Radio Access and SpectrumFP7FutureNetworksClusterhttp://www.ictras.eu/

5G radio network architectureThis white paper is an outcome of the Radio Access and Spectrum clusterprojects funded by the European Commission under the Seventh FrameworkProgramme in the area of Future Networks. It presents a view from theEuropean research community on the architecture aspects of 5G mobile andwireless communication systems.

1. IntroductionTheuseofmobilecommunicationnetworkshasincreasedsignificantlyinthepastdecades,intermsofcomplexityofapplications,theirrequiredcapacities,andheterogeneityofdevicetypes.Sofar,thistrendhasalwaysbeenmetbysignificanttechnologicaladvancementsandwillcontinuetoincrease.By2020,Europehastopavethewayforanewgenerationofconvergedwiredandwirelesscommunicationnetworks,whichhastobedevelopedanddeployedtomoveforwardtoafuturenetworkedsociety.Inthiswhitepaper,wepresentourperspectiveonsucha5Gradioaccessnetworkandfocusespeciallyonthearisingchallengesandnewtechnologiesthatenableustomeetthesechallenges.

Lookingbackatthedevelopmentof3G(UMTS,HSPA)and4G(LTE,LTEAdvanced)itisclearthatthesegenerationsofmobilenetworksfocusedoncreatingnewphysicalradiotransmissionschemesinordertomeetnewcapacityrequirements.Fromourpointofview,5Gnetworksshouldconsiderbothwirelessandwiredpartstargetingafullyintegratedsolution.Furthermore,inordertoaddresstheuserorientedchallenges,weforeseeacontinuedevolutionofexistingfunctions,e.g.,networkdensificationintoultradensenetworksanddevicetodevicecommunications,aswellasdevelopmentofnewfunctionssuchasmovingnetworksandmassivemachinecommunications.Thisrequiresautointegrationandselfmanagementcapabilitieswellbeyondtodaysselforganisingnetworkfeatures,whichhavetobereflectedinthearchitecturallayertoachievetheirfullpotential.Additionally,ultrareliablecommunicationsputverystringentlatencyandreliabilityrequirementsonthearchitecture.

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Thewhitepaperisorganisedasfollows.InSection2wedescribethemostimportantchallengesfor5Gnetworks,includingtheriseofnumberofdevices,increasedrequirementsforcapacity,energyefficiency,infrastructureissuesandvaryingservicerequirementsandcharacteristics.InSection3wepresentnewtechnologiesthatwillenableussolvethesechallenges,withafocusonnetworkfunctionvirtualisation,cooperativecommunications,automatednetworkorganisation,flexiblebackhauling,aswellasadvancedtrafficmanagementandoffloading.WeconcludewithsomefinalremarksinSection4.

2. ChallengesInthissectionwegiveanoverviewofthemainissueschallengefor5Gsystems,asdrivenbythefastchangingmobilenetworkevolutionandtheforecastexpansionofusecasesandapplications.

2.1 Broad variation of requirements and service characteristicsThemainchallengesfor5Gsystemarethecontinuedevolutionofmobilebroadbandandtheadditionofnewservicese.g.,massivesensorcommunicationandvehiculartoanythingcommunication,requiringshortersetuptimesanddelay,aswellasreducedsignallingoverheadandenergyconsumption[Bal13].Mobilebroadbandofthefuturewillhavesignificantlyincreasedtrafficvolumesanddatatransmissionsrates,butalsomanymoreusecases.Theyincludenotonlytrafficbetweenhumansandbetweenhumanandthecloud,butalsobetweenhumans,sensors,andactuatorsintheirenvironment,aswellasbetweensensorsandactuatorsthemselves.Somenewkeyapplicationswithdisruptivecharacteristicsfollow[METISprojecthttps://www.metis2020.com/].

Firstly,massivemachinecommunications(MMC)isenvisioned,whosemainchallengesare[Fal13,Oss13]:i)tosupport10100timesmoredevicesthantodayii)toallowverylongbatterylifetimes(ontheorderof5+years)ofthewirelessdeviceiii)toincurminimumsignallingoverheadiv)toenablelowcostwirelessdevicesv)tosupportefficienttransmissionofsmallpayloadswithfastsetupandlowlatency.Atthesame,itisdesirabletohave99.999%coverage,whileenergyconsumptionandcostfortheinfrastructureshouldnotincrease.Despitesomeinitialworksdone,e.g.,in3GPP[3gpp22.368],theservicerequirementsforthesemachinetypecommunicationstorealisetheInternetofThings(IoT)arestillnotfullyunderstood.ThisismostlyduetothemultidisciplinarynatureofIoTapplicationsandthecurrentlackoftrulymassiveandlargescaledeploymentsofsmartobjectsinaneconomicallysustainablemanner.

Secondly,safetycriticaldomains,whichtraditionallyhadtheirprivateinfrastructure,willincreasinglyusemobilebroadbandnetworks.Exampleapplicationsare:Assisteddrivingviavehicletovehicle(V2V)andvehicletoinfrastructure(V2I)communications,withautomatedbreakingincaseofaccidentoradversetrafficconditionsaheadpublicprotectionanddisasterrelief(PPDR)systemspublictransportationautomationandcontrol,e.g.,theInternationalUnionofRailwaysisconsideringLTEasadualusetechnologytocomplementandextendtherailwaysflavourofGSM,calledGSMR,currentlystandardisedinEuropeforsignallingin

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highspeed/highavailabilitytrainsautomaticcontrolofsmartgridelements,e.g.,substationsandelectricvehiclechargingstations,tobalancenetworkloadandmitigateinstabilitycausedby,e.g.,theintroductionofrenewablesourcesofenergy.Suchsafetycriticalapplicationstypicallyrequireextremelyshortsetuptimesandlowdelays.Also,itisimportanttoachievewideareacoverage,whichcanbedoneonlythroughasmoothandefficientintegrationwithalternativetechnologies,suchassatellitecommunicationsandaerialbasestations[ABSOLUTEprojecthttp://www.absoluteproject.eu/].

Finally,wecanexpectafurthergrowthofmobilecloudbasedapplications,whichhaveuniquecharacteristicsintermsoflatencyandbandwidth.Infact,themostcomplexapplications(e.g.,speechrecognition,navigation)areoften,itnotalways,offloadedtoacloudserver,soastoreducetheprocessingandenergyburdenofmobiledevices.Whilethiseffectivelymakesthesmartphoneortabletleaner,itstressestheimportanceofareliable,lowlatency,highbandwidthconnectiontotheInternet.

Afinalnoteismadeonvideostreaming,whichisalreadythebiggestcontributortoworldwidetraffictoday,atleastinthefixedpartoftheInternet,andisexpectedtoshifttomobilebroadbandconnectionassoonasthecurrenttechnologiesandbillingplanswillallowthis.Moreover,thefuturevideoencodingandplaybackadvances,including3D,veryhighqualityencoding,4Kresolution,andmultiangle,willfurtherincreasethecapacityrequirements.

Someexamplesofverydiverserequirementsforsomeusecasesofbusinessandsocialinterestarereportedinthetablebelow.

Requirement Verystrict Intermediate Relaxed

Highbitrate Videoequipments(3DTV,realtimestreamingdevices,remoteconference)

Typicalapplicationsonsmartphonesandtablets,V2I

IoT,V2V

Fastmobility Applicationsrunningonsmartphonesandtabletsontheroad,V2V/I

Everythingelse Homeandofficeappliances,IoT(most)

Highreliability PPDR,IoT(some),V2V/I

Everythingelse

Lowlatency Gameconsoles,IoT(some),V2V/I,PPDR(some)

Web&mobileapps,cloudcomputing

IoT(some)

Lowenergyconsumption

IoTdevices(most) Smartphonesandtablets

Cableddevices

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2.2 Energy efficiencyClassicaldesignsforwirelesscommunications,whichtendtomaximiserate,capacityandcoverage,potentiallyleadtosolutionswhereenergyefficiencydrops.Energyefficiencyisunderstoodfromtwoviewpoints.Ontheonehand,theenergyspentbytheinfrastructuremayincrease,implyinghighoperationalcostsfortheoperatorthatwillindirectlyaffectalsotheinvoiceofthefinalsusers.Ontheotherhand,somecommunicationstrategiesrequirehighcomputationalburdenattheterminalsidehavingnegativeimpactonbatterylifetime.Hence,theintelligentuseofenergybecomesamajornewtargetinadditiontotheclassicaldesigncriteria.

Currentlytwoapproachestoreduceenergyconsumptionontheradiolinkexist.First,smallcellsreducethedistancetotheterminal.Themainchallengesofthisapproacharerelatedtoprovidinganeconomicbackhaulsolutionandtominimisetheadditionaldeploymentcost.ThesecondapproachismassiveMIMO,whereenergyismorefocusedtowardstheuserbymeansofmoredirectivebeams.Inthisway,lessenergyiswastedyieldinginterferenceforotherusersattheend.ThechallengesofmassiveMIMOincludethediffusionofenergyduetoscatteringinNLOSscenarios,limitingtheachievabledirectivity,andthecomplexityofspatialmultiplexingofusers.Bothintheterminalandatthebasestation,thegoalofminimisingtheenergyconsumptionperbitwillrequireaparadigmshiftinwirelesssystemdesigntodramaticallyimproveefficiencyintermsofpowerandspectrumusage.Furtherresearchonimplementationtechnologiesisnecessary,focusedonlowpowerhardwarearchitecturesandenergyefficientsignalprocessing[COSTIC1004http://www.ic1004.org/].Someapproacheshavebeenproposedonmultihopcooperativenetworking,andwirelessnetworkcoding[Car12].

Therearefurtherpotentialsavingsbyoperatingthenetworkwithenergyefficiencyinmind.Nowadaysbasestationsconsumeaconstantpower,regardlessofthetrafficload.Duringoffpeaktraffichours,smallcellsareswitchedoffwhilecoverageismaintainedbymacrocells.Foractivebasestationsservingasingleuser,followingShannonstheorem,themostenergyefficientsituationwouldbetousethefullbandwidthandtoreducepowersothatthethroughputtargetismet.However,aninterferencelimitedmultiuserscenarioismoretypicalinmobilenetworks.ServingmultipleusershavingdifferentsignaltointerferenceratiosinaTDMAfashionsuchasroundrobin,changingthepowerdynamicallywouldresultinunpredictableinterferenceinadjacentcells.ThesameholdsforOFDMA,implyinginhomogeneousinterferenceondifferentfrequencysubbands.Hence,currentPHYandMAClayersdesignneedstechnologyadvances,includingdynamicpowercontrolthatisoptimallycoordinatedamongtheusersandwithsurroundingcellssothatthereisproportionalitybetweenthetrafficandtheenergyconsumption[5GNOWprojecthttp://www.5gnow.eu/].Thereisaneedfornetworkarchitectureadvancesrequiredtoi)includesmallcellsandlargerantennaarraysefficientlyintothenetworkdesign,ii)switchon/offbasestationsdependingonthetrafficloadiii)achievetrafficproportionalityatPHYlayer.

Mobiledeviceswithadvancedcapabilitiessuchassmartphonesortabletsmaypresentimportantrequirementsintermsofenergy,notonlyasfarastransmissionisconcerned(whichdependsforexampleonthedataflows,thetypeofapplicationorthewirelessnetworktopology)

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butalsoregardingothercomponentssuchasCPU,screenoraudiodevicesattheuserequipment.Theoffloadingofapplicationstodayhostedbythemobileterminalstowardstheservingbasestationsora(micro)datacentremayalsocontributetoenergyefficiency[TROPICprojecthttp://www.icttropic.eu/].Thisway,theexecutionofresourcehungryapplicationsisshiftedtoprocessingelementsthathavemoreefficientcomputationalandcachingcapabilities.

Thereisalsoaneedtoreduceenergyconsumptioninthebackhaulnetwork,bothinRANandcore,inordertoreducenetworkoperationalcosts.Energyefficiencyinthebackhaulbecomesincreasinglycriticalastheaccesssegmentofthenetworkconsumesupto90%ofthetotaltelecomnetworkenergycost.Historically,thishugenumberisrelatedtotheuseofcopperwiththeincreasinguseofopticalfibre,theenergyrequirementisreduced.Theaccessnetworkhasadistributed(tree)topologytoaggregatethetraffic.Theenormousheterogeneityoffixedandwirelessfinaldroptechnologies(i.e.e.g.FTTH,PON,AON,WiFi,WiMAX,UWBetc.)makeseconomiesofscaleratherproblematic.Moreunifiedandstandardisedfixedaccesssolutionswouldallowmuchhighervolumes,andtherebyhigherintegrationdensities,muchlowercostandreducedenergyconsumption.Forinstance,theuseofanactiveremotenode,originallyputforwardinEthernetPONs[Chan10],wasrecentlyproposedasacommonplatformforfixedwirelessconvergence[HARPprojecthttp://www.fp7harp.eu/].Thisnodelocatesthenetworkintelligenceclosertotheendusersandperformsstatisticalmultiplexingoftrafficfromfewerusers,whichallowstohandlelocallysometrafficflows(suchasthesignallingbetweencooperativebasestations),thereforereducingthebackhaulloadandenablingamoreenergyefficientoperation.Moreover,suchlowerlevelaggregationrequireslesspowerhungrycircuitrywhich,inturn,alsomakesitpossibletouserenewableenergysourcesonly.

2.3 Network infrastructureSmallaccessnodes,withlowtransmitpowerandnopreciseplanningrequirements,areconceivedtobedenselydeployed,resultinginanUltraDenseNetwork(UDN).Thisapproachwillimprovespectralefficiencybyreducingthedistancebetweentransmittersandreceivers,andtoimprovemacrocellservicebyoffloadingwirelesstraffic,thusfreeingradioresourcesintheaccess.Networkdensificationisawaytoincreasethecapacityanddataratetowards2020.

UDNsareastepfurthertowardslowcost,plugandplay,selfconfiguringandselfoptimisingnetworks.5Gwillneedtodealwithmanymorebasestations,deployeddynamicallyandinaheterogeneousmanner,combiningdifferentradiotechnologiesthatneedtobeflexiblyintegrated.Moreover,amassivedeploymentofsmallaccessnodesinducesseveralchallengessuchasanadverseinterferencescenariooradditionalbackhaulandmobilitymanagementrequirements,which5Gneedstoaddress[CROWDprojecthttp://www.ictcrowd.eu/].3GPPiscurrentlyworkingonsmallcellssolutionstoreducetheintersitedistance[3gpp36.932]but,atthetimebeing,pilotcontaminationandinterferencestilllimitthepossibledensification.Differentlevelsofcoordination/cooperationamongsmallcellsarekeytoenhancethenetworkcapacityandkeepinterferenceatanadequatelevel,tomanagemobilityandspectrum,toensureserviceavailabilityandresponsetononuniformtrafficdistributionbetweenneighbouringaccesspoints.

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Withtheincreasingdensityofnetworks,alsothebackhaulwillbecomemoreheterogeneousandpossiblyalsoscenariodependent(i.e.,fibre,wirelessbackhaulorothernonidealtypesofbackhaulmightbeuseddependingontheiravailability).Inaddition,theconnectivityamongthenetworknodesmaychangeinordertoallowforfastdirectexchangeofdatabetweenthem(whichwillbechallenginginultradensedeployments).Theheterogeneousbackhaulstructurewillalsoinfluencetheoperationoftheradioaccessnetworks,e.g.latencydifferencesonbackhaullinkswillimpactintercellcoordinationandcooperationalgorithms.Therefore,bothradioaccessnetworkandbackhaulnetworkneedtobeawareoflimitationsandcapabilitiesofeachother[TUCAN3Gprojecthttp://www.icttucan3g.eu/].ThismayforinstanceimplyanextendedSONappliedtoradioaccessnetworkswhichalsousesinformationprovidedaboutthebackhaulnetwork.

TherequiredflexibilityofthenetworkitselfwillrequirenewconceptsonnetworkmanagementinthebackhaulsuchastheapplicationofSoftwareDefinedNetworking(SDN)principlesinordertoachievefastreroutingandcongestioncontrol,mainlyintheaccesspart[Kre10,Ahm13].SDNconceptsenableustoadapttheoperationofthebackhaulnetworktotheneedsoftheradioaccessnetwork.Forexample,theselectionofIPbreakoutanchorpointsmaydependonthecurrentbackhaultrafficsituationandQoSrequirementsintheradioaccessnetworks.Furthermore,thesmallerthecellsintheradioaccessnetwork,thehigherthetemporalandspatialtrafficfluctuations.Thisimpliesthatalsothebackhaulnetworkmayexperienceahighervarianceoftraffic.Besides,currenttrendssuggestthatInfrastructureasaService(IaaS)canbesupportedbysmallcellsinordertoofferinnovativeproximityservicesandtoenableaseriesofadvantagesforendcustomers.Withthisapproach,energyscarce,capacitylimitedmobiledevicescanoffloadhighlydemandingcomputationaltasksintoproximalfixedunitsorusethemforstorage.Thisentailsthatnovelmechanismsareneededtoefficientlyallocateresources,understoodinawidesense(radio/computation/storage/energy),includingcontextualinformationmetricsandclusteringtechniquesforsmallcells.

Anotherimportantaspectinthenetworkinfrastructureisrelatedtotheexposureofenduserstoelectromagneticfield(EMF).ThereistodayapublicconcernconcerningEMFinducedbywirelessnetworks.Byreducingthedistancebetweenreceiversandtransmitters,smallcellsenabletheminimisationofthepoweremittedbythemobilesphonesandthetotalEMFexposurebecause,currently,themostimportantcontributionislinkedtotheuserequipment.5Garchitecturecombiningsmallcells,heterogeneousnetworksandoffloadingshouldinherentlyenableminimisingthehumanEMFexposure[LEXNETprojecthttp://www.lexnetproject.eu/].

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3. New enabling technologiesInthissectionweintroducethemostpromisingenablingtechnologiesthatareexpectedtobeusedextensivelyin5Gradionetworkstotacklethechallengesidentifiedabove.

3.1 Network Functions Virtualisation (NFV)NetworkFunctionsVirtualisation(NFV)[Nfv12]referstotheimplementationofnetworkfunctionsinsoftwarerunningongeneralpurposecomputing/storageplatforms.Thisapproachallowsthedeploymentofnetworkfunctionsindatacentresandtoleveragefromvirtualisationtechniques.Bycontrast,thestateoftheartistoimplementnetworkfunctionsondedicatedandapplicationspecifichardware.Hence,themainmotivationforNFVistoleveragefromtheeconomyofscaleofhighvolumehardwareplatforms,toreducetimetomarketandinnovationcycleswithintelecommunicationnetworksthroughsoftwareupdatesratherthanhardwareupdates,andtoexploitnoveldatacentretechnology.NFVhasrecentlyattractedsignificantinterestfromtheindustry,whichhasledtothecreationofadedicateindustrystudygroupatETSI.

ImplementingnetworkfunctionsinsoftwareonstandardITplatformswillallowfornewflexibilitiesinoperatingandmanagingmobilenetworks.Inmobilenetworks,NFViscurrentlydiscussedinthecontextofvirtualisingthecorenetwork[You13]aswellaslogicallycentralisingthebasebandprocessingwithintheRAN,socalledCloudRAN(CRAN)[Gua10].CRANstillrequiresspecialisedhardwareindatacentresinordertosatisfythehardrealtimerequirementsinmobilenetworks.Furthermore,CRANdoesnotallowforafunctionaldecompositionwhichimpliesthattheRANfunctionsaredecomposedinindividualmoduleswhichmaythenbemanagedandoperatedondifferent(virtual)machinesandprovidedbydifferentsoftwarevendors.WhileCRANenablesbothfullcentralisationanddistributionof(digital)RANfunctions,thisneedsnottobethecasewithageneralNFVimplementationwhereonlyasubsetofallmodulesmaybeimplementedcentrallyortheradioaccesspointsimplementallfunctionsbasedongeneralpurposehardware[iJOINProjecthttp://www.ictijoin.eu].

Anotherimportanttopicinmobilenetworkswhichmaybeimprovedbyimplementingnetworkfunctionsinadatacentreisresilience.Thisallowsforreassigningfunctionsbetweeneithervirtualorrealmachines.Forexample,ratherthanrunningfunctionsinadatacentre,theymayberuninaRadioAccessPoint(RAP)atlowercomputationalcomplexity[TROPICprojecthttp://www.icttropic.eu/].Furthermore,NFVandimplementingmobilenetworkfunctionsindatacentresallowsmoreflexibilityintermsofresourcemanagement,assignment,andscaling.Thishasalsoanimpactontheenergyefficiencyofnetworksasonlytherequiredamountofresourcesmaybeusedandoverprovisioningofresourcescanbeavoided.ThisresourceorchestrationcouldreusemanagementalgorithmsalreadydevelopedintheITworldinordertoexploitresourcesasefficientlyaspossible.

Asmentioned,NFVisalreadyappliedoncorenetworksandfirsttrialsareperformeddemonstratingthatcriticalmobilenetworkfunctionssuchasMME,HGW/PGW,orHSScanbe

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implementedonstandardITplatforms.Acriticalenablerofthisdevelopmentis,besidesvirtualisationtechnologies,theavailabilityofhighspeedIPnetworksandthepossibilitytomanagethemmoreflexiblythroughSDN.InterestonthelatterisconfirmedbytherecentfoundationofaworkinggrouponwirelessandmobilewithintheOpenNetworkingFoundation,whichistheorganisationthathasstandardisedOpenFlow.IncaseofRANs,NFVmaybemoredifficulttoapplyasitiseitherapplieddirectlywithinnetworknodessuchasRAPsoratmorecentralisedlocationswhichrequireshighperformanceconnectionsbetweenRAPsanddatacentres.ThoseconnectionsmaynotbeavailableatalllocationswhichimposesnewchallengesonimplementationsofNFVinRANandmanagingnetworkscomposedofheterogeneousnetworknodes(macro,metro,andpicocells),heterogeneousbackhaulconnectivity(opticalfibre,DSL,wireless),aswellasheterogeneouslocationofRANfunctions[Sab13,Ber13,Ros14].

3.2 Cooperative communicationsRecently,multihoprelaycommunicationhasbeengainingglobalacceptanceasoneofthemostpromisingtechnologiesinnextgenerationwirelesscellularnetworks[She09,Wij09,Loa10].Presentdaycellularsystemshaveasingledirectlinkbetweenthebasestationandtheterminal.Inamultihopwirelessnetwork,thecommunicationtakesplaceoveroneormorelinks(hops)toformamultihoppathbetweenthetransmitterandthereceiver.MultihopcooperativenetworkshavethecapabilitytoincreasethecapacitydensityandtoreduceenergyconsumptionbybringingtheRANclosertotheenduser[ABSOLUTEprojecthttp://www.absoluteproject.eu/].Comparedtotheexistinglayeredprotocols,whichincludemechanismssuchasretransmissionsormultipleacknowledgements,multihopnetworksovercomesuchinefficienciesandpreventthesemechanismsfromscalingasrequiredforhighcapacitydensityaccessnetworks.However,multihopnetworksoftensufferathroughputpenaltysincethenodesoperateinahalfduplexmodeandthereforenecessarilyintroduceinefficiencyinspectrumusage,asmultipletimeslotsarerequiredtoreceiveandthenrelaytheinformation.Anotherproblemisthelatencyduetomultiplehops.Ontheotherhand,wirelessnetworkcodinghasthepotentialofnaturallyadaptingtoproblemsrelatedtodense,cloudlike,massivelyinteractingnetworksofnodes,sinceitisanexampleofthegeneralconceptofnetworkawarephysicallayer:functionslikerouting,conventionallyperformedathighlayersoftheprotocolstack,aremoreefficientlycarriedoutatthephysicallayer,whichhasthecapabilityofprocessingsignalsdirectlyandwithoutlossofinformation.Bylookingatmultiplecommunicationflowsjointly,insteadofasingleflowatthetime,wirelessnetworkcodingcanovercometheefficiencyandlatencyissuesmentionedbeforeforgeneralmultihopnetworks.

Furthermore,storingthedataattheedgeofthenetwork,i.e.,caching,willbeapromisingwayofreachinghighcapacityin5Gsystems[MOTOprojecthttp://www.fp7moto.eu/].Infact,inspiteofincreasingthewirelessnetworkcapacitybyemployingadvancedPHYtechniques,highdataratesmightstillnotbeachievableduetothelimitedbackhaul.SincebasestationshavetoserveusersbybringingtheirrequestedcontentfromtheInternetthroughthebackhaul,thecapacityofthisbackhaulshouldalsobeinthesameorderofthewirelessnetworkcapacity,inordertoavoid

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ratebottlenecks,whichareespeciallyevidentindenselydeployedsmallcellscenarioswherelowratebackhaullinksarepreferredinsteadoffibreopticconnectionsduetodeploymentandoperationalcosts.Equipping(smallcell)basestationsinsuchdeploymentswithstorageunitsandproactivelycachingthecontentdefinitelyhelpstomitigatethisbottleneck[Bas13]andbenefitscanbebroughtfurtherbyenablingdirectaccessofUEcachesandleveragingsocialnetworksviadevicetodevice(D2D)communications.Moreover,whenusingnonidealbackhaul,e.g.xDSL,thethroughputisveryasymmetrical,stronglylimitingtheusertrafficandaffectingthelatencyoftheinterbasestationcommunicationneededformostoftheapplicablecoordinatedmultipoint(CoMP)techniques.Theoretically,thislimitationcanbeovercomebyconsideringovertheairmeshcommunicationforsignallingbetweenbasestations[COSTIC1004http://www.ic1004.org/].However,thistypeofcommunicationinherentlyrequiresresearchonmanytomanynetworkarchitecturesandprotocols,whicharefundamentallydifferentfromtheexistingonetomanyapproach.

Thelargeuserdatatrafficdemandinconventionalwirelesscommunicationsystemstendstoincreasethenumberofrequiredaccesspointsorbasestationsperareainanetwork,inducinganadversescenariowherecommunicationsareseverelyaffectedbyinterference.Onewayofimprovingthespectralefficiencyofthesystemistheuseofadvancedcoordination/cooperativeschemesamongtransmittersinordertocombatthegeneratedinterference.InLTEAdvancedanditsevolutionstheseschemesareknownasCoMP.Coordinationoftransmittersbyexchangingcontrolplanemessagesandinterferencealignmentbasedtransmissionsolutionsareunderinvestigation[5GNOWprojecthttp://www.5gnow.eu/].AnotherwayofimprovingthespectralefficiencyistoenhancethespatialreuseofradioresourceswhenD2Dcommunicationisallowedforterminalsinthesameradiorange.Thissolutiongeneratesadditionalinterference,buttheinvolvedterminalsemploymuchlesspowerthanthebasestations,whichmeansthatalowerlevelofinterferencecanbeexpected.

Alsoconcerningthelimitationsrelatedtotheenhancedintercellinterferencecoordination(eICIC)mechanismsinthecaseofsmallcelldeployments,currentresearchsuggestsamoreflexibleinterferencecoordinationapproachintimefrequencyandpowerdomainswhich,whenusedinconjunctionwithasimpleICICbasedRadioResourceManagement(RRM),hasthemeritofsignificantlyincreaseduserthroughput.Forinstance,eachbasestationcancreateitsowncollaboratingmicrocluster,composedbythe(mutually)interferingbasestationsindownlink:theinformationonresourceandpowerallocationisdistributedbyeachcelltoitsmicroclusterpeerstooptimiseaccesstocommonresources.Thiscouldbefurtherextendedbysharingwithineachmicrocluster,e.g.,theuserlocationorotherrelevantinformation.

Weconcludebynotingthatthemobilenetworkinfrastructuresarecurrentlyevolvingtoreducetherange,hencethesizeandcomplexity,ofbasestations,whileincreasingthenumberandbandwidthofthephysicalconnectionsbetweensmallercellsites.Thewidedeploymentofopticalcommunicationsnetworks,withfibreconnectionsclosertotheendusers,makesensealsoforwidebandconnectionsbetweensmallcells,changingthecurrentbasicconceptoftrafficscaledcellulardeploymenttoamodernviewofopportunisticspectrumaccessbasedcooperativenetworking[Car12].Inconjunctiontothiscooperativesmallcellsscenario,the

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terminalwillbeactingasalocalaccessenabler,managingradiocommunicationsnotonlyfromtheuserbutalsofromsurroundingsmartobjects.Radionetworkarchitecturescanthenconsidertheroaminguserdevice(onthebus,inthestreet,insidethecar,athome,etc.)asanIoTrelaynodeabletoprovidecoverageextensionandtoactasagatewaytotheInternetfortheIPenabledsmartobjects.

3.3 Automated Network OrganisationCurrent trends in the definition of 5G wireless systems rely on evolving heterogeneous networks where macrocells are overlaid with small cells to deliver improved spectral efficiency and coverage within an area. Such coexistence imposes difficulties to the traditional network planning, where new site locations are set based on expensive and limited tests often based on propagation models that may be inaccurate. Furthermore, achieving and maintaining optimal performance in future cellular systems will become virtually impossible with manual configuration, optimisation, and maintenance due to their incremental densification, which involves a rise of the number of parameters involved, as well as latency and accuracy limitations. In the past, automated network organisation has been addressed via, e.g., SON proposals, which have emerged as a possible solution for the issues mentioned above. SON has shown itself as a paradigm that can reduce OPEX and CAPEX while yielding optimal performanceinLTE[Ham12].

Selfconfiguration,automaticneighbourrelation,selforganisedcarrierselection,andselfhealingmechanismsareexamplesofautomatednetworkorganisationtechniques.Selfconfiguration,forinstance,(i.e.,automaticconfigurationofemissionpower,antennatilt,etc.)allowsnewlyaddedbasestationstobeselfconfiguredinlinewitha"plugandplay"paradigm,whichisparticularlyimportantinthecaseofsmallcells.Inaddition,asfarascostsfornetworkplanninganddeploymentareconcerned,suchtechniquescanalsoalleviatetheburdenofoperatorscomingfrommanuallymanagingneighbourrelations.Indeed,automaticneighbourrelation(ANR)wasthefirstSONtechniquetobeincludedintheLTEspecifications.Incontrasttolegacynetworks,whereasignificantamountoftimeandresourcesisneededtoidentifyfailingbasestationsandfixsuchasituation,5Gsystemsshouldhaveselfhealingfunctionsbuiltin.Thiswillallowthemtodetectfailingbasestationsimmediatelyandtotakefurthermeasureswhileensuringnosignificantdegradationofservicefortheusers.Finally,selforganisingcarrierselectionandinterferencemanagementwillhelptoreducecostandimproveservicereliability.

Automationseemstheonlysensibleapproachtocosteffectivemanagementoffutureoperationallycomplexheterogeneousmobileaccessnetworks.Thekeyenablingelementisaunifiedselfmanagementsystem,whichcontrolsthecomplexnetworkenvironmentasasingleentity.Thisselfmanagementsystemshallenablethenetworkoperatortospecifynetworkorientedobjectivesregarding,e.g.,desiredservicecoverage,resourceefficiencyandqualityofexperience,andshalleffectuatetheseobjectivesintheunifiedandautomatedoptimisationoftheunderlyingintegratedaccessnetworks.Theselfmanagementshallthenperformresourcemanagementandtunetheradioparametersofmobileaccessnetworksinline

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withnetworkoperatordefinedtargets.

Suchaunifiedmanagementsystemwillprovideconsiderablegainstotheoperatorsintermsof(i)enhancedresourceefficiency,implyingincreasedcapacityandhencedelayedinvestmentsinnetworkexpansionsand/orequipmentupgrades(ii)improvedmanageabilityandhenceloweroperationalcosts(iii)enhancedperformanceintermsofserviceavailability,seamlesssessioncontinuityanduserlevelqualityofexperienceand(iv)enhancedconfigurationflexibility,thereforesupportingdifferentresourceutilisationstrategiesandfasttransitionsincaseofredefinitionofnetworktargets.Interestingly,automatisednetworkcontrollersandSDNtechniquesofferapromisingpracticalparadigmtoimplementaunifiedmanagementsystem.Furthermore,anSDNbasedapproachgoesbeyondamereunifiedmanagementsystem,andindeedprovidestoolsforjointlyorchestratingradioandbackhaulresourcesondemand,workingatthetimescaleofIPflows[CROWDprojecthttp://www.ictcrowd.eu/].However,theapplicabilityandsuitabilityofSDNforfuturedensewirelessnetworksiscurrentlyunderinvestigation,andindustrygradesoftwareandinterfacesforSDNoperationarestilltobestudiedanddesigned,asmentionedinSection3.1.

Thefollowingisadescriptionofthetwokeyelementsforfutureunifiedselfmanagementsystem[SEMAFOURprojecthttp://www.fp7semafour.eu/].

ThefirstelementistheintegratedSONManagement,aspresentedinthefigurebelow.Thetoppartdepictsaserviceprovider,whichmaintainsaServiceLevelAgreement(SLA)withanetworkoperator,contractuallyformalisingtheiragreementregardingperformanceandtariffs.Integratingsuchperformanceobligationswithitsownbusinessstrategy,theoperatorformulatesitsnetworkorientedobjectivesandprovidestheseasaninputtotheintegratedSONmanagementlayer,whichservesasitsinterfacetotheselfmanagementsystem.Thekeypurposesofthislayerare(i)totransformtheseobjectivesintodedicatedexecutionpoliciesforspecificSONfunctions(ii)tosuperviseandcoordinatetheseSONfunctionsand(iii)tomonitorandanalysetheirperformanceaccordingtotheobjectives,providinginputtoperiodicoperatorreports,SONmanagement,SONfunctionsanddecisionsupportsystems.

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ThesecondelementisSingle/multiRAT/layerSONfunctions.ThesewillresideatthefunctionallayerbelowtheintegratedSONmanagement(seefigureabove).TheseSONfunctionscontrolthephysicalnetworkresourcesindifferentRATsandlayersandcanbeimplementedinadistributedfashioninthenetworkelements,orinacentralisedfashioninthenetworkmanagementsystem.NumerousSONfunctionshavebeendevelopedsofar,includingmobilityrobustnessoptimisation(MRO)andmobilityloadbalancing(MLB)SONfunctions.TheymostlyfocusonsingleRAT/layerscenarios.FutureSONfunctionswillbeneededtotargetmultiRAT/layerSONfunctions,addressingamongstothersadvancedtrafficsteeringbetweenWiFiand3G/LTEcellularlayers,dynamicallocationofspectrumoverRATsandlayers,andtheautomated(de)activationandtuningofsitesectorisation.

3.4 Flexible backhauling3Gand4Gusedifferentbackhaulingtechnologies(e.g.opticalfibre,microwavelinksorevenasatellitelink)butineverycasethebackhaulisseenasprovidingenoughQoS(qualityofservice)andasmuchcapacityastheRANmayrequire.5GRANbecomesmoreheterogeneous,thusrequiringflexibletopologyandperformancefromthebackhaul[e.g.iJOINProjecthttp://www.ictijoin.eu].Thebackhaulfor5Gneedshigherflexibilityalsotounlockthepotentialofincreased,moreefficientandmoreflexiblespectrumusageandtosupportnewapplications.

Carrieraggregationandtheuseofmorefrequencybandswillleadtoanincreasedmobilespectrumusagein5G.Newradiotechnologieswillenablesignificantlyhigherspectrumefficiencybyusingintersitecoordinatedmultipoint,smallcellsandmassiveMIMO.Sharingofboth,thespectrumandthemobileinfrastructurewillenablestatisticalmultiplexinggainsforthespectrumusageaswellasincreaseddensityofbasestationsperoperatorwithouttheneedfornewsites.Bothapproachesneedasharedbackhaulnetworkusedbymobileoperators.Sharingwillfurtherincreasethecurrentheterogeneityofthenetworkandwillbringnewspecificrequirements.Lastbutnotleast,newapplicationssuchasthetactilemobileInternet[Fet13]forthesupportofIoTwillalsohaveabigimpactonthemobilebackhaulevolution.

Themobilebackhaulevolutionfor5Gisexpectedtofollowfivemajortrends:Opennetworkarchitecture,endtoendsupportforQoSandsecurity,significantlyhigherdatarates,reducedlatencyandnetworkassistedsynchronisation.Opennetworksenableasharedinfrastructureinwhichmultipleoperatorscontributetoasharedoverallnetwork.Inafirststep,operatorsintegratetheirproprietaryinfrastructureintotheoverallnetwork.Virtualisationtoolsaretheninstalledenablingtheformationofcoexistingvirtualsubnetworks.Inathirdstep,theoverallnetworkresourcesaredynamicallydistributedamongtheoperators.Distributioncanbemanagedusinganeutralbrokertradingthepriceofmutualresourceutilisationbythesubnetworksaccordingtoofferanddemand.

The5GRANneedstoverifyactivelyanddynamicallythesupportedQoSandtheavailablecapacityinthebackhaul.Signallingbetweenthebackhauland/orrealtimeQoSmeasurementsperformedbytheRANwillbeessentialtoguaranteeQoStoendusers.Thenativesupportfor

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MMCwillrequirelowerlatency.Guaranteedbandwidth,latencyandendtoendsecurityunaffectedbyotherusersdemandswillbeneeded.Forlowerlatency,forexample,instantaneoushandoverisneeded.ButthedatatransferoverX2orsimilarevolvedinterfacestothenewservingbaseincludesprivateuserdata.Thisisalsotrueforcoordinatedmultipoint[Fri12].Obviously,thecurrentsecurityarchitectureneedstoberevised.

Dataratesofthe5Gairinterfacewillbeincreasedbyafactor1000,comparedtoLTE.Thebackhaulwillfollowthistrend,obviously.Evenmore,intersitecoordinatedmultipointenablesagainoffactor3byexchanginguserdataintheclustersbetween3cellsatdistantsites,onaverage.Accordingly,thefactorbecomes3000inthe5Gbackhaul.Several100Gb/spersitewillbeneeded[Jun13].Althoughtechnologiesexistorarealreadydeveloped,costisanissue.Lowcostandhighperformancebackhaulsolutionswillalsobeneededforsmallcells,bothforLOSandNLOSdeploymentscenarios[SODALESprojecthttp://www.fp7sodales.eu/].

Minimiseddelayisadriverforthebackhaulevolution.ConsideringLTE,handoverlatencyisduetoframingdelaysandtheX2interface[Dim09],duetothecentralisedsecurityarchitecture[Fri12].Adistributedsecuritywillbeneededtoprotectprivateuserdatawhilereducingthelatency.Further,thehoplengthsbetweennodescanbeminimisedbydistributingtheintelligenceinthenetwork.Activeswitcheswillbeplacedinallaggregationnodessothatsignalscanberoutedthroughtheshortestpathtootherports.Flexibledistributedvirtualisationforcoordinatedmultipointisdescribedin[Kre10].Extendedwithendtoendencryptionandguaranteedbandwidth,itisagoodexamplewhatisneededalsofortheIoT.

Synchronisationisneededforhigherspectralefficiencyusingcoordinatedmultipointandtominimisedelays[SODALESprojecthttp://www.fp7sodales.eu/].GPSsynchronisationisanexampleofadistributedapproach,see[Irm11].HowevernetworkoperatorscurrentlyprefertheIEEE1588precisiontimeprotocol(PTP)overthebackhaulbecauseitisapplicablealsotoindoordeployments.Notethatthereferenceclockispassedoverseveralaggregationnodesfromagrandmastertoeachbasestation.Nativesupportfornetworkwidesynchronisationisthereforeneededineachaggregationnode[Jun13].

3.5 Advanced traffic management & offloadingTohandletheexplosionofmobilewirelessdataoffloadingtechniqueshavebeenproposedtoimprovetheuserexperienceforcellularservicesinoverloadedareas.OffloadingtechniquestowardstheendusereitherthroughWiFi(outdoor)infrastructuresandfemtocellsarecurrentlybeingapplied.NewstandardssuchasSelectiveIPTrafficOffloading(SIPTO),LocalIPAccess(LIPA)andIPflowmobility(IFO)arebeingproposedtooptimisethedatatransferfromtothemobiledevicestotheInternet.

However,thenetworkdensificationenvisagedin5Gisactuallyintroducingaparadigmshiftthatthenextgenerationtrafficoffloadingtechniqueswillhavetotakeintoaccount.Networkuplinkanddownlinkasymmetrywillincreasein5G,hencetheywillneedtobeconsideredastwoindependentconnections.Infact,manymobilesmayfindmoreefficientenergyand

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throughputwisetoassociatetotwodifferentPointsofAccess(PoAs)foruplinkanddownlinkcommunications,respectively.MoreworkisrequiredtobetterunderstandhowtoassigntraffictoeachRANunderrealisticnetworkloadingmodelsanddealingwithdiversetypesoftraffic(e.g.,balancingQoSfordata,voiceoverIPandvideostreamingforinstance).Today,dataoffloadingmodifiestheservicerate,whichmakesthenetrateoptimisationproblemforallusersunderdynamictrafficoffloadingverycomplex.Furtheroffloadingisonlypossibleifoneusestheuserequipmentasarelaywithinacell,oracrossseveralcells,buildingupontherecentdevelopmentsinD2DcommunicationsinanLTEAinfrastructure.StrategiesforD2Dpathestablishment,orformanagingopportunisticD2Dcommunicationsneedtobefurtherinvestigated[MOTOprojecthttp://www.fp7moto.eu/].

Furthermore,5Gnetworkswiththedrasticincreaseintrafficloadandnumberofdevicesconnectedwillstarttoexperiencealsoabackhaulbottleneckbeyondthecurrentdatacapacityshortageexperiencedbycustomersinthewirelessaccesssegment.Offloadingtechniqueswillalsoneedtoincreasinglycombineandcoordinatemassiveantennaconfigurationswithstrategiesfordecreasingtheloadonthebackhaul,e.g.,throughfemtocatching,outofbandcontentloading,andincreasedD2Dopportunisticcommunications.Thedensificationoftheinfrastructureincludesanadditionalchallengeforeffectivemanagementofoffloadingmobility,intermsofnetworkassociation.Tomakerealtimedecisionsregardingselectiveoffloading,increasedapplication,deviceandsubscriberawarenessarerequiredtoeffectivelymanagethewholeprocess.Consistentuserexperienceandservicecontinuityindependentofthedataoffloadingsolutionsimplemented(femto,WiFi,opportunistic,IPflowmobility,IPlayermanagement)demandstransparentsignonsolutionsacrossmanagedandheterogeneousnetworkinfrastructures.Hence,roamingagreementmanagementacrossWiFinetworksisanimportantissue.Seamlesssessionhandoveradditionallydemandsnetworkreadinesspriortodevicereadiness.

4. Final remarksAsuddenchangeofthesystemrequirementsisexpectedinthenextyears,whichneedsanequallyfastreactiontoadaptthenetworkarchitectureandprotocolssoastoefficientlysupporteachusecase.Unfortunately,thetypicaldurationofthelifecyclefromtheanalysisofrequirementstothefullscaledeploymentofatechnologyisintheorderof710yearsinthetelecommunicationsindustry,whichisincompatiblewiththedesiredtimehorizonofsomeusecase.Forexample,manyIoTapplicationscouldbringsignificanteconomicandsocialimpactalreadytodaysincetheyarebasedonmaturetechnology(exceptforcommunications).

5Gsolutionswillhavetoenableserviceawareoptimalcoverage,capacity,andreliabilitywithlowestcostandenergyconsumption[METISprojecthttps://www.metis2020.com/].Differentscenariosmayrequiredifferentgroupingoffunctionstonetworkelements.Thetradeoffbetweencentralisingnetworkfunctions(whosemainbenefitsare:resourcepooling,easierdeploymentandmanagement,andglobaloptimisation)anddecentralisingthemtowardsthenetworkedges(achievingfasterreaction,incrementalcommissioning,andpotentiallylower

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signallingoverhead)mustbeinvestigated,e.g.,whichfunctionsmaybecombinedincommonRANelementstoachieveoptimalperformance.

Inaddition,itisnotonlyamatteroffurtheroptimisationoftheradioconnectionitself.Thenewclassofservicesandserviceprovidersrequiresamoreopenandserviceawarenetworkstructureinordertocustomisethenetworkresourcesandthemanagementofthenetwork(orpartsthereof).Itisalsoverylikelythatthereisnosinglenetworkarchitecturethatcansupportallofthe5Gscenariosinacostefficientmanner:onesizefitsallislikelynotviable!

Thefuturenetworkarchitecturewillalsobescenarioandtestcasespecific,i.e.,itmaybedifferentinareaswithlowcelldensitycomparedtoultradensedeployments,suchasMegaCities.ThefutureRANarchitecturewillincludedenselydeployedheterogeneousradioaccessnodesprovidedbynetworkoperators,accessnodesprivatelyinstalled,andevenmovingaccessnodes.ManyofthenodeswillsupportmultipleheterogeneousRATsandsoftwaredefinedinterfaces.Furthermore,datacommunicationsandmanagementofthenetworkwillbebasedonaunifiedallIPnetwork.

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[Fal13]M.Fallgren,andB.Timus(Eds.),Scenarios,requirementsandKPIsfor5Gmobileandwirelesssystem,availableonlineathttp://goo.gl/64tKZt,May2013.

[Oss13]A.Osseiran,V.Braun,T.Hidekazu,P.Marsch,H.SchottenH.Tullberg,M.A.Uusitalo,andM.Schellman,TheFoundationoftheMobileandWirelessCommunicationSystemfor202andBeyond,Proc.IEEEVehic.TechConf,Dresden,Germany,June2013.

[3gpp22.368]3GPPTS22.368,ServicerequirementsforMachineTypeCommunications(MTC)Stage1,version12.2.0,Mar.2013.

[Car12]N.Cardona(Ed.),COSTIC1004PositionPaperonHorizon2020,availableonlineathttp://goo.gl/jiHzBv,May2012.

[Cha10]C.A.Chan,M.Attygalle,A.Nirmalathas,"RemoteRepeaterBasedEPONWithMACForwardingforLongReachandHighSplitRatioPassiveOpticalNetworks,"IEEE/OSAJournalofOpticalCommunicationsandNetworking,vol.2,no.1,pp.2837,January2010.

[3gpp36.932]3GPPTR36.932ScenariosandRequirementsforsmallcellsenhancementsforEUTRAandEUTRAN,version12.1.0,Mar.2013.

[Kre10] W.Kreher,J.Mller,V.Jungnickel,S.Jaeckel,F.Bauermeister,A.Forck,T.Haustein,CoordinatedMultipointTransmissionCOMPDataandSignallingonX2InterfaceusingAdditionalVLANIdentifier,EuropeanPatentEP20100178477,2010.

[Ahm13]H.AliAhmad,C.Cicconetti,A.delaOliva,V.Mancuso,M.R.Sama,P.Seite,andS.Shanmugalingam,AnSDNbasedNetworkArchitectureforExtremelyDenseWirelessNetworks,IEEESoftwareDefinedNetworksforFutureNetworksandServices,November2013.

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[Nfv12]NetworkFunctionsVirtualisation:AnIntroduction,Benefits,Enablers,Challenges&CallforAction,WhitePaper,October2012,Darmstadt,Germany.

[You13]F.Z.Yousaf,J.Lessmann,P.Loureiro,andS.Schmid,SoftEPCDynamicInstantiationofMobileCoreNetworkEntitiesforEfficientResourceUtilization,IEEEInternationalConferenceonCommunications,June2013,Budapest,Hungary.

[Gua10]H.Guan,T.KoldingandP.Merz,"DiscoveryofCloudRAN,"inCloudRANWorkshop,April2010.

[Sab13]D.Sabella,P.Rost,Y.Sheng,E.Pateromichelakis,U.Salim,P.GuittonOuhamou,M.DiGirolamo,andG.Giuliani,RANasaService:ChallengesofdesigningaflexibleRANarchitectureinacloudbasedheterogeneousmobilenetwork,2013FutureNetworksSummit,July2013,Lisbon,Portugal.

[Ber13]C.J.Bernardos,A.DeDomenico,J.Ortin,P.Rost,andD.Wbben,Challengesofdesigningjointlythebackhaulandradioaccessnetworkinacloudbasedmobilenetwork,2013FutureNetworksSummit,July2013,Lisbon,Portugal.

[Ros14]P.Rost,C.J.Bernardos,A.DeDomenico,M.DiGirolamo,M.Lalam,A.Maeder,D.Sabella,andD.Wbben,Cloudtechnologiesforflexible5Gradioaccessnetworks,IEEECommunicationsMagazine,May2014.

[She09]G.Shen,J.Liu,D.Wang,J.Wang,andS.Jin,Multihoprelayfornextgenerationwirelessnetworks,BellLabsTechnicalJournal,Vol.13,No.4,pp.175194,2009.

[Wij09]C.Wijting,K.Doppler,K.KallioJarvi,T.Svensson,M.Sternad,G.Auer,N.Johansson,J.Nystrom,M.Olsson,A.Osseiran,M.Dottling,J.Luo,T.LestableandS.Pfletschinger,KeytechnologiesforIMTAdvancedmobilecommunicationsystems,IEEEWirelessCommunications,vol.16,No.3,pp.7685,June2009.

[Loa10]K.Loa,C.C.Wu,S.T.Sheu,Y.Yuan,M.Chion,D.HuoandL.Xu,IMTadvancedrelaystandards[WiMAX/LTEupdate],IEEECommunicationsMagazine,vol.48,no.8,August2010.

[Bas13]E.Batu,M.BennisandM.Debbah,LivingontheEdge:TheroleofProactiveCachingin5GWirelessNetworks,IEEECommunicationsMagazine(toappear).

[Ham12]S.Hmlinen,H.Sanneck,C.Sartori,LTESelfOrganisingNetworks(SON):NetworkManagementAutomationforOperationalEfficiency,JohnWiley&Sons(Publisher),ISBN:1119970679,2012.

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Acronyms3GPP ThirdGenerationPartnershipProject MMC MassiveMachineCommunicationsANR AutomaticNeighbourRelation MME MobilityManagementEntityAON ActiveOpticalNetwork MRO MobilityRobustnessOptimisationCAPEX CapitalExpenditures NFV NetworkFunctionVirtualisationCoMP CoordinatedMultipoint NLOS NonLineofSight

CPU CentralProcessingUnit OFDMAOrthogonalFrequencyDivisionMultipleAccess

CRAN CloudRadioAccessNetwork OPEX OperationalExpendituresD2D DevicetoDevice PGW PacketdatanetworkGatewayDSL DigitalSubscriberLine PON PassiveOpticalNetworkeICIC enhancedIntercellInterferenceCoordination PPDR PublicProtectionDisasterReliefEMF ElectromagneticField PTP PrecisionTimeProtocolFTTH FibreToTheHome QoS QualityofServiceGPS GlobalPositioningSystem RAN RadioAccessNetworkHGW HomeGateway RAP RadioAccessPointHSPA HighSpeedPacketAccess RRM RadioResourceManagementHSS HomeSubscriberServer SDN SoftwareDefinedNetworkIaaS InfrastructureasaService SIPTO SelectiveIPTrafficOffloadingIEEE InstituteofElectricalandElectronicsEngineers SON SelfOrganisingNetworkIFO IPFlowmobility TDMA TimeDivisionMultipleAccessIP InternetProtocol UDN UltraDenseNetwork

IT InformationTechnology UMTSUniversalMobileTelecommunicationsSystem

LIPA LocalIPAccess UWB UltraWidebandLOS LineofSight V2I VehicletoInfrastructureLTE LongTermEvolution V2V VehicletoVehicleMAC MediumAccessControl WiFi WirelessFidelity

MIMO MultipleInputMultipleOutput WiMAXWorldwideInteroperabilityforMicrowaveAccess

MLB MobilityLoadBalancing

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AcknowledgmentsRASclusterchairedbyPauloMarques(CRSihttp://www.ictcrsi.eu/)

WhitepapereditedbyClaudioCicconetti(CROWD)withcontributionsfrom:

GerhardWunder(5GNOWhttp://www.5gnow.eu/)IsabelleBucaille(ABSOLUTEhttp://www.absoluteproject.eu/)NarcsCardona(COSTIC1004http://www.ic1004.org/)VincenzoMancuso,MartinDrxlerandAntoniodelaOliva(CROWDhttp://www.ictcrowd.eu/)TharmRatnarajahandLarsDittmann(HARPhttp://www.fp7harp.eu/)PeterRostandAlbertBanchs(iJOINhttp://www.ictijoin.eu/)JoeWiart(LEXNEThttp://www.lexnetproject.eu/)AfifOsseiranandHugoTullberg(METIShttps://www.metis2020.com/)OscarLazaroandDamienLavaux(MOTOhttp://www.fp7moto.eu/)EjderBatuandMrouaneDebbah(NEWCOM#http://www.newcomproject.eu/)ColinWillcock(SEMAFOURhttp://fp7semafour.eu/)VolkerJungnickel(SODALEShttp://www.fp7sodales.eu/)FeliciaLobillo,AntonioPascualIserte,AdrinAgustn,OlgaMuoz,andMarianaGoldhamer(TROPIChttp://www.icttropic.eu/)JosepVidal,DavidChavez,andFranciscoJavierSimReigadas(TUCAN3Ghttp://www.icttucan3g.eu/)

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