LTE的多址接入技术外文翻译.docx

中文2180字201x届本科毕业设计（外文潮译）学院：专业：姓名:学号：指导教师：完成时间：二。一四年三月LTE的多址接入技术1.TE的多址接入OFDM传输正交频分复用(OFDM)是一种多载波传输技术，已被采纳为3gpplong长期演化(LTE)的下行链路传输方案，也可用于其他几个无线技术，例如：wimax和DVB广播技术。它的特点是在一个频域内分布着许多带有间隔的子载波f=lTu其中，Tll是每个子载波的调制符号时间。如图2-1所示，“OFDM子载波间隔”。OFDM的传输是基于块的。每个OFDM符号间隔之间，调制符号是并行发送的。调制符号可以通过调制字母表得到，如QPSK,16QAM或64QAM,对于3GPP组织LTE,子载波间隔是相等的为15kHz。另一方面，子载波的数目取决于传输带宽，在一个IOMHZ的频谱分配下，600个子载波可以有序传输。当然,带宽减小了，子载波数目也相应减少，带宽增加了，子载波数目也相应增加。图2-1C)FDM子载波间隔在OFDM传输时，物理资源经常被描述成一个时域一频域的网格坐标图。在这个坐标图里一列对应一个OFDM子载波，一行对应一个OFDM子载波。如图2-2所示，“OFDM时频网格”。尽管子载波的频谱有重叠，但在理想情况下，是对OFDM子载波解调后不引起任何干扰的，这是因为对每一个子载波间隔的特殊选择，让它等于相应的解调符号率。 time图2-2 OFDM时频网格以一定的频率於=NX/进行采样的OFDM信号，是该SiZe-N的逆离散傅立叶变换（IDFT）的调制符号块ao,a,.aN0因此，OFDM调制可以通过IDFT处理再到数字-模拟的转换来实现。（见图2-3,“OFDM调制”）。在实际中，OFDM调制是以快速傅立叶反变换（IFFT）方式实现简单和快速的处理，通过选择IDFTsizeN等于2小（m为整数）。在接收端，对接收信号以为二NX/的频率采样，高效的FFT处理是用来实现OFDM的解调和检索调制符号块ao,a,.aN°（参见图2-4,“OFDM解调”）。Frequency to time domain conversion图2-3 OFDM调制ThetofrequencydomainconversionSamplingdtratenTs图24OFDM解调正如上面提到的，一个无干扰的OFDM信号可以解调出无任何子载波间干扰的信号。然而，在一个时间色散信道的情况下（如多径无线信道），子载波之间的正交性丢失，造成符号间干扰（ISI）。这是因为，解调器相关区间的一条路径将与不同路径的符号边界有重叠。（见图25,“时间的分散性和相应的接收信号。图2-5次分散和相应的接收信号要解决这个问题，使OFDM信号在无线信道传播时对时间色散完全不敏感,所谓的插入循环前缀通常被使用。如图2-6所示，“插入循环前缀:循环前缀插入就意味着OFDM符号的最后部分（第N个CP）被复制并且被插入到OFDM块的开始部分。因此，OFDM符号的长度从TU到7u+7bp,其中TCP=NCPTU是循环前缀的长度。作为一个结果，OFDM符号率是减少的。因此，在时间色散信道里,只要时间色散的跨度小于循环前缀的长度，子载波的正交性就能被保持。>CyclicprefixpermitstofacilitatedemodulationThecyclicprefixtransformtheclassicalchannelconvolutionintoacyclicconvolutionwhichpermitseasydemodulationafterFFT图2-6插入循环前缀循环前缀插入的缺点是，在整个信号带宽没有减少，OFDM符号率减少的情况下，就意味着在吞吐量方面有相应的损失。OFDM调制组合（IFFT处理），一个（分散的）无线信道，以及解调（FFr处理）可以被看作是一个频域信道。如图2-7,“频域模型的OFDM传输接收”，其中每个OFDM符号的时间期间，N个不同的调制码元被发送，每一个在相应的子载波上，在对比单一宽带载波系统时,如WCDMAWhere,每个调制符号被传输在整个带宽上。RadioChannelresponseh(t)图27频率的OFDM传输接收域模型在频道左上，调制符号ak被缩放和相位转移，通过复杂的信道系数"k（频域）。在接收端，解调后允许发送的信息准确解码。在接收端需要一个频域的信道抽头估计"0,"1,，"N-1。这可以通过在OFDM时频网格内以一定规律的间隔插入已知参考符号来实现，有时也称作导频符号或导频器。运用参考符号的相关知识，接收机可以估计信道抽头（频域）用于解码的必要。OFDM信号带宽一个OFDM信号的带宽等于NX/,这就是说：子载波数乘以子载波间隔数。另一方面，通过设置这个传输符号从一侧组相邻子载波到零，这个基带被减少到NcXH；其中NC是非空子载波数目。然而，OFDM信号的频谱脱落到基本带宽以外的速度是很慢的，尤其比一个WCDMA信号慢的多。因此，在实际中，一个OFDM需要10%的保护间隔。这也就是说，举个例子，在一个5MHZ的频谱分配中，OFDM基本带宽NcX/大约是4.5MHZ。做一个假设，例如，为LTE选择一个15KHZ的子载波间隔，那么，在5MHZ内应对应于300个子载波。DFTOFDM传输离散的傅里叶变换扩展的正交频分复用（DFTS-OFDM）已被用作LTE上行链路的传输方案。DFTS-OFDM传输的基本原理在图2-8,“DFT的OFDM信号生成”中说明。类似于OFDM调制，DFrS-OFDM依赖于基于块的信号生成。在DFTS-OFDM中，一个M调制符号块来自于一些调制字母表，比如，QPSK或者16QAM,第一次被应用到sizemDTF0这个DFr输出被应用到一个SiZe-N的逆DFT的连续输入当中。其中，N>M且未使用的输入（N-M）设置为零。和OFDM一样，每个传输块插入一个循环前缀。图28 DFT的C)FDM信号的产生与图2-8,“DFT的OFDM信号生成”相比，基于IFFTOFDM调制的实现，很显然，DFTS-OFDM可以看作是OFDM调制之前的DFT运算。如果DFT的M的大小等于IDFT的N的大小，那么级联DFT和IDFT的块图2-8“DFT的OFDM信号生成”将完全抵消。如果M小于N且IDFT的剩余输入被设置为零，贝打DFT的输出将是一个低功率变化的信号，类似于一个单载波信号。此外，不同块大小为m的瞬时带宽发送的信号可以是多种多样的，允许灵活的带宽分配。与DFTS-OFDM的主要好处想比，多载波传输方案，如OFDM,减少变化的瞬时发射功率，对提高功率放大器效率是可能的。功率的变化一般根据测得的峰值平均功率比(PRPA)来判断。定义为在峰值功率一个OFDM符号的平均信号功率的归一化。对于DFTS-OFDM,PRPA明显降低，相比OFDM,再考虑到移动终端的电源能力，这种传输技术在上行链路的传输中是非常有用的。DFTS-OFDM信号解调的基本原理如图2-9所示，“DFT的OFDM解调”。这些操作和图2-9"DFT的OFDM解调”基本上是相反的。即SiZe-n离散傅里叶变换处理中，和接受信号不对应的频率采样会被移除。Timetofrequencydomainconversion(I：A/D Synchro R黑;Ye二 Tr 二3nftsnff discardSiZe M d"mtIDFT discard图29DFTSOFDM调制LTEmultipleaccesstechniques1.TEmultipleaccessOFDMtransmissionOrthogonalFrequencyDivisionMultiplexing(OFDM)isamulticarriertransmissiontechniquethathasbeenadoptedasthedownlinktransmissionschemeforthe3GPPLong-TermEvolution(LTE)andisalsousedforseveralotherradiotechnologies,e.g.WiMAXandtheDVBbroadcasttechnologies.Itischaracterizedbyatightfrequency-domainpackingofthesubcarrierswithasubcarrierspacing/=l7u,whereTuistheper-subcarriermodulation-symboltime.(SeeFigure2-1,0FDMsubcarrierSPaCing").OFDMtransmissionisblock-based.DuringeachOFDMsymbolinterval,modulationsymbolsaretransmittedinparallel.Themodulationsymbolscanbefromanymodulationalphabet,suchasQPSK,16QAM,or64QAM.For3GPPLTE,thebasicsubcarrierspacingequals15kHz.Ontheotherhand,thenumberofsubcarriersdependsonthetransmissionbandwidth,withintheorderof600subcarriersincaseofoperationina10MHzspectrumallocationandcorrespondinglyfewer/moresubcarriersincaseofsmaller/largeroveralltransmissionbandwidths.Figure2-1OFDMsubcarrierspacingThephysicalresourceincaseofOFDMtransmissionisoftenillustratedasatime-frequencygridwhereacolumncorrespondstooneOFDMsymbol(time)andarowcorrespondstooneOFDMsubcarrier,asillustratedin(seeFigure2-2,“OFDMtime-frequencygrid).Intheidealcase,despitethefactthatthespectrumofneighborsubcarriersdooverlap,theOFDMsubcarriersdonotcauseanyinterferencetoeachotherafterdemodulationduetothespecificchoiceofasubcarrierspacingfequaltothemodulationsymbolrate.X3u8nb4=Figure 2-2 OFDM time-frequency gridAnOFDMsignalsampledataratefi=/V×sthesze-NInverseDiscreteFourierTransform(IDFT)oftheblockofmodulationsymbolsao,a,.aN-.Thus,OFDMmodulationcanbeimplementedbymeansofIDFTprocessingfollowedbydigital-to-analogconversion(seeFigure2-3,“OFDMmodulation).Inpractice,theOFDMmodulationcanbeimplementedbymeansofInverseFastFourierTransform(IFFT)easyandfastprocessing,byselectingtheIDFTsizeNequalto2mforsomeintegerm.Atthereceiver,bysamplingthereceivedsignalattherateJs=N×f,efficientFFTprocessingisusedtoachieveOFDMdemodulationandretrievetheblockofmodulationsymbolsao,a,.aN-seeFigure2-4,“OFDMdemodulation").Figure2-3OFDMmodulationFrequency to time domain conversionSerial Io parallel conversionFigure2-4OFDMdemodulationTimetofrequencydomainconversionSamplingdtrale=TAsmentionedabove,anuncorruptedOFDMsignalcanbedemodulatedwithoutanyinterferencebetweensubcarriers.However,incaseofatime-dispersivechannel(suchasmultipathradiochannels),theorthogonalitybetweenthesubcarriersislost,causingInterSymbolInterference(ISI).Thereasonforthisisthatthedemodulatorcorrelationintervalforonepathwilloverlapwiththesymbolboundaryofadifferentpath(seeFigure2-5,“TimedispersionandcorrespondingreceivedsignaF,)Figure2-5TimedispersionandcorrespondingreceivedsignalTodealwiththisproblemandmakeanOFDMsignaltrulyinsensitivetotimedispersionontheradiochannel,so-calledCyclicPrefixinsertionistypicallyusedincaseofOFDMtransmission.Asillustratedin(seeFigure2-6,“CyclicPrefixinsertion,),cyclic-prefixinsertionimpliesthatthelastpartoftheOFDMsymbol(thelastNcpsymbols)iscopiedandinsertedatthebeginningoftheOFDMblock,increasingthusthelengthoftheOFDMsymbolfrom7uto7u+7cp,where7cp=NCP,Tuisthelengthofthecyclicprefix.TheOrDMsymbolrateasisreducedasaconsequence.Thus,subcarrierorthogonalityispreservedincaseofatime-dispersivechannel,aslongasthespanofthetimedispersionis典蛆钮由油地y<邸由JengthFigure2CyclicPrefixinsertion+,+;h(0土*h(n-±l×b(i)=h(i)*ai+n(i)convolutionwhichpermitseasydemodulationafterFFTUsefulOFCMsymbolThedrawbackofcyclic-prefixinsertionisthatitimpliesacorrespondinglossintermsofthroughputastheOFDMsymbolrateisreducedwithoutacorrespondingreductionintheoverallsignalbandwidth.ThecombinationofOFDMmodulation(IFFTprocessing),a(time-dispersive)radiolid,dlkdOIChaD二Dade10dFali9n(FFTlPlpressing)ethendpmain央阳节I，一“stratedoese3-可啸器忖酒/呷'嘲廊modelpringiea(fh；C|roMsmbHg向林riNierOVertIen<飒W的)tin")嗝ITa¾Imn>n11e<eacho-÷divensudiffer11modulationSymbOcorrespondingsub-band,incontrasttosinglee5andearnersystems,suchasaWCDMAwhereeachmodulationsymbol夕(TanSNedovertheentirebandwidth.N。lnMntEf11c.-differentusnujdifferentsubcarrs1.ocalizedORejected阮3GPPduetopoorchannelst.performanceandsensitivitytofrequencyoffsetFigure2-7FreqUel¾do福能M>delofOFDMtransmissionreceptiogFromDFT(SizeM)Q0YOSizeNO-Onfrequencychannelk,modulationsymbolakisscaledandphaserotatedbythecomplex(frequency-domain)channelcoefficientHk.Atthereceiverside,toallowforproperdecodingofthetransmittedinformationafterdemodulation,thereceiverneedsanestimateofthefrequency-domainchanneltapsHo,H,.mHn-.Thiscanbedonebyinsertingknownreferencesymbols,sometimesalsoreferredtoaspilotsymbolsorpilots,atregularintervalswithintheOFDMtime/frequencygrid.Usingknowledgeaboutthereferencesymbols,thereceivercanestimatethe(frequency-domain)channeltapsnecessaryforthedecoding.OFDMsignalbandwidthThebasicbandwidthofanOFDMsignalequalsN×f,i.e.thenumberofsubcarriersmultipliedbythesubcarrierspacing.Ontheotherhand,bysettingthesymbolstobetransmittedonagroupofsidecontiguoussubcarrierstozero,thebasicbandwidthisreducedtoNc×fwhereNcisthenumberofnon-nullsubcarriers.However,thespectrumofanOFDMsignalfallsoffslowlyoutsidethebasicOFDMbandwidthandespeciallymuchslowerthanforaWCDMAsignal.Thus,inpractice,typicallyintheorderof10%guard-bandisneededforanOFDMsignal,implyingthat,asanexample,inaspectrumallocationof5MHz,thebasicOFDMbandwidthNcMHz.Assuming,forexample,asubcarrierspacingof15kHzasselectedforLTE,thiscorrespondsto300subcarriersin5MHz.DFTSOFDMtransmissionDiscreteFourierTransformSpreadOFDM(DFTS-OFDM)isatransmissionschemethathasbeenselectedastheuplinktransmissionschemeforLTE.ThebasicprincipleofDFTS-OFDMtransmissionisillustratedin(seeFigure2-8,“DFTSOFDMsignalgeneration").SimilartoOFDMmodulation,DFTS-OFDMreliesonblock-basedsignalgeneration.IncaseofDFTS-OFDM,ablockofMmodulationsymbolsfromsomemodulationalphabet,e.g.QPSKor16QAM,isfirstappliedtoasize-MDFT.TheoutputoftheDFTisthenappliedtoconsecutiveinputsofasize-VinverseDFTwhereN>Mandwherethe(N-Af)unusedinputsoftheIDFTaresettozero.AlsosimilartoOFDM,acyclicprefixisinsertedforeachtransmittedblock.Comparing(seeFigure2-8,DFTSOFDMsignalgeneration),withtheIFFT-basedimplementationofOFDMmodulation,itisobviousthatDFTS-OFDMcanalternativelybeseenasOFDMmodulationprecededbyaDFToperation.IftheDFTsizeMequalstheIDFTsizeN,thecascadedDFTandIDFTblocksof(seeFigure2-8,44DFTSOFDMsignalgeneration),willcompletelycancelouteachother.However,ifMissmallerthanNandtheremaininginputstotheIDFTaresettozero,theoutputoftheIDFTwillbeasignalwithlowpowervariations,similartoasingle-carriersignal.Besides,byvaryingtheblocksizeMtheinstantaneousbandwidthofthetransmittedsignalcanbevaried,allowingforflexible-bandwidthassignment.ThemainbenefitofDFTS-OFDM,comparedtoamulti-carriertransmissionschemesuchasOFDM,isreducedvariationsintheinstantaneoustransmitpower,implyingthepossibilityforincreasedpower-amplifierefficiency.ThepowervariationsaregenerallymeasuredbythePeak-to-Average-PowerRatio(PAPR),definedasthepeakpowerwithinoneOFDMsymbolnormalizedbytheaveragesignalpower.ThePAPRissignificantlylowerforDFTS-OFDM,comparedtoOFDM,makingthusthistransmissiontechniqueveryusefulintheuplinkconsideringthetransmitpowercapabilitiesofthemobileterminal.ThebasicprincipleofDFTS-OFDMsignaldemodulationisillustratedin(seeFigure2-9,44DFTSOFDMdemodulation“).TheoperationsarebasicallythereverseofthosefortheDFTS-OFDMsignalgenerationof(seeFigure2-9,44DFTSOFDMdemodulation"),i.e.size-NDFT(FFT)processing,removalofthefrequencysamplesnotcorrespondingtothesignaltobereceived,an抬一端加温SeDFTprocessing.(foikiinconversion