通信專業(yè)外文翻譯

通信工程專業(yè)畢業(yè)設(shè)計外文資料翻譯第1頁譯文正交頻分復(fù)用技術(shù)介紹正交頻分復(fù)用是一個多載波調(diào)制技術(shù)。其主要思想是將信道分成若干正交子信道將高速數(shù)據(jù)信號轉(zhuǎn)換成并行低速子數(shù)據(jù)流調(diào)制到在每個子信道上進行傳輸。正交信號能夠經(jīng)過在接收端采取相關(guān)技術(shù)來分開這么能夠降低子信道之間相互干擾。每個子信道上信號帶寬小于信道相關(guān)帶寬所以每個子信道上能夠看成平坦性衰落從而能夠消除碼間串?dāng)_。而且因為每個子信道帶寬僅僅是原信道帶寬一小部分信道均衡變得相對輕易。因為這種技術(shù)具備在雜波干擾下傳送信號能力所以經(jīng)常會被利用在輕易受外界干擾或者抵抗外界干擾能力較差傳輸介質(zhì)中。現(xiàn)在正交頻分復(fù)用技術(shù)已經(jīng)被廣泛應(yīng)用于廣播式音頻、視頻領(lǐng)域和民用通信系統(tǒng)主要應(yīng)用包含非對稱數(shù)字用戶環(huán)路、歐洲電信標(biāo)準(zhǔn)協(xié)會數(shù)字音頻廣播、數(shù)字視頻廣播、高清楚度電視、無線局域網(wǎng)等。正交頻分復(fù)用并不是才發(fā)展起來新技術(shù)其應(yīng)用已經(jīng)有40余年歷史在上個世紀(jì)60年代就已經(jīng)有些人提出了使用平行數(shù)據(jù)傳輸和頻分復(fù)用概念。70年代韋斯坦和艾伯特等人應(yīng)用離散傅里葉變換和離散傅里葉逆變換方法研制了一個完整多載波傳輸系統(tǒng)叫做正交頻分復(fù)用系統(tǒng)。正交頻分復(fù)用是一個特殊多載波傳輸方案它應(yīng)用離散傅里葉變換和離散傅里葉逆變換方法處理了產(chǎn)生多個相互正交子載波以及從子載波中恢復(fù)原信號問題。這就處理了多載波傳輸系統(tǒng)發(fā)送和傳送難題。應(yīng)用快速傅里葉變換和快速傅里葉逆變換更是使多載波傳輸系統(tǒng)復(fù)雜度大大降低。從此正交頻分復(fù)用技術(shù)開始走向?qū)嵱?。不過應(yīng)用正交頻分復(fù)用系統(tǒng)依然需要大量繁雜數(shù)字信號處理過程而當(dāng)初還缺乏數(shù)字處理功效強大元器件發(fā)射機和接收機振蕩器穩(wěn)定性以及射頻功率放大器線性要求等原因也是正交頻分復(fù)用技術(shù)實現(xiàn)制約條件。所以正交頻分復(fù)用技術(shù)遲遲沒有得到快速發(fā)展。80年代集成電路取得了突破性進展大規(guī)模集成電路讓快速傅里葉變換和快速傅里葉逆變換實現(xiàn)不再是難以逾越障礙一些其它難以實現(xiàn)困難也都得到了處理自此正交頻分復(fù)用走上了通信舞臺逐步邁向高速數(shù)字移動通信領(lǐng)域。進入90年代因為技術(shù)可實現(xiàn)性正交頻分復(fù)用應(yīng)用包括到了利用移動調(diào)頻和單邊帶信道進行高速數(shù)據(jù)通信陸地移動通信高速數(shù)字用戶環(huán)路非對稱數(shù)字用戶環(huán)路高清楚度數(shù)字電視和陸地移動廣播等各種通信系統(tǒng)。1999年國際電氣與電子工程師協(xié)會經(jīng)過了一個無線局域網(wǎng)標(biāo)準(zhǔn)IEEE802.lla其中正交頻分復(fù)用調(diào)制技術(shù)被采用為物理層標(biāo)準(zhǔn)使得傳輸速率能夠達(dá)54Mbps。這么可提供25Mbps無線ATM接口和10Mbps以太網(wǎng)無線幀結(jié)構(gòu)接口并支持語音、數(shù)據(jù)、圖像業(yè)務(wù)。這么速率完全能滿足室內(nèi)、室外各種應(yīng)用場所。歐洲電信組織寬帶射頻接入網(wǎng)局域網(wǎng)標(biāo)準(zhǔn)通信工程專業(yè)畢業(yè)設(shè)計外文資料翻譯第2頁HiperiLAN2也把正交頻分復(fù)用定為它物理層標(biāo)準(zhǔn)調(diào)制技術(shù)。正交頻分復(fù)用有許多關(guān)鍵技術(shù)。1時域和頻域同時。正交頻分復(fù)用系統(tǒng)對定時和頻率偏移敏感尤其是實際應(yīng)用中可能與頻分多址、時分多址和碼分多址等多址方式結(jié)合使用時時域和頻率同時顯得尤為主要。與其它數(shù)字通信系統(tǒng)一樣同時分為捕捉和跟蹤兩個階段。在下行鏈路中基站向各個移動終端廣播式發(fā)同時信號所以下行鏈路同時相對簡單較易實現(xiàn)。在上行鏈路中來自不一樣移動終端信號必須同時抵達(dá)基站才能確保子載波間正交性?；疽勒崭饕苿咏K端發(fā)來子載波攜帶信息進行時域和頻域同時信息提取再由基站發(fā)回移動終端方便讓移動終端進行同時。詳細(xì)實現(xiàn)時同時將分為時域同時和頻域同步也能夠時頻域同時進行同時。2信道估量。在正交頻分復(fù)用系統(tǒng)中信道估量器設(shè)計主要有兩個問題一是導(dǎo)頻信息選擇。因為無線信道經(jīng)常是衰落信道需要不停對信道進行跟蹤所以導(dǎo)頻信息也必須不停傳送。二是現(xiàn)有較低復(fù)雜度又有良好導(dǎo)頻跟蹤能力信道估量器設(shè)計。在實際設(shè)計中導(dǎo)頻信息選擇和最好估量器設(shè)計通常又是相互關(guān)聯(lián)因為估計器性能與導(dǎo)頻信息傳輸方式關(guān)于。3信道編碼和交織。為了提升數(shù)字通信系統(tǒng)性能信道編碼和交織是通常采取方法。對于衰落信道中隨機錯誤能夠采取信道編碼對于衰落信道中突發(fā)錯誤可以采取交織。實際應(yīng)用中通常同時采取信道編碼和交織深入改進整個系統(tǒng)性能。在正交頻分復(fù)用系統(tǒng)中假如信道衰落不是太深均衡是無法再利用信道分集特征來改進系統(tǒng)性能因為正交頻分復(fù)用系統(tǒng)本身具備利用信道分集特征能力通常信道特征信息已經(jīng)被正交頻分復(fù)用這種調(diào)制方式本身所利用了。不過正交頻分復(fù)用系統(tǒng)結(jié)構(gòu)卻為在子載波間進行編碼提供了機會形成編碼正交頻分復(fù)用。編碼能夠采取各種碼如分組碼、卷積碼等卷積碼效果要比分組碼好。4降低峰均功率比。因為正交頻分復(fù)用信號時域上表現(xiàn)為N個正交子載波信號疊加當(dāng)這N個信號恰好均以峰值占相加時正交頻分復(fù)用信號也將產(chǎn)生最大峰值該峰值功率是平均功率N倍。盡管峰值功率出現(xiàn)概率較低但為了不失真地傳輸這些高峰均功率比信號發(fā)送端對高功率放大器線性度要求很高且發(fā)送效率極低接收端對前端放大器以及模數(shù)轉(zhuǎn)換器線性度要求也很高。所以高峰均功率比使得正交頻分復(fù)用系統(tǒng)性能大大下降甚至直接影響實際應(yīng)用。為了處理這一問題人們提出了基于信號畸變技術(shù)、信號擾碼技術(shù)和基于信號空間擴展等降低正交頻分復(fù)用系統(tǒng)峰均功率比方法。作為通信方面應(yīng)用正交頻分復(fù)用存在很多技術(shù)優(yōu)點。1在窄帶帶寬下也能夠發(fā)出大量數(shù)據(jù)。正交頻分復(fù)用技術(shù)能同時分開最少1000個數(shù)字信號而且在干擾信號周圍能夠安全運行這種能力將直接威脅到現(xiàn)在已經(jīng)開始通信工程專業(yè)畢業(yè)設(shè)計外文資料翻譯第3頁流行碼分多址技術(shù)深入發(fā)展和壯大正是因為具備了這種特殊信號穿透能力使得正交頻分復(fù)用技術(shù)深受歐洲通信營運商以及手機生產(chǎn)商喜愛和歡迎。(2)正交頻分復(fù)用技術(shù)能夠連續(xù)不停地監(jiān)控傳輸介質(zhì)上通信特征突然改變。因為通信路徑傳送數(shù)據(jù)能力會隨時間發(fā)生改變所以正交頻分復(fù)用能動態(tài)地與之相適應(yīng)而且接通和切斷對應(yīng)載波以確保連續(xù)地進行成功通信。(3)正交頻分復(fù)用能夠自動地檢測到傳輸介質(zhì)下哪一個特定載波存在高信號衰減或干擾脈沖然后采取適宜調(diào)制方法來使指定頻率下載波進行成功通信。(4)正交頻分復(fù)用技術(shù)尤其適合使用在高層建筑物、居民密集和地理上突出地方以及將信號散播地域。高速數(shù)據(jù)傳輸及數(shù)字語音廣播都希望降低多徑效應(yīng)對信號影響。(5)正交頻分復(fù)用技術(shù)最大優(yōu)點是反抗頻率選擇性衰落或窄帶干擾。在單載波系統(tǒng)中單個衰落或干擾能夠造成整個通信鏈路失敗不過在多載波系統(tǒng)中僅僅有很小一部分載波會受到干擾。對這些子信道還能夠采取糾錯碼來進行糾錯。(6)能夠有效地反抗信號波形間干擾適適用于多徑環(huán)境和衰落信道中高速數(shù)據(jù)傳輸。當(dāng)信道中因為多徑傳輸而出現(xiàn)頻率選擇性衰落時只有落在頻帶凹陷處子載波以及其攜帶信息受影響其余子載波未受損害所以系統(tǒng)總誤碼率性能要好得多。(7)經(jīng)過各個子載波聯(lián)合編碼具備很強抗衰落能力。正交頻分復(fù)用技術(shù)本身已經(jīng)利用了信道頻率分集假如衰落不是尤其嚴(yán)重就沒有必要再加時域均衡器。經(jīng)過將各個信道聯(lián)合編碼則能夠使系統(tǒng)性能得到提升。(8)正交頻分復(fù)用技術(shù)抗窄帶干擾性很強因為這些干擾僅僅影響到很小一部分子信道。(9)信道利用率很高這一點在頻譜資源有限無線環(huán)境中尤為主要當(dāng)子載波個數(shù)很大時系統(tǒng)頻譜利用率趨于2Baud/Hz。即使正交頻分復(fù)用有上述優(yōu)點不過一樣其信號調(diào)制機制也使得其信號在傳輸過程中存在著一些劣勢。(1)對相位噪聲和載波頻偏十分敏感。這是正交頻分復(fù)用技術(shù)一個非常致命缺點整個正交頻分復(fù)用系統(tǒng)對各個子載波之間正交性要求格外嚴(yán)格任何一點小載波頻偏都會破壞子載波之間正交性引發(fā)符號間干擾一樣相位噪聲也會造成碼元星座點旋轉(zhuǎn)、擴散從而形成信道間干擾。而單載波系統(tǒng)就沒有這個問題相位噪聲和載波頻偏僅僅是降低了接收到信噪比而不會引發(fā)相互之間干擾。(2)峰均比過大。正交頻分復(fù)用信號由多個子載波信號組成這些子載波信號由不一樣調(diào)制符號獨立調(diào)制。同傳統(tǒng)恒包絡(luò)調(diào)制方法相比正交頻分復(fù)用調(diào)制存在一個很高峰值因子。因為其信號是很多個小信號總和這些小信號相位是由要傳輸數(shù)據(jù)序列決定。對一些數(shù)據(jù)這些小信號可能同相而在幅度上疊加在一起從而產(chǎn)生很大通信工程專業(yè)畢業(yè)設(shè)計外文資料翻譯第4頁瞬時峰值幅度。而峰均比過大將會增加模數(shù)轉(zhuǎn)換器和數(shù)模轉(zhuǎn)換器復(fù)雜性而且會降低射頻功率放大器效率。同時在發(fā)射端放大器最大輸出功率就限制了信號峰值這會在正交頻分復(fù)用頻段內(nèi)和相鄰頻段之間產(chǎn)生干擾。(3)所需線性范圍寬。因為正交頻分復(fù)用系統(tǒng)峰值平均功率比大對非線性放大更為敏感故正交頻分復(fù)用調(diào)制系統(tǒng)比單載波系統(tǒng)對放大器線性范圍要求更高。通信工程專業(yè)畢業(yè)設(shè)計外文資料翻譯第5頁原文OrthogonalfrequencydivisionmultiplexingtechnologyintroductionOFDMisamulticarriermodulationtechnique.Themainideais:dividesthechannelintoseveralorthogonalsubchannels,thehigh-speeddatasignalintoalowflowsubdataparallelism,modulationtotransmitineachsubchannel.Orthogonalsignalcanbeseparatedbytheuseofrelatedtechnologiesatthereceivingend,thusreducingthemutualinterferencebetweenthechannel.Relativebandwidthsignalbandwidthofeachsubchannelisshorterthanthechannel,soeachsubchannelcanbeseenflatfadingchannel,whichcaneliminateintersymbolinterference.Andbecausethebandwidthofeachchannelisonlyasmallpartofthechannelbandwidth,channelequalizationbecomesrelativelyeasy.Becauseofthistechnologyhastheabilitytotransmitsignalsinclutter,itisoftenusedinthetransmissionmediumisvulnerabletooutsideinterferenceorresistinterferenceabilityispoorerin.Atpresent,theorthogonalfrequencydivisionmultiplexingtechniquehasbeenwidelyappliedtobroadcastaudio,videoandcivilcommunicationsystem,includingthemainapplication:digitalsubscriberloop,nonsymmetryoftheEuropeanTelecommunicationsStandardsInstituteofdigitalaudiobroadcasting,digitalvideobroadcasting,high-definitiontelevision,wirelesslocalareanetwork.Thenewtechnologyoforthogonalfrequencydivisionmultiplexingisnotdevelopment,itsapplicationhasahistoryof40years,60yearsinthelastcentury,ithasbeenputforwardtheconceptofusingparalleldatatransmissionandfrequencydivisionmultiplexing.In70,WeinseinandAlbertetal.ApplicationofdiscreteFuLiyetransformandthediscreteFuLiyetransformisdevelopedformulticarriertransmissionsystemofacomplete,calledorthogonalfrequencydivisionmultiplexingsystem.Orthogonalfrequencydivisionmultiplexingisaspecialmulticarriertransmissionscheme,whichusingthediscreteFuLiyetransformanddiscreteFuLiyetransformsolutiontorestoretheoriginalsignaltogenerateapluralityofmutuallyorthogonalsubcarriersandsubcarrierinquestion.Thiswouldsolvetheproblemofmulticarriertransmissionsystemtransmissionandtransfer.ApplicationofthefastFuLiyetransformandfastFuLiyetransformisthecomplexityofmulticarriertransmissionsystemisgreatlyreduced.Fromtheorthogonalfrequencydivisionmultiplexingtechnologytopractical.Buttheapplicationoforthogonalfrequencydivisionmultiplexingsystemstillneedsalotofdigitalsignalprocessingprocedureiscomplicated,andtherewasalackofdigital通信工程專業(yè)畢業(yè)設(shè)計外文資料翻譯第6頁processingpowerfulcomponents,restrictingfactorsandorthogonalfrequencydivisionmultiplexingtechnologyofthetransmitterandthereceiveroscillatorstabilityandlinearpoweramplifierrequirements.Therefore,thetechnologyoforthogonalfrequencydivisionmultiplexinghasnotobtainedtherapiddevelopment.In80,theintegratedcircuittobeabreakthrough,realizelarge-scaleintegratedcircuittomakethefastFuLiyetransformandfastFuLiyetransformisnotinsurmountableobstacles,someotherdifficultiesofimplementationhavebeensolved,sinceorthogonalfrequencydivisionmultiplexingonthecommunicationstage,graduallymovingtowardshighspeeddigitalmobilecommunicationfield.Enter90age,therealizationofthetechnology,applicationoforthogonalfrequencydivisionmultiplexinginvolvestheuseofmobileFMandsinglesidebandchannelhighspeeddatacommunication,mobilecommunication,highspeeddigitalsubscriberloop,asymmetricdigitalsubscriberloop,high-definitiondigitalTVandlandmobileradiocommunicationsystem.In1999,theInternationalAssociationofelectricalandelectronicengineersthroughtheIEEE802.llawirelessLANstandardone,theorthogonalfrequencydivisionmultiplexingmodulationtechniqueisadoptedasthestandardofphysicallayer,thetransmissionratecanbeupto54Mbps.Inthisway,EthernetwirelessframestructureofwirelessATMinterfaceand10Mbpsinterfaceprovides25Mbps,andsupportforvoice,data,videoservice.Thisratecansatisfythevariousapplicationsofindoor,outdoor.LANstandardHiperiLAN2broadbandradioaccessnetworkEuropeanTelecommunicationsOrganizationoftheorthogonalfrequencydivisionmultiplexingforphysicallayermodulationtechnologyit.Orthogonalfrequencydivisionmultiplexingmanykeytechnology.(1)thetimedomainandfrequencydomainsynchronization.Orthogonalfrequencydivisionmultiplexingsystemissensitivetothetimingandfrequencyoffset,inparticularmaybeusedinactualapplicationcombinedwithFDMA,TDMAandCDMA,FDMA,timeandfrequencysynchronizationisveryimportant.Aswithotherdigitalcommunicationsystems,synchronizationisdividedintotwostagestocaptureandtrack.Inthedownlink,basestationtoeachmobileterminaltobroadcasttypesynchronoussignal,therefore,thedownlinksynchronizationisrelativelysimple,iseasytorealize.Intheuplink,signalsfromdifferentmobileterminalmustbesynchronizedtothebasestation,inordertoensuretheorthogonalitybetweensubcarriers.Thebasestationaccordingtotheextractionofeachmobileterminaltothesubcarriertocarrytheinformationofthetimedomainandfrequencydomainsynchronizationinformation,thenthebasestationtransmittedbacktothemobileterminal,sothatthemobileterminalsynchronization.Concreteimplementation,synchronizationwillbe通信工程專業(yè)畢業(yè)設(shè)計外文資料翻譯第7頁dividedintosynchronoustimesynchronizationandfrequencydomain,timedomainandfrequencydomainatthesametimesynchronizationcanalsobe.(2)Channelestimation.Inorthogonalfrequencydivisionmultiplexingsystem,thedesignofchannelestimatorhastwomainproblems:oneistheselectionofpilotinformation.Sincethewirelesschannelisoftenafadingchannel,needtocontinuetotrackthechannel,thepilotinformationmustalsobecontinuousconveyor.Thetwoistodesignboththechannelestimatorwithlowcomplexityandgoodpilottrackingcapability.Inpracticaldesign,thedesignofpilotinformationselectionandoptimalestimatorisusuallyrelatedtoeachother,becausethetransmissionperformanceandpilotinformationestimatorof.(3)Channelcodingandinterleaving.Inordertoimprovetheperformanceofdigitalcommunicationsystem,channelcodingandinterleavingisacommonlyusedmethod.Forfadingrandomerrorsinthechannel,thechannelcodingforfading;bursterrorchannel,cantheinterleaving.Inpracticalapplication,usuallyatthesametime,channelcodingandinterleaving,tofurtherimprovetheperformanceofthewholesystem.Inorthogonalfrequencydivisionmultiplexingsystem,ifthechannelfadingisnottoodeep,theequilibriumisnolongerusediversitychanneltoimprovetheperformanceofthesystem,becausetheorthogonalfrequencydivisionmultiplexingsystemitselfhastheabilitytoutilizethechanneldiversitycharacteristics,informationthegeneralchannelcharacteristicshavebeenorthogonalfrequencydivisionmultiplexingthismodulationisused.Butthestructureoforthogonalfrequencydivisionmultiplexingsystemisencodedinintercarrierprovidesopportunities,theformationofcodedorthogonalfrequencydivisionmultiplexing.Codecanbeusedinavarietyofcode,suchascode,convolutionalcode,convolutionalcodestoscoregoodresults.(4)Reducingthepeaktoaveragepowerratio.TheorthogonalfrequencydivisionmultiplexingsignalintimedomainontheperformanceofthesuperpositionofNorthogonalsubcarriersignal,whentheNsignalisinthepeakofovertime,orthogonalfrequencydivisionmultiplexingsignalwillproducethemaximumpeak,thepeakpowerisNtimestheaveragepower.Althoughtheprobabilityofpeakpowerofthelower,butinordernottotransmitthesignaldistortionofthepeaktoaveragepowerratio,thesendingendonlinearhighpoweramplifierhighdegreerequirementsandthetransmissionefficiencyisverylow,thereceivingendofthefront-endamplifierandADClinearityrequirementsareveryhigh.Therefore,theperformanceofhighpeaktoaveragepowerratiooftheorthogonalfrequencydivisionmultiplexingsystemisgreatlydecreasedorevendirectlyaffecttheactualapplication.Inordertosolvethisproblem,peopleputforwardthesignaldistortiontechniques,signalscramblingtechniquesandsignalspaceexpansionbasedonreductionoforthogonalfrequency通信工程專業(yè)畢業(yè)設(shè)計外文資料翻譯第8頁divisionmultiplexingsystempeakaveragepowerratiobasedmethod.Asanapplicationofcommunication,OFDMhasmanytechnicaladvantages.(1)Inthenarrowbandwidthcanbealotofdatasent.Orthogonalfrequencydivisionmultiplexingtechnologycanalsoseparatedatleast1000digitalsignal,andcansafelyrunaroundininterferencesignal,thisabilitywillbeadirectthreattothefurtherdevelopmentofthecurrenthasbeguntoCDMAtechnologypopularandexpands,itisbecauseofthespecialsignalpenetrationcapabilityoftheorthogonalfrequencydivisionmultiplexingtechnologybyEuropeantelecommunicationsoperatorsandmobilephonemanufacturerswelcomedandaccepted.(2)Asuddenchangeinorthogonalfrequencydivisionmultiplexingtechnologycanconstantlymonitorthecommunicationcharacteristicsofthetransmissionmedium.Becauseoftheabilitytotransmitdatacommunicationpathwillchangeovertime,sotheorthogonalfrequencydivisionmultiplexingcandynamicallyadapt,andconnectanddisconnectthecorrespondingcarriertoensureongoingsuccessfulcommunication.(3)Orthogonalfrequencydivisionmultiplexingcanautomaticallydetectthetransmissionmediumunderwhichaspecificcarrierhashighsignalattenuationandinterferencepulse,andthentaketheappropriatemeasurestomakethespecifiedcarrierfrequencymodulationofthesuccessfulcommunication.(4)Orthogonalfrequencydivisionmultiplexingtechniqueisespeciallysuitableforuseinhigh-risebuildings,highlightingthedenselypopulatedandgeographicalplaceandsignalsofspreadingarea.Datacommunicationanddigitalaudiobroadcastinghighhopetoreducemultipatheffectsonsignal.(5)Thebiggestadvantagesoforthogonalfrequencydivisionmultiplexingtechniqueistocombatfrequencyselectivefadingornarrowbandinterference.Insinglecarriersystems,asinglefadingorinterferencecancausetheentirecommunicationlinkfailure,butinamulticarriersystem,onlyaverysmallpartofcarrierinterference.Thesubchannelcanalsouseerrorcorrectingcodesforerrorcorrection.(6)Caneffectivelyresisttheinterferencebetweenthesignalwaveform,highspeeddatatransmissioninmultipathenvironmentandfadingchannels.Whenthechannelbecauseofmultipathtransmissionfrequencyselectivefading,onlyfellonthebanddepressionsubcarrierandcarryinformationaffected,subcarrierotherunimpaired,thereforebetterBERperformancethanthegeneral.(7)Throughthejointcodingofeachsubcarrier,hasastrongabilityofantifading.Orthogonalfrequencydivisionmultiplexingtechnologyitselfhastakenadvantageof通信工程專業(yè)畢業(yè)設(shè)計外文資料翻譯第9頁frequencydiversitychannelfading,ifnotserious,thereisnoneedtoaddthetimedomainequalizer.Throughthevariouschannelcoding,itcanimprovetheperformanceofsystem.(8)Orthogonalfrequencydivisionmultiplexingtechnologyofnarrowbandinterferenceisverystrong,becausethesedisturbancesaffectonlyasmallfractionofthesubchannel.(9)Thechannelutilizationrateishigh,thisisanimportantpointinthespectrumisalimitedresourceinwirelessenvironment;whenthesubcarriernumberislarge,thesystemspectrumutilizationtendsto2Baud/Hz.Althoughorthogonalfrequencydivisionmultiplexinghastheseadvantages,butalsoitsmodulationmechanismalsomakesthesignalinthetransmissionprocessexistsomedisadvantages.(1)Isverysensitivetothephasenoiseandcarrierfrequ