With Contributions of the following partners (in order of ap(3)

A state of the art

1.1Cellularnetworks

implemented,dueforexampletousermobility.Finally,somespeci cQoSrequirementshavesometimestobeensured.Forexample,whenauserreceivespacketsfromdifferentradiotransmittersatthesametime,i.e.theuserisinsofthandover,thepacketarrivalsatthetransmittershavetobesynchronized(incellularnetworks,thisrequirementimpliesstringentboundsforpacketdelayintheUTRAN).

Asinwire-linenetworks,traf cisrandomlygeneratedbyusers:atanytime,acalloratransfercanbegeneratedbetweenasourceandadestination.Incellularnetworks,forthedownlinkforexample,thetransferisinitiatedfromabasestationtoamobileattachedtothisbasestation.Inad-hocnetworks,thesourceandthedestinationmaybeanynodesofthenetwork.Whenthecallorthetransferiscompleted,therequirednetworkresourcesarefreed.Inwirelessnetworks,thistraf cdemandisalsode nedbythemobilitypatternsofeachuser/node:eachuserfollowshis/herowntrajectory(mostoftenindependentlyofotherusermovements)resultinginatimevaryingsource-to-destinationdistanceandroutingpath.

1.0.0.2RadioresourcemanagementRadioresourcesconsistofbandwidthandpower.Theachiev-ablerateforasource-destinationpairdependsinacomplicatedmanner,notonlyonthewaytheseresourcesaresharedamongallsource-destinationpairs,butalsoonthehighlyvariablepropagationcharacteristics,i.e.fading.Therearebasicallytwoapproachestohandlefadingdependingontheconsideredapplication: rst,onemayadapttheusedradioresourcestocompensateforthevaryingchannelqualityandtomaintaina xedrate,asincircuit-switchedvoicenetworks;second,onemaydynamicallyadapttheratesoastomatchthe uctuationsinthechannelquality.Thelatterapproachiswell-suitedfordatatransferapplicationsthatdonothaveastringentraterequirement.

Radioresourcesmanagementinanactualsystemmustalsosatisfysomedesignconstraintsdependingontheconsideredtypeofnetwork.Incellularnetworksforexample,radioresourcesmayoftenbemanagedinacentralizedmanner,whereasinWLANorAdHocnetworks,byde nition,theabsenceofsignalingproceduresimposesadecentralized(andthuslessef cient)waytoshareresources.

1.1Cellularnetworks

Cellularnetworksaredividedupintocells,eachcellbeingservedbyoneormoreradiotransceiverscompos-ingabasestation.Theradioresourcemanagementcanthenbeperformedinacentralizedmanneratthebasestationlevelorperhapsevenatahigherlevelinthenetwork(e.g.theRNCinUMTSnetworks)toallowforspeci cfunctionalities,suchasmacrodiversity.

Cellularsystemsare rstde nedbytheirradioaccesstechnology.Fromthe rstanalogsystemsdeployedinthe70’stotheOFDM/MIMObased4Gproposals,theaccesstechnologyhasconstantlyevolved,notonlytooptimizetheuseofthebandwidth,butalsotosupportdifferenttypesofapplications.A rstissuetoaddressisthedesignofanoptimalradiointerfacebasedontheconsideredapplication.Forexample,traditionalvoicetraf cprovestobebettersupportedbyCDMAsystemsthanbyTDMAsystems,whereasrecentadvancesinscheduling,suchasthedevelopmentofschedulersthattakeadvantageoffastfadingvariations,makeTDMAaccess,ratherthanCDMA,moresuitedtodelaytolerantdatatraf c.Nevertheless,thereisnoconsensusonthewayradioresourcesshouldbeshared,andbuildinganef cientmulti-serviceairinterfaceisstillthemajorissueinwirelessnetworkdesign.

Cellularnetworkswereinitiallydesignedtosupportcircuitservices.Theperformanceofsuchservicesde-pendsonmanyparameters,suchasthetypeofradioaccessorthewayadmissioncontrolisperformed.Unlikeusualtelephonenetworks,theQoSinwirelessnetworksisperceivednotonlythroughtheprobabilitythecall

A state of the art

1.2WirelessLANs

isadmittedbutalsothroughtheprobabilitytheQoSisensuredduringtheentirecall.TheprobabilitythattheQoSisdegradedduringthecallisoftentermedoutageprobability.Indeed,anoutagecanoccurevenundersevereadmissioncriterium,due,forexample,tousermobility.Wemayconsidertwokindsofmobility,intraandinter-cellmobility.The rstkindmainlyconsistsinshadowingandRayleighfading,whereasthesecondkindconsistsinwideusermovementsimplyinghardhand-overs.PredictingandcharacterizingmobilityisanimportanttasksincemobilityrepresentsthemaincontributioninoutageandQoSdegradation.AreviewofmobilitycharacterizationsispresentedinSection10.Theadmissionschemehastorealizeagoodtrade-offbetweenblockingandoutageprobabilities.ThesenotionsarefurtherdiscussedinSection3.Astate-of-the-artonadmissioncontrolinCDMAnetworksanditsperformanceevaluationispresentedinSections2,4and

9.Inconjunctionwithadmissioncontrol,powercontrolalgorithmsareusedtoadaptthetransmittedpowertothechannelqualityvariations.Referto[46]foranexhaustivereviewoftheliteraturedealingwithpowercontrol.

Inadditiontocircuitservices,wirelessnetworksareexpectedtosupportawidevarietyofdatatransfers.Thiskindoftraf c,withoutdelayorraterequirements,opensupthepotentialforde ningvariousschedulingdisciplinessoastomaximizecertainperformanceindicators.Areviewofexistingresultsonsuchschedulingdisciplinesandtheirpacketand owlevelperformanceispresentedinSection5.

Recently,economicmodelinghasbeenproposedasanalternativewayofresourcemanagementinwirednetworks.Thesenewideas,suchascongestionpricingorauction-basedresourcereservation,arenowadaptedtothecontextofwirelessnetworks.RefertoSections6and7forareviewonthistopic.

Finally,itmightbeimportanttoensurespeci cQoSrequirementsintheaccessnetwork,e.g.lowpacketdelaytoallowmacrodiversity.ThearchitectureaswellastheenvisagedQoSmechanismoftheUTRAN(theaccessnetworkofUMTSnetworks)arediscussedinSections8and9.

1.2WirelessLANs

WLANsaredesignedwithacompletelydifferentphilosophythancellularnetworks.TheinitialmotivationoftheIEEE802.11WorkingGroupwasthedevelopmentofaradiointerfacethatcouldreplicatethetraditionalEthernet-basedlocaldatanetworksinwirelessnetworks.Themajorconstraintofsuchaninterfacewasthattheradioresourceneededtobesharedinadecentralizedmanner,withoutanysignalingprocedures.TheWorkingGroupdevelopeda rststandard,the802.11standard,thatactuallycanbeseenasamodi edversionofEthernet,atleastfortheMAClayer.Manyotherstandardshavebeenproposedsince,thatadapttheMACprotocolstothephysicallayerevolutionorsupportnewfunctiona …… 此处隐藏：6328字，全部文档内容请下载后查看。喜欢就下载吧 ……