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1、外文原文CodedivisionmultipleaccessCodedivisionmultipleaccess(CDMA)isachannelaccessmethodusedbyvariousradiocommunicationtechnologies.ItshouldnotbeconfusedwiththemobilephonestandardscalledcdmaOne,CDMA2000(the3GevolutionofcdmaOne)andWCDMA(the3GstandardusedbyGSMcarr
2、iers),whichareoftenreferredtoassimplyCDMA,anduseCDMAasanunderlyingchannelaccessmethod.Oneoftheconceptsindatacommunicationistheideaofallowingseveraltransmitterstosendinformationsimultaneouslyoverasinglecommunicationchannel.Thisallowsseveraluserstoshareabandof
3、frequencies(seebandwidth).Thisconceptiscalledmultipleaccess.CDMAemploysspread-spectrumtechnologyandaspecialcodingscheme(whereeachtransmitterisassignedacode)toallowmultipleuserstobemultiplexedoverthesamephysicalchannel.Bycontrast,timedivisionmultipleaccess(TD
4、MA)dividesaccessbytime,whilefrequency-divisionmultipleaccess(FDMA)dividesitbyfrequency.CDMAisaformofspread-spectrumsignalling,sincethemodulatedcodedsignalhasamuchhigherdatabandwidththanthedatabeingcommunicated.StepsinCDMAModulationEachuserinaCDMAsystemusesad
5、ifferentcodetomodulatetheirsignal.ChoosingthecodesusedtomodulatethesignalisveryimportantintheperformanceofCDMAsystems.Thebestperformancewilloccurwhenthereisgoodseparationbetweenthesignalofadesireduserandthesignalsofotherusers.Theseparationofthesignalsismadeb
6、ycorrelatingthereceivedsignalwiththelocallygeneratedcodeofthedesireduser.Ifthesignalmatchesthedesireduser'scodethenthecorrelationfunctionwillbehighandthesystemcanextractthatsignal.Ifthedesireduser'scodehasnothingincommonwiththesignalthecorrelationshouldbeasc
7、losetozeroaspossible(thuseliminatingthesignal);thisisreferredtoascrosscorrelation.Ifthecodeiscorrelatedwiththesignalatanytimeoffsetotherthanzero,thecorrelationshouldbeasclosetozeroaspossible.Thisisreferredtoasauto-correlationandisusedtorejectmulti-pathinterf
8、erence.Ingeneral,CDMAbelongstotwobasiccategories:synchronous(orthogonalcodes)andasynchronous(pseudorandomcodes).Codedivisionmultiplexing(SynchronousCDMA)SynchronousCDMAexploitsmathematicalproper
