外文翻译--基于MIMO-OFDM系统的正交空时分组码【适用于毕业论文外文翻译】

外文资料（一）OnOrthogonalSpace-TimeBlockCodesforMIMO-OFDMSystemsSpace-timecodeinmobilecommunicationsystem,andorthogonaldesinginmultiple-antennasscnemearedicsussed.Bythemethods,dataisencodedusingaspace-timeblockcodeandissplitintoseveralstreamswhicharesimultaneouslytransmittedbyantennas.Soamaximum-likelihooddecodingalgorithmcanbeusedatthereceivertoachievethemaximumdiversityorderIntroductionMostworkonwirelesscommunicationshadfocusedonhavinganantennaarrayatonlyoneendofthewirelesslink—usuallyatthereceiver.SeminalpapersbyGerardJ.FoschiniandMichaelJ.Gans[1],Foschini[2]andEmreTelatar[3]enlargedthescopeofwirelesscommunicationpossibilitiesbyshowingthatforthehighly-scatteringenvironmentsubstantialcapacitygainsareenabledwhenantennaarraysareusedatbothendsofalink.Analternativeapproachtoutilizingmultipleantennasreliesonhavingmultipletransmitantennasandonlyoptionallymultiplereceiveantennas.ProposedbyVahidTarokh,NambiSeshadriandRobertCalderbank,thesespace–timecodes(STCs)achievesignificanterrorrateimprovementsoversingle-antennasystems.TheiroriginalschemewasbasedontrelliscodesbutthesimplerblockcodeswereutilizedbySiavashAlamouti,andlaterVahidTarokh,HamidJafarkhaniandRobertCalderbanktodevelopspace–timeblock-codes(STBCs)[4].STCinvolvesthetransmissionofmultipleredundantcopiesofdatatocompensateforfadingandthermalnoiseinthehopethatsomeofthemmayarriveatthereceiverinabetterstatethanothers.InthecaseofSTBCinparticular,thedatastreamtobetransmittedisencodedinblocks,whicharedistributedamongspacedantennasandacrosstime.Whileitisnecessarytohavemultipletransmitantennas,itisnotnecessarytohavemultiplereceiveantennas,althoughtodosoimprovesperformance.ThisprocessofreceivingdiversecopiesofthedataisknownasdiversityreceptionandiswhatwaslargelystudieduntilFoschini's1998paper.AnSTBCisusuallyrepresentedbyamatrix.Eachrowrepresentsatimeslotandeachcolumnrepresentsoneantenna'stransmissionsovertime.Here,sijisthemodulatedsymboltobetransmittedintimeslotifromantennaj.TherearetobeTtimeslotsandnTtransmitantennasaswellasnRreceiveantennas.Thisblockisusuallyconsideredtobeof'length'TThecoderateofanSTBCmeasureshowmanysymbolspertimeslotittransmitsonaverageoverthecourseofoneblock.Ifablockencodesksymbols,thecode-rateisTkrOnlyonestandardSTBCcanachievefull-rate(rate1)—Alamouti'scodeOrthogonalitySTBCsasoriginallyintroduced,andasusuallystudied,areorthogonal.ThismeansthattheSTBCisdesignedsuchthatthevectorsrepresentinganypairofcolumnstakenfromthecodingmatrixisorthogonal.Theresultofthisissimple,linear,optimaldecodingatthereceiver.Itsmostseriousdisadvantageisthatallbutoneofthecodesthatsatisfythiscriterionmustsacrificesomeproportionoftheirdatarate(seeAlamouti'scode).Moreover,thereexistquasi-orthogonalSTBCsthatachievehigherdataratesatthecostofinter-symbolinterference(ISI).Thus,theirerror-rateperformanceislowerboundedbytheoneoforthogonalrate1STBCs,thatprovideISIfreetransmissionsduetoorthogonality.HigherorderSTBCsTarokhetal.discoveredasetofSTBCsthatareparticularlystraightforward,andcoinedthescheme'sname.Theyalsoprovedthatnocodeformorethan2transmitantennascouldachievefull-rate.Theircodeshavesincebeenimprovedupon(bothbytheoriginalauthorsandbymanyothers).Nevertheless,theyserveasclearexamplesofwhytheratecannotreach1,andwhatotherproblemsmustbesolvedtoproduce'good'STBCs.Theyalsodemonstratedthesimple,lineardecodingschemethatgoeswiththeircodesunderperfectchannelstateinformationassumption.4transmitantennasTwostraightforwardcodesfor4transmitantennasare:*1*2*3*4*2*1*4*3*3*4*1*2*4*3*2*112342143341243212/14ccccccccccccccccccccccccccccccccC，and222222222222*22*11*11*22*3*3*11*22*22*11*3*333*1*233214/3,4ccccccccccccccccccccccccccccCThesecodesachieverate-1/2andrate-3/4respectively,asfortheir3-antennacounterparts.C4,3/4exhibitsthesameunevenpowerproblemsasC3,3/4.AnimprovedversionofC4,3/4is1*2*32*1*33*1*23214/3,40000ccccccccccccCwhichhasequalpowerfromallantennasinalltime-slots.DecodingOneparticularlyattractivefeatureoforthogonalSTBCsisthatmaximumlikelihooddecodingcanbeachievedatthereceiverwithonlylinearprocessing.Inordertoconsideradecodingmethod,amodelofthewirelesscommunicationssystemisneeded.Attimet,thesignalitrreceivedatantennajis:jtitniijjtnsrT1,whereαijisthepathgainfromtransmitantennaitoreceiveantennaj,itsisthesignaltransmittedbytransmitantennaiandjtnisasampleofadditivewhiteGaussiannoise(AWGN).Themaximum-likelihooddetectionruleistoformthedecisionvariables)()(11irRtjintnjjtitTRwhereδk(i)isthesignofsiinthekthrowofthecodingmatrix,εk(p)=qdenotesthatspis(uptoasigndifference),the(k,q)elementofthecodingmatrix,fori=1,2...nTandthendecideonconstellationsymbolsithatsatisfieshCChhCrhCChhCrrhCrcWHWHwHwHWHwHwRe2minargminargminargˆ22,withtheconstellationalphabet.Despiteitsappearance,thisisasimple,lineardecodingschemethatprovidesmaximaldiversity.References[1]GerardJ.FoschiniandMichael.J.Gans(January1998).“Onlimitsofwirelesscommunicationsinafadingenvironmentwhenusingmultipleantennas”.Wire