《电子与通信工程专业英语》课件第1章

Unit1TheCommunicationProcessNEWWORDSANDPHRASES

NOTES EXERCISES

参考译文

EXTENSIVETEXT

Today,communicationentersourdailylivesinsomanydifferentwaysthatitisveryeasytooverlookthemultitudeofitsfacets.Thetelephonesatourhands,theradiosandtelevisionsinourlivingrooms,thecomputerterminalswithaccesstotheInternetinourofficesandhomes,andournewspapersareallcapableofprovidingrapidcommunicationsfromeverycorneroftheglobe.Communicationprovidesthesenseforshipsonthehighseas,aircraftinflight,androcketsandsatelliteinspace.

Communicationsthroughawirelesstelephonekeepsacardriverintouchwiththeofficeorhomemilesaway.Communicationkeepsaweatherforecasterinformedofconditionsmeasuredbyamultitudeofsensors.Indeed,thelistofapplicationsinvolvingtheuseofcommunicationinonewayoranotherisalmostendless.

Inthemostfundamentalsense,communicationinvolvesimplicitlythetransmissionofinformationfromonepointtoanotherthroughasuccessionofprocesses,asdescribedhere:

(1)

Thegenerationofamessagesignal:voice,music,picture,orcomputerdata.

(2)

Thedescriptionofthatmessagesignalwithacertainmeasureofprecision,byasetofsymbols:electrical,aural,orvisual.

(3)

Theencodingofthesesymbolsinaformthatissuitablefortransmissionoveraphysicalmediumofinterest.

(4)

Thetransmissionoftheencodedsymbolstothedesireddestination.

(5)

Thedecodingandreproductionoftheoriginalsymbols.

(6)

There-creationoftheoriginalmessagesignal,withadefinabledegradationinquality;thedegradationiscausedbyimperfectionsinthesystem.

Thereare,ofcourse,manyotherformsofcommunicationthatdonotdirectlyinvolvethehumanmindinrealtime.Forexample,incomputercommunicationsinvolvingcommunicationbetweentwoormorecomputers,humandecisionsmayenteronlyinsettinguptheprogramsorcommandsforthecomputer,orinmonitoringtheresults.

Irrespectiveoftheformofcommunicationprocessbeingconsidered,therearethreebasicelementstoeverycommunicationsystems,namely,transmitter,channel,andreceiver,asdepictedinFig1.1.Thetransmitterislocatedatonepointinspace,thereceiverislocatedatsomeotherpointseparatefromthetransmitter,andthechannelisthephysicalmediumthatconnectsthem.Thepurposeofthetransmitteristoconverterthemessagesignalproducedbythesourceofinformationintoaformsuitablefortransmissionoverthechannel.However,asthetransmittedsignalpropagatesalongthechannel,itisdistortedduetochannelimperfection.Moreover,noiseandinterferingsignals(originatingfromothersources)areaddedtothechanneloutput,withtheresultthatthereceivedsignalisacorruptedversionofthetransmittedsignal.Thereceiverhasthetaskofoperationonthereceivedsignalsoastoreconstructarecognizableformoftheoriginalmessagesignalforauser[1].Fig1.1Elementsofacommunicationsystem

Therearetwobasicmodesofcommunication:

(1)

Broadcasting,whichinvolvestheuseofasinglepowerfultransmitterandnumerousreceiversthatarerelativelyinexpensivetobuild.Hereinformation-bearingsignalsflowonlyinonedirection.

(2)

Point-to-pointcommunication,inwhichthecommunicationprocesstakesplaceoveralinkbetweenasingletransmitterandareceiver.Inthiscase,thereisusuallyabidirectionalflowofinformation-bearingsignals,whichrequirestheuseofatransmitterandreceiverateachendofthelink.

Thebroadcastingmodeofcommunicationisexemplifiedbyradioandtelevision,andtheubiquitoustelephoneprovidesthemeansforoneformofpoint-to-pointcommunication.

Allthesedifferentcommunicationsystemsaswellasothersnotmentionedhereshareacommonfeature:Theunderlyingcommunicationprocessineachandeveryoneofthemisstatisticalinnature.Indeed,itisforthisimportantreasonthatmuchofthisbookisdevotedtothestatisticalunderpinningsofcommunicationsystems.

1．PrimaryCommunicationResources

Inacommunicationsystem,twoprimaryresourcesareemployed:transmittedpowerandchannelbandwidth.Thetransmittedpoweristheaveragepowerofthetransmittedsignal.Thechannelbandwidthisdefinedasthebandoffrequenciesallocatedforthetransmissionofthemessagesignal.Ageneralsystemdesignobjectiveistousethesetworesourcesasefficientlyaspossible.Inmostcommunicationchannels,oneresourcemaybeconsideredmoreimportantthantheother.Wemaythereforeclassifycommunicationchannelsaspowerlimitedorbandlimited.Forexample,thetelephonecircuitisatypicalband-limitedchannel,whereasaspacecommunicationlinkorsatellitechannelistypicallypowerlimited.

Whenthespectrumofamessagesignalextendsdowntozeroorlowfrequencies,wedefinethebandwidthofthesignalasthatupperfrequencyabovewhichthespectralcontentofthesignalisnegligibleandthereforeunnecessaryfortransmittinginformation.Forexample,theaveragevoicespectrumextendswellbeyond10kHz,thoughmostoftheaveragepowerisconcentratedintherangeof100to600Hz,andabandfrom300to3100Hzgivesgoodarticulation.Accordingly,wefindthattelephonecircuitsthatrespondwelltothislatterrangeoffrequenciesgivequitesatisfactorycommercialtelephoneservice.

Anotherimportantpointthatwehavetokeepinmindistheunavoidablepresenceofnoiseinacommunicationsystem.Noisereferstounwantedwavesthattendtodisturbthetransmissionandprocessingofmessagesignalsinacommunicationsystem.Thesourceofnoisemaybeinternalorexternaltothesystem.

Aquantitativewaytoaccountfortheeffectofnoiseistointroducesignal-to-noiseratio(SNR)asasystemparameter.Forexample,wemaydefinetheSNRatthereceiverinputastheratiooftheaveragesignalpowertotheaveragenoisepower,bothbeingmeasuredatthesamepoint.ThecustomarypracticeistoexpresstheSNRindecibels(dBs),definedas10timesthelogarithm(tobase10)ofthepowerratio.Forexample,signal-to-noiseratiosof10,100and1,000correspondto10,20,and30dBs,respectively.

2．SourceofInformation

Thetelecommunicationsenvironmentisdominatedbyfourimportantsourcesofinformation:speech,music,pictures,andcomputerdata.Asourceofinformationmaybecharacterizedintermsofthesignalthatcarriestheinformation.Asignalisdefinedasasingle-valuedfunctionoftimethatplaystheroleoftheindependentvariable;ateveryinstantoftime,thefunctionhasauniquevalue.Thesignalcanbeone-dimensional,asinthecaseofspeech,music,orcomputerdata;two-dimensional,asinthecaseofpictures;three-dimensional,asinthecaseofvideodata;andfour-dimensional,asinthecaseofvolumedataovertime.Inthesequel,weelaborateondifferentsourcesofinformation.

(1)Speechistheprimarymethodofhumancommunication.Specifically,thespeechcommunicationprocessinvolvesthetransferofinformationfromaspeakertoalistener,whichtakesplaceinthreesuccessivestages:

①Production.Anintendedmessageinthespeaker’smindisrepresentedbyaspeechsignalthatconsistsofsounds(i.e.,pressurewaves)generatedinsidethevocaltractandwhosearrangementisgovernedbytherulesoflanguage.

②Propagation.Thesoundwavespropagatethroughtheairataspeedof300m/s,reachingthelistener’sears.

③Perception.Theincomingsoundsaredecipheredbythelistenerintoareceivedmessage,therebycompletingthechainofeventsthatculminateinthetransferofinformationfromthespeakertothelistener.

Thespeech-productionprocessmaybeviewedasaformoffiltering,inwhichasoundsourceexcitesavocaltractfilter.Thevocaltractconsistsofatubeofnonuniformcross-sectionalarea,beginningattheglottis(i.e.,theopeningbetweenthevocalcords)andendingatthelip.Asthesoundpropagatesalongthevocaltract,thespectrum(i.e.,frequencycontent)isshapedbythefrequencyselectivityofthevocaltract;thiseffectissomewhatsimilartotheresonancephenomenonobservedinorganpipes.Theimportantpointtonotehereisthatthepowerspectrum(i.e.,thedistributionoflong-termaveragepowerversusfrequency)ofspeechapproacheszeroforzerofrequencyandreachesapeakintheneighborhoodofafewhundredhertz.Toputmattersintoproperperspective,however,wehavetokeepinmindthatthehearingmechanismisverysensitivetofrequency.Moreover,thetypeofcommunicationsystembeingconsideredhasanimportantbearingonthebandoffrequenciesconsideredtobe“essential”forthecommunicationprocess.Forexample,asmentionedpreviously,abandwidthof300to3100Hzisconsideredadequateforcommercialtelephoniccommunication.

(2)

Thesecondsourceofinformation,music,originatesfrominstrumentssuchasthepiano,violin,andflute.Thenotemadebyamusicalinstrumentmaylastforashorttimeintervalasintheprocessingofakeyonapiano,oritmaybesustainedforalongtimeintervalasintheexampleofafluteplayerholdingaprolongednote.Typically,musichastwostructures:amelodicstructureconsistingofasetofsimultaneoussounds.Likeaspeechsignal,amusicalsignalisbipolar.However,amusicalsignaldiffersfromaspeechsignalinthatitsspectrumoccupiesamuchwiderbandoffrequenciesthatmayextenduptoabout15kHz.Accordingly,musicalsignalsdemandamuchwiderchannelbandwidththanspeechsignalsfortheirtransmission.

NEWWORDSANDPHRASES

aural adj. 听觉的

degradation n. 降级，退化

imperfection adj. 不完整性，非理想性

ubiquitous adj. 无所不在的；普通的

underpinning n. 基础，支柱，支撑

articulation n. 清晰度

decibel n. 分贝

logarithm n. [数]对数

vocaltract 声道

decipher vt. 译解(密码等)，解释

glottis n. 声门

resonance n. 共鸣，谐振，共振

prolonged adj. 延长的，拖延的

melodic adj. 有旋律的，调子美妙的

powerspectrum 功率谱

NOTES

[1]Thereceiverhasthetaskofoperationonthereceivedsignalsoastoreconstructarecognizableformoftheoriginalmessagesignalforauser.

“soasto”这里翻译为“如此……以至……”。“recognizableform”译为“可识别的形式”。

本句可译为：接收器的任务是对接收到的信号进行重建，使其成为用户能辨识的原始信息信号的形式。

EXERCISES

I．Translatethefollowingwordsorphrases.

spectrumofamessagesignal goodarticulation thehighseas

signal-to-noiseratio(SNR) independentvariable vocaltract

点到点通信 光栅扫描 消隐脉冲

II．TranslatethefollowingparagraphsintoChinese.

(1)

Thetransmitterislocatedatonepointinspace,thereceiverislocatedatsomeotherpointseparatefromthetransmitter,andthechannelisthephysicalmediumthatconnectsthem.Thepurposeofthetransmitteristoconverterthemessagesignalproducedbythesourceofinformationintoaformsuitablefortransmissionoverthechannel.However,asthetransmittedsignalpropagatesalongthechannel,itisdistortedduetochannelimperfection.Moreover,noiseandinterferingsignals(originatingfromothersources)areaddedtothechanneloutput,withtheresultthatthereceivedsignalisacorruptedversionofthetransmittedsignal.

(2)

Aquantitativewaytoaccountfortheeffectofnoiseistointroducesignal-to-noiseratio(SNR)asasystemparameter.Forexample,wemaydefinetheSNRatthereceiverinputastheratiooftheaveragesignalpowertotheaveragenoisepower,bothbeingmeasuredatthesamepoint.ThecustomarypracticeistoexpresstheSNRindecibels(dBs),definedas10timesthelogarithm(tobase10)ofthepowerratio.Forexample,signal-to-noiseratiosof10,100and1,000correspondto10,20,and30dBs,respectively.

(3)

Thesecondsourceofinformation,music,originatesfrominstrumentssuchasthepiano,violin,andflute.Thenotemadebyamusicalinstrumentmaylastforashorttimeintervalasintheprocessingofakeyonapiano,oritmaybesustainedforalongtimeintervalasintheexampleofafluteplayerholdingaprolongednote.Typically,musichastwostructures:amelodicstructureconsistingofasetofsimultaneoussounds.Likeaspeechsignal,amusicalsignalisbipolar.However,amusicalsignaldiffersfromaspeechsignalinthatitsspectrumoccupiesamuchwiderbandoffrequenciesthatmayextenduptoabout15kHz.Accordingly,musicalsignalsdemandamuchwiderchannelbandwidththanspeechsignalsfortheirtransmission.

参考译文

第一单元通信过程

今天，通信以许多不同的方式进入到了我们的日常生活，以至于可以很容易地忽视它的许多方面。我们手中的电话、客厅里的收音机和电视、办公室和家里接入到互联网的计算机终端以及报纸都能够从地球的每个角落(为我们)提供快速的通信。通信为公海上的船只、飞行中的飞行器、空中的火箭和卫星提供了感知能力。通过无线电话可使汽车驾驶员与几英里外的办公室或家人保持通信联系。通信可使天气预报人员通过多个传感器获得天气情况。通信的方式各种各样，通信应用的例子几乎不胜枚举。最基本的，通信无疑涉及了信息通过一系列的过程从一个点到另一个点的传输，其过程描述如下：

(1)信息的产生：话音、音乐、图片或计算机数据。

(2)通过一系列的符号——电的、听觉的或视觉的方式，使用一定的测量精度对信息符号进行描述。

(3)以适合物理介质传输的形式对这些符号进行编码。

(4)把编码后的符号传输到期望的目的地。

(5)原始符号的解码和再现。

(6)原始信息信号的再生，这种再生信号的质量下降是可界定的，是由系统的非理想性引起的。

当然，还有很多种通信形式不需要人的头脑的实时参与。例如，在计算机通信中两个或多个计算机通信时，人类的决定只是在安装程序或对电脑发出命令或监视结果时才参与到通信过程中。

不考虑正在讨论的通信过程的形式，每一个通信系统都有三个基本元素，即发射器、信道和接收器，如图1.1所示。发射器在空间上位于通信的一端，接收器位于和发射器分开的另一端，信道是连接发射器和接收器的物理介质。发射器的目的是把信源产生的信息转换为适合信道传输的信号形式。但是，随着在信道中的传播，被传输的信号会因信道的非理想性而失真。而且，噪声和干扰信号(来自于其他信源)会叠加在信道输出端，结果使得接收到的信号是恶化后的传输信号。接收器的任务是对接收到的信号进行重建，使其成为用户能辨识的原始信息信号的形式。

图1.1通信系统的构成通信有两种基本的模式：

(1)广播方式。这种方式使用一个大功率的发射器和多个接收器，(这里的)接收器的制造是相对廉价的。这里承载信息的信号只向一个方向传输。

(2)点对点通信。在这种方式中，通信过程在单个发射器和接收器之间的链路上发生。在这种情况下，承载信息的信号通常是双向传输的，这需要使用链路两端的发射器和接收器。收音机和电视是广播通信方式的例子，普遍使用的电话提供了一种点对点通信的手段。

所有这些不同的通信过程以及此处未提及的通信过程都有一个共同的特点：在本质上，潜在的通信过程都是(遵循)统计学(特征)的。实际上，正是由于这个重要原因，本书的大部分内容都致力于(阐述)通信系统的统计学基础。

1．主要的通信资源

在通信系统中，使用了两种主要的资源：传输功率和信道带宽。传输功率是传输信号的平均功率。信道带宽定义为分配给信息信号传输用的频带。一般系统的设计目标是尽量高效地利用这两种资源。在大多数通信信道中，一种资源可能会被认为比另一种资源更重要。因此，我们可以把通信信道分为功率受限或波段受限两种。比如，电话电路是典型的波段受限信道，而空间通信链路或卫星信道是典型的功率受限信道。当信息信号的频谱向下扩展到低频或0时，我们把信号带宽定义为上限频率，高于上限频率的信号频谱分量可以忽略，因此对于信息的传输也是不必要的。举例来说，普通话音频谱可以适当地扩展到10

kHz以外，尽管大部分的平均功率集中在100

Hz～600

Hz之间，但是300

Hz～3100

Hz的波段就可以提供良好的话音清晰度。因此，我们发现电话电路对300

Hz～3100

Hz频率范围响应良好，能够提供十分满意的商业电话业务。我们必须牢记的另一个要点是：通信系统中不可避免地存在着噪声。噪声指的是通信系统中可能干扰信息信号传输和处理的多余波。噪声可来源于系统内部或外部。

对噪声的影响进行定量分析的方法是引入信噪比(SNR)作为系统参数。举例来说，我们可以在接收器输入端把SNR定义为同点测量的平均信号功率与平均噪声功率的比值。习惯的做法是用分贝(dB)来表示SNR，分贝定义为功率比值的对数(以10为底)的10倍。比如，信噪比为10、100和1000时，系统的分贝数分别为10

dB、20

dB和30

dB。

2．信息源

电信环境是由四个重要的信息源决定的：话音、音乐、图片和计算机数据。信息源的特性可以用携带信息的信号来定义。信号定义为时间的单值函数，时间是自变量；在每一个时刻函数有一个唯一的值。信号可以是一维的，如话音、音乐或计算机数据。图片信号是二维的；视频数据信号是三维的；随时间变化的大量数据是四维的。接下来，我们详细说明不同的信息来源。

(1)话音是人类交流的基本手段。特别地，话音交流过程包含从说者到听者的信息的传输，具体包括三个连续的阶段：

①话音产生阶段。说话者头脑中的想法由包含了声音(即压力波)的话音信号表达出来，这些声音由声道产生，其排列组合受控于语言规则。

②传输阶段。声波以300m/s的速度在空气中传播，到达听者的耳朵。

③理解阶段。接收的声音由收听者解释成接收信息，通过完成一连串的事件，最终实现信息从说者到听者的传送。话音产生过程可以看做一种过滤形式，在此过程中，声源激励声道过滤器。声道包括一个截面不均匀的管道，开始于声门(即声带间的开口)，终止于嘴唇。当声音沿着声道传播时，频谱(即频率分量)会受到声道频率选择的整形，这种作用有些类似于风琴管道的共鸣。这里要重点提及的是话音的功率谱(即长期平均功率随频率的分布)在0频率附近逼近0，而在数百赫兹频率附近达到峰值。但是，为了对事物进行正确的观察，我们必须牢记听力机制对频率是非常敏感的。而且，通信系统类型对频率的波段有重要的影响，而频率对通信过程是必不可少的。比如，就像前面提到的，300

Hz～3100

Hz的带宽对于商业电话通信是能够满足需要的。

(2)第二个信源是音乐，它是由乐器，比如钢琴、小提琴和长笛发出的。乐器发出的音可能持续一个短的时间，如敲击一个钢琴键，或可以持续一个长的时间，如长笛演奏者吹出的长音。典型地，乐声有两个结构：一个旋律结构包含了一系列同时发出的声音。类似话音信号，音乐是双极性信号。但是，音乐信号不同于话音信号，因为它的频谱占据了更宽的频率波段，此频率可扩展到大约15kHz。因此，音乐信号的传输需要比话音信号更宽的信道带宽。

EXTENSIVETEXT

Shannon’sInformationCapacityTheorem

Thegoalofacommunicationsystemdesigneristoconfigureasystemthattransportsamessagesignalfromasourceofinterestacrossanoisychanneltoauserattheotherendofthechannelwiththefollowingobjective:

Themessagesignalisdeliveredtotheuserbothefficientlyandreliably,subjecttocertaindesignconstraints:allowabletransmitpower,availablechannelbandwidth,andaffordablecostofbuildingthesystem.

Inthecaseofadigitalcommunicationsystem,reliabilityiscommonlyexpressedintermsofbiterrorrate(BER)orprobabilityofbiterrormeasuredatthereceiveroutput.Clearly,thesmallertheBER,themorereliablethecommunicationsystemis.AquestionthatcomestomindinthiscontextiswhetheritispossibletodesignacommunicationsystemthatoperateswithzeroBEReventhroughthechannelisnoisy.Inanidealsetting,theanswertothisquestionisanemphaticyes.TheanswerisembodiedinoneofShannon’scelebratedtheorems,whichiscalledtheinformationcapacitytheorem.

LetBdenotethechannelbandwidth,andletSNRdenotethereceivedsignal-to-noiseratio.Theinformationcapacitytheoremstatesthatideallythesetwoparametersarerelatedas

C

=

Blb(1

+

SNR)b/s(1)

WhereCistheinformationcapacityofthechannel.Theinformationcapacityisdefinedasthemaximumrateatwhichinformationcanbetransmittedacrossthechannelwithouterror;itismeasuredinbitspersecond

(b/s).ForaprescribedchannelbandwidthBandreceivedSNR,theinformationcapacitytheoremtellsusthatamessagesignalcanbetransmittedthroughthesystemwithouterrorevenwhenthechannelisnoisy,providedthattheactualsignalingrateRinbitspersecond,atwhichdataaretransmittedthroughthechannel,islessthantheinformationcapacityC.

Unfortunately,Shannon’sinformationcapacitytheoremdoesnottellushowtodesignthesystem.Nevertheless,fromadesignpointofview,thetheoremisveryvaluableforthefollowingreasons:

(i)

TheinformationcapacitytheoremprovidesaboundonwhatrateofdatatransmissionistheoreticallyattainableforprescribedvaluesofchannelbandwidthBandreceivedSNR.Onthisbasis,wemayusetheratio

asameasureoftheefficiencyofthedigitalcommunicationsystemunderstudy.Thecloserηistounity,the

moreefficientthesystemis.

(ii)

Equation(1)providesabasisforthetrade-offbetweenchannelbandwidthBandreceivedSNR.Inparticular,foraprescribedsignalingrateR,wemayreducetherequiredSNRbyincreasingthechannelbandwidthB,hencethemotivationforusingawidebandmodulatedscheme(e.g.pulse-codemodulation)forimprovednoiseperformance.

(iii)

Equation(1)providesanidealizedframeworkforcomparingthenoiseperformanceofonemodulationschemeagainstanother.

ADigitalCommunicationProblem

Whenwespeakofadigitalcommunicationsystemhavingalowbiterrorrate,say,theimplicationisthatonlyasmallfractioninalongstreamofbinarysymbolsisdecodedinerrorbythereceiver.Theissueofthereceiverdeterminingwhetherabinarysymbolsentoveranoisychannelisdecodedinerrorornotisoffundamentalimportancetothedesignofdigitalcommunicationsystems.Itisthereforeappropriatebrieflytodiscussthisbasicissuesoastomotivatethestudyofcommunicationsystems.

Supposewehavearandombinarysignal,m(t),consistingofsymbols1