1.GSM Basic Knowledge

7/27/2019 1.GSM Basic Knowledge

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Global System for Mobile

System Overview


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Evolution of Mobile

Communication Mobile communication existed half a

century ago, but it was in the 1980s that it

was really developed

The main goal of mobile communication is

to realize communication among any

objects at any time, and in any place.


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Evolution of Mobile

CommunicationLand cellular mobile communication

system (PLMN) has gone through 3 stages:

1. First Generation --Analog Mobile

Telephone System

2. Second Generation---Digital Mobile

Communication System

3. Third Generation---IMT-2000


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First Generation --Analog Mobile Telephone Syste

Adopts cellular networking technology

Currently practical, high capacity systems

worldwide include:1North-Americans AMPS2North Europeans NMT-450/9003Britains TACS Operating frequency bands locate around

450MHz and 900MHz with carrier spacing less

than 30 kHz


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Analog cellular mobile communication

system has following fatal weaknesses:

A) There is no public air interface between

various systems;

B) It can not adapt itself to the digitization of

fixed networks, and digital bearer servicesare hard to develop;


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C) It has a low frequency availability, thusunable to meet large capacity requirements;

D) It has a low degree of safety, thus easy tobe eavesdropped, and easy to be copiedwith false handsets.

Analog cellular mobile communicationsystem has been replaced by digitalcellular mobile communication in China.


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Second GenerationDigital

Mobile Communication System Due to the various defects of analog

systems such as TACS, mobile telephone

systems were developed in the 1990s

embodied by digital transmission, TDMA

and narrow-band CDMA, which are called

the second generation mobile telephonesystem.


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Types of TDMA Systems

The comparatively mature and most typical

system in the TDMA series are: the pan-

European GSM, American D-AMPS and

the Japanese PDC.

1DAMPS was formally launched into

commerical application in 1993 after the

American Electronic Industry Association(EIA) completed its technical

standardization in 1989


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Types of TDMA Systems

2The Japanese JDC (now renamed as PDC)

had its technical standards completed in

1990. It was put into service in 1993, but isrestricted to Japan only.

3The special mobile communication group

(SMG) of CEPT laid down the phase 1standard ofGSM in 1988, with the working

frequency band around 900MHz. It was

launched into commercial use in 1990.


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Main Features of GSM (1)

(1) Spectrum efficiency: due to the adoption ofhigh-efficiency modulator, channel coding,

interleaving, equalization and voice coding

technologies, the system has a high spectrum

efficiency.

(2) Capacity: due to the increase of the transmission

bandwidth of each channel, the requirement ofcofrequency reuse carrier-to-interference ratio is

lowered to 9dB,


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so the cofrequency reuse mode of the

GSM system can be narrowed down

to 4/12 or 3/9 or even less (for a

analog system, it is 7/21). GSM

system capacity is 3~5 times higher

than that of a TACS system.


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(3) due to digital transmission technology,GSM system has a strong anti-interference

capability compared with analog system,thus the voice quality is guaranteed.

(4) Interfaces openness: the open interfacesprovided by GSM standards refer not onlyto air interface(Um), but also A interfaceand, to some degree, Abis interface.


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(5) Security: security is guaranteed with

authentification, encryption and TMSI

(6) Interconnection with ISDN, PSTN, etc.:interconnections with other networks

normally use the existing standard interfaces,

such as ISUP or TUP.

(7) Roaming: roaming is realized on basis of

SIM cards.



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Summary of TDMA System

The common features of the above three types ofproducts are digitization/TDMA, good voicequality, good security, ability to transmit data, and

automatic roaming. Each of these 3 types of systems has its own

merits. The PDC system has a high spectrumavailability, and the DAMPS system has the

greatest capacity, while GSM is the most matureof all technologies. Besides, it is based on OSI,with open technical standards, thus have beenapplied on the greatest scale in the world..


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N-CDMA System

The NCDMA (narrow band code division

multiple access) system is a mobile

communication system developed mainly bycompanies leaded by the QuacommCo. on

basis of IS95. The specifications of North

American digital cellular systems are drawn

up by the American Telecom Industry

Association(TIA). This system has been

adopted by China Unicom in 2001


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Third GenerationIMT-2000 With the ever-lasting increase in subscribers

and the development of digital

communications, the second generationmobile telephone systems has gradually

exposed its shortcomings:

-Narrow bandwidth:unable to provide variousbroadband information services such as

high-speed data and television pictures.


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Third GenerationIMT-2000

With the development of sciences and

technologies as well as communication

services, what is needed is an

integrated service system that can

integrate all current mobile telephone

system functions and provide multiple

services


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Features of IMT-2000

Including multiple systems;

With a high degree of consistency in

worldwide design;

Compatibility between IMT-2000 services

and fixed networks;

High quality;

Small portable terminals used worldwide.


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GSM System Architecture


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Block Diagram of GSM System

BSC

BT

S

B

T

S

HLR/AUC EIR SC

ISDN

PLMN

PSTN

PSPDN

OMC

BSS(1)

BSSn

MS

A interface

BS interfaceAbis interface

Um interface

Fig.1-1 GSM Mobile Communication Network Structure

MSC/VLR


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BSS: Base Station Subsystem

BSC: Base Station Controller

BTS: Base Transceiver Station MSC:Mobile Services Switching Center

OMC:Operation and Maintenance Center

AUC:Authentication Center EIR:Equipment Identification Register


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HLR:Home Location Register

VLR:Visitor Location Register

MS:Mobile Station

ISDN:Integrated Services Digital Network

PSTN:Public Switched Telephone Network

PSPDN:Packet Switched Public

DataNetwork

PLMN

Public Land Mobile Network


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Mobile Station

Mobile stations are not fixed to one

subscriber. On any mobile station in the

system, we can identify the subscriber withthe SIM card (Subscriber Identity Module).

The personal identification number (PIN)

can be used to prevent unauthorized use ofthe SIM card.


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Mobile Station

Each mobile station has its ownidentification number, i.e., the internationalmobile equipment identifier (IMEI). IMEI

mainly consists of the type permission codeand the related manufacturer productnumber.

Each mobile subscriber has its owninternational mobile subscriber identifier(IMSI), which is stored in the SIM card andin HLR.


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Network and Switching

Subsystem(NSS) The network and switching sub-system

mainly includes switching functions of the

GSM system, and database functions usedfor subscriber data and mobility

management as well as safety management.

It manages the communications amongGSM mobile subscribers and those between

GSM mobile subscribers and other

communication network subscribers.


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Network and Switching

SubsystemNSS


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NSS Architecture

The network sub-system is divided into six

function units:

Mobile Service Switching Center (MSC)

Home location Register (HLR)

Visitor Location Register (VLR)

Authentification Center (AUC)

Equipment Identification Register (EIR)

Operation and Maintenance Center (OMC)


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Mobile Services Switching

Center (MSC) As the core of a network, MSC provides switching

functions, and connects mobile subscribers with

fixed network subscribers, or with mobilesubscribers. Thus, it provides interfaces to fixed

networks (such as PSTN, ISDN, etc.) and interfaces

for interconnection with other MSCs.


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Functions of MSC

MSC obtains all the data for processingsubscriber call requests from 3 types of

databases (HLR, VLR and AUC). MSC can provides a series of services for

subscribers:

- Telecom services, such as phone, fax, and emergent calls- Bearer services

- Supplementary services, such as call transfer, callrestriction and videoconferencing.


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Visitor Location RegisterVLR VLR stores all related information of mobile

subscribers having entering into its coverage area,

which enables MSC to set up incoming/outgoing

calls. It can be taken as a dynamic subscriberdatabase. VLR obtains and stores necessary data

from the HLR of a mobile subscriber. Once a

mobile subscriber leaves the coverage area of this

VLR, it will be re-registered in another VLR, the

temporarily recorded data of this mobile

subscriber stored in the original VLR will be

deleted.


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Home Location RegisterHLR As the central database of the GSM system, HLR stores

the related data of all existing mobile subscribers

controlled by the same HLR. One HLR can control several

mobile switching areas or the whole mobilecommunication network and the important static data of all

subscribers are stored in the HLR, including IMSI, access

capability, subscriber type and supplementary services.

Furthermore, HLR also stores and provides MSC(A) with(dynamic) information of the MSC(B) area into which a

mobile station has roamed, so that any incoming call is

immediately sent to the called subscriber on a selected path.


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Authentication CenterAUC As a function unit of HLR, AUC is specially used

for security management of the GSM system. AUC

stores authentication information and encryptionkeys for subscriber authentication, encryption of

voice, data, signaling messages on radio interfaces,

preventing unauthorized subscribers access and

guaranteeing the safety of mobile subscribercommunication.


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Equipment Identification

RegisterEIR EIR stores the international mobile equipment

identifier (IMEI) of mobile equipment. By

checking 3 types of lists, i.e., white lists, blacklists, and gray lists, it respectively lists the mobile

equipment identifiers that are authorized, that

should be monitored in case of faults, and that are

unauthorized in case of theft. Service operatorscan use such information to locate the location of

a stolen mobile station and block it.


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BSS System Architecture


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ZXG-10 BSS Structure

MS

Ater interfaceBS interface

Um interface

BSC

BIE

BTS

BTS

BIE

Abisinterface

SM SM TC

A interface

MSC

OMC

Q3 interface

Fig.1-3 BSS Structure

TC: TransCoder

SM: SubMultiplexing

BIE: Base station

Interface Equipment


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Base Transceiver Station (BTS)

It is the radio tranmission part of the

base station system. Controlled by

BSC, it serves cell radio transceiverequipment, handles conversion

between BSC and radio channels, and

performs radio transmission between

BTS and MS via air interfaces as well

as related control functions.


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Base Station Controller(BSC)

As the control part of BSS, BSC performs

the switching function in BSS.

- BSC may be connected with multiple BTSs at one end, andMSC and OMC at the other end. BSC mainly manages

radio network and radio resources, supervises and manages

radio base station, controls the establishment, connection

and disconnection of radio links in MS and BTS and thelocation updating, hand-over and paging of mobile station,

provides functions such as voice encoding, transcoding,

rate adaptation, as well as the operation and maintenance

functions of BSS.


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TransCoder (TC)

TC mainly completes voice conversion

between the 16kbit/s RPE-LTP (regular

pulse excited long-term prediction) codesand 64kbit/s A-law PCM codes. In a typical

application pattern, ZXG10-TC is located

between MSC and BSC.


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GSM System External Interfaces

GSM

Operator

Exter

nalNetw

ork

Sub

scr

iber

OSS

NSS

BSS

MS


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Functional Layers of GSM

OAM

Service carrier

CM

MM

RR

Subscriber

Transmission


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Transmission: data transmission function,

providing methods of carrying subscriber data and

transmitting signalings between different entitiesin various segments along the communication path.

RR: radio resources management, setting up and

releasing stable connections between mobile

stations and MSC at the call setup stage, which ismainly performed by MS and BSC;


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MM: refers to mobility and safety management,

mobile station processingenvironment

changing, making choices of cells possiblybelonging to different networks, so that the calling

subscriber is able to set up a valid process;

infrastructures are required to manage subscriber

location data (location updating);


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CM: refers to communication management, i.e., under

subscriber requests, setting up connections between

subscribers, maintaining and releasing calls (which canbe divided into CCcall control, SSM

supplementary service management, and SMS

short messages service);

OAM: Operation, administration and maintenance

platform, providing operation methods for operators.

The service is supplied by the transmission layer

directly.


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Protocol Stack Structure of

GSM System

CM

MM

RR

LAPDm

MS

RR

LAPDm

Um

LAPD

BTSM

LAPD

Abis

RR

BTSM SCCP

MTP3

BSSAP

BTS BSC

MTP2

SCCP

MTP3

BSSAP

MTP2

CM

MM

MSC

A


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A Interface Signaling Protocol Model1 The A interface is used in the message

between BSC and MSC as well as the

message coming into/out of MS Layer1Physical and electrical parameter and channel

architecture, defining the physical layer structure of

MSC~BSC.

It is realized by employing the first level of the messagetransfer part (MTP) in the common channel signaling

system NO.7 (CSS7), adopting 2Mbit/s PCM digital link

as the transmission link


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A Interface Signaling Protocol Model2 Layer2: defining the data link layer and the

network layer, namely MTP2, MTP3 and

SCCP.- MTP2 is a variant of HDLC protocol.

- MTP3 and SCCP (signal connection

controlling part) chiefly implement suchfunctions as signaling route selection.


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A Interface Signaling Protocol Model3 Layer3: Application layer, including BSS

application part (BSSAP) and BSS operation

and maintenance application part(BSSOMAP), maintains and manages the

resources and the connections in BSS as well

as controls both the connection and thedisconnection of service.


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Abis Interface Signaling Protocol Model (1)

The Abis interface complies with the

requirements in 08.5X series of GSM

standards. Layer1: Physical layer, usually adopts the

2Mbit/sPCM link in accordance with the

specifications stipulated in ITU-T G.703and G.704


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The data link layer employs the LAPD protocol,

which is a point to multi-point communication

protocol. LAPD also utilizes the frame structure

including the flag field, the control field, the

information field, the check field and the flag

sequence. The service access point identification

(SAPI) and the terminal equipment identification(TEI) are both found in the flag field, used to

discriminate respectively the service and the entity

to access.


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Frame Structure of LAPD

FLAG ADDR CONTL MESSAGE FCS FLAG

SAPI TEI N(S) N(R)


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Frame Structure of LAPD

FLAG: Flag

ADDR: Address

FCS: Frame check sequence SAPI: Service access point identifier

TEI: Terminal equipment identifier

N(S): Sending frame No. N(R): Receiving frame No.


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Frame Structure of LAPDm


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Layer3: The upper layer part, mainly

transmitting the application part of BTS,

including the radio link management (RLM)function and the operation and maintenance

function.

Through the Abis interface, BSC providessuch signaling control information as BTS

configuration, BTS monitoring, BTS testing,

and service control.


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Um Interface Signaling Protocol Model (1)

Layer1Transmission layer (physical

layer), the lowest layer of Um interface,

provides transmission channel for radiolink and provides differently functional

logic channels (control channel and

traffic channel ) for higher layer.


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Layer2Data link layer, provides

reliable dedicated data link for and between

MS and BTS. Its based on link accessprotocol of D channel (LAPD), but add

some protocols of mobile applications that

are unique to GSM (LAPDm, the differenceis that no FCS and sync flag in LAPDm)


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Layer3application layer, mainly

performs controlling and management

functions. It includes three sub-layers (CM,MM and RR), each realizes call control,

supplementary service management and

short message management respectively.


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GSM900890~915MHZ (upward)935~960MHZ (downward)

Duplex interval is 45MHZ, working bandwidth is

25MHZ, and carrier frequency interval is 200KHZ.

GSMDCS18001710-1785MHZ (upward)

1805-1880MHZ (downward)

Duplex interval is 95MHZ, working bandwidth is75MHZ, and carrier frequency interval is 200kHZ.

EGSM900: 880~915MHZ (upward)

925~960MHZ (downward)

GSM Wireless Operating Band

Timeslot in Time domain


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Timeslot in Time-domain

and Frequency-domain


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Types of Burst Pulse(1)

NB(Normal Burst): used for traffic channel

and control channels except for RACH,

SCH, FCCH. AB(Access Burst): Transmitted on RACH

channel and used as access request made by

MS to BTS. AB is the sole short BPsequence defined by GSM protocol.


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Types of Burst Pulse(2)

SB (Synchronization Burst): Transmitted

on SCH channel and used for initial

synchronization seizing by MS.

FB (Frequency Correction Burst) : Used

for carrier frequency correction of MS.

DB (Dummy Burst): Has the same

format with NB, mainly used for bit filling


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Channel types

In a GSM system, channels are divided into

logical and physical channels. Time slots are

basic physical channels; thus each carrierfrequency contains 8 physical channels.

Physical channels of radio sub-systems

support logical channels which can be

subdivided funcationally into traffic channels

(TCH) and control channels (CCH).


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Traffic Channel (TCH)

Traffic channel carries encoded speech or

subscriber data, including full-rate traffic channel

and half-rate traffic channel: Full-rate traffic channel (TCH/F): total rate is 22.8kbit/s

Half-rate traffic channel (TCH/H): total rate is 11.4kbit/s

1) Speech channel TCH/FS: full-rate speech traffic channel

TCH/HS: half-rate speech traffic channel

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


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Control Channel

Control channel carries signaling or

synchronized data. There are three types of

control channels:- Broadcasting channels (BCH)

Broadcasting channels are used only as

downward channels, i.e., one-waytransmission from the base station to mobile

stations. They can be divided into three

types of channels:


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Control Channel-BCH

FCCH: frequency correction channel, carrying

information used in MS frequency calibration.

SCH: synchronization channel, carrying informationof MS frame synchronization and base

Transceiver station (BTS) identification.

BCCH: broadcasting control channel; this channel

broadcasts general BTS information. Amongtransceivers at each base station, there is always

one transceiver that contains such a channel, so as

to broadcast system information to mobile stations.


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Common Control ChannelCCCH

CCCH are shared by all mobile stations in the

network. There are 3 types of such channels:

-PCH: paging channels, used by a base station to page mobilestations (downward).-RACH: random access channel, used by mobile stations for

random access network application, i.e., requesting the

allocation of SDCCH channels (upward).-AGCH: access granted channel, used by a base station to

respond to random access requests of mobile stations, i.e.,

to assign one SDCCH or directly assign one TCH

(downward).


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Dedicated Control Channel(DCCH)

In application, the base station will assign it

to a mobile station, so as to make point-to-

point transmission between the base stationand the mobile station.

SDCCH: a stand-alone dedicated control

channel, used to transmit such informationas channel assignment.


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Dedicated Control Channel(DCCH)

SACCH: slow-speed associated control channel, usedin combination with one traffic channel or one SDCCH, to

send some specific subscriber information, e.g.,power and

frame adjustment control information, measurement data,

etc.

FACCH: fast associated control channel, combiningwith one traffic channel to carry the same signals as

SDCCH, but it is assigned only when SDCCH has not

been assigned. Call connection is realized via frames

borrowed from traffic channels to transmit such commands

as hand-over

Ch l C bi ti


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Channel Combination

1) tCHFull TCH/F + FACCH/F + SACCH/TF2) tCHHalf TCH/H)+FACCH/H+ SACCH/TH

3) tCHHalf2 TCH/H + FACCH/H+ SACCH/TH+ TCH/H

4) mainBCCH FCCH + SCH + BCCH + CCCH

5) bCCHCombined FCCH+SCH+BCCH+CCCH+

SDCCH/4+ SACCH/C4

6) bCH BCCH + CCCH

7) sDCCH SDCCH/8+ SACCH/C8

8) bCCHwithCBCH FCCH+SCH+BCCH+CCCH+ SDCCH/4

+ SACCH/C4 + CBCH9) sDCCHwithCBCH SDCCH/8+SACCH/C8+CBCH


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Structure of 26-Frame Traffic Channel

Structure of 51-Frame Control Channel


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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

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 SSCHBBCCH CCCCH CCCH=PCH+AGCH+RACHRRACH DSDCCHASACCH/C Iidle

BCCH+CCCH

BCCH+CCCH

8 SDCCH/8

8 SDCCH/8

BCCH+CCCH+4SDCCH/4

BCCH+CCCH+4SDCCH/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)


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Channel Assignment inside Cells Small capacity cell with only 1 TRX

TN0:FCCH+SCH+CCCH+BCCH+SDCCH/4(0,_,3)

+SACCH/C4(0,_,3);TN1_7: TCH/F+FACCH/F+SACCH/TF

The medium-size cell with 4 TRXs

1TN0 group: FCCH+SCH+BCCH+CCCH;2 SDCCH/8(0,_,7)+SACCH/C8(0,_,7);

29 TCH/F+FACCH/F+SACCH/TF


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Channel Assignment inside Cells

Large-size cell with 12 TRXs

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

1 TN2 group, 1 TN4 group and 1 TN6 group:

BCCH+CCCH;

5 SDCCH/8(0,_,7)+SACCH/C8(0,_,7);

87 TCH/F+FACCH/F+SACCH/TF

Traffic


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Traffic

Traffic refers to the numbers of subscribers the network cansupport and is usually described as follows:

A=nT/3600

where,

n- calls made by a subscriber within an hourT- average duration of each call(in seconds)

A - Traffic, in Erlang

If one call is made by a subscriber within an hour and last 120

seconds, the traffic is calculated as: A=1120/3600=33mErl

For convenience of engineering calculation, the traffic is usually

defined as 25mErl per subscriber.


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GOS

Grade of ServiceGOSGOS refers to thedegree of network congestion or call loss rate.

GOS=2% means that 98% of subscribers can

make calls successfully and 2% of subscribers

will end up with unsuccessfulness.

For network operators, 2-5% of GoS is adopted.

E l B T ffi T bl


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Erlang-B Traffic Tables

Abbreviated - For P.02 Grade of Service Only

#TrunksErlangs #TrunksErlangs #Trunks #TrunksErlangs #TrunksErlangs #TrunksErlangs #TrunksErlangs #TrunksErlangs1 0.0204 26 18.4 51 41.2 76 64.9 100 88 150 136.8 200 186.2 250 235.8

2 0.223 27 19.3 52 42.1 77 65.8 102 89.9 152 138.8 202 188.1 300 285.7

3 0.602 28 20.2 53 43.1 78 66.8 104 91.9 154 140.7 204 190.1 350 335.7

4 1.09 29 21 54 44 79 67.7 106 93.8 156 142.7 206 192.1 400 385.9

5 1.66 30 21.9 55 44.9 80 68.7 108 95.7 158 144.7 208 194.1 450 436.1

6 2.28 31 22.8 56 45.9 81 69.6 110 97.7 160 146.6 210 196.1 500 486.4

7 2.94 32 23.7 57 46.8 82 70.6 112 99.6 162 148.6 212 198.1 600 587.2

8 3.63 33 24.6 58 47.8 83 71.6 114 101.6 164 150.6 214 200 700 688.2

9 4.34 34 25.5 59 48.7 84 72.5 116 103.5 166 152.6 216 202 800 789.3

10 5.08 35 26.4 60 49.6 85 73.5 118 105.5 168 154.5 218 204 900 890.6

11 5.84 36 27.3 61 50.6 86 74.5 120 107.4 170 156.5 220 206 1000 999.1

12 6.61 37 28.3 62 51.5 87 75.4 122 109.4 172 158.5 222 208 1100 1093

13 7.4 38 29.2 63 52.5 88 76.4 124 111.3 174 160.4 224 210

14 8.2 39 30.1 64 53.4 89 77.3 126 113.3 176 162.4 226 212

15 9.01 40 31 65 54.4 90 78.3 128 115.2 178 164.4 228 213.9

16 9.83 41 31.9 66 55.3 91 79.3 130 117.2 180 166.4 230 215.9

17 10.7 42 32.8 67 56.3 92 80.2 132 119.1 182 168.3 232 217.9

18 11.5 43 33.8 68 57.2 93 81.2 134 121.1 184 170.3 234 219.9

19 12.3 44 34.7 69 58.2 94 82.2 136 123.1 186 172.4 236 221.9

20 13.2 45 35.6 70 59.1 95 83.1 138 125 188 174.3 238 223.9

21 14 46 36.5 71 60.1 96 84.1 140 127 190 176.3 240 225.9

22 14.9 47 37.5 72 61 97 85.1 142 128.9 192 178.2 242 227.9

23 15.8 48 38.4 73 62 98 86 144 130.9 194 180.2 244 229.9

24 16.6 49 39.3 74 62.9 99 87 146 132.9 196 182.2 246 231.8

25 17.5 50 40.3 75 63.9 100 88 148 134.8 198 184.2 248 233.8

Erlangs


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Suggested Channel Configuration

Based on 2% GoS, Zhongxing adopts

followed traffic model.

TRXs 1 2 3 4 5 6 7

CCHs 1 2 2 2 3 3 3

TCHs 7 14 22 30 37 45 53Traffic(Erl) 2.94 8.2 14.9 22 28 35.5 43

Training Sequence of GSM


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BinaryTSC Hexadecimal

BN61 - BN86

0 970897 00100101110000100010010111

1 B778B7 00101101110111100010110111

2 10EE90E 01000011101110100100001110

3 11ED11E 01000111101101000100011110

4 6B906B 00011010111001000001101011

5 13AC13A 01001110101100000100111010

6 29F629F 1010011110110001010011111

7 3BC4BBC 11101111000100101110111100

Training Sequence of GSM

Channel Encoding


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Channel Encoding In order to enhance channel anti-interference

capacity and transmission quality, specialredundancy technologies should be adopted to

increase the bulk of transmitted information which

can be inserted at a certain pattern (encoding) at

the sending end and extracted at an agreed pattern(decoding) at the receiving end. This is called the

encoding/decoding process of channels.

Commonly used channel coding methods are: 1)

convolutional coding; 2) Fire coding; 3) parity

check coding.

Convolutional coding


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50bit(Ia) 132bit(Ib) 78bit(Ic)

50 3 132 4

1

78

260bit/20ms

X2 X3 X4

Fire code

+

+

Protection

bits

456bit

78bit

378bit

Block

coding

Convolutional

coder

G0

G1

136bit

189bit

53bit

O/E

g


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Channel Interleaving

In radio telecommunication, error bits often occurs in theburst pulse sequence because of deep signal fading. Check

and correction of errors cannot be efficiently done merely

by means of channel encoding mentioned above.

Interleaving technique is thus adopted, by which the

continuous bits in an information block are segmented and

transmitted individually according to certain rules. That is,

the originally continuous block in the transmission process

becomes discontinuous, forming a group of interwoven

message transmitting blocks, which are to be recovered(de-interleaving) into the original information blocks at the

receiving end.


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456bit

Block A

0

8

1

1

9

2

2

10

3

3

11

4

4

12

5

5

13

6

6

14

7

7

15

8

456bit

Block B

0

8

1

1

9

2

2

10

3

3

11

4

4

12

5

5

13

6

6

14

7

7

15

8

456bit

Block A+1

0

8

1

1

9

2

2

10

3

3

11

4

4

12

5

5

13

6

6

14

7

7

15

8

456bit

Block B+1

0

8

1

1

9

2

2

10

3

3

11

4

4

12

5

5

13

6

6

14

7

7

15

8

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

Even N+3Even N+2Even N+1

Even N

Odd N+4Odd N+5

Odd N+6 Odd N+7

116-bit block 116-bit block 116-bit block 116-bit block


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Two aims can be achieved by adopting DTX mode. One is tolower the total interference level in the air, and the other is to

save transmitter power. The DTX mode and the normal mode

are optional, since the former will slightly lower the

transmission quality.

DTX


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TA

Signals sent by a base station on the broadcastingchannel help mobile stations to synchronize with the

base station. After connection is set up between a

mobile station and the base station, the base station

will make continuous tests, and provide the timeadvance (TA) on the SACCH channel to all mobile

stations twice every second according to the BS-MS-

BS broadcasting delay. Mobile stations will make self-

adaptive frame adjustment according to time advance

so that the time of mobile station transmission to the

base station matches that of base station reception.

Frequency Hopping(1)


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To enhance system anti-interference ability, the GSM

system adopts the frequency hopping technology.

Frequency hopping refers to the hopping of carrier

wave frequencies according to a certain sequence in a

very wide frequency range. Data of controlinformation are converted into base band signals after

modulation, which are then sent into carrier wave

modulation. Afterwards, the carrier frequency

changes under the control of pseudo-random codes,the sequence of which is frequency-hopping sequence.

Finally, when filtered by radio filter, the carrier is

transmitted to and radiated by antenna.


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Informationmodulation

Up conversion

Synchronouscircuit

Frequency hoppingsequence generator Variable frequencysynthesizer

I n f o r m a t i o ndemodulation Down conversion

transmission

receiving


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Frequency hopping is to avoid external

interference. In other words, it is to prevent

or greatly reduce co-channel interferenceand frequency selective fading effect by

converting frequencies to an extent that

interference cannot catch up with

There are two frequency hopping modes:

base band frequency hopping and radio

1.GSM Basic Knowledge

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Transcript of 1.GSM Basic Knowledge