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Which is Better:CDMA or GSM?There has been a lot of debate recently regarding 'CDMA vs G5M' in theindustryas well as media. 50 to sort things out for EFY readers, here is a briefreport on the subject

.RICHA SHARMA

The rapid technological strides inglobal mobile telephony havechanged interpersonal commu-nication forever. With this, the vari-ous technologies that have come to theforefront internationally are global sys-tems for mobile communication (GSM)and code-division multiple access(CDMA).

To understand these, it is impor-tant to know about radio communica-tion. Radio communication is the wire-

less transmission of signals, by modu-

lation of electromagnetic waves withfrequencies below those of light. Mo-bile communication is an advancedversion of radio communication.

Mobile communication works on

the cellular communication techniques.Cell-based mobile service was formu-

lated in USA at Bell Labs in the early70s but introduced first by Nordic Mo-bile Telephone (NMT) in 1981 for com-mercial use.

Since then, several advancements

have taken place in this field. In thebeginning of the cellular communica-tion era, we had analogue mobile tele-phone systems. Later on, we had digi-tal systems. With these advancements,there was requirement of a commonstandard for mobile telecommunica-

tions to coordinate the complicatedtask of specifying the newstandardised network.

The GSM standard was set up anddeveloped by many European coun-tries. The result is a cellular systemthat has been implemented through-out Europe and now in every part of

36 . OCTOBER 2006 . ELECTRONICS FOR YOU

the world. GSM was formed in 1982and field-tested in 1986.

GSM is an open, digital cellulartechnology used for transmitting mo-

consists of base station controllers

(BSCs) and base transceiver stations(BTSs).BSc. It provides control functions

bile voice and data services. It

differs from first-generationwireless systems in that it usesdigital technology.

GSM's componentsMobile station. The part of the net-

\

work that the subscriber can see.

It consists of the mobile equipment ...

(ME) and an electronic card called .'subscriber identity module' (SIM). jThe SIM card, plugged into theME, identifies the MS subscriberand provides the information regard-ing the service that the subscribershould receive. Mobile equipment canbe purchased from anywhere, but SIMis provided by the GSM network pro-vider.

Base station system. The part of the network that provides radio inter-connection between the MS and the

land-based switching equipment. It NokiaN90

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Switching and control of GSM network

PSTN/ISDN PUBLIC DATA NETWORK

and physical links between the MSCand BTS. Control functions include

hand over, cell configuration data andcontrol of radio-frequency power lev-els in base transceiver stations. A num-

ber of BSCs are served by an MSe.BTS. It handles the radio interface

to the mobile station. The BTS isneeded to service each cell in the net-

work. A group of BTS is controlled bya BSe.

Nehvork switching system. It con-sists of the mobile services switchingcentre (MSC) and its associated sys-tem--control databases-and providesfor interconnection between the GSM

network and the public-switched tele-phone network (PSTN).

Operations and maintenance sys-tem. It enables the network provider

38 . OCTOBER 200n . FI F('TAn."rc "no> vn..

to configure and maintain the network from a central location.

How the GSM systemworks?The frequency bandwidths specifiedfor the GSM services within the vari-ous countries are: GSM-400, GSM-800,GSM-900, GSM-1800 and GSM-R. Of these, GSM-900 and GSM-1800 areused in most parts of the world.

The GSM-900 has receive (uplink)frequency range of 895-915 MHz andtransmit (downlink) frequency rangeof 935-960 MHz. This frequency bandis divided into 124 pairs of simplexchannels with separation of 200 kHz.Of these, a particular range of simplexchannels is issued to a particular net-work provider like Airtel, Hutch or

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26/36 CellularWirelessDataTransportTerminology

TDMA Time-division multiple access is a2G technology.

Generalpacketradioserviceis a2.5Gnetworkthat supportsdatapackets.EnhanceddataGSM environmentis a ,

! 3G digital network. :ii Code-divisionmultiple access is a2G technology developedby

Qualcommthat is transitioning to 3G.

, WidebandCOMA(also known asi universalmobiletelecommunicationI system (UMTS))is a 3Gtechnology., On November 6, 2002, NTTDoCoMo,

Ericsson, Nokiaand Siemensagreedon licencing arrangementsforW-CDMA,which should set abenchmarkfor royalty rates.

A 3G technology, 1xRTTis the firstphaseof CDMA2000.

GSM Globalsystem for mobilecommunications is a 2G digitalcellphonetechnology.

GPRS

EDGE

COMA

W-CDMA (UMTS)

CDMA2000 1xRTT

CDMA2000 1xEV-DO

CDMA2000 1xEV-DV

Voice anddata upto 9.6 kbps

Voice and data. This Europeansystem usesthe 900MHz and 1.8GHzfrequencies.In theUnitedStatesit operatesin the 1.9GHzPCSband up to 9.6 kbps.

I Dataup to 115 kbps;the AT&TWireless, GPRSnetworkwill transmitdataat40 to

60 kbps.

Dataup to 384 kbps

i Voiceanddata. UMTSisbeingdesigned toi offerspeedsof at least144kbpstousersini fast-movingvehicles

Voice and data up to 144 kbps

Deliversdata on a separatechannel.

Integratesvoice and data on thesame channel.

Dataup to 2.4 Mbps

Voice and dataup to 2.4 Mbps

Source:Wlkipedia

Idea.

The GSM uses a digital air inter-face. The GSM systems convert ana-logue voice signals into digitalvoice signals for transmission. A singleGSM RF carrier channel can supportup to eight MS subscribers simulta-neously. The transmission rate is

270 kbps.The digital signals are transmitted

by Gaussian minimum shift keyingtechnique (GMSK) in GSM. WithGMSK, the phase change, which rep-resents the change from a digital T ora '0,' occurs over a period of time andtherefore addition of high-frequencycomponents to the spectrum is re-duced. With GMSK, the phase changeis not instantaneous but spread-out.

TDMA techniqueA technique called 'time-division mul-tiple access' (TDMA) is used by GSM.

In this technique, each simplex chan-nel occupies the carrier for one-eighthof the time. Time is divided into dis-

crete periods called' timeslots: Thetimeslots are arranged in a sequenceand conventionally numbered '0' to 7.'Each repetition of this sequence iscalled a TDMA frame. Each MS tele-

phone call occupies one timeslot ('0'to 7') within the frame until the call isterminated, or a handover occurs. TheTDMA frames are then built into fur-

ther frame structures according to thetype of the channel.

The data output from the MSC is64kbps PCM, which has to beconverted into a form specifiedby the GSM for transmission over theentire air interface, Le., between theBSS and the MS. For this purpose, atranscoder (XCDR) is used. It useslinear predictive coding (LPC) toencode the speech at a rate of 13 kbps

i Low battery consumption, but transmissionis one-way, and its speed palesnext to 3Gtechnologies.

Populararound the globe. Worldwideroaming in about 180 countries,but GSM'sshort messagingservice (GSM-SMS)onlytransmits one-way, and can delivermessagesonly up to 160 characterslong.

Messagesnot limited to 160 characters.

May betemporary solutionfor operatorsunableto getW-CDMAlicences

Although behindTDMAin numberofsubscribers, this fast-growing technologyi hasmorecapacitythanTDMA.

Likely to be dominant outsidethe UnitedStates, andthereforegood for roamingglobally. Commitmentsfrom US operatorsare currently lacking,though AT&TWirelessperformed UMTStests in 2002. Primarilytobe implementedin Asia-Pacific region

Proponentssay migrationfrom TDMA issimpler with CDMA2000than W-CDMA,andthat spectrum use is more efficient. ButW-CDMAis likely to be more common inEurope.

(see CDMA2000 1xRTTabove)

(see CDMA2000 1xRTTabove)

plus 3kbps control data. The 3kbpscontrol data is called 'transcoder rate

adaption unit data' (TRAU data). TheTRAU data is removed from the

speech data before its transmission onthe air interface.

So how does a call from a GSMmobile reach its destination? When a

number is dialed from the GSMmobile using any network, thespeech signal is transformed into thedigital signal. This signal, transmittedinto the air interface, is then receivedby the GSM antenna in the locationfrom where the call is beingmade. There is BTS along withthe antenna, which is connectedeither directly to the BSS or via sev-eral other BTS.

The speech data is transferred tothe BSS and from there it goes to theMSC of the network provider. Now if the call has been made to a number of

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