电子类专业英语电子教案第三课.ppt

0 概述 第一代移动通信系统从80年代中期开始 以模拟传输和采用频分复用 FDMA 这样的简单复用技术为特征 第二代系统使用数字技术 出现于90年代初 通过语音压缩使容量增加三倍左右 第三代系统是对第二代系统的扩展 系统容量扩大到原来第一代系统的10倍以上 1 2 3 用户稀少 降低有线电话高昂基础设施费用的一个方法 Thetelecommunicationsindustryfacestheproblemofprovidingtelephoneservicestoruralareas wherethecustomerbaseissmall butthecostofinstallingawiredphonenetworkisveryhigh Onemethodofreducingthehighinfrastructurecostofawiredsystemistouseafixedwirelessradionetwork Theproblemwiththisisthatforruralandurbanareas largecellsizesarerequiredtogetsufficientcoverage Thispresentsextraproblemsastherearelongdelaytimesinmultipathsignalpropagation IntroductiontoMobileCommunications 4 这种技术的码元速率很高 会导致多径引起码间干扰的问题 CurrentlyGlobalSystemforMobiletelecommunications GSM technologyisbeingappliedtofixedwirelessphonesystemsinruralareasofAustralia However GSMusestimedivisionmultiplexaccess TDMA whichhasahighsymbolrateleadingtoproblemswithmultipathcausinginter symbolinterference 5 目的是改进蜂窝单元的容量 抗多径干扰以及灵活性 各自分别适合于不同的具体应用 Severaltechniquesareunderconsiderationforthenextgenerationofdigitalphonesystems withtheaimofimprovingcellcapacity multipathimmunity andflexibility TheseincludeCDMAandCOFDM Boththesetechniquescouldbeappliedtoprovideafixedwirelesssystemforruralareas However eachtechniquehasdifferentproperties makingitmoresuitedforspecificapplications 6 而对COFDM作为一种移动通信系统的传输方法却研究甚少 COFDMiscurrentlybeingusedinseveralnewradiobroadcastsystemsincludingtheproposalforhighdefinitiondigitaltelevision HDTV anddigitalaudiobroadcasting DAB However littleresearchhasbeendoneintotheuseofCOFDMasatransmissionmethodformobiletelecommunicationssystems 7 基站和移动站都知道用于调制发送数据的码 InCDMA alluserstransmitinthesamebroadfrequencybandusingspecializedcodesasabasisofchannelization Boththebasestationandthemobilestationknowthesecodesusedtomodulatethedatasent 8 传输以这样的方法进行 载波之间相互正交因而 每个用户分配到若干载波在其中发送数据 OFDM COFDMallowsmanyuserstotransmitinanallocatedband bysubdividingtheavailablebandwidthintomanynarrowbandwidthcarriers Eachuserisallocatedseveralcarriersinwhichtotransmittheirdata Thetransmissionisgeneratedinsuchawaythatthecarriersusedareorthogonaltooneanother thusallowingthemtobepackedtogethermuchcloserthanstandardfrequencydivisionmultiplexing FDM ThisleadstothehighspectralefficiencyofOFDM COFDM 9 目前的第二代欧洲数字系统 GSM 预期在21世纪初达到饱和 导致了设计大容量通信网络的需要 Theexpansionoftheuseofdigitalnetworkshasledtotheneedforthedesignofnewhighercapacitycommunicationsnetworks Thedemandforcellular typesystemsinEuropeispredictedtobebetween15and20millionusersbytheyear2000 andisalreadyover30million 1995 intheU S Wirelessserviceshavebeengrowingatarategreaterthan50 peryear withthecurrentsecond generationEuropeandigitalsystems GSM beingexpectedtobefilledtocapacitybytheearly2000s 1 以每年超过50 的速度 ThirdGenerationWirelessNetworks 10 随着更广泛的业务需求如视频会议 互联网服务 数据网络 多媒体等 Thetelecommunicationsindustryisalsochanging withademandforagreaterrangeofservicessuchasvideoconferencing Internetservices datanetworks andmultimedia Thisdemandforhighercapacitynetworkshasledtothedevelopmentofthethirdgenerationtelecommunicationssystems 11 其目标是提供更大的灵活性 更大的容量 以及更紧密集成的业务 OneoftheproposedthirdgenerationtelecommunicationssystemsistheUniversalMobileTelecommunicationsSystem UMTS withtheaimofprovidingmoreflexibility highercapacity andamoretightlyintegratedservice ThissectionfocusesontheservicesandaimsoftheUMTS Othersystemsaroundtheworldarebeingdeveloped howevermanyofthesetechnologiesareexpectedtobecombinedintotheUMTS 12 60 以上的用户从他们的居住地访问互联网 那里的带宽常限于28 8kbps TheWorldWideWeb WWW hasbecomeanimportantcommunicationsmedia asitsusehasincreaseddramaticallyoverthelastfewyears Thishasresultedinanincreaseddemandforcomputernetworkingservices Inordertosatisfythis telecommunicationssystemsarenowbeingusedforcomputernetworking Internetaccessandvoicecommunications AWWWsurveyrevealedthatmorethan60 ofusersaccesstheInternetfromresidentiallocations wherethebandwidthisoftenlimitedto28 8kbps 13 这就需要有更加集成化的业务 提供更快的速度 以及对于各种业务更通用的界面 ThisrestrictstheuseoftheInternet preventingtheuseofrealtimeaudioandvideocapabilities Higherspeedservicesareavailable suchasintegrated servicesdigitalnetwork ISDN Theseprovidedataratesuptofivetimesasfast butatamuchincreasedaccesscost Thishasledtothedemandofamoreintegratedservice providingfasterdatarates andamoreuniversalinterfaceforavarietyofservices Theemphasishasshiftedawayfromprovidingafixedvoiceservicetoprovidingageneraldataconnectionthatallowsforawidevarietyofapplications suchasvoice Internetaccess computernetworking etc 14 TheincreasedrelianceoncomputernetworkingandtheInternethasresultedinanincreaseddemandforconnectivitytobeprovided anywhere anytime leadingtoanincreaseinthedemandforwirelesssystems Thisdemandhasdriventheneedtodevelopnewhighercapacity highreliabilitywirelesstelecommunicationssystems 15 通过提供大容量和综合无线网络克服现有无线系统的缺点 Thedevelopmentanddeploymentofthirdgenerationtelecommunicationsystemsaimtoovercomesomeofthedownfallsofcurrentwirelesssystemsbyprovidingahighcapacity integratedwirelessnetwork Therearecurrentlyseveralthirdgenerationwirelessstandards includingUMTS cdmaOne IMT2000 andIS 95 16 看来还会出现更多的标准 Manymobileradiostandardshavebeendevelopedforwirelesssystemsthroughouttheworld withmorestandardslikelytoemerge EvolutionofTelecommunicationSystems 17 还存在用户容量小的问题 同时由于所用的无线电接口简单 也不够安全 以模拟传输和采用频分复用 FDMA 这样的简单复用技术为特征 Thefirstgenerationsystemswereintroducedinthemid1980 s andcanbecharacterizedbytheuseofanalogtransmissiontechniques andtheuseofsimplemultiplexaccesstechniquessuchasFrequencyDivisionMultipleAccess FDMA 2FirstgenerationtelecommunicationssystemssuchasAdvancedMobilePhoneService AMPS onlyprovidedvoicecommunications Theyalsosufferedfromalowusercapacity andsecurityproblemsduetothesimpleradiointerfaceused 18 使容量增加了三倍左右 Secondgenerationsystemswereintroducedintheearly1990 s andallusedigitaltechnology Thisprovidedanincreaseintheusercapacityofaroundthreetimes Thiswasachievedbycompressingthevoicewaveformsbeforetransmission 19 是在复杂性方面对第二代系统的扩展 系统容量预计将扩大到原来第一代系统的10倍以上 Thirdgenerationsystemsareanextensiononthecomplexityofsecondgenerationsystemsandareexpectedtobeintroducedaftertheyear2000 Thesystemcapacityisexpectedtobeincreasedtoovertentimesoriginalfirstgenerationsystems ThisisgoingtobeachievedbyusingcomplexmultiplexaccesstechniquessuchasCodeDivisionMultiplexAccess CDMA oranextensionofTDMA andbyimprovingflexibilityofservicesavailable 20 当今许多分离的系统和业务如 将互相结合 因而 给出现有业务和网络向融入统一的第三代网络这一目标的发展情况 Table10 1andTable10 2showsomeofthemajorcellularmobilephonestandardsinNorthAmericaandEurope Figure10 1showstheevolutionofcurrentservicesandnetworkstotheaimofcombiningthemintoaunifiedthirdgenerationnetwork Manycurrentlyseparatesystemsandservicessuchasradiopaging cordlesstelephony satellitephones privateradiosystemsforcompanies etc willbecombinedsothatalltheseserviceswillbeprovidedbythirdgenerationtelecommunicationssystems 3 21 使固定业务和无线业务融合在一起 在无线和有线环境下提供更加统一的大容量网络 ThemainaimsoftheUniversalMobileTelecommunicationsSystemistoprovideamoreunifiedhighcapacitynetwork inwirelessandwiredenvironments UMTSwillenablefixedandwirelessservicestoconverge Theyaretobethreemainchannelcapacityconnections amobilerateof144kbps aportablerateof384kbpsandanin buildingrateof2Mbps OverallAimsofUMTS 22 根据所要求的数据率 服务质量 可靠性和允许的误码率 BER 实时传输速率来分类 在延迟宽容度和允许的误码率方面 ManyserviceshavebeenidentifiedfortheUMTS whichcanbecategorizedbasedonthedataraterequired qualityofservice reliabilityandallowablebiterrorrate BER andrealtimetransferrate Eachoftheserviceshasdifferentcharacteristicsintermsofdelaytoleranceandallowablebiterrorrates Table10 3showscharacteristicsforsomeoftheUMTSservices Teleservices 23 Thedatacharacteristicswilldeterminethemostsuitabletransmissionmethods Thetypeofdataassociatedwitheachservicedeterminesthetypeofenvironmentinwhichtheservicecanbesupported 业务所支持的环境类型 24 任何地点 任何时间 的服务 TheaimoftheUMTSsystemsistoprovidean anywhere anytime service thustheoperatingenvironmentwillvarydependingontheuserlocation Theenvironmentinwhichthewirelesssystemmustoperateaffectsthesystemcapacityandtypeofservicesthatcanbeprovided Table10 4listssomeoftheenvironmentsinwhichUMTSwillberequiredtoprovidecoverage Themaximumsupporteddatarateforeachenvironmentisrelatedtothecellsizerequiredtoprovideadequatecoveragefortheenvironment UMTSEnvironments 25 因为传播效应如多径和衰落迫使大的单元只能使用较低的数据率 AcellularnetworkisrequiredtoensurethattheUMTScanprovideahighcapacitynetwork Aswithanycellularsystem thetotalcapacityofthenetworkisdependentonthesizeofthecellsused Thesmallerthecellsaremade thelargerthetotalcapacity However thecellsizeislimitedbytheamountofinfrastructurethatcanbesetup Thecellsizealsodeterminesthemaximumchannelcapacityforeachcell aspropagationeffectssuchasmultipathandfadingforcelargecellstohavealowerdatarate CellTypes 26 大的单元还必须为大量用户服务 每个用户只能使用相应于小单元的低数据率 Largecellsalsohavetoservicealargenumberofusers andsincethecellcapacityisapproximatelyfixed eachusercanonlyhaveareduceddatarate withrespecttoasmallercell Inordertooptimizethecellularnetworkthreecelltypesareused Thesearethepico cell microcell andmacro cell Thethreecelltypestradeoffcellsizewithtotalcapacityandservices Table10 5showsthethreecelltypesusedintheUMTSsystemandsomeofthecellcharacteristics 27 每种蜂窝单元的覆盖范围大小和类型导致所面临的无线电传播问题 Thesizeandtypeofcoverageofeachcelleffectstheradiopropagationproblemsthatwillbeencountered Thiswilldeterminethemostsuitableradiotransmissiontechniquetouse 28 在一个大范围无线环境中提供灵活 按需分配的2Mbps宽带业务的要求 将要求无线电接入技术方面的变革 OneoftheaimsidentifiedforUMTSistoprovideawirelessinterfacecomparabletowiredconnections Therequirementtoprovidewidebandservicesupto2Mb s withflexible ondemandallocationoftransmissioncapacityinalargerangeofradioenvironments willcallforarevolutionintheradioaccesstechniquesused RadioInterface 29 涉及到对CDMA和TDMA性能比较的研究 看来CDMA最有可能成为支持所要求高数据率的候选技术 Theradiointerfaceiscurrentlyundergoingsubstantialresearch withtherelativeperformanceofCDMAandTDMAbeinginvestigated CurrentlyCDMAappearstobethemostlikelycandidateforsupportingthehighdataraterequired However othertechniquessuchasCOFDMandhybridsolutionsmayalsobeappropriateforUMTS 30 限于高昂的基础设施成本 蜂窝网络只能覆盖有限区域 OneoftheaimsoftheUMTSistoprovideaccess anywhere anytime However cellularnetworkscanonlycoveralimitedareaduetothehighinfrastructurecosts Forthisreason satellitesystemsformanintegralpartoftheUMTSnetwork Satelliteswillbeabletoprovideanextendedwirelesscoveragetoremoteareasandtoaeronauticalandmaritimemobiles SatelliteNetworking 31 一个完整的集成解决方案将要求移动电话是双重模式的 同时允许与轨道上运行的卫星和地面上的蜂窝式系统通信 Thelevelofintegrationofthesatellitesystemswiththeterrestrialcellularnetworksisunderstudy Afullyintegratedsolutionwillrequiremobilestobedualmodeterminalsthatwouldallowcommunicationswithorbitingsatellitesandterrestrialcellularnetworks LowEarthOrbit LEO satellitesarethemostlikelycandidatesforprovidingworldwidecoverage 32 因此提出了宽带 broadband 无线网络的要求 向全世界实际上所有地方提供具有较大带宽的 highbandwidth 双向通信 Currentlyseverallowearthorbitsatellitesystemsarebeingdeployedforprovidingglobaltelecommunications TheseincludetheTeledesicSystem whichisscheduledtobeginoperationbytheendof2002with288satellites toprovidehighbandwidthtwo waycommunicationstovirtuallyanywhereintheworld However theTeledesicSystemwillnotbeabletomeeteven20 ofthedemand thustheneedforbroadbandwirelessnetworks 33 建立系统并投入运行 正在开始超出容量极限 Acrosstheglobe eachregionismovingtomakethirdgenerationsystemshappen Japanislookingathavingasystemupandrunningbyyear2000 Thisisdrivenbythefactthatdemandformobilecommunicationshasbeensogreatthattheirsecondgenerationcellularnetworksarestartingtorunoutofcapacity ItisexpectedthatEuropewillhaveawidebandCDMAsystembytheyear2005 TheU S isexpectedtoimplementathirdgenerationsystemsomewherefrom2000to2010 TimetableforSystemImplementation 34 在基础设施方案公布以前 Manufacturersarecreatingseveralstandardstomeetrequirementsineachsectoroftheworld Todate themajorityofsystemsarebasedonCDMAstandards Beforeinfrastructurerollsout thirdgenerationswillbedevelopedonaregionalbasis 35 其目的是将地区性系统纳入统一的标准 ThisprocessisbeingguidedbytheInternationalTelecommunicationsUnion s ITU efforttocreatetheIMT2000standard ITUwillproducetheIMT2000standardbytheyear2000 withtheaimofcombiningtheregionalsystemsintoaunifiedstandard 36 对于21世纪的用户 理想的情况是在移动和固定网络接入的服务能力方面应该没有区别 Futurecommunicationswillbedrivenbytheneedtoprovideamoreintegratedhighcapacity widecoverageservice Forthe21stcenturyuserthereshouldideallybenodistinctioninservicecapabilitybetweenmobileorfixednetworkaccess Thiswillbeachievedusingavarietyoftechnologiesincludingsatellitecommunications advancedradionetworkingtechniques andhighspeedfixednetworks Conclusion 37 Operationoftheradarisbasedonthemeasurementofthetimeittakesforapulsetransmittedfromanantennatogetreflect