外文翻译--瑞利衰落环境下单蜂窝多载波DS-CDMA系统的频谱效率.doc

附录B外文文献及译文Spectralefficiencyofasingle-cellmulti-carrierDS-CDMAsysteminRayleighfadingP.Varzakas,G.S.TombrasLaboratoryofElectronics,DepartmentofPhysics,UniversityofAthens,GR-15784Athens,GreeceReceived8June2005；receivedinrevisedform12September2005；accepted11March2006Abstract:Thespectralefficiencyofamulti-carrierdirect-sequencecode-divisionmultiple-access(MC/DSCDMA)systemoperatinginaRayleighfadingenvironmentisinvestigatedandevaluatedintermsofthetheoreticallyachievablechannelcapacity(intheShannonsense)peruser,estimatedinanaveragesense.。Thisshortpapercoversoperationoftheconsideredsystemoverbroadcastcommunicationrandomlytime-varyingchannelsasapplicabletowirelessradionetworksandsingle-cellindoormobilesystemsandleadstothederivationofaclosed-formexpressionfortheachievedspectralefficiency.Furthermore,therelationbetweenthenumberoftheemployedsub-carriersandtheachievedspectralefficiencyisrevealed.Keywords:Spectralefficiency；Channelcapacity；Multi-carriermodulation；Code-divisionmultipleaccess；Rayleighfading；1.IntroductionBasedonorthogonalfrequency-divisionmultiplexing(OFDM),multi-carrierdirectsequencecode-divisionmultiple-access(MC/DS-CDMA)hasbeenproposedandinvestigatedinthecontextofhighdataratecommunicationovertime-variantchannels1.InsuchaMC/DS-CDMAsystem,thedatasequence,multipliedbyaspreadingsequence,modulatesasetofNorthogonalfrequencycarriersratherthanasingleone,asitisthecaseinconventionalDS-CDMA.Inthispaper,weconsidertheoperationofasingle-cellMC/DS-CDMAsysteminaRayleighfadingenvironmentandevaluateitsspectralefficiencyintermsofthetheoreticallyachievablechannelcapacity(intheShannonsense)peruser,ontheconditionthatthisisestimatedinanaveragesense.Suchanapproachisjustifiedbythefactthatthespectralefficiencyachievedbyanymultipleaccesscommunicationsystemdependsonthephysicalmodelofthefadingradioenvironment,i.e.,onwhatisknownabouttheparticularfadingchannels.Forexample,ifnothingisknownforthefadingstatisticsofachannelthenitscountablecapacity(inbits/s)willbedictatedbytheminimumfadesignal-to-noiseratio(SNR),ming,andwill,thus,tendtozeroasmingontheotherhand,iffadingstatisticsisknown,thenaveragecapacityformulacanbederivedafterthedistributionofthefadingSNRgforafixedtransmissionrate2-4.Thepresentedanalysisreferstothedownlinktransmission(broadcastchannel)forafixednumberKofsimultaneoususers,reflectingastaticmodelofoperation.Then,followingthemethoddescribedin5-7,channelcapacityisestimatedassuminganequalpowercase,meaningthatallusersreceiveequalaveragesignalpowerwhenanappropriatepowercontrolschemeisapplied,incombinationtothepath-diversityachievedbyaconventionalcoherentmaximal-ratiocombining(MRC)RAKEreceiverandthephysicalfrequencydiversitypotentialprovidedbyfrequency-divisionmultiplexingonasetoforthogonalcarriers.However,thisapproachdoesnotdealwiththeproblemofthe“capacityregion”,i.e.,thesetofinformationratesatwhichsimultaneouslyreliablecommunicationofeachusermessagesispossible8,9；itismerelybasedonestimatingtheaveragechannelcapacityperuser,consideringthesystemsinherentdiversitypotentialinconjunctiontothepath-diversityobtainedbyMRCRAKEreception.Hence,thederivedspectralefficiencyexpressiondoesnotindicatethesystemsmaximumvaluein(bits/s/Hz),butitrepresentsanoptimisticupperbound,inaveragesense,forpracticalmodulationandcodingschemes5-7.2.SystemdescriptionandchannelcapacityperuserInMC/DS-CDMA,eachusersdatasymbolistransmittedinparalleloverNorthogonalcarrierfrequencies(sub-carriers),multipliedbyaspreadingsequenceuniquetoeachuser.Then,thereceivedsignalwillbetheoutputsumofalltheseN“branches”,whilethetotallyallocatedsystembandwidth,MC/DSW,assumingnoguardbandbetweenadjacentfrequencybandsandastrictlyband-limited“chip”sequencewithbandwidthMC/DSW=P,mc/dsGWwillbeequaltoMC/DSW=mc/dsNW=p,mc/dsNGW(1)Wherep,mc/dsGistheprocessinggainappliedfordirect-sequence(DS)spreadtransmissionandWistheuserunspreadsignalbandwidth.However,reliabletransmissionofeachusersignalovereachoftheNsub-carriersclearlydependsonthelevelofcooperationamongtheKusersofthesystem,i.e.,themultipleaccessinterference(MAI)power,whileinter-carrierinterference(ICI)isconsideredminimum.Thus,thenthchannelcapacityperuser,i,nC,i.e.eachusers(1ik)conditionalchannelcapacity(intheShannonsense)overthenthsub-carrier,(1nN),willbegivenbytheShannonHartleytheoremwhenarbitrarilycomplexcodinganddelayisappliedandthetotalMAIpower,causedbyevenasmallnumberofinterferingusers,tendstobeGaussiandistributed10.Inthisrespect,consideringtheprofoundresemblancetotheoperationofaspreadspectrumsystemwithKsimultaneoususerstransmittingwiththesameaveragepowerandthechannelcapacityperuserexpressioninanon-fadingadditivewhiteGaussiannoise(AWGN)environment4,6,itcanbereadilywrittenthati,ni,nmc/ds20mc/dsn,MAIpCWlog(1),1ik；1nNnWP=+(2)Where0nisthenoisepowerspectraldensity,i,nPistheaveragereceivedpowerovertheconsiderednthchannel,andkn,MAIj,ni,niipp(k1)p=-(3)istheGaussiandistributedMAIpower(fori,nPp=,1ik,1nN)overthischannel.Then,consideringthetotaloftheNsub-carriersoverwhich,theuseriinformationdata(spreadbyp,mc/dsG)istransmitted,weconcludethatthetotallyassignedbytheMC/DSCDMAsystemchannelcapacitypereachuseriwillbeNi,ni,MC/DSi,nmc/ds2n10mc/dsi,npCCNWlog(1)nw(k1)p=+-(4)瑞利衰落环境下单蜂窝多载波DS-CDMA系统的频谱效率希腊雅典大学物理系电子实验室收稿日期：2005年6月8日；收到修改稿日期：2005年9月12日；接受日期：2006年3月11日摘要：对运行在瑞利衰落环境下的多载直接序列码分多址(MC/DSCDMA)系统的频谱效率进行了研究，以及在一般意义下，对理论上单用户可达到的信道容量(香农意义)进行了评价。本文涵盖了适用于无线网络和室内移动系统的随机时变广播通信信道在系统中的运行情况；推导出了频谱效率封闭表达式的偏差。此外，还揭示了应用的多载波数目与能达到的频谱效率之间的关系。关键词：频谱；信道容量；多载波调制；码分多址；瑞利衰落；1.介绍在正交频分复用(OFDM)的基础上，提出了多载波直接序列码分多址(MC/DS-CDMA)，并对通过时变信道中高速通信进行了研究1。在这个MC/DS-CDMA系统中，数据序列与扩频序列相乘，去调制N个正交频率载波，而不像在传统DS-CDMA中调制单个载波。在本文中,我们考虑运行在瑞利衰落环境下的单蜂窝MC/DS-CDMA系统的运行情况，根据香农理论上可能达到的单用户信道容量来评估频谱效率，并且假设是在平均意义上进行评价。该方法被这样的事实证明了，多径通信能够达到的频谱效率都依赖于无线衰落环境下的物理模型，依赖于对那些特殊衰落信道的了解。例如，如果对一个信道的衰落统计值一无所知，那么可数容量被强行规定为最小信噪比ming，所以理所当然容量趋进于零，即ming0。另一方面，如果知道衰落统计值，那么可以根据固定传输速率2-4下衰落信噪比的分布推导出平均容量公式。上面的分析涉及到K个用户同时使用下行链路传输(广播信道)，反映出一个运行的静态模型。然后，根据5-7描述的方法，信道容量是在假定功率相等的情况下被估计的采用适当的发射功率控制方案，由传统相干MRCRAKE分集接收机接收分路信号，由一系列正交载波频分复用提供物理频分电压，使所有用户接收到的信号平均功率相等。但这种方法不能解决区域容量的问题，即在各用户同时进行可靠通信时设置传信率是可能的8-9；它仅考虑由MRCRAKE接收到的分路信号的内在差异，在此基础上估计每用户的平均信道容量。因此，这样得到的频谱效率(bits/s/Hz)表达式不能表明系统的最大值，但它代表了一般意义上实用模块化和编码方案的最优上界5-7。2.系统说明和单用户信道容量在MC/DS-CDMA中，各用户的数