通信專業(yè)外文翻譯

通信工程專業(yè)畢業(yè)設(shè)計(jì)外文資料翻譯第1頁譯文 正交頻分復(fù)用技術(shù)簡介正交頻分復(fù)用是一種多載波調(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)換成并行的低速子數(shù)據(jù)流調(diào)制到在每個(gè)子信道上進(jìn)行傳輸。正交信號可以通過在接收端采用相關(guān)技術(shù)來分開這樣可以減少子信道之間的相互干擾。每個(gè)子信道上的信號帶寬小于信道的相關(guān)帶寬因此每個(gè)子信道上的可以看成平坦性衰落從而可以消除碼間串?dāng)_。而且由于每個(gè)子信道的帶寬僅僅是原信道帶寬的一小部分信道均衡變得相對容易。由于這種技術(shù)具有在雜波干擾下傳送信號的能力因此常常會被利用在容易受外界干擾或者抵抗外界干擾能力較差的傳輸介質(zhì)中。目前正交頻分復(fù)用技術(shù)已經(jīng)被廣泛應(yīng)用于廣播式的音頻、視頻領(lǐng)域和民用通信系統(tǒng)主要的應(yīng)用包括非對稱的數(shù)字用戶環(huán)路、歐洲電信標(biāo)準(zhǔn)協(xié)會的數(shù)字音頻廣播、數(shù)字視頻廣播、高清晰度電視、無線局域網(wǎng)等。正交頻分復(fù)用并不是才發(fā)展起來的新技術(shù)其應(yīng)用已有40余年的歷史在上個(gè)世紀(jì)60年代就已經(jīng)有人提出了使用平行數(shù)據(jù)傳輸和頻分復(fù)用的概念。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的方法研制了一個(gè)完整的多載波傳輸系統(tǒng)叫做正交頻分復(fù)用系統(tǒng)。正交頻分復(fù)用是一種特殊的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的方法解決了產(chǎn)生多個(gè)互相正交的子載波以及從子載波中恢復(fù)原信號的問題。這就解決了多載波傳輸系統(tǒng)發(fā)送和傳送的難題。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)的復(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?。但是?yīng)用正交頻分復(fù)用系統(tǒng)仍然需要大量繁雜的數(shù)字信號處理過程而當(dāng)時(shí)還缺乏數(shù)字處理功能強(qiáng)大的元器件發(fā)射機(jī)和接收機(jī)振蕩器的穩(wěn)定性以及射頻功率放大器的線性要求等因素也是正交頻分復(fù)用技術(shù)實(shí)現(xiàn)的制約條件。因此正交頻分復(fù)用技術(shù)遲遲沒有得到迅速發(fā)展。80年代集成電路獲得了突破性進(jìn)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的實(shí)現(xiàn)不再是難以逾越的障礙一些其它難以實(shí)現(xiàn)的困難也都得到了解決自此正交頻分復(fù)用走上了通信的舞臺 逐步邁向高速數(shù)字移動(dòng)通信的領(lǐng)域。進(jìn)入90年代由于技術(shù)的可實(shí)現(xiàn)性正交頻分復(fù)用的應(yīng)用涉及到了利用移動(dòng)調(diào)頻和單邊帶信道進(jìn)行高速數(shù)據(jù)通信陸地移動(dòng)通信高速數(shù)字用戶環(huán)路非對稱數(shù)字用戶環(huán)路高清晰度數(shù)字電視和陸地移動(dòng)廣播等各種通信系統(tǒng)。1999年國際電氣與電子工程師協(xié)會通過了一個(gè)的無線局域網(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) 使得傳輸速率可以達(dá)54Mbps。這樣可提供25Mbps的無線ATM接口和10Mbps的以太網(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)。這樣的速率完全能滿足室內(nèi)、室外的各種應(yīng)用場合。歐洲電信組織的寬帶射頻接入網(wǎng)的局域網(wǎng)標(biāo)準(zhǔn)通信工程專業(yè)畢業(yè)設(shè)計(jì)外文資料翻譯第2頁HiperiLAN2也把正交頻分復(fù)用定為它的物理層標(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(shí)域和頻域同步。正交頻分復(fù)用系統(tǒng)對定時(shí)和頻率偏移敏感特別是實(shí)際應(yīng)用中可能與頻分多址、時(shí)分多址和碼分多址等多址方式結(jié)合使用時(shí)時(shí)域和頻率同步顯得尤為重要。與其它數(shù)字通信系統(tǒng)一樣 同步分為捕獲和跟蹤兩個(gè)階段。在下行鏈路中基站向各個(gè)移動(dòng)終端廣播式發(fā)同步信號所以下行鏈路同步相對簡單較易實(shí)現(xiàn)。在上行鏈路中來自不同移動(dòng)終端的信號必須同步到達(dá)基站才能保證子載波間的正交性。基站根據(jù)各移動(dòng)終端發(fā)來的子載波攜帶信息進(jìn)行時(shí)域和頻域同步信息的提取再由基站發(fā)回移動(dòng)終端以便讓移動(dòng)終端進(jìn)行同步。具體實(shí)現(xiàn)時(shí)同步將分為時(shí)域同步和頻域同步也可以時(shí)頻域同時(shí)進(jìn)行同步。2信道估計(jì)。在正交頻分復(fù)用系統(tǒng)中信道估計(jì)器的設(shè)計(jì)主要有兩個(gè)問題一是導(dǎo)頻信息的選擇。由于無線信道常常是衰落信道需要不斷對信道進(jìn)行跟蹤因此導(dǎo)頻信息也必須不斷的傳送。二是既有較低的復(fù)雜度又有良好的導(dǎo)頻跟蹤能力的信道估計(jì)器的設(shè)計(jì)。在實(shí)際設(shè)計(jì)中導(dǎo)頻信息選擇和最佳估計(jì)器的設(shè)計(jì)通常又是相互關(guān)聯(lián)的因?yàn)楣烙?jì)器的性能與導(dǎo)頻信息的傳輸方式有關(guān)。3信道編碼和交織。為了提高數(shù)字通信系統(tǒng)性能信道編碼和交織是通常采用的方法。對于衰落信道中的隨機(jī)錯(cuò)誤可以采用信道編碼對于衰落信道中的突發(fā)錯(cuò)誤可以采用交織。實(shí)際應(yīng)用中通常同時(shí)采用信道編碼和交織進(jìn)一步改善整個(gè)系統(tǒng)的性能。在正交頻分復(fù)用系統(tǒng)中如果信道衰落不是太深均衡是無法再利用信道的分集特性來改善系統(tǒng)性能的因?yàn)檎活l分復(fù)用系統(tǒng)自身具有利用信道分集特性的能力一般的信道特性信息已經(jīng)被正交頻分復(fù)用這種調(diào)制方式本身所利用了。但是正交頻分復(fù)用系統(tǒng)的結(jié)構(gòu)卻為在子載波間進(jìn)行編碼提供了機(jī)會 形成編碼正交頻分復(fù)用。編碼可以采用各種碼如分組碼、卷積碼等卷積碼的效果要比分組碼好。4降低峰均功率比。由于正交頻分復(fù)用信號時(shí)域上表現(xiàn)為N個(gè)正交子載波信號的疊加當(dāng)這N個(gè)信號恰好均以峰值占相加時(shí)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的N倍。盡管峰值功率出現(xiàn)的概率較低但為了不失真地傳輸這些高峰均功率比的信號發(fā)送端對高功率放大器的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)換器的線性度要求也很高。因此高的峰均功率比使得正交頻分復(fù)用系統(tǒng)的性能大大下降甚至直接影響實(shí)際應(yīng)用。為了解決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(kuò)展等降低正交頻分復(fù)用系統(tǒng)峰均功率比的方法。作為通信方面的應(yīng)用正交頻分復(fù)用存在很多技術(shù)優(yōu)點(diǎn)。1在窄帶帶寬下也能夠發(fā)出大量的數(shù)據(jù)。正交頻分復(fù)用技術(shù)能同時(shí)分開至少1000個(gè)數(shù)字信號而且在干擾的信號周圍可以安全運(yùn)行這種能力將直接威脅到目前已經(jīng)開始通信工程專業(yè)畢業(yè)設(shè)計(jì)外文資料翻譯第3頁流行的碼分多址技術(shù)的進(jìn)一步發(fā)展和壯大正是由于具有了這種特殊的信號穿透能力使b5E2RGbCAP得正交頻分復(fù)用技術(shù)深受歐洲通信營運(yùn)商以及手機(jī)生產(chǎn)商的喜愛和歡迎。(2>正交頻分復(fù)用技術(shù)能夠持續(xù)不斷地監(jiān)控傳輸介質(zhì)上通信特性的突然變化。由于通信路徑傳送數(shù)據(jù)的能力會隨時(shí)間發(fā)生變化所以正交頻分復(fù)用能動(dòng)態(tài)地與之相適應(yīng)并且接通和切斷相應(yīng)的載波以保證持續(xù)地進(jìn)行成功的通信。(3>正交頻分復(fù)用可以自動(dòng)地檢測到傳輸介質(zhì)下哪一個(gè)特定的載波存在高的信號衰減或干擾脈沖然后采取合適的調(diào)制措施來使指定頻率下的載波進(jìn)行成功通信。(4>正交頻分復(fù)用技術(shù)特別適合使用在高層建筑物、居民密集和地理上突出的地方以及將信號散播的地區(qū)。高速的數(shù)據(jù)傳播及數(shù)字語音廣播都希望降低多徑效應(yīng)對信號的影響。(5>正交頻分復(fù)用技術(shù)的最大優(yōu)點(diǎn)是對抗頻率選擇性衰落或窄帶干擾。在單載波系統(tǒng)中單個(gè)衰落或干擾能夠?qū)е抡麄€(gè)通信鏈路失敗但是在多載波系統(tǒng)中僅僅有很小一部分載波會受到干擾。對這些子信道還可以采用糾錯(cuò)碼來進(jìn)行糾錯(cuò)。(6〉可以有效地對抗信號波形間的干擾 適用于多徑環(huán)境和衰落信道中的高速數(shù)據(jù)傳輸。當(dāng)信道中因?yàn)槎鄰絺鬏敹霈F(xiàn)頻率選擇性衰落時(shí)只有落在頻帶凹陷處的子載波以及其攜帶的信息受影響其他的子載波未受損害因此系統(tǒng)總的誤碼率性能要好得多。(7〉通過各個(gè)子載波的聯(lián)合編碼具有很強(qiáng)的抗衰落能力。正交頻分復(fù)用技術(shù)本身已經(jīng)利用了信道的頻率分集如果衰落不是特別嚴(yán)重就沒有必要再加時(shí)域均衡器。通過將各個(gè)信道聯(lián)合編碼則可以使系統(tǒng)性能得到提高。(8＞正交頻分復(fù)用技術(shù)抗窄帶干擾性很強(qiáng)因?yàn)檫@些干擾僅僅影響到很小一部分的子信道。(9〉信道利用率很高這一點(diǎn)在頻譜資源有限的無線環(huán)境中尤為重要當(dāng)子載波個(gè)數(shù)很大時(shí)系統(tǒng)的頻譜利用率趨于2Baud/Hz。雖然正交頻分復(fù)用有上述優(yōu)點(diǎn)但是同樣其信號調(diào)制機(jī)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(gè)非常致命的缺點(diǎn)整個(gè)正交頻分復(fù)用系統(tǒng)對各個(gè)子載波之間的正交性要求格外嚴(yán)格任何一點(diǎn)小的載波頻偏都會破壞子載波之間的正交性引起符號間干擾同樣相位噪聲也會導(dǎo)致碼元星座點(diǎn)的旋轉(zhuǎn)、擴(kuò)散從而形成信道間干擾。而單載波系統(tǒng)就沒有這個(gè)問題相位噪聲和載波頻偏僅僅是降低了接收到的信噪比而不會引起互相之間的干擾。(2〉峰均比過大。正交頻分復(fù)用信號由多個(gè)子載波信號組成這些子載波信號由不同的調(diào)制符號獨(dú)立調(diào)制。同傳統(tǒng)的恒包絡(luò)的調(diào)制方法相比 正交頻分復(fù)用調(diào)制存在一個(gè)很高的峰值因子。因?yàn)槠湫盘柺呛芏鄠€(gè)小信號的總和這些小信號的相位是由要傳輸?shù)臄?shù)據(jù)序列決定的。對某些數(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(jì)外文資料翻譯第4頁的瞬時(shí)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)換器的復(fù)雜性而且會plEanqFDPw降低射頻功率放大器的效率。同時(shí)在發(fā)射端放大器的最大輸出功率就限制了信號的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。由于正交頻分復(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)對放大器的線性范圍要求更高。通信工程專業(yè)畢業(yè)設(shè)計(jì)外文資料翻譯第5頁原文OrthogonalfrequencydivisionmultiplexingtechnologyintroductionOFDMisamulticarriermodulationtechnique.Themainideais:dividesthechannelDXDiTa9E3dintoseveralorthogonalsubchannels,thehigh-speeddatasignalintoalowflowsubdataRTCrpUDGiTparallelism,modulationtotransmitineachsubchannel.Orthogonalsignalcanbeseparated5PCzVD7HxAbytheuseofrelatedtechnologiesatthereceivingend,thusreducingthemutualinterferencejLBHrnAILgbetweenthechannel.RelativebandwidthsignalbandwidthofeachsubchannelisshorterthanxHAQX74J0Xthechannel,soeachsubchannelcanbeseenflatfadingchannel,whichcaneliminateLDAYtRyKfEintersymbolinterference.AndbecausethebandwidthofeachchannelisonlyasmallpartofZzz6ZB2Ltkthechannelbandwidth,channelequalizationbecomesrelativelyeasy.BecauseofthisdvzfvkwMI1technologyhastheabilitytotransmitsignalsinclutter,itisoftenusedinthetransmissionrqyn14ZNXImediumisvulnerabletooutsideinterferenceorresistinterferenceabilityispoorerin.AtEmxvxOtOcopresent,theorthogonalfrequencydivisionmultiplexingtechniquehasbeenwidelyappliedtoSixE2yXPq5broadcastaudio,videoandcivilcommunicationsystem,includingthemainapplication:6ewMyirQFLdigitalsubscriberloop,nonsymmetryoftheEuropeanTelecommunicationsStandardskavU42VRUsInstituteofdigitalaudiobroadcasting,digitalvideobroadcasting,high-definitiontelevision,y6v3ALoS89wirelesslocalareanetwork.Thenewtechnologyoforthogonalfrequencydivisionmultiplexingisnotdevelopment,M2ub6vSTnPitsapplicationhasahistoryof40years,60yearsinthelastcentury,ithasbeenputforward0YujCfmUCwtheconceptofusingparalleldatatransmissionandfrequencydivisionmultiplexing.eUts8ZQVRdIn70,WeinseinandAlbertetal.ApplicationofdiscreteFuLiyetransformandthesQsAEJkW5TdiscreteFuLiyetransformisdevelopedformulticarriertransmissionsystemofacomplete,GMsIasNXkAcalledorthogonalfrequencydivisionmultiplexingsystem.OrthogonalfrequencydivisionTIrRGchYzgmultiplexingisaspecialmulticarriertransmissionscheme,whichusingthediscreteFuLiye7EqZcWLZNXtransformanddiscreteFuLiyetransformsolutiontorestoretheoriginalsignaltogeneratealzq7IGf02Epluralityofmutuallyorthogonalsubcarriersandsubcarrierinquestion.ThiswouldsolvethezvpgeqJ1hkproblemofmulticarriertransmissionsystemtransmissionandtransfer.ApplicationofthefastNrpoJac3v1FuLiyetransformandfastFuLiyetransformisthecomplexityofmulticarriertransmission1nowfTG4KIsystemisgreatlyreduced.FromtheorthogonalfrequencydivisionmultiplexingtechnologytofjnFLDa5Zopractical.ButtheapplicationoforthogonalfrequencydivisionmultiplexingsystemstillneedstfnNhnE6e5alotofdigitalsignalprocessingprocedureiscomplicated,andtherewasalackofdigitalHbmVN777sL通信工程專業(yè)畢業(yè)設(shè)計(jì)外文資料翻譯第6頁processingpowerfulcomponents,restrictingfactorsandorthogonalfrequencydivisionV7l4jRB8Hsmultiplexingtechnologyofthetransmitterandthereceiveroscillatorstabilityandlinear83lcPA59W9poweramplifierrequirements.Therefore,thetechnologyoforthogonalfrequencydivisionmZkklkzaaPmultiplexinghasnotobtainedtherapiddevelopment.AVktR43bpwIn80,theintegratedcircuittobeabreakthrough,realizelarge-scaleintegratedcircuittoORjBnOwcEdmakethefastFuLiyetransformandfastFuLiyetransformisnotinsurmountableobstacles,2MiJTy0dTTsomeotherdifficultiesofimplementationhavebeensolved,sinceorthogonalfrequencygIiSpiue7Adivisionmultiplexingonthecommunicationstage,graduallymovingtowardshighspeeduEh0U1Yfmhdigitalmobilecommunicationfield.Enter90age,therealizationofthetechnology,applicationoforthogonalfrequencyIAg9qLsgBXdivisionmultiplexinginvolvestheuseofmobileFMandsinglesidebandchannelhighspeedWwghWvVhPEdatacommunication,mobilecommunication,highspeeddigitalsubscriberloop,asymmetricasfpsfpi4kdigitalsubscriberloop,high-definitiondigitalTVandlandmobileradiocommunicationooeyYZTjj1system.In1999,theInternationalAssociationofelectricalandelectronicengineersthroughBkeGuInkxItheIEEE802.llawirelessLANstandardone,theorthogonalfrequencydivisionmultiplexingPgdO0sRlMomodulationtechniqueisadoptedasthestandardofphysicallayer,thetransmissionratecanbe3cdXwckm15upto54Mbps.Inthisway,EthernetwirelessframestructureofwirelessATMinterfaceandh8c52WOngM10Mbpsinterfaceprovides25Mbps,andsupportforvoice,data,videoservice.Thisratecanv4bdyGioussatisfythevariousapplicationsofindoor,outdoor.LANstandardHiperiLAN2broadbandJ0bm4qMpJ9radioaccessnetworkEuropeanTelecommunicationsOrganizationoftheorthogonalXVauA9grYPfrequencydivisionmultiplexingforphysicallayermodulationtechnologyit.bR9C6TJscwOrthogonalfrequencydivisionmultiplexingmanykeytechnology.pN9LBDdtrd(1>thetimedomainandfrequencydomainsynchronization.OrthogonalfrequencyDJ8T7nHuGTdivisionmultiplexingsystemissensitivetothetimingandfrequencyoffset,inparticularmayQF81D7bvUAbeusedinactualapplicationcombinedwithFDMA,TDMAandCDMA,FDMA,timeand4B7a9QFw9hfrequencysynchronizationisveryimportant.Aswithotherdigitalcommunicationsystems,ix6iFA8xoXsynchronizationisdividedintotwostagestocaptureandtrack.Inthedownlink,basestationwt6qbkCyDEtoeachmobileterminaltobroadcasttypesynchronoussignal,therefore,thedownlinkKp5zH46zRksynchronizationisrelativelysimple,iseasytorealize.Intheuplink,signalsfromdifferentYl4HdOAA61mobileterminalmustbesynchronizedtothebasestation,inordertoensuretheorthogonalitych4PJx4BlIbetweensubcarriers.Thebasestationaccordingtotheextractionofeachmobileterminaltoqd3YfhxCzothesubcarriertocarrytheinformationofthetimedomainandfrequencydomainE836L11DO5synchronizationinformation,thenthebasestationtransmittedbacktothemobileterminal,soS42ehLvE3Mthatthemobileterminalsynchronization.Concreteimplementation,synchronizationwillbe501nNvZFis通信工程專業(yè)畢業(yè)設(shè)計(jì)外文資料翻譯第7頁dividedintosynchronoustimesynchronizationandfrequencydomain,timedomainandjW1viftGw9frequencydomainatthesametimesynchronizationcanalsobe.xS0DOYWHLP(2>Channelestimation.Inorthogonalfrequencydivisionmultiplexingsystem,theLOZMkIqI0wdesignofchannelestimatorhastwomainproblems:oneistheselectionofpilotinformation.ZKZUQsUJedSincethewirelesschannelisoftenafadingchannel,needtocontinuetotrackthechannel,thedGY2mcoKtTpilotinformationmustalsobecontinuousconveyor.ThetwoistodesignboththechannelrCYbSWRLIAestimatorwithlowcomplexityandgoodpilottrackingcapability.Inpracticaldesign,theFyXjoFlMWhdesignofpilotinformationselectionandoptimalestimatorisusuallyrelatedtoeachother,TuWrUpPObXbecausethetransmissionperformanceandpilotinformationestimatorof.7qWAq9jPqE(3>Channelcodingandinterleaving.InordertoimprovetheperformanceofdigitalllVIWTNQFkcommunicationsystem,channelcodingandinterleavingisacommonlyusedmethod.ForyhUQsDgRT1fadingrandomerrorsinthechannel,thechannelcodingforfading。bursterrorchannel,canMdUZYnKS8Itheinterleaving.Inpracticalapplication,usuallyatthesametime,channelcodingand09T7t6eTnointerleaving,tofurtherimprovetheperformanceofthewholesystem.Inorthogonale5TfZQIUB5frequencydivisionmultiplexingsystem,ifthechannelfadingisnottoodeep,theequilibriums1SovAcVQMisnolongerusediversitychanneltoimprovetheperformanceofthesystem,becausetheGXRw1kFW5sorthogonalfrequencydivisionmultiplexingsystemitselfhastheabilitytoutilizethechannelUTREx49Xj9diversitycharacteristics,informationthegeneralchannelcharacteristicshavebeenorthogonal8PQN3NDYyPfrequencydivisionmultiplexingthismodulationisused.ButthestructureoforthogonalmLPVzx7ZNwfrequencydivisionmultiplexingsystemisencodedinintercarrierprovidesopportunities,theAHP35hB02dformationofcodedorthogonalfrequencydivisionmultiplexing.CodecanbeusedinavarietyNDOcB141gTofcode,suchascode,convolutionalcode,convolutionalcodestoscoregoodresults.1zOk7Ly2vA(4>Reducingthepeaktoaveragepowerratio.TheorthogonalfrequencydivisionfuNsDv23KhmultiplexingsignalintimedomainontheperformanceofthesuperpositionofNorthogonaltqMB9ew4YXsubcarriersignal,whentheNsignalisinthepeakofovertime,orthogonalfrequencydivisionHmMJFY05dEmultiplexingsignalwillproducethemaximumpeak,thepeakpowerisNtimestheaverageViLRaIt6skpower.Althoughtheprobabilityofpeakpowerofthelower,butinordernottotransmitthe9eK0GsX7H1signaldistortionofthepeaktoaveragepowerratio,thesendingendonlinearhighpowernaK8ccr8VIamplifierhighdegreerequirementsandthetransmissionefficiencyisverylow,thereceivingB6JgIVV9aoendofthefront-endamplifierandADClinearityrequirementsareveryhigh.Therefore,theP2IpeFpap5performanceofhighpeaktoaveragepowerratiooftheorthogonalfrequencydivision3YIxKpScDMmultiplexingsystemisgreatlydecreasedorevendirectlyaffecttheactualapplication.IngUHFg9mdSsordertosolvethisproblem,peopleputforwardthesignaldistortiontechniques,signaluQHOMTQe79scramblingtechniquesandsignalspaceexpansionbasedonreductionoforthogonalfrequencyIMGWiDkflP通信工程專業(yè)畢業(yè)設(shè)計(jì)外文資料翻譯第8頁divisionmultiplexingsystempeakaveragepowerratiobasedmethod.WHF4OmOgAwAsanapplicationofcommunication,OFDMhasmanytechnicaladvantages.aDFdk6hhPd(1>Inthenarrowbandwidthcanbealotofdatasent.OrthogonalfrequencydivisionozElQQLi4Tmultiplexingtechnologycanalsoseparatedatleast1000digitalsignal,andcansafelyrunCvDtmAfjiAaroundininterferencesignal,thisabilitywillbeadirectthreattothefurtherdevelopmentofQrDCRkJkxhthecurrenthasbeguntoCDMAtechnologypopularandexpands,itisbecauseofthespecial4nCKn3dlMXsignalpenetrationcapabilityoftheorthogonalfrequencydivisionmultiplexingtechnologybyijCSTNGm0EEuropeantelecommunicationsoperatorsandmobilephonemanufacturerswelcomedandvfB1pxanfkaccepted.(2>AsuddenchangeinorthogonalfrequencydivisionmultiplexingtechnologycanJbA9VhEou1constantlymonitorthecommunicationcharacteristicsofthetransmissionmedium.BecauseofX7Ahr18pJItheabilitytotransmitdatacommunicationpathwillchangeovertime,sotheorthogonalb3zqXLCqXofrequencydivisionmultiplexingcandynamicallyadapt,andconnectanddisconnectthepZyytu5rc5correspondingcarriertoensureongoingsuccessfulcommunication.DVyGZezsrM(3>OrthogonalfrequencydivisionmultiplexingcanautomaticallydetecttheRQxPvY3tFstransmissionmediumunderwhichaspecificcarrierhashighsignalattenuationand5MxX1IxuU9interferencepulse,andthentaketheappropriatemeasurestomakethespecifiedcarrierjIw5xs0v9Pfrequencymodulationofthesuccessfulcommunication.xEve2buwnw(4>OrthogonalfrequencydivisionmultiplexingtechniqueisespeciallysuitableforuseinKAvmyVYxCdhigh-risebuildings,highlightingthedenselypopulatedandgeographicalplaceandsignalsofYwuu4FszRTspreadingarea.DatacommunicationanddigitalaudiobroadcastinghighhopetoreducecstDApWA6Amultipatheffectsonsignal.(5>ThebiggestadvantagesoforthogonalfrequencydivisionmultiplexingtechniqueistoqotL69pBkhcombat frequency selectivefadingor narrowbandinterference.Insinglecarriersystems,aEksTCSTCzXsinglefadingorinterferencecancausetheentirecommunicationlinkfailure,butinamultiSgs28CnDOEcarriersystem,onlyaverysmallpartofcarrierinterference.Thesubchannelcanalsouse6craEmRE2kerrorcorrectingcodesforerrorcorrection.(6>Caneffectivelyresisttheinterferencebetweenthesignalwaveform,highspeeddatak8qia6lFh1transmissioninmultipathenvironmentandfadingchannels.Whenthechannelbecauseofy3qrGQOGwImultipathtransmissionfrequencyselectivefading,onlyfellonthebanddepressionsubcarrierMZpzcAiHKoandcarryinformationaffected,subcarrierotherunimpaired,thereforebetterBER0VoHIjMIZ5performancethanthegeneral.(7>Throughthejointcodingofeachsubcarrier,hasastrongabilityofantifading.dRoQe3gJeMOrthogonalfrequencydivisionmultiplexingtechnologyitselfhastakenadvantageofrNnYJNKKts通信工程專業(yè)畢業(yè)設(shè)計(jì)外文資料翻譯第9頁frequencydiversitychannelfading,ifnotserious,thereisnoneedtoaddthetimedomainFJn6fxdLH9equalizer.Throughthevariouschannelcoding,itcanimprovetheperformanceofsystem.TFmfLhHMWP(8>Orthogonalfrequencydivisionmultiplexingtechnologyofnarrowbandinterferenceis7Blnh0bNbwverystrong,becausethesedisturbancesaffectonlyasmallfractionofthesubchannel.lxlvNKFOpd(9>Thechannelutilizationrateishigh,thisisanimportantpointinthespectrumisaztkEju9PETlimitedresourceinwirelessenvironment。whenthesubcarriernumberislarge,thesystemNpjMPeCQTAspectrumutilizationtendsto2Baud/Hz.Althoughorthogonalfrequencydivisionmultiplexinghastheseadvantages,butalsoits1ljUlY6R8hmodulationmechanismalsomakesthesignalinthetransmissionprocessexistsomefhi3RIASmXdisadvantages.(1>Isverysensitivetothephasenoiseandcarrierfrequency.Thisisorthogonalscibnr4TBEfrequencydivisionmultiplexingtechnologyisaveryfatalflaw,theorthogonalfrequencyG8hjTbyUQkdivisionmultiplexingsystemisstrictlyontheo