通信工程专业外文翻译--码分多址

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1、外文原文CodedivisionmultipleaccessCodedivisionmultipleaccess(CDMA)isachannelaccessmethodusedbyvariousradiocommunicationtechnologies.ItshouldnotbeconfusedwiththemobilephonestandardscalledcdmaOne,CDMA2000(the3GevolutionofcdmaOne)andWCDMA(the3GstandardusedbyGSM

2、carriers),whichareoftenreferredtoassimplyCDMA,anduseCDMAasanunderlyingchannelaccessmethod.Oneoftheconceptsindatacommunicationistheideaofallowingseveraltransmitterstosendinformationsimultaneouslyoverasinglecommunicationchannel.Thisallowsseveraluserstoshar

3、eabandoffrequencies(seebandwidth).Thisconceptiscalledmultipleaccess.CDMAemploysspread-spectrumtechnologyandaspecialcodingscheme(whereeachtransmitterisassignedacode)toallowmultipleuserstobemultiplexedoverthesamephysicalchannel.Bycontrast,timedivisionmulti

4、pleaccess(TDMA)dividesaccessbytime,whilefrequency-divisionmultipleaccess(FDMA)dividesitbyfrequency.CDMAisaformofspread-spectrumsignalling,sincethemodulatedcodedsignalhasamuchhigherdatabandwidththanthedatabeingcommunicated.StepsinCDMAModulationEachuserina

5、CDMAsystemusesadifferentcodetomodulatetheirsignal.ChoosingthecodesusedtomodulatethesignalisveryimportantintheperformanceofCDMAsystems.Thebestperformancewilloccurwhenthereisgoodseparationbetweenthesignalofadesireduserandthesignalsofotherusers.Theseparatio

6、nofthesignalsismadebycorrelatingthereceivedsignalwiththelocallygeneratedcodeofthedesireduser.Ifthesignalmatchesthedesireduser'scodethenthecorrelationfunctionwillbehighandthesystemcanextractthatsignal.Ifthedesireduser'scodehasnothingincommonwiththesignalt

7、hecorrelationshouldbeasclosetozeroaspossible(thuseliminatingthesignal);thisisreferredtoascrosscorrelation.Ifthecodeiscorrelatedwiththesignalatanytimeoffsetotherthanzero,thecorrelationshouldbeasclosetozeroaspossible.Thisisreferredtoasauto-correlationandis

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