03-WLWiMAXMS-InitiatedHHOWPdownloadV1

MSInitiatedHardHandoverinWiMAXDetailedAnalysisusingMACManagementMessagesThiswhitepaperdiscussesindetailsvariousMACManagementMessagesrelatedtoMSinitiatedHardHandover.ExamplesfromrealWiMAXnetworksarepresentedusingWirelessLogixRFoptimizationtools.11/4/2008

MSInitiatedHardHandoverinWiMAX

2008WirelessLogix,Inc.

Nov.4

2

MSInitiatedHardHandoverinWIMAX

DetailedAnalysisusingMACManagementMessages

DocumentVersion :1.0

Date :November4,2008

Authors :MohammadKawser&RubiantoSatrio

WirelessLogix,Inc.

500N.CentralExpressway,Suite210

Plano,TX75074



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MSInitiatedHardHandoverinWIMAX

1. IntroductionMobileWiMAXbasedontheIEEE802.16e2005standards[1]isoneofthemostpromisingtechnologiesfor broadband wireless communication. IEEE 802.16e provides enhancements to 802.162004 tosupportsubscriberstationsmovingatvehicularspeed.ThephysicallayerofMobileWiMAXisbasedonOFDMA technology.Key advantagesof802.16e include tolerance tomultipath and selfinterference,scalable channel bandwidth, orthogonal uplink multiple access, support for spectrallyefficient TDD,frequencyselectivescheduling, fractional frequencyreuse, fineQualityofService (QoS)andadvancedantenna technology. The new technologies employed forMobileWiMAX result in lower equipmentcomplexity and simpler mobility management due to all IP core network and provide many otheradvantagesoverCDMAbased3Gsystems.

InIEEE802.16estandards,threedifferenttypesofhandovercanbesupported;HardHandover(HHO),Macro Diversity Handover (MDHO), and Fast Base Station Switching (FBSS). HHO is the simplesthandovertechnique.Theothertwowhicharemorecomplicatedareregardedasoptional.HHOcanbeeitherMSinitiatedorBSinitiated.However,itisMSinitiatedHHOwhichcommonlycausesmobilityonWiMAXnetworkatpresent.VariousMACManagementMessagesbetweenMSandBSplaykeyrolebehindthedecisionandfinaloccurrenceofsuchahandover.

ThiswhitepaperpresentsadetailoverviewofthefunctionofMACManagementMessageswithregardto the general MSinitiated HHO process. It expands on of the topics discussed in the previouswhitepaperfromWirelessLogix[2].

2. MACManagementMessagesRelatedtoMSInitiatedHHOThefollowingMACManagementMessagescarryinformationrelatedtoMSInitiatedHHOandareusedduringthehandoverprocess:

DCD:DownlinkChannelDescriptorThismessagecontains informationabout the typeofhandover supported, thehysteresismargin,andthetriggerconditionsandactions.

MOB_NBRADV:NeighborAdvertisementMessageThismessagespecifiesalistofneighborBSalongwiththeirinformationsuchasPreambleIndexandBSID.

MOB_SCNREQ:ScanningIntervalAllocationRequestThis message is used by MS to request allocation of scanning interval. It specifies a list of BS,scanningtype,scanningduration,etc.

MOB_SCNRSP:ScanningIntervalAllocationResponseThis message is used by BS to allocate scanning interval and/or instruct MS to report themeasurementresults.



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MOB_SCNREP:ScanningResultReportThismessage isusedbyMS toreportthescanning/measurementresultsto itsservingBS (usuallymeanCINRand/ormeanRSSI).

MOB_MSHOREQ:MSHORequestMessageThismessageissentbyMStoinitiatehandover.ItcontainsalistofneighborBStobeconsideredforhandoveralongwiththeirinformationsuchasPreambleIndex,measuredCINRand/orRSSI,serviceleverprediction,andarrivaltimedifference.

MOB_BSHORSP:BSHOResponseMessageThismessageisusedbyServingBStorecommendanewlistofneighborBSstoMSalongwiththeirinformationsuchasPreambleIndex,BSID,servicelevelprediction,HOIDforranging,timeforMSto send RNGREQ message to Target BS, and if and how long the Serving BS will retain theconnection.

MOB_HOIND:HOIndicationMessageThismessageisusedbyMStoindicateitsdecisiontoperformhandover.

RNGREQ:RangingRequestMessageMSperformsrangingontargetBS.

RNGRSP:RangingResponseMessageA successful RNGRSP message indicates a successful handover. The subsequent DCD messageshowingtheBS IDoftheTargetBS indicated inMOB_HOINDmessagecanconfirmthesuccessfulhandover.

The picture below shows these MAC Management Messages in WirelessLogix postprocessing toolXCAPX.



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3. HHOParametersinsideMACManagementMessages3.1. DCD:DownlinkChannelDescriptor

BStransmitsDCDatperiodicintervals.DCDmaybroadcastthefollowinginformationwithregardtoMSinitiatedHHO.HOTypeSupport:ItspecifiesifHHO,MDHOandFBSSaresupported.HysteresisMargin:HysteresismargingivenindBisusedbytheMStoconsideraneighborBSasatargetBS.WhentheCINRofaneighborBSislargerthanthesumoftheCINRofthecurrentservingBSandthehysteresismarginforthetimetotriggerduration,thentheneighborBSisincludedinthelistofpossibletargetBSinMOB_MSHOREQ.TimetoTriggerduration:Itspecifies inmillisecondsthetimeforMStodecideselectionofaneighborBSasapossibletargetBS.

Trigger:MSkeepsmeasuringservingandneighborBS.ThetriggerfieldspecifiesthatMSshouldtriggerparticularactionsbasedonthemeasurement.Theactioncanbe

BeginscanningofneighborBSbysendingMOB_SCNREQ SendHORequestusingMOB_MSHOREQ SendMOB_SCNREPaftertheendofeachscanninginterval



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Theactionmaybetriggeredwhen

MeasuredCINRorRSSIofneighborBSisgreaterorlessthanabsolutevalue MeasuredCINRorRSSIofservingBSisgreaterorlessthanabsolutevalue MeasuredCINRorRSSIofneighborBSisgreaterorlessthanthatofservingBSbyrelativevalue

Thevalue for theaction trigger is specified.Also, triggeraveragingduration, i.e. the time inmsoverwhichthemeasurementsareaveraged,isspecified.Morethanonetriggercanbeprovided.3.2. MOB_NBRADV:NeighborAdvertisementMessage

MOB_NBRADVmessage specifiesa listofneighborBSalongwith their information. ItalsomaintainsConfigurationChangeCountwhichisincrementedeverytimetheneighborBSinformationischanged.



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TheinformationofaneighborBSmayinclude:

PreambleIndex Leastsignificant24bitsofBSID Frequencyassignment BSEIRP DCD/UCDConfigurationChangeCount SchedulingServiceSupported:UGS,rtPS,nrtPS,BE,ertPS HOProcessOptimization

MOB_NBRADV message can specify trigger action for any neighbor BS from its list if the triggerpropertiesfortheparticularneighborBSaredifferentfromthetriggerpropertiesspecifiedinDCD.

3.3. MOB_SCNREQ:ScanningIntervalAllocationRequest

WhenMS justifies scanningofneighborBS, itmay requestallocationof scanning intervalby sendingMOB_SCNREQ message. MS may decide to begin scanning when corresponding trigger conditions(whicharementionedinDCDorMOB_NBRADV)aremet.ScanningallowsMStochecktheavailabilityandsuitabilityofneighborBSas targets forHO.Duringascaninterval,theservingBSbufferstheincomingpacketstotheMS.Duringnormaloperationperiods,theservingBSsendsthequeuedpacketstotheMS.MS may perform association with neighbor BS which are the potential HO targets during scanningintervals.AssociationhelpsMStoestablishbasicrelationshipswithneighborBS.ThishelpsreducethetimetosynchronizeandregisterwiththetargetBS.MOB_SCNREQ message gives a list of BS requested for scanning by referring to a MOB_NBRADVmessage. The Configuration Change Count forMOB_NBRADV field gives the Configuration ChangeCountofthereferredMOB_NBRADV.For each of the requested BS, Neighbor_BS_Index value in the MOB_SCNREQ message gives thepositionoftheBSinthelistofneighborBSavailableonthereferredMOB_NBRADVmessage.Thus,thedescriptionoftherequestedneighborBScanbeavoidedinthismessage.MOB_SCNREQ message specifies scanning type for each requested BS which can be any of thefollowing,

ScanningwithoutAssociation:MSonlymonitorssignalqualityandreportstotheservingBSbutitdoesnotperformassociation.

ScanningwithAssociation level0:MSperformsassociationduringscanning intervalsbutonlywithrangingonacontentionbasis.Dedicatedrangingregionisnotallocated.

ScanningwithAssociation level1:MSperformsassociationduringscanning intervalsthroughcoordinationof servingBS.ServingBSnotifies theneighborBSand theneighborBSallocates



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ranging opportunities. ServingBS sends association parameters overMOB_SCNRSPmessagewhichincludeRendezvousTime,CDMAcodeandTransmissionopportunityoffset.ThusMScansendrangingrequesttotheneighborBSinthededicatedrangingregion.

Scanning with Association level 2: MS performs association during scanning intervals withnetwork assisted response. ServingBS coordinates association like ScanningwithAssociationlevel1. Additionally,rangingresponses fromdifferentneighborBSaresent to theservingBSoverthebackbonenetworkandtheservingBSforwardstherangingresponsestoMStogetherusingMOB_ASCREPmessage.

MOB_SCNREQmessagealsospecifiesthefollowing,ScanDuration:Numberofframesrequestedasscanningperiod.InterleavingInterval:Theperiodofnormaloperationwhichisinterleavedbetweenscanningintervals.ScanIteration:ThenumberofrepeatedscanningintervalsrequestedbyMS.MOB_SCNREQmessagecanalsorequestaBSwithoutreferringtoanyMOB_NBRADVmessage.Inthiscase,foreachBS,MOB_SCNREQmessageprovidesthewhole48bitsofBSID.



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3.4. MOB_SCNRSP:ScanningIntervalAllocationResponse

BSsendsMOB_SCNRSPmessageitselforinresponsetoMOB_SCNREQmessagefromMS.MOB_SCNRSPallocatesscanningintervalorsimplyinstructsMStoreportwithoutallocatingscanning.MOB_SCNRSP message gives a list of neighbor BS recommended for scanning by referring to aMOB_NBRADV message. The Configuration Change Count for MOB_NBRADV field gives theConfigurationChangeCountofthereferredMOB_NBRADV.Foreachof the recommendedBS,Neighbor_BS_Indexvalue in theMOB_SCNRSPmessagegives thepositionoftheBSinthelistofneighborBSavailableonthereferredMOB_NBRADVmessage.Thus,thedescriptionoftherecommendedneighborBScanbeavoidedinthismessage.MOB_SCNRSPmessage specifies scanning type for each recommended BSwhich can be any of thefollowing,

ScanningwithoutAssociation ScanningwithAssociationlevel0 ScanningwithAssociationlevel1 ScanningwithAssociationlevel2

MOB_SCNRSPmessagealsospecifiesthefollowing:Scanduration:Numberofframesrecommendedasscanningperiod. Ifthisduration issettozeroandMOB_SCNRSP issent in response toMOB_SCNREQ, it indicates that the request foranallocationofscanningintervalisdenied.Reportmode:ItspecifieshowMSshouldreportmeasurementresults.Itcanbeasfollows:

MSdoesnotreport. MSreportstheresultofthemeasurementtoServingBSperiodically.Inthiscase,MOB_SCNRSP

alsospecifiesReportPeriod. MSreportstheresultofthemeasurementtoServingBSaftereachmeasurement.

Reportmetric:Itspecifiesreportquantity.ItisusuallymeanCINRand/ormeanRSSI.StartFrame: ItspecifieswhenScanningIntervalstartsand it ismeasuredfromtheframe inwhichtheMOB_SCNRSPmessagewasreceived.InterleavingInterval:TheperiodofnormaloperationwhichisinterleavedbetweenScanningintervals.ScanIteration:ThenumberofrepeatedscanningintervalrequestedbyMS.



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IfMOB_SCNRSPmessagerecommendsaneighborBSforScanningwithAssociationlevel1orScanningwithAssociationlevel2,thenitprovidesthefollowingadditionalparameters.RendezvousTime:ItspecifiesanoffsetthatgiveswhentheneighborBSshouldprovidenoncontentionbasedrangingopportunity.Thisoffset iscalculated inunitsofframedurationfromtheframe inwhichMOB_SCNRSPmessageistransmitted.CDMAcode:ItspecifiesauniquecodetobeusedinrangingforassociationwiththeneighborBS.Transmissionopportunityoffset:ItspecifiesnumberofsymboldurationtobeusedforassociationwiththeneighborBS.MOB_SCNRSPmessagecanalsogiveBSswithoutreferringtoanyMOB_NBRADVmessage.Inthiscase,foreachBS,MOB_SCNRSPmessageprovidesthewhole48bitsofBSID.

3.5. MOB_SCNREP:ScanningResultReport

MSmaytransmitaMOB_SCNREPmessagetoreportthescanningresultstoitsservingBS.MOB_SCNREP message gives a list of neighbor BS with measurement results by referring to aMOB_NBRADV message. The Configuration Change Count for MOB_NBRADV field gives theConfigurationChangeCountofthereferredMOB_NBRADV.



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ForeachoftheneighborBS,Neighbor_BS_IndexvalueintheMOB_SCNREPmessagegivesthepositionof the BS in the list of neighbor BS available on the referred MOB_NBRADV message. Thus, thedescriptionoftherecommendedneighborBScanbeavoidedinthismessage.ThereportmetricisusuallymeanCINRand/ormeanRSSI.Thereportmodecanbe,

Periodic:MSreportsaccordingtoscanreportperiodspecifiedinMOB_SCNRSP. Eventtriggered:MSreportsaftereachscanningperiodasindicatedintheMOB_SCNRSP.

MOB_SCNREPmessagecanalsogiveBSswithoutreferringtoanyMOB_NBRADVmessage.Inthiscase,foreachBS,MOB_SCNREPmessageprovidesthewhole48bitsofBSID.

3.6. MOB_MSHOREQ:MSHORequestMessage

WhenMS justifies handover to neighbor BS, itmay send anMOB_MSHOREQmessage in order toinitiatehandover.MSmaydecidesendingMOB_MSHOREQwhencorrespondingtriggerconditionsaremetwhicharementionedinDCDorMOB_NBRADV.MSsendsanMOB_MSHOREQmessageinordertoinitiatehandover.MOB_MSHOREQmessagegivesalist of neighbor BS to be considered for handover by referring to a MOB_NBRADV message. TheConfiguration Change Count for MOB_NBRADV field gives the Configuration Change Count of thereferredMOB_NBRADV.ForeachoftheneighborBS,Neighbor_BS_IndexvalueintheMOB_SCNREPmessagegivesthepositionof the BS in the list of neighbor BS available on the referred MOB_NBRADV message. Thus, thedescriptionoftherecommendedneighborBScanbeavoidedinthismessage.MOB_MSHOREQmessagemay includethefollowing informationforeachneighborBSconsideredforhandover.

PreambleIndex MeasuredMeanCINR MeasuredMeanRSSI Servicelevelprediction:ThisindicatesthelevelofservicetheMScanexpectfromthisBS.Itcan

be likenoservicepossible,someservice isavailableforoneorseveralserviceflowsoraMACconnectioncanbeestablishedforeachserviceflowwithQoSspecified.

ArrivalTimeDifference: It indicatesthedelayofdownlinksignaloftheneighborBSrelativetotheservingBSasmeasuredbytheMS.

MOB_MSHOREQmessagemayalsoincludemeanCINRand/ormeanRSSIofservingBS.MOB_MSHOREQmessagecangiveBSwithoutreferring toanyMOB_NBRADVmessage. In thiscase,foreachBS,MOB_SCNREPmessageprovidesthewhole48bitsofBSID.



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3.7. MOB_BSHORSP:BSHOResponseMessage

After reception of MOB_MSHOREQ message, serving BS checks the list of candidate BS andmeasurementreport.Then,servingBSmaysendhandoverrequesttooneormorepotentialtargetBSsover backbone network. The neighbor BSs, after reception of handover request, may send contextrequesttothecontextserver.Thus,theycheckonQoSofexistingconnectionofMS.Theyalsocheckontheir available resources and then send handover responsemessages to serving BS. The serving BS,based on handover responses, recommends a new list of neighbor BSs and notifies MS overMOB_BSHORSPmessage.MOB_BSHORSPmessagealsoprovidesthe following information foreachneighborBSrecommendedforhandover.PreambleIndexBSIDServicelevelprediction:ThisindicatesthelevelofserviceMScanexpectfromthisBS.Itcanbelikenoservicepossible,someservice isavailableforoneorseveralserviceflowsoraMACconnectioncanbeestablishedforeachserviceflowwithQoSspecified.

HOprocessoptimization:ItspecifieswhichstepsofnetworkentrywouldberequiredduringHO.



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HO_ID: It is an ID assigned for use in initial ranging with the target BS. This ID is available ifHO_ID_included_indicatoristrue.

ActionTime:ServingBSdecidesthisparametercontactingwithpotentialTargetBSsoverthebackbone.Action Time specifies the number of frames until the Target BS allocates a dedicated transmissionopportunityforMStosendRNGREQmessage.Thisactuallymeansthatchannelparametersobtainedby theMSduringAssociationof thatBSstayvalidandcanbe reusedduringactualNetworkReentrywithoutprecedingCDMAbasedInitialRanging.

ResourceRetain Flag/Time:TheResourceRetainFlag indicateswhether the servingBSwill retainordeletetheconnectioninformationoftheMSuponreceptionofMOB_HOINDmessageforhandover.Iftheflagissetto1,theservingBSretainstheconnections,MACstatemachineandPDUsassociatedwiththeMS until the expiration of Resource Retain Time. This temporary reservation of resources helpsresumeservicespromptly,ifthehandoverdoesnotsucceedandMSreturnstotheoldservingBS.

3.8. MOB_HOIND:HOIndicationMessage

After reception of MOB_BSHORSP message, MS may decide to perform handover and then sendMOB_HOINDmessagetoservingBS.Inthiscase,HO_IND_typefieldissettoServingBSRelease.MOB_HOINDmessageincludes:



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PreambleIndex BSID Ranging_Params_valid_indication: IfMS rangingparameters forTargetBSare specified in the

MOB_HOINDmessage, then thisparametermay indicate if thoseMS rangingparameters forTargetBSarevalidornot.

3.9. RNGREQ:RangingRequestMessage

After sending MOB_HOINDmessage to serving BS, MSmay perform initial rangingwith target BS.Rangingcanbereservationbasedormaybe firstcontentionbased, ifneeded.AfterMSsynchronizeswiththetargetBS, itmayreceivededicatedrangingopportunity inFast_Ranging_IEfieldoverULMAPmessagefromtargetBS.ThenMSsends RNGREQmessagetotargetBSwhichincludes,

ServingBSID:ThisistheBSIDoftheservingBSwhichMSisstillconnectedwith.

Ranging Purpose Indication: This indicates the purpose of ranging which is here, MS is currentlyattemptinghandover.

HO_ID:It isthesameIDwhichwasprovided inMOB_BSHORSPmessage.TheID isusedhere in initialrangingwiththetargetBS.

MSMACAddress:IfHO_IDisomitted,thenthisfieldisusedtogiveMACAddressofMS.



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3.10. RNGRSP:RangingResponseMessage

The target BS sends RNGRSP message in response to RNGREQ message. This can be followed bynetworkentryprocedure.RNGRSPmessageincludes:

HOprocessoptimization:ThetargetBSmaycollectinformationaboutserviceflow,statemachineetc.from servingBSvia context serverwithout involvementofMS [3].This stephelpsavoidunnecessarysteps innetworkentry.HOprocessoptimization fieldspecifieswhichstepsofnetworkentrycouldbeskipped.

HO_ID:It isthesameIDwhichwasprovided inMOB_BSHORSPmessageandthereafterused inRNGREQmessage.

Resource Retain Flag: TheResourceRetain Flag indicateswhether theold servingBS is retainingordeletingtheconnection informationoftheMS.Thistemporaryreservationofresourceshelpsresumeservicespromptly,ifthehandoverfailsandMSreturnstotheoldservingBS.



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3.11. DCDfromNewBS



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The DCD message, shortly after handover attempt, should be received from the target BS if thehandoverhasbeensuccessful.Thus,iftheservingBSshownontheDCDhasBSIDassameasthetargetBSIDspecified intheMOB_HOINDmessage,then theHOhasbeensuccessful.However, iftheywerenotsame,HOwouldbedeemedtohavefailed.

References

1. IEEE,P802.16Rev2/D5Standards,June2008.2. WirelessLogix,MissingNeighborAnalysisforWiMAXNetworks,July2008.3. YanZhangandHsiaoHwaChen,MobileWiMAX:TowardBroadbandWirelessMetropolitanArea

Networks,2007.

AbouttheAuthors

MohammadKawserisaTechnologyConsultanttoWirelessLogix,Inc.HeisalsocurrentlyanAssistantProfessorattheElectricalandElectronicEngineeringDepartmentatIslamicUniversityofTechnology,Bangladesh.HisworkexperiencealsoincludesQualcomm,Inc.HereceivedMSdegreefromVirginiaTech,USAin2005andBSdegreefromBangladeshUniversityofEngineeringandTechnology,Bangladeshin1999,bothinElectricalEngineering.

RubiantoSatrioisVPofEngineeringatWirelessLogix,Inc.HereceivedMSEdegreeinElectricalEngineeringfromtheUniversityofTexasatAustinin1994.

MoreInformation:

Forquotesorcustomsolutionpricing:[email protected]:[email protected]

WirelessLogix,Inc.500NorthCentralExpressway,Suite210Plano,TX75074Phone:(469)2416100Fax:(469)2416199www.wirelesslogix.com

03-WLWiMAXMS-InitiatedHHOWPdownloadV1

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