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IEEE802.11ah:ALongRange802.11WLANatSub1GHzWeipingSun,MunhwanChoiandSunghyunChoiDepartmentofECEandINMC,SeoulNationalUniversity,Seoul,Korea;email:{weiping,mhchoi}@mwnl.snu.ac.kr,schoi@snu.ac.krReceived10April2013;Accepted14May2013AbstractIEEE802.11ahisanemergingWirelessLAN(WLAN)standardthatdefinesaWLANsystemoperatingatsub1GHzlicense-exemptbands.Thankstothefavorablepropagationcharacteristicsofthelowfrequencyspectra,802.11ahcanprovidemuchimprovedtransmissionrangecomparedwiththeconven-tional802.11WLANsoperatingat2.4GHzand5GHzbands.802.11ahcanbeusedforvariouspurposesincludinglargescalesensornetworks,extendedrangehotspot,andoutdoorWi-Fiforcellulartrafficoffloading,whereastheavailablebandwidthisrelativelynarrow.Inthispaper,wegiveatechnicaloverviewof802.11ahPhysical(PHY)layerandMediumAccessControl(MAC)layer.Forthe802.11ahPHY,whichisdesignedbasedonthedown-clockedoperationofIEEE802.11ac’sPHYlayer,wedescribeitschannelizationandtransmissionmodes.Besides,802.11ahMAClayerhasadoptedsomeenhancementstofulfilltheexpectedsystemrequirements.Theseenhancementsincludetheimprovementofpowersavingfeatures,supportoflargenumberofstations,efficientmediumaccessmechanismsandthroughputenhancementsbygreatercompactnessofvariousframeformats.Throughthenumericalanalysis,weevaluatethetransmissionrangeforindoorandoutdoorenvironmentsandthetheoreticalthroughputwithnewlydefinedchannelaccessmechanisms.Keywords:Sub1GHz,IEEE802.11ah,smartgrid,longrangeWLAN.JournalofICTStandardization,Vol.1,83–108.doi:10.13052/jicts2245-800X.125c2013RiverPublishers.Allrightsreserved.84W.Sun,M.ChoiandS.Choi1IntroductionIEEE802.11WirelessLocalAreaNetwork(WLAN),whichiscurrentlyoperatingat2.4GHzand5GHzbands,hasbeenoneofthemostpopularwirelesstechnologiesinindoorenvironmentsthankstoitshighdatarates,easydeploymentandlowcost.However,thehighfrequencybandshaveputlimitsonthetransmissionrangesofthe802.11WLANsystem,andhence,thesystemhasbeentreatedwithindifferenceinoutdoorenvironments.Furthermore,theexcessiveutilizationandirregulardeploymentshaveresultedinthesaturationofthe2.4GHzand5GHzspectra.Thesituationwillworsenifmorenewlydesignedsystemsactascompetitors.DuetothedrawbacksencounteredbythecurrentIEEE802.11WLANsandtheincreasingdemandsforubiquitouswirelessaccess,IEEE802.11workinggrouphastriggeredanewproject,calledIEEE802.11ah,thatat-temptstoenactan802.11standardatsub1GHzlicense-exemptbandsforcost-effectiveandlargescalewirelessnetworks.Asdescribedin[1],withinthe802.11workinggroup,therearefivestepsindevelopinganewstandardamendment:1)discussionofnewideasintheWirelessNextGen-erationStandingCommittee,2)developmentofthepurposeandscopefortheamendmentinastudygroup,3)draftinganamendmentinataskgroup,4)approvalofthedraftbytheworkinggroup,and5)reviewbyaspon-sorballotpool,andapprovalandratificationofthedraftbytheIEEEStandardsAssociationboard.Thesub1GHzstandardizationactivityiscur-rentlyinstep3,andthecorrespondingtaskgroupiscalledTaskGroupah(TGah).Acriticaldeficiencyencounteredbythe802.11ahsystemisthescarcityoftheavailablespectrainthesub1GHzIndustrial,Scientific,andMedical(ISM)bands,suchthatincreasingthespectralefficiencyhasbeenoneofthemainconcernsinsystemdesign.Inordertoincreasethesystemthroughput,TGahhasdesignedanewPhysical(PHY)layerbasedontheIEEE802.11ac[2],whichisanotheramendmentofthe802.11familydesignedforhighthroughputWLAN.Besides,apartfromthePHYlayerdesign,therealsohavebeensomeeffortsmadeinMediumAccessControl(MAC)layertoincreasethesystemthroughput.Moreover,whendesigninganew802.11systematsub1GHzbands,thereisnoneedtomaintainthebackwardcompatibilitywiththeother802.11systems,sincethenewsystemwilloperateatentirelydifferentfre-quencybands,thusallowingthe802.11ahtodefinesomenewcompactframeformatstoreducetheprotocoloverheadwithoutconsideringthebackwardcompatibility.IEEE802.11ahSub1GHzWirelessLAN85Ontheotherhand,thankstothefavorablepropagationcharacteristicsofsuchalowfrequencyspectrum,the802.11ahsystemissupposedtopro-videmuchimprovedtransmissionrangecomparedwiththecurrent802.11WLANsoperatingat2.4GHzand5GHzbands.Low-costandlargecoveragepropertiesmakethe802.11ahsystemhighlyattractiveforlargescalesensornetworks,e.g.,smartgrid,inwhichthenumberofinvolveddevicesinagivennetworkcouldbemuchlargerthanthatofthecurrent802.11system.Ontheotherhand,thetargetdevicesinthesensornetworksarelikelytobebattery-powereddevices,andhence,thepowersavingfeaturesbecomemuchmorecriticaltotheperformanceof802.11ahsystem.Inordertocopewithsuchexpectedrequirements,someenhancementshavebeenconsideredin802.11ahMAClayerdesignintermsofpowersavingandsupportoflargenumberofstations.InthispaperwefirstintroducethecurrentactivitiesandstatusofIEEE802.11ahproject,andthenelaboratetheusecasesof802.11ahforhelpingtheunderstandingoftheintentionofitsvarioustechniques.Afterwards,wewilldescribethefeaturesrelatedwithPHYlayerintermsofchannelizationandtransmissionmodes,whicharebasedonthe10timesdown-clockedversionof802.11acPHY.MAClayertechniques,forwhichtheagreementsarealreadymadeaspartofthe802.11ahdraft,willbedescribedfromfouraspects,namely,supportoflargenumberofstations