电子信息科学专业英语Special-English-Unit11

Unit11MobileCommunications11-1TheMobileTelephoneSystemWirelesstelephonescomeintwobasicvarieties:cordlessphonesandmobilephones(sometimescalledcellphones).Cordlessphonesaredevicesconsistingofabasestationandhandsetsoldasasetforusewithinthehome.Theseareneverusedfornetworking.无绳电话手机移动电话电话听筒11-1TheMobileTelephoneSystemMobilephoneshavegonethroughthreedistinctgenerations,withdifferenttechnologies:AnalogvoiceDigitalvoiceDigitalvoiceanddata(Internet,e-mail,etc.)(1G)(2G)(3G)11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceThissystemusedasinglelargetransmitterontopofatallbuildingandhadasinglechannel,usedforbothsendingandreceiving.Totalk,theuserhadtopushabuttonthatenabledthetransmitteranddisabledthereceiver.Suchsystems,knownaspush-to-talksystems,wereinstalledinseveralcitiesbeginninginthelate1950s.使……工作使……不工作11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceInthe1960s,IMTS(ImprovedMobileTelephoneSystem)wasinstalled.Itusedtwofrequencies,oneforsendingandoneforreceiving,sothepush-to-talkbuttonwasnolongerneeded.…IMTSsupported23channelsspreadoutfrom150MHzto450MHz.Duetothesmallnumberofchannels,usersoftenhadtowaitalongtimebeforegettingadialtone.Allinall,thelimitedcapacitymadethesystemimpractical.不实用11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceAdvancedMobilePhoneSystem(AMPS)AllthatchangedwithAMPS(AdvancedMobilePhoneSystem),inventedbyBellLabsandfirstinstalledintheUnitedStatesin1982.11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceAdvancedMobilePhoneSystem(AMPS)Inallmobilephonesystems,ageographicregionisdividedupintocells,whichwhythedevicesaresometimescalledcellphones.InAMPS,thecellsaretypically10to20kmacross;indigitalsystems,thecellsaresmaller.地理的Whythemobilephonesarealsocalledcellphones?11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceAdvancedMobilePhoneSystem(AMPS)Eachcellusessomesetoffrequenciesnotusedbyanyofitsneighbors.Thekeyideathatgivescellularsystemsfarmorecapacitythanprevioussystemsistheuseofrelativelysmallcellsandthereuseoftransmissionfrequenciesinnearby(butnotadjacent)cells.相邻的蜂窝系统11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceAdvancedMobilePhoneSystem(AMPS)WhereasanIMTSsystem100kmacrosscanhaveonecalloneachfrequency,anAMPSsystemmighthave10010-kmcellsinthesameareaandbeabletohave10-15callsoneachfrequency,inwidelyseparatedcells.Thus,thecellulardesignincreasesthesystemcapacitybyatleastanorderofmagnitude,moreasthecellsgetsmaller.数量级Whatistheadvantageofcellularsystems?11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceAdvancedMobilePhoneSystem(AMPS)TheideaoffrequencyreuseisillustratedinFigure1(a).Thecellsarenormallyroughlycircular,buttheyareeasiertomodelhashexagons.InFigure1(a),thecellsareallthesamesize.六边形Howdoesacellularsystemwork?11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceAdvancedMobilePhoneSystem(AMPS)Theyaregroupedinunitsofsevencells.Eachletterindicatesagroupoffrequencies.Noticesthatforeachfrequencyset,thereisabufferabouttwocellswidewherethatfrequencyisnotreused,providingforgoodseparationandlowinterference.缓冲区Howdoesacellularsystemavoidfrequencyconflicts?11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceAdvancedMobilePhoneSystem(AMPS)Inanareawherethenumberofusershasgrowntothepointthatthesystemisoverloaded,thepowerisreused,andtheoverloadedcellsaresplitintosmallermicrocellstopermitmorefrequencyreuse,asshowninFigure1(b).超载的11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceAdvancedMobilePhoneSystem(AMPS)Telephonecompaniessometimescreatetemporarymicrocells,usingportabletowerswithsatellitelinksatsportingevents,rockconcerts,andotherplaceswherelargenumbersofmobileuserscongregateforafewhours.临时的卫星链路聚集11-1TheMobileTelephoneSystem1.First-GenerationMobilePhones:AnalogVoiceAdvancedMobilePhoneSystem(AMPS)Atthecenterofeachcellisabasestationtowhichallthetelephonesinthecelltransmit.Thebasestationconsistsofacomputerandtransmitter/receiverconnectedtoanantenna.天线11-1TheMobileTelephoneSystem2.Second-GenerationMobilePhones:DigitalVoiceThefirstgenerationofmobilephoneswasanalog,thesecondgenerationwasdigital.Foursystemsareinusein2G:D-AMPS,GSM,CDMA,andPDC.PDCisusedonlyinJapanandisbasicallyD-AMPSmodifiedforbackwardcompatibilitywiththefirst-generationJapaneseanalogsystem.向下兼容11-1TheMobileTelephoneSystem2.Second-GenerationMobilePhones:DigitalVoice(2)GSM—TheGlobalSystemforMobileCommunicationIntheGSMsystem,frequencydivisionmultiplexingisused,witheachmobiletransmittingononefrequencyandreceivingonahigherfrequency.Asinglefrequencypairissplitbytime-divisionmultiplexingintotimeslotssharedbymultiplemobiles.FDM频分复用TDM时分复用11-1TheMobileTelephoneSystem2.Second-GenerationMobilePhones:DigitalVoice(3)CDMA—CodeDivisionMultipleAccessCDMAiscompletelydifferentfromAMPS,D-AMPS,andGSM.Insteadofdividingtheallowedfrequencyrankintoafewhundrednarrowchannels,CDMAallowseachstationtotransmitovertheentirefrequencyspectrumallthetime.分割11-1TheMobileTelephoneSyste