码分多址-通信工程专业外文翻译

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1、外文原文CodedivisionmultipleaccessCodedivisionmultipleaccess(CDMA)isachannelaccessmethodusedbyvariousradiocommunicationtechnologies.ItshouldnotbeconfusedwiththemobilephonestandardscalledcdmaOne,CDMA2000(the3GevolutionofcdmaOne)andWCDMA(the3GstandardusedbyGSMcarriers),whichareoftenreferredtoassimpl

2、yCDMA,anduseCDMAasanunderlyingchannelaccessmethod.Oneoftheconceptsindatacommunicationistheideaofallowingseveraltransmitterstosendinformationsimultaneouslyoverasinglecommunicationchannel.Thisallowsseveraluserstoshareabandoffrequencies(seebandwidth).Thisconceptiscalledmultipleaccess.CDMAemployss

3、pread-spectrumtechnologyandaspecialcodingscheme(whereeachtransmitterisassignedacode)toallowmultipleuserstobemultiplexedoverthesamephysicalchannel.Bycontrast,timedivisionmultipleaccess(TDMA)dividesaccessbytime,whilefrequency-divisionmultipleaccess(FDMA)dividesitbyfrequency.CDMAisaformofspread-s

4、pectrumsignalling,sincethemodulatedcodedsignalhasamuchhigherdatabandwidththanthedatabeingcommunicated.StepsinCDMAModulationEachuserinaCDMAsystemusesadifferentcodetomodulatetheirsignal.ChoosingthecodesusedtomodulatethesignalisveryimportantintheperformanceofCDMAsystems.Thebestperformancewilloccu

5、rwhenthereisgoodseparationbetweenthesignalofadesireduserandthesignalsofotherusers.Theseparationofthesignalsismadebycorrelatingthereceivedsignalwiththelocallygeneratedcodeofthedesireduser.Ifthesignalmatchesthedesireduser'scodethenthecorrelationfunctionwillbehighandthesystemcanextractthatsignal.

6、Ifthedesireduser'scodehasnothingincommonwiththesignalthecorrelationshouldbeasclosetozeroaspossible(thuseliminatingthesignal);thisisreferredtoascrosscorrelation.Ifthecodeiscorrelatedwiththesignalatanytimeoffsetotherthanzero,thecorrelationshouldbeasclosetozeroaspossible.Thisisreferredtoasauto-co

7、rrelationandisusedtorejectmulti-pathinterference.Ingeneral,CDMAbelongstotwobasiccategories:synchronous(orthogonalcodes)andasynchronous(pseudorandomcodes).Codedivisionmultiplexing(SynchronousCDMA)SynchronousCDMAexploitsmathematicalproper