附錄B外文文獻(xiàn)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(xué)物理系電子實(shí)驗(yàn)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(duì)運(yùn)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(jìn)行了研究，以及在一般意義下，對(duì)理論上單用戶可達(dá)到的信道容量(香農(nóng)意義)進(jìn)行了評(píng)價(jià)。本文涵蓋了適用于無(wú)線網(wǎng)絡(luò)和室內(nèi)移動(dòng)系統(tǒng)的隨機(jī)時(shí)變廣播通信信道在系統(tǒng)中的運(yùn)行情況；推導(dǎo)出了頻譜效率封閉表達(dá)式的偏差。此外，還揭示了應(yīng)用的多載波數(shù)目與能達(dá)到的頻譜效率之間的關(guān)系。關(guān)鍵詞：頻譜；信道容量；多載波調(diào)制；碼分多址；瑞利衰落；1.介紹在正交頻分復(fù)用(OFDM)的基礎(chǔ)上，提出了多載波直接序列碼分多址(MC/DS-CDMA)，并對(duì)通過(guò)時(shí)變信道中高速通信進(jìn)行了研究1。在這個(gè)MC/DS-CDMA系統(tǒng)中，數(shù)據(jù)序列與擴(kuò)頻序列相乘，去調(diào)制N個(gè)正交頻率載波，而不像在傳統(tǒng)DS-CDMA中調(diào)制單個(gè)載波。在本文中,我們考慮運(yùn)行在瑞利衰落環(huán)境下的單蜂窩MC/DS-CDMA系統(tǒng)的運(yùn)行情況，根據(jù)香農(nóng)理論上可能達(dá)到的單用戶信道容量來(lái)評(píng)估頻譜效率，并且假設(shè)是在平均意義上進(jìn)行評(píng)價(jià)。該方法被這樣的事實(shí)證明了，多徑通信能夠達(dá)到的頻譜效率都依賴于無(wú)線衰落環(huán)境下的物理模型，依賴于對(duì)那些特殊衰落信道的了解。例如，如果對(duì)一個(gè)信道的衰落統(tǒng)計(jì)值一無(wú)所知，那么可數(shù)容量被強(qiáng)行規(guī)定為最小信噪比ming，所以理所當(dāng)然容量趨進(jìn)于零，即ming0。另一方面，如果知道衰落統(tǒng)計(jì)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(dǎo)出平均容量公式。上面的分析涉及到K個(gè)用戶同時(shí)使用下行鏈路傳輸(廣播信道)，反映出一個(gè)運(yùn)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(jì)的采用適當(dāng)?shù)陌l(fā)射功率控制方案，由傳統(tǒng)相干MRCRAKE分集接收機(jī)接收分路信號(hào)，由一系列正交載波頻分復(fù)用提供物理頻分電壓，使所有用戶接收到的信號(hào)平均功率相等。但這種方法不能解決區(qū)域容量的問(wèn)題，即在各用戶同時(shí)進(jìn)行可靠通信時(shí)設(shè)置傳信率是可能的8-9；它僅考慮由MRCRAKE接收到的分路信號(hào)的內(nèi)在差異，在此基礎(chǔ)上估計(jì)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(dá)式不能表明系統(tǒng)的最大值，但它代表了一般意義上實(shí)用模塊化和編碼方案的最優(yōu)上界5-7。2.系統(tǒng)說(shuō)明和單用戶信道容量在MC/DS-CDMA中，各用戶的數(shù)