中英对照频谱效率

频谱效率频谱效率（Spectralefficiency、Spectrumefficiency）是指在数位通信系统中的带宽限制下，可以传送的资料总量。在有限的波频谱下，物理层通信协议可以达到的使用效率有一定的限度。链路频谱效率数字通信系统的链路频谱效率（ Linkspectralefficiency ）的单位是bit/s/Hz ，或(bit/s)/Hz （较少用，但更准确）。其定义为净比特率（有用信息速率，不包括纠错码）或最大吞吐量除以通信信道或数据链路的带宽（单位：赫兹）。调制效率定义为净比特率（包括纠错码）除以带宽。频谱效率通常被用于分析数字调制方式的效率， 有时也考虑前向纠错码（forwarderror correction, FEC）和其他物理层开销。在后一种情况下， 1个“比特”特指一个用户比特，FEC的开销总是不包括在内的。例1：1kHz带宽中可以传送毎秒1000bit的技术，其频谱效率或调制效率均为1bit/s/Hz。例2：电话网的48,000bit/s

V.92调制解调器在模拟电话网上以 56,000bit/s 的下行速率和的上行速率传输。经由电话交换机的滤波，频率限制在 300Hz到3,400Hz之间，带宽相应为 3400-300=3100Hz 。频谱效率或调制效率为56,000/3,100=18.1bit/s/Hz （下行）、48,000/3,100=15.5bit/s/Hz （上行）。使用FEC的架空调变方式可达到最大的频谱效率可以利用标本化定理来求得，信号的字母表(计算机科学)利用符号数量M来组合、各符号使用N=log2Mbit来表示。此情况下频谱效率若不使用编码间干涉的话，无法超过2Nbit/s/Hz的效率。举例来说，符号种类有8种、每个各有3bit的话，频谱效率最高不超过6bit/s/Hz 。在使用前向错误更正编码的情形时频谱效率会降低。 比如说使用1/2编码率的FEC时，编码长度会变为1.5倍，频谱效率会降低50%。频谱效率降低的同时FEC可以改善信号的SN比(并非一定会有改善)。对某个SN比通信回来说、在完全没有传输错误，且编码与调变方式皆处于理想的状况时， 其频谱效率的上限可哈特利定理得出。比如说 SN比1即分贝为0时，无论编码与调变方式如何变化，频谱效率不会超过1bit/s/Hz。Goodput（应用层情报使用的量）比一般在此计算的吞吐量还小，其原因为有封包再次传送、超传输协议的架空造成的。频谱效率这个用语，会产生数值越大的话可以使周波数频谱产生更有效的误解产生。比如手机因为频谱扩散与使用FEC技术使得频谱效率低下，但SN比不好有时还是可以正常通信。因此可以使用到比周波带宽数还多的链结、以整体来看其效果可以弥补频谱效率低下的缺点还有过之。如同后面会提到的，具有较为合适尺度代表”单位带宽利用率”单位的bit/s/Hz存在，这是属于分码多工(CDMA)的技术并已成为数位手机的基本构成技术。但是电话线路与有线电视网等由于没有频道相互干扰的问题，其使用的基本上皆为其 SN比下最大频谱效率。? 系统频谱效率无线网络是以系统频谱效率'在有限的无线周波数带宽下可以同时支援的客户数与服务进行量化。其单位为bit/s/Hz/areaunit、bit/s/Hz/cell、bit/s/Hz/site等进行计量。有可以把系统能同时支援使用者的吞吐量与goodput的总量以通信回路的带宽(Hz)来表示。这并不单影响使用单一通信回路的技术，多元连接手法与无线资源管理技术也受到影响，特别是动态无线资源管理可以得到改善。定义最大goodput时，会排除掉通信回路间的相互干渉与冲突，高阶通讯协定的架空也是忽略不计的。手机网络的容量也是以1MHz周波数带宽上可以同时最大连接线数来表示，即Erlang/MHz/cell、Erlangs/MHz/sector、Erlangs/MHz/km2等单位。这个数值也影响到讯息编码技术（数据压缩）、在类比电话网络也有使用。例:以频分多址(FDMA)与固定频道分配(FCA)为基础的手机系统在频率再利用系数是4的时候、各基地局可以利用的是所有频谱的1/4。根据此推算、最大系统频谱效率（bit/s/Hz/site）是链结频谱效率的1/4。各基地局使用3个扇形天线将讯号分为3扇区时，被称为4/12再利用模式。各部份可以使用全频谱的1/12，因此系统的频谱效率（bit/s/Hz/cell或bit/s/Hz/sector）为链结频谱效率的1/12。即使链结频谱效率（bit/s/Hz）偏低，以”系统频谱效率”的観点来看，并不一定代表编码效率不好。例如、分码多工(CDMA)频谱扩散为单一通信回路(即只有依未使用者)时，频谱效率是不好的，但是由于在同一带宽中有复数的通信回路存在，因次系统频谱效率非常好。例: 以W-CDMA3G手机系统来说、打电话时最大压缩 8,500bit/s 时、会造成5MHz 带宽的扩散，此时此连接的吞吐量为 8,500/5,000,000=0.0017bit/s/Hz 。在这情形下同扇区内可以有同时容纳 100通电话(有声音)的进行。由于各基地局以3个方向的扇形天线区分为 3个扇区，在频谱扩散后、频率再利用系数会变的比 1还小。此时的系统频谱效率为 1·100·0.0017=0.17bit/s/Hz/site 亦或 0.17/3 = 0.06 bit/s/Hz/cell （也可换算成bit/s/Hz/sector ）。频谱效率可以使用固定/动态频道分配、电力控制、 即被称为 LinkAdaptatio的无线资源管理技术来进行改善。比较表如下表1：一般通信系统的频谱效率数值一般通信系统每秒频道的频道的带链结频谱效系统频谱效率带宽R宽B典型的频率再利的频谱效率服规格率R/B一般R/B/K数值用系数1/K务(Mbit/s)(MHz)(bit/s/Hz)(bit/s/Hz/site)0.013·8第二世代手GSM1993时隙=0.20.521/70.17机(2G)0.104最大0.384最大1.922.75GGSM+EDGE0.21/70.33通常0.20通常1.00IS-136HS+最大0.384最大1.922.75G0.21/70.45EDGE通常0.27通常1.35第三世代手W-CDMAFDD传到手机时传到手机时51/70.51机(3G)1997最大0.384最大0.077传到手机时传到手机时3.5GHSDPA200751/70.71最大14.4最大2.883.5G HSOPAOFDMA传到手机时 10 传到手机时 1/7 0.71最大100最大5第三世代携传到手机时传到手机时CDMA20001x1.251/70.51帯电话(3G)最大0.144最大0.115IEEEWi-Fi802.11a/g200最大5420最大2.71/30.93IEEE802.11nWi-FiDraft2.0最大144.420最大7.221/32.4200720WiMAXIEEE96(1.75,4.81/41.2802.1620043.5,7...)0.576～0.34～数位广播DAB1.1521.7120.671/50.08～0.17DAB+SFN0.576～1.7120.34～数位广播1.1520.67最大31.67最大4.0数位电视DVB-T通常22.08通常2.81/50.55数位电视DVB-T+SFN最大31.678最大4.0通常22.0通常2.80.68～数位电视DVB-H5.5～1181.41/50.14～0.28数位电视DVB-H+SFN5.5～1180.68～1.4光纤用数位256-QAM3866.3316.33电视TVSpectralefficiencySpectralefficiency,spectrumefficiencyorbandwidthefficiencyreferstotheinformationratethatcanbetransmittedoveragivenbandwidthinaspecificcommunicationsystem.Itisameasureofhowefficientlyalimitedfrequencyspectrumisutilizedbythephysicallayerprotocol,andsometimesbythemediaaccesscontrol(thechannelaccessprotocol).LinkspectralefficiencyThelink spectral efficiency ofadigital communicationsystemismeasuredinbit/s/Hz,[1]or,lessfrequentlybutunambiguously,in(bit/s)/Hz.Itisthenetbitrate(usefulinformationrateexcludingerror-correctingcodes)ormaximumthroughputdividedbythebandwidthinhertzofacommunicationchanneloradatalink.Alternatively,thespectralefficiencymaybemeasuredininbit/symbol,whichisequivalenttobitsperchanneluse(bpcu),implyingthatthenetbitrateisdividedbythesymbolrate(modulationrate)orlinecodepulserate.Linkspectralefficiencyistypicallyusedtoanalysetheefficiencyofadigitalmodulationmethodorlinecode,sometimesincombinationwithaforwarderrorcorrection(FEC)codeandotherphysicallayeroverhead.Inthelattercase,a"bit"referstoauserdatabit;FECoverheadisalwaysexcluded.Themodulationefficiencyinbit/sisthegrossbitrate(includinganyerror-correctingcode)dividedbythebandwidth.Example1:Atransmissiontechniqueusingonekilohertzofbandwidthtotransmit1,000bitspersecondhasamodulationefficiencyof1(bit/s)/Hz.Example2:AV.92modemforthetelephonenetworkcantransfer56,000bit/sdownstreamand48,000bit/supstreamoverananalogtelephonenetwork.Duetofilteringinthetelephoneexchange,thefrequencyrangeislimitedtobetween300hertzand3,400hertz,correspondingtoabandwidthof3,400-300=3,100hertz.Thespectralefficiencyormodulationefficiencyis56,000/3,100=18.1(bit/s)/Hzdownstream,and48,000/3,100=15.5(bit/s)/Hzupstream.AnupperboundfortheattainablemodulationefficiencyisgivenbytheNyquistrateorHartley'slawasfollows:ForasignalingalphabetwithMalternativesymbols,eachsymbolrepresentsN=log2Mbits.Nisthemodulationefficiencymeasuredinbit/symbolorbpcu.Inthecaseofbasebandtransmission(linecodingorpulse-amplitudemodulation)withabasebandbandwidth(oruppercut-offfrequency)Bthesymbolratecan,Bsymbols/sinviewtoavoidintersymbolinterference.Thus,notexceed2N=14thespectralefficiencycannotexceed2N(bit/s)/Hzinthebasebandtransmissioncase.Inthepassbandtransmissioncase,asignalwithpassbandbandwidthWcanbeconvertedtoanequivalentbasebandsignal(usingundersamplingorasuperheterodynereceiver),withuppercut-offfrequencyW/2.Ifdouble-sidebandmodulationschemessuchasQAM,ASK,PSKorOFDMareused,thisresultsinamaximumsymbolrateofWsymbols/s,andinthatthemodulationefficiencycannotexceedN(bit/s)/Hz.Ifdigitalsingle-sidebandmodulationisused,thepassbandsignalwithbandwidthWcorrespondstoabasebandmessagesignalwithbasebandbandwidthW,resultinginamaximumsymbolrateof2Wandanattainablemodulationefficiencyof2N(bit/s)/Hz.Example3:An16QAMmodemhasanalphabetsizeofM=16alternativesymbols,withN=4bit/symbolorbpcu.SinceQAMisaformofdoublesidebandpassbandtransmission,thespectralefficiencycannotexceedN=4(bit/s)/Hz.Example4:The8VSB(8-levelvestigialsideband)modulationschemeusedintheATSCdigitaltelevisionstandardgivesN=3bit/symbolorbpcu.Sinceitcanbedescribedasnearlysingle-sideband,themodulationefficiencyiscloseto2N=6(bit/s)/Hz.Inpractice,ATSCtransfersagrossbitrateof32Mbit/sovera6MHzwidechannel,resultinginamodulationefficiencyof32/6=5.3(bit/s)/Hz.Example5:ThedownlinkofaV.92modemusesapulse-amplitudemodulationwith128signallevels,resultinginN=7bit/symbol.Sincethetransmittedsignalbeforepassbandfilteringcanbeconsideredasbasebandtransmission,thespectralefficiencycannotexceed2(bit/s)/Hzoverthefullbasebandchannel(0to4kHz).Asseenabove,ahigherspectralefficiencyisachievedifweconsiderthesmallerpassbandbandwidth.Ifaforwarderrorcorrectioncodeisused,thespectralefficiencyisreducedfromtheuncodedmodulationefficiencyfigure.Example6:Ifaforwarderrorcorrection(FEC)codewithcoderate1/2isadded,meaningthattheencoderinputbitrateisonehalftheencoderoutputrate,thespectralefficiencyis50%ofthemodulationefficiency.Inexchangeforthisreductioninspectralefficiency,FECusuallyreducesthebit-errorrate,andtypicallyenablesoperationatalowersignaltonoiseratio(SNR).AnupperboundforthespectralefficiencypossiblewithoutbiterrorsinachannelwithacertainSNR,ifidealerrorcodingandmodulationisassumed,isgivenbytheShannon-Hartleytheorem.Example7:IftheSNRis1timesexpressedasaratio,correspondingto0decibel,thelinkspectralefficiencycannotexceed1(bit/s)/Hzforerror-freedetection(assuminganidealerror-correctingcode)accordingtoShannon-Hartleyregardlessofthemodulationandcoding.Notethatthegoodput(theamountofapplicationlayerusefulinformation)isnormallylowerthanthemaximumthroughputusedintheabovecalculations,becauseofpacketretransmissions,higherprotocollayeroverhead,flowcontrol,congestionavoidance,etc.Ontheotherhand,adatacompressionscheme,suchastheV.44orV.42biscompressionusedintelephonemodems,mayhowevergivehighergoodputifthetransferreddataisnotalreadyefficientlycompressed.Thelinkspectralefficiencyofawirelesstelephonylinkmayalsobeexpressedasthemaximumnumberofsimultaneouscallsover1MHzfrequencyspectruminerlangspermegahertz,orE/MHz.Thismeasureisalsoaffectedbythesourcecoding(datacompression)scheme.Itmaybeappliedtoanalogaswellasdigitaltransmission.Inwirelessnetworks,thelinkspectralefficiencycanbesomewhatmisleading,aslargervaluesarenotnecessarilymoreefficientintheiroveralluseofradiospectrum.Inawirelessnetwork,highlinkspectralefficiencymayresultinhighsensitivitytoco-channelinterference(crosstalk),whichaffectsthecapacity.Forexample,inacellulartelephonenetworkwithfrequencyreuse,spectrumspreadingandforwarderrorcorrectionreducethespectralefficiencyin(bit/s)/Hzbutsubstantiallylowertherequiredsignal-to-noiseratioincomparisontonon-spreadspectrumtechniques.Thiscanallowformuchdensergeographicalfrequencyreusethatcompensatesforthelowerlinkspectralefficiency,resultinginapproximatelythesamecapacity(thesamenumberofsimultaneousphonecalls)overthesamebandwidth,usingthesamenumberofbasestationtransmitters.Asdiscussedbelow,amorerelevantmeasureforwirelessnetworkswouldbesystemspectralefficiencyinbit/s/Hzperunitarea.However,inclosedcommunicationlinkssuchastelephonelinesandcableTVnetworks,andinnoise-limitedwirelesscommunicationsystemwhereco-channelinterferenceisnotafactor,thelargestlinkspectralefficiencythatcanbesupportedbytheavailableSNRisgenerallyused.SystemspectralefficiencyorareaspectralefficiencyIndigitalwirelessnetworks,thesystemspectralefficiencyorareaspectralefficiencyistypicallymeasuredin(bit/s)/Hzperunitarea,(bit/s)/Hzpercell,or(bit/s)/Hzpersite.Itisameasureofthequantityofusersorservicesthatcanbesimultaneouslysupportedbyalimitedradiofrequencybandwidthinadefinedgeographicarea.Itmayforexamplebedefinedasthemaximumthroughputorgoodput,summedoverallusersinthesystem,dividedbythechannelbandwidth.Thismeasureisaffectednotonlybythesingleusertransmissiontechnique,butalsobymultipleaccessschemesandradioresourcemanagementtechniquesutilized.Itcanbesubstantiallyimprovedbydynamicradioresourcemanagement.Ifitisdefinedasameasureofthemaximumgoodput,retransmissionsduetoco-channelinterferenceandcollisionsareexcluded.Higher-layerprotocoloverhead(abovethemediaaccesscontrolsublayer)isnormallyneglected.Example8:Inacellularsystembasedonfrequency-divisionmultipleaccess(FDMA)withafixedchannelallocation(FCA)cellplanusingafrequencyreusefactorof4,eachbasestationhasaccessto1/4ofthetotalavailablefrequencyspectrum.Thus,themaximumpossiblesystemspectralefficiencyin(bit/s)/Hzpersiteis1/4ofthelinkspectralefficiency.Eachbasestationmaybedividedinto3cellsbymeansof3sectorantennas,alsoknownasa4/12reusepattern.Theneachcellhasaccessto1/12oftheavailablespectrum,andthesystemspectralefficiencyin(bit/s)/Hzpercellor(bit/s)/Hzpersectoris1/12ofthelinkspectralefficiency.Thesystemspectralefficiencyofacellularnetworkmayalsobeexpressedasthemaximumnumberofsimultaneousphonecallsperareaunitover1MHzfrequencyspectruminE/MHzpercell,E/MHzpersector,E/MHzpersite,or(E/MHz)/m2.Thismeasureisalsoaffectedbythesourcecoding(datacompression)scheme.Itmaybeusedinanalogcellularnetworksaswell.Lowlinkspectralefficiencyin(bit/s)/Hzdoesnotnecessarilymeanthatanencodingschemeisinefficientfromasystemspectralefficiencypointofview.Asanexample,considerCodeDivisionMultiplexedAccess(CDMA)spreadspectrum,whichisnotaparticularlyspectralefficientencodingschemewhenconsideringasinglechannelorsingleuser.However,thefactthatonecan"layer"multiplechannelsonthesamefrequencybandmeansthatthesystemspectrumutilizationforamulti-channelCDMAsystemcanbeverygood.Example9:IntheW-CDMA3Gcellularsystem,everyphonecalliscompressedtoamaximumof8,500bit/s(theusefulbitrate),andspreadoutovera5MHzwidefrequencychannel.Thiscorrespondstoalinkthroughputofonly8,500/5,000,000=0.0017(bit/s)/Hz.Letusassumethat100simultaneous(non-silent)simultaneouscallsarepossibleinthesamecell.Spreadspectrummakesitpossibletohaveaslowafrequencyreusefactoras1,ifeachbasestationisdividedinto3cellsbymeansof3directionalsectorantennas.Thiscorrespondstoasystemspectrumefficiencyofover1×100×0.0017=0.17(bit/s)/Hzpercellorsector.Thespectralefficiencycanbeimprovedbyradioresourcemanagementtechniquessuchasefficientfixedordynamicchannelallocation,powercontrol,linkadaptationanddiversityschemes.Acombinedfairnessmeasureandsystemspectralefficiencymeasureisthefairlysharedspectralefficiency.ComparisontableExamplesofnumericalspectralefficiencyvaluesofsomecommoncommunicationsystemscanbefoundinthetablebelow.Spectralefficiencyofcommoncommunicationsystems.SystemspectralNetLinkefficieLaBandwidspectralTypicalncyuncbitrateRpethBperreuseApproServiceStandardhedrcarrierefficiencycarrierR/Bfactorx.yea(Mbit/s)(MHz)1/K((R/B)/r((bit/s)/Hz)K)((bit/s)/Hzper1GcellularNMT4501980.00120.02511GcellularAMPS1980.0096[citatio3nneeded]0.0301990.013×82GcellularGSMtimeslots=0.210.1041990.013×32GcellularD-AMPStimeslots=0.03010.0392