white paper on 5g radio network architecture
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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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[Nfv12]NetworkFunctionsVirtualisation:AnIntroduction,Benefits,Enablers,Challenges&CallforAction,WhitePaper,October2012,Darmstadt,Germany.
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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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