通信工程专业外文翻译码分多址.docx

通信工程专业外文翻译码分多址.docx

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通信工程专业外文翻译码分多址

外文原文

Codedivisionmultipleaccess

Codedivisionmultipleaccess（CDMA）isachannelaccessmethodusedbyvariousradiocommunicationtechnologies.ItshouldnotbeconfusedwiththemobilephonestandardscalledcdmaOne,CDMA2000（the3GevolutionofcdmaOne）andWCDMA（the3GstandardusedbyGSMcarriers）,whichareoftenreferredtoassimplyCDMA,anduseCDMAasanunderlyingchannelaccessmethod.

Oneoftheconceptsindatacommunicationistheideaofallowingseveraltransmitterstosendinformationsimultaneouslyoverasinglecommunicationchannel.Thisallowsseveraluserstoshareabandoffrequencies（seebandwidth）.Thisconceptiscalledmultipleaccess.CDMAemploysspread-spectrumtechnologyandaspecialcodingscheme（whereeachtransmitterisassignedacode）toallowmultipleuserstobemultiplexedoverthesamephysicalchannel.Bycontrast,timedivisionmultipleaccess（TDMA）dividesaccessbytime,whilefrequency-divisionmultipleaccess（FDMA）dividesitbyfrequency.CDMAisaformofspread-spectrumsignalling,sincethemodulatedcodedsignalhasamuchhigherdatabandwidththanthedatabeingcommunicated.

StepsinCDMAModulation

EachuserinaCDMAsystemusesadifferentcodetomodulatetheirsignal.ChoosingthecodesusedtomodulatethesignalisveryimportantintheperformanceofCDMAsystems.Thebestperformancewilloccurwhenthereisgoodseparationbetweenthesignalofadesireduserandthesignalsofotherusers.Theseparationofthesignalsismadebycorrelatingthereceivedsignalwiththelocallygeneratedcodeofthedesireduser.Ifthesignalmatchesthedesireduser'scodethenthecorrelationfunctionwillbehighandthesystemcanextractthatsignal.Ifthedesireduser'scodehasnothingincommonwiththesignalthecorrelationshouldbeasclosetozeroaspossible（thuseliminatingthesignal）;thisisreferredtoascrosscorrelation.Ifthecodeiscorrelatedwiththesignalatanytimeoffsetotherthanzero,thecorrelationshouldbeasclosetozeroaspossible.Thisisreferredtoasauto-correlationandisusedtorejectmulti-pathinterference.

Ingeneral,CDMAbelongstotwobasiccategories:

synchronous（orthogonalcodes）andasynchronous（pseudorandomcodes）.

Codedivisionmultiplexing（SynchronousCDMA）

SynchronousCDMAexploitsmathematicalpropertiesoforthogonalitybetweenvectorsrepresentingthedatastrings.Forexample,binarystring1011isrepresentedbythevector（1,0,1,1）.Vectorscanbemultipliedbytakingtheirdotproduct,bysummingtheproductsoftheirrespectivecomponents（forexample,ifu=（a,b）andv=（c,d）,thentheirdotproductu·v=ac+bd）.Ifthedotproductiszero,thetwovectorsaresaidtobeorthogonaltoeachother.SomepropertiesofthedotproductaidunderstandingofhowW-CDMAworks.

EachuserinsynchronousCDMAusesacodeorthogonaltotheothers'codestomodulatetheirsignal.Anexampleoffourmutuallyorthogonaldigitalsignalsisshowninthefigure.Orthogonalcodeshaveacross-correlationequaltozero;inotherwords,theydonotinterferewitheachother.InthecaseofIS-9564bitWalshcodesareusedtoencodethesignaltoseparatedifferentusers.Sinceeachofthe64Walshcodesareorthogonaltooneanother,thesignalsarechannelizedinto64orthogonalsignals.Thefollowingexampledemonstrateshoweachuser'ssignalcanbeencodedanddecoded.

AsynchronousCDMA

Whenmobile-to-baselinkscannotbepreciselycoordinated,particularlyduetothemobilityofthehandsets,adifferentapproachisrequired.Sinceitisnotmathematicallypossibletocreatesignaturesequencesthatarebothorthogonalforarbitrarilyrandomstartingpointsandwhichmakefulluseofthecodespace,unique"pseudo-random"or"pseudo-noise"（PN）sequencesareusedinasynchronousCDMAsystems.APNcodeisabinarysequencethatappearsrandombutcanbereproducedinadeterministicmannerbyintendedreceivers.ThesePNcodesareusedtoencodeanddecodeauser'ssignalinAsynchronousCDMAinthesamemannerastheorthogonalcodesinsynchronousCDMA（shownintheexampleabove）.ThesePNsequencesarestatisticallyuncorrelated,andthesumofalargenumberofPNsequencesresultsinmultipleaccessinterference（MAI）thatisapproximatedbyaGaussiannoiseprocess（followingthecentrallimittheoreminstatistics）.GoldcodesareanexampleofaPNsuitableforthispurpose,asthereislowcorrelationbetweenthecodes.Ifalloftheusersarereceivedwiththesamepowerlevel,thenthevariance（e.g.,thenoisepower）oftheMAIincreasesindirectproportiontothenumberofusers.Inotherwords,unlikesynchronousCDMA,thesignalsofotheruserswillappearasnoisetothesignalofinterestandinterfereslightlywiththedesiredsignalinproportiontonumberofusers.

AllformsofCDMAusespreadspectrumprocessgaintoallowreceiverstopartiallydiscriminateagainstunwantedsignals.SignalsencodedwiththespecifiedPNsequence（code）arereceived,whilesignalswithdifferentcodes（orthesamecodebutadifferenttimingoffset）appearaswidebandnoisereducedbytheprocessgain.

SinceeachusergeneratesMAI,controllingthesignalstrengthisanimportantissuewithCDMAtransmitters.ACDM（synchronousCDMA）,TDMA,orFDMAreceivercanintheorycompletelyrejectarbitrarilystrongsignalsusingdifferentcodes,timeslotsorfrequencychannelsduetotheorthogonalityofthesesystems.ThisisnottrueforAsynchronousCDMA;rejectionofunwantedsignalsisonlypartial.Ifanyoralloftheunwantedsignalsaremuchstrongerthanthedesiredsignal,theywilloverwhelmit.ThisleadstoageneralrequirementinanyasynchronousCDMAsystemtoapproximatelymatchthevarioussignalpowerlevelsasseenatthereceiver.InCDMAcellular,thebasestationusesafastclosed-looppowercontrolschemetotightlycontroleachmobile'stransmitpower.

[edit]EfficientPracticalutilizationofFixedFrequencySpectrum

Intheory,CDMA,TDMAandFDMAhaveexactlythesamespectralefficiencybutpractically,eachhasitsownchallenges–powercontrolinthecaseofCDMA,timinginthecaseofTDMA,andfrequencygeneration/filteringinthecaseofFDMA.

TDMAsystemsmustcarefullysynchronizethetransmissiontimesofalltheuserstoensurethattheyarereceivedinthecorrecttimeslotanddonotcauseinterference.Sincethiscannotbeperfectlycontrolledinamobileenvironment,eachtimeslotmusthaveaguard-time,whichreducestheprobabilitythatuserswillinterfere,butdecreasesthespectralefficiency.Similarly,FDMAsystemsmustuseaguard-bandbetweenadjacentchannels,duetotheunpredictabledopplershiftofthesignalspectrumbecauseofusermobility.Theguard-bandswillreducetheprobabilitythatadjacentchannelswillinterfere,butdecreasetheutilizationofthespectrum.

FlexibleAllocationofResources

AsynchronousCDMAoffersakeyadvantageintheflexibleallocationofresourcesi.e.allocationofaPNcodestoactiveusers.InthecaseofCDM（synchronousCDMA）,TDMA,andFDMAthenumberofsimultaneousorthogonalcodes,timeslotsandfrequencyslotsrespectivelyisfixedhencethecapacityintermsofnumberofsimultaneoususersislimited.Thereareafixednumberoforthogonalcodes,timeslotsorfrequencybandsthatcanbeallocatedforCDM,TDMA,andFDMAsystems,whichremainunderutilizedduetotheburstynatureoftelephonyandpacketizeddatatransmissions.ThereisnostrictlimittothenumberofusersthatcanbesupportedinanasynchronousCDMAsystem,onlyapracticallimitgovernedbythedesiredbiterrorprobability,sincetheSIR（SignaltoInterferenceRatio）variesinverselywiththenumberofusers.Inaburstytrafficenvironmentlikemobiletelephony,theadvantageaffordedbyasynchronousCDMAisthattheperformance（biterrorrate）isallowedtofluctuaterandomly,withanaveragevaluedeterminedbythenumberofuserstimesthepercentageofutilization.Supposethereare2Nusersthatonlytalkhalfofthetime,then2NuserscanbeaccommodatedwiththesameaveragebiterrorprobabilityasNusersthattalkallofthetime.ThekeydifferencehereisthatthebiterrorprobabilityforNuserstalkingallofthetimeisconstant,whereasitisarandomquantity（withthesamemean）for2Nuserstalkinghalfofthetime.

Inotherwords,asynchronousCDMAisideallysuitedtoamobilenetworkwherelargenumbersoftransmitterseachgeneratearelativelysmallamountoftrafficatirregularintervals.CDM（synchronousCDMA）,TDMA,andFDMAsystemscannotrecovertheunderutilizedresourcesinherenttoburstytrafficduetothefixednumberoforthogonalcodes,timeslotsorfrequencychannelsthatcanbeassignedtoindividualtransmitters.Forinstance,ifthereareNtimeslotsinaTDMAsystemand2Nusersthattalkhalfofthetime,thenhalfofthetimetherewillbemorethanNusersneedingtousemorethanNtimeslots.Furthermore,itwouldrequiresignificantoverheadtocontinuallyallocateanddeallocatetheorthogonalcode,timeslotorfrequencychannelresources.Bycomparison,asynchronousCDMAtransmitterssimplysendwhentheyhavesomethingtosay,andgoofftheairwhentheydon't,keepingthesamePNsignaturesequenceaslongastheyareconnectedtothesystem.

Spread-spectrumcharacteristicsofCDMA

Mostmodulationschemestrytominimizethebandwidthofthissignalsincebandwidthisalimitedresource.However,spreadspectrumtechniquesuseatransmissionbandwidththatisseveralordersofmagnitudegreaterthantheminimumrequiredsignalbandwidth.Oneoftheinitialreasonsfordoingthiswasmilitaryapplicationsincludingguidanceandcommunicationsystems.Thesesystemsweredesignedusingspreadspectrumbeca