无线通信 Lecture 4 Cellular Cont.

1Lecture4:CellularFundamentalsChapter3-Continued2I.AdjacentChannelInterferenceTwomajortypesofsystem-generatedinterference:1)Co-ChannelInterference(CCI)–discussedinlastlecture2)AdjacentChannelInterference(ACI)AdjacentChannelInterference(ACI)ImperfectRxfiltersallowenergyfromadjacentchannelstoleakintothepassbandofotherchannels3actualfilterresponsedesiredfilterresponse4Thisaffectsbothforward&reverselinksForwardLink→base-to-mobileinterference@mobileRxfroma______Tx(anothermobileoranotherbasestationthatisnottheonethemobileislisteningto)whenmobileRxis___awayfrombasestation.signalfrombasestationisweakandothersaresomewhatstrong.ReverseLink→mobile-to-baseinterference@basestationRxfromnearbymobileTxwhendesiredmobileTxisfarawayfrombasestation5Near/FarEffectinterferingsourceisnearsomeRxwhendesiredsourceisfarawayACIisprimarilyfrommobilesinthesamecellsomecell-to-cellACIdoesoccuraswell→butasecondarysourceControlofACIdon’tallocatechannelswithinagivencellfromacontiguousbandoffrequenciesforexample,usechannels1,4,7,and10foracell.nochannelsnexttoeachother6maximizechannelseparationseparationofasmanyasNchannelbandwidthssomeschemesalsoseektominimizeACIfromneighboringcellsbynotassigningadjacentchannelsinneighboringcells78Originally666channels,then10MHzofspectrumwasadded666+166=832channels395VCplus21CCperserviceprovider(providersA&B)395*2=790,plus42controlchannelsProviderAisacompanythathasnottraditionallyprovidedtelephoneserviceProviderBisatraditionalwirelineoperator21VCgroupswith≈19channels/groupatleast21channelseparationforeachgroup9forN=7→3VCgroups/cellForexample,choosegroups1A,1B,and1Cforacell–sochannels1,8,15,22,29,36,etc.areused.∴≈57channels/cellatleast7channelseparationforeachcellgrouptohavehighqualityoncontrolchannels,21cellreuseisusedforCC’sinsteadofreusingaCCevery7cells,asforVC’s,reuseevery21cells(aftereverythreeclusters)greaterdistancebetweencontrolchannels,solessCCI10usehighqualityfiltersinbasestationsbetterfiltersarepossibleinbasestationssincetheyarenotconstrainedbyphysicalsizeandpowerasmuchasinthemobileRxmakesreverselinkACIlessofaconcernthanforwardlinkACIalsotruebecauseofpowercontrol(discussedbelow)choiceofmodulationschemesdifferentmodulationschemesprovidelessormoreenergyoutsidetheirpassband.11PowerControltechniquetominimizeACIbasestation&MSCconstantlymonitormobilereceivedsignalstrengthmobileTxpowervaried(controlled)sothatsmallestTxpowernecessaryforaqualityreverselinksignalisused(lowerpowerforthecloserthemobileistothebasestation)alsohelpsbatterylifeonmobile12dramaticallyimprovesadjacentchannelS/Iratio,sincemobilesinothercellsonlytransmitathighenoughpowerastransmittercontrols(notatfullpower)mostbeneficialforACIonreverselinkwillseelaterthatthisisespeciallyimportantforCDMAsystems13III.Trunking&GradeofService(GOS)Trunkedradiosystem:radiosystemwherealarge#ofusersshareapoolofchannelschannelallocatedondemand&returnedtochannelpooluponcallterminationexploitstatistical(random)behaviorofuserssothatfixed#ofchannelscanaccommodatelarge#ofusersTrade-offbetweenthenumberofavailablechannelsthatareprovidedandthelikelihoodofaparticularuserfindingnochannelsavailableduringthebusyhouroftheday.14trunkingtheoryisusedbytelephonecompaniestoallocatelimited#ofvoicecircuitsforlarge#oftelephonelinesefficientuseofequipmentresources→savingsdisadvantageisthatsomeprobabilityexiststhatmobileuserwillbedeniedaccesstoachannelblockedcall:accessdenied→“blockedcallcleared”delayedcall:accessdelayedbycallbeingputintoholdingqueueforspecifiedamountoftime15GOS:measureoftheabilityofuseraccesstoatrunkedsystemduringthe_______hourspecifiedasprobability(Pr)thatcallisblockedordelayeddesignedtohandlethebusiesthour→typically______Erlang:unitlessmeasureoftrafficintensitye.g.0.5erlangs=1channeloccupied30minutesduring1hourTable3.3,pg.78→trunkingtheorydefinitions16“Offered”TrafficIntensity(A)Offered?→notnecessarilycarriedbysystem(someisblockedordelayed)eachuserAu=λHErlangs(alsocalledρinqueueingtheory)λ=trafficintensity(averagearrivalrateofnewcalls,innewrequestspertimeunit,saycalls/min).H=averagedurationofacall(alsocalled1/µinqueueingtheory)systemwithUusers→A=UAu=UλHErlangscapacity=maximumcarriedtraffic=CErlangs=(equaltototal#ofavailablechannelsthatarebusyallthetime)17ErlangBformulaCallsareeitheradmittedorblockedA=totalofferedtrafficC=#channelsintrunkingpool(e.g.acell)AMPSdesignedforGOSof2%blockedcallcleared(denied)→BCC18capacitiestosupportvariousGOSvaluesNotethattwicethecapacitycansupportmuchmorethantwicetheload(twicethenumberofErlangs).19ErlangCformulasblockedcalldelayed→BCD→putintoholdingqueueGOSisprobabilitythatacallwillstillbeblockedevenifitspendstimeinaqueueandwaitsforuptotsecondsequations(3.17)to(3.19)inbook20GraphicalformofErlangBformulas21GraphicalformofErlangCformulas22Example:Findhowmanyuserscanbesupportedinacellcontaining50channelsfora2%GOS(BlockedCallsCleared)iftheaverageusercallstwice/hrwithanaveragecalldurationof5minutes.WhatisthecorrespondingCfromthefigure?WhatisA(TrafficInten