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TPRC2015:43rdResearchConferenceonCommunications,InformationandInternetPolicy

(Revisedversion,Nov.2015.Newerandextendedversionunderpeerreviewinjournal)

ComparisonbetweenBenefitsandCostsofOffloadofMobileInternetTrafficViaVehicularNetworks

AlexandreK.Ligo†‡a,JonM.Peha†b,PedroFerreira†c,JoãoBarros‡d†CarnegieMellonUniversity,USA

‡UniversityofPorto,Portugal

AbstractDedicated Short Range Communications (DSRC) is an emerging technology thatconnects automobiles with each other and with roadside infrastructure. The U.S.DepartmentofTransportationmaysoonmandatethatcarsbeequippedwithDSRCtoenhancesafety.Thisworkfindsthatiftheydo,thenDSRCnetworkscouldalsobeanimportantnewwaytoprovideInternetaccessinurbanareasthatismorecost-effective than expanding the capacity of cellular networks. By combining oursimulation model with data collected from an actual vehicular network that isoperating inPorto,Portugal,weestimatehowmuchInternettrafficcanbecarriedonvehicularnetworksthatwouldotherwisebecarriedbycellularnetworksunderavariety of conditions. We then compare the benefits of cost savings of reducedcellularinfrastructureduetooffloadwiththecostoftheDSRCvehicularnetwork,todeterminewhethertheformerexceedsthelatter.AlthoughwefindthatthebenefitsfromInternettrafficalonearenotenoughto justifyauniversalmandatetodeployDSRCinallvehicles, i.e.benefitofInternetaccessaloneis lessthantotalcosts, themajorityofDSRC-relatedcostsmustbeincurredanywayifsafetyistobeenhanced.Thus, soon after a mandate to put DSRC in new vehicles becomes effective, thebenefitsof Internetaccess throughvehicularnetworks indenselypopulatedareaswould be significantly greater than the remaining costs, which are the costs ofroadside infrastructure that can serve as a gateway between DSRC-equippedvehiclesandtheInternet.Moreover,benefitof InternetaccesswouldexceedDSRCinfrastructurecostinregionswithlowerandlowerpopulationdensitiesovertime.

Keywords:mobileInternet,vehicularnetworks,mobiledataoffload,DedicatedShortRangeCommunications,DSRC,benefit-costanalysis,socialwelfare

aAlexandreK.Ligo,Ph.D.Student,CarnegieMellonUniversityandUniversityofPorto,aligo@cmu.edubJonM.Peha,Professor,CarnegieMellonUniversity,peha@cmu.edu,www.ece.cmu.edu/~peha/bio.htmlcPedroFerreira,Professor,CarnegieMellonUniversity,pedrof@cmu.edu,pedro-ferreira.orgdJoãoBarros,Professor,UniversityofPorto,jbarros@fe.up.pt,web.fe.up.pt/~jbarros

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1 IntroductionStandardizedtechnologynowexiststhatwouldsupportvehicularnetworks,whicharemeshnetworksrunningInternetprotocolssuchasIP.Theroutersinavehicularnetworkareplacedinautomobiles,mostofwhicharemoving,andininfrastructureplacednearroadsforthispurpose.Thistechnologymaysoonbewidelydeployed,primarilyasawayofenhancingautomotivesafety.Extensiveresearchisbeingdoneonthepotentialsafetybenefitsofvehicularnetworks(Kenney2011;U.S.DepartmentofTransportation2015;Mecklenbraukeretal.2011).Thispaperinvestigatesanentirelydifferentuseofvehicularnetworks–asanewwaytoprovideInternetaccess,especiallyformobiledevices.

Thereisstrongmotivationtofindnewcost-effectiveapproachesasInternettrafficovermobilenetworkshasbeengrowingsteadily(Sandvine2014).Somepredictthatgrowthwillbesustained;Ciscoforecastsatenfoldincreaseinmobiletrafficoverthenextfiveyears(Cisco2015).Partofthegrowthisexpectedtocomefromin-vehicleInternetusage,bothbecausevehicleoccupantswillincreasinglyusesmartphonesandlaptops,andbecauseofvehicularinfotainmentplatformssuchasAppleCarPlay,AndroidAuto,andcarmaker-proprietarysystems.Expandingcapacityofcellularnetworkstomeetsuchdemandgrowthwouldbecostly,asitwouldrequiresignificantlymorespectrum,capital,orboth.However,ifpartofthetrafficcouldbeoffloaded,i.e.deviatedfromthemacrocellularnetworkstoalternativenetworks,thenthedemandgrowthmightbemetwhileaddingfewernewcellsandthereforeincurringlowercosts.VehicularnetworksareapossiblealternativeforbringingInternetaccesstodevicesinautomobiles,aswellasdevicescarriedbypedestriansorplacedinlocationsnearroads.

Thispapershowsthatundersomeimportantcircumstances,vehicularnetworkscanprovideInternetaccessatlowercoststhanwouldbeincurredintoday’scellularnetworks.ThepaperanalyzesthecostsandbenefitsofInternetaccessthroughvehicularnetworksthatuseanemergingtechnologycalledDedicatedShortRangeCommunications(DSRC).ThedevelopmentofDSRCtechnologyisprimarilymotivatedbyroadsafetyapplicationssuchastheexchangeofwarningmessagesbetweenvehiclesenrouteofcollisionratherthanInternetaccess.TheUnitedStatesDepartmentofTransportation(USDOT)isexpectedtoproposerulemakingin2016tomandateDSRCinallnewvehicles(U.S.DepartmentofTransportation2015),andtheU.S.FederalCommunicationsCommission(FCC)hasalreadyallocated75MHzofspectrumforIntelligentTransportationSystems(U.S.FederalCommunicationsCommission;Kenney2011).TheDSRCstandardsallowpartofthe75MHzallocatedintheU.S.(and50MHzthathasbeensimilarlyallocatedintheEuropeanUnion)tobeusedforapplicationsotherthansafety(UzcateguiandAcosta-Marum2009;CampoloandMolinaro2013;Kenney2011).Non-safetyapplicationsincludevehicleandroad-relatedservicessuchasnavigationandtollcollection,aswellasInternetaccess(Zeadallyetal.2010;CampoloandMolinaro2013).

Ourcost-benefitanalysiswillinformimportantdecisionsregardingwhetherresourcesshouldbeinvestedinvehicularnetworksforthepurposeofInternetaccess,ratherthanjustvehicularsafety.ThisincludesdecisionsaboutwhetherDSRC-equippedroadsideinfrastructureshouldbedeployed,whethervehiclesshouldbeequippedwithDSRCdevices,andwhetherspectrumshouldbeallocatedforintelligenttransportationsystems.Wedeterminewhetherdecisionstoincurtheseinvestmentswouldincreasesocialwelfarebycomparingtherelevantcoststobenefits,andignoringanysunkcosts.OnedecisioniswhethertoinvestinroadsideinfrastructureforInternetaccess.InscenarioswhereDSRCspectrumhasalreadybeenallocated,asisthecaseinseveralregionsworldwide(Zeadally

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etal.2010),andwherethereisalreadyamandatetodeployDSRCOnboardUnits(OBUs)forsafetypurposes,thenspectrumandOBUcostsaresunk.Inthesescenarios,thedeploymentofroadsideinfrastructureexclusivelyforInternetaccesswouldincreasesocialwelfareifandonlyifthebenefitofInternetaccessexceedsRSUcoste.Thispaperwilldeterminewhenthatisthecase,andwhichfactorsaremostinfluential.Inparticular,wefindthatdeploymentofthisinfrastructureforInternetaccessindenseurbanareasislikelytoincreasesocialwelfarefairlysoonafteramandatetoputOBUsinvehiclesbecomeseffective.OtherdecisionsincludewhethertoallocatespectrumandmandateOBUsinthefirstplace,ifthesestepsarenottakenforsafetyreasons.InsituationsinwhichbenefitofInternetaccessexceedsalltypesofDSRCcost,thensocialwelfareisincreasedbymandatingDSRCdevicesinallvehiclesandallocatingspectrumregardlessofwhethertherearesafetyorothertypesofbenefit.Thispaperwillalsodeterminewhenthisisthecase.

SomecarriersandresearchersareconsideringuseoffixedWi-Fihotspotsthatoffloadtrafficfromstationarydevicesthatareincloseproximity,orvehiculardatatrafficthatistoleranttodelays(AT&T2015;Comcast2013;Comcast2015;Balasubramanian,Mahajan,andVenkataramani2010;Eriksson,Balakrishnan,andMadden2008;K.Leeetal.2010;Balasubramanianetal.2008),andtherehasbeenresearchontheresultingeconomicimpact(Markendahl2011;J.Leeetal.2014).However,vehicularnetworksoffernewopportunitiesforInternetaccessthatarequitedifferentfromwhatispossiblewithWi-Fihotspots,andthisrequiresnewanalysis.

ThebenefitsofvehicularnetworksaredifferentfromWi-Fihotspotsbecausethetrafficcarriedisdifferent.Wi-FiisoftenagoodsolutionforuserswhoarestationaryfortheperiodwhentheyareaccessingtheInternet,butitisofteninadequateforuserswhoaccesstheInternetwhilemoving.OnereasonisthatWi-Fihotspotsrequire1-8secondsforauthentication(Bychkovskyetal.2006;Murray,Dixon,andKoziniec2007),whichmustoccurbeforeanewconnectioncanbeestablished.Thisisoflittlevaluetocarstravellingathighspeeds.Incontrast,DSRClinkscanbeestablishedinjust300milliseconds(IEEE2010a;MussabbirandYao2007).Thus,whilevehicularnetworkscouldservesomeofthesameusersasWi-Fi,e.g.apedestrianwhoisnearbothaWi-Fihotspotandabusystreet,vehicularnetworkscanbringInternetaccesstomanyusersinmovingvehiclesthatarenotservedwithWi-Fi.Inaddition,thecostsassociatedwithvehicularnetworksarequitedifferentfromthecostsoftypicalWi-Finetworks,whicharegenerallymicrocellular.AsshowninFigure1,vehicularnetworksbasedonDSRCaremeshnetworkscomprisedofvehicle-to-vehicle(V2V)andvehicle-to-infrastructure(V2I)wirelesslinks,whichmeansthatinformationcantravelalongmultihoppathsfromvehicletovehicletovehiclebeforefinallyreachingfixedinfrastructure.Inameshnetwork,thedevicesusedtoaccessthenetworkalsoserveasinfrastructurethatbringsaccesstoothers.Asaresult,arelativelysmallnumberoffixedDSRCroadsideunits(RSUs)canconnectalargenumberofvehicle-borneDSRCunitstotheInternet.ItalsohelpsthatDSRClinkscanbelongerthantypicalWi-Fihotspots,i.e.upto1000-meterdistancesiftherearenoobstructions,and250-350metersinclutteredurbanareasf.AlthoughfarfewerfixeddevicesareneededtocoveranareaswithavehicularnetworkthanwithWi-Fi,thosefixedDSRCdevicesarealsomore

eTheestimationofbenefitsandcostsassumesthattheroadsideinfrastructureinquestionisusedonlyforInternetaccess,andnotforsafetyandotheruses.ThereareotherpossibilitiestoincreasesocialwelfarewheninfrastructureusedforsafetyandInternetisshared,whichissubjectforfuturework.fAsmeasuredinthecityofPorto,Portugal.

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expensive,inpartbecausetheymustoperateoutdoorsinhostileconditions,andinpartbecausetheyarenotcurrentlymassproduced.

Figure1.DSRC-basedcommunications.V2IandV2Vlinkscanbeusedtodisseminatesafetymessages,andalsoforwardIPpacketsbetweenvehiclesandroadsideinfrastructureforInternetapplications.MultipleV2Vhopsthroughintermediatevehiclescanbeusedfortwoendpointstoconnect

OurmethodologyconsistsofcombiningageneralsimulationmodelthatwedevelopedwithdatacollectedfromanactualvehicularnetworkthatisoperatinginPorto,Portugal.ThefirststepistoestimatehowmuchvehicularInternettrafficthatwouldotherwisebecarriedbycellularproviderscaninsteadbecarriedbyaDSRC-basednetworkunderavarietyofconditions.Toachievethis,wedevelopedsoftwarethatsimulatestherateatwhichdataistransferredbetweenvehiclesandRSUs.Oursimulationemploysrealisticrepresentationsoftheelementsofavehicularnetworkthatgreatlyaffectthroughputrates,includingthelocationofvehiclesandRSUs,thesignallossbetweendevices,andtheDSRCprotocolitself.Someofthatrealismcomesfrommeasurementdatatakenfromthecity-scaletrialinPortugal.Forexample,ourmodelsofvehicletrafficpatternsarebasedinpartonlocationdatacollectedfrom900busesandtaxisbetween2012and2015.

ThenextstepistoestimatecostsandbenefitsofInternetaccessthroughvehicularnetworksundergivenconditions.Today,nearlyalltrafficfrommobiledevicesmustbecarriedoveramacrocelltothenearestcellulartower(asdiscussedabove,lesscostlyalternativessuchasWi-Fihotpotssometimesexistforstationarydevices,butusuallynotfordevicesthataremoving).Inacapacity-limitedcellularnetwork,areductionoftrafficfrommobiledevicesthatmustbecarriedinthebusyhourallowseachcelltowertoprovideadequatecapacityoveralargerarea,therebyreducingthenumberofcostlytowersthatacellularoperatorneedstocoveragivenregion.WedefinethebenefitofInternetaccessthroughvehicularnetworksinagivenscenarioasthecostsavingsfromreducingthenumberofcelltowers.ThisiscomparedtothecostsofDSRCRSUs,spectrumorOBUs.Inthisanalysis,weconsiderawiderangeofvaluesforimportantfactorssuchaspopulationdensity,DSRCpenetration,datarateperDSRC-equippedvehicle,andvariousunitcosts.

Thispaperisorganizedasfollows.Section2describestheDSRCnetworkoperatinginPortoforInternetaccess,andwhichdataisbeingusedfromitforthispaper.Section3explainsthesimulationmodel,thebenefit-costanalysisandtheirunderlyingassumptions.Section4containstheresultsthatarerelevanttoanswertheresearchquestionsproposed.Section5endsthepaperwiththeconclusions,aswellasthelimitationsandopportunitiesforfuturework.

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2 PortoVehicularNetworkforInternetAccessandDatasetPortoisthesecondlargestcityinPortugal,witha2011populationof237,000inanareaof41.4km2(InstitutoNacionaldeEstatistica2011).InSeptember2014,theurbanbusauthoritycompanyofPortostartedofferingfreeWi-Fiserviceforitspassengers24hoursaday,7daysaweek.Eachofits477urbanbuseshasanOBUequippedwithaWi-Fihotspotforpassengerstoconnectto.AllbushotspotsshareasingleWi-FinetworkIDwithopenaccess:uponactivatingaWi-Fienabledsmartphoneorcomputerinthebus,apassengercanuseittofindthecommonWi-Finetworkname,andafteropeningawelcomewebpage,he/shehasInternetaccesswithnopasswordrequired.

EachbusOBUhasitshotspotcoupledtoarouterthatrelaysallpassengertrafficto/fromtheInternet.Eachpacketisrelayedthroughoneoftwopossiblepaths.ThepreferredisthroughtheuseofDSRC,forwhichtherewere27RSUsgdeployedatfixedlocationsofthecitysuchastrafficlightsh:busescanconnecttoRSUseitherdirectlyorthroughmultihopconnectionsusingotherbuses.IfnoRSUiswithinrangeofasingleormultihopconnection,thentheOBUtransferdatathroughthecommercialLTEnetwork.BothOBUsandRSUsaredeployedandmaintainedbyVeniamNetworks.

AsofMarch2015,over2.7TBweretransferredbyPortobuspassengersthroughDSRCandcellular.Overthefirstquarterof2015acompoundmonthlygrowthrateof35%wasobserved.TheobservedvolumetransferredthoughDSRCvarieswithlocation,withthemajorityoftheRSUsbeingconcentratedindowntown,wheretheratiobetweenthenumberofbytestransferredthroughDSRCandthetotalnumberofbytescanreachasmuchas70%atpeakhours.

Metadataregardingthebusnetworkstateandusageiscollectedandstoredperiodically.ThatincludesbytestransferredthroughDSRCandcellular,signalstrengthbetweentheendpointsofthewirelesslinks,andGPSpositionsofthebuses.

Moreover,thereisdataabouttaximobilityinPorto.Ofthecityestimatedtotalof800taxis,GPSpositionsof435vehicleswerecollectedduringonemonthin2012andsharedforuseinthispaper.

ThedatafromthePortobusnetworkandtaxipositionrecordsthatwereusedinthispaperissummarizedinTable1.AsdescribedinSection3,realbusandtaxipositionsareusedtosimulateaDSRCnetworkwithvaryingconditionsofpopulation,OBUpenetration,andloadofInternetdata.RealmeasurementsofDSRCsignalswerecomparedwiththephysicalmodelofthesimulation.

gAsofMarch2015.hTherearemoreDSRCRSUsdeployedinthemetropolitanregionofPorto,namelyintheharborarea.Nevertheless,theseareforcommunicationswithcargotrucksandhavenotbeenusedforoffloadingofInternettraffic.See(Ameixieiraetal.2014):thesametypeofRSUandOBUequipmentareusedinboththetruckandbusnetworks.

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Table1.Datausedfortheanalysis

DatafrombusDSRCnetworkcollectedfromOctober2014toMarch2015

DataItem NumberofObservations Description

Wi-Fisessions 477buses:106sessions PerWi-Fisession:numberofbytestransferred,startandendtimes,andanidentifierofthebus

Datavolume/position/signalper15-secondperbus

477buses:240*106datapoints

Per15-secondinterval,perbus:bytestransferredoverDSRC,bytestransferredovercellular,dateandtime,GPSposition,receivedsignalstrengthfromRSU(ifV2I-connected)orpeerbus(ifV2V-connected),identifierofthebus,identifierofthepeerifV2IorV2V-connected

RSUpositions 27RSUs PerRSU:GPSpositionandheight

DatafromtaxiscollectedinMarch2012

DataItem Value Description

Positionpersecondpertaxi

435taxis:120*106datapoints

Persecond,pertaxi:time,GPSposition,andanidentifierofthevehicle

3 MethodologyTheanalysisinthispaperevaluatesthevolume,benefits,andcostsofInternetaccessthoughvehicularnetworks,underseveralscenariosrepresentingdistinctvaluesoftheparametersthatmostaffectresults.Theanalysisisperformedintwomainsteps,asillustratedinFigure2.

Figure2.Summaryofsteps,inputsandoutputsofthemethodology

ThefirststepinthemethodologyistoestimatepotentialofInternetaccessthroughavehicularnetwork.ToachievethiswedevelopedawirelessnetworksimulationmodelwhichsimulatestherateatwhichdataistransferredbetweenvehiclesandRSUsthroughsingleormultiplehops.Thatmodelmakesuseofrepresentationsoftheelementsthatmostinfluencethethroughputrates:locationsofRSUsandvehicles,signallossbetweenthem,andmultiplevehiclesandRSUsexchangingdatasimultaneouslyinthesamearea,atwhichcompetitionfortheuseofthewirelessmediumishandedbytheDSRCandInternetprotocols.Portodataisusedinthreeways:first,busandtaxiGPSpositionsfromPortoare

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usedtodeterminethepositionsofthevehiclesinthesimulation.Second,thereceivedsignalmeasuredinthebusesisusedtoverifywhetherthesimulatedsignalloss(whichinfluencestransmissionrangesandinterference)iscompatiblewithmeasuredloss,onaverage.Third,measureddataratesthroughDSRCfromPortoarecomparedwiththesimulatedrates,inordertoverifywhetherthelatterisareasonableapproximationofrealdataratesthroughDSRCundersimilarconditionsofnumberofvehiclesandRSUs,anddatarates.

OnepossiblequestionabouttheaboveiswhydataratesthroughDSRCaresimulated,whenmeasuredratesfromarealvehicularnetworkareavailable.TheansweristhatweareinterestedinlearningthepotentialofInternetaccessthroughvehicularnetworkswithcharacteristicsthatdifferfromthePortonetwork,withrespecttothequantityorconcentrationofvehiclesandRSUs,typesofvehicles,volumeofInternettrafficdemanded,availablebandwidthforInternetaccess,etc.WiththemodelwesimulateconditionsthatrepresentcitiesotherthanPorto,andfutureperiodswithhigherpenetrationofDSRCdevicesorhigherInternettraffic.

ThesecondstepistousethedataratethroughDSRCtoestimatethebenefitandcostofInternetaccess.ToaccomplishthisweconsiderthebenefitasthesavingsaccruedfromthedifferencebetweenthenumberofmacrocellulartowersthatwouldbenecessaryifthereisnoInternetaccessthroughDSRC,andthe(lower)numberoftowersnecessarywhenpartofthetotaltrafficisoffloaded.Costsofvehicularnetworksareofthreetypes:DSRCOBUs,spectrum,andRSUs.Whilethequantityofonboarddevicesandamountofspectrumareamongthedefinitionsthatcharacterizeascenarioofanalysis,theamountofinfrastructuredeployedforeachscenarioisestimatedattheoptimalquantityofRSUsthatmaximizesthedifferencebetweenbenefitofInternetaccessandinfrastructurecost.

Locationcharacteristics,i.e.whetheragivenareaisurban,suburbanorrural,influencebothsteps.Dataratesareinfluencedbysignalpropagationcharacteristics,whichdifferbetweenurbanandruralareas.Moreover,thosedataratesareonlyrelevantwherethecellularnetworksarecapacity-limited,whichalsoisaconditiontypicalforurbanareas.Ontheotherhand,thosesamedataratesareexpectedtobehigherinurbanareas,becauseofthehigherpopulationdensities.Therefore,resultsarelikelytobemoresubstantialinurbanareas,whichmakethemtheprimaryfocusofthisanalysis.

EachstepisdescribedinSections3.1and3.2,respectively,andthenumericalvalueschosenforthebasecasescenarioanditsvariationsaredescribedinSection3.3.

3.1 NetworkSimulationThesimulationmodelrepresentsawirelessnetworkofDSRCRSUsconnectedtotheInternet,andvehiclesequippedwithDSRCOBUsthatexchangeInternettrafficwiththoseRSUs.TransfersofdatapacketsaresimulatedbetweenpairsofvehiclesandbetweenvehiclesandRSUs.Atanygiventime,packetstreamsflowbetweeneachconnectedvehicleandoneRSUwhichservesasagatewaytotheInternet,eitherdirectlyorthroughmultiplehopswithothervehiclesactingasrelays.Thesedatatransfersaresimulatedatthetransport,network,linkandphysicallayersusingthens-3networksimulator(“Ns-3NetworkSimulator”2015).Aparticularsetofparametersusedinarunofthesimulationisreferredinthispaperasasimulationscenario.

Theunderlyingassumptionsofthenetworksimulationmodelaredescribedbelowinthefollowingorder.First,mobilityandnetworktopology,theuseofDSRCspectrum,the

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estimationofthroughputrates,andendpointsfortrafficflowsaredescribed.Then,thedescriptionisseparatedbycommunicationlayer,beginningwiththetransportlayerandthenproceedsonebyoneuntilthephysicallayer.

VehiclemobilityandRSUlocations.Resultsobviouslydependonthelocationsofvehiclesineverytimeinterval.ArealisticmodelofvehiclepositionsisderivedfromthelogsofvehicleGPSreadingsfromPorto.GPSreadingsarecollectedeverysecondfortaxis,soeveryfifthreadingmarksthebeginningofatimeinterval.GPSreadingsforbusesarecollectedevery15seconds,sowegetpositionsinterpolatedevery5seconds.ThepositionsofvehiclesotherthanbusesarealsoderivedfromtheGPSlogsoftaxis.Especiallyinurbanareas,mobilitypatternsofprivatevehiclesarelikelytobesimilartothoseoftaxis,althoughperhapsnotidentical.Vehiclemobilityissimulatedasaseriesof“snapshot”positionsin5-secondintervals,meaningthatrepresentationsofvehiclesarecreatedinthesimulationwithstaticpositions.Then,communicationsbetweenvehiclesandRSUsaresimulated,andthethroughputratesareestimated,representingawirelessnetworkwithnon-movingnodescommunicatingfor5seconds.Afterthesimulationruncompletesandthroughputratesarecalculatedforonetimeinterval,theprocessrepeatsforthenext5-secondinterval:thepositionsofthevehiclesarechangedtorepresentthenetworktopologyforthenext5seconds,thecommunicationssimulationandthroughputrateestimationisperformedagainforthereferredinterval,andsoon.

ResultsalsodependonthelocationsofRSUs.ThesimulationacceptsRSUdensityasaninputvariable,andthenplacesRSUswheretheyarelikelytodothemostgood.Thus,RSUsshouldbesetinplaceswithalargenumberofvehiclesatpeakhours.Morespecifically,agivennumberkofRSUsareplacedusingthek-meansclusteringheuristic(Moore2001),withpeak-hourvehiclepositionsastheinput.Thealgorithmisapopularapproachtodivideanumberofobservations(vehiclelocations,inourcase)intokregions,andfindtheoptimalcentroidforeachregion,withrespecttominimizingthedistancebetweeneachobservationandthecentroid.ThepositionsfoundforthecentroidarethenusedtoplacetherepresentationsoftheRSUsinthesimulationbeforeitisrun.Foreachsimulationscenario,thenetworkissimulatedmultipletimeswithinfrastructuredensityrangingfrom0to10RSUs/km2.

VehicleandRSUantennasareplacedinatri-dimensionalspace.XandYcoordinatesrepresentlongitudeandlatitude,respectively,andaregivenbytheGPSdata.Zcoordinatesrepresenttheheightofantennas.AllRSUantennashaveaheightof7meters,whichistheaverageheightofPortoRSUsasinMarch2015.Busantennashaveaheightof3meters(averageofsingledeckbusesinPorto),andallothervehicleshaveheightof1.5meters(whichisconsistentwithpreviousworkin(Bobanetal.2011)).

UseofDSRCspectrumforInternetaccess.75MHzofspectrumallocatedforDSRCisusedinseven10MHzchannels,ofwhichoneisreservedforcontrolandmanagementofallchannels,andtwoothersarereservedforsafetyapplications(IEEE2010b).WeassumethefourremainingchannelsareavailableforInternetaccess,andeachvehicleOBUandeachRSUisequippedwithfourradios.

Itisassumedthateachpacketstreamsflowusesonechanneli.Thechanneltobeusedateachhopoftheflowischosenastheleastusedchannelintheareasimulated.

iMorechannelsmaybeallocatedifthedatatobesentinaflowexceedsthecapacityofthechannel,butthisisnotthecasefortheresultspresentedinthispaper.

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Estimationofthroughputofthevehicularnetwork.ThethroughputrateviaDSRCforeachvehicleisdefinedasthedatathroughputachievablewhenthatvehiclereceivesdatafromaRSUitisconnectedto(eitherthroughasingleormultiplehops).WeassumethatthetrafficsentdownstreamtoanygivencarequalsthesumofthethroughputovertheDSRCnetworktothatcarandthethroughputoverthecellularnetworktothatcar.Thesameisassumedfortraffictravelingupstreamfromeachcar.Theseassumptionsareaccurateiftheamountoftrafficthatislostandtheamountoftrafficthatisunnecessarilysentonbothnetworksarebothnegligible.Thisisreasonableaslongasthecellularnetworkisalwaysavailableandhasenoughcapacitytocarryalltrafficthatcannotbecarriedoverthevehicularnetwork.

Steady-statethroughputthroughDSRCareestimatedforeach5-secondintervalbasedonthepositionsofallvehiclesatthebeginningoftheinterval.Thissimplifyingassumptionignoresthefactthatvehiclesmovecontinuouslyduringtheinterval,sothroughputwouldactuallychangegraduallyratherthanjumpevery5seconds.Thisformofanalysismaymisssomeofthefluctuationsindatarateasobservedbyamovingvehicle,butitallowsforagoodapproximationofthroughputwhenaveragedovermanytimeintervalsaslongasvehiclescanswitchoffbetweenthevehicularnetworkandaubiquitouscellularnetworkasneededsothatdataratefluctuationshavelittleeffectonthetotalamountoftrafficsentandreceived.Thisisareasonablefirst-orderestimateifthetimetoestablishV2VandV2Ihopsisnegligible,andthisswitchingtimewithDSRCisexpectedtoberoughly300milliseconds(IEEE2010b;MussabbirandYao2007).Toestimatesteady-statethroughputinagiventimeinterval,thesimulationisfirstrunforanextendedwarm-upperiodbeforestatisticsaregathered.Thewarm-uppartofthesimulationrunsfor8seconds,andafterthatstatisticsarecalculatedforthedatareceivedinonesecond.This8-secondwarm-upperiodwasobtainedbyexperimentation–allscenariossimulatedresultedinthroughputclosetothemeanafterthatperiod,andmostdosolessthan1secondafterthebeginningoftheinterval.

EachDSRC-equippedvehicleistheendpointofoneandonlyonebidirectionalflow,whileeachRSUmaybetheendpointforzero,oneormoreflows,uptothenumberofvehicles.However,anyvehiclecanalsoserveasarelayfordataofaflowthathasanothervehicleasadestination,incaseofmultihopcommunications.Protocol-specificdataincludeacknowledgmentsandretransmissionsinalllayers.However,thoseprotocolmessagesarenotconsideredinthestatistics–onlythenumberofapplication-layerdatabytesreceivedandsentbythevehicleperunitoftimeisconsideredinthethroughput.

Endpointsfortraffic.EachRSUisagatewaytotheInternetwhichagivenvehicleconnectsto.Weonlymodelthetrafficonthevehicularnetwork,i.e.betweenvehiclesandRSUs,sowetreattheRSUasifitweretheendpointofatransport-layerconnectionratherthanmerelyagateway.

Transportlayer.Ateachinterval,aTransmissionControlProtocol(TCP)connectionissimulatedbetweeneachvehicleandRSUitconnectsto.TCPisusedbecauseitisthemostcommontransportprotocolusedintheInternet(Raoetal.2011;Zambelli2009).

TheTCPMaximumSegmentSize(MSS)usedis2244bytes,whichisthemaximumsizeofthepacketthattheIEEE802.11linklayersupportswithoutfragmentation(2304bytes),minus60bytesforthelinkandIPheaders(WangandHassan2008).ThatMSSisroughlysimilartotypicalvaluesTCPconnectionstraversing802.11networks.

Networklayer.IPpacketsareroutedbetweentwoendpointsthroughthepathwiththesmallestnumberofhops.IfthereareneighboringRSUs,theonewiththehighestreceived

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signalischosenastheendpoint.Otherwise,thevehiclesearchesitsneighborsinrandomsequence,fora2-hoppathtoanyRSU.Thesearchstopsforthefirstpathfound.Ifnoneisfound,thenthesearchcontinuesfora3-hoppath.Ifnonepathisfound,thenitisassumedthevehicleisunreachablebyanyRSU.Thismethodisasimplificationbecauseroutingalgorithmsinvehicularnetworksareanongoingresearchtopic(Meireles2015;LiandWang2007;Meireles,Steenkiste,andBarros2012;Wisitpongphanetal.2007).

Linklayer.Themediaaccesscontrol(MAC)sublayerintheDSRClinklayeristheonespecifiedintheIEEE802.11pamendment(IEEE2010a)oftheIEEE802.11standards.AllpacketstransferredinallhopshavethesameprioritywithrespecttotheIEEE802.11puserprioritylevels.

Physicallayer.Thereceiversensitivitythresholdis-94dBm.Ahopisusedbetweentwonodesonlyifsignalstrengthatthereceiverexceeds15dBabovethesensitivitythreshold.ThisisthecriteriadeterminedempiricallyinthebusnetworkofPortoastheminimumqualityforthepairsofnodestotransferdata.Whenthehopisused,packetsarereceivedatanerrorratethatalsodependsonthesignal-to-interference-plus-noiseratio(SINR),asdescribedin(LacageandHenderson2006)and(“Ns-3NetworkSimulator”2015).

Thetransmittedpoweris14.6dBm,obtainedfrommeasurementsattheequipmentoutput,whichisalsoconsistentwith(Cardoteetal.2012)and(Bai,Stancil,andKrishnan2010),andthegainsofthetransmissionantennasare16dBiand5dBifortheRSUsandvehicles,respectively,whichareconsistentwiththesettingsoftheequipmentinthePortobusnetwork.

Thereceivedsignaliscalculatedaccordingtothepropagationlossmodelfrom(Meiniläetal.2009)(urbanmicrocellB1variant).ItwasthepreferredmodelbecauseitisvalidfortheDSRCband(5.9GHz),anditexplicitlymodelstwoothercharacteristicsthatarerelevantinvehicularnetworks:whetherthosenodesareinline-of-sight(LOS)ornon-LOS(NLOS)(Meiniläetal.2009;Zhaoetal.2006),andtheantennaheightsofvehiclesandRSUs(Mecklenbraukeretal.2011;Meiniläetal.2009).ForLOS,thelossLisgiven(indB)as

𝐿 = 𝑃𝐿$%& + 𝑁~(0, 𝜎)

where𝑃𝐿$%& = 𝐿/ + 10𝑛𝑙𝑜𝑔6/(𝑑)isthepathlosscalculatedasareferenceloss𝐿/andafunctionofthedistanced(meters)andthepathlossexponentnrepresentingthedegreeofattenuation.NisaGaussianrandomvariablewithzeromeanandrepresentslarge-scalefadingeffectssuchasshadowingoftheLOSpathbyobstacles.ForLOSthevaluesare

𝑛 =2.27𝑓𝑜𝑟𝑑 < 𝑑>?4𝑓𝑜𝑟𝑑 ≥ 𝑑>?

𝐿/ =41 + 20𝑙𝑜𝑔6/

𝑓5 ∗ 10D

𝑓𝑜𝑟𝑑 < 𝑑>?

9.45 − 17.3𝑙𝑜𝑔6/ ℎ6 − 1 − 17.3𝑙𝑜𝑔6/ ℎI − 1 + 2.7𝑙𝑜𝑔6/𝑓

5 ∗ 10D𝑓𝑜𝑟𝑑 ≥ 𝑑>?

𝜎 = 3

wherefistheDSRCfrequencyinHz,h1andh2aretheheightsofthevehiclesand/orRSU,and𝑑>? = 4 ℎ6 − 1 ℎI − 1 𝑓/𝑐 (cisthespeedoflightinm/s).

ForNLOS,

𝐿 = 𝑃𝐿$%& + 20 − 12.5𝑛 + 10𝑛𝑙𝑜𝑔6/ 𝑑 + 3𝑙𝑜𝑔6/𝑓

5 ∗ 10D+ 𝑁~(0, 𝜎)

11

𝑛 = 𝑚𝑎𝑥(2.8 − 0.0024𝑑, 1.84)

𝜎 = 4

Othermodels(Zhaoetal.2006)providesimilarpathlossandshadowingparametersnotsubstantiallydifferentfromthoseshownabove.

EachintervaleachlinkisassumedtobeinLOSorNLOSaccordingwithprobabilityProb(LOS)estimatedas(Calcevetal.2007)

𝑃𝑟𝑜𝑏(𝐿𝑂𝑆) =𝑑 − 300300

𝑓𝑜𝑟𝑑 < 300

0𝑜𝑡ℎ𝑒𝑟𝑤𝑖𝑠𝑒

(Asplundetal.2006)and(Meiniläetal.2009)proposeexpressionswhichresultssimilarLOSprobability.

Inadditiontopathlossandshadowing,somemodelsincludezero-meanrandomvariablestorepresentfast-fadingeffectssuchasmultipathpropagationandDopplerspread(Mecklenbraukeretal.2011).Inoursimulationmodel,theestimatedpathlossandshadowingcomponentsareassumedtobeconstantovereach5-secondinterval,andtheeffectoffast-fadingisassumedasnegligible,asweestimateaveragelossesacrossmanylinksratherthanpredictthelossofaparticularlink.

ThedifferencebetweenthemediansimulatedlossandthemedianlossmeasuredinPortobusesislessthan5dBformostdistancesshorterthan200meters,whichshowstheassumedmodelisareasonableapproximationfortheobservedloss.Forexample,atadistanceof100mbetweenaRSUandabus,themedianmeasuredlossis92dBwhilethesimulatedlossis95dB.Morethan95%ofthehopsobservedinthePortonetworkareshorterthan200meters.

3.2 Benefit-CostAnalysisThesecondstepofthemethodologyistouseDSRCthroughputtoestimatebenefitsandcostsofInternetaccessatpeak-hours.Ourdefinitionsofcostsandbenefitareindependentofwhoincursthosecostsandwhoderivesthosebenefits.Thisallowsustoquantifytheimpactofdeployinganewkindofinfrastructureontotalsocialwelfarewithoutmakinganyassumptionsaboutthingslikewhopaysforbuildingandoperatingtheroadsideinfrastructure,whethertheoperatorofroadsideinfrastructurechargesfortheservice,whopaysfortheservice,orhowmuch.Goodanswerstothequestionscanbefoundifandonlyifanewsystemwouldincreaseoverallsocialwelfare.

WedefinethebenefitofInternetaccessthroughvehicularnetworksasthenetpresentvalueofcostsavings,whichwederiveunderthefollowingassumptions.Allmacrocellularcarriersintheregionbeinganalyzedareassumedtobecapacity-limitedinsteadofcoverage-limited.Inacoverage-limitedsystem,acarrierdeploystheminimumnumberoftowerstomeetcoveragerequirements,andtherewillstillbemorecapacitythanneededeveninthepeakhour.InternetaccessthroughDSRCisnotvaluableinaregionthatalreadyhasexcessunusedcapacity.Incontrast,inacapacity-limitedsystem,acarrierdeploysenoughtowerstomeetcapacityrequirements,whichmeansthesystemisexpectedtooperateatfullcapacityduringpeakhours.Therefore,Internetusageinvehiclesasanewsourceofmobiletrafficshouldbemeteitherviacapacityexpansionofthemacrocellularnetworks,orviaoffload.Toservemoreusersorhigherrateperuser,acapacity-constrainedcarrierthatisalreadyusingcurrenttechnologythroughoutthespectrumavailabletoit

12

mustdeploynewtowers,resultinginasmallerareapercelltodelivermorecapacityperarea.

Besidesdeploymentofnewtowers,therearetwootherwaystoincreasemacrocellularcapacitythatmaypreservetheexistingmacrocellulartopology.Oneistheacquisitionofmorespectrum,whichincreasesthecapacitypertower.Theotherwayischangingtheefficiencyofthetechnologyemployedpertower,suchasmigratingfrom3Gto4Gequipment,oraddingequipmenttoincreasethenumberofsectorspertower.Sincenetworkdesignerswillgenerallychoosetheapproachforexpandingcapacitythatismostcost-effectiveatthetime,themarginalcostofincreasingcapacityislikelytobesimilarforallavailableapproaches(TanandPeha,2015).Weassumeforthisanalysisthatthedeploymentofnewtowersisthepreferredmethodtoincreasemacrocellularcapacity.Carriersdodeploytowerswhentheyneedcapacity,inpartbecausespectrumisdifficultandcostlytoobtain,andcarriersthatneedmorecapacityinaregionareoftenalreadyusingcurrenttechnologythere(Clarke2014)andhaveoftendeployedthemaximumnumberofsectorsallowedbythattechnology.

ItisassumedthatineveryintervaldeviceswillsendasmuchtrafficaspossibleovertheDSRCnetwork.TheamountoftrafficcarriedthroughDSRCequalsthereductionintheamountoftrafficcarriedthroughcellular,meaningthatdevicesswitchbetweentheDSRCandmacrocellularnetworkwithnegligibledisruption,withnodatabeinglostortransmittedinduplicitythroughbothnetworks.

WedefinethebenefitofInternetaccessthroughvehicularnetworksinagivenscenarioasthenetpresentvalueofcostsavingsfromreducingthenumberofcelltowersthatwouldotherwisebeneededtocarrythatpeak-hourtrafficifitwasnotcarriedthroughDSRC,assumingcellularcarriersarelimitedbycapacity.

TheNPVofthebenefitofInternetaccessperkm2is

𝑁𝑃𝑉𝐵 = 𝜌[\]^_`ab^c[ ∗ 𝐶`ab^c

where𝜌[\]^_`ab^c[isthetotalnumberofmacrocelltowers“saved”perunitofareaduetoInternetaccessthroughvehicularnetworksand𝐶`ab^c istheaverageNPVpermacrocelltower.

Whencalculatingtherelationshipbetweencostandcapacity,weassumethatifthereissufficientcapacitydownstreamthenthereisalsosufficientcapacityupstream,andthatcarriersareusingFrequencyDivisionDuplexing(FDD)sospectrumcanbelabeledaseitherupstreamordownstream.Thisisreasonablebecausedownstreamtrafficrateshavebeengrowingfasterthanupstreamrates(Sandvine2014),andmosttier-1carrierscurrentlyuseFDD(Engebretson2012).Inacellularnetwork,themaximumdownstreamcapacity𝑏𝑝𝑠_𝑚𝑎𝑥ghiinbitspersecondperunitofareaisgivenby

𝑏𝑝𝑠_𝑚𝑎𝑥ghi = 𝑠[^j`ac ∗𝑏𝑤𝐹𝑅

∗ 𝜌`ab^c[ ∗ 𝑁[^j`ac[

where𝑠[^j`ac istheaveragedownstreamspectralefficiencyinbitspersecondperhertzpersector,bwisthetotalbandwidthpermacrocellularcarrierusedfordownstreamtransmission,𝐹𝑅 ≥ 1isthefrequencyreusefactor,𝜌`ab^c[isthenumberoftowersperkm2

and𝑁[^j`ac[isthenumberofsectorspermacrocellulartower.

Inordertoserveallfluctuationsofdemand,themaximumcapacityshouldequalorexceedthedataratedemandatpeakhours.Therefore,if𝑏𝑝𝑠1ghi isthepeak-hour,downstream

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dataratedemandperunitofareafrommacrocellswhennoInternetaccessthroughDSRCtakesplace,thenumberoftowersnecessary𝜌1perunitofareais

𝑏𝑝𝑠1ghi = 𝑠[^j`ac ∗𝑏𝑤𝐹𝑅

∗ 𝜌1 ∗ 𝑁[^j`ac[

Let𝑏𝑝𝑠2ghi alsobethedataratedemandfrommacrocells,butwhenpartofthetrafficiscarriedthroughDSRC,tobeservedby𝜌2towers.Thedifferencebetween𝑏𝑝𝑠1ghi and𝑏𝑝𝑠2ghi isthetrafficoffloadedperunitofarea:

𝑏𝑝𝑠1ghi − 𝑏𝑝𝑠2ghi = 𝑏𝑝𝑠𝑂𝑓𝑓 = 𝑠[^j`ac ∗𝑏𝑤𝐹𝑅

∗ 𝜌[\]^_`ab^c[ ∗ 𝑁[^j`ac[

thenthenumberofsavedmacrocelltowersis

𝜌[\]^_`ab^c[ =𝑏𝑝𝑠𝑂𝑓𝑓 ∗ 𝐹𝑅

𝑠[^j`ac ∗ 𝑏𝑤 ∗ 𝑁[^j`ac[

wherebpsOffisthedownstreamDSRCthroughput.

ThetotalcostofInternetaccessthroughDSRCperkm2𝑁𝑃𝑉𝐶 consistsofthreetypesofcosts:

𝑁𝑃𝑉𝐶 = 𝑁𝑃𝑉𝐶m&n + 𝑁𝑃𝑉𝐶%>n + 𝑁𝑃𝑉𝐶&o^j`cph

where𝑁𝑃𝑉𝐶m&n, 𝑁𝑃𝑉𝐶%>n and𝑁𝑃𝑉𝐶&o^j`cpharetheNPVperkm2ofthecostsofRSUs,OBUsandDSRCspectrum,respectively,andaregivenas

𝑁𝑃𝑉𝐶m&n = 𝜌m&n ∗ 𝐶m&n

𝑁𝑃𝑉𝐶%>n = 𝜌%>n ∗ 𝐶%>n

𝑁𝑃𝑉𝐶&o^j`cph = 𝜌&o^j`cph ∗ 𝐶&o^j`cph

where𝜌m&n isthenumberofRSUsforInternetaccessdeployedperunitofarea,whichisassumedtobeindependentandnotsharedwithRSUsdeployedforsafetyorpurposesotherthanInternetaccess,𝜌%>n isthenumberofOBUsdeployedperunitofarea,𝜌&o^j`cphistheamountofDSRCspectruminMHztimesthepopulationdensity,and𝐶m&n, 𝐶%>n, 𝐶&o^j`cpharetheNPVperRSU,OBU,andMHzofspectrumperperson(alsoknownasthecostperMHz-pop),respectively.

Thecomparisonbetweenthebenefitandcostsdefinedabovedependsonthedecisiontobemade,i.e.someofthecoststhatarerelevantforonedecisionmaybeirrelevantforanother.Forexample,inthecontextofasafetymandate,DSRCspectrumisallocatedandOBUsarepurchasedforsafetyreasons.Inthiscase,spectrumandOBUcostsaresunkwithrespecttoInternetaccess,andadecisiontodeployRSUinfrastructureincreasessocialwelfareifandonlyifbenefitofInternetaccessthroughDSRCexceedsRSUcosts.

However,ifthereisnosafetybenefitderivedfromthemandate,thenspectrumandOBUcostsarenotsunk,andsocialwelfarewillincreaseonlyifbenefitofInternetaccessexceedsallDSRCcosts:RSUs,OBUsandspectrum.

Inthisanalysis,weassumeparametersthataffecttheNPVofcostandbenefitarestatic,andwillusenumericalvaluesthatarereasonablefordecision-makersthatarelookingseveralyearsintothefuture.Inreality,someoftheseparametersarechangingovertime,althoughinwaysthataresometimeshardtopredictmorethanafewyearsintothefuture.Ingeneral,weexpectthatDSRCthroughputwillincreaseovertime,becauseboththenumberofDSRC-equippedvehiclesontheroadandthedatarateperDSRC-equippedvehicleare

14

likelytoincreaseovertime.Asthroughputincreases,thebenefitsandthecostsofInternetaccessthroughDSRCbothincrease,e.g.moreRSUsaredeployedtocarrymoretraffic.AslongasnetbenefittendstoincreasewithDSRCthroughput,andthroughputincreasesovertime,thenaresultthatbenefitsexceedcostsinagivenscenariogenerallymeansthatdeploymentinthecurrentplanninghorizonwillincreasesocialwelfare.Ontheotherhand,ifnetbenefitincreaseswithDSRCthroughputandcostsarefoundtoexceedbenefitsinthecurrentplanninghorizon,thatdoesnottelluswhetherornotcostswillstillexceedbenefitsinthefuture.

3.3 BaseCaseScenarioThisSectiondescribesthebasecasenumericalvaluesfortheassumptionsusedintheestimatesofthethroughputofInternetaccessviaDSRC,benefitandDSRCcosts.Someofthosevaluesaresubjecttouncertainty,changewithlocation(suchasthepopulationdensity),and/orareexpectedtoevolveintime(suchaspenetrationandtrafficpervehicle).Therefore,wewillalsoconsiderscenarioswhereoneassumptionvalueisvariedatatimefromitsbasecasevalueinordertoobservetheeffectofeachassumptionontheresults.

Themonetaryvaluesthatfollowareinconstant2014dollarsj.BenefitandcostNPVsarecalculatedatarealdiscountrateof7%overahorizonof10years.ThediscountrateisconsistentwiththeraterecommendedbytheU.S.OfficeofManagementandBudgetforbenefit-costanalysisoffederalprograms(OfficeofManagementandBudget1992).Otheranalysesusesimilarrates(HallahanandPeha2009;Markendahl2011;MarkendahlandMäkitalo2010;Hardingetal.2014).The10-yearhorizonislongenoughtoevaluatethelifetimecostsofthemainelementsofthemodel.Forexample,RSUlifetimewasestimatedtobe10yearsinanalysisfortheU.S.Dept.ofTransportation(Wrightetal.2014).AnOBUlifetimeof10yearsisconsistentwithestimatesfromtheU.S.DOTfortheaveragelifetimeofcarsintheU.S.(Santosetal.2011).Althoughsomecostssuchasmacrocellulartowersareincurredforalongerhorizon,becauseofthe7%discountrate,theirNPVisprimarilydeterminedinthefirst10years.

Thebasecasevaluesare:

Populationdensity.Wemakethesimplifyingassumptionthatpopulationdensityisconstantthroughouttheregionbeinganalyzed.Forthebasecasethepopulationdensityischosenas5000people/km2,whichisrepresentativeofPorto(5,600)(InstitutoNacionaldeEstatistica2011)whereourmeasurementsweretaken,aswellaslargecitiessuchasBoston(5000people/km2),Chicago(4,600),Miami(4,300)(UnitedStatesCensusBureau2015),London(5,000)(UKOfficeforNationalStatistics2012),andTokyo(5,900)(TokyoMetropolitanGovernment2014).Populationdensitycanbemuchgreater,e.g.Paris(21000people/km2)(INSEE2013)oritcanbenegligible.

Numberofvehiclesontheroadatpeakhourspercapita.AssumedasinTable2,whichiscalculatedastheproductofvehiclesownedpercapita,fractionoftimevehicleisinuse,andratioofpeak-hourusagetoaverageusage:

jWhenvaluesarebasedonoldersources,theyareadjustedusingtheU.S.ConsumerPriceIndex.

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(i) ThenumberofvehiclesownedpercapitaintheU.S.varieswithpopulationdensityandwasestimatedusingdatafrom(UnitedStatesCensusBureau2015),andisshowninTable2k.

(ii) TheU.S.NationalHouseholdTravelSurvey(NHTS)reportsthatavehicleisusedfor57minutesperday,onaverage(Santosetal.2011).

(iii) AlsofromtheNHTS,weestimatedtheratiobetweenthenumberofvehiclestravellingatpeakhoursandthenumberofvehicletravellingatalltimesoftheday,as1.94(calculatedfromFigure12of(Santosetal.2011)).

Table2.Numberofvehiclesontheroadatpeakhourspercapita,asafunctionofpopulationdensity

Peopleperkm2

Numberofvehiclesownedpercapita

Numberofvehiclesontheroadatpeakhourspercapita

10 1 0.077200 0.8 0.0611000 0.65 0.0502000 0.6 0.0463000 0.55 0.0425000 0.44 0.03412000 0.22 0.017

Numberofbuses/Numberofvehiclesontheroad.Inthebasecase,weassume1.4%ofthevehiclesontheroadarebuses.BasedonNHTSdata,about10%oftotalpassenger-kmperyearintheU.S.aretravelledinpublictransit(calculatedfromTable7of(Santosetal.2011)).Assuming1.5astheaveragecaroccupancy(NHTS,Table16)and11astheaveragebusoccupancyintheU.S.(Puchalsky2005),wecalculatedtheaverageratiobetweenthenumberofbusesandthetotalnumberofvehiclesontheroadas10%*1.5/11≈1.4%.Itisassumedthatthisratioappliesforpeakhours,andmostpublictransitkmaretraveledinbuses.

DSRCPenetration.Thebase-casevalueofpenetrationofDSRCOBUsinvehiclesis25%.Thisisreasonableforadecision-makerlooking5to10yearsaheadinthecontextofagovernmentmandatetodeployDSRCinvehicles.TheU.S.DepartmentofTransportationforecaststhatallnewscarswouldbesoldwithOBUswithin3yearsafterasafetymandateiseffective(Hardingetal.2014).TheaveragelifetimeofanewcarintheU.S.is11years(Wile2014),soaslongasabout9%ofallvehiclesarereplacedeachyear,itisreasonabletoexpectpenetrationwillreach25%afterafewyearsfollowingthemandate–indeed,(Hardingetal.2014)estimateis5to6years.

DatatrafficperDSRC-equippedvehicleontheroad.Forthebasecase,weassumethatinany5-secondintervalduringthepeakhour,50%oftheDSRC-equippedvehiclesontheroadareendpointsfordatabeingcontinuallyat800kbps(totaldownstreamandupstream).Theremainingvehiclesarenotendpointsfortraffic,althoughtheymayrelaypacketsforothervehiclesinmultihopconnections.ThustheaveragetrafficperDSRC-equippedvehicleontheroadis400kbps.ThisisconsistentwiththeDeutscheTelekom kWeusedthedatafromtheAmericanCommunitySurvey2013,atthecountylevel,availableatcensus.gov/programs-surveys/acs/.TheU.S.averageisroughly0.9.

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predictionthatvehiculartrafficwillreach5GB/monthinthe“nextyears”(DeutscheTelekom2013),iftheaveragevehicleisontheroad57minperdayasdiscussedabove,andaveragedatarateisthesamewheneverthevehicleisinuse.Inreality,dataratesvaryfromvehicletovehicleatanygiventime,butsinceRSUsaretypicallyinrangeofdozensofDSRC-equippedvehiclesatalltimesduringpeakhour,thissimplifyingassumptionshouldhavelimitedeffectonaggregatethroughput.

Shareofdownstreamtraffic.Weassumethatwhileavehicleistransferringdata,90%ofthedataflowsinthedownstreamdirection(RSUtovehicle).InPortoDSRCnetwork92%ofasessionvolumeisdownstream,onaveragel,and(Sandvine2014)reportsasimilarratioforthemonthlyusagepermobiledeviceintheU.S.

Unitcostofmacrocellulartower.Thebase-caseassumptionforNPVofcostpermacrocelltowerover10yearsis$750,000.Wherecarriersareleasingspaceonexistingcelltowers,thiscostincludesleasingfees.Wherecarriersbuildtheirowntowers,adecadeofleasingfeesisreplacedbyCAPEX.A10-yearNPVof$750,000isroughlyconsistentwithsomepreviousestimatesthatvarybetween$650,000(FederalCommunicationsCommission2010a),$800,000(HallahanandPeha2011),and$900,000(Newman2008),in2014dollars.

Macrocellularspectrumefficiency.Weassumedthedownstreamaverageefficiencyofamacrocellas1.4bps/Hz/sectorforthebasecasevalue,whichisanacceptedvalueforLTE-FDDrel.8,assessedbythe3GPP(Sesia,Toufik,andBaker2011).Somedeviceswillbemorespectrallyefficient,suchasthoseusingLTE-Awhichisexpectedtohaveanefficiencyof2.4bps/Hz/sectorormore(Sesia,Toufik,andBaker2011),whileusageoflessefficientdevicesalsocontinues(withefficienciesbelow1bps/Hz/sector,asestimatedin(Clarke2014)).

Sectorspermacrocell.Weassumedeachmacrocellisdividedin3sectors,whichisconsistentwithatypicalmacrocellconfiguration(Sheikh2014).

Macrocellularbandwidth.Weassumedthatanynewtowerdeployedinacapacity-limitedregionwouldbeconstrainedbythebandwidthavailablefordownlink,andwouldoperateoveradownlinkbandwidthof70MHzpersectorinthebasecase.Atier-1providerisestimatedtoholdroughly30MHzofdownlinkspectrumforLTE,onaverage(Goldstein2015),andspectruminuseforLTEisestimatedasabouthalfoftotalspectrumformobilebroadbandm.Substantialamountsofnewspectrumareexpectedtobeallocatedbyregulators(FederalCommunicationsCommission2010b),butitseffectiveusemaytakeseveralyearsforreallocation,auctioningandactualdeployment.Thuswechose70MHzasabasevalueofdownlinkspectrumforthenextfewyears.

Macrocellularfrequencyreusefactor.Weassumedafrequencyreusefactorinmacrocellsof1.Thisisconsistentwithatypicalmacrocellularnetworkconfigurationwithcurrenttechnology(WannstromandMallinson2014).

UnitcostofDSRCRSU.WechosetheaverageNPVover10yearsofaDSRCRSUas$14,000.ThisisbasedonU.S.DOTestimates(averageannualcostbetween$2,000-3,000(Wrightetal.2014),includingreplacementcostsevery5to10years).However,inSection4wewillconsidervariationsofmorethan50%fromthebasecasevalue,asconditionsaboutinfrastructureavailabilitymayvary.Forexample,theCityofPortodeployedRSUsfora

lAsinMarch2015.mSeee.g.(Clarke2014),Table3,or(SprintNextelCorporation2011).

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Capexofbetween$1,200-4,000,byplacingRSUsinexistingstructures(trafficpoles,buildings,etc.)alreadyownedbythecityandalreadyequippedwithenergyandbackhaulaccess.ThecostperRSUcouldbealsobelowerifRSUsdeployedforInternetaccessaresharedforsafetyorvice-versa,thoughsharingdependsonmanyissues,includingwhethertheoptimalplacementofRSUsforInternetaccessmatchestheplacementforsafetycommunicationsandwhetherdevicesforInternetaccessanddevicesthataresafety-criticalareplacedundersharedcontrol.ForthebasecasevalueofthecostperRSU,nosharingisassumed.

Ontheotherhand,costscanbesignificantlyhigherifnewpoles,energyandcommunicationsinfrastructurehastobebuiltentirely(Wrightetal.2014).

UnitCostofDSRCOBU.Forthecaseofthevehiculardevice,ourbase-caseassumptionforNPVofthecostofaDSRCOBUis$350.ThisisbasedonU.S.NHTSAestimates(Hardingetal.2014)consideringfourradiointerfacesandantennaspervehicle.However,massproductionofOBUscoulddriveunitcostsdown,especiallyinthecontextofamandate.Becauseofthis,wevarytheNPVperOBUfrom$350downto$50inSection4.

UnitcostofDSRCspectrum.ForthecostofDSRCspectrum,wechoseavalueof$0.10perMHzperpopulation(MHz-pop).Thisvalueisuncertain,asthecostofspectrumdependsonfrequency(Keransetal.2011;TanandPeha2015;Alotaibi,Peha,andSirbu2015;Peha2013),andthemarketvalueabove5GHzisnotwell-established.

4 ResultsThisSectionpresentsthesimulatedDSRCthroughput,benefitandcostresultsforthebasecasescenario,andhowthoseresultsvaryifbasecasevalueschangeeitherbecauseofvariationsacrosscitiesorregions(suchaspopulationdensity),anticipatedchangesovertime(suchaspenetrationortrafficpervehicles),oruncertaintyaboutthebasecasenumericalassumptions.Section4.1showsthroughput,benefitandcostsforthebasecasescenario,whereinSections4.2to4.7wediscussthevariationsinvaluesofpopulationdensity,penetration,trafficpervehicle,unitcostsofDSRC,andunitcostandbandwidthofmacrocellulartowers.

4.1 BaseCaseScenario–Benefit,CostsandVolumeofInternetAccessThroughDSRC

Figure3showsthroughputasafunctionofRSUsperkm2underbasecaseassumptions.Thethroughputincreaseswithhigherquantitiesofinfrastructure,asthenumberofvehiclesthatcanconnecttoaRSUincreases.However,themarginalgainsinoffloadratedecreaseastheRSUdensityexceeds2perkm2.ThismattersbecausewhileincreasingRSUdensityincreasesDSRCthroughputandthereforebenefit,italsoincreasescost.ThiscanbeseeninFigure4,whichshowsbothbenefitandcostsasafunctionofRSUdensityunderthesameassumptions.Figure4showsthatforthebasecasevalues,themaximumdifferencebetweenbenefitsandcostsoccursatinfrastructuredeploymentof1RSUperkm2.Atthisoptimalquantityofinfrastructure,benefitsexceedthecostofRSUsby50%.IfthespectrumhasalreadybeenallocatedandtheOBUsarealreadybeingpurchased,asislikelytooccurifDSRCisdeemedtobeimportantforsafetyapplications,thenthosearesunkcosts.

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Consequently,thebenefitsofdeployingRSUsexceedthecosts,anddoingsowillincreasesocialwelfare.However,Figure4alsoshowsthatbenefitofInternetaccessisconsiderablylessthanthecostofOBUs,muchlessthecombinedcostofRSUs,OBUs,andspectrum.Thus,thevalueofdeploymentofvehicularnetworksforInternetaccessalone,i.e.withoutconsiderationoftheimprovementsinhighwaysafety,arenotsufficienttojustifythedeploymentofOBUsandtheallocationofspectruminthebasecasescenario.

Infrastructure Density (RSUs/km2)

Figure3.Averagetrafficofferedandoffloadrateatapeakhour,forthebasecasescenario

Infrastructure Density (RSUs/km2)

Figure4.Benefitandcostforvaryinginfrastructuredensity,forthebasecasescenario

0.5 1 2 3 4 6 8

Peak

-hou

r Tra

ffic

(Mbp

s/km

2 )

0

4

8

12

16

20

Offloaded

Offered: 400 kbps per DSRC-equipped vehicle

0.5 1 2 3 4

Bene

fit a

nd C

ost N

PV (U

SD/k

m2 )

0K

50K

100K

150K

200K

Benefit

Cost: RSU+OBU+Spectrum

Cost: RSU+OBU

Cost: RSU

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Thenumberofsimulatedobservationsishighenoughsuchthatstatisticalsignificanceinthisgraph(andthegraphsthatfollow)issufficienttosupportconclusions.Forthebasecasescenario,foreachvalueofRSUdensity,1000vehiclesweresimulatedina20km2regionovera50-secondperiod.Wecalculateaveragethroughputovertimeforeachofthevehiclessimulated.Ifwemakethesimplifyingassumptionthatthethroughputsofthese1000vehiclesaremutuallyindependent,thenthemeanthroughputacrossallvehiclesis170kbps,whichisabout40%ofofferedload,andtheconfidenceintervaliswithin7%ofthemean.Thesamplestandarddeviationis180kbps.ThesamplestandarddeviationisaslargeasthesamplemeanbecausethedistributionofDSRCthroughputisclosetobimodal;withbasecaseassumptions,offloadiszeroin30%oftheobservations,wherevehiclescannotconnecttoaRSUeitherthroughsingleormultihoppaths,andDSRCthroughputequalstheofferedtrafficin30%oftheobservations.

4.2 PopulationDensitySection4.1showedthatdeployingRSUscanincreasesocialwelfareinthebaselinecase,whichcorrespondstoadenselypopulatedcitysuchasPorto,Chicago,orLondon.However,thatmaynotbethecaseeverywherebecauseboththecostsandthebenefitsofInternetaccessthroughvehicularnetworksarelikelytodependgreatlyonpopulationdensity.Thehigherthepopulationdensityinanarea,itisexpectedthatthenumberofvehiclesownedbythatpopulation,andthenumberofvehiclesontheroadatpeakhours,willbothincrease.Thereforeitisexpectedthatmorein-vehicleOBUswillbeused,andmoreRSUswillbedeployedforthosevehiclestoconnectto.Ononehand,thismakesDSRCcostsofOBUandRSUsincreasewithpopulationdensity.Ontheotherhand,throughputperunitofarea,andhencethebenefit,arealsoexpectedtoincrease.Thus,thisSectionexaminestheeffectofpopulationdensityonbenefitandcost.

Figure5andFigure6showthroughput,benefitandcostsasafunctionofpopulationdensity.Trafficpervehicle,penetration,unitcostsandspectrumparametersareheldconstantatbasecasevaluesforallpopulationdensities.BenefitandthecostofRSUsinFigure6dependonthequantityofRSUsforeachpopulationdensity,whichischosenasfollows.ForthevaluesofpopulationdensityinwhichtheNPVofbenefitofInternetaccessexceedstheNPVofcostofRSUs,thenumberofRSUschosenisthequantitythatmaximizesthedifferencebetweentheNPVofbenefitandtheNPVofRSUcost(thethroughputforeachscenarioisactuallysimulatedforadiscretesetofRSUquantitiesn.AlinearfitisperformedsuchthatfractionalRSUquantitiesarealsopossible).ForthepopulationdensityvaluesinwhichtheNPVofbenefitislowerthantheNPVofRSUcostforanyquantityofRSUs,theoptimalquantityisobviouslyzero,leadingtozerobenefitofInternetaccessandzeroRSUcostforthosepoints.However,forthesepointsweinsteadcalculatethequantityofRSUsasalinearextrapolationfromthepopulationdensityrangewhichtheNPVofbenefitisgreaterthantheNPVofRSUcost.ThisshowshowfarfromRSUcoststhebenefitofInternetaccesswouldbeinthose“negative”regions,thoughitdoesnotshowtheoptimalcost(becauseoptimalRSUcostandbenefitwouldbothbezerointhisregion).

Figure5showsthatofferedtrafficincreasesrapidlyasafunctionofpopulationdensity,whichisexpectedconsideringconstanttrafficpervehiclebutanincreasingquantityof

nEachscenarioissimulated12times,onetimeforeachofthefollowingvaluesofRSUdensity:0.25,0.5,0.75,1,1.25,1.5,2,3,4,6,8,and10RSUs/km2.Thenumberofpointswaschosenforsimulationtimereasons,sincethens-3modeliscomputationallyexpensive.

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vehiclesforhigherpopulationdensities.DSRCthroughputalsoincreaseswithpopulationdensity,thoughatalowerpacethanofferedtraffic.ThishappensbecausetheincreasedInternettrafficoriginatedfromagreaternumberofvehiclesperkm2causesmorecompetitionfortheuseofthewirelessmedium,andlimitsoffload.

Figure6showsthatbenefitincreasesfasterthanRSUcost.ThereasonisthatthroughputgrowsroughlyproportionallytopopulationbuttheoptimalnumberofRSUsrisesatalowerpace:for12000people/km2,thedifferencebetweenbenefitandRSUcostisfourtimeshigherthanfor5000people/km2areas.ThedensityatwhichbenefitofInternetaccessisequalorgreaterthanRSUcostdependsonpenetration,trafficandunitcosts;forthebasecasevaluesoftheassumptions,the“threshold”valueis4000people/km2.IfdecisionsaboutwhethertodeployRSUsaremadeonacity-widebasis,thismeanscitieswithpopulationdensitiesatleastasgreatasChicagoorPortoowouldbenefitfromRSUdeployment,atleastincaseswherethereisalreadyspectrumallocatedandamandateofDSRCOBUsforsafetypurposes.However,RSUscouldbedeployedwithinanareamuchsmallerthanacity,andmanycitieswithmoremodestpopulationdensityhavesomeneighborhoodswithpopulationdensityover4000peopleperkm2.

Population Density (people/km2)

Figure5.Averagetrafficofferedandoffloadrateatapeakhourforvaryingpopulationdensitiesandotherparametersfixedatbasecasevalues,optimalRSUquantityateachpoint(i.e.atRSUquantitythatmaximizestheNPVofbenefitminustheNPVofcostforeachpopulationdensity:1to2RSUs/km2)

oSee(GoverningInstitute2015)forpopulationdensitiesofthoseandothercities.

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Figure6.Benefitandcostforvaryingpopulationdensities(andotherparametersatbasecasevalues),andoptimalRSUquantityateachpoint

Figure6showsthatbenefitgrowsfasterthanRSUcostforawiderangeofpopulationdensities.However,thetrendisdifferentforOBUcost,whichgrowsmuchfasterthanbenefit.ThereasoncanbeseenfromFigure7,whichshowsvehicleownershipandvehicleusageasafunctionofpopulationdensity,usingbasecaseassumptionsofvehicleownershippercapita,timeontheroadpervehicleandpeakhourratio.Ownershipreferstothetotalnumberofvehiclesavailableperunitofarea.Intheeventofamandate,ownershipdetermineshowmanyvehicleswillhaveOBUsinstalled,andthetotalOBUcost.Ontheotherhand,vehicleusageisthenumberofvehiclesontheroadatpeakhours.AvehicleequippedwithDSRCwillonlyhavetrafficcarriedwhileontheroad,andonlythepeak-hourthroughputisrelevantforbenefit.Thus,Figure7helpsexplainwhyOBUcostsaresignificantlyhigherthanbenefitofInternetaccess,underamandatescenario.Overlocationswithincreasingpopulationdensities,andunderuniformDSRCpenetration,vehicleownershiprisesfasterthanvehicleusage,makingOBUcostsrisefasterthanbenefits,atleastforbasecasevaluesoftheotherparameters.Thismaynotbetrueforallassumptions.Forexample,ifOBUscostless,thentotalOBUcostwouldgrowmoreslowlywithrespecttopopulationdensity,buttheOBUcostswouldneedtobelowerthatbaselinebyoneorderofmagnitudeforOBUcostsnottoincreasefasterthanbenefits.

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Figure7.Averagevehicleownershipandusage(i.e.vehiclesontheroadatpeakhours)forpopulationdensityranges

Ontheotherhand,Figure8showswhathappensiftheratiobetweenthequantityofDSRC-equippedvehiclesinuseandthetotalquantityofvehiclesownedisdifferentthaninthebasecaseassumption.Forthisgraph,thepopulationdensityisheldinthebasecasevalue(5000people/km2),aswellaspenetration,trafficpervehicle,unitcosts,spectrumcharacteristicsandnumberofvehiclesowned.Whatisvariedisthenumberofvehiclesontheroadatpeakhourpercapita,meaningtheratiobetweenthatandthenumberofvehiclesownedchanges.Theratiovalueof0.08correspondstothebasecase,andislikelytovaryamongcitieswithcomparablepopulationdensities,inpartduetofactorsliketheavailabilityofpublictransportation(EuropeanCommission2012).AsFigure8shows,thenetbenefitofdeployingRSUswillbegreaterinacitywherealargerfractionofcarsareontheroadinpeakhours.

Ifvehiclesareequippedvoluntarilyratherthanbecauseofamandate,thenFigure8isrelevantforadifferentreason.Ifadoptionisvoluntary,ownersofvehiclesthatareofteninusearemorelikelytoadopt,andthiswouldalsohavetheeffectofincreasingtheratioofDSRC-equippedvehiclesontheroadatpeakhourtototalcarsthatisshowninFigure8.Thus,ifmanyoftheDSRCequippedcarsaredrivenextensively,asiscertainlythecasefortheDSRC-equippedbusesandtaxisinPorto,thenthiswillalsoincreasethenetbenefitofdeployingRSUs.

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Figure8.BenefitandcostforvaryingratiosbetweenthequantityofDSRC-equippedvehiclesinuseandthetotalquantityofvehiclesowned(andotherparametersatbasecasevalues),andoptimalRSUquantityateachpoint(1to2RSUs/km2)

4.3 PenetrationLikepopulationdensity,OBUpenetrationislikelytoaffectbenefitandcosts,althoughunlikepopulation,penetrationmayincreaserapidlyovertime.Withhigherpenetration,itisexpectedthatboththenumberofDSRC-equippedvehiclesandthenumberofDSRC-equippedvehiclesontheroadatpeakhourswillincrease.ThereforeitisexpectedthatmoreRSUsforthosevehiclestoconnecttowillbenecessary.Ononehand,thismakesDSRCcostsofOBUandRSUsincreasewithpenetration.Ontheotherhand,DSRCthroughputperunitofarea,andhencethebenefit,arealsoexpectedtoincrease.ThisSectionexaminestheeffectofpenetrationonbenefitandcost.

Figure9andFigure10showthroughput,benefitandcostsasafunctionofOBUpenetration,assumingthepopulationdensity,quantityofvehicles,trafficpervehicle,unitcostsandspectrumparametersareheldconstantatthebasecasevaluesforallvaluesofpenetrationconsidered.BenefitandthecostofRSUsdependonthequantityofRSUsforeachpenetration,whichischoseninthesamewayasinthepreviousSection.

Figure9showsthetotalamountoftrafficofferedincreasesrapidlyasafunctionofpenetration,whichisexpectedconsideringanincreasingquantityofvehiclesforhigherpenetrations.TheDSRCthroughputisalsohigher.Ifpenetrationincreasesovertimeasexpected(especiallyifthereisamandate),thenDSRCthroughputwillincreaseovertime.Sincebenefitisdefinedasafunctionofthroughput,RSUsareexpectedtobedeployedonlyinareaswherethepotentialratesarehighenoughforbenefittoexceedRSUcost,aslongasspectrumandOBUcostsaresunk.Therefore,thegrowthofDSRCthroughputovertimewouldeventuallycausethepotentialbenefitofInternetaccesstoexceedthecostofRSUsinregionswherethisisnotinitiallythecase.

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Figure9.Averagetrafficoffloaded(andoffered)atapeakhour:varyingDSRCpenetration(andotherparametersatbasecasevalues),optimalRSUquantityateachpoint(0.8to1.6RSUs/km2)

Figure10showsbenefitandcostsasafunctionofpenetration,withallparametersexceptpenetrationandpopulationdensityinthebasecasevalues.Thetophorizontalaxisshowspenetrationforalowerpopulationdensity(2000people/km2),whilethebottomhorizontalaxisshowspenetrationforthebasecasepopulationdensity(5000).Figure10showsthatasOBUpenetrationincreases,benefitincreasesfasterthanRSUcost.Thus,itisamatterofwaitinguntilpenetrationishighenoughthatbenefitofInternetaccesswouldexceedRSUcost.Ifpenetrationishighenough,itwillremainhighenough.Ontheotherhand,incitieswhereRSUdeploymentdoesnotresultinbenefitexceedingRSUcostwithinthecurrentplanninghorizon,thismaychangeafterafewyearsaspenetrationincreases.Moreover,benefitwillexceedRSUcostsoonerforcitieswithhigherpopulationdensity.UnderthenumericalassumptionsofFigure10,benefitofInternetaccessexceedsRSUcostswhenpenetrationis0.19orgreaterinacitywithpopulationdensityof5000,andwhenpenetrationis0.37orgreaterinacitywithpopulationdensityof2000peopleperkm2.

AlthoughbenefitofInternetaccessincreasesfasterthanRSUcostsaspenetrationincreases,OBUcostincreasesmuchfasterthanbenefit.ThereasonisthatpenetrationaffectsOBUcostandofferedtrafficlinearly,buttheformerincreasesmuchfasterthanthelatter,atleastforbasecasevalues.Moreover,benefitdependsonDSRCthroughput,whichincreaseslowerthanofferedtraffic.EvenifallofferedtrafficwerecarriedthroughDSRC,benefitwouldbenomorethantwiceascurrentlyestimated,andOBUcostwouldstillincreasefasterwithpenetration.

Thisresultmeansthatforthebasecaseassumptions,OBUcostfarexceedsbenefitofInternetaccessregardlessthepenetration.Andaspenetrationisexpectedtoincreaseovertime,thenthedifferencebetweenOBUcostandbenefitisalsolikelytoincrease.Inthissituation,iftherewereamandatewithnobenefitsotherthanInternetaccess,whichcouldonlybetrueifDSRChadnosafetybenefitswhatsoever,thensocialwelfarewoulddecrease.

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Figure10.BenefitandcostforvaryingvaluesofDSRCpenetration(andotherparametersatbasecasevalues),andoptimalRSUquantityateachpoint.Eachxaxisreferstoadifferentpopulationdensity

4.4 CostperOnboardUnit(OBU)InordertoinvestigatewhetherbenefitofInternetaccessthroughDSRCwouldeverexceedallcosts,includingtheOBUcostthatdominatedinthebasecase,thisSectionexaminestheeffectoftheOBUunitcostontotalbenefitandcost.

Figure11showsbenefitandcostsasafunctionofOBUunitcost,forthebasecasevaluesofpopulationdensity,thequantityofvehicles,penetration,traffic,RSUandmacrocellularunitcosts,andspectrumparameters.ThequantityofRSUsischosentomaximizethedifferencebetweenbenefitandRSUcost.IfamandatewastobejustifiedbyInternetaccessonly,thenbenefitofInternetaccessaloneshouldexceedallDSRCcosts.Figure11showsthattotalOBUcostwouldexceedRSUandspectrumcostscombinedundertheseassumptions,andthatthesumofRSUandOBUcostswouldexceedbenefitofInternetaccessevenifthecostperOBUfallsbymorethan80%from$350to$50.

ItisonlypossibleforthecostperOBUtodecreaseovertherangeshowninFigure11ifDSRCismass-deployedatascalecomparabletoWi-Fi.InthephysicallevelDSRCisspecifiedbytheIEEE802.11pstandard(IEEE2010a),whichismostlyanadaptationoftheWi-Fi802.11astandardforthe5.9GHzband.Wi-Firadioswithantennascurrentlycostnomorethanafewtensofdollars,andperhapsDSRCOBUcostscoulddropifitismassproduced.Butevenifthishappens,Figure11showsthatbenefitstilldoesnotexceedtotalOBUcost.

However,ifthereisamandateinwhichspectrumisalreadyallocatedandOBUsarepurchased,thenspectrumandOBUcostsaresunk.Inthisscenario,sincebenefitofInternetaccessexceedsRSUcostforbasecaseassumptions,RSUdeploymentforInternetaccessdoesincreasesocialwelfare.

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Figure11.BenefitandcostforvaryingNPVperOBU(andotherparametersatbasecasevalues),andoptimalRSUquantity(1RSU/km2)

4.5 InternetTrafficperVehicleItisimportanttoconsiderdifferentvaluesfordataratepervehicle,bothbecausethereareuncertaintiesinanypredictionoffuturedatarate,andbecausedatarateisgenerallyexpectedtoincreaserapidlyovertime(Cisco2015;Clarke2014).ThisSectionexaminestheeffectoftrafficpervehicleonbenefitandcost.

Figure12showsthroughputasafunctionoftrafficpervehicle,assumingthepopulationdensity,quantityofvehicles,penetration,unitcostsandspectrumparametersareheldconstantinthebasecasevaluesforallvaluesoftrafficconsidered.Foranincreaseinthetrafficpervehicle,DSRCthroughputincreases,thoughwithadecreasingmarginalgain.Figure12suggeststhatDSRCthroughputisstillgrowingfortrafficpervehicleashighasfourtimesthebasecasevalue.Iftrafficpervehicleincreasesovertime,thenDSRCthroughputislikelytoincreaseovertimeaswelleveniftrafficgrowsasmuchthewiderangeshowninFigure12,underbasecasevaluesfortheotherassumptions.

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Figure12.AveragetrafficofferedandoffloadrateatapeakhourforvaryingtrafficperDSRC-equippedvehicleontheroad,optimalRSUquantityateachpoint(0.9to2RSUs/km2)

Figure13showsbenefitandcostsasafunctionoftrafficpervehicle,assumingthepopulationdensity,quantityofvehicles,penetration,unitcostsandspectrumparametersareheldconstantinthebasecasevaluesforallvaluesoftrafficconsidered.BenefitofInternetaccessandthecostofRSUsdependonthequantityofRSUsforeachvalueoftrafficpervehicle,whichischosentomaximizethedifferencebetweenbenefitandRSUcost,asexplainedfurtherinSection4.2.Forhigherdatarates,bothbenefitandRSUcostarehigheraswell,andthedifferencebetweenthemisalsohigherthanwithlowerdatarates.InpreviousSectionsitisshownthatbenefitexceedsRSUcostforlocationswithpopulationdensityabove4000peopleperkm2,withthebasecaseassumptionoftrafficpervehicle.SinceFigure13showsthatthedifferencebetweenbenefitofInternetaccessandRSUcostincreaseswithtrafficpervehicle,andiftrafficwillincreaseovertimeassomepredict,thenbenefitwouldexceedRSUcostinlocationswithpopulationdensitiesbelow4000peopleperkm2overtime,i.e.inareaswhichpopulationdensitiesthatwerenothighenoughtoresultinsignificantoffloadsoonafterthemandateiseffective.

Figure13alsoshowsthat,underthebasecasescenariofortheotherassumptions,benefitofInternetaccessexceedsRSUcostforatrafficpervehicleabove250kbpsatpeakhours.Thiscorrespondstoamonthlyusageof3GBpervehicle(also,underbasecaseassumptions).Thus,deployingRSUswouldstillresultinbenefitexceedingRSUcostsoonafterthemandatebecomeseffectiveinthedensely-populatedurbanarearepresentedbyourbasecaseifdatarateisabouthalfwhatsomearecurrentlypredicting.

TheaveragedatarateofaDSRC-equippedvehiclemayalsoexceedtheaveragedatarateofallvehiclesifvehicleownerspurchaseOBUsvoluntarily,ratherthanonlyinresponsetoamandate.Theownerswhoadoptvoluntarilywouldbetheoneswhobenefitthemost.IfownersarechargedforInternetservicebasedonusage,thenmoreownersofvehicleswithhighervolumesofInternettrafficwouldoptin,andaveragedataratescouldbemuchgreaterthanthebasecase.Forexample,abuscompanyofferingInternetservicefor

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passengers(suchastheoneinPorto)mightvoluntarilyinstallOBUsassoonasRSUsareoperatingbecausethebuscompanyexpectsadataratepervehiclethatiswellaboveaverage,andcarryingthattrafficoveracellularnetworkwouldbeexpensive.Thus,foragivenOBUpenetrationrate,benefitofInternetaccesswillexceedcostsatalowerpopulationdensityifthereisasignificantlevelofvoluntaryadoptionofOBUs.

Average Traffic per Vehicle (kbps)

Figure13.BenefitandcostforvaryingvaluesoftrafficperDSRC-equippedvehicleontheroad(andotherparametersatbasecasevalues),andoptimalRSUquantityateachpoint

4.6 CostperRoadsideUnit(RSU)IfthecostperRSUislowerthaninthebasecase,thenitmaybeworthwhiletodeploymoreRSUstoincreasetotalthroughput.Ontheotherhand,ifRSUsaresignificantlymoreexpensivethaninthebasecase,thenthatmaypreventdeploymentandresultinnobenefitatall.ThisSectionexaminestheeffectofRSUunitcostsontotalbenefitandcost.

Figure14andFigure15showthroughput,benefitandcostsasafunctionofRSUunitcost.Thebasecasevaluesofpopulationdensity,thequantityofvehicles,penetration,traffic,OBUandmacrocellularunitcosts,andspectrumparametersareassumed.ThequantityofinfrastructureforeachvalueofRSUunitcostischosentomaximizethedifferencebetweenbenefitandRSUcost,asexplainedfurtherinSection4.2.ThecostperRSUaffectsthatoptimalquantityofRSUs,whichinfluencesDSRCthroughput.ThisisshowninFigure14:ifthecostperRSUislowerthanthebasecasevalue($14,000),thentheDSRCthroughputishigherandviceversa.However,evenwiththatvariationinthroughput,Figure15showsthatthetotalbenefitandcostresultsarerobusttoawidevariationofcostsperRSU.Evenifthiscostis30%higher(orlower)thanthebasecase,benefitofInternetaccesswillstillexceedtotalRSUcost.

However,thatresultmightchangeifthecostperRSUisradicallydifferentthanthebasecase.Forexample,ifRSUsaredeployedbybusinessesinplacesthatrequireexpensivepolesorlackofaccesstocommercialpowerorcommunications,thenthecostperRSU

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mightbemuchhigher,andFigure15showsthatbenefitofInternetaccessislowerwhentotalRSUcostifitsunitcostishigherthan$20,000perRSUandotherassumptionsareatbasecase.Ontheotherhand,ifthedecisiontodeployRSUsaremadebyamunicipalitythatalreadyhaspole,energyandbackhaulinfrastructureavailable,costperRSUmaybelow,andRSUdeploymentmightbebeneficialevenforlessdenselypopulatedcitiesthanthe“threshold”densityshowninSection4.2forbasecaseassumptions,aslongasspectrumandOBUcostsaresunkunderamandate.

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Figure14.AverageoffloadrateatapeakhourforvaryingPVperRSU,andoptimalRSUquantityateachpoint(1.3to0.8RSU/km2)

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4.7 MacrocellularCostsMacrocellularcostsareexpectedtoinfluencebenefitandcostsintheoppositewayastheDSRCunitcostsanalyzedinthepreviousSection.Figure16showsbenefitandcostsasafunctionoftheunitcostpermacrocellulartower,forwhichisassumedthebasecasevaluesofpopulationdensity,thequantityofvehicles,penetration,traffic,DSRCcostsandspectrumparameters.IftheNPVofthecostpermacrocellulartowerishigherthanthebasecaseassumption,thenbenefitofInternetaccessexceedsRSUcostinlesspopulatedareasthaninthebasecasescenario.Ontheotherhand,ifmacrocellularcostislowerthaninthebasecase,thanthebenefitmightbelowerthaninthebasecase.However,Figure16showsthatthefindingsinpreviousSectionsdonotchangesubstantiallyifthecostpermacrocellulartowerchangesoverarangeof20%beloworabovethebasecasevalue.

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Figure16.BenefitandcostforvaryingNPVpermacrocellulartower,andoptimalRSUquantity(1RSU/km2)

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Figure17.Benefitandcostforvaryingbandwidthavailableformacrocells,andoptimalRSUquantityateachpoint(1RSU/km2)

Benefitsandcostsmayalsobeinfluencediftheamountofspectrumacarrierhasavailablevariesfromthebasevalue.Figure17showsbenefitandcostsasafunctionofthebandwidthavailablepercarrier,andbasecasevaluesofpopulationdensity,quantityofvehicles,penetration,traffic,unitcostsandDSRCspectrum,andindicatesthatbenefitofInternetaccessexceedsRSUcostifasmuchas20%morebandwidthpercarrierisinuse.Spectrumholdingsforcellularservicemayincreaseovertimeaslongasthegrowing

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demandformobileInternettriggerdecisionstoreallocatespectrumfromotherusestocellular–forexample,in2010theU.S.NationalBroadbandPlanrecommendedincreasingtheamountofspectrumavailableforbroadbandby500MHz.However,spectrumreallocationsarenotfrequentandtakeyearstobecomeeffective–65MHzwereauctionedin2015,beingthefirstsignificantadditiontomobilespectrumsince2008intheU.S.(Clarke2014).Therefore,overagivenperiodtheamountofspectrummayincreaselessthantherapidgrowthexpectedfortrafficpervehicle(whichincreasesbenefit,asshowninSection4.5),whichsuggeststhatthegrowthincellularspectrumisnotlikelytochangeourestimatesthatbenefitofInternetaccessexceedsRSUcostforbasecasevaluesoftheotherassumptions.Theaverageamountpercarriermayalsoincreaseifthereisareductioninthenumberofcarrierscompetinginaregion.However,Figure17showsthisisalsounlikelytobelargeenoughtochangeourconclusionswithinaplanninghorizon.

5 ConclusionsInthispaperweanalyzebenefitsandcostsofInternetaccessthroughDSRC.WefindthatiftherehasalreadybeenamandatetodeployDSRCinnewvehicles,thenthedeploymentofRSUsforInternetaccessincreasessocialwelfarefordenseurbanareaswhenOBUpenetrationisrepresentativeofafewyearsafteramandatebecomeseffectiveandpeak-hourInternettrafficpervehicleiscompatiblewithforecastsforthenextyears,andevenifthoseRSUsarenotsharedwithsafetyorotherapplications.Moreover,RSUdeploymentislikelytobecomewelfareenhancinginthefutureformanyless-populatedareasaswell,aslongaspenetrationorInternettrafficincreasesovertime.

BenefitisdefinedasthecostsavingsfromreducingthenumberofmacrocellulartowersthatwouldotherwisebeneededtocarrytrafficwhichisoffloadedthroughDSRC,andthecostsarethoseofDSRCRSUs,spectrumandOBUs.UnderamandatetodeployOBUs,ourresultsshowthatOBUcostismuchgreaterthanspectrumandRSUcosts,andOBUcostaloneexceedsthebenefitofInternetaccessthroughDSRC.Thus,ifDSRChadnosafetybenefitswhatsoever,thenmandatingOBUsandallocatingspectrumforthoseOBUswoulddecreasesocialwelfare.

However,ithasbeenestimatedthatanOBUmandatewillaccruesignificantroadsafetybenefits(Hardingetal.2014),whichhasmotivatedtheallocationofDSRCspectrumandthepossibilityofamandatetodeployDSRCinallnewvehiclesintheU.S.Ifthismandateoccurs,thenthedecisionofwhethertouseDSRCnetworksforInternetaccessbecomesadecisionaboutwhethertodeployroadsideinfrastructurethatcanserveasagatewaytotheInternet.Forthisdecision,bothOBUandspectrumcostswouldbesunk,andifbenefitofInternetaccessexceedsRSUcost,thenadecisiontodeployRSUinfrastructurewouldincreasesocialwelfare.OurresultsshowthatbenefitdoesexceedRSUcostunderbasecaseassumptions,whichcorrespondtodenseurbanareas.

Benefitandcostsarebothaffectedbypopulationdensity.Ifallelseisequal,benefitofInternetaccessthroughDSRCminusthecostofRSUsisgreaterwhenpopulationdensityisgreater.Withbasecaseassumptions,benefitexceedsRSUcostinlocationswithpopulationdensityabove4000peopleperkm2,i.e.onlyinfairlydenselypopulatedurbanareas.However,thisshouldchangeovertime.UnderanOBUmandate,thevolumeoftrafficpervehicleandOBUpenetrationarebothlikelytoriserapidlybeyondourbaselineassumptionsinthecomingyears,andourresultsshowthateitherofthesechangeswould

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increasebenefitofInternetaccessminusRSUcosts.Thus,ifallassumptionsareclosetobasecasevaluesexceptOBUpenetrationandtrafficpervehicle,thenbenefitwillexceedthecostofRSUsinregionswithlowerandlowerpopulationdensitiesovertime,andthedeploymentofRSUswillbecomesocial-welfare-enhancingovermoreofthecountry.However,therewillremainareaswheredeploymentofRSUsdoesnotenhancesocialwelfare,includingthoseruralareaswherepopulationdensityissolowthatcellularnetworksarenotcapacity-limited,i.e.theyhaveexcesscapacityanddon’tneedoffload.

RSUcostalsoaffectswhetherdeploymentofRSUswouldincreasesocialwelfare,andRSUcostvariesfromcommunitytocommunity.Forexample,allelsebeingequal,benefitsofInternetaccessthroughDSRCminusRSUcostswillbelowerwheretheproviderhastoacquireinfrastructure(poles,backhaul,etc.)thanwhereRSUsaredeployedbyamunicipalitythatalreadyhasinfrastructureavailable,orwherepartoftheRSUcostisincurredforanotherpurpose,e.g.agivenRSUissharedforsafetyandInternettraffic.

Likeanymodelofacomplexsystem,ouranalysisisbasedonanumberofsimplifyingassumptions,someofwhichwemayexplorefurtherinfutureresearch,suchasthevariabilityintrafficpervehicleandamongvehicletypes,andthedynamicsoftraffic,penetrationandcostsovertime.However,theconclusionthatbenefitexceedsRSUcostinurbanareasbutislowerthanthesumofRSU,spectrumandOBUcostsissufficientlyrobustthatasmallchangeofaround20%inanyoftheseassumptionswouldnotchangeit.Ifrealitydiffersfromthebasecaseevenmorethanthis,thisismostlikelyeitherbecauseofourassumptionaboutamandateorourassumptionaboutmobiletrafficlevels.TheU.S.Dept.ofTransportationhasnotmadeafinaldecisionaboutamandate,andwhateveritdecides,othercountriesmaydecidedifferently.IfOBUsarenotmandated,thenpenetrationcouldbelowerthanthebasecase.Moreover,ownersofvehicleswillopttopurchaseOBUsiftheirindividualbenefitexceedstheirindividualcosts.CarsthatwereequippedwithDSRCOBUsbecauseofthishighbenefitmaydifferfromcarsthatwereequippedwithDSRCduetoablanketmandate,anditisthelatterthatarebestreflectedinourbaselineassumptions.Thecomparisonbetweenbenefitsandcostswithoutamandateisasubjectforfuturework.

Theotherassumptionthatmayvarydramaticallyfromthebasecaseassumptionisthetrafficpervehicle.Throughputratefromvehiclesandothermobiledevicesobviouslydependsontheamountoftrafficflowingtoandfrommobiledevices.Whiletherehavebeenprominentpredictionsthatdataratesassociatedwithmobiledeviceswillincreaserapidlyandexponentially(Cisco2015),andproductsareemergingthatwouldgeneratethistraffic,theactualdemandisunknown.Ifdataratesaresubstantiallyhigherorlowerthanourbaselinesestimateof5GBpermonthpervehicle,thenthepopulationdensityrequiredforthebenefitofInternetaccessthroughDSRCtoexceedthecostofRSUsmaybemoreorlessthanourestimated4000peopleperkm2,respectively.

6 AcknowledgmentsThisworkissupportedundertheCMU-PortugalPartnership(scholarshipSFRH/BD/51564/2011),thePortugalFoundationfortheScienceandTechnology(ref.UID/EEA/50008/2013),andtheFutureCitiesProject(EuropeanCommissionEUFP7undergrantnumber316296).TheauthorsalsothanktheInstitutodeTelecomunicações–Porto,VeniamNetworks,themunicipalityofPortoandSTCP,forthedataandsupportprovided.

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