPRS Support Node)

and the GGSN (Gateway GPRS Support Node). To support GPRS, the

GSM network is added with nine GPRS interfaces whose names begin with

G and thirteen new protocols. For details, refer to the logic structure

diagram of the GPRS network. Compared with the CS service, the SGSN in

the PS service is equivalent to the MSC in CS, responsible for sending the

packet data via the BSS wireless route to the terminal. The GGSN

connected to the external Packet Switched Packet Data Network (PSPDN)

is equivalent to the GMSC in the CS domain and it provides multiple

different interconnection interfaces to support the connection with packet

data networks such as IP, X.25 and private line networks. As viewed from

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the Internet in the IP world, the GGSN plays the role of a router.

Fig. 1 Logic Structure Diagram of the GPRS Network

To support GPRS, the existing GSM equipment should be changed as follows:

1) The relevant GPRS subscription information should be added to the

HLR.

2) The software of the MSC/VLR should be modified so as to support the

interoperability with the SGSN.

3) The charging system should be changed according to the GPRS features

so as to support the traffic charging function of packet data.4) Upgrade the BSS software: Keep the BTS hardware unchanged and add

multiple new protocols for the Um interface so as to support the GRR and GMM

functions; add the Packet Control Unit (PCU) to the BSS, so as to provide the frame

relay interface for the connection of the BSS with the SGSN, complete the base station

control function for the packet service and cooperate with the circuit control module on

the BSC to share radio resources. The PCU is generally embedded in the BSC as a

function module or it can be an independent PCU device if there is little redundant

processing capability of the BSC.

5) The mobile terminal equipment should be redesigned. By their ability to

support multiple time slots, CS service and PS service, they can be divided into multiple

levels and types.

6) The PS service-related data and the function to manage the new nodes

should be added to the OMC.

7) The SMSC should be added with the support for transmission of short

messages over packet channels.

2.2.2 Positions of the CCU and the PCU

In the BSS system, two function entities should be added to implement the

GPRS function: PCU (Packet Control Unit) and CCU (Channel Codec Unit).

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The PCU is located in the BSC and the CCU is located in the BTS. The PCU

provides the Gb interface to connect the SGSN while the CCU provides the

Um interface to connect the MS.

In the BSS, the PCU and the CCU may be distributed in one of the three

modes as depicted in the following figure: In Case A, the mode is similar to the

CDPD network and is seldom used in the GSM network; in Case B, the mode

considers both the Gb interface and the Abis interface, it is currently the mode

mostly applied by vendors and the PCU can be embedded in the BSC system

or can be an independent entity beside the BSC; in Case C, the PCU is

generally an independent entity externally placed beside the SGSN system but

CS data are transmitted between the BSC system and the PCU (similar to the

CS data provided via the MSC, the multi-the utilization rate of the trunk circuitbetween the BSC system and the SGSN system is not high though the multi-

slot capability improves the transmission rate).

BTS BSC site GSN site

BSC site GSN site

BSC site GSN site

CCU

PC UCCU

BTS

CCU

CCU

BTSCCU

CCU

Um

Abis

Gb

A

B

C

PC U

PC U

key: c ircuit switching function (16 or 64 Kbps)

packet switching function

Gb

Different modes of distributing the PCU and the CCU in the network

Fig. 2 Logic Structure Diagram of the GPRS Network

2.2.3 Solution of Implementation of the BTS in the GPRS Network

There are four coding schemes for GPRS data: CS-1 ~ CS-4, whose data

rates are 9.05 kb/s, 13.4 kb/s, 15.6 kb/s and 21.4 kb/s in turn.

As regards the implementation of the BTS in the GPRS network, both

vendors in China and foreign vendors take two steps:

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The first step is to implement CS-1 and CS-2 with uplink open loop power

control.

The second step is to implement CS-3 and CS-4 with uplink closed loop power

control and downlink power control.

BTSBSC

(PCU)2M linkSupporting CS-1

and CS-2

CS-1 and CS-2 seize one 16 kb/s link

Fig. 3 Implementation of CS-1 and CS-2 for the BTS in the First Phase

In the implementation solution in the first phase, the existing hardware platform

of the BTS does not need to be changed. Instead, only its software including

baseband control and baseband processing software needs to be changed. On

the Abis interface, CS-1 or CS-2 data can be encapsulated in a PCU frame and

take up one 16 kb/s link.

BTS

Supporting CS-1 ~ CS-4

BSC

(PCU)2M link

CS-1 and CS-2 seize one 16 kb/s link

CS-3 and CS-4 seize two16 kb/s links

Fig. 4 Implementation of CS-3 and CS-4 for the BTS in the Second Phase

On the Abis interface, CS-3 or CS-4 data need to take up two 16 kb/s

links and so they are implemented in the second phase.

gsm&gprs basic

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