外文翻譯--通信系統(tǒng)簡介.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通信系統(tǒng)簡介人們常說我們正生活在一個信息時代，通信技術(shù)對信息的產(chǎn)生，存儲與轉(zhuǎn)換有著至關(guān)重要的作用。任何通信系統(tǒng)的都是通過信道將信息從信源傳送到目的地，圖1所示為一個簡單的系統(tǒng)。來自信源的信息一般是不能通過信道直接傳輸?shù)模虼嗽谝欢艘玫奖环Q為是發(fā)射機的裝置，另一端要用到被稱為是接收機的裝置。圖1簡單通信系統(tǒng)信號源或信息信號可以是模擬的或數(shù)字的。常見的例子是模擬音頻信號、視頻信號及數(shù)據(jù)。信息源常被描述為信號所占用的頻率范圍，例如，電話質(zhì)量的語音信號，包含著300Hz3kHz的頻率范圍，而模擬高保真音樂信號大概需要20Hz20kHz的頻率范圍。數(shù)字信號源于音頻或視頻信號，或由數(shù)據(jù)組成（如文字和字符）。數(shù)字信號可以有任意帶寬，這要取決于每秒鐘傳送的比特數(shù)和所采用的將二進制1和0轉(zhuǎn)換成電信號的方法。通信信道可以是任何媒介：一對導(dǎo)體、一條光纖或者是我們生活的自由空間。有時信道可以直接承載信息。例如，語音信號可以直接有一對雙絞線電話電纜來承載。另一方面，自由空間