《通信工程专业英语（第二版）》课件Unit 12

Unit12TextAMultiplexingandMultipleAccessMultiplexingandMultipleAccessI.MultiplexingBecauseoftheinstallationcostofacommunicationschannel,suchasamicrowavelinkoracoaxialcablelink,itisdesirabletosharethechannelamongmultipleusers.Providedthatthechannel’sdatacapacityexceedsthatrequiredtosupportasingleuser,thechannelmaybesharedthroughtheuseofmultiplexingmethods.Multiplexingisthesharingofacommunicationschannelthroughlocalcombiningofsignalsatacommonpoint.Twotypesofmultiplexingarecommonlyemployed:frequency-divisionmultiplexingandtime-divisionmultiplexing.TextTextWordsNotesDiscussionHome(1)Frequency-divisionmultiplexing(FDM)Infrequency-divisionmultiplexing(FDM)[1],theavailablebandwidthofacommunicationschannelissharedamongmultipleusersbyfrequencytranslating,ormodulating,eachoftheindividualusersontoadifferentcarrierfrequency.Assumingsufficientfrequencyseparationofthecarrierfrequenciesthatthemodulatedsignalsdonotoverlap,recoveryofeachoftheFDMsignalsispossibleatthereceivingend.Inordertopreventoverlapofthesignalsandtosimplifyfiltering,eachofthemodulatedsignalsisseparatedbyaguardband,whichconsistsofanunusedportionoftheavailablefrequencyspectrum.Eachuserisassignedagivenfrequencybandforalltime.TextTextWordsNotesDiscussionHomeWhileeachuser’sinformationsignalmaybeeitheranalogordigital,thecombinedFDMsignalisinherentlyananalogwaveform.Therefore,anFDMsignalmustbetransmittedoverananalogchannel.ExamplesofFDMarefoundinsomeoftheoldlong-distancetelephonetransmissionsystems,includingtheAmericanN-andL-carriercoaxialcablesystemsandanalogpoint-to-pointmicrowavesystems.IntheL-carriersystemahierarchicalcombiningstructureisemployedinwhich12voicebandsignalsarefrequency-divisionmultiplexedtoformagroupsignalinthefrequencyrangeof60to108kilohertz.(tobecontinued)TextTextWordsNotesDiscussionHome(Continued)Fivegroupsignalsaremultiplexedtoformasupergroupsignalinthefrequencyrangeof312to552kilohertz,correspondingto60voicebandsignals,and10supergroupsignalsaremultiplexedtoformamastergroupsignal.IntheL1carriersystem,deployedinthe1940s,themastergroupwastransmitteddirectlyovercoaxialcable.Formicrowavesystems,itwasfrequencymodulatedontoamicrowavecarrierfrequencyforpoint-to-pointtransmission.IntheL4system,developedinthe1960s,sixmastergroupswerecombinedtoformajumbogroupsignalof3,600voicebandsignals.TextTextWordsNotesDiscussionHome(2)Time-divisionmultiplexingMultiplexingalsomaybeconductedthroughtheinterleavingoftimesegmentsfromdifferentsignalsontoasingletransmissionpath—aprocessknownastime-divisionmultiplexing(TDM)[2].Time-divisionmultiplexingofmultiplesignalsispossibleonlywhentheavailabledatarateofthechannelexceedsthedatarateofthetotalnumberofusers.WhileTDMmaybeappliedtoeitherdigitaloranalogsignals,inpracticeitisappliedalmostalwaystodigitalsignals.Theresultingcompositesignalisthusalsoadigitalsignal.TextTextWordsNotesDiscussionHomeInarepresentativeTDMsystem,datafrommultipleusersarepresentedtoatime-divisionmultiplexer.AscanningswitchthenselectsdatafromeachoftheusersinsequencetoformacompositeTDMsignalconsistingoftheinterleaveddatasignals.Eachuser’sdatapathisassumedtobetime-alignedorsynchronizedtoeachoftheotherusers’datapathsandtothescanningmechanism.Ifonlyonebitwereselectedfromeachofthedatasources,thenthescanningmechanismwouldselectthevalueofthearrivingbitfromeachofthemultipledatasources.Inpractice,however,thescanningmechanismusuallyselectsaslotofdataconsistingofmultiplebitsofeachuser’sdata;thescannerswitchisthenadvancedtothenextusertoselectanotherslot,andsoon.Eachuserisassignedagiventimeslotforalltime.TextTextWordsNotesDiscussionHomeMostmoderntelecommunicationssystemsemploysomeformofTDMfortransmissionoverlong-distanceroutes.Themultiplexedsignalmaybesentdirectlyovercablesystems,oritmaybemodulatedontoacarriersignalfortransmissionviaradiowave.ExamplesofsuchsystemsincludetheNorthAmericanTcarriersaswellasdigitalpoint-to-pointmicrowavesystems.InT1systems,introducedin1962,24voicebandsignals(orthedigitalequivalent)aretime-divisionmultiplexedtogether.(tobecontinued)TextTextWordsNotesDiscussionHome(Continued)Thevoicebandsignalisa64-kilobit-per-seconddatastreamconsistingof8-bitsymbolstransmittedatarateof8,000symbolspersecond.TheTDMprocessinterleaves248-bittimeslotstogether,alongwithasingleframe-synchronizationbit,toforma193-bitframe.The193-bitframesareformedattherateof8,000framespersecond,resultinginanoveralldatarateof1.544megabitspersecond.FortransmissionovermorerecentT-carriersystems,T1signalsareoftenfurthermultiplexedtoformhigher-data-ratesignals—againusingahierarchicalscheme.TextTextWordsNotesDiscussionHome2.MultipleAccessMultiplexingisdefinedasthesharingofacommunicationschannelthroughlocalcombiningatacommonpoint.Inmanycases,however,thecommunicationschannelmustbeefficientlysharedamongmanyusersthataregeographicallydistributedandthatsporadicallyattempttocommunicateatrandompointsintime.Threeschemeshavebeendevisedforefficientsharingofasinglechannelundertheseconditions;theyarecalledfrequency-divisionmultipleaccess(FDMA),time-divisionmultipleaccess(TDMA),andcode-divisionmultipleaccess(CDMA)[3].Thesetechniquescanbeusedaloneortogetherintelephonesystems,andtheyarewellillustratedbythemostadvancedmobilecellularsystems.TextTextWordsNotesDiscussionHome(1)Frequency-divisionmultipleaccessInFDMAthegoalistodividethefrequencyspectrumintoslotsandthentoseparatethesignalsofdifferentusersbyplacingtheminseparatefrequencyslots.Thedifficultyisthatthefrequencyspectrumislimitedandthattherearetypicallymanymorepotentialcommunicatorsthanthereareavailablefrequencyslots.Inordertomakeefficientuseofthecommunicationschannel,asystemmustbedevisedformanagingtheavailableslots.Intheadvancedmobilephonesystem(AMPS),thecellularsystememployedintheUnitedStates,differentcallersuseseparatefrequencyslotsviaFDMA.(tobecontinued)TextTextWordsNotesDiscussionHome(Continued)Whenonetelephonecalliscompleted,anetwork-managingcomputeratthecellularbasestationreassignsthereleasedfrequencyslottoanewcaller.AkeygoaloftheAMPSsystemistoreusefrequencyslotswheneverpossibleinordertoaccommodateasmanycallersaspossible.Locallywithinacell,frequencyslotscanbereusedwhencorrespondingcallsareterminated.Inaddition,frequencyslotscanbeusedsimultaneouslybymultiplecallerslocatedinseparatecells.Thecellsmustbefarenoughapartgeographicallythattheradiosignalsfromonecellaresufficientlyattenuatedatthelocationoftheothercellusingthesamefrequencyslot.TextTextWordsNotesDiscussionHome(2)Time-divisionmultipleaccessInTDMAthegoalistodividetimeintoslotsandseparatethesignalsofdifferentusersbyplacingthesignalsinseparatetimeslots.Thedifficultyisthatrequeststouseasinglecommunicationschanneloccurrandomly,sothatonoccasionthenumberofrequestsfortimeslotsisgreaterthanthenumberofavailableslots.Inthiscaseinformationmustbebuffered,orstoredinmemory,untiltimeslotsbecomeavailablefortransmittingthedata.Thebufferingintroducesdelayintothesystem.IntheIS54cellularsystem,threedigitalsignalsareinterleavedusingTDMAandthentransmittedina30-kilohertzfrequencyslotthatwouldbeoccupiedbyoneanalogsignalinAMPS[4].Bufferingdigitalsignalsandinterleavingthemintimecausessomeextradelay,butthedelayissobriefthatitisnotordinarilynoticedduringacall.TheIS54systemusesaspectsofbothTDMAandFDMA.TextTextWordsNotesDiscussionHome(3)Code-divisionmultipleaccessInCDMA,signalsaresentatthesametimeinthesamefrequencyband.Signalsareeitherselectedorrejectedatthereceiverbyrecognitionofauser-specificsignaturewaveform,whichisconstructedfromanassignedspreadingcode.TheIS95cellularsystememploystheCDMAtechnique.InIS95ananalogspeechsignalthatistobesenttoacellsiteisfirstquantizedandthenorganizedintooneofanumberofdigitalframestructures.Inoneframestructure,aframeof20milliseconds'durationconsistsof192bits.Ofthese192bits,172representthespeechsignalitself,12formacyclicredundancycheckthatcanbeusedforerrordetection,and8formanencoder"tail"thatallowsthedecodertoworkproperly.Thesebitsareformedintoanencodeddatastream.(tobecontinued)TextTextWordsNotesDiscussionHome(Continued)Afterinterleavingoftheencodeddatastream,bitsareorganizedintogroupsofsix.Eachgroupofsixbitsindicateswhichof64possiblewaveformstotransmit.Eachofthewaveformstobetransmittedhasaparticularpatternofalternatingpolaritiesandoccupiesacertainportionoftheradio-frequencyspectrum.Beforeoneofthewaveformsistransmitted,however,itismultipliedbyacodesequenceofpolaritiesthatalternateatarateof1.2288megahertz,spreadingthebandwidthoccupiedbythesignalandcausingittooccupy(afterfilteringatthetransmitter)about1.23megahertzoftheradio-frequencyspectrum.Atthecellsiteoneusercanbeselectedfrommultipleusersofthesame1.23-megahertzbandwidthbyitsassignedcodesequence.TextTextWordsNotesDiscussionHomeCDMAissometimesreferredtoasspread-spectrummultipleaccess(SSMA),becausetheprocessofmultiplyingthesignalbythecodesequencecausesthepowerofthetransmittedsignaltobespreadoveralargerbandwidth.Frequencymanagement,anecessaryfeatureofFDMA,iseliminatedinCDMA.Whenanotheruserwishestousethecommunicationschannel,itisassignedacodeandimmediatelytransmitsinsteadofbeingstoreduntilafrequencyslotopens.TextTextWordsNotesDiscussionHomeaccessn.进入；使用权；通路desirableadj.令人满意的；值得要的modulatev.调节；（信号）调制overlapn.&v.重叠；重复；部分重叠simplifyvt.简化；使单纯；使简易assignvt.分配；[计][数]赋值WordsWordsTextWordsNotesDiscussionHometransmitvt.传输；传播；遗传hierarchicaladj.分层的；分级的kilohertzn.[物]千赫jumboadj.&n.巨大的；庞然大物segmentv.&n.分割；段；部分compositen.&adj.&vt.合成物；合成的；使合成WordsTextWordsNotesDiscussionHomeinterleavev.[计]交错，交叉存取alignvt.使结盟；使成一行；匹配mechanismn.机制；原理；技巧slotn.&vt.位置；跟踪；开槽于sporadicallyadv.零星地；偶发地simultaneouslyadv.同时地WordsTextWordsNotesDiscussionHomeattenuateadj.&vt.减弱的；细小的；使减弱buffern.&vt.[计]缓冲区；缓冲durationn.持续，持续的时间，期间polarityn.[物]极性；两极；对立WordsTextWordsNotesDiscussionHome[1]FrequencyDivisionMultiplexing（FDM）：中文名是频分多路复用，就是指用不同频率传送各路消息，以实现多路通信。这种方法也叫频率复用。TextWordsNotesDiscussionNotesHome[2]TDM就是时分复用模式。时分复用是指一种通过不同信道或时隙中的交叉位脉冲，同时在同一个通信媒体上传输多个数字化数据、语音和视频信号等的技术。TextWordsNotesDiscussionHome[3]CDMA是指一种扩频多址数字式通信技术，通过独特的代码序列建立信道，可用于二代和三代无线通信中的任何一种协议。CDMA是一种多路方式，多路信号只占用一条信道，极大提高带宽使用率，应用于800MHz和1.9GHz的超高频（UHF）移动电话系统。TextWordsNotesDiscussionHome[4]AMPS是第一代蜂窝技术，使用单独的频带，或者说“信道”，为每次对话服务（详见FDMA）。它因此需要相当的带宽来支持一个大数量的用户群体。在通用术语中，AMPS常常被当作更早的“0G”改进型移动通信服务，只不过AMPS使用更多的计算功率来选择频谱、切换到PSTN线路的通话、以及处理登记和呼叫建立。TextWordsNotesDiscussionHomeQuestionsfordiscussion1.Inthispassage,whatdoesmultiplexingmean?2.Whichtwokindsofmultiplexingarefrequentlyusedinorderformultipleuserstosharethecommunicationschannel?3.Whatarethethreeschemesdevisedforefficientsharingofasinglechannel?TextWordsNotesDiscussionHomeAnswerstoquestionsfordiscussionTextWordsNotesDiscussionHome1.Whatdoesmultiplexingmean?Bothfrequency-divisionmultiplexing(FDM)andtime-divisionmultiplexing(TDM)arefrequentlyusedtoenableuserssharethechannel.Theformermeanstheavailablebandwidthofacommunicationschannelissharedamongmultipleusersbyfrequencytranslating,ormodulating,eachoftheindividualusersontoadifferentcarrierfrequencyandthelatterreferstotheprocessinwhichmultiplexingisconductedthroughtheinterleavingoftimesegmentsfromdifferentsignalsontoasingletransmissionpath.TextWordsNotesDiscussionHome2.Whichtwokindsofmultiplexingarefrequentlyusedinorderformultipleuserstosharethecommunicationschannel?Bothfrequency-divisionmultiplexing(FDM)andtime-divisionmultiplexing(TDM)arefrequentlyusedtoenableuserssharethechannel.Theformermeanstheavailablebandwidthofacommunicationschannelissharedamongmultipleusersbyfrequencytranslating,ormodulating,eachoftheindividualusersontoadifferentcarrierfrequencyandthelatterreferstotheprocessinwhichmultiplexingisconductedthroughtheinterleavingoftimesegmentsfromdifferentsignalsontoasingletransmissionpath.TextWordsNotesDiscussionHome3.Whatarethethreeschemesdevisedforefficientsharingofasinglechannel?Thethreeschemesarecalledfrequency-divisionmultipleaccess(FDMA),time-divisionmultipleaccess(TDMA),andcode-divisionmultipleaccess(CDMA).Thesetechniquescanbeusedaloneortogetherintelephonesystems.TextWordsNotesDiscussionHomeThankyou!Unit12TextBOrthogonalFrequency-divisionMultiplexingOrthogonalFrequency-divisionMultiplexing1Orthogonalfrequency-divisionmultiplexing(OFDM)isamethodofencodingdigitaldataonmultiplecarrierfrequencies.OFDMhasdevelopedintoapopularschemeforwidebanddigitalcommunication,usedinapplicationssuchasdigitaltelevisionandaudiobroadcasting,DSLInternetaccess,wirelessnetworks,powerlinenetworks,and4Gmobilecommunications.TextTextWordsNotesDiscussionHomeOFDMisafrequency-divisionmultiplexing(FDM)schemeusedasadigitalmulti-carriermodulationmethod.Alargenumberofcloselyspacedorthogonalsub-carriersignalsareusedtocarrydataonseveralparalleldatastreamsorchannels.Eachsub-carrierismodulatedwithaconventionalmodulationscheme(suchasquadratureamplitudemodulationorphase-shiftkeying)atalowsymbolrate,maintainingtotaldataratessimilartoconventionalsingle-carriermodulationschemesinthesamebandwidth.TextTextWordsNotesDiscussionHomeTheprimaryadvantageofOFDMoversingle-carrierschemesisitsabilitytocopewithseverechannelconditions(forexample,attenuationofhighfrequenciesinalongcopperwire,narrowbandinterferenceandfrequency-selectivefadingduetomultipath)withoutcomplexequalizationfilters.ChannelequalizationissimplifiedbecauseOFDMmaybeviewedasusingmanyslowlymodulatednarrowbandsignalsratherthanonerapidlymodulatedwidebandsignal.(tobecontinued)TextTextWordsNotesDiscussionHome(Continued)Thelowsymbolratemakestheuseofaguardintervalbetweensymbolsaffordable,makingitpossibletoeliminateintersymbolinterference(ISI)2andutilizeechoesandtime-spreading(onanalogueTVthesearevisibleasghostingandblurring,respectively)toachieveadiversitygain,i.e.asignal-to-noiseratioimprovement.Thismechanismalsofacilitatesthedesignofsinglefrequencynetworks(SFNs),whereseveraladjacenttransmitterssendthesamesignalsimultaneouslyatthesamefrequency,asthesignalsfrommultipledistanttransmittersmaybecombinedconstructively,ratherthaninterferingaswouldtypicallyoccurinatraditionalsingle-carriersystem.TextTextWordsNotesDiscussionHome1.ExampleofapplicationsThefollowinglistisasummaryofexistingOFDM-basedstandardsandproducts.TextTextWordsNotesDiscussionHomeWired·ADSLandVDSL3

broadbandaccessviaPOTScopperwiring·DVB-C2,anenhancedversionoftheDVB-CdigitalcableTVstandard·Powerlinecommunication(PLC)·ITU-TG.hn,astandardwhichprovideshigh-speedlocalareanetworkingofexistinghomewiring(powerlines,phonelinesandcoaxialcables)·TrailBlazertelephonelinemodems·MultimediaoverCoaxAlliance(MoCA)homenetworking·DOCSIS4

3.1BroadbanddeliveryTextTextWordsNotesDiscussionHomeWireless·ThewirelessLAN(WLAN)radiointerfacesIEEE802.11a,g,n,acandHIPERLAN/2·ThedigitalradiosystemsDAB/EUREKA147,DAB+,DigitalRadioMondiale,HDRadio,T-DMBandISDB-TSB·TheterrestrialdigitalTVsystemsDVB-TandISDB-T·TheterrestrialmobileTVsystemsDVB-H,T-DMB,ISDB-TandMediaFLOforwardlink·Thewirelesspersonalareanetwork(PAN)ultra-wideband(UWB)IEEE802.15.3aimplementationsuggestedbyWiMediaAllianceTextTextWordsNotesDiscussionHomeTheOFDMbasedmultipleaccesstechnologyOFDMAisalsousedinseveral4Gandpre-4Gcellularnetworksandmobilebroadbandstandards:·ThemobilitymodeofthewirelessMAN/broadbandwirelessaccess(BWA)standardIEEE802.16e(orMobile-WiMAX)·Themobilebroadbandwirelessaccess(MBWA)standardIEEE802.20·Thedownlinkofthe3GPPLongTermEvolution(LTE)fourthgenerationmobilebroadbandstandard.TheradiointerfacewasformerlynamedHighSpeedOFDMPacketAccess(HSOPA),nownamedEvolvedUMTSTerrestrialRadioAccess(E-UTRA)TextTextWordsNotesDiscussionHome2.KeyfeaturesSummaryofadvantages·Highspectralefficiencyascomparedtootherdoublesidebandmodulationschemes,spreadspectrum,etc.·Caneasilyadapttoseverechannelconditionswithoutcomplextime-domainequalization.·Robustagainstnarrow-bandco-channelinterferenceTextTextWordsNotesDiscussionHome·Robustagainstintersymbolinterference(ISI)andfadingcausedbymultipathpropagation·EfficientimplementationusingfastFouriertransform(FFT)·Lowsensitivitytotimesynchronizationerrors·Tunedsub-channelreceiverfiltersarenotrequired(unlikeconventionalFDM)·Facilitatessinglefrequencynetworks(SFNs)(i.e.transmittermacrodiversity)TextTextWordsNotesDiscussionHomeSummaryofdisadvantages·SensitivetoDopplershift·Sensitivetofrequencysynchronizationproblems·Highpeak-to-average-powerratio(PAPR),requiringlineartransmittercircuitry,whichsuffersfrompoorpowerefficiency·Lossofefficiencycausedbycyclicprefix/guardintervalTextTextWordsNotesDiscussionHomeOrthogonalityConceptually,OFDMisaspecializedFDM,theadditionalconstraintbeingthatallcarriersignalsareorthogonaltooneanother.TextTextWordsNotesDiscussionHomeInOFDM,thesub-carrierfrequenciesarechosensothatthesub-carriersareorthogonaltoeachother,meaningthatcross-talkbetweenthesub-channelsiseliminatedandinter-carrierguardbandsarenotrequired.Thisgreatlysimplifiesthedesignofboththetransmitterandthereceiver;unlikeconventionalFDM,aseparatefilterforeachsub-channelisnotrequired.TextTextWordsNotesDiscussionHomeTheorthogonalityrequiresthatthesub-carrierspacingisHertz,wheresecondsistheusefulsymbolduration(thereceiver-sidewindowsize),andisapositiveinteger,typicallyequalto1.Therefore,withsub-carriers,thetotalpassbandbandwidthwillbe(Hz).TextTextWordsNotesDiscussionHomeTheorthogonalityalsoallowshighspectralefficiency,withatotalsymbolrateneartheNyquistratefortheequivalentbasebandsignal(i.e.nearhalftheNyquistrateforthedouble-sidebandphysicalpassbandsignal).Almostthewholeavailablefrequencybandcanbeutilized.OFDMgenerallyhasanearly'white'spectrum,givingitbenignelectromagneticinterferencepropertieswithrespecttootherco-channelusers.TextTextWordsNotesDiscussionHomeAsimpleexample:Ausefulsymbolduration=1mswouldrequireasub-carrierspacingof1kHz(oranintegermultipleofthat)fororthogonality.N=1,000sub-carrierswouldresultinatotalpassbandbandwidthof=1MHz.Forthissymboltime,therequiredbandwidthintheoryaccordingtoNyquistis(i.e.,halfoftheachievedbandwidthrequiredbyourscheme).Ifaguardintervalisapplied,Nyquistbandwidthrequirementwouldbeevenlower.(tobecontinued)TextTextWordsNotesDiscussionHome(Continued)TheFFTwouldresultinN=1,000samplespersymbol.Ifnoguardintervalwasapplied,thiswouldresultinabasebandcomplexvaluedsignalwithasamplerateof1MHz,whichwouldrequireabasebandbandwidthof0.5MHzaccordingtoNyquist.However,thepassbandRFsignalisproducedbymultiplyingthebasebandsignalwithacarrierwaveform(i.e.,double-sidebandquadratureamplitude-modulation)resultinginapassbandbandwidthof1MHz.Asingle-sideband(SSB)orvestigialsideband(VSB)modulationschemewouldachievealmosthalfthatbandwidthforthesamesymbolrate(i.e.,twiceashighspectralefficiencyforthesamesymbolalphabetlength).Itishowevermoresensitivetomultipathinterference.TextTextWordsNotesDiscussionHomeOFDMrequiresveryaccuratefrequencysynchronizationbetweenthereceiverandthetransmitter;withfrequencydeviationthesub-carrierswillnolongerbeorthogonal,causinginter-carrierinterference(ICI)(i.e.,cross-talkbetweenthesub-carriers).Frequencyoffsetsaretypicallycausedbymismatchedtransmitterandreceiveroscillators,orbyDopplershiftduetomovement.WhileDopplershiftalonemaybecompensatedforbythereceiver,thesituationisworsenedwhencombinedwithmultipath,asreflectionswillappearatvariousfrequencyoffsets,whichismuchhardertocorrect.Thiseffecttypicallyworsensasspeedincreases,andisanimportantfactorlimitingtheuseofOFDMinhigh-speedvehicles.(tobecontinued)TextTextWordsNotesDiscussionHome(Continued)InordertomitigateICI5insuchscenarios,onecanshapeeachsub-carrierinordertominimizetheinterferenceresultinginanon-orthogonalsubcarriersoverlapping.Forexample,alow-complexityschemereferredtoasWCP-OFDM(WeightedCyclicPrefixOrthogonalFrequency-DivisionMultiplexing)consistsofusingshortfiltersatthetransmitteroutputinordertoperformapotentiallynon-rectangularpulseshapingandanearperfectreconstructionusingasingle-tappersubcarrierequalization.OtherICIsuppressiontechniquesusuallyincreasedrasticallythereceivercomplexity.TextTextWordsNotesDiscussionHomeorthogonaln.&adj.正交直线；[数]正交的encodevt.（将文字材料）译成密码carriern.运送者；带菌者；货架attenuationn.[物]衰减；变薄；稀释

blurv.涂污；使暗淡；玷污

trailblazern.开拓者；开路的人；先驱者

WordsWordsTextWordsNotesDiscussionHomeadaptv.使适应；改编

；适应fadev.褪色；凋谢；逐渐消失macrodiversityn.宏分集cyclicadj.环的；循环的；周期的constraintn.[数]约束；局促；强制

vestigialadj.退化的；残余的；发育不全的

WordsTextWordsNotesDiscussionHomesubcarriern.[电子][通信]副载波；辅助波

suppressionn.抑制；镇压；[植]压抑

drasticallyadv.彻底地；激烈