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外文翻译--通信系统简介.doc

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外文翻译--通信系统简介.doc

1IntroductiontoCommunicationSystemItisoftensaidthatwearelivingintheinformationage.Communicationtechnologyisabsolutelyvitaltothegeneration,storage,andtransmissionofthisinformation.Anycommunicationsystemmovesinformationfromasourcetoadestinationthroughachannel.Figure1illustratesthisverysimpleidea.Theinformationfromthesourcewillgenerallynotbeinaformthatcantravelthroughthechannel,soadevicecalledatransmitterwillbeemployedatoneendandareceiverattheother.Figure1simplecommunicationsystemThesourceorinformationsignalcanbeanalogordigital.Commonexamplesareanalogaudio,videosignalsanddigitaldata.Sourcesareoftendescribedintermsofthefrequencyrangethattheyoccupy.Telephonequalityanalogvoicesignals,forinstance,containfrequenciesfrom300Hzto3kHz,whileanaloghighfidelitymusicneedsafrequencyrangeofapproximately20Hzto20kHz.Digitalsourcescanbederivedfromaudioorvideosignalscanhavealmostanybandwidthdependingonthenumberofbitstransmittedpersecond,andthemethodusedtoconvertbinaryonesandzerosintoelectricalsignals.Acommunicationchannelcanbealmostanythingapairofconductors,anopticalfiberorafreespacethatwelive.Sometimesachannelcancarrytheinformationsignaldirectly.Forexample,anaudiosignalcanbecarrieddirectlybyatwistedpairtelephonecable.Ontheotherhand,aradiolinkthroughfreespacecannotbeuseddirectlyforvoicesignals.Suchsituationrequiretheuseofacarrierwavewillbealtered,ormodulatedm,bytheinformationsignalsinsuchawaythattheinformationcanberecoveredatthedestination.Whenacarrierisused,theinformationsignalisalsoknownasthemodulatingsignals.Technologyisatthecoreofmanynewandemergingdigitalinformationproductsandapplicationsthatsupporttheinformationsociety.Suchproductsandapplicationsoftenrequirethecollection,sometimesinrealtime.Theabilityoftechnologytohandlerealworldsignalsdigitallyhasmadeitpossibletocreateaffordable,innovativeandhighqualityproductsandapplicationsforlargeconsumermarketforexampledigitalcellularmobilephone,digitaltelevisionandvideogames.Theimpactofisalsoevidentinmanyotherareas,suchasmedicineandhealthcare.Forexampleinpatientmonitorsforintensivecare,digitalXrayappliances,advancedcardiologyandbrainmappingsystemsandsoon,digitalaudio,forexampleCDplayersaudiomixersandelectronicmusicandsoon.Andpersonalcomputersystemsforexampledisksforefficientdatastorageanderrorcorrection,moderns,soundcardsandvideoconferencingandsoon.Mostofthemajorcitiesinthedomesticbusstopartificialvoice.Everyoneofthekeypointsfromthedriverorattendanttostopbyvoice.Butsometimesduetovariousfactorssuchasweather,vehiclecongestion,SourceTransmitterReceiverDestination2flightattendantsarefeelingtheeffectsofthechanges.Therebeinggiventhestationsreportingstations,especiallyforpassengersnotfamiliarwiththetopographyofthecity,causingalotofunnecessarytrouble.Wellthusaffecttheimageofacityconstructionwindow,thendevelopedautomaticstopsysteminevitable.AsrequiredbeforethedockingsystembusGPSinformationlatitudeandlongitudeinformation,etc.,longitudeandlatitudeinformationgeneratedbythedistancebetweenbusstopswiththemessagethatthisisgoingtoexperiencethetedious,usethemicrocontrollerdifficulttoachieve,andwhenusingchips,thepropersolutionofthisproblem.Usingradianspersecondinthemathematicsdealingwithmodulationmakestheequationsimpler.Ofcourse,frequencyisusuallygiveninhertz,ratherthaninradianspersecond,whenpracticaldevicesarebeingdiscussed.Itiseasytoconvertbetweenthetwosystemspersecond,whenpracticaldevicesarebeingdiscussed.ItiseasytoconvertbetweenthetwosystemsbyrecallingfrombasicACtheory,ω2πf.Inmodulation,theparametersthatcanbechangedareamplitudeE,frequencyω,andphaseθ.Combinationsarealsopossible.Forexample,manyschemesfortransmittingdigitalinformationusebothamplitudeandphasemodulation.Multiplexingisthetermusedincommunicationstorefertothecombiningoftwoormoreinformationsignals.Whentheavailablefrequencyrangeisdividedamongthesignals,theprocessisknownasfrequencydivisionmultiplexingFDM.Radioandtelevisionbroadcasting,inwhichtheavailablespectrumisdividedamongmanysignals,areeverydayexamplesofFDM.Therearelimitationstothenumberofsignalsthatcanbecrowdedintoagivenfrequencyrangebecauseeachrequiresacertainbandwidth,Forexample,atelevisionchannelonlyoccupiessgivenbandwidthof6MHzin68MHzbandwidthofVHF.ParallelDSPchiptoenhancetheperformanceofatraditionalimprovedthroughtheuseofmultiplyaddunitsandtheHarvardstructure,itgoesfarbeyondthecomputationalcapabilitiesofthetraditionalmicroprocessor.Areasonableinferenceischipoperationsbyincreasingthenumberofmodulesandthecorrespondingnumberofbuslinkingcomputationalmodules.Thechipcanbedoubledtoenhancetheoveralloperationalcapacity.Ofcourse,suchaninferencetwopreconditionsmustbemetFirst,thememorybusbandwidthasnecessarytomeettheincreaseinthenumberofenhanceddatathroughputInaddition,variousfunctionalunitsinvolvedintheparallelschedulingalgorithmisitscomplexitycanbeachieved.AnalternativemethodforusingasinglecommunicationchanneltosendmanysignalsistousetimedivisionmultiplexingTDM.Insteadofdividingtheavailablebandwidthofthechannelamongmanysignals,theentirebandwidthisusedforeachsignal,butonlyforasmallpartofthetime.Anonelectronicexampleisthedivisionofthetotalavailabletimeonatelevisionchannelamongthevariousprogramstransmitted.Eachprogramusesthewholebandwidthofthechannel,butonlyforpartofthetime.ItiscertainlypossibletocombineFDMandTDM,Forexample,theavailablebandwidthofa3communicationsatelliteisdividedamonganumberoftransmitterreceivercombinationscalledtransponders.ThisisanexampleofFDM.AsingletranspondercanbeusedtocarryalargenumberofdigitalsignalsusingTDM.Thiscoursepresentsatopdownapproachtocommunicationssystemdesign.Thecoursewillcovercommunicationtheory,algorithmsandimplementationarchitecturesforessentialblocksinmodernphysicallayercommunicationsystemscodersanddecoders,filters,multitonemodulation,synchronizationsubsystems.Thecourseishandson,withaprojectcomponentservingasavehicleforstudyofdifferentcommunicationtechniques,architecturesandimplementations.Thisyear,theprojectisfocusedonWLANtransceivers.Attheendofthecourse,studentswillhavegonethroughthecompleteWLANSystemOnaChipdesignprocess,fromcommunicationtheory,throughalgorithmandarchitectureallthewaytothesynthesizedstandardcellRTLchiprepresentation.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,自由空间无线链路不能直接用于承载语音信号,这就需要使用一个载波信号,它的频率能够使语音信号通过信道传输或传播。载波有信息信号改变或调制,这样信息才能够在接受端被恢复。当使用载波时信息信号被称为调制信号。通信技术是许多新的和正在涌现的支撑信息社会的数字信息产品和应用的核心,这些产品和应用要求收集,分析,传输,显示和存储现实世界的信息,有时候则需要实时实现。通信技术数字化地处理现实世界信息的能力,使得广大的消费市场如数字蜂窝移动电话,数字电视和视频游戏等等。创造可生产的,新的,高指令的产品和应用成为可能,DSP在其它领域的影响力也十分明显的,例如医学,健康监护,病人监护,数字X光机,先进的心脏和脑电图系统等等,以及数字音频,例如CD播放器,音频混频器和电子音乐和个人计算机系统,例如有效数据存储的磁盘,误差校正,调制解调器,声卡和视频会议等等。当前国内主要大城市的公交车大都采用人工语音报站,即每到一站由司机或者乘务员按语音键来进行报站。但有时由于受到各种因素如雨雪天路滑、车上拥挤、乘务员心情的变化等的影响,会出现报错车站,漏报车站的情况,给乘客特别是不熟悉本市地形的乘客带来了不必要的麻烦,进而会影响到一个城市的窗口形象工程建设,于是开发研制自动报站系统成为必然。系统中由于要求对接收到的公交车GPS定位信息(经纬度信息等)进行处理,由经纬度信息生成站牌与公交车之间的距离信息,这之间会经历繁琐的计算,用一般的微控制器难于实现,而当采用处理芯片时,这一问题就很好解决了。调制在数学上采用弧度单位,能释放流程变得简单。当然,在讨论实际装置时,频率通常用赫兹来表示,而不是用弧度每秒来表示,运用交流基本理论ω2πf,两者之间很容易转换。信源转换器接收机信宿

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