An Adaptive Beamforming Time With Round Robin MAC algorithm for reducing energy

17 Pages·2017·1.52 MB·English

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Article
An Adaptive Beamforming Time With Round Robin
MAC algorithm for reducing energy consumption in
MANET
VincenzoInzillo1,(cid:5),FlorianoDeRango1,•,AlfonsoArizaQuintana2,† andAmilcareF.
Santamaria1,(cid:63)
1,(cid:5) UniversityofCalabria,DIMES;[email protected]
1,• UniversityofCalabria,DIMES;[email protected]
1,(cid:63) UniversityofCalabria,DIMES;[email protected]
2,† UniversityofMalaga;[email protected]
VersionOctober24,2018submittedtoPreprints
Abstract: TheuseofSmartAntennaSystems(SAS)inpervasiveenvironmentssuchtheMobileAd
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hocNetworks(MANET)hasbeenpromotedasthebestchoicetoimproveSpatialDivisionMultiple
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Access(SDMA)andthroughput. Althoughdirectionalcommunicationsareexpectedtoprovidegreat
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advantagesintermsofnetworkperformance,directionalMAC(MediumAccessControl)protocols
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introduceseveralissues. Oneofthemostknownproblemsinthiscontextisrepresentedbythefact
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that, attemptingtosolveoratleastmitigatetheproblemsintroducedbythesekindsofantennas
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especiallyatMAClayer,alargeamountofenergyconsumptionisachieved;forexample,dueto
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excessiveretransmissionsintroducedbyveryfrequentlyissuesuchasdeafnessandhandoff. The
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expedientsproposedinordertoreducethesedrawbacksattemptingtolimitbeamformingtimeof
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nodesincooperationwithaRound-Robinscheduling,cangranthighperformanceintermsoffairness
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andthroughput. Howevertheoverallenergyconsumptioninthenetworkisnotefficentduetothe
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staticapproach. Inviewofthis, weproposeanAdaptiveBeamformingTimewithRound-Robin
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MACprovidingforadynamicassignmentofthebeamformingtimewiththepurposetolimitthe
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wasteofenergyofnodes.
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Keywords: SmartAntennaSystems,MANET,MAC,EnergyConsumption,Beamforming,Round
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Robin
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1. Introduction
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Inthelatestresearchstudies,relatingtowirelessnetworkenvironments,oneofthemostsignificant
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and,atthesametime,criticalissueisrepresentedbythemanagementofenergyconsumptionofnodes
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thatcouldhighlylimittheoverallnetworkperformancewithreferencetoprotocolsandapplication
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fields. Inthisregard,severalaspectsshouldbeconsideredinordertoaddresstheproblemsimpliedby
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mainfeaturesofthiskindsofnetworksthatsignificantlyaffectthebehavioursatphysical,Medium
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Access Control (MAC) and routing layers. For instance, let us consider Mobile Ad hoc Networks
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(MANET); they are self-organized networks in which mobile nodes can move independently; the
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nodesusuallymoveaccordingtoacertainmobilitypatternmodelandthemovementofanodeisnot
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necessarilyrelatedtothemovementofothernodesinthenetwork.Usually,inMANET,omnidirectional
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antennasareusedforcommunicationamongnodesbothfortransmissionaswellasforreception;
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thisapproachresultsinverylimitedperformancerelatingtophysical,linkandroutinglayerstatistics
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[1-3]. Omnidirectionalantennas,alsoknownasisotropicantennas,radiateandreceiveequallywell
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inalldirections. Mainadvantagesofomnidirectionalantennasinclude: easeofconfigurationand
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implementation,lowdesigningcost,veryandsimplearchitecture(hardware-less). Forinstance,in
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cellularsystems,theyallowtoamplifycellsignalsfrommultiplecarrierswithdifferentcelltowersin
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multiplelocations.Nevertheless,despitefromthesefewbenefitstheyintroduceaconsiderablenumber
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ofdrawbackssuchas: limitedrangeandcoverage(impliedbylowgain),highenergyconsumption,
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highinterferenceprobability(especiallyindensenetworks),veryhighperformancedependencyonthe
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environmentsinwhichtheyareemployed(indoororoutdoor);nonetheless,omnidirectionalantennas
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cannotexploitthebenefitsofcross-polarizationbecausetheyareverticallypolarized. Morespecifically,
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thislastissue,contributestoincreasetheprobabilityofinterferencebetweencommunicatingnodes
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inthechannel[4]. Fromatopologicalpointofview,thisapproachimpliesthat,thesignalgenerated
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fromthetransmitterT,reachdesireduserswithonlyasmallpercentageoftheoverallenergysentout
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intotheenvironment. Duetothishugenumberoflimitations,omnidirectionalantennasmaynotbe
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efficientduetointerferencecausedbythetransmissionofpacketsinalldirections(otherthantarget
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direction)andlimitedrangeofcommunications.
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Figure1.Interferencecausedbyomnidirectionalantennas.
The Figure 1 illustrates a wireless network scenario in which nodes use omnidirectional
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antennas to perform communications. In particular, the transmitter node T sends to the receiver
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Racommunicationsignalbyusinganomnidirectionalantenna;Rattemptstocapturethesignalwith
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thesameantennamodel. Becausethetransmittersignalisradiatedinalldirectionswiththesame
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intensity,iftherearesuchnodesintheneighboringofthetransmitter/receiver(N andN )ispossible
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thattheradiatedsignaliscapturedbythesenodesthat, inturn, mayattemptacommunicationat
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thesametime. Inthiscase,interferencesandcollisionscanoccur;theseissuescouldenhanceasthe
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mobilityofnodesincreases. Nevertheless,inthiscase,becausenodesradiateinthesamewaytoward
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alldirections,ahugewasteofthebatterylifeofnodesiscertainlyachieved. Themajorityofthese
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issuescouldbepartiallymitigatedusingdirectionalantennas. Directionalantennasmaybeuseful
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toincreasenetworkefficiencybydirectingthetransmittedpowerinthedesired/intendeddirection.
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Directionalantennashaveagreatnumberofadvantagesoveromnidirectionalantennasinadhoc
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networking. Byfocusingenergyonlyintheintendeddirection,directionalantennascanincreasethe
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potentialforspatialreuseandcanprovidelongertransmissionandreceptionrangesforthesame
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amount of power. Increased spatial reuse and longer range translates into higher ad hoc network
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capacity(moresimultaneoustransmissionsandfewerhops),andlongerrangealsoprovidesimproved
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connectivity[5-7]. Differentkindsofissueshavetobeinvestigatedwhendirectionalcommunications
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occurwithrespecttothetraditionalomnidirectionalcase;problemssuchasthehiddenterminaland
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thedeafnessproblemhavetobeproperlyhandledaswellashandoffissueimpliedbymobilityof
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nodes.
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(a) Deafnessproblem (b) Handoffproblem
Figure2.DirectionalantennascommonissuesinMANET
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ReferringtodirectionalMACcommunicationsinwhichDirectionalRequesttoSend(DRTS)and
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Directional Clear to Send (DCTS) are used to perform a transmission/reception flow, a particular
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node (deaf node) that is engaged in a certain communication, but at the same time is solicited as
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receiverbyanothersourcenodearisesthedeafnessproblem(Fig. 2(a)). Thenodethatexperiencesthe
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deafness(thenodeAinFig. 2(a))couldtrytoretransmitmanytimesMAClayerpackets,resulting
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in a large amount of collisions and considerable increase of the network overhead. Furthermore,
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due to the recurring retransmission attempts from deaf node, a large waste of energy could take
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place, andconsequentlythisnodeconsumesitsbatterylifeinaveryshorttime, highlydegrading
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theoverallthroughputofthenetwork. Anothercommonissuewhileusingdirectionalantennasis
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given by the handoff problem that is usually implied by the mobility of nodes in the network. In
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Fig. 2(b)anexampleofhandoffisillustrated: thetransmitternodeTiscommunicatingwithanode
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R;duringthecommunicationRmovesinthepositionR’andexitoutofthetransmitterbeamand
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consequentlythecommunicationfailsandthebeamsneedtobere-pointed. Inthiscase,ifthenode
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in the position R’ can still be reached by T through a beam switching. If a proper mechanism of
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synchronizationandnodepositionrefreshingisnotprovided,thedirectionalbeamremainstunedfor
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alongtimeinanundesireddirectionduetonodemovement;forthisreason,again,alotofenergy
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consumptionoccurs. Mostresearchers,inordertomitigatetheseundesiredphenomenaindirectional
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contexts,demonstratedthat,throughtheemploymentofSmartAntennaSystems(SAS)insteadofthe
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moretraditionaldirectionalantennas,itispossibletocreateanefficientsystem,exploitingtheSpatial
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DivisionMultiplexing(SDMA)techniquethatthiskindofdeviceswellprovided. UsingSAS,higher
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spatialreuseandbetterlinkreliabilitycanbeachieved[8-9]. IncontextsinwhichdirectionalSmart
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AntennaSystemsareused,thebeamformingissuehavetobedeeplyinvestigated.TheuseofDRTSand
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DCTSframesinassociationwithaDirectionalNetworkAllocatorVector(DNAV)information,helps
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todecreasethelargeamountofcollisionsthatusuallyoccurswhenusingomnidirectionalantennas,
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butinenvironmentsinwhichSASareemployeditmightnotbeenoughtoprovidetheseexpedients
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[10-12]. Inthepresentpaper,weproposetoreducetheseissuesbyenhancingtheworksproposedin
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[13-14]usingSASalongwithaRound-Robinschedulinginordertoaddressmoredetailedchallenges
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such as the queue and the time slice problem involved by the use of the Round-Robin. The main
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purposeoftheworkistolimitthemassiveenergyconsumptioninthenetworkand,simultaneously
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toimprovetheoverallperformancewhenDirectionalMACprotocolsareexecutedinmedium-high
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mobilityenvironments.
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2. StateofArt
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RelatingtoMAClayercommunications, themostcausesofexcessiveenergyconsumptionin
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mobilenetworkscenariosincludetheuseofomnidirectionalantennas,dataprocessing,highprotocol
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overhead,highlevelofinterferencesinthechannel. DataprocessingimpliesthelargeusageofCentral
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ProcessingUnit(CPU),memory,harddrive,etc. Topartiallysolvethisissuethemostactualsolution
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istofindatradeoffonenergyconsumptionbetweendataprocessingandradiocommunication[16].
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Forexample,datacompressiontechniquesareintroducedin[15]tominimizepacketlengthandsoto
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obtainanenergysavinginradiocommunication,butthecostofcomputationisincreased. Inwork
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[17]authorsalsohighlightthelargeprotocoloverheadintroducedbythiskindofsystems. Generally,
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tomakeaMACprotocolenergyefficient,atleastoneofthefollowingguidelinesareused:
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• Reducingcollisionsandretransmissions:OneofthemostcommonobjectiveofMACprotocols
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inordertoavoidcollisionssothattwointerferingnodesdonottransmitatthesametime. The
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simplestwaysforcollisionavoidanceinageneralnetworkincludecodedivisionmultipleaccess
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(CDMA),timedivisionmultipleaccess(TDMA),andfrequencydivisionmultipleaccess(FDMA).
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Sincecollisionavoidancemaytranslateintoasubstantialoverhead,whichwillburnmoreenergy,
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tradeoffsmustbeexploredtoachievereasonablesolution.
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• Reducing overhearing: Wireless mobile nodes deplet battery life because they overhear the
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transmissionsoftheirneighbors. Therefore,themobilenodesreceiveallpacketsthathittheir
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receivers. One possible solution to this problem is the introduction of a control channel for
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the transmission of control signals that will wake up the nodes only when needed. Authors
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in[18]proposetobroadcastaschedulethatcontainsthedatatransmissionstartingtimesfor
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eachmobilenodes. Inwork[19]twoschemesareproposedinordertomitigatethedeafness
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causedbypersistenthearingofdataandforhandlingtheShortRetryLimit(SRL)indirectional
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environments.
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• Minimize control overhead: Protocol overhead should be reduced as much as possible,
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especiallyfortransmittingshortpackets[20-22]. Duetothelargechannelacquisitionoverhead,
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smallpacketshavedisproportionatelyhighenergycosts. Whenmobilenodesrequestmultiple
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transmissionslotswithasinglereservationpacket,thecontroloverheadforreservationcanbe
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reduced[23]. ApacketdeliveryschedulingalgorithmandtwoMACprotocolsinwhichnodes
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usesdirectionalsectorizedantennasareproposedin[24]. Theschedulerandtheprotocolsare
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designedwiththepurposetopreventtheco-siteinterferenceproblemthatcouldariseinsome
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directionalcontextsalsobylimitingtheoveralloverheadinthenetwork.
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• Reducingbeamformingtime: In[25]authors,proposeaToneDMACmechanismthatenable
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thetransmissionofspecialpackets(Out-ofbandtones)bynodesinomnidirectionalmode;these
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tonescanbeprocessedbyneighborsreducingconsiderablythelargebackofftimeintroducedby
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deafness. In [14] a sectorized antenna model based on a round-robin scheduling algorithm
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is presented in order to reduce the impact of the deafness in directional communication
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environments. TheRound-Robinmechanismwasimplementedbyanalgorithm,thatmanages
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theassignmentofthebeamtowardacertainsectoralsohandlingtheincomingframesthatcould
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nottemporallybetransmittedinthechannel(incasetheyareoutsidefromthecurrentactive
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sector)byusingwaitqueues.
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Figure3.Round-RobinMACprinciple.
TheFigure3illustratestheRound-RobinMAC(RR-MAC)principle. TheplaneisdividedintoN
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equalsectors,eachonehavingacertainamplitudew(sectorwidth)andabeamformingduration
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time T (sectortime)withi = 1...N; N isthenumberofsectorsinwhichtheplaneisdivided.
138 i
Notethatallsectorshavethesamewidthaswellasthesamesectortime;eachnodethatbelongs
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toacertainsectorbeamformswithanangleα untilthesectortimeisreached,thenitswitchesto
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thenextsector. Thesectorinwhichthebeamiscurrentlyactiveisdefinedcurrentactivesector.
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2.1. MobilityissuesdirectionalMACworks
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Otherapproaches,suchasthework[26],attempttoreducethehandoffissuethroughanefficient
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beamsystemcontrol; asimilarworkispresentedin[27]; inthispaper, authorstrytomitigatethe
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handoffproblembyproposingapredictivelocationmodelinordertoadvancethefutureposition
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movements of the nodes. Nevertheless, these works are suitable for Vehicular ad Hoc Network
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(VANET)environmentsanddonotproperlyaddressthedeafnessproblemwhichisveryrelevantin
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MANET.In[28]weintroduceapredictivelocationmechanismthatalsoprovidesforaframescheduler
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withpriorityinordertoreducethehandoffproblem. However,inthiscase,theenergyconsumption
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ofnodesisnotconsidered.
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3. OmnidirectionalandDirectionalantennasvsRound-RobinMAC
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As largely mentioned in the previous sections, networks containing nodes equipped with
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omnidirectionalantennasarepronetohighwasteofenergybecausetheshapeoftheradiationpattern
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thatdoesnotbeamformonlyaspecificdirection. Althoughdifferentproposalsthereexistwiththe
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aimtosolveoratleast,mitigatetheissuesderivedfromtheuseofomnidirectionalanddirectional
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antennasatMAClayer,amoredeeplyanalysisisrequiredforhighlightingtheissuesthatneedtobe
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addressed. Forexample,letusconsiderthework[14],thatattemptstomitigatetheoverallnetwork
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energyconsumptionusingaSASantennamodulealongwithaRound-Robinscheduleralgorithm.
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Basically,itworksfineundertwomainassumptions:
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Assumption1. nodesareequippedwithhigh-efficienthardwareantennas(SAS).
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Assumption2. datatrafficandnodesareuniformlydistributedamongsectors.
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Assumption3. thesizeofthesectorqueuesarefixedandequalsforallsectors.
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Inordertoevaluatetheimplicationsofthefirstassumptionitcanbeusefultoanalyzethebehavior
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oftheRR-MACincasetheomnidirectionalantennaisusedinplaceofdirectionalorSAStechnologies.
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Forthispurpose,weevaluate,throughtheuseoftheOmnet++Networksimulator[29],theenergy
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consumptionofnodesofthreedifferentrunconfigurationsusingthesamesimulationparameters
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describedin[14](themostofthethemareillustratedintable1insubsection5.2ofthepresentpaper);
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inordertoaccomplishtheanalysis,energysimulationmoduleshavebeeninsertedintoeachmobile
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nodeforallowingtheemulationofthebatterylifetimebehavior. Theinitialenergyvalueofeachnode
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wassetto300J,theshutdownenergyvaluewassetto0J;inthisway, anodeshutdownswhena
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nodecompletelydepletesitsbatterylife. Inthefirstconfigurationrunnodesareequippedwiththe
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classicalomnidirectionalantenna;inthesecond,nodesusetheomnidirectionalantennatogetherwith
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theRR-MACscheduling;thelatestrunconfigurationissimilartothesecondexceptfromthefactthat
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inthiscasetheantennaistheSASmoduledesignedin[30].
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Figure4.Residualcapacityprogression.
TheFigure4depictstheresidualcapacity(averagedbyallnodes)plotcomparisonbetweenthe
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threeconsideredcases. Aswecanexpect,whenusingtheomnidirectionalantennawithoutanyenergy
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savingmechanism,nodesdeplettheirbatterylifeveryquickly;whentheomnidirectionalantennas
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areusedincooperationwithRR-MACtheaveragedepletiontimeisalmostdoubled(t=190s)and
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improvessignificantlywhenSASareemployedbynodes. ThisismainlyduetothefactthatRR-MAC
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limitsthebeamformingtimeforeachsectortranslatingintoareductionofthenumbercollisionsand
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interferences. Then,weinvestigateabouttheassumptions2and3. Inparticular,theseassumptions
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implythatthesystemcouldbeaffectedfromthequeuesizeandthewaitingqueuetimeproblems[31]
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thatcanaffecttheoverallnetworkconsumptioninthenetwork. Weanalyzetheseissuesbycreating
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twomorerunconfigurations: inthefirstonemobilenodesareuniformlydistributedinthenetwork
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scenario;alsotheactivitydegreeofthenodesisuniformlydistributedamongsectors;inthesecond,
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instead,nodesareperiodicallyconcentratedinacertainsector(randomlychosen)ofthesectorized
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planeandtheactivitydegreeofnodesisunbalancedamongsectors. Thenumberofsectorsinthe
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planevariesfrom3to8whilethequeuesizeisevaluatedbasedonthefollowingequation:
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Queuesize = Q×[N+w (N )] w ∈W (1)
Q s Q Q
ThequeuesizeisfunctionofthenumberofnodesNandthenumberofsectorsN intheplane.
189 s
Qisavaluechosenintheinterval0.1 ≤ Q ≤ 0.25inordertomaximizethePacketDeliveryRatio
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performanceof[14]. Thenumberofsectorsisweightedbythetermw thatvariesfrom1to6asthe
191 Q
numberofsectorvalueincreases. Inparticular,giventhenumberofsectorarrayS = [3,4,5,6,7,8],the
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weightvectorW isexpressedby:
193 Q
W = [1,2,3,4,5,6] (2)
Q
Inordertoassesstheimpactofthetwosimulationscenariosrelatingtothequeueissues, we
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comparethewaitingqueuetimeofthetworunningconfigurationsinfunctionofthenumberofsectors
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andqueuesize.
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(a) Averagewaitingqueuetimevsnumberofsectors (b) AveragewaitingqueuetimevsQueuesize
Figure5.QueueissueswithRound-Robinscheduling.
The Figure 5 illustrates the average waiting queue time in function of the number of sectors.
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ThewaitingqueuetimeisaveragedbyvaryingtheQparameterfrom0.1to0.25. InFig. 3itcanbe
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observedthatthewaitingqueuetimeinthecaseofnotuniformdistributednodesremainshigherthan
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theuniformcaseindependentlyfromthenumberofsectorvalue;inparticular,thegapbetweenthe
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waitingqueuetimeofthetwoconsideredcasesseekstogrowforsectornumbervalueshigherthan
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5. ThecurvesinFig. 4areplottedinfunctionoftheQparameterthatrepresentsaproperlyindex
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ofthequeuesize;asitcanbededucedfromeq. 1,thehigheristheQvalue,thehigheristhequeue
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size. AsithappensinFig. 3thewaitingqueuetimeinthecaseofnotuniformtrafficliesabovethe
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uniformcasecurveindependentlyfromthequeuesize. However,asopposedtoFig.3,thedifference
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betweencurveshaskeptalmostconstantasthequeuesizeincreases. Intheuniformtrafficcaseitcan
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benotedhowthewaitingtimeslightlydecreasesforthehighestvaluesofQwhile,inthenotuniform
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case,thedecreasecorrespondingtothesamevaluesiscloselynegligible. Insummary,asthesizeof
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thequeuegrowsupthewaitingtimeinthenotuniformcasedoesnottendtogetsmaller,orrather,
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thedecreasingrelatedtohighqueuesizevaluesisnotsignificant. ThetrendsevaluatedinFig. 5are
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certainlyjustifiedbyoneofthemostcommonissuesimpliedbyaRound-Robinalgorithmscheduling:
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thetimesliceproblem[32-33];basically,thetimeslicerepresentsthequantumassignedtoeachsector
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(thesectortime)inequalportions;therefore,thecommunicationsarehandledinacircularwayamong
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sectorswithoutpriority. Incaseofnotuniformdistributedtrafficifmostofmobilenodesarefocused
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inaspecificsectoroftheplane,thecommunicationsrelatedtothatnodesareenqueueduntilthebeam
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isallowedinthespecificsector;asconsequence,thelargestisthequantumassignedtoeachsectorthe
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largestisthewaitingqueuetime. Inthesameway,asobservedin[14]ifthequantumassignedtoeach
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sectoristoolow,thesystemwillprovidebadperformanceintermsofoverallthroughput;asimilar
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behaviorisproducedifthequantumassignedtoeachsectorisaveragedinaspecificintervalwith
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increasingthenumberofsectorswhichresultsinlargestwaitingqueuetimevaluesasseeninFig. 3.
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4. Proposedmodel
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4.1. ComparisonwithRR-MACandmotivations
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Asexplainedinthesection3themainchallengesrelatedtotheRRapproacharereferredtothe
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sizeofthequeuesandthedelayproducedbystatictimesliceswhichcannotbemodeledinfunction
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ofthetrafficinthenetwork. However,asobservedin[13,14]thequantumoftimeassignedforthe
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beamformingprocessisthesameforeachsectoraswellastheamplitudeofthewidth. Therefore,
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inthework[14]emergesthatapropersetofthesectortimevalueaffectstheoverallperformance
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morethanthesectorwidthchoice;thisimpliesthattimeslotassignedtosectorsneedstobecarefully
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assignedinordertoimprovethesystemefficiency. Forthesereasonsweproposeanapproachthat
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modifiestheoriginalRound-Robinalgorithmformulationrelativetotheevaluationofthesectortime
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whilekeepingunchangedthesectorizationoftheplaneandthusthewidthofthesectors. Inorderto
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understandthemodificationsintroducedwithrespecttotheRound-RobinMACalgorithmwebriefly
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recallthemathematicalformulationofthislatter:
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α = α =....= α
i j
T = T =....= T
i j s
(3)
α (T) = α (T) =....= α(T)
i i j j
∀i,j =1,2..N, i (cid:54)= j
FromformulationgivenbyEq. 3itiseasytoobservethatallsectorshavethesamewidthand
234
thesamesectortimeT . TheuseofthisapproachhelpstoenhancetheMACperformanceintermsof
235 s
reductionofcollisionsandfairnessimproving,resultinginaoveralldecreaseofthetotalamountof
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energywastedbynodes. Unfortunately,RR-MACisastaticmodelanddoesnotadaptitselftothe
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trafficchannelconditions;thiscouldrepresentalimitinscenariosinwhichnodesareconcentrated
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inacertainsectorasverifiedintheprevioussection. Thegoalofthepresentworkistoimprovethe
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efficiencyoftheRound-Robinschedulerintermsofenergymanagementbyprovidinganadaptive
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beamforming time for each sector that takes into account the size of the waiting queue for each
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sector. WedenotedthisnovelapproachasAdaptiveBeamformingTimewithRound-RobinMAC
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(ABT-RRMAC).Tofigureouttherelevanceofusinganadaptivesectortimeassignmentletusconsider
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thesituationdiscussedinsec. 3inwhichtwodifferentmobilenodestrafficdistribution(uniformand
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notuniform)inthenetworkarecompared:
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(a) Uniformnodedistribution (b) Notuniformnodedistribution
Figure6.UniformnodedistributionvsnotuniformusingRound-Robinapproach.
The Fig. 6(a) represents a network scenario in which mobile nodes are almost uniformly
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distributedamongsectorsofthesectorizedplane;thecommunicationsareruledbytheRound-Robin
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scheduling;therefore,thecurrentactivesectoristhesector2,thenthebeamisactiveinthatsector. As
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noticedfromtheanalysisaccomplishedinsec. 3,assumingauniformactivitydegreeofnodes,inthis
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case,thewaitingqueuetimeisacceptableaswellastheMAClayerperformance;inFig. 9bitcanbe
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observedthattheoverallnodedistributionisunbalancedamongsectorsaswellastheactivitydegree
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ofnodes. Morespecifically,mostofthemobilenodesaremainlydistributedinthesector1which,
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asassumption,hasalsothehighestactivitydegreeamongsectors;supposingthatthecurrentactive
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sectorisagainthesector2andthatsectorcontainsaverysmallnumberofnodeswithrespecttothe
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sector1togetherwithaverylowburdenofactivecommunications,itmayoccuranunder-useofthe
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allocatedresourcesinthatsector;inparticular,duetothestaticsectortimereservationassignment
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provided by the Round-Robin algorithm (the assigned sector time is the same for all sectors), the
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overallbeamformingprocessbecomesinefficientresultinginaconsiderabledecreasingoftheMAC
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performanceespeciallyintermsofthroughputandenergyconsumption. Indeed,inFig. 6bitiseasy
259
toobservethat,duetounbalanceddispositionofnodesamongsectors,theenergyconsumptionin
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sectorsS ,S ,S ,S ,S ,S (representingthe80%oftheoverallplane)isquiteinefficientbecausethe
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beam,inthatsectorsisperformedforatimeT thatismuchhigherthantherequiredquantumoftime
262 s
requiredforemptyingthequeues. Importanttohighlightthat,becauseweareanalyzingtheworst
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conditioncase,weassumethatthescenariosillustratedinFig. 6(b)remainunchangedforalongtime
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orchangeveryslowly.
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4.2. ABT-RRMACformulation
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InlightofthepreviousconsiderationsourABT-RRMACapproachmodifiestheoriginalRound
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Robinalgorithmformulationregardingtheevaluationofthesectortimewithoutaffectingtheplane
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sectorizationandthusthewidthofthesectors. Basically,theideaistoassignaportionoftimeforeach
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sectorthatisproportionaltothesizeofthequeues,thatinthiscaseisnotfixedforeachsectorandcan
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varydynamically.
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Figure7.AdaptiveBeamformingTimeRR-MACexample.
InFigure7,anexampleoftheABT-RRMACapplicationisshown.Theplaneisnormallysectorized
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intoequalsamplitudesectorsaswellastheRoundRobinMAC.However,inthiscase,weconsider
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trafficpatternthatisnotuniformlydistributedamongnodes;inFig.7,itisassumedthat,twoparticular
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sectorsdenotedS andS respectivelyhavedifferentsizeoftheframewaitingqueues;inthisregard,
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thetermsize denotesthesizeofthewaitingqueuerelatedtothei-thsector. Whileconsideringthe
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situationinFigure7,assumingthatT andT arethebeamforming(sector)timesrelatedtothesector
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xandyrespectively,theABT-RRMACassignsthebeamformingtimesasfollows:
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 T1 = ∑iN=si1zseiqz.i.e.qi ×T (cid:54)= Ts
⇒ T (cid:54)= T (cid:54)=..(cid:54)= T ⇐⇒ size (cid:54)= size (cid:54)=..(cid:54)= size
 TN = ∑iNs=.i.z1.esqiNzeqi ×T 1 2 N q1 q2 qN
Where:
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T = ∑N Ti (4)
N
i=1
ThetermTdenotesthemeanbeamformingtimeaveragedbyallsectors.Initially,thebeamforming
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timesarethesameforallsectorsandsettoT (RR-MAC);afteratrainingphasesetto10s(thetime
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of convergence of the RR-MAC), the mean beamforming time and then sector times are updated
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periodicallyinorderthatforeachsectorisassignedaquantumoftimethatisproportionaltothesize
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ofthequeueofthatsectormultipliedbythemeanbeamformingtime. Inthisway,sectortimescanbe
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differentandunbalancedamongthemandthemajorfractionoftimeisassignedtothesectorhaving
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thebiggestqueuesize. Forinstance,ifweconsidertheexampleofFigure7,becausesize >size
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thentheABT-RRMACwillassignsectortimessuchthat T > T . Themotivationofthischoiceis
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duetofactthatsize >size theservingqueuerateofS islowerthantheservingqueuerateofS ;
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consequently,S needsforabeamformingtimehigherthanS inordertoemptyitsqueue. Among
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otherthings,thischoicewilloptimizetheenergyconsumptionofnodesinthenetwork;infact,the
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undesiredwasteofenergycausedbytheclassicRound-Robinduetothestaticmodelislimitedbythe
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dynamicbeamformingtimeassignment.
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4.3. ABT-RRMACimplementation
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The ABT-RRMAC algorithm is implemented in the Omnet++ Network Simulator in the
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DcfUpperMACmodule,thatisthemainclassinwhichthemostimportantoperationsatMAClayerare
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provided,suchasframeandcollisionsmanagement;therefore,asexplainedin[14]thesectorizationof
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theplaneismanagedbytheSASantennamodule(PhasedArraymodule). Thefollowingpseudo-code
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enhancestheoriginalRound-RobinMACformulation:
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Algorithm1ABT-RRMACpseudo-code(part1)
1: procedureINIT(numSectors)
2: numQueues←numSectors
3: CREATEQUEUES(numQueues)
4: endprocedure
1: procedureASSIGNSECTORTIME(trainingPeriod,updatePeriod,numQueues)
2: ifSim.Time()<trainingPeriod||Sim.Time()<updatePeriodthen
3: averageSectorsTime=0;
4: fori=1;i<numQueues;i++do
5: sectorTime[i]=Ts;
6: endfor
7: else
8: averageSectorsTime=computeAverageSectorsTime()
9: fori=1;i<numQueues;i++do
10: sectorTime[i]= sizeqi ×averageSectorsTime;
11: endfor ∑iN=1sizeqi
12: endif
13: endprocedure
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 24 October 2018 doi:10.20944/preprints201810.0547.v1
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Algorithm2ABT-RRMACpseudo-code(part2)
14: procedureSTARTTRANSMIT(frame)
15: intframeSector=getFrameSector(frame);
16: intcurrentActiveSector=getCurrentActiveSector();
17: ifcurrentActiveSector!=frameSectorthen
18: queueSector[frameSector].insert(frame)//queuetheframe
19: else
20: transmissionQueue.insert(frame);
21: transmitFrame←queueSector[activeSector].front()
22: queueSector[activeSector].pop()
23: CONFIGUREANTENNA(activeSector)
24: endif
25: endprocedure
1: procedureSTARTRECEPTIONSTATE
2: CONFIGUREANTENNA(omnidirectional)
3: MACinreceptionMode
4: SCHEDULEEVENT(CSMATimer)
5: endprocedure
1: procedureRECEPTIONFRAME(frame)
2: orientation←GETORIENTATION(frame)
3: sector←GETSECTOR(orientation)
4: CONFIGUREANTENNA(sector)
5: endprocedure
1: procedureRECEIVEFRAMEFROMUPPERLAYERS(frame)
2: sector←GETSECTORFRAME(frame)
3: queueSector[sector].push_back(frame)
4: endprocedure
1: procedureMACPROCCESS
2: INIT(NumSectors)
3: STARTRECEPTIONSTATE
4: loop
5: WaitEvent
6: ifEventIsUpperFramethen
7: RECEIVEFRAMEFROMUPPERLAYERS(frame)
8: elseifEventIsLowerFramethen
9: RECEPTIONFRAME(frame)
10: elseifEndTransmissionthen
11: STARTRECEPTIONSTATE
12: elseifEndCSMA∧QueueSector(cid:54)=empty then
13: STARTTRANSMIT(frame)
14: else
15: STARTRECEPTIONSTATE
16: endif
17: endloop
18: endprocedure
Atthebeginningoftheprocess,aftertheplaneissectorizedintoequalwidthsectors,thesame
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quantum of time is assigned for all sectors (Round-Robin). If the trainingPeriod has elapsed the
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averageSectorsTimedenotingthemeanbeamformingtimeaveragedbyallsectors,canbecomputed;
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rememberthatthisparameterisupdatedperiodicallyafteranupdatePeriod(thatwesetto10s)has
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passed;fromthefirsttimethattheaverageSectorsTimeiscomputed,thebeaformingtimesofthesectors
303
areassignedaccordingtoexpressiongivenbyEquation4. Thewholeoftheseoperationsareincluded
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inAssignSectorTimefunction. ThetransmissionofframesisruledbyStartTransmitfunctioninwhich
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thesystemchecksifthesectorforwhichisdestinedthecurrentframeisthecurrentactivesector. Iftrue,
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itinsertstheframeinthetransmissionqueueandtransmittheframe,otherwisetheframeisqueuedin
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itswaitingsectorqueue,anddelayeduntilitssectordoesnotbecomethecurrentactivesector. The
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