DO AALOHA Slotted ALOHA for Ad Hoc Netw orking Using Smart Antennas Harkirat Singh and Suresh Singh Department of Computer Science Portland State Uni ersity Portland OR harkiratsingh cs - Description

pdxedu Abstract This paper de velops novel slotted ALOHA pr oto col Dir ectionOfArrival ALOHA or use in ad hoc networks wher nodes ar equipped with smart antennas The pr otocol elies on the ability of the antenna and DO algorithms to identify the dir ID: 35286 Download Pdf

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DO AALOHA Slotted ALOHA for Ad Hoc Netw orking Using Smart Antennas Harkirat Singh and Suresh Singh Department of Computer Science Portland State Uni ersity Portland OR harkiratsingh cs

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pdxedu Abstract This paper de velops novel slotted ALOHA pr oto col Dir ectionOfArrival ALOHA or use in ad hoc networks wher nodes ar equipped with smart antennas The pr otocol elies on the ability of the antenna and DO algorithms to identify the dir

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DO AALOHA Slotted ALOHA for Ad Hoc Netw orking Using Smart Antennas Harkirat Singh and Suresh Singh Department of Computer Science Portland State Uni ersity Portland OR harkiratsingh cs






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DOA-ALOHA:SlottedALOHAforAdHocNetworkingUsingSmartAntennasHarkiratSinghandSureshSingh,DepartmentofComputerSciencePortlandStateUniversity,Portland,OR97201harkirat,singh@cs.pdx.eduAbstract—ThispaperdevelopsanovelslottedALOHAproto-col(Direction-Of-ArrivalALOHA)foruseinadhocnetworkswherenodesareequippedwithsmartantennas.TheprotocolreliesontheabilityoftheantennaandDOAalgorithmstoidentifythedirectionofthedesiredsignalandthedirectionoftheinterfererstomaximizeSINR(SignaltoInterferenceandNoiseRatio)atthereceiver.TheperformanceoftheprotocolisevaluatedusingjointsimulationinOPNETandMatlab.WecomparetheperformanceofournewprotocolagainstrecentdirectionalMAC(MediumAccessControl)[3]protocol.WeshowthatDOA-ALOHAachievessignicantlyhigherthroughputthan[3]despiteitssimplicity.Theimpactofusingdifferentnumberofantennaelementsisalsostudiedforthisenvironment.I.INTRODUCTIONRecently,therehasbeenincreasinginterestindevelopingMACprotocolsforuseinadhocnetworkswherenodesareequippedwithdirectionalantennas.Antennamodelsusedincludesectoredxedbeamantennas,idealizedadaptivear-rayantennas,andsteerabledirectionalantennas.Aspreviousresearchershaveshown,usingdirectionalantennasincreasesthroughputbecauseofbetterspatialreuseofthespectrum(see[10],[1],[3],[6]).However,wenotethatthesepreviousworkshavenotfullyexploitedthebenetsofadaptivearrayantennas(orsmartantennas)suchastheabilitytoformnullsinthedirectionofinterferers(resultinginhighSINR)andtheabilitytodeterminethedirectionoftransmitters(DirectionofArrival).Weshowthatbyexploitingthesecapabilitiesofsmartantennas,asimpleprotocolcanyieldthroughputsthatare2x–4xhigherthanoneoftherecentprotocols[3].Wealsonotethatoursimulationsuserealisticantennamodelsunliketheidealizedmodelsusedinmany(withtheexceptionof[6])papersand,despitethis,ourprotocoloutperformsmostoftheseexistingprotocols.Thekeyprobleminexploitingthecapabilitiesprovidedbyasmartantennaatareceiverisindeterminingthedirectionoftheinterferingsignals(soastoplacenullsinthosedirections)andformingabeamtowardsthetransmitter.Inordertodothis,wehavedevelopedamodiedversionoftheslottedALOHAprotocolinwhichasmallinitialportionoftheslotisusedforndingthedirectionofvarioustransmitters.Thisisdonebyrequiringeachtransmittertotransmitapuretonetowardsitsintendedreceiverforashortintervalpriortotransmittingthepacket.ThereceiverrunsaDOA(DirectionOfArrival)algorithmwhichprovidesinformationThisworkisfundedbytheNSFundergrantANIR-0125728.aboutthereceivedsignalstrengthanddirectionofthedifferenttransmitters.Thisinformationisthenusedateachreceivertoguidebeamforming(beamandnulls)fortheremainingdurationoftheslot.Uponcorrectpacketreception,areceiversendsanACKusingthealreadyformedbeams.WeimplementedsmartantennainMatlabandinterfaceditwiththephysicallayerofOPNET.Inourstudyweusedrealisticantennapatternswiththesidelobes.Wecarriedoutextensivesimulationsandobtainveryhighthroughputinthesingleaswellasmulti-hopcase.Theremainderofthepaperisorganizedasfollows:inthenextsectionwedescribeoursystemmodel.SectionIIIdescribesthecontextforthisworkinrelationtootherresearch.SectionIVdescribeourprotocolDOA-ALOHAinmoredetail.SectionVsummarizeskeyresults.II.SYSTEMMODELWeassumethateachnodeisequippedwithasmartantennasystemwhichiscomposedofalineararrayofelements.Forsimplicityweassumethattheantennaarrayisperpendiculartothex-yplaneinwhichthenodeslie.Thereasonforthisassumptionisthatthebeamformedbytheantennaissymmetricabouttheantennaaxisandisthusindependentofthedirectioninwhichanodeis“facing”1.Figure1providesaschematicofanadaptivearraysmartantennasystem.Asillustratedinthegure,theantennaelementsareseparatedfromeachotherbyaknowndistance.Weassumethatthetransmitterislocatedfarenoughawayfromthereceiverthatallthesignals\n  arrivingatthedifferentantennaelementsareparallel.However,sincetheelementsareseparatedbydistance,thephaseofthedifferentsignalsisdifferent.Let \rdenotethephaseandgainthatisaddedtoeachsignal\n  .Then  ,theoutputsenttothereceiver,canbewrittenas,\n   \n  ! "  $#!%\n   '&)(+*-,+.(0/2143'57698! "  where䀀:;=isthephasepropagationfactor,@isthewavelength,#isanarbitrarygainconstant,and "  isAWGNnoise.Theweights \rusedinthispaperonlyshiftthephaseofthesignalandleavetheamplitudeuntouched.1Ifweplacetheantennaarrayinthex-yplanethenthebeampatternformedinsomedirectionwilldependontherelativealignmentoftheantennawithrespecttothexandyaxes.Thismakestheanalysisfarmoredifcultwithoutgaininganyadditionalgenerality. Ifthedesireddirectionis,thentherepresentationfortheweightsis, &*,+.(/1'8Foramorecomprehensivediscussion,pleasesee[5].InFigure2weshowthedifferentantennapatternsformedbyalineararrayof\n \relementswhen.Wenotethatasthenumberofelementsincreases,thebeamwidthbecomesnarroweranddirectivityoftheantennaincreases.Further,wenotethat,ratherthanonebeam,usingalineararrayresultsintwobeamsthatcanleadtogreaterinterference.Aswenotedintheintroduction,anotherbenecialfeatureofsmartantennasistheabilityoftheseantennastoformnullsingivendirections.Infact,givenelements,anantennacanformupto\rnulls.However,theshapeofthedesiredbeamcanchangedependingonthenumberofandthedirectionofthenulls.InthisworkweusetheMMSE(MinimumMeanSquareError)algorithmtodetermineweights toformnullsappropriately[5].Figure3illustratestwocaseswhenusing antennaelementswith!beingthedesireddirection.Intherstcase,weareformingonlytwonullswhereasinthesecondcaseweareformingsixnulls.Ascanbeseen,theshapeofthebeamandsidelobeschanges.wwww123MSReceiverS1(t)S2(t)S3(t)SM(t)dVariable gain andAntenna elementsSignal received fromqFig.1.Schematicofasmartantenna(adaptivelineararray).III.RELATEDWORKRecentlytherehavebeenseveralpapersthathavelookedattheproblemofMACdesignforadhocnetworkswherenodesareequippedwithdirectionalantennas.Thedirectionalantennamodelsusedincludeswitchedbeamantennas(thean-tennaissectoredandoneofthesesectorsisuseddependingthedirectionofthecommunicatingnode),multi-beamantennas(heremorethanonebeamcanbeusedsimultaneously),andadaptiveantennaarrays(herethebeamcanbemadetopointinanydirectionasdescribedinsectionII).[2]developsslottedscheduling-basedMACprotocolsfornodesequippedwithdirectionalantennas.Thedirectionalantennaconsideredisamulti-beamadaptivearrayantenna(MBAA)whichiscapableofformingmultiplebeams.Theprotocolsassumethatnodescanengageinseveralsimulta-neoustransmissions.Thekeycontributionofthepaperisthedevelopmentofaneighbortrackingschemethatisthenusedtoscheduletransmissionsbyeachnodeinadistributedway. 0.005 0.01 0.0153021060240902701203001503301800Array Pattern with 8 Elements DOA = 45 DegreesTheta(Degree)Normalized Pattern 0.005 0.01 0.015 0.02 0.0253021060240902701203001503301800Array Pattern with 16 Elements DOA = 45 DegreesTheta(Degree)Normalized PatternFig.2.Antennapatternsfor8and16antennaelements.[11]alsodevelopsMACprotocolsforthisscenario.Theyassumethatthegainofthedirectionalantennaequalsomnidirectionalantenna.Anantennaiscomprisedof4antennaelementswhichcantransmitin90degsectorsonly.NodesaredependentonGPSorsomeotherdevicetohavepositioninformationofthesenderandreceiver.TheypresentaMACprotocolbasedonDirectionalRTS/CTSoracombinationofDRTS/O-CTS(OmnidirectionalCTS).Theprotocolassumesthatifatransmissionishappeninginsomedirectionthenitwilldeferalltransmissionsinthatdirection.Similarly,[4],[12]consideraswitchedbeamantennamodelwithnonullingorDOAinformation.Theyuseadirectionalantennaatthereceiver.AsecondprotocolbasedonDRTS/DCTSassumestwoseparatechannels:onefordataandanotherforsignaling.Thesepapersdonotconsidernullingornarrowerbeamwidths.In[3]anovelmulti-hopRTSisproposedtoestablishlinksbetweendistantnodesand1(directionalgain)isassumedtobehigherthan(omnidirectionalgain).ThedirectioninwhichthemainlobeistobeorientedisdeterminedbytheMACprotocol(whichinturnisprovidedthisinformationbythenetworklayerwhichisassumedtobeneighbor-aware).Theauthorsnotethatnodealignmentnegatesthebenetsachievedduetodirectionalantennas,however,unaligned 0.2 0.4 0.6 0.8 13021060240902701203001503301800Desired = 45 deg, Nulls = 25, 70 deg 0.2 0.4 0.6 0.8 13021060240902701203001503301800Desired = 45 deg, Nulls = 10, 20, 30, 70, 80, 90 degFig.3.Patterswith8antennaelementsand2or6nulls.routesenhancesthespatialreuse.Theyshowthattheirprotocolhasa4-5xthroughputascomparedwith802.11.In[8],[7]theauthorsassumethateachnodemaintainsneighborAngle-SINRtable(AST)andtheyprovidealinkstatebasedtable-drivenroutingandMACprotocol.BasedonASTanodecalculatesanafnityforananglewhichprovidesmaximumSINR.BasedonthisaNLSTableisformed.Nodesinthebeamformedregionremainintheomnimodebuttheymakenullsinthedirectionofongoingtransmissions.[9]usesdirectionaltransmissionsforcontrolanddatapack-ets.Itusesadirectional-NAVtablefortransmissionschedulingandcollisionavoidance.However,theydonotexploitthecapabilitiesofthesmartantennas,suchasbeamsteeringandtheplacementofnullsinthedirectionofinterferers.In[6]anodecachesAOAinformationbasedonsignalsreceivedandnodesremaininpromiscuousmodetocachesignals.802.11specicationssaythatRTSneedstobetrans-mitted7times,soanodewilltransmit4directionalRTSandremainingthe3asomni-directionalRTSifthereisnoresponsetothedirectionalRTS.Acircularantennawith6elementsisassumed,andanodeiscapableofelectronicallysteeringtheboresighttowardsaspecicdirection.Aconstantbeamwidthof45degassumed.However,itwasobservedthatastheboresightchanges,thesidelobepatternchangesdrastically.Thelessonisthattheeffectsofsideandbacklobescannotbeignoredintheevaluationofnetworkperformancewithdirectionalantennas.[6]showsthatusinganidealantennaresultsinamaximumthroughputof2.2Mbpswhileusingarealisticantennahasamaximumthroughputofonly1.4Mbps.Thisfact,thatantennapatternsmatterinevaluatingMACbehavior,isonethathaslargelybeenignoredbyagreatmanyauthors.Followingthelessonof[6],weuserealisticantennapatternsinourstudies.IV.PROTOCOLDESCRIPTION:DOA-ALOHAInthissectionwedescribethebehaviorofourprotocol.However,beforedoingthis,weneedtomakethefollowingassumptions:(1)Nodesareawareoftheangularlocationofeachoftheirneighbors(asin[3])sincethisinformationisneededattransmitterstoformdirectedbeamstowardsreceivers;(2)Forsimplicity,weassumethatallnodesusethesameconstanttransmitpower.Packet TransmissionDOAACKOne SlotFig.4.StructureofaslotinDOA-ALOHA.Figure4showstheformoftheslotsusedinDOA-ALOHAandasshown,eachslotisbrokenintothreeminislots.Ouralgorithmworksasfollows:1)TherstminislotinaslotiscalledtheDOA-minislotanditisherethatanodeidentiestheangulardirectionofalltransmittersthatitcanhear.Alltransmitterstrans-mitasimpletone(i.e.,asinewave)duringtheDOA-minislottowardstheirintendedreceivers.Thesignalreceivedatsomereceiveristhusthecomplexsumofallofthesetones.ThereceiverrunsaDOAalgorithm(suchasMUSIC[5])todeterminetheangulardirectionofeachofthetransmittersandthereceivedpowerfromeachtransmitter.2)OnceareceiverdeterminestheDOAofalltransmittersitcanhear,itformsitsdirectedbeamtowardstheonethathasthemaximumpowerandformsnullsinalltheotheridentieddirections.3)Thesecond(andlargest)minislotisthepackettransmis-sionslotanditisherethatthepacketsaretransmitted.Afterthereceiverhasformeditsbeamandnullsasde-scribedabove,itreceivesthepacketfromthetransmitter.Afterreceivingthepacket,itlooksattheheaderandrejectsthepacketifitwasnottheintendeddestination.4)ThelastminislotistheACKslotwherethereceivertransmitsanACKusingthealreadyformedbeamtothesender(ifthepacketwasnotrejectedandcorrectly received).WhenatransmitterdoesnotreceiveanACK,itretransmitsthepacketatalatertime(thisisexactlyasinALOHA).abdcNode d mistakenly formsa beam towards a becausea's signal is strongerthan b's signal at da has a packet for cb has a packet for dFig.5.Falsebeamforming.Theintuitionbehindthereceiverbeamforminginthedirec-tionofthemaximumsignalisthat,becauseofthedirectivityoftheantenna,thereisahighprobabilitythatitistheintendedrecipientforthepacket.However,wenotethatincases,asinFigure5,thereceiverincorrectlybeamformstowardsbecause'ssignalisstrongerthan's.Whilethisisnotaseriousprobleminmostcases,wecanenvisionscenarioswherethetransmissiongetsstarvedduetoalargevolumeoftrafc.Anoptimizationwehavethereforeimplementedisasingle-entrycacheschemewhichworksasfollows:Ifanodebeamformsincorrectlyinagiventimeslot,itremembersthatdirectioninasingle-entrycache.Inthenextslot,ifthemaximumsignalstrengthisagaininthedirectionrecordedinthesingle-entrycache,thenthenodeignoresthatdirectionandbeamformstowardsthesecondstrongestsignal.–Ifthenodereceivesapacketcorrectly(i.e.,itwastheintendedrecipient),itdoesnotchangethecache.–Ifitreceivesapacketincorrectly,itupdatesthecachewiththisnewdirection.Ifthereisnopacketinaslotfromthedirectionrecordedinthecache,thecacheisreset.ThissimplemechanismensuresthatincasessimilartoFigure5,connectionsarenotstarved.However,wecanconstructmorecomplexscenarioswhereasingle-entrycachewillfailtopreventstarvation.Inthesecases,moresophisticatedmultiple-entrycachingschemesarerequired.However,inoursimula-tions,weonlyusethesingle-entrycachingschemebecausetheprobabilityofmorecomplexscenariosresultinginstarvationareveryrare.V.RESULTSOPNETprovidesanexcellentphysicallayermodelbuthasadrawbackinthatithasaveryidealisticdirectionalantennamodel.Toovercomethisdrawbackweimplementedthesmartantennamodel(foralineararrayofantennaelements)inMatlabandinterfaceditwiththephysicallayerofOPNET.WeinvokeMatlabtocompute 's(sectionII)basedonactualreceivedsignalstrength97  ateachantennaelementTABLEIOPNETSIMULATIONPARAMETERS.SimulationParametersBackgroundNoise+ambientNoise-143dBPropagationmodelFreespaceBandwidth1,000kHzMinfrequency2,402MHzDataRate2000kbpsCarrierSensingThreshold+3dBMinimumSINR9dBBitErrorBasedonBPSKModulationcurveMaximumradiorange250masobtainedfromOPNET.WealsomodiedOPNET'sradiopipelinestagewiththesimulationparametersdisplayedinTableI.WeevaluatetheperformanceofDOA-ALOHAusing5x5mesh(asusedin[3])withfourpre-denedows.Figure6showsthenetworktopologyandowsusedfortwooftheseexperiments.Forthethirdexperiment,weusedarandomnodeplacementonthegridwhereanode'spositionisrandomlyshiftedinthex-axisandy-axisbyaddingadisplacementrandomlyselectedfrom[-150m,+150m]andtheowsareasinFigure6(b).ThetrafcisCBR(ConstantBitRate)whichincreases(perow)from75kbpsto2Mbps.Thepacketsizeis512bytes.Figure7plotstheaggregatethroughputasafunctionofthedatarateofoneow(forFigure6(a))fortwoantennasystems–onewith8elementsandonewith16elements.Figure8doesthesameforFigure6(b)andFigure9correspondstotherandommeshtopologycase.Weused10differentcasesforrandomows(Figure6(b))andrandomlyselectednodes.Inordertomakethecomparisonasfairaspossible,weusedtheexactsameparametersinourexperimentsasthosedescribedin[3].(a) Four flows (some alignment)(b) Randomly selected flowsFig.6.5x5gridtopologyusedtocompareperformancewith[3].Weobservethatusing16antennaelementsasopposedto8elementsmakesabigdifferenceinaggregatethroughput.Thisisbecausethebeamwidthwhenusing16elementsissmallerthanwhenusing8elementswhichresultsinmoresimultaneoustransmissions/slot.FortheowsinFigure6(a),(whenowsarealigned),wedidnotnoticemuchdifferenceintheperformanceof16and8antennaelementsbutforFigure6(b)andforrandomtopologieswedoseeasignicantdifference.Thereasonisthatwhenowsarenotaligned,thereisagreaterpotentialforspatialreusewith16antennaelements(duetoitssmallerbeamwidth).Wenotethatthe TABLEIIMeshFigure6(a)MeshFigure6(b)RandomMesh16Elements8Elements16Elements8Elements16Elements8Elements()()()()()()DOA-ALOHA2500kbps21503000200042503750[3](\n  )800kbps100010000200400600800100012001400160018002000050010001500200025003000Sending rate (Tx) vs Aggregate Throughput Sending Rate (Kbps) Aggregate Throughput (Kbps)8 elements(a) Fig.7.Performanceofourprotocolin6(a)02004006008001000120014001600180020000500100015002000250030003500Sending rate (Tx) vs Aggregate Throughput Sending Rate (Kbps) Aggregate Throughput (Kbps)8 elements(b) Fig.8.Performanceofourprotocolin6(b)beamwidthusedin[3]is.Inourcase,thelineararraycreatestwosymmetricbeamsandwedenebeamwidthforourprotocolasthesumofthesetwobeams.For16antennaelements,wenotedanaveragebeamwidthof foreachofthetwosymmetricbeamsformedwithalineararraythusgivingusaneffectivebeamwidthof (addingthetwobeams).Theeffectivebeamwidthwhenusing8antennaelementsisapproximately\r .TableIIsummarizesourresultsandcomparesthemwith[3].Weobservethatourprotocolis2x–3xbetterwhenweuse8elementsandismuchbetter(3x–4x)for16elements.VI.CONCLUSIONInthispaperwehavepresentedDOA-ALOHA,aversionofslottedALOHAthatusesDOAinformationatthereceivertobeamforminawaythatmaximizesSINR.Wecomparetheperformanceofourprotocolagainst[3]andshowthatourprotocolhasathroughputof2x–4xhigherthanthe[3].In0200400600800100012001400160018002000050010001500200025003000350040004500Sending rate (Tx) vs Aggregate Throughput Sending Rate (Kbps) Aggregate Throughput (Kbps)8 elementsFig.9.Performanceofourprotocolinrandomgridtopologies.thefutureworkwewillexaminetheperformanceof802.11whenusingsmartantennas.ACKNOWLEDGEMENTSWewouldliketothankOPNETfortheirtechnicalsupport.REFERENCES[1]J.YouA.Nasipuri,S.YeandR.Hiromoto.Amacprotocolformobileadhocnetworksusingdirectionalantennas.InIEEEWCNC,2000.[2]LichunBaoandJ.J.Garcia-Luna-Aceves.Transmissionschedulinginadhocnetworkswithdirectionalantennas.InACM/SIGMOBILEMobiCom2002,23–28Sep2002.[3]RomitRoyChoudhury,XueYang,RamRamanathan,andNitinH.Vaidya.Usingdirectionalantennasformediumaccesscontrolinadhocnetworks.InACM/SIGMOBILEMobiCom2002,23–28Sep2002.[4]ZhuochuanHuangandChien-ChungShen.Acomparisonstudyofomnidirectionalanddirectionalmacprotocolsforadhocnetworks.InIEEEGlobecom2002,2002.[5]J.C.LibertiandT.S.Rappaport.SmartAntennasforWirelessCommunications.PrenticeHall,1999.[6]RajivBagrodiaMineoTakai,JayMartinandAifengRen.Directionalvirtualcarriersensingfordirectionalantennasinmobileadhocnet-works.InACM/SIGMOBILEMobiHoc2002,Oct2002.[7]S.Roy,D.Saha,S.Bandyopadhyay,T.Ueda,andS.Tanaka.Anetwork-awaremacandroutingprotocolforeffectiveloadbalancinginadhocwirelessnetworkswithdirectionalantenna.InACMMobihoc'03,1–3June2003.[8]S.HorisawaS.Bandyopadhyay,K.HausikeandS.Tawara.Anadaptivemacanddirectionalroutingprotocolforadhocwirelessnetworksusingesparantenna.InACM/SIGMOBILEMobiHoc2001,Oct2001.[9]GentianJakllariThanasisKorakisandLeandrosTassiulas.Amacprotocolforfullexploitationofdirectionalantennasinad-hocwirelessnetworks.InACMMobihoc'03,1–3June2003.[10]JamesWardandJr.R.T.Compton.Improvingtheperformanceofaslottedalohapacketradionetworkwithanadaptivearray.IEEETransactionsonCommunications,40(2):292–300,February1992.[11]V.ShankarkumarY.B.KoandN.H.Vaidya.Mediumaccesscontrolprotocolsusingdirectionalantennasinadhocnetworks.InIEEEINFOCOM2000,March2000.[12]ChavalitSrisathapornphatZhuochuanHuang,Chien-ChungShen.Amacprotocolbasedondirectionalantennaandbusy-toneforadhocnetworks.InIEEEMILCOM2002,2002.