电子信息与通信工程专业英语课件chapter8

电子信息与通信工程专业英语Chapter8WirelessCommunication8.1WirelessLANs1Wirelesscommunicationisoneofthefastest-growing2technologies.Thedemandforconnectingdeviceswithouttheuseofcablesisincreasingeverywhere.WirelessLANscanbefoundoncollegecampuses,inofficebuildings,andinmanypublicareas.weconcentrateon3twopromisingwirelesstechnologiesforLANs:IEEE802.11wirelessLANs,sometimescalledwirelessEthernet,andBluetooth,atechnologyforsmallwirelessLANs.Althoughbothprotocolsneedseverallayerstooperate,weconcentratemostlyonthephysicalanddatalinklayers.8.1.1IEEE802.11IEEEhasdefinedthespecificationsforawirelessLAN,calledIEEE802.11,whichcoversthephysicalanddatalinklayers.Architecture:Thestandarddefinestwokindsofservices:thebasicserviceset(BSS)andtheextendedserviceset(ESS).Basicserviceset:IEEE802.11definesthebasicserviceset(BSS)asthebuildingblockofawirelessLAN.Abasicservicesetismadeof4stationaryormobilewirelessstationsandanoptionalcentralbasestation,knownastheaccesspoint(AP).TheBSSwithoutanAPisastand-alonenetworkandcannotsenddatatootherBSSs.Itiscalledanadhocarchitecture.Inthisarchitecture,stationscanformanetworkwithouttheneedofanAP;theycanlocateoneanotherandagreetobepartofaBSS.ABSSwithanAPissometimesreferredtoasaninfrastructurenetwork.Extendedserviceset:Anextendedserviceset(ESS)ismadeupoftwoormoreBSSswithAPs.Inthiscase,theBSSsareconnectedthroughadistributionsystem,whichisusuallyawiredLAN.ThedistributionsystemconnectstheAPsintheBSSs.IEEE802.11doesnotrestrictthedistributionsystem;itcanbeanyIEEELANsuchasanEthernet.Notethattheextendedservicesetusestwotypesofstations:mobileandstationary.ThemobilestationsarenormalstationsinsideaBSS.ThestationarystationsareAPstationsthatarepartofawiredLAN.WhenBSSsareconnected,thestationswithinreachofoneanothercancommunicatewithouttheuseofanAP.However,communicationbetweentwostationsintwodifferentBSSsusuallyoccursviatwoAPs.TheideaissimilartocommunicationinacellularnetworkifweconsidereachBSStobeacellandeachAPtobeabasestation.NotethatamobilestationcanbelongtomorethanoneBSSatthesametime.8.1.2MACSublayerIEEE802.11definestwoMACsublayers:thedistributedcoordinationfunction(DCF)andpointcoordinationfunction(PCF).WewilldiscussthephysicallayerimplementationslaterinthepaperandnowconcentrateontheMACsublayer.OneofthetwoprotocolsdefinedbyIEEEattheMACsublayeriscalledthedistributedcoordinationfunction(DCF).DCFusesCSMA/CAastheaccessmethod.Thepointcoordinationfunction(PCF)isanoptionalaccessmethodthatcanbeimplementedinaninfrastructurenetwork(notinanadhocnetwork).Itisimplementedontopof5theDCFandisusedmostlyfortime-sensitive5transmission.PCFhasacentralized,contention-freepollingaccessmethod.TheAPperformspollingforstationsthatarecapableofbeingpolled.Thestationsarepolledoneafteranother,sendinganydatatheyhavetotheAP.TogiveprioritytoPCFoverDCF,anothersetofinterframespaceshasbeendefined:PIFSandSIFS.TheSIFSisthesameasthatinDCF,butthePIFS(PCFIFS)isshorterthantheDIFS.Thismeansthatif,atthesametime6,astationwantstouseonlyDCFandanAPwantstousePCF,theAPhaspriority.DuetothepriorityofPCFoverDCF,stationsthatonlyuseDCFmaynotgainaccesstothemedium.Topreventthis,arepetitionintervalhasbeendesignedtocoverbothcontention-free(PCF)andcontention-based(DCF)traffic.Therepetitioninterval,whichisrepeatedcontinuously,startswithaspecialcontrolframe,calledabeaconframe.Whenthestationshearthebeaconframe,theystarttheirNAVforthedurationofthecontention-freeperiodoftherepetitioninterval.8.1.3PhysicalLayerWediscusssixspecifications,asshowninTable8-1.Table8-1

PhysicallayersIEEETechniqueBandModulationRate(Mbps)802.11FHSS2.4GHzFSK1and2DSSS2.4GHzPSK1and2InfraredPPM1and2802.11aOFDM5.725GHzPSKorQAM6to54802.11bDSSS2.4GHzPSK5.5and11802.l

lgOFDM2.4GHzDifferent22and54IEEE802.11FHSSusesthefrequency-hoppingspreadspectrum(FHSS)7method.FHSSusesthe2.4GHzISMband.Thebandisdividedinto79subbandsof1MHz(andsomeguardbands).Apseudorandomnumbergeneratorselectsthehoppingsequence.Themodulationtechniqueinthisspecificationiseithertwo-levelFSKorfour-levelFSKwith1or2bits/baud,whichresultsinadatarateof1or2Mbps.IEEE802.11DSSSusesthedirectsequencespreadspectrum(DSSS)7method.DSSSusesthe2.4GHzISMband.ThemodulationtechniqueinthisspecificationisPSKat1Mbaud/s.Thesystemallows1or2bits/baud(BPSKorQPSK),whichresultsinadatarateof1or2Mbps.IEEE802.11infraredusesinfraredlightintherangeof800to950nm.Themodulationtechniqueiscalledpulsepositionmodulation(PPM).Fora1Mbpsdatarate,a4bitsequenceisfirstmappedintoa16-bitsequenceinwhichonlyonebitissetto1andtherestaresetto0.Fora2Mbpsdatarate,a2bitsequenceisfirstmappedintoa4bitsequenceinwhichonlyonebitissetto1andtherestaresetto0.Themappedsequencesarethenconvertedtoopticalsignals;thepresenceoflightspecifies1,theabsenceoflightspecifies0.IEEE802.11aOFDMdescribestheorthogonalfrequency-divisionmultiplexing(OFDM)methodforsignalgenerationina5GHzISMband.OFDMissimilartoFDM,withonemajordifference:Allthesubbandsareusedbyonesourceatagiventime.Sourcescontendwith8oneanotheratthedatalinklayerforaccess.Thebandisdividedinto52subbands,with48subbandsforsending48groupsofbitsatatimeand4subbandsforcontrolinformation.TheschemeissimilartoADSL.Dividingthebandintosubbandsdiminishestheeffectsofinterference.Ifthesubbandsareusedrandomly,securitycanalsobeincreased.OFDMusesPSKandQAMformodulation.Thecommondataratesare18Mbps(PSK)and54Mbps(QAM).IEEE802.1lbDSSSdescribesthehigh-ratedirectsequencespreadspectrum(HR-DSSS)methodforsignalgenerationinthe2.4GHzISMband.HR-DSSSissimilartoDSSSexceptfortheencodingmethod,whichiscalledcomplementarycodekeying(CCK).CCKencodes4or8bitstooneCCKsymbol.Tobebackwardcompatiblewith9DSSS,HR-DSSSdefinesfourdatarates:1,2,5.5,and11Mbps.ThefirsttwousethesamemodulationtechniquesasDSSS.The5.5MbpsversionusesBPSKandtransmitsat1.375Mbaud/swith4bitCCKencoding.The11MbpsversionusesQPSKandtransmitsat1.375Mbpswith8bitCCKencoding.IEEE802.11gisanewspecificationwhichdefinesforwarderrorcorrectionandOFDMusingthe2.4GHzISMband.Themodulationtechniqueachievesa22Mbpsor54Mbpsdatarate.Itisbackwardcompatiblewith802.l1b,butthemodulationtechniqueisOFDM.8.1.4BluetoothBluetoothisawirelessLANtechnologydesignedtoconnectdevicesofdifferentfunctionssuchastelephones,notebooks,computers(desktopandlaptop),cameras,printers,coffeemakers,andsoon.ABluetoothLANisanadhocnetwork,whichmeansthatthenetworkisformedspontaneously;thedevices,sometimescalledgadgets,findeachotherandmakeanetworkcalledapiconet.ABluetoothLANcanevenbeconnectedtotheInternetifoneofthegadgetshasthiscapability.ABluetoothLAN,bynature,cannotbelarge.Iftherearemanygadgetsthattrytoconnect,thereischaos.Bluetoothtechnologyhasseveralapplications.Peripheraldevicessuchasawirelessmouseorkeyboardcancommunicatewiththecomputerthroughthistechnology.Monitoringdevicescancommunicatewithsensordevicesinasmallhealthcarecenter.Homesecuritydevicescanusethistechnologytoconnectdifferentsensorstothemainsecuritycontroller.Conferenceattendeescansynchronizetheirlaptopcomputersataconference.BluetoothwasoriginallystartedasaprojectbytheEricssonCompany.ItisnamedforHaraldBlaatand,thekingofDenmark(940-981)whounitedDenmarkandNorway.BlaatandtranslatesittoBluetoothinEnglish.Today,BluetoothtechnologyistheimplementationofaprotocoldefinedbytheIEEE802.15standard.Thestandarddefinesawirelesspersonal-areanetwork(PAN)operableinanareawiththesizeofaroomorahall.Bluetoothdefinestwotypesofnetworks:piconetandscattemet.ABluetoothnetworkiscalledapiconet,orasmallnet.Apiconetcanhaveuptoeightstations,oneofwhichiscalledtheprimary;therestarecalledsecondaries.Allthesecondarystationssynchronizetheirclocksandhoppingsequencewiththeprimary.Notethatapiconetcanhaveonlyoneprimarystation.Thecommunicationbetweentheprimaryandthesecondarycanbeone-to-oneorone-to-many.Althoughapiconetcanhaveamaximumofsevensecondaries,anadditionaleightsecondariescanbeintheparkedstate.Asecondaryinaparkedstateissynchronizedwiththeprimary,butcannottakepartincommunicationuntilitismovedfromtheparkedstate.Becauseonlyeightstationscanbeactiveinapiconet,activatingastationfromtheparkedstatemeansthatanactivestationmustgototheparkedstate.Piconetscanbecombinedtoformwhatiscalledascatternet.Asecondarystationinonepiconetcanbetheprimaryinanotherpiconet.Thisstationcanreceivemessagesfromtheprimaryinthefirstpiconet(asasecondary)and,actingasaprimary,deliverthemtosecondariesinthesecondpiconet.Astationcanbeamemberoftwopiconets.2.4GHzbandwidth.ThismeansthatthereisapossibilityofinterferencebetweentheIEEE802.1lbwirelessLANsandBluetoothLANs.Newwordsandphrasescampus [ˈkæmpəs] n. 〈美〉校园，学校范围内；大学ethernet [ˈi:θənet] n. 【计】以太网(局域网)bluetooth [ˈblu:tu:θ] n. 蓝牙architecture [ˈɑrkiˌtektʃər] n. 构造，结构【自】(电子计算机的)架构，体系结构stationary [ˈsteiʃ(ə)n(ə)ri] adj. 不动的，静止的，不变的；固定的，装定的optional [ˈɔpʃ(ə)nəl] adj. 可自由选择的；随意的，任意的；非强制的access [ˈækses] n. 进路，入口；捷径，门路；检查孔distribution [ˌdistriˈbju:ʃ(ə)n] n. 分布，配置，分布状态cell [sel] n. 蜂窝coordination [kəuˌɔ:(r)dinˈeiʃ(ə)n] n. 协作，协调；同等关系；调整；配合implementation [ˌimplimənˈteiʃ(ə)n] n. 执行，履行；贯彻，落实contention [kənˈtenʃ(ə)n] n. 斗争，竞争，争论priority [praiˈɔriti] n. 优先(权)，重点beacon [ˈbi:kən] n. 灯塔，信标；信号所，望楼hop [hɔp] v. 跳跃pseudorandom [ˌpsju:dəuˈrændəm] n. 【计】伪随机的direct [diˈrekt] adj. 直截了当的，直率的，明白的infrared [ˌinfrəˈred] adj. 【物】红外线的；红外区的；产生红外辐射的，对红外辐射敏感的n.

红外线；红外区presence [ˈprez(ə)ns] n. 在，存在，实在absence [ˈæbs(ə)ns] n. 不在，缺席，缺勤orthogonal [ɔ:ˈθɔg(ə)n(ə)l] adj. 直角的，互相垂直的；正交的；直交的diminish [diˈminiʃ] v. 减少，下降，减轻，缩小，缩短randomly [ˈrændəmli] adv. 随机地，随机backward [ˈbækwə(r)d] adj. 倒的；反的，相反的forward [ˈfɔ(r):wə(r)d] adj. 前方的；前面的correction [kəˈrekʃ(ə)n] n. 修正，改正；校正；矫正spontaneously [spɔnˈteiniəsli] adv. 自发地gadget [ˈgædʒit] n. 小机件，小配件；小装置piconet [ˈpikənet] n. 微型网peripheral [pəˈrif(ə)rəl] n. 外围设备chaos [ˈkeiɔs] n. 混乱attendee [əˌtenˈdi:] n. 参加者laptop [ˈlæpˌtɔp] n. 笔记本计算机，膝上型计算机，膝上型便携计算机Notes1.本节涉及无线通信领域的无线局域网部分。2.

fastest-growing组合词类型：动词最高级+动词现在分词(名词形式)=形容词。此句可译为：无线通信是发展最快的技术之一。3.

concentrate…on…，把……集中在……上。此句可译为：虽然这两种协议需要在几个层中运行，但这里集中讨论物理层和数据链路层。4.

ismadeof由……组成。knownastheaccesspoint(AP)作为限制性定语从句，采用就近原则进行翻译，用于限定anoptionalcentralbasestation。此句可译为：一个基本服务集由固定的或移动的无线站点和一个可选的、称为访问点(accesspoint，AP)的中央基站组成。5.

ontopof在……之上，在……的上面；逼近；胜任(工作等)；掌握(情况等)。time-sensitive时间相关的。此句可译为：点协调功能(pointcoordinationfunction，PCF)是一种可以在基础设施网络中(不在特定网络中)实现的可选访问方式。它在DCF上实现，主要用于时间敏感的传输。6.

atthesametime

同时，兼有，既……又……。插入语，较口语化。此句译为：这意味着如果在同一时间一个站点想只使用DCF，而一个想使用PCF，那么AP就获得优先权。7.

frequency-hoppingspreadspectrum(FHSS)，跳频扩频。directsequencespreadspectrum(DSSS)直接序列扩频。8.

contendwith作斗争，各个源在数据链路层访问时会互相竞争。9.

compatiblewith与……不矛盾，与……兼容。为了与DSSS向后兼容，HR-DSSS定义了4种数据速率：1，2，5.5，11(Mbps)。8.2WirelessWAN:CellularTelephony1Wirelesstechnologyisusedincellulartelephonyandsatellitenetworks.Wediscusstheformerinthissectionaswellasexamplesofchannelizationaccessmethods.Wealsobrieflydiscusssatellitenetworks,atechnologythateventuallywillbelinkedtocellulartelephonytoaccesstheInternetdirectly.Cellulartelephonyisdesignedtoprovidecommunicationsbetweentwomovingunits,calledmobilestations(MSs),orbetweenonemobileunitandonestationaryunit,oftencalledalandunit2.Aserviceprovidermustbeabletolocateandtrackacaller,assignachanneltothecall,andtransferthechannelfrombasestationtobasestationasthecallermovesoutofrange.Tomakethistrackingpossible,eachcellularserviceareaisdividedintosmallregionscalledcells3.EachcellcontainsanantennaandiscontrolledbyasolarorACpowerednetworkstation,calledthebasestation(BS).Eachbasestation,inturn,iscontrolledbyaswitchingoffice,calledamobileswitchingcenter(MSC).TheMSCcoordinatescommunicationbetweenallthebasestationsandthetelephonecentraloffice.Itisacomputerizedcenterthatisresponsibleforconnectingcalls,recordingcallinformation,andbilling.Cellsizeisnotfixedandcanbeincreasedordecreaseddependingonthepopulationofthearea.Thetypicalradiusofacellis1to12mi.High-densityareasrequiremore,geographicallysmallercellstomeettrafficdemandsthandolow-densityareas4.Oncedetermined,cellsizeisoptimizedtopreventtheinterferenceofadjacentcellsignals.Thetransmissionpowerofeachcelliskeptlowtopreventitssignalfrominterferingwiththoseofothercells.8.2.1Frequency-ReusePrincipleIngeneral,neighboringcellscannotusethesamesetoffrequenciesforcommunicationbecauseitmaycreateinterferencefortheuserslocatednearthecellboundaries.However,thesetoffrequenciesavailableislimited,andfrequenciesneedtobereused.AfrequencyreusepatternisaconfigurationofNcells,Nbeingthereusefactor5,inwhicheachcellusesauniquesetoffrequencies.Whenthepatternisrepeated,thefrequenciescanbereused.Thereareseveraldifferentpatterns.Thenumbersinthecellsdefinethepattern.Thecellswiththesamenumberinapatterncanusethesamesetoffrequencies.Wecallthesecellsthereusingcells.AsFigure8.1shows,inapatternwithreusefactor4,onlyonecellseparatesthecellsusingthesamesetoffrequencies.Inthepatternwithreusefactor7,twocellsseparatethereusingcells.Figure

8.1

FrequencyreusepatternsToplaceacallfromamobilestation,thecallerentersacodeof7or10digits(aphonenumber)andpressesthesendbutton.Themobilestationthenscanstheband,seekingasetupchannelwithastrongsignal,andsendsthedata(phonenumber)totheclosestbasestationusingthatchannel.ThebasestationrelaysthedatatotheMSC.TheMSCsendsthedatatothetelephonecentraloffice.Ifthecalledpartyisavailable,aconnectionismadeandtheresultisrelayedbacktotheMSC.Atthispoint,theMSCassignsanunusedvoicechanneltothecall,andaconnectionisestablished.Themobilestationautomaticallyadjustsitstuningtothenewchannel,andcommunicationcanbegin.Whenamobilephoneiscalled,thetelephonecentralofficesendsthenumbertotheMSC.TheMSCsearchesforthelocationofthemobilestationbysendingquerysignalstoeachcellinaprocesscalledpaging.Oncethemobilestationisfound,theMSCtransmitsaringingsignaland,whenthemobilestationanswers,assignsavoicechanneltothecall,allowingvoicecommunicationtobegin.Handoff:Itmayhappenthat,duringaconversation,themobilestationmovesfromonecelltoanother.Whenitdoes,thesignalmayeweak.Tosolvethisproblem,theMSCmonitorsthelevelofthesignaleveryfewseconds.Ifthestrengthofthesignaldiminishes,theMSCseeksanewcellthatcanbettermodatethecommunication.TheMSCthenchangesthechannelcarryingthecall(handsthesignalofffromtheoldchanneltoanewone).HardHandoff:Earlysystemsusedahardhandoff.Inahardhandoff,amobilestationonlycommunicateswithonebasestation.WhentheMSmovesfromonecelltoanother,communicationmustfirstbebrokenwiththepreviousbasestationbeforecommunicationcanbeestablishedwiththenewone.Thismaycreatearoughtransition.SoftHandoff:Newsystemsuseasofthandoff.Inthiscase,amobilestationcancommunicatewithtwobasestationsatthesametime.Thismeansthat,duringhandoff,amobilestationmaycontinuewiththenewbasestationbeforebreakingofffromtheoldone.Onefeatureofcellulartelephonyiscalledroaming.Roamingmeans,inprinciple,thatausercanhaveaccesstocommunicationorcanbereachedwherethereiscoverage.Aserviceproviderusuallyhaslimitedcoverage.Neighboringserviceproviderscanprovideextendedcoveragethrougharoamingcontract.Thesituationissimilartosnailmailbetweencountries.Thechargefordeliveryofaletterbetweentwocountriescanbedivideduponagreementbythetwocountries.8.2.2FirstGenerationCellulartelephonyisnowinitssecondgenerationwiththethirdonthehorizon.Thefirstgenerationwasdesignedforvoicecommunicationusinganalogsignals.Wediscussonefirst-generationmobilesystemusedinNorthAmerica,AMPS.Advancedmobilephonesystem(AMPS)isoneoftheleadinganalogcellularsystemsinNorthAmerica.ItusesFDMAtoseparatechannelsinalink.AMPSoperatesintheISM800MHzband.Thesystemusestwoseparateanalogchannels,oneforforward(basestationtomobilestation)communicationandoneforreverse(mobilestationtobasestation)communication.Thebandbetween824and849MHzcarriesreversecommunication;thebandbetween869and894MHzcarriesforwardcommunication.Eachbandisdividedinto832channels.However,twoproviderscanshareanarea,whichmeans416channelsineachcellforeachprovider.Outofthese416,21channelsareusedforcontrol,whichleaves395channels.AMPShasafrequencyreusefactorof7;thismeansonlyone-seventhofthese395trafficchannelsareactuallyavailableinacell.TransmissionAMPSusesFMandFSKformodulation.VoicechannelsaremodulatedusingFM,andcontrolchannelsuseFSKtocreate30kHzanalogsignals.AMPSusesFDMAtodivideeach25MHzbandinto30kHzchannels.8.2.3SecondGenerationToprovidehigher-quality(lessnoise-prone)6mobilevoicecommunications,thesecondgenerationofthecellularphonenetworkwasdeveloped.Whilethefirstgenerationwasdesignedforanalogvoicecommunication,thesecondgenerationwasmainlydesignedfordigitizedvoice.Threemajorsystemsevolvedinthesecondgeneration.Wewilldiscusseachsystemseparately.D-AMPS:TheproductoftheevolutionoftheanalogAMPSintoadigitalsystemisdigitalAMPS(D-AMPS).D-AMPSwasdesignedtobepatible7withAMPS.Thismeansthatinacell,onetelephonecanuseAMPSandanotherD-AMPS.D-AMPSwasfirstdefinedbyIS-54(InterimStandard54)andlaterrevisedbyIS-136.Band:D-AMPSusesthesamebandsandchannelsasAMPS.Transmission:EachvoicechannelisdigitizedusingaverycomplexPCMandcompressiontechnique.Avoicechannelisdigitizedto7.95Kbps.Three7.95KbpsdigitalvoicechannelsarecombinedusingTDMA.Theresultis48.6Kbpsofdigitaldata;muchofthisisoverhead.Thesystemsends25framespersecond,with1944bitsperframe.Eachframelasts40ms(1/25)andisdividedintosixslotssharedbythreedigitalchannels;eachchannelisallottedtwoslots.Eachslotholds324bits.However,only159bitscomesfromthedigitizedvoice;64bitsareforcontroland101bitsareforerrorcorrection.Inotherwords,eachchanneldrops159bitsofdataintoeachofthetwochannelsassignedtoit.Thesystemadds64controlbitsand101error-correctingbits.TheGlobalSystemforMobileCommunication(GSM)isaEuropeanstandardthatwasdevelopedtoprovideacommonsecond-generationtechnologyforallEurope.Theaimwastoreplaceanumberofpatiblefirst-generationtechnologies.

Bands:GSMusestwobandsforduplexcommunication.Eachbandis25MHzinwidth,shiftedtoward900MHz.Eachbandisdividedinto124channelsof200kHzseparatedbyguardbands.Transmission:Eachvoicechannelisdigitizedandcompressedtoa13Kbpsdigitalsignal.Eachslotcarries156.25bits.Eightslotsshareaframe(TDMA).Twenty-sixframesalsoshareamultiframe(TDMA).Reuse:Becauseofthecomplexerrorcorrectionmechanism,GSMallowsareusefactoraslowas3.Oneofthedominantsecond-generationstandardsinNorthAmericanisInterimStandard95(IS-95).ItisbasedonCDMAandDSSS.8.2.4ThirdGenerationThethirdgenerationofcellulartelephonyreferstoacombinationoftechnologiesthatprovideavarietyofservices.Ideally,whenitmatures,thethirdgenerationcanprovidebothdigitaldataandvoicecommunication.Usingasmallportabledevice,apersonshouldbeabletotalktoanyoneelseintheworldwithavoicequalitysimilartothatoftheexistingfixedtelephonenetwork.Apersoncandownloadandwatchamovie,candownloadandlistentomusic,cansurftheInternetorplaygames,canhaveavideoconference,andcandomuchmore.Oneoftheinterestingcharacteristicsofathird-generationsystemisthattheportabledeviceisalwaysconnected;youdonotneedtodialanumbertoconnecttotheInternet.Thethird-generationconceptstartedin1992,whenITUissuedablueprintcalledtheInternetMobileCommunication2000(IMT-2000).Theblueprintdefinessomecriteriaforthird-generationtechnologyasoutlinedbelow:Voicequalitycomparabletothatoftheexistingpublictelephonenetwork;Datarateof144Kbpsforaccessinamovingvehicle(car),384Kbpsforaccessastheuserwalks(pedestrians),and2Mbpsforthestationaryuser(officeorhome);Supportforpacket-switchedandcircuit-switcheddataservices;Abandof2GHz;Bandwidthsof2MHz;InterfacetotheInternet.Newwordsandphraseschannelization [ˌtʃænəlaiˈzeiʃən] n.通信信道的选择eventually [iˈventjuəli] v. 终于，最后cellular [ˈseljulə] adj. 蜂窝状的antenna [ænˈtenə] n. 天线population [ˌpɔpjuˈleiʃən] n. 人口；全体居民；特定人(动物)群solar [ˈsəulə] adj. 太阳的，日光的；太阳能的billing [ˈbiliŋ] n. 开具账单；营业额radius [ˈreidjəs] n. 半径(距离)interference [ˌintəˈfiərəns] n. 干涉；干预；介入，(无线电信号的)干扰adjacent [əˈdʒeisənt] adj. (地区、建筑、房间等)与……毗连的；邻近的boundary [ˈbaundəri] n. 分界线；界线，范围reused [ˈri:ˈju:zd] n. 可复的automatically [ˌɔ:təˈmætikəli] adv. 自动地；无意识地，不自觉地，机械地handoff [ˈhændɔf] n. 越区切换monitor [ˈmɔnitə] n. 监视器，监听器；检测器

modate [əˈkɔmədeit] vt. 容纳；提供空间rough [rʌf] adj. 粗糙的；不平的roaming n. 漫游coverage [ˈkʌvəridʒ] n. 提供的数量；覆盖范围(或方式)contract [kənˈtrækt] n. 契约，合同snail [sneil] n. 蜗牛generation [ˌdʒenəˈreiʃən] n. 代，一代，一辈horizon [həˈraɪzn] n. 地平线；范围，界限，眼界forward [ˈfɔ:wəd] adv. 向前[向末]端；前进reverse [riˈvə:(r)s] n. 相反evolution [ˌi:vəˈlu:ʃ(ə)n] n. 演变；进化；发展duplex [ˈdju:pleks] adj. 双工portable [ˈpɔ:təbl] adj. 手提式的；便于携带的；轻便的vehicle [ˈvi:ik(ə)l] n. 交通工具，车辆；传播媒介，手段Notes1.本节涉及无线广域网中的移动通信内容。2.

between…，orbetween…并列状语从句。此句译为：移动电话被设计用来在两个称为移动站点(MS)的移动单元之间，或者在一个移动单元和一个固定单元(通常称为地面信元)之间提供通信。3.

Tomakethistrackingpossible，用动词不定式表示目的，作为状语。isdividedinto，被分成。此句译为：为了使这种追踪变得可能，每个移动电话服务区域都被分割成小的地区，称为信元。4.

high-density，高密度。more,geographicallysmallercells，双重定语，修饰cells，译为：更多、地理位置更小的单元格。与低人口密度的地区相比，高人口密度的地区需要更多的地理范围和更小的信元来满足通信量的需求。5.

Nbeingthereusefactor，N是复用因子，插入语句，解释N。频率复用的模式是N个信元的配置，N是复用因子(reusefactor)，代表每个信元使用的唯一频率集。当这个模式重复时，频率即可复用。6.

higher-quality(lessnoise-prone)，高质量(低噪声倾向的)。7.patible后向兼容。此句译为：D-AMPS设计为向后兼容AMPS。例句：ComputersrunningtheclientcomponentsforpatibledocumentlibrariesmusthaveoneofthefollowingWebbrowsersinstalled.运行向后兼容文档库客户端组件的计算机必须安装下列Web浏览器。8.3SatelliteNetworks1Asatellitenetworkisacombinationofnodes,someofwhicharesatellites,thatprovidescommunicationfromonepointontheEarthtoanother.Anodeinthenetworkcanbeasatellite,anEarthstation,oranend-userterminalortelephone.Althoughanaturalsatellite,suchastheMoon,canbeusedasarelayingnodeinthenetwork,theuseofartificialsatellitesispreferredbecausewecaninstallelectronicequipmentonthesatellitetoregeneratethesignalthathaslostitsenergyduringtravel2.AnotherrestrictiononusingnaturalsatellitesistheirdistancesfromtheEarth,whichcreatealongdelayincommunication.Satellitenetworksarelikecellularnetworksinthattheydividetheplanetintocells.SatellitescanprovidetransmissioncapabilitytoandfromanylocationonEarth,nomatterhowremote.Thisadvantagemakeshigh-qualitycommunicationavailabletoundevelopedpartsoftheworldwithoutrequiringahugeinvestmentinground-basedinfrastructure.Anartificialsatelliteneedstohaveanorbit,thepathinwhichittravelsaroundtheEarth.Theorbitcanbeequatorial,inclined,orpolar,asshowninFigure8.2.Figure8.2SatellitecategoriesBasedonthelocationoftheorbit,satellitescanbedividedintothreecategories:geostationaryEarthorbit(GEO),low-Earth-orbit(LEO),andmiddle-Earth-orbit(MEO)3.Basedonthelocationoftheorbit,satellitescanbedividedintothreecategories:geostationaryEarthorbit(GEO),low-Earth-orbit(LEO),andmiddle-Earth-orbit(MEO)3.GEOSatellites:Line-of-sightpropagationrequiresthatthesendingandreceivingantennasbelockedontoeachother'slocationatalltimes(oneantennamusthavetheotherinsight)4.Forthisreason,asatellitethatmovesfasterorslowerthantheEarth'srotationisusefulonlyforshortperiods.Toensureconstantcommunication,thesatellitemustmoveatthesamespeedastheEarthsothatitseemstoremainfixedaboveacertainspot.Suchsatellitesarecalledgeostationary.Becauseorbitalspeedisbasedonthedistancefromtheplanet,onlyoneorbitcanbegeostationary.Thisorbitoccursattheequatorialplaneandisapproximately22

000mifromthesurfaceoftheEarth.ButonegeostationarysatellitecannotcoverthewholeEarth.Onesatelliteinorbithasline-of-sightcontactwithavastnumb