外文翻譯--瑞利衰落環(huán)境下單蜂窩多載波DS-CDMA系統(tǒng)的頻譜效率.doc

附錄B外文文獻及譯文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)瑞利衰落環(huán)境下單蜂窩多載波DS-CDMA系統(tǒng)的頻譜效率希臘雅典大學物理系電子實驗室收稿日期：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：對運行在瑞利衰落環(huán)境下的多載直接序列碼分多址(MC/DSCDMA)系統(tǒng)的頻譜效率進行了研究，以及在一般意義下，對理論上單用戶可達到的信道容量(香農意義)進行了評價。本文涵蓋了適用于無線網(wǎng)絡和室內移動系統(tǒng)的隨機時變廣播通信信道在系統(tǒng)中的運行情況；推導出了頻譜效率封閉表達式的偏差。此外，還揭示了應用的多載波數(shù)目與能達到的頻譜效率之間的關系。關鍵詞：頻譜；信道容量；多載波調制；碼分多址；瑞利衰落；1.介紹在正交頻分復用(OFDM)的基礎上，提出了多載波直接序列碼分多址(MC/DS-CDMA)，并對通過時變信道中高速通信進行了研究1。在這個MC/DS-CDMA系統(tǒng)中，數(shù)據(jù)序列與擴頻序列相乘，去調制N個正交頻率載波，而不像在傳統(tǒng)DS-CDMA中調制單個載波。在本文中,我們考慮運行在瑞利衰落環(huán)境下的單蜂窩MC/DS-CDMA系統(tǒng)的運行情況，根據(jù)香農理論上可能達到的單用戶信道容量來評估頻譜效率，并且假設是在平均意義上進行評價。該方法被這樣的事實證明了，多徑通信能夠達到的頻譜效率都依賴于無線衰落環(huán)境下的物理模型，依賴于對那些特殊衰落信道的了解。例如，如果對一個信道的衰落統(tǒng)計值一無所知，那么可數(shù)容量被強行規(guī)定為最小信噪比ming，所以理所當然容量趨進于零，即ming0。另一方面，如果知道衰落統(tǒng)計值，那么可以根據(jù)固定傳輸速率2-4下衰落信噪比的分布推導出平均容量公式。上面的分析涉及到K個用戶同時使用下行鏈路傳輸(廣播信道)，反映出一個運行的靜態(tài)模型。然后，根據(jù)5-7描述的方法，信道容量是在假定功率相等的情況下被估計的采用適當?shù)陌l(fā)射功率控制方案，由傳統(tǒng)相干MRCRAKE分集接收機接收分路信號，由一系列正交載波頻分復用提供物理頻分電壓，使所有用戶接收到的信號平均功率相等。但這種方法不能解決區(qū)域容量的問題，即在各用戶同時進行可靠通信時設置傳信率是可能的8-9；它僅考慮由MRCRAKE接收到的分路信號的內在差異，在此基礎上估計每用戶的平均信道容量。因此，這樣得到的頻譜效率(bits/s/Hz)表達式不能表明系統(tǒng)的最大值，但它代表了一般意義上實用模塊化和編碼方案的最優(yōu)上界5-7。2.系統(tǒng)說明和單用戶信道容量在MC/DS-CDMA中，各用戶的數(shù)