Efficient Medium Access Arbitration Among Interfering WBANs Using Latin Rectangles

1.9 MB·

by  Mohamad Jaafar Ali

ffi
E cient Medium Access Arbitration Among Interfering WBANs Using Latin Rectangles
MohamadJaafarAlia,HassineMounglaa,b,MohamedYounisc,AhmedMehaouaa
aLIPADE,UniversityofParisDescartes,SorbonneParisCite´,Paris,Frane
bUMR5157,CNRS,InstituteMines-Telecom,Te´le´comSudParis,Nano-InnovCEASaclay,France
cDepartmentofComputerScienceandElectricalEngineering,UniversityofMaryland,BaltimoreCounty,UnitedStates
Abstract
The overlap of transmission ranges among multiple Wireless Body Area Networks (WBANs) is referred to as coexistence. The
interferenceismostlikelytoaffectthecommunicationlinksanddegradetheperformancewhensensorsofdifferentWBANssimul-
taneouslytransmitusingthesamechannel. Inthispaper,weproposeadistributedapproachthatadaptstothesizeofthenetwork,
7i.e.,thenumberofcoexistingWBANs,andtothedensityofsensorsformingeachindividualWBANinordertominimizetheimpact
1
ofco-channelinterferencethroughdynamicchannelhoppingbasedonLatinrectangles. Furthermore,theproposedapproachopts
0
to reduce the overhead resulting from channel hopping, and lowers the transmission delay, and saves the power resource at both
2
sensor-andWBAN-levels. Specifically,weproposetwoschemesforchannelallocationandmediumaccessschedulingtodiminish
n
the probability of inter-WBAN interference. The first scheme, namely, Distributed Interference Avoidance using Latin rectangles
a
J(DAIL), assigns channel and time-slot combination that reduces the probability of medium access collision. DAIL suits crowded
areas,e.g.,highdensityofcoexistingWBANs,andinvolvesoverheadduetofrequTehins atrtciclhe haans bneeenl achceoptpedp foirn ingcluasitont ihn ea fuWturBe isAsuNe of cthois ojorurdnailn. Caotnotenrt ias nfindal as presented, with the exception of pagination.
7
MOVASSAGHIetal.:WIRELESSBODYAREANETWORKS:ASURVEY 7
2sensors. Thesecondscheme,namely, CHIM,takesadvantageoftherelativelylowerdensityofcollocatedWBANstosavepower
byhoppingamongchannelsonlywheninterferenceisdetectedattheleveloftheindividualnodes. WCOeMPpARrISeOsNOeFnONtE-aHOnPSaTAnRTNaAEBTlWLyEOtRIiIKIcANaDlMmULTIo-HdOPeNElTWORK[40]
]that derives the collision probability and network throughput. The performance of DComApaIrisLoncriaterniad CHSItaMrNetwoirsks further validated throMuultig-HohpNetworks
NIsimulations. EnergyConsumption Ftwinroaiflrnolsrnmmcooaidtnteitsoosinstti.henentcPllyDosAreewqpuirliolrxebiemmliotowyre.tTophoethwneeordPeDtsoAfu,bretthheearbaplewowatyoe,rhtuoraswneedsvmetirot, Ttihnhefoeyrnmwoaidltleiohsnatvhfearottmoarfeootrchwleoarsrednsontdoteotso.thnelyPthDeAircoownnsuinmfeormmoartieonenbeurgtyalsaos
Thestarnetworkpresentstheleastpossibledelaypresentin Dependentonhowthenetworkisconfigured.Intermsofdelay,
[cs.Keywords: WBAN,Interference,ChannelHopping,TDMA,LatinSquare TINnrotaednresfemFreiasnislcuioerneaDnedlaMyobility thSwOcraoeaninpntnlh.syspomhetrhirisgsfeoshthirfeomaarnitlpeafaodrrsoewnmnefoeared,artdenihneyewdcrsr.ieelalnawssbaioenyrgatfofrtfohetmehcetaetmPdhDeoaAnuPdnD,tatAhoseftrheirenqertsueetirriofesefrotetrnhanlencysenma.esitswinsoigorlkne tqSeoTrhenfuihceneieioccrnngnkeptoyfileaydergr,ofaetuefcwsrrohteeircftdnalhn.cotnoohessOude.emtevsniteasierntsshtoiyweooatnoinhndrliektsysePrakttmhrrDeeaapeAnttdisniilmnavoctvaowietnoltvliarvnegenedgledas.tythtto.hheneeiticsrfeainimnleefiiodtgirhgmnbaoatoedtriseontnhhoeadtsheersfto,foeutcgbhthees

powerisrequired.Throughtheuseofrelays,thetransmission
11. Introduction heatwillbedistributedandaconvenientheatwillbeobtained
forthesurroundingareaofthetransmittingsensor.Asper
v thelatestversionoftheIEEEstandardproposedforWBANs
91.1. WBANsOverview iWnBFAebNrusatraynd2a0r1d2sc[3o]m,polnialynttcwoomhmoupnsicaarteiosnu.pPproorpterdietianryIEsyEsE-
5 The recent technological advances in wireless communica- tweomusldcobueldapursoebmleomre,atshathnetywwoohuolpdsn,obtubtethsetannidnaterdr--ocpoemrapbliialintty.
0
tion, microelectronics have enabled the development of low- D.CommunicationArchitectureofWBANs
8 ThecommunicationarchitectureofWBANscanbesepa-
0power, intelligent, miniaturized sensor nodes to be implanted Coordinator ratedintothreedifferenttiersasfollows:
Tier-1:Intra-WBANcommunication
.inorattachedtothehumanbodies. Inter-networkingtheseex- • Tier-2:Inter-WBANcommunication
1 • Tier-3:Beyond-WBANcommunication
citing and evolving devices is referred to as a WBAN and is Fig•.2illustratesthesecommunicationtiersinanefficient,
0 component-basedsystemforWBANs.InFig.2,thedevices
7revolutionizing remote health monitoring and telemedicine in are scattered all over the body in a centralized network
architecturewheretheexactlocationofadeviceisapplication-
1general. Inessence,aWBAN isawirelessshort-rangecommu- specific[41].However,asthebodymaybeinmotion(e.g.
running,walking)theidealbodylocationofsensornodesis
:nicationnetworkconsistingofasinglecoordinatorandafinite notalwaysthesame;hence,WBANsarenotregardedasbeing
v static[8].
inumber of low-power wireless sensor devices. These sensors netTwieorr-k1:inItenrtarac-tiWonBAofNnocdoemsmaunndicthaetiiornre–speTciteivr-e1trdaenpsimctisssitohne
Xenable continual monitoring of the physiological state of the FigurFieg.12.:CComommunimcatiuonnTiiecrsaitniaoWniraelerscshBoidtyeAcrteuaNreetwoorfkWBANsr2aniglleusstr(a∼tes2WmBetAerNs)cinomamndunaicroautinodntwheithhinumaanWbBoAdyN.Faingd.
rbodyinstationaryormobilityscenarios.AWBANalsoincludes blood sugar, blood pressure, body temperature, oxbseyentwsgoereesnnatrheesuWsaedBtuAtoN-foarnwdaritdsbmoudlytispilgentailesrst.oIanPTeiersro-1n,avlaSreiarvbeler
a ration,CO2conupcoentnhetrchaoticieoonft,heeontec-h.opTorhtheetwcoo-hlolpetocptoelodgy.dWahetnac(PaS)n,lotchateedninbTeier-1.Theprocessedphysiologicaldatais
a coordinator that collects the measurements of the individual allnodesinthenetworkaredirectlyconnectedtothesink, thentransmittedtoanaccesspointinTier-2.
forwardedtoahthoenseptwiotraklisfcoonrsimdereeddtoichaavleaasonsee-hsospmstaretnopto.logy. Tier-2:Inter-WBANcommunication–Thiscommunication
sensorsandsharesthemwitharemotemonitoringstation(com- InaWBAN,thecoordinatorisknownasthesinknodeto tierisbetweenthePSandoneormoreaccesspoints(APs).
mandcanter)[1]. WBANs are bwnohedicechsoaarmlelcnioonndnegesctteaidlnk.tocHraocewcaeesvsserip,noiinngtaslyvmiauplotit-ehheorrpvnaaordcsehsii.vteTceatub;rleethTeveheeinrAbPecsopclaaencxebdiessct-roantseigdiecraeldlyaisnpaartdoynfatmheicinefnravsirtrouncmtuernet,otor
IIIprovidesacomparisonbetweenamulti-hopnetworkand handleemergencysituations.Tier-2communicationaimsto
WBANscanserveinvariousapplicationssuchasubiquitous tencewillbecomahoopentera-ahnosspmeisrstsaiiroontouhpsaosloiagsyhsi[gu4h0ee]r.idTnehliastyhtaanbedleulosphwoecwrostrtamhnastminmisusgilotiny- eibanetrearsccc.oensEnseecdlteiWnvBdeaAinlNysliwfeithasvawreiollusasnectewllourlkars,nwethwicohrkcsananedastihlye
millionsensorspwoweerrceomepasrteidmtoathteedonet-hoopbsetarutospeolodgyi.nTh2e0mu0lt9i- aInntedrneatr[5e].pThreem-oretechnologiessupportedbyaWBAN,
health care, telemedicine, entertainment, sports and military hopconfigurationinvolvesoverheadsalongwithitsnetwork theeasierforthemtobeintegratedwithinapplications.
dicted to reacho4pe8rat5ionm;asilinlciroeansingbthyen2um0b1er8of[h1op]s.couTldhleeadctooamTmheupnariacdigamtsioofninter-WBANcommunicationaredivided
[2]. Basically,theWBAN sensorsmayobservetheheart(elec- highcomplexity.Morespecifically,usingrelaysinWBANs intotwosubcategoriesasfollows:
architectureofiasnsivstosilnvreidnucgingWtheBcoAncNentrsaticonoomfthpetoranssemsissioonfpotwherreetiIenfrrasstraucsturielb-asedarchitecture–Thearchitectureshown
trocardiography) and the brain electrical (electroencephalo- fromthesourcetoitsdestination.Thus,thefurtherapartthe inFig.3isusedinmostWBANapplicationsasitfacilitates
lustratedinFigusourrceea1nd.destinationareindistance,thehighertransmission dynamicdeploymentinalimitedspacesuchasahospital
graphs) activities as well as vital signs and parameters like
• Tier-1: Intra-WBAN communication – This tier reflects
theinteractionoftheWBAN nodes(sensorsandcoordina-
Emailaddresses:[email protected](MohamadJaafarAli),
tor). Sensorsforwardtheirmeasurementstothecoordina-
[email protected](HassineMoungla),
tor. Thecoordinatorprocessesthecollectedphysiological
[email protected](MohamedYounis),
[email protected](AhmedMehaoua) dataandthentransmitstoanaccesspointinTier-2
PreprintsubmittedtoAdHocNetworksELSEVIER January30,2017
MOVASSAGHIetal.:WIRELESSBODYAREANETWORKS:ASURVEY 1669
Fig.9.SuperframeStructureofIEEE802.15.6[10]
TABLEVI by high signal attenuation and distortion. Due to the mobile
LISTOFFREQUENCYBANDSFORIEEE802.15.6[28]
natureofWBANs, theWBANsmaychangetheirpositionrela-
Description FrequencyBand
Implant 402-405MHz tivetoeachotherandtheindividualsensorsinthesameWBAN
On-body 13.5MHz
OOnn--bbooddyy 5-504M00HMz(HHzBC) maychangetheirlocationrelativetoeachother. Suchdynamic
On-body 600MHz
On-body 900MHz natureandtheabsenceofcoordinationaswellasthesynchro-
On-body 2.4GHz
On-body 3.1-10.6GHz nizationamongstWBANs,maketheallocationofaglobalcen-
tralizedentitytomanageWBANscoexistenceandmitigatethe
appliedtothesimulatedormeasuredchannelgaindatato
describefadinginWBANs,havebeencompared[48–52]. interferenceunsuitableforWBANs[2,6].
Themostcommonlyattempteddistributionfitislognormal,
followedbyNakagami-mthenRicean.Whilstthebest-fitfor Recently,theIEEE802.15.6standard[4]hasemergedasthe
aparticulardistributionatanygivFenigatutermept2is:mRosatdofitoenco-channelinterferencebetweenWBANsandotherwireless
Weibull,lognormalandgamma11.EventhoughNakagami- Fig.10.InterferenceinWBANs defactomediumaccesscontrolprotocolforWBANs. Thestan-
networks
mismostoftenusedasafit,ithasasmallersuccessrate;
andRiceanhasconsiderablysmallersu•ccesTsriateerco-m2p:aredIn(tIneterr--WWBABNAInNterfercenocem)[5m4,u55n].iHcoawteiveor,ndue–toTthheis communica- dardrequiresthesystemtofunctionproperlywithinthetrans-
toNakagami-m.TheRayleighdistribution,hotwieovenr,istaiepororisnatbureetowfaeWeBnANtahndeitschoigohrmdobiilnityaittoisrinfaeansidbleotone or more ac- mission range of up to 3 meters when up to 10 WBANs are
fidettfaoilrsmoofstthisscecnoamripoasriasnodnecnavnirboenmfuernthtserwshtuerdceieeidtsiisnsu[s2e4pd]..oThientamsllu.olctiaptTleeaiWegBlroAb-Na2lsco[c5o6rod].inmIantomcratsoeuscnownhitrecorleacctooei-xolioscnteantecdaeWiammBoAnsgNsstto interconnect collocated. It also has to support 60 sensors in a 6m3 space
A.Interference WBANswuciostenhflthicevt;saaasmrtehieochaaucntinsveelnp(seeirmiotidlwasrcfoarnerqoukveensrclaie(ps.c)M,etroalrnelsoumveilsr,asiworin,tshecthatnec,) (256 sensors in a 3m3). The standard recommends the use of
SinceaWBANismostlikelytoencounterotherWBANs, increaseinthenumberofWBANsthatcancoexistinshort time division multiple access schemes (TDMA) as an alterna-
iInEtEerE-W8B02A.N15.i6nttearsfkeregnrcoeupisreoqfutihreesutthme•ossytsiTtmempioerttoran-fcu3en.c:TtiohneCppareorfrxoiemrmgitaynicvoefedeeragcrhad–oatthioeTnr,.tEhhveiencsoimntmciauesneirscawtihorenerelflisnmkeacclalntnsusmufbfmeerredical facility, tive solution to avoid co-channel interference within WBANs.
properlywithinatransmissionrangeofupto3nmueterrsswehse,nudpoocftWoBrAsN,saarenddeplofyaedminiealcyhomthere’smvicibnitey,rtshe,reacenivedd can be viewed Tothisend,italsoproposesthreemechanismsforinter-WBAN
to10WBANsareco-located[3].Thedifferenttypesofradio signalstrengthoftheinterferingsignalcanbequitehigh,
interferencethatcanbeencounteredinWBaAsNsraeremshoowtnewchoicmhafmfectasnthedpecrfoermnatnecerosfaparticularWBAN[54]. co-channelinterferencemitigation,namely,beaconshifting,ac-
inFig.10.Onetypeofinterferenceoccurswhennumerous
peoplewearingWBANdevicesstepintoeachothersrange, B.DataRatesandPowerRequirements tivesuperframeinterleavingandchannelhopping[6,7],where
whichmakescoordinationinfeasibl1e.(o2ff.-bodCyoint-erCferhenacen).nelInterferenceamongstWBANs
Collisionfromexternalsensorsmayalsoleadtothistype OneofthemainconstraintsinWBANsistheirlimited asuperframeisdefinedasatimeperiodconsistingoftwosuc-
ofinterference[53].Additionally,theunDpreudicetabtleonattuhreeoifrpsoowecrisauplplyn.aFitgu.1r1es,hotwhseacionmpdariivsonidbeutwaelenWpowBerANsmaymove cessiveframes,anactiveframededicatedforsensorstransmis-
posturalbodymovementsleadstonetworkseasilymovinginto requirementsanddataratesinWBANscomparedtoother
andoutofeachothersrange[42].tTohiwsisasuredbsecoemaescehvenotwhireelerssitnechcnorloogiwes.dAcecodrdiangrlye,aWsBAsNupcrohtocoalssreaquirheospital’slobby. sions and an inactive period dedicated for coordinators trans-
morecrucialinthecaseofwirelesstechnologieswithhigher higherpowerefficiencywhencomparedtotheotherexisting
coverageareas. The interferencperotomcolas.yTheosecnscoursrinWbBeAcNasuaresceapaoblfeotfhtraensmsitmtingaller number of missions.
amGoenngestramlluyltiinptleerfWerBenAcNesotchcautrscowexhciesnthinnaoe-nacconhoroedtihnleasrt’i,sonvii.ceeixniis.tt,ys1d[f6e5aw7ta]i.KinnIbnpaZsewsisIniedGneacrpeaB,anctgehEemeoaEdfkaedtraa[ttao3rartq]aeut,eio[tsef4fraion]fm-ebtwo1hdMyKabbnnipot/sdsetitnsohva1ea0cryaMnpsfbruuoiltam/msr berofcoexist- • Beaconshifting: aWBAN’scoordinatormaytransmitits
ingWBANs. EvenwhenfewWBANscoexist,suchinterference
11consideringthosedistributionstested10mormaoryetimaeffs[2e4]cttheceondmoscmopeu.Fnigi.c11asthioowsnthlatinthekcsurrbenyttedchenoclorgeiesamseientgthesignal-to- beaconsatdifferenttimeoffsetsbyincludingashiftingse-
quence field in its beacons that is not used by neighbor
interference-plus-noise-ratio(SINR)ofthereceivedsignaland
coordinators.
thusmayhaveanadverseimpactonthereliabilityofWBANs.
The overlap among the transmission ranges of multiple collo- • Superframe interleaving : a WBAN’s coordinator ex-
cated WBAN-based networks may cause medium access colli-
changesinformationwithitscounterpartsinotherWBANs
sion. Basically, a link in one WBAN will suffer interference
inordertomaketheactiveperiodofitssuperframenotto
due to transmissions on the same channel made by a device
overlapwiththeactiveperiodsofothersuperframes.
mounted on another person in the vicinity. Thus, the inter-
WBAN interference may affect the communication links and • Channel hopping : a coordinator may change its oper-
degradetheperformanceaswellasincreasethepacketlossof ating channel periodically by choosing a particular chan-
each individual WBAN. In addition, inter-WBAN interference nel hopping sequence that should not be used by nearby
drains the most valuable resource of WBAN’s sensor, namely, WBANs.
power,whiletryingtocompeteforabettersignal-to-noise-ratio
However, the first two mechanisms require distributed guard
(SNR); it also leads to throughput degradation. Thus, the co-
timecomputationthatmakestimesynchronizationveryhardto
channelinterferencebecomesoneofthemostimportantprob-
achieve across WBANs. Therefore, we exploit the unlicensed
lemsthathappenwhenmultipleindependentanduncoordinated
international spectrum available in the ZIGBEE standard and
WBANsworkconcurrentlyinthecloseproximityofeachother.
pursuetheapproachofspectrumallocationtoresolvetheprob-
Therefore,interferencemitigationisofveryimportancetoim-
lemofinter-WBANinterferencemitigationnotonlythroughthe
prove the performance of the whole network [2, 5]. In addi-
channelbutalsothechanneltotime-slothopping.
tion,theresourceconstrainednatureofWBANsintermsoflim-
itedpowersupply,sensor’sandantenna’ssize,thetransmission 1.3. ContributionandOrganization
range,andthequalityofservice(QoS)requirementsmakethe Due to their social and highly mobile nature, WBANs may
applicationofadvancedantennaandpowercontrolmechanisms coexist in the proximity of each other and due to the absence
that proved their efficiency in interference mitigation in cellu- ofcoordinationamongstthem, theactiveperiodsofthecorre-
lar and wireless sensor networks unsuitable for WBANs. The sponding superframes may overlap with each other, hence the
protocolsproposedforthesenetworksdonotconsiderthestrin- co-channel interference may happen. Particularly, when some
gentpropertiesofWBANs,forinstance,powercontrolmecha- sensors of the interfering WBANs simultaneously access the
nismsarenotsuitableforWBANsbecausetheyrequiredifferent samechannel,thecorrespondingtransmissionsofthesesensors
levelsoftransmissionpower,whilst,longerlifetimeofsensors experiencemediumaccesscollision. Inthispaper, weaddress
batteries is required. The simple design and shape of sensor’s thisproblemusingLatinsquaresandproposetwoschemesfor
antenna make signal processing very hard in WBANs because channel allocation and medium access scheduling to diminish
oftheinhomogeneousnatureofthehumanbodycharacterized the probability of inter-WBAN interference. The first scheme,
2
namely,DAIL,assignschannelandtime-slotcombinationsthat [8,9]proposedanadaptiveinter-WBANinterferencemitigation
reduce the probability of medium access collision. No inter- scheme, where sensors transmissions are classified according
WBAN coordinationisneededinDAIL.Despitebeingveryef- to the QoS parameters. In their scheme, each interfering pair
fective, DAIL involves overhead due to frequent channel hop- of WBANs generates a table of interfering sensors and deter-
ping at the coordinator and the sensors. The second scheme, mines an interference region (IR) between them. An orthogo-
namely, CHIM, takes advantage of the relatively lower den- nalchannelisallocatedtoeachsensorinIRthathasthehighest
sity of collocated WBANs and pursues hopping among chan- priorityaccordingtoQoSclassification; whilesensorsthatare
nelsonlywheninterferenceisdetectedinordertosavepower. notbelongingtoIRcantransmitinthesametimeperiod. This
Insummary,wecontributethefollowing: schemereducesthenumberofassignedchannelsandimproves
thethroughput. In[10],thesameauthorsadaptchannelassign-
• DAIL, a distributed scheme which enables time-based
menttovariousmobilityscenariosofWBANsusingpredictive
channel hopping using Latin rectangles in order to mini-
models of their next location. In addition, their proposed al-
mizethemediumaccesscollisionandavoidtheco-channel
gorithmallocatesatransmissiontimesuchthattheinterference
interferenceamongstcoexistingWBANs.
isbetteravoidedtoextendtheWBAN lifetimeandimprovethe
spatial reuse and throughput. Meanwhile, an adaptive scheme
• CHIM,adistributedschemethatallocatesarandomchan-
basedonsocialinteractionispresentedin[11]tooptimizethe
nel to each WBAN, and provisions backup time-slots for
transmissiontimeallocation.SuchschemefactorsintheWBAN
failed transmission. The backup time-slots are scheduled
mobility,thedensityofsensorsinaWBAN,thereceivedsignal
in a way that is similar to DAIL. CHIM enables only a
strength indicator (RSSI), and the heterogeneous traffic load
sensorthatexperiencescollisionstohoptoanalternative
in order to minimize the power consumption and increase the
backupchannelinitsallocatedbackuptime-slot.
throughput.
• An analytical model that derives bounds on the collision
probabilityandthroughputforsensorstransmissions. On the other hand, some approaches tried to avoid interfer-
ence by assigning conflict free channels. Obviously this ap-
Thesimulationresultsandthemathematicalanalysisshowthat proachdoesnotsuitdynamicenvironmentswhereWBANsac-
our approach can significantly reduce the number of medium cidently becomes in range of each other. Nonetheless, con-
accesscollisionsamongtheindividualtransmissionsofthedif- flictfreechannelassignmentcanfitscenarioswherethesetof
ferentcoexistingWBANsaswellasreducepowerconsumption. WBANs that may coexist can be predicted beforehand. The
Moreover, DAILandCHIM donotrequireanymutualcoordi- popular methodology for channel assignment in this case is
nation among the individual WBANs. The organization of the to use graph coloring. Huang et al. [12] proposed a dis-
paper is as follows. Section 2 sets our work from other ap- tributed scheme that maps the channel allocation as a graph
proachesintheliterature.Section3describesthesystemmodel coloring problem. In their scheme, they mapped the time-
andprovidesabriefoverviewofLatinsquares. Sections4and slots to different colors and further proposed a coloring algo-
5,receptively,describeDAILandCHIM indetail. Theperfor- rithm to determine a color assignment for each node. The
manceofDAILandCHIMareanalyzedin6and7,respectively. coordinators exchange control messages to achieve a conflict-
Section8presentsthesimulationsresultsofDAIL andCHIM. free coloring. Meantime, Cheng et al. [13] proposed a ran-
Finally,thepaperisconcludedinSection9. dom incomplete coloring scheme (RICC) to realize conflict-
2. RelatedWorks free channel allocation without involving heavy computation.
Although RICC allows for higher spatial reuse of channel al-
Co-channelinterferenceavoidanceandmitigationhavebeen
location, itisstillfarfromtheoptimalasitonlyconsidersthe
subjecttoextensiveresearchinrecentyears. Inthecontextof
scheduling at the WBAN level but not at the node-level, and
WBANs, published techniques can be categorized as resource
hencethespatialreuseisnotoptimallyutilized. Similarly,Seo
allocation,powercontrol,somesolutionsarealsobasedonin-
et al. [14] presented a coloring-based scheduling method to
corporationofmultiplemediumaccessarbitrationmechanisms
avoidtheinter-WBANinterferencebyallocatingdifferenttime-
and link adaptation. In the balance of this section, we sum-
slots to adjacent WBANs and by allocating more time-slots
marizethemajorworkineachcategoryandcomparewithour
to traffic-intensive WBANs to increase the total throughput.
approach. WealsodiscusspreviousworkthatemployedLatin
Overall, channel and time resource allocation protocols may
squaresinmediumaccessarbitration.
work efficiently in dynamic environment and under high in-
2.1. ResourceAllocation
terferenceconditions,i.e.,highlymobileanddenselydeployed
Counteringinterferencebycarefulresourcemanagementand WBANs. These protocols may support an acceptable level of
medium access scheduling is obviously a viable option. Re- QoS requirements. Graph-based resource allocation protocols
source allocation, e.g., channels and time, is an effective way are unsuitable for topology with high-frequent changes, e.g.,
foravoidingradioco-channelinterferenceandmediumaccess WBANs, because of the incurred cost due to update and mes-
collision. However, the main problem in channel assignment sage exchanges; therefore these protocols may be not only in-
solutions is the limited number of available channels, espe- efficientbutalsodetrimentalforhealthcareapplicationsindy-
cially,whenthereishigh-densityofcoexistingWBANs. Some namic environment. Moreover, they do not even support an
approaches have pursued this methodology. Movassaghi et al. acceptablelevelofQoSrequirementstosensors.
3
2.2. PowerControl tion across coexisting WBANs. However, game-based proto-
colsdonotsupportQoSandarecharacterizedbylongdelays.
One of the most design issues in WBANs is the scarce en-
2.3. LinkAdaptation
ergy resource of their sensors. Thus, saving energy by adjust-
Interference,inessence,affectstheindividualwirelesslinks.
ing transmission power of the radio transceiver is very crucial
Therefore, one way to mitigate the effect of interference is to
toextendthelifetimeoftheWBAN. InWBANs,thelinkqual-
ity is affected by different channel conditions such as fading, adjustthelinkparameters. Linkadaptationprotocolsopttody-
namically match the modulation and coding parameters, e.g.,
pathlossandshadowingthatchangefrequentlyduetothebody
datarate,modulationscheme,etc.,ofthetransmittedsignalto
movements. Therefore, the transmission power can be adap-
thechannelconditions,namely,thepathloss,SINR,RSSI,etc.
tivelycontrolledbasedonitslinkstateinordertoimprovethe
Theseprotocolsinvariablyrequiresomechannelstateinforma-
relaibility while minimizing the energy consumption. Kim et
tion at the transmitter. For example, the link data rate could
al. [15]presentedalinkstateestimationtransmitpowercontrol
protocol (TPC) to improve the energy efficiency and the link changebasedontheSINRofthechannel. Yangetal.,[21]pro-
posed several schemes such as adaptive modulation, adaptive
reliabilityinWBANs. Theirprotocoladaptsthetransmitpower
datarates,anddutycycles. ThecoordinatorofaWBAN selects
accordingtoshort-termandlong-termlink-stateestimationsof
RSSI under different mobility scenarios of the patient. Mean- theappropriateschemeforusebythesensorsbasedonthelevel
of experienced interference. Whilst, Moungla et al. [22] pur-
time, Chen et al. [16] proposed a power allocation algorithm
sued a tree-based topology design for a single mobile WBAN
based on genetic algorithms to mitigate inter-WBAN interfer-
to ensure reliable data delivery. WBAN sensors share a small
enceandincreasethepowersavingswhileensuringQoSguar-
number of channels, whereas, the relay nodes share the most
antees. Yet,theiralgorithmdidnotconsiderthemobilitywhich
numberofchannelstoimprovethedataflowacrosstheWBAN.
leadstoalongconvergencetime. InHOSS[17], lowinterfer-
To mitigate the interference among relays, adaptive data rate
ingsensorstransmitonthesamechannel;whilehighinterfering
anddutycycleareusedtoimprovethedatarateandensurean
sensors transmit using orthogonal channels, at the same time,
energyefficientandreliablecommunicationwithinaWBAN.
theremainingsensorsareassociatedwithanenergyharvesting
Arelayisdefinedasanintermediatenodeconnectingasen-
model to gather the wireless broadcast energy from other sen-
sortoacoordinator. AsstatedintheZIGBEEstandard[3],[4],
sorstransmissions. HOSScapturestheinterferencesignaltobe
the two-hop communication is a very promising solution as
usedasasourceofenergyandmitigatingtheinterferenceatthe
it exploits the spatial diversity to improve WBAN energy effi-
sametime. Consequently,thehigherthenumberofcoexisting
ciency and reliability as well as to enable better WBAN’s in-
WBANsimpliesthehighertheamountofharvestedenergyand
terference mitigation. Dong et al. [23] proposed a two-hop
thehigherthenetworklifetime.
cooperativeschemewhereaTPCmechanismisintegratedinto
Zou et al. [18] proposed a Bayesian non-cooperative game
sensor and relay nodes to prolong their batteries lifetimes and
basedpowercontrolapproachtomitigateinter-WBAN interfer-
mitigate the interference by increasing the SINR of data pack-
ence.TheymodeledWBANsasplayersandactivelinksastypes
ets received at the WBAN coordinator. Moreover, in [24] the
of players in the Bayesian model to maximize the through-
same authors implemented a two-hop communication scheme
put and energy efficiency of WBANs. Meanwhile, Dong et
(LRCS) for a WBAN. LRCS considered two relay nodes and
al. [19] proposed a non-cooperative social-based game theo-
providesa3-linkdiversitygaintothecoordinator,whereinse-
retic transmit power control scheme to maximize the through-
lectioncombining,thecoordinatorselectsthebestofthethree
put amongst coexisting WBANs so that the average transmis-
links according to best link channel gain. Overall, link adap-
sion power is minimized. Meantime, Moovasi et al. [20]
tation protocols do not suite highly mobile and densely de-
proposedanon-cooperative game-theoreticapproachtoinves-
ployed WBANs because of the fast-changing channel condi-
tigate the problem of joint relay selection and power control
tions. However,theseprotocolsmayintroducesomeadditional
in WBANs. To better mitigate the interference and ensure en-
energycosttotherelayingnodesandlatencytothepacketde-
ergy efficient communication, each sensor seeks a strategy to
livery that may be unacceptable in time-sensitive health-care
selecttheappropriatetransmissionpowerlevelandthebestre-
applications.
layinordertoensureshortdelayandjitterinadistributedman-
2.4. UseofMultipleAccessProtocols
ner. Overall, the published protocols have shown that power
control is robust to different body mobility scenarios and suit With contention-based MAC, sensors within a WBAN can
highly populated environment with WBANs. Although link- decide their medium access pattern without the need for any
state based protocols do not require negotiation and message timesynchronization. However,theindividualperformanceof
exchange among WBANs, they significantly consume energy thesesensorsmaybedegradedbecauseofthehighoverheadin-
andprovidepoorQoSsupport. Theseprotocolsarenotrecom- curredduetoaddressingthemediumaccesscollisionswhenthe
mendedfordynamicenvironmentsbecausethelinkstatevaries densityofWBANsishigh;suchoverheadismainlyduetotime
very frequently due to body movements. On the other hand, and energy consumption during backoffs. On the other hand,
game-basedpowercontrolprotocolsdonotsupportthemobil- contention-free protocols use time synchronization to achieve
ity of WBANs. Nonetheless, these protocols support dynamic collision-free transmissions and high throughput. However,
channel conditions, e.g., varying channel gain, and do not re- oneofthemajorlimitationsofcontention-freeapproachisthe
quire message exchange, which reduces the energy consump- needfortimesynchronizationwhichisverycostlytoachievein
4
WBANs. Another limitation is the time-slot allocation, which 2.5. MediumSharingusingLatinSquare
becomeschallengingparticularlywhenthedifferentcoexisting
WBANs employ non-similar duty cycles, i.e., the number of Ju et al. [36] proposed a noteworthy approach although it
activesensorsinaperiodoftimeisnotconsistentamongthese is not geared for WBAN. Basically, they presented a multi-
WBANs. Contention-freeMACisreliableandenergy-efficient channeltopologytransparentalgorithmthatleveragestheprop-
[11] in relatively low-density WBANs, though extra energy is erties of Latin squares for scheduling the transmissions in a
consumed for their periodic synchronization and control mes- multi-hop packet radio network. Wireless nodes are consid-
sages. eredtobeequippedwithonetransmitterandmultiplereceivers.
It is worth noting that in [37] Latin squares are used in cel-
Chenetal.,[26]proposedatwo-layerMACprotocoltodeal
lular networks for the sub-carrier allocations to users, where
withtheinterferenceofWBANs(2L-MAC).TheWBAN’scoor-
a user could be allocated multiple virtual channels. Each vir-
dinatorschedulestransmissionswithinitsWBANusingTDMA, tualchannelhopsoverdifferentsub-carriersatdifferentorthog-
andacarrier sensemechanismtodeal withinter-WBAN inter-
onal frequency division multiplexing (OFDM) symbol times.
ference. 2L-MAC reduces packet collisions, transmission de-
Basically, users are allocated multiple sub-carrier-to-OFDM-
layandenergyconsumptioninsensors. Meantime,Deylamiet
symbol-time combinations to avoid inter-cell interference. In
al. [25] proposed a dynamic coexistence management mecha-
DAIL, sensors are simply allocated a single channel to time-
nismasanextensiontotheIEEE802.15.4standard[4]which
slotcombinationandthissimplifiesinter-WBAN coordination
makestheIEEE802.15.4-basedWBANsabletodetectandmit-
andtimesynchronization.In[38],multipletopology-dependent
igatetheinterference. Meanwhile,intheapproachofHuanget
transmissionschedulingalgorithmshavebeenproposedtomin-
al. [27],thecoordinatoremployscarriersensemultipleaccess
imize the TDMA frame length in multi-hop packet radio net-
withcollisionavoidance(CSMA/CA)toadaptivelyadjustsuper-
worksusingGaloisfieldtheoryandLatinsquares.Forasingle-
framelengthaccordingtothelevelofinterference,andbeacons
channelnetwork,themodifiedGaloisfielddesignandtheLatin
tocontrolthemediumaccesswithinitsWBAN.Similarly,Zhou
squaredesignfortopology-transparentbroadcastschedulingis
et al. [28] proposed an opportunistic scheduling scheme (OS-
proposed. Modified Galois field design obtains much smaller
MAC)byapplyingheuristicschedulingtoimprovetheWBAN
frame length than the existing scheme while the Latin square
performance. OS-MAC dynamically adjusts the superframe
designcanevenachievepossibleperformancegainwhencom-
lengthaccordingtothechannelstatus. Ontheotherhand,Liu
paredwiththemodifiedGaloisfielddesign. Inone-hoprather
etal. [29]proposedaTDMA-basedMACprotocol(CA-MAC)
thanmulti-hopcommunicationscheme,likeDAIL,usingLatin
tomaintainanacceptedlevelofQoSwhileatthesametimeen-
squares yields better schedules the medium access and conse-
suringenergy-efficientcommunication. CA-MACdynamically
quently significantly diminishes the inter-WBAN interference.
adjuststhetransmissionorderandtransmissiondurationofthe
Like[36],DAILandCHIMexploitthepropertiesofLatinrect-
sensornodesaccordingtodynamicchannelconditionsandtraf-
angles to generate a predictable interference-free transmission
ficcharacteristicsofWBANsunderdifferentmobilityscenarios.
schedule for all sensors within a WBAN. However, DAIL and
Grassi et al. [30] employed a mechanism that reschedules CHIMdependontheexistenceofonlyoneratherthanmultiple
beacons to avoid active period overlapping and hence reduc- receivers within each node and one-hop rather than multi-hop
ing the interferences amongst WBANs. The rescheduling may communication. UnlikeDAIL,CHIMminimizesthefrequency
causesignificanttransmissiondelayifthereisalargenumberof of channel switching significantly, i.e., CHIM applies channel
WBANs. Meantime,Deylamietal. [31]proposedadistributed switchingonlywhenasensorexperiencesinterference.
approach that collaboratively arranges the active durations of Compared to the related work covered in this section, our
thesuperframesofthecoexistingWBANssothatinterferenceis approach can be viewed as combing two solution strategies,
minimizedandthechannelusageismaximized.Similarly,Kim multi-channelandtime-slotadjustment. Inthispaper, weem-
etal. [32]proposedadistributedasynchronousinter-WBANin- ployLatinrectanglesforinterferencemitigationamongstmul-
terference avoidance scheme (AIIA) that includes the timing tiplecoexistingWBANs.Tomitigateinterference,ourapproach
offset and dynamically adjust the location of the TDMA pe- optstoexploittheavailabilityofmultiplechannelsintheZIG-
riodtopreventinter-WBANinterferencebyoverlappingTDMA BEE standard and employs Latin rectangles as the underlying
transmissionsbetweennearbyWBANs. AIIAadaptstothelevel schemeforchannelandtime-slotallocationtosensorswhileen-
ofinterferenceinthemobileWBANenvironmentandimproves ablingautonomousschedulingofthemediumaccess. Bylever-
thecoordinationtimewithoutincurringsignificantcomplexity aging the properties of Latin rectangles, our approach reduces
overhead. SuchtimesharingbasedsolutionsinwhichWBANs theprobabilityofmediumaccesscollisionandprovidesbetter
interleavetheiractiveperiodsthroughnegotiationorcontention usage of the limited resources of WBANs. Since the potential
areineffectivewhentheloadinWBANsisheavyanddutycycle formediumaccesscollisiongrowswiththeincreaseinthecom-
of WBAN is high. Both contention-free and contention-based munication range and the density of sensors in the individual
approaches are suitable for relatively low-density of WBANs WBANs, we propose two schemes. The first scheme, namely,
with low-occupancy channels and few sensors [33, 34, 35]. DAILsuitscrowdedareaswhereahighdensityofWBANsco-
However, these approaches are unsuitable for highly mobile exists. On the other hand, the second scheme, CHIM, takes
WBANs with high-density of sensors and with heavy traffic advantageoftherelativelylowerdensityofcoexistingWBANs
load. tosavethepowerresourceatbothsensor-andWBAN-levels.
5
Table1:Symboldescription
WBANk kthWBANs
Crdq coordinatorofqthWBANs
Si,k ithsensorofkthWBANs
Ck adefaultchannelofkthWBANs
BKC(Si,k) abackupchannelpickedbyith
sensorofkthWBANs
3. SystemModelandPreliminaries
3.1. SystemandProblemModels
WeassumeNTDMA-basedWBANsmaycoexistintheclose
proximityofeachother. Asanexample,whenanumberofpa-
tientsmovearoundinahospital’shallorcorridor. EachWBAN Figure3:Collisionscenariosatsensor-andcoordinator-levels
isformedofonecoordinatordenotedbyCrdanduptoK wire- 5-A. In IMB frame, each interfering sensor may use a backup
less sensors. Each sensor generates its sensed data according channelwithinitsallocatedbackuptime-slot.
toapredeterminedsamplingrateandtransmitsatamaximum 3.1.2. AcknowledgmentPacketsCollision
daterateof250Kb/susingtheunlicensed2.4GHzband. Inour
In similar way, a coordinator’s acknowledgment packet ex-
approach, we assume that all ZIGBEE standard channels are
periencesacollisionatthereceivingsensorwhenthislatteris
availableforsensorsandthecoordinatorwithineachWBAN at
in the transmission or radio range of another active sensor or
anytime. Furthermore,allcoordinatorshavericherpowersup-
coordinator. When S receives from its corresponding Crd ,
ply than sensors and are not affected by the frequent channel i,k k
whileatthesametime,anotherS orCrd transmitsusingthe
hopping. j,q q
samechannelthatS uses,i.e.,acollisionoccursunderthefol-
i,k
Due to the WBAN’s unpredictable motion pattern, it is very
lowingcondition: S isintransmissionrangeofCrd orS ;
i,k q j,q
hardtoachieveinter-WBANscoordinationortohaveacentral
andCrd orS transmitsusingthesamechannelemployedby
q j,q
unit to mitigate the potential interference when some of them
S .
i,k
areincloseproximityofeachother. Basically,theco-channel
interference happens when multiple WBANs concurrently ac- 3.2. LatinSquares
cessthesamechannelandtheircorrespondingradiotransmis- Inourapproach,wehaveemployedthespecialpropertiesof
sion ranges overlap. In essence, the medium access collision Latin squares to allocate interference mitigation channels. In
may arise due to the collisions amongst the concurrent trans- thissection,weprovideabriefoverviewofLatinsquares.
missionsoftheseWBANs. Sincedatapacketsmaybelostdue
totheco-channelinterference,andhenceacknowledgmentsare Definition 1. A Latin square is a K ×K matrix, filled with K
requiredtoconfirmsuccessfulreception. Time-outsareusedto distinct symbols, each symbol appearing once in each column
detectreceptionfailureatthecorrespondingreceivers. Wenote andonceineachrow.
that collisions may take place at the level of data or acknowl-
edgement packets as shown in Figure 3 and explained below. Definition2. Twodistinct K ×K LatinsquaresE=(ei,j)and
Table1showssymbolsdescriptions. F = (fi,j), so that ei,j and fi,j ∈ {1,2,...K}, are said to be or-
thogonal,iftheK2orderedpairs(e , f )arealldifferentfrom
3.1.1. DataPacketsCollision eachother. Moregenerally,theseti,Oj =i,j{E ,E ,E ,...,E }of
1 2 3 r
Essentially, asensor’sdatapacketexperiencesacollisionat distinct Latin squares E is said to be orthogonal, if every pair
thereceivingcoordinatorwhenthislatterisinthetransmission inOisorthogonal.
orradiorangeofanotheractivesensororcoordinator. Whena
sensor denoted by S of WBAN transmits to its correspond- Definition3. AnorthogonalsetofLatinsquaresoforderKis
i,k k
ing coordinator denoted by Crd while another sensor S or ofsize(K-1),i.e.,thenumberofLatinsquaresintheorthogonal
k j,q
coordinatorCrd of another WBAN transmits using the same familyis(K-1),iscalledacompleteset[36].
q q
channelthatS employs,i.e.,acollisionoccursunderthefol-
i,k
lowing condition: Crd is in range of Crd or S ; and Crd Definition 4. A M ×K Latin rectangle is a M ×K matrix G,
k q j,q q
orSj,qtransmitsusingthesamechannelusedbysensorSi,k. In filled with symbols aij ∈ {1,2,...,K}, such that each row and
essence, such collision may be experienced in both DAIL and eachcolumncontainsonlydistinctsymbols.
CHIM schemes,whenC = C ;i.e.,bothWBAN andWBAN
k q k q Theorem1. IfthereisanorthogonalfamilyofrLatinsquares
happentopickthesamechannelforintra-WBAN communica-
oforderK,thenr≤ K−1[39]
tion,inwhichcaseS orCrd couldbesendingadataorac-
j,q q
knowledgementpackets,respectively. Inaddition,anothercol-
EandF areorthogonalLatinsquaresoforder3,becauseno
lision scenario may be experienced only in the case of CHIM
twoorderedpairswithinE(cid:46)(cid:47)Faresimilar.
scheme when C = BKC(S ) or C = BKC(S ); i.e., the
k j,q q i,k
ctthihmaaten-hnsaelsolthbwaeseitbnheiaenlnltohpceiacItkeMeddBtbofyrasWemnBesAowrNhSkicij,shqewoqfiullaWlbBteoAdtNehfieqnsieandmitiesncbsheaaccntkinuoepnl E=123 231 312F=132 213 321E(cid:46)(cid:47)F=231,,,132 231,,,213 312,,,321
6
According to Theorem 1, the number of WBANs using or- transmissionsthathappeninthesametime-slot.Thus,thenum-
thogonalLatinsquaresisupperboundedbyK-1,thus,Kshould berofcollisionsdependsonthenumberofcoexistingWBANs,
be large enough so that, each WBAN can pick an orthogonal i.e., the corresponding interfering sensors and the number of
Latin square with high probability. According to the orthogo- orthogonal Latin rectangles used by the interfering WBANs.
nalityproperty,eachWBAN willbeassignedauniquechannel Therefore, the medium access collision among the transmis-
allocation pattern that does not look like the pattern of other sions of the different sensors is completely avoided, iff, the
WBANs to avoid the co-channel interference, i.e., they do not numberoforthogonalLatinrectanglesislargerthanthenumber
share the same symbol positions, each in its own Latin rect- of those sensors competing to transmit in the same time-slot.
angleandconsequently,nootherWBAN inthenetworkwould Otherwise, DAIL extends the number of columns in the Latin
simultaneouslysharethesamepatternwithWBAN allthetime. rectanglewhichisdirectlyrelatedtothelengthoftheWBAN’s
i
Generally, our approach makes it highly improbable for two superframeby addingextratime-slotsto lowertheprobability
transmissions to collide. Nonetheless, a medium access colli- ofmediumaccesscollisions. Forexample,ifthenumberofco-
sionmaystillhappenwhen(i)twoormorecoexistingWBANs existingWBANsisN andthenumberofLatinrectanglesisP,
in the proximity of each other randomly pick the same Latin eachofsize16×K,whereKdenotesthenumberofcolumnsin
rectangle,or(ii)whenthenumberofWBANscoexistinginthe theLatinandatthesametimethenumberoftime-slotsinthe
proximity of each other is larger than the number of ZIGBEE superframe.IfN>max(16,K),theneachWBANwillextendthe
standard channels forming the Latin rectangle which is 16 in numberofcolumnsintheLatin,i.e.,thenumberoftime-slotsin
ourcase. thesuperframefromK toK+XT,whereXT=N-max(16,K).
Basically, a particular WBAN randomly picks one Latin Doingso,suchsensorswillhaveahigherprobabilitytonotpick
square from the orthogonal set of Latin squares. According the same channel in the same time-slot and hence the number
totheirproperties,LatinsquaresensurethataparticularWBAN ofcollisionsisminimized. Algorithm1providesahighlevel
willnotshareachannelamongstthecoexistingWBANsthatuse summaryofDAIL.
distinctLatinsquareschosenfromthesameorthogonalset.The
Latinsquaresizewillbedeterminedaccordingtothelargestbe- Algorithm1ProposedDAILScheme
tweenthenumberofsensorsthatformseachWBAN,denotedby input :NWBANs,Ksensors/WBAN,CoordinatorCrd,MZIGBEEchannels,
Kandthenumberofchannelsthatdeterminesthenumberofthe LatinrectangleR,framelengthFL
rowsintheLatin,denotedbyM.IntheIEEE82.15.6standard 1 BEGIN
2 FL=K // defaultsettingoftheframelength
[4],thenumberofchannelsislimitedto16,i.e.,themaximum
3 ifN>Kthen
numberofrowsintheLatinis16. Therefore,thepossiblenum- 4 FL=N // Crdincreasesthenumberoftime-slotsinthesuperframe
ber of simultaneous transmissions that can be scheduled is no 5 endif
morethan16. Duetothelimitednumberofpossiblechannels, 6 EachWBAN’sCrdrandomlypicksaLatinrectangleRofsizeM×FL
7 END
DAIL addresses this limitation by employing Latin rectangles
ratherthanLatinsquares. Inotherwords,DAILdoesnotfixthe
number of columns in the Latin square, i.e., the value of K is 4.2. DAILSuperframeStructure
notfixedto16.HenceDAILsupportsK > M,i.e.,morethan16
Each WBAN’s superframe is delimited by two beacons and
transmissionscanbescheduled.UnlikeDAIL,CHIMoptimizes
composedoftwosuccessiveframes: (i)theactiveframewhich
the time-slot allocation within the superframe and the channel
is dedicated for all sensors within a WBAN and, (ii) the inac-
hoppingisdecidedonlywhentheinterferenceisexperienced.
tive frame which is designated for the WBAN’s coordinator as
4. DistributedInterferenceAvoidanceusingLatinRectan- showninFigure5. ThroughassumingM=16channelsofthe
glesScheme(DAIL) ZIGBEE are available at each WBAN, it is still needed to de-
terminethenumberoftime-slotsformingeachrowoftheLatin
4.1. AlgorithmDescription
rectangleandconsequentlythesizeofeachsuperframe.Infact,
DAILoperatesinadistributedmannerandsuitscrowdedarea
thesuperframesizedependsonhowbigthetime-slotwhichis
withalargenumberofpatients,eachmaycarryasingleWBAN
based on the protocol in use and on the number of time-slots
andmaymovetowardsotherpatients, e.g., inahospital’scor- requiredwhichisdeterminedbythedifferentsamplingratesof
ridor. Through using Latin rectangles, DAIL assigns a chan-
thesensors. Generally,thesuperframesizeisdeterminedbased
nel and time-slot combinations that reduce the probability of
onthehighestsamplingrateandthesumofthenumberofsam-
mediumaccesscollision. InDAIL,eachcoordinatorrandomly
ples for all sensors in a time period. DAIL requires the same
selectsoneorthogonalLatinrectanglethroughwhichitassigns
superframesizeforallWBANssothatmediumaccesscollision
auniquesymboltoeachsensorwithinitsWBAN.Accordingto
couldbebetteravoidedbypickingtherightvalueforK,where
itsassignedsymbol,asensordeterminesitstransmissionsched-
K is the number of time-slots to be made in the superframe,
ule which is formed of a sequence of channel and time-slot
respectively,intheLatinrectangle.
combinations. Thatmeanseachsensordeterminesitshopping
4.3. IllustrativeExample
pattern, i.e., which hopping channel to use in which time-slot
withineverysuperframe. We illustrate our approach through a scenario of 3 coexist-
DAILenablesdifferentcoexistingsensorstohopamongdis- ingWBANs,whereeachcircumferencerepresentstheinterfer-
tinctchannelstoavoidthecollisionamongtheircorresponding ence range as shown in Figure 3. Furthermore, each WBAN
7
Time-slots
1 2 3 4
1 W B G R
C
h 2 B G R W
a
n
ne 3 G R W B
ls
4 R W B G
Figure4:A4×4channeltotime-slotassignmentLatinsquare Figure5:ProposedsuperframestructureforDAIL
is assigned M = 4 channels and consists of L = 4 sensors,
in turn, each sensor is assigned a symbol from the set K =
{1,2,3,4} ⇐⇒ {G,B,R,W}. Here, we assume that each sensor
requires only one time-slot to transmit its data in each super-
frame. Basedonthisscenario,anypairofsensorsareinterfer-
ingwitheachother, i.e., theytransmitusingthesamechannel
atthesametimeifbothsensorsareintheintersectionoftheir
correspondinginterferenceranges.
However, as shown in Figure 3, 4th sensor of WBAN de- Figure6:ProposedsuperframestructureforCHIM
1
sityofcoexistingWBANs.
notedbyS andS areinterfering,also,S andS .There-
1,4 2,4 3,1 2,3
fore,inDAIL,eachWBANpicksadistinctLatinrectanglefrom 5. Channel Hopping for Interference Mitigation Scheme
anorthogonalsetasfollows: WBAN picksE,WBAN picksF (CHIM)
1 2
andWBAN3 picksJ,whereEandFareconsideredasin(3-B). Like DAIL, CHIM leverages the special properties of Latin
Assume3sensors,u,vandwofWBAN1,WBAN2 andWBAN3 squarestoenablepredictablechannelhoppinginordertoavoid
are, respectively, assigned symbols B, R and G in Latin rect- theco-channelinterferenceamongstcoexistingWBANs. How-
angles E, F and J. Thus, the distinct positions of symbol B in ever, CHIM takes advantage of the relatively lower density of
EcorrespondstothetransmissionpatternofuinWBAN1’ssu- interferingWBANstosavethepowerresourceatbothsensors-
perframe,similarlyforvandwinWBAN2andWBAN3,respec- and WBAN-levels. Basically, CHIM enables only sensors that
tively. However, B=2 in E, R=3 in F and G=1 in J, therefore, experiencecollisionstohopamongbackupchannels,eachinits
the transmission patterns for u, v and w are, respectively, rep- allocatedbackuptime-slot. CHIMimposeslessoverheadatthe
resented by B, R, and G of the matrix shown in Figure 4. As sensor-andcoordinator-levelsbecauseofonlysensorsthatex-
clearlyseeninthisfigurethatu,vandwneithersharethesame periencecollisionsarerequiredtousetheirpre-computedtrans-
channel nor the same time-slot, i.e., no collision occurs at all. missionschedules(i.e.,acombinationofabackupchanneland
DAILiscompletelydistributedchannelallocationandmedium a time-slot). As pointed out, the co-channel interference hap-
accessschedulingschemethatsuitshighdensityofWBANsand pens if the concurrent transmissions of sensor nodes and co-
does not require coordination among coordinators. DAIL as- ordinator nodes in the different coexisting WBANs collide. To
signsWBAN sensorschannelandtime-slotcombinationstodi- mitigate the potential co-channel interference, CHIM exploits
minishtheprobabilityofinter-WBANinterference,i.e.,reduces theavailabilityofmultiplechannelsintheZIGBEEstandardto
theprobabilityofmediumaccesscollision. Nonetheless,DAIL allocate each WBAN a default channel and in the case of in-
drainsthepowerresourceofthewholeWBANswhensomeof terference, it enables the individual interfering sensors to hop
their corresponding sensors do not face any collision. For ex- among the remaining channels in a pattern that is predictable
ample, as an estimate of power cost of a WBAN consisting of within each WBAN and random to the other WBANs. To do
up to L sensors, L < K, is L × HE, where HE is the power that, CHIM modifies the superframe structure and extends its
consumption resulting from a channel hopping in each super- size by adding extra interference mitigation backup time-slots
frame. Therefore, the mean power cost per sensor is denoted andemploysthepropertiesoftheLatinrectanglesforchannel
meanEC= L×HE. allocationtothesensorswithineachindividualWBAN.
K
Morespecifically,DAILimposesoneachWBAN’ssensorto 5.1. CHIMSuperframeStructure
hop among the available channels whether that sensor experi- LikeDAIL,inCHIM,eachWBAN’ssuperframeiscomposed
ences collision or not. Although the interference-free sensors of successive active and inactive frames as shown in Figure
do not need to hop among the channels, and hence the power 6, however the active frame is further divided into two parts,
is wasted at both the sensor- and the WBAN- levels. Another the TDMA data-collection part and the interference mitigation
shortcominginDAIListhatnomorethan16transmissionscan backup(IMB)interferencemitigationpartwhereeachpartcon-
be scheduled and thus, this limits the number of transmitting sistsofK time-slots. Figure6showsCHIM superframestruc-
sensors,i.e.,ifmorethan16WBANscoexist,andthenthecol- ture. In the TDMA data-collection part, each sensor transmits
lisionsmayarise.ToovercomesuchissuesinDAIL,wepropose the sensed data packet to the coordinator in its assigned time-
a distributed scheme, namely, CHIM, to reduce the number of slotusingthedefaultchannel. However,intheIMBpart,each
mediumaccesscollisionsandoverheadatthecoordinator-and interferingsensorretransmitsthesamedatapackettothecoor-
sensor-levelsaswellastosavepoweroftherelativelylowden- dinatorinitsallocatedbackuptime-slotusingthepriori-agreed
8
uponthechannel. Ininterference-freeconditions,theWBAN’s However,ifthesensordoesnotreceivetheacknowledgment
coordinator remains tuned to the default operation channel. If packetbeforethetimerexpires,thetransmissionisconsidered
communicationwithaparticularsensorS failsduringitsdes- unsuccessful due to the collision and subsequently, it should
i
ignated time-slot, the coordinator will tune to the S ’s backup use its allocated backup channel and time-slot to mitigate the
i
channelduringS ’stime-slotintheIMBpartoftheactiveframe interference. Basically,thesensorthatexperiencesinterference
i
ofthesuperframe. However, duringtheinactiveframe, allthe shouldwaituntiltheTDMApartcompletesandthenitswitches
activesensorswithineachWBANturnofftheirradiotransceiver itscurrentdefaultchanneltotheallocatedinterferencemitiga-
off and sleep, whilst, the corresponding coordinator transmits tion channel at the beginning of its backup time-slot through
allthedatacollectedtoacommandcenter. which it retransmits the data packet to its corresponding coor-
Yet,itisstillrequiredtodeterminethelengthofeachframe. dinator. Infact,thepacketdeliveryfailureresultsfromthedata
LikeDAIL,thelengthoftheTDMApartisdeterminedaccord- or acknowledgment packets collisions at both the sensor- or
ing to the largest sampling rate and the sum of the number of coordinator-levels, respectively. That means, the desired data
samplesforallsensorswithinaWBAN inatimeperiod. How- packetislostatthecoordinatorduetotheinterferencethatthe
ever,CHIMrequiresthelengthoftheTDMApartforallWBANs transmitting sensor experienced from the simultaneous trans-
tobeofthesamesizesothatthemediumaccesscollisioncould missionsofsensorsorcoordinatorsinothercoexistingWBANs;
be better in the backup part. Thus, CHIM states that 2 × K ortheacknowledgmentpacketislostatthedesiredsensordue
time-slotsshouldbemadeintheactiveframe,i.e.,Ktime-slots to the interference that the desired sensor experienced from
forms the TDMA part and K time-slots forms the IMB backup the simultaneous transmissions of the sensors or coordinators
part.Whilst,theinactiveframedirectlyfollowstheactiveframe in other coexisting WBANs sharing the same channel with the
andwhoselengthdependsonthedutycycleschemeusedbythe desired sensor. Therefore, depending on the timeouts and the
individualsensorswithineachWBAN. acknowledgment packets, the interfering nodes, i.e., sensors
5.2. NetworkSetup andcoordinators, resolvethemediumaccesscollisionissuein
the same way. Algorithm 2 summarizes the proposed CHIM
Atthesetupofthenetwork,eachcoordinatorofaWBANran-
scheme.
domlyselectsasingleM×KLatinrectanglefromthesetofthe
orthogonal Latin rectangles and one default channel. Initially,
Algorithm2ProposedCHIMScheme
thecoordinatorcommandsalltheindividualsensorswithinits
input :NWBANs,KSensors/WBAN,OrthogonalLatinrectangleOLR
WBAN toutilizethecommondefaultchannelalongtheTDMA
8 Stage1:NetworkSetup
datacollectionpart. Inaddition,eachWBAN’scoordinatoras- 9 fori=1toN
signsasinglesymboltoeachsensorwithinitsWBAN.These- 10 CrdirandomlypicksasingleDFCi&OLRiforitsWBANi
lected symbol from the set {1,2,..., K} determines the trans- 11 fork=1toK
mission schedule for each sensor. In other words, the symbol 12 CrdiallocatesBKCk,i&BKTSk,itoSk,ifromOLRi
reflects position hopping in the rectangle relates one interfer- 13 Stage2:Sensor-levelInterferenceMitigation
ence mitigation channel and one backup time-slot. Thereby, 14 fori=1toN
theWBAN’scoordinatorformsthesequenceofaninterference 15 fork=1toK
mitigationchannel andonebackuptime-slot combinationsfor 16 Sk,itransmitsPktk,iinTSk,itoCrdionDFCiinTDMAi
eachsensorwithinitsWBAN.Inaddition, eachWBAN’scoor- 17 ifAckk,iissuccessfullyreceivedbySk,ionDFCi
dinator reports the sequence of combinations or the transmis- 18 Sk,iswitchestoSLEEPmodeuntilthenextsuperframe
sion schedule corresponding to each sensor within its WBAN 19 else
through beacons. Subsequently, each sensor will be informed 20 Sk,iwaitsitsdesignatedBKTSk,iwithinIMBipart
aboutitsallocatedinterferencemitigationbackupchanneland 21 Sk,iretransmitsPktk,iinBKTSk,itoCrdionBKCk,i
22 Stage3:Coordinator-levelInterferenceMitigation
backup time-slot to be used within the IMB part of the super-
23 fori=1toN
frame.
24 fork=1toK
5.3. NetworkOperationunderCHIM 25 ifCrdisuccessfullyreceivedPktk,iinTSk,ionDFCi
CHIM relies on the acknowledgment packets and the time- 26 CrditransmitsAckk,iinTSk,itoSk,ionDFCi
outstodetectthepacketcollisionthatoccursatboththecoor- 27 else
dinators and sensors. In the superframe’s TDMA active part, 28 CrdiwilltunetoBKCk,itoreceivefromSk,iinIMBi
eachindividualsensortransmitsitspackettothecoordinatorin 29 CrdireceivesPktk,iinSk,i’sBKTSk,ionBKCk,i
its allocated time-slot using the default channel. Immediately
after this transmission, that sensor sets a timer and then waits
6. DAILAnalysis
for the acknowledgment. If the sensor receives the acknowl-
edgment packet from the WBAN’s coordinator, the transmis- We analyze the performance of DAIL mathematically and
sion is considered successful, and then the sensor turns its ra- consider a multi-channel TDMA-basedWBANs. The super-
diotransceiveroffandsleepsuntilthenextsuperframe. There- frames of each individual WBAN are constructed as from one
fore,allthesensorsthatsucceedintheirtransmissionswillnot M×K matrix. Withinthesuperframe,eachsensormaybeas-
switchtonewchannelsandwillnotoccupybackuptime-slots signed M time-slots to transmit its data packet according to a
intheIMBpart. uniquechanneltotime-slotassignmentpattern. Thesepatterns
9
aregeneratedfromtheorthogonalfamilyof M×K Latinrect- 6.2. CollisionProbability
angles. However, all the individual sensors of a WBAN share WeconsiderasensorS ofWBAN issurroundedbyOinter-
i i
onecommonM×KLatinrectangle,wherethechanneltotime- feringsensorsv ofdifferentcoexistingWBAN inthevicinity,
j j
slot assignment pattern of each sensor corresponds to a single where j = 1,2,...,O and i (cid:44) j. For simplicity, we assume,
symbolpatternintheLatinrectangle,asshowninFigure4. eachsensortransmitsonedatapacketineachtime-slot. How-
6.1. InterferenceBound ever,sensorS successfullytransmitsitsdatapacketintime-slot
i
Inthissubsection,wedeterminetheworst-casecollisionpat- t, on channel h to the coordinator, if, none of the O neighbors
ternfortheindividualsensor. transmits its data packet using the same time-slot on the same
channelassensorS . Werepresentthenumberofsensorsthat
Definition5. LetEandFbetwoorthogonalM×KLatinrect- i
transmit their packets in the same time-slot as sensor S as a
angles. Symbol e from E is assigned to sensor u, and symbol i
randomvariabledenotedbyX.Whenxpacketsaretransmitted
f from F is assigned to sensor v. Then, there exists a collision
in the same time-slot as S , then, the probability of event X=x
at the jth slot on ith channel for u and v, if the ordering (e,f) i
isdenotedbyPr(X=x)anddefinedbydefinedbyeq. 1below.
ofbothrectanglesappearsatith row, jth column,whichmeans
[E ]=eand[F ]= f.
i,j i,j
Pr(X=x)=CO+1×ωx×(1−ω)O−x
Theorem2. Iftwosensorsareassignedtwodistinctsymbolsin x (1)
×(min(M,K)/K)x∀x ≤ O
thesameLatinrectangle;therewillbenocollisionamongtheir
transmissions. If they are assigned symbols from two distinct Whereωistheusefactor, definedastheratioofthetimethat
orthogonal Latin rectangles, then, they will face at most one asensorisinusetothetotaltimethatitcouldbeinuse. Now,
collisionineverysuperframe. supposeYsensorsoutofXsensorsscheduletheirtransmissions
Proof: From the definition of Latin rectangles, because ev- accordingtothesameLatinrectangleassensorSi,i.e. youtof
xsensorsselectsymbolpatternsfromthesameLatinrectangle
ery symbol occurs exactly one time in each row and exactly
asS .
onetimeineachcolumn,anytwotime-slotassignmentpatterns i
constructed from the same Latin rectangle will not have any
(cid:16) (cid:17)
overlapintheirpatternsandsotheywillnothaveanycollision Pr(Y =y|X=x)= CK+1×CZ−K /CZ−1
y x−y x
(2)
with each other. Based on Definition 2, hence, the ordering
∀x≤O&∀y ≤ x
(e,f)foranypairoforthogonalLatinrectangles, where, eand
f ∈ {1,2,..., K}, can only appear one time, which means that Where Z = K ×m is the total number of symbol patterns in
thesesensorswillonlyhaveoneopportunityofcollision. theorthogonalLatinrectanglesfamily. However,theseYsen-
sorswillnotimposeanycollisionwithS ’stransmission,since
Theorem3. InanetworkofNWBANs,eachsensorhasachan- i
they (Y sensors) use the same Latin rectangle as S . On the
i
neltotime-slottransmissionpatterncorrespondingtoasymbol
other hand, X − Y sensors may collide with the transmission
patternchosenfromoneoftheKthsetoforthogonalLatinrect-
fromsensorS tothecoordinatoronthesamechannel,thenthe
i
angles. Let us consider a sensor denoted by “S” surrounded
conditional probability of transmission collision is denoted by
by maximum number of O WBANs, i.e., O sensors from other
(collTx)anddefinedbyeq. 3below.
WBANs,whichmeans,Osensorsmaycoexistinthecommuni-
cationrangeofS.Then,SmayexperienceatmostOcollisions.
Pr(collTx| Y =y&X=x)
Additionally,sensorSmayfaceaminimalnumberofcollisions
whichequaltomax(O-K+1,0). =1−Pr(succTx | Y =y&X=x) (3)
=1−((min(M,K)−1)/min(M,K))x−y
Proof: Based on Theorem 2, each neighboring sensor can
createatmostonecollisiontoS.Intheworstcase, allOsen- Wheremin(M,K)representsthenumberoftransmissiontime-
sors are within the range of communication of S. The trans- slotsforeachsensorineachsuperframe. Then,theprobability
missionspatternsofOsensorsareconstructedfromLatinrect- of a successful data packet transmission from sensor S to the
i
anglesthataredifferentfromtheLatinrectangleutilizedbyS. coordinatorisdenotedbyλasfollows:
Subsequently,themaximumnumberofpossiblecollisionsexpe-
rienced by S is O. Now, to count the minimal number of colli- (cid:88)O (cid:88)x
sionsforS,itisrequiredtofindthemaximumnumberofsensors λ= Pr(Y =y,X=x)×
that construct their transmission patterns from the same Latin x=0 y=0
rectangle,whichisK,i.e.,Ksensorswillhavenocollisionac- Pr(succTx |Y =y&X=x)=
cording to Theorem 2. Also, Theorem 2 proves that there ex- (cid:88)O (cid:88)x
istsatmostonecollisionforeachpairofsensorsconstructing Pr(Y =y | X=x)×Pr(X=x)×
theirtransmissionpatternsfromtwodifferentorthogonalLatin x=0 y=0 (4)
rectangles. Therefore, eachoftheremainingsensors(O-K+1) Pr(succTx | Y =y&X=x)=
will cause one collision to S because they belong to different (cid:88)O (cid:88)x
orthogonalLatinrectangles. Asaresult,theminimumnumber (COxCyK−1CZx−−yK)/(CZx−1)×ωx×(1−ω)O−x×
of collisions for sensor S surrounded by O sensors is equal to x=0 y=0
max((O-K+1),0). (min(M,K)/K)x×((min(M,K)−1)/min(M,K))x−y
10