apis training gsm system overview - base station system

8/3/2019 Apis Training GSM System Overview - Base Station System

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7.1 INTRODUCTION. 2

7.2 TRANSMISSIONNElWORK 3

7.3 BSCFUNCflONALffY 4

7.4 BTSFUNcnONAUfY 5

APIS Training (, Seminars

GSM System Overview - BaseStation System, ev. no. 000207

Copyright @APIS TechnicalTraining AB 2000. All rights reserved



7.1 Introduction

The base station system BSS) can be seenas a bridge that connects heMS (through the radio interface) to the MSC/VLR and hence othertelecommunicationusers. The BSS is responsible or the radio-related

functions to establishand maintain he radio link to the MS. In the BSSthere s also a handover reparationunction to determinewhich cell is thebest for the connectionwith the MS to make correct handoverdecisions.The BSS also manageshe radio network resources nd cell configurationdata.

The functional division of the BSS s basically done between he BTS andthe BSC. The BTS comprises the radio transmission and receptionequipment for the communication o and from the MS over the airinterface.The BSC s in chargeof the radio interfacemanagementhroughthe control of the BTS and the MS, mainly by handling allocation and

release f radio channels.

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GSM SystemOverview- Base Station System, rev. no. <XX>207

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7.2 Transmission etwork

The GSM recommendations nly specifies he fundamentalnecessities f

the BSS.The following is mentioned:

The BSC is a network componentwith functions for controlling one orseveralBTSs.The BTS is a network componentwhich servesone cell. It is controlledby a BSC and can consist of one or several TRXs with or without

commoncontrol equipment.

This makes the interpretation very flexible resulting in a variety ofsolutions,especially or the transmissionnetwork, promotedby differentsuppliers.Different structures or the transmissionnetwork are differentlyefficient dependingon the concentrations nd geographical istribution ofthe subscribers. he star-shaped onfiguration could be suitable or a city

while the chain/tree s better for a widespreadarea e.g. a coastline or

highway.

Transmission network prlndples

Another ssuedifferentiating the implementations rom different suppliers

is the Transcoderand Rate Adaptation Unit. The TRAU is the unit thattransforms he speech it rate rom 64 kbps (in the fonD speechs normallycoded n telecommunication ystems) o 13 kbps (in the form the speechscoded by the GSM-system), or adapts the bit rate in case of datainformation. The TRAU is considereda part of the BTS, even thoughGSM specifies that it can be placed remotely. (According to thespecification t can not be placed n the MSC. This is to keep the MSCcompliantwith ISDN, only allowing 64 kbps ime slots o be switched.)

APISTraining, Seminars

GSM System Overview - BaseStationSystem. ev. no. 000207

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Let us take a look at the capacity demand or the transmission rom theBTS, via the BSC and onwards o the MSC, and vice versa.The followingnumberof time slotson a PCM-link is needed:

for the decodedspeech which is actually PCM-coded- 64 kbps is

neededper Traffic Channel (TCH). One time slot on the PCM-link

carriesoneTCH.for the speechcoded by the GSM-speech odec, our TCHs (13 kbpseach) s carriedby one ime slot n the PCM-link.for the control infonnation (channels ot carrying traffic) sent betweenthe BTS and BSC even more PCM-capacity s needed.Dependingonsupplier, control signalling for betweenone and four carriers can befitted into a time slot on the PCM-link.

As transmissioncosts are a considerablepart of the total cost for anoperator most suppliers have chosen to implement a remote TRAU.Depending on the BSC philosophy of the supplier, the TRAU is either

placedat the BSC site or at the MSC site. t is, howeveralwayscontrolledfrom the BTS.

7.3

The main roles of the BSC are he management f the radio channels n theair interface,and he management f handovers.n other words, he BSCcontrols he radio network and ensures estpossibleutilization of the radioresources. hese unctionswere nitially intended o be ncluded n the MSC.

However, he complexity of the radio systemncreased uring the nitialspecificationof GSM, which led to a separation f the radio ntelligence romthe MSC.

Genera]The BSC must perform he following duties:. manage he transmission etwork.. supply switching for the connectionof the MSs to the MSC. The BSC

switches64 kbps time slots to and from different radio links handledbythe TRXs in the BTSs.

. manage he stockof radio channels.. initiate and control the handovers etween he different cells.. manage he radio network and control he BTSs connectedo it.

Radio channelmanagementThe configuration of the radio channels s done by assigning ogicalchannels o the availableRF resourcesn the BTSs. The physicalchannelscould be assignedone of seven channelcombinations (as listed in the

APIS Training (, Seminars

GSM System Overview - Base Station System. rev. no. 000207

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alsoSM-specification). The radio channe] managementestablishing nd maintainingconnectionso the MSs.

involves

The BSC is responsibleor allocation,assignment, upervisionand releaseof the radio channels. t must also administrate ransmissionof systeminformation and t must administrate he pagingon request rom the MSC.

Also, the frequencyhopping management nd the dynamic power controlof the MS and BTS is controlled from the BSC. Handover s handled fnecessary,according to the outcome from the handover preparationproceduren the BSC.

Radionetwork managementThe BSC administrateshe cell descriptiondata, suchas dentities and ypeof cell, and he configurationof frequencies nd output power of the TRXsin the BTSs.

As part of the radio network management he BSC sends cell specific

system nformation to the MS's, containing a list of neighbouringcells onwhich the MSs should perform measurements. hese measurements reused o assist he BSC n evaluating he need or handovers.By evaluatingevent ecordingsand statistics hose ists and the handoverparameters anbe revised thus tuning and improving the performanceand quality of theradio network.

BTS controlThe control, supervisionand maintenance f the BTS needed,depends nthe equipmentused.Loading of software, collecting alarms and statistics

could be part of the control. The messages oncerning he control are senton the signalling inks between he BSC and he BTSs.

7.4 BTS functionality

The BTS has the equipmentneeded o transmit and receive on the radiochannelsallocated o take care of traffic and signalling in one cell. Themain responsibility s to provide connectionswith the MSs over the airinterface.The BTS can consist of severalTRXs (one per carrier) whose

functions can be divided into Control, Signal processing,Transmittingpart, Receivingpart and Synchronisation.Other functions commonly, butnot necessarily, sed are combining and dividing the signals to and fromthe antenna(s), nd a batterybackup n caseof power ailure.

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Control

All the signalling to the MS as well as to the BSC during different traffic

cases s handled rom here.Connection o the BSC could also concern hecontrol of a remoteTRAU and he associated iscontinuousTransmission(DTX) function. The control function also handlesquality measurementswith calculation of the Timing Advance value (TA). alamlS and other

statistics.

Signal ~rocessingThe signal processing performed in the TRX is channel coding,interleaving, ciphering, burst fonnatting and also multiplexing of thelogical channels onto the physical channelsbefore transmission.Afterreception the corresponding channel decoding, de-interleaving and

deciphering akesplace.

Transmitting 12art-TxThe Tx includes functions for GMSK modulation (i.e. transformingbitsinto radio waves),RF generationand power amplification. A combiner sused n order o combine he signals rom severalTxs to feed one antenna.

If applicable he Txs can alsoperform requencyhopping.

ReceivingRaft-RxRx has functions for receptionand demodulation transforming he radiowaves nto bits) of the incoming signal. If antennadiversity is used wo

signals,one rom eachantenna,will be fed to the signalprocessing.

SYnchronisationDue to the TDMA structure synchronisation of the air interface is essential

for the reliability of the network. The synchronisation could be retrieved

from the PCM link connecting the BTS to the BSC, and is used for two

reasons. One is to provide the TRX with a frequency reference but also to

increment the TDMA frame counter according to the TDMA structure.

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GSM SystemOverview- BaseStationSystem. ev. no. 000207

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apis training gsm system overview - base station system

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