下载

1下载券

加入VIP
  • 专属下载券
  • 上传内容扩展
  • 资料优先审核
  • 免费资料无限下载

上传资料

关闭

关闭

关闭

封号提示

内容

首页 Solutions_2nd_ed

Solutions_2nd_ed.pdf

Solutions_2nd_ed

tjwyl2000
2011-06-21 0人阅读 举报 0 0 0 暂无简介

简介:本文档为《Solutions_2nd_edpdf》,可适用于高等教育领域

SolutionManualforMobileCommunicationsndedJochenHSchiller,FreieUniversitätBerlin,Germanyschillercomputerorg,wwwjochenschillerdeIntroductionGoodsourcesforsubscribernumbersandotherstatisticsare,eg,wwwgsmworldcom,wwwgpporg,wwwgpporg,wwwemcdatabasecom,wwwgcouk,wwwregtpde…Today’sGSMoperatorsaddthenewGairinterfacesofUMTStotheirexistingGSMGPRSinfrastructurenetworksCurrentGSMGPRSnetworksalreadyofferpacketandcircuitswitcheddatatransmissionfollowingtheReleaseofUMTSTheoperatorshavetoinstallnewradioaccessnetworks,ie,antennas,radionetworkcontrolleretcasdescribedinchapterThesituationissimilarforoperatorsusingcdmaOne(IS)technologyHowever,theseoperatorsgoforcdmaasthissystemallowsthemtoreusetheiralreadyexistinginfrastructureThus,basedontheseparationofthemobilephonesystemsinto(veryroughly)CDMAandGSMoperatorswillleadtotwodifferentmajorGsystems,cdmaandUMTS(andtheirfuturereleases)Rightnow,itdoesnotseemthatthereisaplaceforathirdGsystemCurrentTDMAoperatorsmightmovetoEDGEenhancedsystemsorjointheUMTSsystemHowever,itisstillopenwhatwillhappeninChina–theChinesesystemTDSCDMAwaspushedbythegovernment,butnetworksanddevicesarestillmissingCurrently,themajorityofChinesesubscribersuseGSM,someoperatorsofferCDMAWirelessTransmissionCheckalsotheWRCsthattrytoharmonizeglobalfrequencyplansBelowMHzradiowavesfollowtheground(groundwavepropagation)Onefactorforthisisdiffraction(wavesareboundtowardsobstaclesthathavesizesintheorderofthewavelength),anotherfactoristhecurrentinducedintheEarth’ssurface,whichslowsthewavefrontneartheearth,causingthewavefronttotiltdownwardSeveralreasonsmakelowfrequenciesunusableincomputernetworks:•LowerfrequenciesalsomeanlowerdataratesaccordingtoNyquistShannonastheavailablebandwidthisless•LowerfrequenciesalsorequirelargeantennasforefficienttransmissionandreceptionThismightworkforsubmarines,notformobilephones•LowerfrequenciespenetratematerialmoreeasilyThusSDMismoredifficult–cellsizewouldincreasedramaticallyandfrequencyreusewouldbealmostimpossibleFrequenciesintheTHzrange,eg,infrared,visiblelight,areeasilyblockedbyobstaclesand,thus,donotinterferewithothertransmissionsInthiscase,onlythestandardsafetyregulationsapply(eg,laseremission)MostradiosystemsstaywellbeyondGHzasitisnotthatsimpletogeneratehigherfrequencies(inthelowerTHzrange)TheclassicalEuropeanapproachwasbasedonstandardisationandregulationbeforeanyproductswereavailableTheEUgovernmentsfoundedETSItoharmonizeallnationalregulationsETSIcreatedthestandards,allcountrieshadtofollowIntheUScompaniesdevelopsystemsandtrytostandardizethemorthemarketforcesdecideuponsuccessTheFCC,eg,onlyregulatesthefairnessamongdifferentsystemsbutdoesnotstipulateacertainsystemTheeffectsofthetwodifferentapproachesaredifferentMany“governmental”standardsinEuropefailedcompletely,eg,HIPERLAN,somesucceededonlyinEurope,eg,ISDN,andhowever,somesoonbecameaworldwidesuccessstory,eg,GSMFormostsystemstheUSapproachworkedbetter,firstsomeinitialproducts,thenstandardsOnegoodexampleisthewirelessLANfamily,agoodcounterexampleisthemobilephonemarket:severaldifferent,incompatiblesystemstrytosucceed,manyfeatures,wellestablishedinEuropesincemanyyears,arenotevenknownintheUS(freeroaming,MMS,GPRSroaming,nochargesforbeingcalledetc)Computers,incontrastto,eg,TVsets,travelaroundtheworldaslaptops,PDAsetcCustomerswanttousethemeverywhereThusitisveryimportanttobeabletousebuiltinWLANadaptersaroundtheglobewithoutreconfigurationandwithoutlicensingFurthermore,itismuchcheaperforWLANmanufacturerstodesignasinglecommonsystemcomparedtomanydifferentsystemsforprobablysmallmarketsNoLossfreetransmissionofanaloguesignalsisnotpossibleAttenuation,dispersionetcwillalwaysdistortthesignalAdditionally,eachdigitalsignalistransmittedas“bundle”ofanaloguesinewaves(thinkofFourier!)Aperfectdigitalsignalwithrectangularshaperequiresaninfinitenumberofsinewavestobepreciselyrepresented(thedigitalsignalcanberepresentedasinfinitesumofsinewavesaccordingtoFourier)However,nomediumcantransmitinfinitehighfrequenciesThus,thedigitalsignalcanneverbetransmittedwithoutanylossWithoutanyadditional“intelligence”directionalantennasarenotusefulinstandardmobilephonesasusersmaynotwanttodirectthephonetoacertainantennaUsersmove,rotate,flipthephonesetcPhonesareinbags,pockets,…whileoperatedhandsfreeThereisnochanceofdirectedtransmissionHowever,newdevelopmentscomprisingfastsignalprocessorsandmultipleantennasmayexploitdirectedcharacteristicsofantennas(beamforming)Thereareseveralwaysofimprovingthegainofanantenna:rightdimensioning(eg,halfthewavelength),multipleantennasplusasignalprocessorcombiningthesignals,activeandpassivecomponentsattachedtotheantenna(comparewithtraditionalTVantennas,satellitedishesetc)Problems:attenuation,scattering,diffraction,reflection,refractionExceptforattenuationallothereffectscandivertthewavesfromastraightlineOnlyinvacuumandwithoutgravitationaleffectsradiowavesfollowastraightlineWithoutreflectionradioreceptionintownswouldbealmostimpossibleAlineofsightalmostneverexistsHowever,reflectionisthemainreasonformultipathpropagationcausingISIISImitigation:largeenoughguardspacesbetweensymbolslowsymbolrate(usedinOFDM:distributethesymbolstreamonmanydifferentcarriers),channelestimationcalculatethenstrongestpathsandadaptthereceiveraccordinglyUsinghigherfrequenciesreducestheeffectsofmultipathpropagationandthusISI(wavesmoreandmorebehavelikelight)ThehigherthesymbolratethestrongertheISIIfsendersandorreceiversmovefastthechancesforISIarehigherbecausethelocationofobstacleschanges,hencethenumber,magnitude,andtimingofthesecondarypulses–itisdifficulttofollowthesignalsandadjustthedelaysforrecombinationISIlowersthebandwidthofaTDMschemeastheguardspacesrequiresometimeSeveralmechanismsexisttomitigatenarrowbandinterference(whichmightbecausedbyothersenders,too):•DynamicFrequencySelection:SenderscansensethemediumforinterferenceandchooseafrequencyrangewithlowernointerferenceHiperLANandhusethisschemeNetworkoperatorscanalsothisschemetodynamicallyassignfrequenciestocellsinmobilephonesystemsDFShasarelativelylowcomplexity•Frequencyhopping:Slowfrequencyhopping(severalsymbolsperfrequency)mayavoidfrequencieswithinterferencemostofthetimewithacertainprobabilityThisschememaybeusedinGSMFurthermore,wirelesssystemscanusethisprincipleformultiplexingasitisdoneinBluetoothsystems(stillslowhoppingasBluetoothsendsmanysymbols,indeedawholepacket,onthesamefrequency)Fasthoppingschemestransmitasymboloverseveralfrequencies,thuscreatingaspreadspectrumFHsystemshavemediumcomplexityMaintopicissynchronisationofthedevices•Directsequencespreadspectrum:DataisXORedwithachippingsequenceresultinginaspreadsignalThisisdoneinallCDMAsystems,butalsoinWLANsusing,eg,Barkersequencesforspreading(eg,b)Thesignalisspreadoveralargespectrumand,thus,narrowbandinterferenceonlydestroysasmallfractionofthesignalThisschemeisverypowerful,butrequiresmorepowerfulreceiverstoextracttheoriginalsignalfromthemixtureofspreadsignalsWorldwideregulationalwaysusesFDMforseparatingdifferentsystems(TV,WLAN,radio,satellite,…)Thus,allradiosystemsmustmodulatethedigitalsignalontoacarrierfrequencyusinganaloguemodulationThemostprominentsystemisthetraditionalradio:allmusicandvoiceusefrequenciesbetween,eg,HzandkHzHowever,manydifferentradiostationswanttotransmitatthesametimeTherefore,alltheoriginalsignals(whichusethesamefrequencyrange)mustbemodulatedontodifferentcarrierfrequenciesOthermotivationsfordigitalmodulationareantennaandmediumcharacteristicsImportantcharacteristicsfordigitalmodulationarespectralefficiency,powerefficiencyandrobustnessTypicalschemesareASK,PSK,FSKThereceivermay“check”thedistancebetweenthereceivedpointandtheneighbouringpointsThereceiverthenchoosestheclosestneighbourandassumesthatthesenderoriginallytransmitteddatarepresentedbythechosenpointThemorepointsaPSKschemeusesthehigherarechancesthatinterference(noise)shiftsatransmitted“point”ontoanotherIfthegapsbetweenthepointsaretoosmall,inparticularsmallerthannoiseaddedduringtransmission,chancesareveryhighthatthereceiverwillmapreceiveddataontothewrongpointintheconstellationdiagram(pleasenote:dataiscodedusingPSK,thepointsintheconstellationdiagramrepresentcodes,thesecodesarethentransmitted–itisjustsimplertothinkin“points”…)Mainbenefits:veryrobustagainstinterference,inherentsecurity(ifthespreadingcodeisunknownitisverydifficulttotapthetransmission),basisforCDMAtechnologies,canbeusedinthe“background”ofexistingsystemsifthesignallevelislowenoughSpreadingcanbeachievedbyXORingabitwithachippingsequenceorfrequencyhoppingGuardspacesarenowtheorthogonalityofthechippingsequencesorhoppingpatternsThehighertheorthogonality(well,thatisnotverymathematical,butintuitive),thelowerthecorrelationofspreadsignalsorthelowerthecollisionprobabilityoffrequencyhoppingsystemsDSSSsystemtypicallyuserakereceiversthatrecombinesignalstravellingalongdifferentpathsRecombinationresultsinastrongersignalcomparedtothestrongestsignalonlyThemainreasonisthesupportofmoreusersCellularsystemsreusespectrumaccordingtocertainpatternsEachcellcansupportamaximumnumberofusersUsingmorecellsthusresultsinahighernumberofusersperkm²Additionally,usingcellsmaysupportuserlocalisationandlocationbasedservicesSmallercellsalsoallowforlesstransmissionpower(thuslessradiation!),longerruntimeformobilesystems,lessdelaybetweensenderandreceiverWell,thedownsideisthetremendousamountofmoneyneededtosetupaninfrastructurewithmanycellsTypically,eachcellholdsacertainnumberoffrequencybandsNeighbouringcellsarenotallowedtousethesamefrequenciesAccordingtocertainpatterns(clusteretc)cellularsystemsreusefrequenciesIfthesystemdynamicallyallocatesfrequenciesdependingonthecurrentload,itcanreactuponsuddenincreaseintrafficbyborrowingcapacityfromothercellsHowever,the“borrowed”frequencymustthenbeblockedinneighbouringcellsTDMFDMsystemshaveahardupperlimitofsimultaneoususersThesystemassignsacertaintimeslotatacertainfrequencytoauserIfalltimeslotsatallfrequenciesareoccupiednomoreuserscanbeacceptedComparedtothis“hardcapacity”aCDMsystemhasasocalled“softcapacity”(comparefillingaboxwithbricksortissues)ForCDMsystemsthesignaltonoiseratiotypicallylimitsthenumberofsimultaneoususersThesystemcanalwaysacceptanadditionaluserHowever,thenoiselevelmaythenincreaseaboveacertainthresholdwheretransmissionisimpossibleInTDMFDMsystemsadditionalusers,ifaccepted,donotinfluenceotherusersasusersareseparatedintimeandfrequency(well,thereissomeinterferencehowever,thiscanbeneglectedinthiscontext)InCDMsystemseachadditionaluserdecreasestransmissionqualityofallotherusers(thespaceforthetissuesintheboxgetstighter)MediumAccessControlStationsinawirednetwork“hear”eachotherIe,thelengthofwiresislimitedinawaythatattenuationisnotstrongenoughtocancelthesignalThus,ifonestationtransmitsasignalallotherstationsconnectedtothewirereceivethesignalThebestexampleforthisistheclassicalEthernet,Base,whichhasabustopologyandusesCSMACDasaccessschemeToday’swirednetworksarestarshapedinthelocalareaandmanydirectconnectionsformingameshinwideareanetworksInwirelessnetworks,itisquiteoftenthecasethatstationsareabletocommunicatewithacentralstationbutnotwitheachotherThisleadintheearlyseventiestotheAlohaaccessscheme(UniversityofHawaii)SowhatisCS(CarrierSense)goodforinwirelessnetworksTheproblemisthatcollisionsofdatapacketscauseproblemsatthereceiver–butcarriersensingtakesplaceatthesenderInwirednetworksthisdoesn’treallymatterassignalstrengthisalmostthesame(ok,withincertainlimits)allalongthewireInwirelessnetworksCSandCDatthesenderdoesn’tmakesense,senderswillquiteoftennothearotherstations’signalsorthecollisionsatthereceiverIncaseofAlohastationsdonotcareaboutotherstationsbutsimplyaccessthemediumiftheyhavetosenddataTherearenostationsexposedasstationsdonotperformcarriersensingHiddenstationsmaycausecollisionsThesameistrueforslottedAlohatheonlydifferencebeingtheslottedcharacterofmediumaccessReservationschemestypicallyworkwithacentralreservationstationwhichcanbeheardbyallothersWithoutthisconditionorequivalentmeansofdistributingreservationsthewholeschemewillnotworkThus,therearenohiddenorexposedterminalsMACAisdesignedtohandlehiddenandexposedterminalsinadistributedWLANwithoutcentralreservationstationHowever,MACAmayfailincaseofasymmetriccommunicationconditionsorhighlydynamictopologies(stationsmaymovefastintocollisionrange)AslongasastationcanreceiveasignalandthesignalarrivesattherighttimetohittherighttimeslotitdoesnotmatterinTDMAsystemsifterminalsarefarornearInTDMAsystemsterminalsmeasurethesignalstrengthandthedistancebetweensenderandreceiverTheterminalsthenadapttransmissionpowerandsendsignalsinadvancedependingonthedistancetothereceiverTerminalsinCDMAsystemshavetoadapttheirtransmissionpowerveryoften(eg,timespersecondinUMTS)sothatallsignalsreceived,eg,atabasestation,havealmostthesamestrengthWithoutthisonesignalcoulddrownothersasthesignalsarenotseparatedintimeTypically,SDMAisperformedorsupportedbyanetworkproviderTheproviderplansthenetwork,ie,placesthebasestationsaccordingtocertaintopologies,geographicsituations,capacityplanningetcIfthesystemisrunning,basestationssupporttheinfrastructureinthedecisionofassigningacertainbasestationtoaterminalThisisoftenbasedonreceivedsignalstrengthorthecurrentcapacityThemobileterminalsupportstheinfrastructurebytransmittinginformationaboutthereceivedsignalstrengthsTheterminalcanfurthermoreinitiatethechangeoftheaccesspointModulation–TransmittersmustshiftallbasebandsignalstoacarrierfrequencyThisistypicallyananalogueprocessandrequiresanaloguecomponentsClassicalreceiversalsoneedfiltersforreceivingsignalsatcertainfrequenciesDependingonthecarrierfrequencydifferentantennasmaybeneededPureTDMAsystemsstayononefrequency,allreceiverscanwaitonthesamefrequencyfordataInFDMAsystemsreceivershavetoscandifferentcarrierfrequenciesbeforetheycanreceivesignalsMACisperformedonmanydifferentlayersTheWRCs(WorldRadioConferences)areusedforworldwidefrequencyassignmentssuchastheGHzrangeforIMTITUcontrolsworldwidefrequencyusageNationalauthoritiesregulatefrequenciesindifferentnationsOnthenextlowerlayersnetworkoperatorsperformMAC:frequenciesusageiscontrolledbynetworkplanningandcurrentloadFinally,basestationsinmobilephonesystemsassignfrequenciestoterminalsdependingonthecurrentavailabilityInWLANsnetworkadministrationassignsfrequenciesthusformingcellsWirelessnetworkscanusedifferentfrequencies,differenttimeslotsorevendifferentcodestoimplementduplexchannelsTypicalwirednetworkssimplyusedifferentwires(however,moreelaboratedschemessuchasechocancellationarefeasible,too)IfcommunicationsystemsusefixedTDMpatternsterminalscanbeverysimpleTheonlyrequirementistostaysynchronisedtobeabletoreceivetherightdataThisisthestandardsysteminclassicaltelecommunicationnetworks(eg,ISDN,PCMsystems,SDHetc)Ethernets,theInternet,wirelessLANsetcworkdemanddrivenHeretheadvantageisthelowoverheadwhenstartingcommunication:terminalsdon’thavetosetupconnectionsreservingtimeslotspriortocommunicationHowever,userstransmitmoreandmoredatacomparedtovoiceMostnetworksoftodayaredatadominated(iftheamountofdataisconsidered,nottherevenue)Thus,datatransmissionshouldbeoptimisedWhileWLANsareoptimisedfordatafromthebeginning(andisochronousaudiotransmissioncausessomeproblems),wideareamobilephonesystemsstartedasalmostvoiceonlysystemsThestandardschemeiscircuitswitched,notpacketswitchedAsmoreanmoredataistransmittedthesenetworkshavetointegratemoreandmoredataorientedtechnologies:GPRSinGSM,IPinthecorenetworkofUMTSetcInterferenceandcountermeasuresin:•SDMA:InterferencehappensifsendersaretooclosetoeachotherTerminalsorbasestationshavetokeepaminimumdistance•TDMA:InterferencehappensifsenderstransmitdataatthesametimeCountermeasuresaretightsynchronisationandguardspaces(timegapbetweentransmissions)•FDMA:InterferencehappensifsenderstransmitdataatthesamefrequencyThus,differentfrequencieshavetobeassignedtosendersbyorganisations,algorithmsinbasestations,commonfrequencyhoppingschemesetcFurthermore,guardbandsbetweenusedfrequencybandstrytoavoidinterference•CDMA:Interferencehappensifsenderstransmitdatausingnonorthogonalcodes,ie,thecorrelationisnotzeroThus,sendersshoulduseorthogonalorquasiorthogonalcodesEveninvacuumradiowaveshavelimitedvelocity:thespeedoflightAssoonasmatterisinthewaywavestravelevenslowerThus,itcanhappenthatasendersensesthemediumidle,startsthetransmissionandjustinamomentbeforethewavesreachanothersenderthissecondsendersensesthemediumidleand

用户评价(0)

关闭

新课改视野下建构高中语文教学实验成果报告(32KB)

抱歉,积分不足下载失败,请稍后再试!

提示

试读已结束,如需要继续阅读或者下载,敬请购买!

评分:

/43

VIP

在线
客服

免费
邮箱

爱问共享资料服务号

扫描关注领取更多福利