东南大学研究生英语雅思写作下学期大作业Word文档格式.docx

东南大学研究生英语雅思写作下学期大作业Word文档格式.docx

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Abstract

Weconsiderahierarchicalnetworkthatconsistsofmobileusers,atwo-tieredcellularnetwork（namelysmallcellsandmacrocells）andcentralrouters,eachofwhichfollowsaPoissonpointprocess（PPP）.Inthisscenario,smallcellswithlimited-capacitybackhaulareabletocachecontentunderagivensetofrandomizedcachingpoliciesandstorageconstraints.Moreover,weconsiderthreedifferentcontentpopularitymodels,namelyfixedcontentpopularity,distance-dependentandload-dependent,inordertomodelthespatio-temporalbehaviorofusers’contentrequestpatterns.Wederiveexpressionsfortheaveragedelayofusersassumingperfectknowledgeofcontentpopularitydistributionsandrandomizedcachingpolicies.Althoughthetrendoftheaveragedelayforallthreecontentpopularitymodelsisessentiallyidentical,ourresultsshowthattheoverallperformanceofcached-enabledheterogeneousnetworkscanbesubstantiallyimproved,especiallyundertheloaddependentcontentpopularitymodel.

Besides,Becauseofthelimitationofresearchconditions,thetotalnetworkdelay,networkcostandoptimizationofnetworkparametersarenotanalyzed.

Keywords:

edgecaching,Poissonpointprocess,stochasticgeometry,mobilewirelessnetworks,5G

Introduction

Itisknownthatcontentcachingin5Gheterogeneouswirelessnetworksimprovesthesystemperformance,andisofhighimportanceinlimited-backhaulscenarios.Mostexistingliteraturefocusesonthecharacterizationofkeyperformancemetricsneglectingthebackhaullimitationsandthespatio-temporalcontentpopularityprofiles.Inthiswork,weanalyzethegainsofcachinginheterogeneousnetworkdeployment,andconsidertheaveragedelayasaperformancemetric.

Firstlyweusepassionpointprocess（PPP）methodtobuildthemodel.Thisheterogeneousnetworkconsistsofmobileterminals（users）,cache-enabledsmallbasestations（SBSs）,macrobasestations（MBSs）andcentralrouters.Inthisnetworksetting,ausermayexperiencedelaysduetodownlinktransmissions,backhaulandcaches.

Moreover,inordertocapturethespatio-temporalcontentaccesspatternsofusers,wesupposefixedcontentpopularity,distance-dependentandload-dependentcontentpopularities.Assumingthatthecontentpopularitydistributionisperfectlyknownatthesmallbasestations,weexplorethreedifferentcachingpoliciesbasedoncontent-popularityandrandomization.Andfinally,wedrawourconclusion.

Methodology

First,ourResearchobjectiveistogettheaveragedelayinacache-enabledtwo-tieredcellularnetworkmodeledbystochasticgeometry,thefigureoneisanillustrationoftheconsideredsystemmodel,themethodologypartincludingthefollowingparts.

Thefirstisthetopology,thecentralrouters,MBSs,SBSs,USERsaremodeledbytheindependentPPPmodels,thesecondisthesignalmodel,forthetractabilityfortheproblem,weassumethatthetypicaluserexperiencetherayleighfadingandthestandardpowerlawpathloss,alsoweassumethatthenetworkisinterference-limited,thatistosaytheinterferencepowerdominatesoverthenoisepower.

Thenitisthecachingmodel,whenauserhasacontentrequest,weassumethattherequestisdrawnfromacontinuouszipfdistribution,inthefunction,thereisacontentpopularityparameter,basedonthisparameter,wehavethreecachingmodels,Fixedmeansthatthecontentpopularityisidenticalforallusers,allSBSsobservethesamedistribution,Distance-dependentmeansthateachuserhasadistance-dependentsteepnessfactor,thefactorisrelatedtotheaveragedistancebetweentheSBSanditsusers.

Basedonthecachingmodels,hereweconsiderthreecachingpolicies:

StdPopmeanstheSBSscachethemostpopularcontentfromthecatalogue,theUnirandmeansthecontentsarecacheduniformlyatrandom,theMixPoppolicymeansthatpartstoragecachethemostpopularcontents,theotherpartcachethecontentatrandom.

Thelastpartisthedelay,ourmetricinthispaper,thedelayincludingthefollowingthreeparts,WhentheBSsdeliverthecontentstotheirintendedmobileusers,itincursthedownlinkdelay,WhenthecontentrequestedarenotcachedintheBSs,theBSshavetogetthecontentformthecentralroutersthroughthebackhaul,thenitcausesdelay,whenthecontentrequestedinthecaches,thecontentneedtobetakenoutfromthecaches,itcausesdelay.

ReportingandDiscussingResults

Inthissection,wenumericallyvalidateourapproximationsderivedintheprevioussection.Theimpactofcriticalsystemparametersarediscussedasfollows.

Fig.（a）ImpactofMBSdensity.（b）Impactofsmallcelldensity.

（c）ImpactoftargetSIR.（d）Impactofstoragesize.

ImpactofMBSdensityλmc:

Thechangeofaveragedelaywithrespectto