外文翻译--通信系统简介.doc

1IntroductiontoCommunicationSystemItisoftensaidthatwearelivingintheinformationage.Communicationtechnologyisabsolutelyvitaltothegeneration,storage,andtransmissionofthisinformation.Anycommunicationsystemmovesinformationfromasourcetoadestinationthroughachannel.Figure1illustratesthisverysimpleidea.Theinformationfromthesourcewillgenerallynotbeinaformthatcantravelthroughthechannel,soadevicecalledatransmitterwillbeemployedatoneendandareceiverattheother.Figure1simplecommunicationsystemThesourceorinformationsignalcanbeanalogordigital.Commonexamplesareanalogaudio,videosignalsanddigitaldata.Sourcesareoftendescribedintermsofthefrequencyrangethattheyoccupy.Telephone-qualityanalogvoicesignals,forinstance,containfrequenciesfrom300Hzto3kHz,whileanaloghigh-fidelitymusicneedsafrequencyrangeofapproximately20Hzto20kHz.Digitalsourcescanbederivedfromaudioorvideosignalscanhavealmostanybandwidthdependingonthenumberofbitstransmittedpersecond,andthemethodusedtoconvertbinaryonesandzerosintoelectricalsignals.Acommunicationchannelcanbealmostanything:apairofconductors,anopticalfiberorafreespacethatwelive.Sometimesachannelcancarrytheinformationsignaldirectly.Forexample,anaudiosignalcanbecarrieddirectlybyatwisted-pairtelephonecable.Ontheotherhand,aradiolinkthroughfreespacecannotbeuseddirectlyforvoicesignals.Suchsituationrequiretheuseofacarrierwavewillbealtered,ormodulatedm,bytheinformationsignalsinsuchawaythattheinformationcanberecoveredatthedestination.Whenacarrierisused,theinformationsignalisalsoknownasthemodulatingsignals.Technologyisatthecoreofmanynewandemergingdigitalinformationproductsandapplicationsthatsupporttheinformationsociety.Suchproductsandapplicationsoftenrequirethecollection,sometimesinrealtime.Theabilityoftechnologytohandlerealworldsignalsdigitallyhasmadeitpossibletocreateaffordable,innovative；andhighqualityproductsandapplicationsforlargeconsumermarketforexample:digitalcellularmobilephone,digitaltelevisionandvideogames.Theimpactofisalsoevidentinmanyotherareas,suchasmedicineandhealthcare.Forexample:inpatientmonitorsforintensivecare,digitalX-rayappliances,advancedcardiologyandbrainmappingsystemsandsoon,digitalaudio,forexample:CDplayers；audiomixersandelectronicmusicandsoon.Andpersonalcomputersystemsforexample:disksforefficientdatastorageanderrorcorrection,moderns,soundcardsandvideoconferencingandsoon.Mostofthemajorcitiesinthedomesticbusstopartificialvoice.Everyoneofthekeypointsfromthedriverorattendanttostopbyvoice.Butsometimesduetovariousfactorssuchasweather,vehiclecongestion,SourceTransmitterReceiverDestination2flightattendantsarefeelingtheeffectsofthechanges.Therebeinggiventhestationsreportingstations,especiallyforpassengersnotfamiliarwiththetopographyofthecity,causingalotofunnecessarytrouble.Wellthusaffecttheimageofacityconstructionwindow,thendevelopedautomaticstopsysteminevitable.AsrequiredbeforethedockingsystembusGPSinformation(latitudeandlongitudeinformation,etc.),longitudeandlatitudeinformationgeneratedbythedistancebetweenbusstopswiththemessagethatthisisgoingtoexperiencethetedious,usethemicro-controllerdifficulttoachieve,andwhenusingchips,thepropersolutionofthisproblem.Usingradianspersecondinthemathematicsdealingwithmodulationmakestheequationsimpler.Ofcourse,frequencyisusuallygiveninhertz,ratherthaninradianspersecond,whenpracticaldevicesarebeingdiscussed.Itiseasytoconvertbetweenthetwosystemspersecond,whenpracticaldevicesarebeingdiscussed.ItiseasytoconvertbetweenthetwosystemsbyrecallingfrombasicACtheory,=2f.Inmodulation,theparametersthatcanbechangedareamplitudeE,frequency,andphase.Combinationsarealsopossible.Forexample,manyschemesfortransmittingdigitalinformationusebothamplitudeandphasemodulation.Multiplexingisthetermusedincommunicationstorefertothecombiningoftwoormoreinformationsignals.Whentheavailablefrequencyrangeisdividedamongthesignals,theprocessisknownasfrequency-divisionmultiplexing(FDM).Radioandtelevisionbroadcasting,inwhichtheavailablespectrumisdividedamongmanysignals,areeverydayexamplesofFDM.Therearelimitationstothenumberofsignalsthatcanbecrowdedintoagivenfrequencyrangebecauseeachrequiresacertainbandwidth,Forexample,atelevisionchannelonlyoccupiessgivenbandwidthof6MHzin68MHzbandwidthofVHF.ParallelDSPchiptoenhancetheperformanceofatraditionalimprovedthroughtheuseofmultiply-addunitsandtheHarvardstructure,itgoesfarbeyondthecomputationalcapabilitiesofthetraditionalmicroprocessor.Areasonableinferenceis:chipoperationsbyincreasingthenumberofmodulesandthecorrespondingnumberofbuslinkingcomputationalmodules.Thechipcanbedoubledtoenhancetheoveralloperationalcapacity.Ofcourse,suchaninferencetwopreconditionsmustbemet:First,thememorybusbandwidthasnecessarytomeettheincreaseinthenumberofenhanceddatathroughput；Inaddition,variousfunctionalunitsinvolvedintheparallelschedulingalgorithmisitscomplexitycanbeachieved.Analternativemethodforusingasinglecommunicationchanneltosendmanysignalsistousetime-divisionmultiplexing(TDM).Insteadofdividingtheavailablebandwidthofthechannelamongmanysignals,theentirebandwidthisusedforeachsignal,butonlyforasmallpartofthetime.Anonelectronicexampleisthedivisionofthetotalavailabletimeonatelevisionchannelamongthevariousprogramstransmitted.Eachprogramusesthewholebandwidthofthechannel,butonlyforpartofthetime.ItiscertainlypossibletocombineFDMandTDM,Forexample,theavailablebandwidthofa3communicationsatelliteisdividedamonganumberoftransmitter-receivercombinationscalledtransponders.ThisisanexampleofFDM.AsingletranspondercanbeusedtocarryalargenumberofdigitalsignalsusingTDM.Thiscoursepresentsatop-downapproachtocommunicationssystemdesign.Thecoursewillcovercommunicationtheory,algorithmsandimplementationarchitecturesforessentialblocksinmodernphysical-layercommunicationsystems(codersanddecoders,filters,multi-tonemodulation,synchronizationsub-systems).Thecourseishands-on,withaprojectcomponentservingasavehicleforstudyofdifferentcommunicationtechniques,architecturesandimplementations.Thisyear,theprojectisfocusedonWLANtransceivers.Attheendofthecourse,studentswillhavegonethroughthecompleteWLANSystem-On-a-Chipdesignprocess,fromcommunicationtheory,throughalgorithmandarchitectureallthewaytothesynthesizedstandard-cellRTLchiprepresentation.4通信系统简介人们常说我们正生活在一个信息时代，通信技术对信息的产生，存储与转换有着至关重要的作用。任何通信系统的都是通过信道将信息从信源传送到目的地，图1所示为一个简单的系统。来自信源的信息一般是不能通过信道直接传输的，因此在一端要用到被称为是发射机的装置，另一端要用到被称为是接收机的装置。图1简单通信系统信号源或信息信号可以是模拟的或数字的。常见的例子是模拟音频信号、视频信号及数据。信息源常被描述为信号所占用的频率范围，例如，电话质量的语音信号，包含着300Hz3kHz的频率范围，而模拟高保真音乐信号大概需要20Hz20kHz的频率范围。数字信号源于音频或视频信号，或由数据组成（如文字和字符）。数字信号可以有任意带宽，这要取决于每秒钟传送的比特数和所采用的将二进制1和0转换成电信号的方法。通信信道可以是任何媒介：一对导体、一条光纤或者是我们生活的自由空间。有时信道可以直接承载信息。例如，语音信号可以直接有一对双绞线电话电缆来承载。另一方面，自由空间