通信與電子信息工程專業(yè)英語(yǔ)課文翻譯 第2單元 聶敏.ppt

Unit2ModulationTechnology,TextA,ModulationTechnology調(diào)制技術(shù),Unit2,原文：Formula(2.1)introducedthebasicfunctionofasinewavewhichalreadyindicatesthethreebasicmodulationschemes(typically,thecosinefunctionisusedforexplanation):譯文：公式（2.1）引入了一個(gè)基本的正弦函數(shù)，它可以表示三種基本調(diào)制方案（通常用余弦函數(shù)來(lái)說(shuō)明）：,Unit2,Unit2,原文：Digitalmodulationisrequiredifdigitaldatahastobetransmittedoveramediumthatonlyallowsforanalogtransmission.譯文：如果數(shù)字信號(hào)必須在只允許模擬信號(hào)傳輸?shù)拿浇橹袀鬏?，則要求進(jìn)行數(shù)字調(diào)制。,Unit2,原文：Oneexampleforwirednetworksistheoldanalogtelephonesystemtoconnectacomputertothissystemamodemisneeded.Themodemthenperformsthetranslationofdigitaldataintoanalogsignalsviceversa.譯文：模擬電話系統(tǒng)是有線網(wǎng)絡(luò)的一個(gè)例子，它需要用一個(gè)調(diào)制解調(diào)器將計(jì)算機(jī)與系統(tǒng)相連。調(diào)制解調(diào)器完成調(diào)制解調(diào)模數(shù)轉(zhuǎn)換的任務(wù)。數(shù)字傳輸在有線局域網(wǎng)或計(jì)算機(jī)內(nèi)部是很有用的。,Unit2,原文：Apartfromthetranslationofdigitaldataintoanalogsignals,wirelesstransmissionrequiresanadditionalmodulation,ananalogmodulationthatshiftsthecenterfrequencyofthebase-bandsignalgeneratedbythedigitalmodulationuptotheradiocarrier.譯文：除了數(shù)模轉(zhuǎn)換外，無(wú)線傳輸還需要一個(gè)附加的模擬調(diào)制，將普通基帶信號(hào)的中心頻率通過(guò)數(shù)字調(diào)制提高形成無(wú)線電載波。,Unit2,原文：Forexampledigitalmodulationtranslatesa1Mbpsbit-steamintoabase-bandsignalwithabandwidthof1MHz.Thereareseveralreasonswhythisbase-bandsignalcannotbedirectlytransmittedinawirelesssystem:譯文：例如，通過(guò)數(shù)字調(diào)制，將一個(gè)1Mbps比特流轉(zhuǎn)換成1MHz帶寬的基帶信號(hào)?；鶐盘?hào)之所有以不能直接在無(wú)線系統(tǒng)中傳輸有如下幾個(gè)原因：,Unit2,原文：Digitaltransmissionisused,forexample,inwiredlocalareanetworksorwithinacomputer.Inwirelessnetworks,however,digitaltransmissioncannotbeused.Here,thebinarybit-steamhastobetranslatedintoananalogsignalfirst.譯文：然而，在無(wú)線網(wǎng)絡(luò)中，二進(jìn)制比特流首先必須被轉(zhuǎn)換成模擬信號(hào)，因此，不一定必須用到數(shù)字傳輸。,Unit2,原文：Thethreebasicmethodsforthistranslationareamplitudeshiftkeying(ASK),frequencyshiftkeying(FSK),andphaseshiftkeying(PSK).Thesearediscussedinmoredetailinthefollowingsections.譯文：三種基本的數(shù)字調(diào)制是幅移鍵控（ASK）、頻移鍵控（FSK）和相移鍵控（PSK）。下面我們要分別詳細(xì)討論。,Unit2,原文：Apartfromthetranslationofdigitaldataintoanalogsignals,wirelesstransmissionrequiresanadditionalmodulation,ananalogmodulationthatshiftsthecenterfrequencyofthebase-bandsignalgeneratedbythedigitalmodulationuptotheradiocarrier.譯文：除了數(shù)模轉(zhuǎn)換外，無(wú)線傳輸還需要一個(gè)附加的模擬調(diào)制，將普通基帶信號(hào)的中心頻率通過(guò)數(shù)字調(diào)制提高形成無(wú)線電載波。,Unit2,原文：Forexampledigitalmodulationtranslatesa1Mbpsbit-steamintoabase-bandsignalwithabandwidthof1MHz.Thereareseveralreasonswhythisbase-bandsignalcannotbedirectlytransmittedinawirelesssystem:譯文：例如，通過(guò)數(shù)字調(diào)制，將一個(gè)1Mbps比特流轉(zhuǎn)換成1MHz帶寬的基帶信號(hào)?；鶐盘?hào)之所有以不能直接在無(wú)線系統(tǒng)中傳輸有如下幾個(gè)原因：,Unit2,原文：Apartfromthetranslationofdigitaldataintoanalogsignals,wirelesstransmissionrequiresanadditionalmodulation,ananalogmodulationthatshiftsthecenterfrequencyofthebase-bandsignalgeneratedbythedigitalmodulationuptotheradiocarrier.譯文：除了數(shù)模轉(zhuǎn)換外，無(wú)線傳輸還需要一個(gè)附加的模擬調(diào)制，將普通基帶信號(hào)的中心頻率通過(guò)數(shù)字調(diào)制提高到無(wú)線電載波頻段。,Unit2,原文：Forexampledigitalmodulationtranslatesa1Mbpsbit-steamintoabase-bandsignalwithabandwidthof1MHz.Thereareseveralreasonswhythisbase-bandsignalcannotbedirectlytransmittedinawirelesssystem:譯文：例如，通過(guò)數(shù)字調(diào)制，將一個(gè)1Mbps比特流轉(zhuǎn)換成1MHz帶寬的基帶信號(hào)?；鶐盘?hào)之所有以不能直接在無(wú)線系統(tǒng)中傳輸有如下幾個(gè)原因：,Unit2,原文：Antennas:Anantennamustbetheorderofmagnitudeofthewavelengthofthesignalinsizetobeeffective.Forthe1MHzsignalintheexamplethiswouldresultinanantennasomehundredmeterhigh,whichisobviouslynotverypracticalforhandhelddevices.With1GHz,antennasofafewcentimetersinlengthcanbeused.譯文：天線：天線必須由有效信號(hào)的波長(zhǎng)大小來(lái)決定。例如，1MHz的信號(hào)，天線就有數(shù)百米高，這顯然對(duì)便攜設(shè)備來(lái)說(shuō)很不實(shí)際。而1GHz，天線則只有幾米高，這是很有用的。,Unit2,原文：Frequencydivisionmultiplexing:Usingonlybase-bandtransmission,FDMcouldnotbeapplied.Analogmodulationshiftsthebase-bandsignalstodifferentcarrierfrequencies.Thehigherthecarrierfrequency,themorebandwidththatisavailableformanybase-bandsignals.譯文：頻分復(fù)用：頻分復(fù)用不用在單獨(dú)的基帶傳輸中。模擬調(diào)制將基帶信號(hào)轉(zhuǎn)換成不同的載波頻率。載波頻率越高，基帶信號(hào)的可用帶寬越寬。,Unit2,原文：Mediumcharacteristics:Path-losspenetrationofobstacles,reflection,scattering,anddiffractiondependheavilyonthewavelengthofthesignal.譯文：介質(zhì)特性：信道損耗、繞射、反射、散射和在信號(hào)波長(zhǎng)緊密相關(guān)的衍射。,Unit2,原文：Dependingontheapplications,therightcarrierfrequencywiththedesiredcharacteristicshastobechosen:longwavesforsubmarines,shortwavesforhandhelddevices,veryshortwavesfordirectedmicrowavetransmissionetc.譯文：根據(jù)實(shí)際應(yīng)用選擇所需合適的載波頻率特性：潛艇采用長(zhǎng)波，便攜設(shè)備采用短波，定向微波傳輸采用超短波等等。,Unit2,原文：Asfordigitalmodulation,threedifferentbasicschemesareknownforanalogmodulation:amplitudemodulation(AM),frequencymodulation(FM),andphasemodulation(PM).Figure2.1showsa(simplified)blockdiagramofaradiotransmitterfordigitaldata.譯文：至于數(shù)字調(diào)制，所知的三種不同的基本模擬調(diào)制類型：幅度調(diào)制（AM）、頻率調(diào)制（FM）和相位調(diào)制（PM）。圖（2.1）表示一個(gè)數(shù)字無(wú)線反射區(qū)的簡(jiǎn)化程序框圖。,Unit2,原文：Thefirststepisthedigitalmodulationofdataintotheanalogbase-bandsignalaccordingtooneoftheschemespresentedinthefollowingsection.譯文：第一步是將數(shù)字調(diào)制數(shù)據(jù)通過(guò)如下部分所示的形式轉(zhuǎn)換成模擬基帶信號(hào)，然后將模擬信號(hào)的中心頻率提高到無(wú)線電載波頻段，最后信號(hào)通過(guò)天線傳輸。,Unit2,原文：Thereceiver(Figure2.2)receivestheanalogradiosignalviaitsantennaanddemodulatesthesignalintotheanalogbase-bandsignalwiththehelpoftheknowncarrier.Thiswouldbeallthatisneededforananalogradiotunedintoaradiostation.(Theanalogbase-bandsignalwouldconstitutethemusic.)譯文：接收機(jī)（圖2.2）通過(guò)天線接收到模擬無(wú)線電信號(hào)，根據(jù)一定的載波對(duì)信號(hào)進(jìn)行解調(diào)轉(zhuǎn)換成模擬基帶信號(hào)，全部應(yīng)用在接收廣播電臺(tái)的模擬廣播。（模擬基帶信號(hào)承載音頻。）,Unit2,原文：Fordigitaldata,anotherstepisneeded.Bitsorframeshavetobedetected,i.e.,thereceivermustsynchronizewiththesender.Howsynchronizationisachieved,dependsonthedigitalmodulationscheme.譯文：對(duì)于數(shù)字信號(hào)來(lái)說(shuō)，另一步是檢測(cè)比特和幀是至關(guān)重要的。例如接收必須和發(fā)射同步。同步怎樣實(shí)現(xiàn)要根據(jù)數(shù)字調(diào)制模式。,Unit2,原文：Aftersynchronization,thereceiverhastodecideifthesignalrepresentsadigital1or0,reconstructingtheoriginaldata.譯文：經(jīng)過(guò)同步，接收機(jī)必須判定是否是由0.1代碼表示的信號(hào)重組成原始數(shù)據(jù)。,Unit2,原文：Thedigitalmodulationschemespresentedinthefollowingsectionsdifferinmanyissues,suchasspectralefficiency(I.e.,howmuchpowerisneededtotransferbitswhichisveryimportantforportabledevicesthatarebatterydependent),androbustnesstomulti-pathpropagation.Noise,andinterference.譯文：數(shù)字調(diào)制模式在下列問(wèn)題中有不同的表示：例如光譜效率（比如，傳輸比特所需用多少能量對(duì)電池終端設(shè)備是非常重要的。）、多徑效應(yīng)、噪聲和干擾。,Unit2,原文：AmplitudeshiftkeyingFigure2.3illustratesamplitudeshiftkeying(ASK),themostsimpledigitalmodulationscheme.Thetwobinaryvalues,1and0,arerepresentedbytwodifferentamplitudes.Intheexample,oneoftheamplitudesis0(representingthebinary0).譯文：振幅鍵控圖2.3所示振幅鍵控（ASK）是最簡(jiǎn)單的數(shù)字調(diào)制方式。二進(jìn)制值0和1由不同的幅度來(lái)表示。例中，一個(gè)幅度是0（代表二進(jìn)制0）。,Unit2,原文:Thissimpleschemeonlyrequireslowlossheavilyinfluencetheamplitude.Inawirelessenvironment,aconstantamplitudecannotbeguaranteed,soASKistypicallynotusedforwirelessradiotransmission.譯文：這種簡(jiǎn)單的方式僅需要低的帶寬，但是容易受到干擾的影響。像多徑傳播，噪聲和信道衰減嚴(yán)重影響幅度所產(chǎn)生的效果。在無(wú)線工作環(huán)境下，不能保證穩(wěn)定的幅度，所以通常ASK不能用于無(wú)線電傳輸。,Unit2,原文：However,thewiretransmissionschemewiththehighestperformance,namelyopticaltransmission,usesASK.Here,alightpulsemayrepresenta1,whiletheabsenceoflightrepresentsa0.譯文：然而，有線傳輸方式有很高的傳輸性能，即光纖傳輸中使用ASK，有光脈沖表示1，同時(shí)無(wú)光表示0。,Unit2,原文：ThecarrierfrequencyinopticalsystemsissomehundredTHz.ASKcanalsobeappliedtowirelessinfraredtransmission,usingadirectedbeamordiffuselight.譯文：光纖系統(tǒng)中載波的頻率達(dá)到數(shù)百THz。ASK也應(yīng)用于無(wú)線紅外線傳輸通過(guò)直連光束或漫射光。,Unit2,原文：FrequencyshiftkeyingAmodulationschemeoftenusedforwirelesstransmissionisfrequencyshiftkeying(FSK)(Figure2.4).ThesimplestformofFSK,alsocalledbinaryFSK(BFSK),assignsonefrequencyf1tothebinary1andanotherfrequencyf2tothebinary0.譯文：頻移鍵控FSK(圖2.4)是常用于無(wú)線傳輸?shù)囊环N調(diào)試方式。最簡(jiǎn)單的頻移鍵控形式，被稱為二進(jìn)制頻移鍵控（BFSK），分配碼元1為頻率f1，碼元0對(duì)應(yīng)頻率f2。,Unit2,原文：AverysimplewaytoimplementFSKistoswitchbetweentwooscillators,onewiththefrequencyf1andtheotherwithf2,dependingontheinput.譯文:實(shí)現(xiàn)FSK最簡(jiǎn)單的辦法是頻率在兩個(gè)振蕩之間轉(zhuǎn)換，根據(jù)輸入，一個(gè)是頻率為f1，另一個(gè)為頻率為f2。,Unit2,原文：Toavoidsuddenchangesinphase,specialfrequencymodulatorswithcontinuousphasemodulation(CPM)canbeused.Suddenchangesinphasecausehighfrequencies,whichisanundesiredside-effect.Asimplewaytoimplementdemodulationisbyusingtwobandpassfilters,oneforf1theotherforotherforf2.譯文：為了防止相位跳變，引入了特殊頻率調(diào)制，連續(xù)相位調(diào)制。相位隨高頻進(jìn)行跳變，這是難免的邊效應(yīng)。簡(jiǎn)單方式上的解調(diào)是使用兩個(gè)帶通濾波，一個(gè)為f1另一個(gè)為f2。,Unit2,原文：Acomputercanthencomparethesignallevelsofthefilteroutputstodecidewhichofthemisstronger.FSKneedsalargerbandwidthcomparedtoASKbutismuchlesssusceptibletoerrors.譯文：抽樣判決器能檢測(cè)出經(jīng)過(guò)濾波輸出的信號(hào)電平是否足夠高。FSK較ASK有較高的帶寬，和更不易產(chǎn)生錯(cuò)誤。,Unit2,Unit2,原文：Thissimplescheme,shiftingthephaseby180eachtimethevalueofdatachanger,isalsocalledbinaryPSK(BSK).譯文：這個(gè)簡(jiǎn)單的方式中，相位每次變化180碼元也隨之改變，這就是二進(jìn)制相移鍵控（BPSK）。,Unit2,原文：AsimpleimplementationofaBPSKmodulatorcouldmultiplyafrequencyfwith+1ifthebinarydatais1andwith-1ifthetransmitter.譯文：BPSK的調(diào)制器實(shí)現(xiàn)是如果是碼元1便乘以頻率（f+1）碼元0則乘以頻率（f-1）。,Unit2,原文：Thiscanbedoneusingaphaselockloop(PLL).ComparedtoFSK,PSKismoreresistanttointerference,butreceiverandtransmitterarealsomorecomplex.譯文：為了數(shù)據(jù)接收的正確性，接收機(jī)在頻率和相位上同發(fā)射機(jī)實(shí)現(xiàn)同步。這將使用鎖相環(huán)（PLL）。PSK較FSK有更好的抗干擾能力，但接收和發(fā)射更復(fù)雜。,Unit2,原文：AdvancedfrequencyshiftkeyingAfamousFSKschemeusedinmanywirelessisminimumshiftkeying(MSK).MSKisbasicallyBFSKwithoutabruptphasechanges,i.e.,itbelongstoCPMschemes,Figure2.6showsanexamplefortheimplementationofMSK.譯文：新型頻移鍵控在大多數(shù)無(wú)線系統(tǒng)中使用的普遍頻移鍵控頻是最小頻移鍵控（MSK）。MSK是基本沒(méi)有相位突變的BFSK,即屬于CPM方式。圖2.6示出MSK實(shí)現(xiàn)的例子。,Unit2,Unit2,原文：Accordingtothefollowingscheme,thelowerorhigherfrequencyischosen(eitherinvertedornon-inverted)togeneratetheMSKsignal:iftheevenandtheoddbitareboth0,thenthehigherfrequencyf2isinverted(I.e.,f2isusedwithaphaseshiftof180);譯文：通過(guò)如下方式，選擇更低或更高的頻率來(lái)產(chǎn)生MSK信號(hào)：如果偶數(shù)為和奇數(shù)位都為0，那么高頻f2部分倒置（即f2其相位180變化）。,Unit2,原文：iftheevenbitis1,theoddbit0,thenthelowerfrequencyf1isinverted.Thisisthecase,e.g.,inthefifthtoseventhcolumnsofFigure2.6;譯文：如果偶數(shù)位是1，奇數(shù)位是0.那么將低頻f1部分倒置。這就是圖2.6例子中第5到第7列的圖形。,Unit2,原文：iftheevenbitis0andtheoddis1,asincolumns1to3,f1istakenwithoutchangingthephase;ifbothbitsare1,thentheoriginalf2istaken.Ahighfrequencyisalwayschosenifevenandoddbitsareequal.Thesignalisinvertediftheoddbitequals0.ThisschemeavoidsallphaseshiftsintheresultingMSKsignal.譯文：如果偶數(shù)位是0奇數(shù)位是1，那么從1到3的波形中f1并不改變相位。如果兩位都是1，則選用原f2。如果奇數(shù)和偶數(shù)位相等通常選用高頻。如果偶數(shù)位等于0信號(hào)翻轉(zhuǎn)。這種方式避免了MSK信號(hào)中的所有相位都翻轉(zhuǎn)。,Unit2,原文：Addingaso-calledGaussianlowpassfiltertotheMSKschemeresultsisGaussianMSK(GMSK),whichisthedigitalmodulationschemeformanyEuropeanwirelessstandards.ThefilterreducesthelargespectrumneededbyMSK.譯文：另外還有一種經(jīng)過(guò)高斯低通濾波的MSK方式形成所謂的高斯最小頻移鍵控（GMSK），這許多歐洲無(wú)線標(biāo)準(zhǔn)中的數(shù)字調(diào)制形式。濾波降低了MSK所需的高頻譜。,Unit2,原文：AdvancedphaseshiftkeyingThesimplePSKschemecanbeimprovedinmanyways.ThebasicBPSKschemeonlyusesonepossiblephaseshiftof180.TheleftsideofFigure2.7showsBPSKinthephasedomain(whichistypicallythebetterrepresentationcomparedtothetimedomaininFigure2.5).譯文：新型相移鍵控簡(jiǎn)單的PSK方式在許多方面可以被改進(jìn)?；镜腂PSK方式僅用一次180度相位轉(zhuǎn)換。圖2.7左邊所示為BPSK相位域（這通常比圖2.5的時(shí)域更具代表性）。,Unit2,原文：therightsideofFigure2.7showsquadraturePSK(QPSK),oneofthemostcommonPSKschemes(sometimesalsocalledquaternaryPSK).譯文：右邊的圖2.7展示了差分相移鍵控（QPSK），是最普通的PSK形式（有時(shí)也稱為四進(jìn)制相移鍵控）。,Unit2,原文：Here,higherbitratescanbeachievedforthesamebandwidthbycodingtwobitsintoonephaseshift.Alternatively,onecanreducethebandwidthandstillachievethesamebitratesasforBPSK.譯文：在相同的帶寬中，一個(gè)相位轉(zhuǎn)換編碼兩比特實(shí)現(xiàn)了高比特率。另外，減小了帶寬還達(dá)到了BPSK相同的比特率。,Unit2,原文：QPSK(andotherPSKschemes)canberealizedintwovariants.Thephaseshiftcanalwaysberelativetoareferencesignal(withthesamefrequency).Ifthisschemeisused,aphaseshiftof0meansthatthesignalisinphasewiththereferencesignal.譯文：QPSK(以及其他PSK方式)能實(shí)現(xiàn)兩種變形。相位變換也同參考信號(hào)相關(guān)（同頻）。如果使用這種方法，0的一個(gè)相位變換意味著參考信號(hào)的相移。,Unit2,原文：AQPSKsignalwillthenexhibitaphaseshiftof45forthedata11,135for10,225for00,and315for01withallphaseshiftsbeingrelativetothereferencesignal.譯文：QPSK信號(hào)將展寬數(shù)據(jù)11的相位變換為45，10的為135,00的為225和01的為315使得所有相位變換通參考信號(hào)相關(guān)。,Unit2,原文：ThetransmitterselectspartsofthesignalasshowninFigure2.28andconcatenatesthem.Toreconstructdata,thereceiverhastocomparetheincomingsignalatthereceiver.譯文：發(fā)射機(jī)選擇部分如圖2.8所示的信號(hào)并串起來(lái)。為了恢復(fù)信號(hào)，接收機(jī)必須把接收的信號(hào)通參考信號(hào)做比較。其的一個(gè)問(wèn)題涉及接收機(jī)產(chǎn)生參考信號(hào)。,Unit2,原文：Transmitterandreceiverhavetobesynchronizedveryoften,e.g.,byusingspecialsynchronizationpatternsbeforeuserdataarrivesorviaapilotfrequencyasreference.譯文：發(fā)射機(jī)和接收機(jī)必須同步，例如，在用戶信號(hào)到達(dá)前使用特殊的同步模式或通過(guò)參考導(dǎo)頻。,Unit2,原文：OnewaytoavoidthisproblemistousedifferentialQPSK(DQPSK).Herethephaseshiftisnotrelativetoareferencesignalbuttothephaseoftheprevioustwobits.Inthiscase,thereceiverdoesnotneedthereferencesignalbutonlycomparestwosignalstoreconstructdata.DQPSKisusedinUSwirelesstechnologiesIS-136andPACSandinJapanesePHS.譯文：為避免這種問(wèn)題的一種方法是使用差分四相鍵控（DQPSK）。相位變換與參考信號(hào)除在前兩個(gè)比特中都不相關(guān)，在這種情況下，接收機(jī)不需要參考信號(hào)，只用比較兩信號(hào)就可以再生數(shù)據(jù)。英國(guó)無(wú)線科技IS-136和日本PHS的PACS都使用DQPSK。,Unit2,原文：Onecouldnowthinkofextendingtheschemetomoreanglearecombinedcoding4bitsperphase/amplitudechange.譯文：現(xiàn)在新型相位變換的方法是擴(kuò)展更多的角度。實(shí)例中，每編譯3比特相位變換8個(gè)角度。此外，PSK結(jié)合ASK的方式就如在9600bps時(shí)延標(biāo)準(zhǔn)的調(diào)制解調(diào)器使用的正交振幅調(diào)制（QAM）（圖2.8的左邊）。每次相位/幅度變換，3種不同振幅和12個(gè)角度編碼4比特。,Unit2,原文：ProblemsoccurforwirelesscommunicationincaseofnoiseorISI.Themorepointsusedinthephasedomain,theharderitistoseparatethem.DQPSKhasbeenprovenasoneoftheefficientschemesundertheseconsiderations.譯文：無(wú)線連接中的問(wèn)題有噪聲和ISI。相位域使用的相位點(diǎn)越多越難分離。DQPSK被證實(shí)是在這些考慮因素下最優(yōu)有效的方式之一。,Unit2,原文：AmoreadvancedschemeisahierarchicalmodulationasusedinthedigitalTVstandardDVB-T.TherightsideofFigure2.8showsa64QAMthatcontainsaQPSKmodulation.譯文：最新型的方式是使用在DVB-T標(biāo)準(zhǔn)的數(shù)字電視中使用的分層調(diào)制。圖2.8右邊的部分展示了用QPSK調(diào)制的64位QAM。,Unit2,原文：A64QAMcancode6bitpersymbol.HerethetwomostsignificantbitsareusedfortheQPSKsignalembeddedintheQAMsignal.IfthereceptionofthesignalisgoodtheentireQAMconstellationcanberesolved.譯文：64位QAM每標(biāo)志編碼6比特。把QPSK信號(hào)中最顯著的兩個(gè)比特嵌入到QAM信號(hào)中。如果信號(hào)接收正確，就能完成QAM的星座圖。,Unit2,原文：Underpoorreceptionconditions,e.g.,withmovingreceivers,onlytheQPSKportioncanberesolved.AhighprioritydatastreaminDVB-TiscodedwithQPSKusingthetwomostsignificantbits.Theremaining4bitsrepresentlowprioritydata.ForTVthiscouldmeanthatthestandardresolutiondatastreamiscodedwithhighpriority,thehighresolutioninformationwithlowpriority.Ifthesignalisdistorted,atleastthestandardTVresolutioncanbereceived.譯文：在接收條件差的情況下，例如，在移動(dòng)接收條件下，只能接收QPSK信號(hào)部分。高優(yōu)先級(jí)數(shù)據(jù)流DVB-T通過(guò)QPSK編碼由兩個(gè)標(biāo)記比特表示，剩下的4個(gè)比特承載低優(yōu)先級(jí)數(shù)據(jù)。如果信號(hào)是扭曲的，最終的電視分辨率也能接收。,Unit2,原文：Multi-carriermodulationSpecialmodulationschemesthatstandsomewhatapartfromtheothersaremulti-carriermodulation(MCM),orthogonalfrequencydivisionmultiplexing(OFDM)orcodedOFDMstandardsIEEE802.11aandHiperLAN2.譯文：多載波調(diào)制特殊的調(diào)制方式與其形式不同的代表有多載波調(diào)制（MCM）、正交頻分復(fù)用、和編碼的正交頻分復(fù)用，都應(yīng)用在歐洲數(shù)字無(wú)線電系統(tǒng)DAB和WLAN標(biāo)準(zhǔn)的IEEE802.11a和HiperLAN2中的內(nèi)容。,Unit2,ThemainattractionofMCMisitsgoodISImitigationproperty.MCMsplitsthehighbitratestreamintomanylowerbitratestreams(seeFigure2.9),eachstreambeingsentusinganindependentcarrierfrequency.MCM的主要影響力是其好的ISI延遲屬性，MCM把高速比特流分割成許多低速比特流（見(jiàn)圖2.9），每個(gè)數(shù)據(jù)流用獨(dú)自的載波頻率傳輸。,Unit2,原文：If,forexample,nsymbols/shavetobetransmitted,eachsubcarriertransmitsn/csymbols/swithcbeingthenumberofsubcarriers.Onesymbolcould,forexamplerepresent2bitsasinQPSK.DAB,forexample,usesbetween192and1536ofthesesubcarrier.ThephysicallayerofHiperLAN2andIEEE802.11auses48subcarriersfordata.譯文：如果n表示傳輸間隔，每個(gè)子載波傳輸n/c個(gè)間隔，其中c表示子載波的數(shù)目。一個(gè)間隔能承載如例QPSK的2比特。DAB用192到1536之間的子載波。HiperLAN2和IEEE802.11a的物理層和使用48的數(shù)據(jù)子載波。,Unit2,原文：Figure2.10showsthesuperpositionoforthogonalfrequencies.Themaximumofonesubcarrierfrequencyappearsexactlyatafrequencywhereallothersubcarriersequalzero.譯文：圖2.10示出正交頻率的疊加。副載波頻率的最大值剛好出現(xiàn)在所有的子載波都為0的頻率處。,Unit2,原文：Usingthisscheme,frequencyselectivefadingonlyinfluencessomesubcarriers,andnotthewholesignalanadditionalbenefitofMCM.Typically,MCMtransmitssymbols.Thishelpsthereceivertohandlemulti-pathpropagation.譯文：使用這種方式，頻率選擇性衰減并不會(huì)影響所有的信號(hào)只是一些子載波，這也是MCM的一個(gè)額外的好處。通常MCM傳輸間隔的防衛(wèi)間隔是在單間隔和群間隔之間。這有助于接收機(jī)處理多信道傳輸。,Unit2,原文：OFDMisaspecialmethodofimplementingMCMusingorthogonalcarriers.Computationally,thisisaveryefficientalgorithmbasedonfastFouriertransform(FFT)formodulation/demodulation.Ifadditionalerror-controlcodingacrossthesymbolsindifferentsubcarriersisapplied,thesystemisreferredtoasCOFDM.譯文：OFDM實(shí)現(xiàn)使用正交載波的MCM的一種特殊方法。計(jì)算上，調(diào)制解調(diào)的基于快速傅里葉變換是一種非常有效是算法。如果貫穿應(yīng)用在不同子載波間隙附加的錯(cuò)誤控制編碼，系統(tǒng)應(yīng)當(dāng)使用COFDM。,Unit2,Unit2ModulationTechnology,TextB,PCM脈沖編碼調(diào)制,Unit2,原文：Pulse-codemodulation(PCM)isamethodusedtodigitallyrepresentsampledanalogsignals,whichwasinventedbyAlecReevesin1937.ItisthestandardfromdigitalaudioincomputersandvariousBlue-ray,CompactDiscandDVDformats,aswellasotherusessuchasdigitaltelephonesystems.譯文：1937年，AlecReeves發(fā)明了一種用數(shù)字信號(hào)表示被抽樣的模擬信號(hào)的方式，即脈沖編碼調(diào)制（PCM）。對(duì)于數(shù)字電話、DVD格式以及計(jì)算機(jī)的各種藍(lán)光壓縮光盤盒數(shù)字音頻而言，PCM是一種標(biāo)準(zhǔn)形式。,Unit2,原文：APCMstreamisadigitalrepresentationofananalogsignal,inwhichthemagnitudeoftheanaloguesignalissampledregularlyatuniformintervals,witheachsamplebeingquantizedtothenearestvaluewithinarangeofdigitalsteps.譯文：PCM數(shù)據(jù)流是模擬信號(hào)的數(shù)字表示，模擬信號(hào)的幅度被間隔規(guī)則抽樣，每個(gè)樣值被量化為數(shù)字量階內(nèi)最近的值。,Unit2,原文：PCMisdependentonthreeseparateoperations,sampling,quantizing,andcoding.Manydifferentschemesforperformingthesethreefunctionshaveevolvedduringrecentyears,andweshalldescribethemainones.Inthesedescriptionsweshallseehowaspeechchanneloftelephonequalitymaybeconveyedasaseriesofamplitudevalues,eachvaluebeingrepresented,thatis,codedasequenceof8binarydigits.譯文：PCM依賴于抽樣，量化和編碼三種單獨(dú)的操作。在二十年內(nèi)演變出了許多不同的實(shí)現(xiàn)能這三種功能的方法，我們將描述最主要的方法。我們將會(huì)了解到語(yǔ)音信道的電話質(zhì)量可以由一系列每個(gè)的幅度值來(lái)表示，如一串編碼的8位二進(jìn)制。,Unit2,原文：Furthermore,weshallprovethataminimumtheoreticalsamplingfrequencyoforder6.8kilohertz(kHz)isrequiredtoconveyavoicechanneloccupyingtherange300Hzto3.4kHz.譯文：進(jìn)一步說(shuō)，我可以證明6.8kHz序列的最小理論抽樣頻率能表示語(yǔ)音信道需占用300Hz到3.4kHz。,Unit2,原文：Practicalequipment,however,normallyuseasamplingrateof8kHz,andif8-digitpersamplevalueareused,thevoicechannelbecomesrepresentedbyastreamofpulsewitharepetitionrateof64kHz.譯文：然而，實(shí)用設(shè)備通常使用的抽樣頻率是8kHz，如果每次抽樣值是8位，語(yǔ)音信道將變成由速率重復(fù)的64kHz的脈沖流表示。,Unit2,原文：Reexaminationofoursimpleexampleshowsusthatthespeechsignalofmaximumfrequency3.4kHzhasbeenrepresentedbyasignaloffrequencyof64kHz.However,ifonly4-digitspersamplevaluehadbeenused,thequalityoffrequencyoftransmissionwoulddrop,andtherepetitionrateofthepulseswouldbereducedto32kHz.譯文：重新檢查抽樣值得出語(yǔ)音信號(hào)的最大頻率3.4kHz已經(jīng)由頻率為64kHz的信號(hào)表示。然而，如果抽樣值僅為4位，傳輸質(zhì)量會(huì)降低，并且根據(jù)脈沖的速率將降低到32kHz。,Unit2,原文：Thusthequalityoftransmissionisdependentonthepulserepetitionrate,andfordigitalcommunicationsystemthesetwovariablesmaybeinterchangedmostefficiently.譯文：因此脈沖重復(fù)率決定傳輸速率，對(duì)數(shù)字通信系統(tǒng)來(lái)說(shuō)這兩個(gè)數(shù)值可以有效的互換。,Unit2,原文：Digitaltransmissionprovidesapowerfulmethodforovercomingnoisyenvironments.Noisecanbeintroducedintotransmissionpathinmanydifferentways:perhapsviaanearbylightingstrike,thesparkingofacarignition譯文：數(shù)字傳輸提供了一種克服自然噪聲的有效方法。傳輸信道中引入噪聲的途徑有很多：也許一個(gè)近的雷擊，汽車點(diǎn)火系統(tǒng)的叫聲，或者通信設(shè)備自身的熱低噪聲水平。,Unit2,原文：system,orthethermallow-levelnoisewiththecommunicationequipmentitself.Itistherelationshipbetweentruesignalandnoisesignal,knownasthesignal-to-noiseratio,whichisofthemostinteresttocommunicationengineer.譯文：真實(shí)信號(hào)和噪聲信號(hào)之間有一定的關(guān)系，比如信噪比，這是通信工程師最感興趣的。,Unit2,原文：Basically,ifthesignalisverylargecomparingtothenoiselevel,thenaperfectmessagetransmissionwilltakeplace;however,thisisnotalwaysthecase.譯文：基本上如果信號(hào)比噪聲電平高很多，這就會(huì)出現(xiàn)理想的信息傳輸，然而這并不常常發(fā)生。,Unit2,原文：Forexample,thesignalfromasatellite,locatedinfarouterspace,isveryweak,andisatalevelonlyslightlyabovethatofthenoise.Alternativeexamplemaybefoundwithinterrestrialsystemwhere,althoughthemessagesignalisstr