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2. TimeDivisionMultipleAccess (TDMA):
Indigitalsystems,continuoustransmissionisnotrequiredbecauseusersdonotuse
theallottedbandwidthallthetime.Insuchsystems,TDMAisacomplimentary
accesstechniquetoFDMA.GlobalSystemsforMobilecommunications(GSM)uses
theTDMAtechnique. InTDMA, theentirebandwidthisavailabletotheuserbutonly
forafiniteperiodoftime. Inmostcasestheavailablebandwidthis dividedintofewer
channelscomparedtoFDMAandtheusersareallottedtimeslotsduringwhichthey
havetheentirechannelbandwidthattheirdisposal.Thisisillustratedinfigure2
TDMArequirescarefultimesynchronizationsinceuserssharethebandwidthinthe
frequencydomain.Sincethenumberofchannelsisless,interchannelinterference is
almostnegligible;hence theguardtimebetweenthechannelsisconsiderablysmaller.
Guardtimeis spacingintimebetweentheTDMAbursts. Incellularcommunications,
whenausermovesfromonecelltoanotherthereisachance thatusercould
experienceacalllossiftherearenofreetimeslotsavailable.TDMAusesdifferent
timeslotsfortransmissionandreception.Thistypeofduplexing isreferredtoas
Timedivisionduplexing (TDD).TDDdoesnotrequireduplexers.
ChannelusagebyTDMAFigure 2
3. CodeDivisionMultipleAccess:
Theworldisdemanding morefromwirelesscommunicationtechnologiesthanever
beforeasmorepeoplearoundtheworldaresubscribing towireless.Add inexciting
Third-Generation(3G)wireless dataservicesandapplications-suchaswireless
email, web, digitalpicturetaking/sending, assisted-GPS positionlocation
applications, videoandaudiostreaming andTVbroadcasting-andwirelessnetworks
aredoingmuchmorethanjustafewyearsago.
Thisis where CDMA technologyfitsin. CDMA consistentlyprovidesbettercapacity
forvoice anddatacommunicationsthanothercommercialmobiletechnologies,
allowing moresubscriberstoconnectatanygiventime,anditisthecommon
platformonwhich3Gtechnologiesarebuilt.
Code-Division Multiple Access,adigitalCellulartechnologythatuses spread-
spectrum techniques.UnlikecompetingsystemssuchasGSM,thatuse TDMA.
CDMA doesnotassignaspecificfrequencytoeachuser. Instead, everyChanneluses
thefullavailablespectrum.Individualconversationsareencodedwithapseudo
randomdigitalsequence,butgivetherighttousebothtoall userssimultaneously.To
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