无线通信基础知识讨论Discussion on basic knowledge of wireless communication.docx
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无线通信基础知识讨论Discussion on basic knowledge of wireless communication.docx
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无线通信基础知识讨论Discussiononbasicknowledgeofwirelesscommunication
无线通信基础知识讨论(Discussiononbasicknowledgeofwirelesscommunication)
Thecoherenttimeisthemaximumtimedifferencerangeforthechanneltoremainconstant.Thecoherentbandwidthissimilartothemaximumfrequencydifferencerangewherethechannelremainsconstant.Tounderstandthisconceptfromtheangleofcomparativeimage:
diversitytimediversityrequirementstwolaunchtimeshouldbegreaterthanthechannelcoherencetime,i.e.ifthelaunchtimeislessthanthecoherencetimeofthechannel,thesignalisthetwolaunchwillexperiencethesamefadingdiversity,antifadingeffectdoesnotexist,thecoherencebandwidthcanbeunderstoodfromfrequencydiversity.
5.Whatarethedefinitionsoffrequencyselectivefading,flatfading,fastfadingandslowfading?
Frequencyselectivefadingandflatfadingaredefinedbyrelativecoherentbandwidth.Thelargerthesignalbandwidthis,themoreobviousthefrequencyselectivefadingis,andthemoresensitivetomultipath,themoreserioustheISIis.
Fastfadingandslowfadingaredefinedbyrelativecoherenttime.Thelargerthesymboltimeorcodetime,thefasterfading(butitcanbeverygoodagainstfrequencyselectivefading).
Thesetwotypesofdeclinearecontradictoryandshouldbeconsideredwhendesigningasystem.
6,wirelesscommunicationbooksareonthetop
7,multipathchannelmodelingrequiresthatmultipathshouldbeindependent.Howshouldindependencebereflectedinchannelmodeling?
ThisshouldalsobethecaseforMIMOchannelsandmultiuserchannels.
Thecorrelationcoefficientsofthetwopathsareassmallaspossible,ormodeledseparatelyaccordingtowhatissaidabove.
8,MIMOchannelmodeling,sometimesrequiredtoconsiderthecorrelationbetweenantennas,andhowthiscorrelationshouldbemodeled?
Firstly,thecorrelationcoefficientmatrixisconstructed,andthentheCholisdecomposed.Thematrixobtainedbythedecompositionismultipliedbythesignalofeachantenna.
Biezswaspublishedat20:
532006-10-25
[quote]or2006-10-2520:
42publishedby[i]lucatt0415[/i]in
(butit'sgoodagainstfrequencyselectivefading).
Thesetwotypesofdeclinearecontradictoryandshouldbeconsideredwhendesigningasystem.
[/quote]
Canyouexplainwhythesetwotypesofdeclinearecontradictory?
Biezswaspublishedat20:
562006-10-25
[quote]or2006-10-2520:
42publishedby[i]lucatt0415[/i]in
Thecorrelationcoefficientsofthetwopathsareassmallaspossible,ormodeledseparatelyaccordingtowhatissaidabove.[/quote]
Therelativecoefficientofthetwopathsassmallaspossibleisourgoal,butI'mnotsurewhattheso-called"individualmodeling"means.Forexample,whatdoesthissinglemodelingmeanforthetraditionalJakesmodel?
Lucatt0415waspublishedat23:
392006-10-25
Canyouexplainwhythesetwotypesofdeclinearecontradictory?
Thereasonsareasfollows:
thecoherencebandwidthrequirementsofsignalbandwidthlessthanthechannelagainstfrequencyselectivefading,whichrequiresbandwidthassmallaspossible,fromthetimeitissymbolictimeorchiptimeaslargeaspossible,butwiththeincreaseofsymbolorofthechipcycle,andwhenthechannelphasedowhenalongtime,itisdifficulttofightagainstfastfadingso,thedesignofthesystemshouldfullyconsidertheapplicationobjectsystem,reasonabledesignparameters.
Lucatt0415waspublishedat23:
442006-10-25
OrinaccordancewiththeMIMOmodelingmethodisbetter,thatis,theuseofcoefficientcorrelationmatrixforCholdecomposition,andthenusethedecomposedmatrixmultipliedbytwosignals,
Individualmodelingdoesnotnecessarilyguaranteethatthetwopathsareuncorrelated.
SWAswaspublishedin2006-10-2608:
46
Largescalefadingismainlypathloss,whichcanbeapproximatedbyfreespacepropagationmodel,butsomecharacteristicsaredifferent,suchasfadingindex,andsoon
Whatarethefactorsassociatedwiththelognormalvarianceofshadowfading?
Howmuchisthetypicalvalue?
OFDMsystemisusedtodiscusstheguidingroleofcoherenttimeandcoherentbandwidthinpracticalsystemdesign.
Inadditiontothefadingcharacteristicsofwirelesschannel,whichisdifferentfromwiredchannel,wemustconsider,suchasmultiusersharedchannel,multipleaccessmethodsarediscussed,thediscussiononthebasisoftheunderstandingofgreathelp.Agoodsubject.Ihopeyouwillhaveaheateddiscussion.
Lucatt0415waspublishedin2006-10-2608:
54
Thethreeeffectsofthewirelesschannelaremissing:
Threeeffectsofwirelesschannel
Shadoweffect,nearfareffectandDopplereffect
(1)shadoweffect:
whentheelectricwaveisdisturbedbyundulatingterrain,buildings,vegetationandotherobstaclesonthepropagationpath,thehalfblindzoneofelectromagneticfieldwillbeproduced
(2):
Nearfareffectduetorandommobilityreceivingusers,mobileusersandthedistancebetweenthebasestationandthenlowchange,ifthemobileuserstransmitpower,thenarriveatthebasestationsignalisstrongifdifferentfromthebasestationnearstrongsignal,farfromthebasestationsignalisweak.Inwhichthebasestationreceiverappearsstrongstrongerandtheweakweakerandstrongweakphenomenon.Thisisthereasonforpowercontrol
(3)Dopplereffect:
becauseofthehighspeedmotionofthemobileuserinthereceivingstate,thepropagationfrequencyisdiffused,andthediffusiondegreeisdirectlyproportionaltotheuser'smovingspeed.
Lcj2212916waspublishedat18:
232006-10-26
Whatarethecharacteristicsofwireless(mobile)channels?
Turnfrom:
[url]
Radiowavesindifferentfrequencybandshavedifferentpropagationmodesandcharacteristics.
ThemobilecommunicationsystemtoworkintheVHFandUHFbands,radiowavepropagationisthemainwayofspace,namelydirectwave,refractionwave,wavescatteringandtheirsynthesiswave.
Inthelandmobilesystem,themobilestationisinthecityorinthebuildingsinthecomplexterrainarea,theantennawillreceivesignalsfrommultiplepathscame,coupledwiththemobilestationitself,thewirelesschannelbetweenthemobilestationandthebasestationischangeableanddifficulttocontrol.
Inanalogwiredchannels,thetypicalsignal-to-noiseratioisabout46dB,thatis,thesignallevelis40000timeshigherthanthenoiselevel,andforthewiredchannel,thequalityoftransmissioniscontrollable.Byselectingtherightmaterialsandcarefulprocessing,youcanensurethatthereisarelativelystableelectricalenvironmentinthewiredtransmissionsystem.Asaresult,thesignal-to-noiseratiousuallydoesnotexceed1~2dBinawiredtransmissionline.Onlywhenalinehasbeenrunningforalongperiodoftime,willthelossofthelinegraduallyreach10~15dB,sointhiscase,thelinewillneedtobeupdated.
Interrestrialmobileradiochannels,asuddendropinsignalintensity,knownasfading,isfrequent,withadepthofabout30dB.
Inurbanenvironments,thenumberofsignalsreceivedbyamobilestationonafastmovingvehiclecandeclinesignificantlywithinasecond.Thiskindoffadingseriouslydeterioratesthequalityofreceivedsignalsandaffectsthereliabilityofcommunications.
Inthecellularmobileenvironment,thesamefrequencyinterferenceisaproblemthatmustbepaidattentionto.Whenfadingoccurs,thesignaltobereceivedmaybeweakerthantheinterferencesignalfromthesamefrequencycellbasestation,sothereceiverwilleasilylockonthewrongsignal.
TheanalogmobilecommunicationadoptsFMsignal,andthecaptureeffectofFMmodehasacertaininhibitoryeffectonthesamefrequencyinterference.ThefadingphenomenonwillsignificantlychangethecharacteristicsofFMsignalandweakenitscaptureeffect.Fordigitaltransmission,fadingwillgreatlyincreasebiterrorrate(BER).
Thefadingcharacteristicsofmobilechannelsdependonthepropagationenvironmentofradiowaves.Differentenvironments,theirpropagationcharacteristicsarenotthesame.
Alargecitywithmanytallbuildings,forexample,hasaverydifferentspreadenvironmentcomparedtoaflatopencountry,andthemobilechannelcharacteristicsvarygreatly.Thecommunicationenvironmentitselfisquitecomplex,whichmakesthemobilechannelcharacteristicsisverycomplex.
Itisspreadindifferentenvironment,leddirectlytothemultipathdelaysizehaveobviousdifference,ifwewanttotransmithigh-speeddata,Tsymbolperiodshouldbeassmallaspossible,butthesmallertheT,theabilitytoresistmultipathdelayisweak,resultinginaOFDM,itisbecausethesymbolperiodinOFDMsystembecauseoftheparalleltransmissionofvariablelength,whichcaneffectivelyresistthemultipathdelay.
Complexandharshpropagationconditionsarethecharacteristicsofmobilechannels.Inordertomaintainacceptabletransmissionqualityundersuchpropagationconditions,avarietyoftechnicalmeasuresmustbeadoptedtocounteracttheadverseeffectsoffading.Therefore,avarietyofantifadingtechnologieshavebeendeveloped,includingdiversity,spreadspectrum/frequencyhopping,equalization,interleavinganderrorcorrectioncoding.Inaddition,thesignaltransmissionmode,suchasmodulationmode,willalsohaveacertainadaptabilitytofadinginthechannel.
[[i]thispostwasfinallyeditedbysreightat19:
142006-10-26on[/i]]
Lcj2212916waspublishedat20:
292006-10-26
Providetworeferencematerials,thebasicknowledgewritteninmoredetail,theXi'anelectronicsdivisionofthebiglessonplans!
!
!
Wemainlydiscusslargescaleandsmallscalefadingandgeneralmodelingofchannel
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