2023年全國碩士研究生考試考研英語一試題真題（含答案詳解+作文范文）_第1頁
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1、CellularwkFromWikipediathefreeencyclopediaTopofacellularradiotowerAcellularwkisaradiowkdistributedoverlareascalledcellseachservedbyatleastonefixedlocationtransceiverknownasacellsitebasestation.Whenjoinedtogetherthesecell

2、sprovideradiocoverageoverawidegeographicarea.Thisenablesalargenumberofptabletransceivers(e.g.mobilephonespagersetc.)tocommunicatewitheachotherwithfixedtransceiverstelephonesanywhereinthewkviabasestationsevenifsomeofthetr

3、ansceiversaremovingthroughmethanonecellduringtransmission.Cellularwksofferanumberofadvantagesoveralternativesolutions:?increasedcapacity?reducedpoweruse?largercoveragearea?reducedinterferencefromothersignalsAnexampleofas

4、implenontelephonecellularsystemisanoldtaxidriversradiosystemwherethetaxicompanyhasseveraltransmittersbasedaroundacitythatcancommunicatedirectlywitheachtaxi.ContentsContents[hide]?1Theconcept?2Cellsignalencoding?3Frequenc

5、yreuse?4Directionalantennasthesignalfromtheothercellswhichusethesamefrequency.ThismeansthatinastardFDMAsystemtheremustbeatleastaonecellgapbetweencellswhichreusethesamefrequency.Inthesimplecaseofthetaxicompanyeachradiohad

6、amanuallyoperatedchannelknobtotunetodifferentfrequencies.Asthedriversmovedaroundtheywouldchangefromchanneltochannel.Thedriversknowwhichfrequencycoversapproximatelywhatarea.Whentheydonotreceiveasignalfromthetransmitterthe

7、ywilltryotherchannelsuntiltheyfindonethatwks.Thetaxidriversonlyspeakoneatatimewheninvitedbythebasestationoperat(inasenseTDMA).[edit]CellsignalencodingTodistinguishsignalsfromseveraldifferenttransmittersfrequencydivisionm

8、ultipleaccess(FDMA)codedivisionmultipleaccess(CDMA)weredeveloped.WithFDMAthetransmittingreceivingfrequenciesusedineachcellaredifferentfromthefrequenciesusedineachneighbouringcell.Inasimpletaxisystemthetaxidrivermanuallyt

9、unedtoafrequencyofachosencelltoobtainastrongsignaltoavoidinterferencefromsignalsfromothercells.TheprincipleofCDMAismecomplexbutachievesthesameresultthedistributedtransceiverscanonecelllistentoit.Otheravailablemethodsofmu

10、ltiplexingsuchaspolarizationdivisionmultipleaccess(PDMA)timedivisionmultipleaccess(TDMA)cannotbeusedtoseparatesignalsfromonecelltothenextsincetheeffectsofbothvarywithpositionthiswouldmakesignalseparationpracticallyimposs

11、ible.TimedivisionmultipleaccesshoweverisusedincombinationwitheitherFDMACDMAinanumberofsystemstogivemultiplechannelswithinthecoverageareaofasinglecell.[edit]FrequencyreuseThekeyacteristicofacellularwkistheabilitytoreusefr

12、equenciestoincreasebothcoveragecapacity.Asdescribedaboveadjacentcellsmustutilizedifferentfrequencieshoweverthereisnoproblemwithtwocellssufficientlyfarapartoperatingonthesamefrequency.Theelementsthatdeterminefrequencyreus

13、earethereusedistancethereusefact.ThereusedistanceDiscalculatedaswhereRisthecellradiusNisthenumberofcellspercluster.Cellsmayvaryinradiusintheranges(1kmto30km).Theboundariesofthecellscanalsooverlapbetweenadjacentcellslarge

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