Why Lte Needs Femto Cells

Published by the Femto ForumMay 2010

www.femtoforum.org

The Best That LTE Can Be Why LTE Needs Femtocells

The Best That LTE Can Be is published by the Femto Forum

May 2010. All rights reserved.

www.femtoforum.org

telephone +44 (0)845 644 5823 fax +44 (0)845 644 5824 email [email protected] PO Box 23 GL11 5WA UK

What is the Femto Forum?

The Femto Forum is the only organisation devoted to promoting femtocell technology worldwide. It is a not-for-profit membership organisation, with membership open to providers of femtocell technology and to operators with spectrum licences for providing mobile services. The Forum is international, representing more than 120 members from three continents and all parts of the femtocell industry, including:

l Major operatorsl Major infrastructure vendorsl Specialist femtocell vendorsl Vendors of components, subsystems, silicon and software necessary to create femtocells

The Femto Forum has three main aims:

l To promote adoption of femtocells by making available information to the industry and the general public;

l To promote the rapid creation of appropriate open standards and interoperability for femtocells;

l To encourage the development of an active ecosystem of femtocell providers to deliver ongoing innovation of commercially and technically efficient solutions.

The Femto Forum is technology agnostic and independent. It is not a standards-setting body, but works with standards organisations and regulators worldwide to provide an aggregated view of the femtocell market.

A full current list of Femto Forum members and further information is available at www.femtoforum.org

The Femto Forum: The Best That LTE Can Be

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Contents

1 ExecutiveSummary 2

2 Introduction 3

3 LTEDriversandHistory 5

4 WhyFemtocellsforLTE? 6

4.1 Business Issues 6

4.2 Indoor coverage issues 8

4.3 Shannons and Coopers laws 10

5 Usesoffemtocellsindifferentenvironments 12

5.1 FemtocellsforResidential/SOHOUse 12

5.2 FemtocellsforEnterpriseUse 13

5.3 FemtocellsforOutdoorUse 13

6 LTEFemtocellStandards,Architectures&Services 13

6.1 Standardization 14

6.2 Architectural Aspects 14

6.3 Service Aspects 16

7 TechnicalDeploymentConsiderations&Challenges 18

7.1 Interference Management 18

7.2 Spectrum 19

7.3 Backhaul 19

8 Conclusions 21


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1. ExecutiveSummary

LTE is being standardized by 3GPP to provide multi-megabit bandwidth, more efficient use ofthe radio network, latency reduction, improvedmobility, and potentially lower cost per bit. LTE femtocells,alsoknownasHomeeNodeB(HeNB) in3GPP,formpartoftheLTEstandard.Alreadydeployedin3Gnetworks,femtocellsarelow-poweraccesspointsthatoperateinlicensedspectrumandprovidemobilecoverageandcapacityoverinternet-gradebackhaul.

Femtocells can strengthen deployments in residential, enterprise, indoor hotspot, and outdoorhotspot environments. This Femto Forum white paper discusses how femtocells may be used to realizethefullpotentialofLTE,provideabetterLTEexperienceforusersincludinghigherbitrates,supportnewservices,offeralternativerolloutmodels,andprovideanimprovementtothemobilebroadband business case by lowering network costs while increasing network capacity.

NumerousmobileoperatorshavepubliclystatedthatfemtocellsareexpectedtoplayaroleinLTE,justas3Gfemtocellsaremakinganeffectivecontributionto3Gnetworkstoday.Thispaperoutlinesthe case for deploying femtocells1inanLTEenvironment,including:

l RealizingthefullpotentialofLTE l Coverage l Throughput l Capacity

l Cost savings l Optimizingthemacrorollout l Operationalsavingsespeciallypower,backhaul,siterental l Churnreductioncontractextension

l Revenue impact l Newrevenuestreamsfromvalue-addedservices

l Location-specifictariffswithoutleakage l Family contracts

To realize the full potential of LTE requires the use of a fine-grained network architecture that includes femtocells from the outset. In addition to this white paper, the Femto Forum has developed a quantitative business case study2 on using femtocells in next generation mobilenetworkdeployments (bothLTEandWiMAXbased).TheFemtoForumstudysuggests there isacompelling business case for LTE femtocells.

1ThesemightbestandaloneLTEfemtocellsordualmodeLTE/3Gfemtocells.2TheBusinessCaseforFemtocellsintheMobileBroadbandEra,FemtoForumwhitepaper,March2010,availablefromwww.femtoforum.org


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2. Introduction

3GPPRelease8specifiestheEvolvedPacketSystem(EPS),whichincludestheEvolvedUTRAN,theLTEradioaccessnetwork;andtheEvolvedPacketCore(EPC),thenextgenerationpacketcore.LTEisdesignedtoprovidemulti-megabitbandwidth,moreefficientuseoftheradionetwork,latencyreduction,andalowercostperbit.Thiscombinationaimstoenhancetheusersexperienceandfurtherdrivethedemandformobilemultimediaservices.WithLTE,peoplewillmorereadilyaccesstheirInternetservicesfrommobiledevices,includingrealtimeandondemandtelevision,blogging,socialnetworkingandinteractivegaming.

Femtocells are low-power access points that operate in licensed spectrum and provide mobile coverage and capacity over internet-grade backhaul. Femtocell applications include residential, enterprise, indoorhotspot,andoutdoorhotspotdeployments.Femtocellsare lower incostthantypical macrocells while retaining full operator management even if they are located on the customer premises.

Figure1:Genericresidentialfemtocellnetworkarchitecture3

Thispaper illustrateswhy realizing thepotentialof LTE requires theuseoffine-grainednetwork architectures that include femtocells from the outset. The benefits of these fine-grained architecturesinclude:

l Improvedperformance,coverageandcapacitythroughtheuseofmore,smallercells. l Compelling business case economics including new revenue streams, potentially faster

time-to-marketforcarrierLTEdeployments,andcostsavingsindeployingLTE. l Next-generation femto-enabled services to enhance the user experience and encourage

LTEadoption.

3Thearchitecturemayvaryforbusinessandpublicfemtocelldeployments(e.g.alternativeIPbackhaultechnologiesmaybeused).


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Thiswhitepaperisstructuredasfollows:

l Section3providesanintroductionandhistoryofLTEandfemtocells. l Section4discussestheneedforfemtocellsinthenextgenerationmobilenetworks. l Section5discussestheusesoffemtocellsindifferentenvironments. l Section6describestheroleofLTEfemtocells,includingarchitecture,services,andstandards. l Section7discussessomeofthetechnicalconsiderationsandchallenges. l Section8providesconclusionsandrecommendations.



3. LTEDriversandHistory

LTE standardization was initiated in 2004. One of the main goals for LTE was to reduce the networkcostperbitasthedatatrafficgrowthisoutpacingthecorrespondinggrowthinrevenue.This cost reduction is possiblewith LTE due to its high spectral efficiency, its all-IP flat network architecture, and its intelligentmanagement capabilities such as Self-Organizing Network (SON) functionality.Thelattertwoarekeyaspectsoffemtocellarchitecturesandhaveallowedasmooth introductionofLTEfemtocellsintothe3GPPstandard.Inaddition,theintentionwastodevelopa universal terrestrial radio access network (UTRAN) which would provide a framework for the evolution of the 3GPP radio-access technology towards a high-data-rate, low-latency and packet-optimizedradio-accesstechnologyasdescribedby3GPP.WithLTE,mobileoperatorswillbeabletoofferabroadbandwirelessexperiencethatrivalsfixed-lineofferings.

InSeptember2006,OFDMAmodulationwasselectedforthedownlinkandSingleCarrierFrequencyDivisionMultipleAccess(SC-FDMA)fortheuplink.ThestandardalsocalledfortheuseofMIMOtoincreasecapacityandprovidespatialdiversity.

LTE isdesigned toprovidehighdata ratesandspectralefficiency. Inaddition to itshigh spectral efficiency,thestandardhasbeenplannedtofacilitatedeploymentinmanydifferentfrequencybandswithverylittlechangetotheradiointerface.Itisresistanttointerferencebetweencellsandspreadstransmissionefficientlyovertheavailablespectrum.ThepeaktheoreticaldownlinkdataratesforLTEareupto326Mbpsovera20MHzchannelwitha4x4MIMOconfiguration.Figure2showsacomparison of LTE with other technologies in terms of peak data rates.

Atthesametime,3GPPhascontinueditsevolutionaryworkonWCDMAwithinReleases5,6,7,8,and9.ThemostsignificantdevelopmentalongthewayhasbeentheintroductionofHSPA/HSPA+,andtheinclusionofhigherordermodulationtechniquessuchas16QAM,64QAM,2x2,4x4MIMO,aswellasmulticarrier.Inaddition,3GPPRelease8addedfemtocellsto3G(WCDMA)aswellasLTE.

While femtocells are a later addition to other standards, LTE standardization took into account femtocell architectures from the beginning, increasing the value of their coverage and capacity offeringtotheoperatorsfromdayone.

Technology Downlink Uplink

EV-DORevA 3.1 Mbps 1.8 MbpsWCDMAR99 384 kbps 384 kbpsHSPA 14.4 Mbps 5.76 MbpsHSPA+ 84 Mbps4 11.5 MbpsLTE 326 Mbps 170 Mbps

Figure2:Peakdatarates5

4 With64QAMmodulation,MIMOanddualcelltransmission.5 Notethatthesefiguresdonotpresentalike-for-likecomparisonbetweenthedifferenttechnologiesforexample,20MHzofspectrumisassumedforLTEversus5MHZforHSPA(acomparisononthebasisofbits/hertz/sectorwouldlookratherdifferent).



4. WhyFemtocellsforLTE?

LTE is theevolutionarypathof choice formanyWCDMAandCDMAnetworks,but somemobileoperatorsarestillnotcertainabouttherighttimeframeorstrategyformarketdeployment.WithLTEproductsbecomingcommerciallyavailable,thedecisionontherighttimeframewilldependonseveralfactorsuniquetoeachoperatorandmarketsuchas:

l Capacitygrowthrateasthetrafficpersubscriberandnumberofconnectionsincreases l Spectrum availability and regulatory environment l Service provisioning strategy l Availabilityofsuitablebackhauloptions l Competitorsmovements

In the past both 2G and 3G were deployed at the macro layer as operators aimed at reaching as many end users as possible for provision of voice and data services. This strategy has resulted today intypically90%orgreaterpopulationcoverageindevelopedcountries6.

WhilesomeproposethatLTEshouldfollowthesamedeploymentstrategyas3G,theFemtoForumbelievesthatincludingfemtocellsaspartofinitialLTEdeploymentsmakessenseinmanyoftodaysmarketsforthefollowingreasons:

l Toimproveperformanceitisimportanttobringtheuserclosertothetransmitter;reductionofthe average size of cells is the most powerful way to increase the capacity of a wireless system (CoopersLaw).

l High data traffic and capacity requirements exist mostly at specific hotspots that are mainly located indoorswherestationaryorportableusage ismostcommon.Deployingthe highercapacityofLTEinitiallyintheseareas(usingfemtocells)andthengraduallyincreasing coveragewhenusers and LTE traffic increases couldhelpoperators reduce their initial LTEspending.

l Although traditional macro base stations will still be essential for providing widespread surfacecoverage,operatorscanusebothindoorandoutdoorfemtocellsfromtheoutsettocarry substantial amounts of data traffic, thereby realizingmajor savings on backhaul andother associated capacity costs. Combining femtocells and macrocells in this way allows operators to build their LTE networks incrementally in line with demand and avoid the need to second guess user uptake.

4.1BusinessIssues

Oneofthemajorindustrychallengeswiththerapidlyincreasingdemandformobilebroadbandisthatdatatrafficgrowthisout-pacingthegrowthinrevenue.AsFigure3illustrates,thereisaclearneedtodramaticallyreducethecostperbitfordata,asgrowthisprojectedtoaccelerateoverthenext3-5years,and thecorresponding revenuemustadequatelycompensateoperators.There isalsoaclearneedtofindwaystoincreaserevenuepersubscriber.

6 SeeforexampleOfcomssixthannualCommunicationsMarketReportformobilepopulationcoveragefiguresintheUK(http://www.ofcom.org.uk/research/cm/cmr09/cmr09.pdf).



Figure3:Traffic/revenuechallenge

Femtocellsallowoperators tocreateamorecompellingmobilebroadband/LTEbusinesscase,asoutlined inFigure4, throughcost savingsandnewrevenues.Femtocells significantly reduce thedeliverycostperbitthroughsavingsinbothCapExandOpEx,andtheywillbeessentialtodrivingdownthecostperbit,reducingtheTotalCostofOwnership(TCO)andmatchingcoststorevenues.Inaddition,femtocellscanpotentiallyimprovetime-to-marketbyenablingquickdeploymentofLTEandprovisioningofnewservices.Finally,femtocellscanenablenewrevenuestreamsthrough,forexample,value-addedservicesandfamilycontracts.

LTE can revolutionise mobile broadband and femtocells can play a role in helping it to deliver itspotentialaccording toSimonSaunders,Chairmanof theFemtoForum. Byadopting femtocells, operators can roll out amuch better performing LTE network than they could with macrobasestationsaloneandatalowercostandwithlessrisk.Allthesefactorsarecrucialinthecurrent uncertain economic environment.

Figure4:BusinesscasedriversforLTEfemtocells



The Femto Forumhas commissioned independent research into the LTE (andWiMAX) femtocellbusinesscase.ConductedbySignalsResearchGroup(SRG),thisstudy2 included two key studies into theopportunitiesforLTE.

In the first study, SRG examined the case of an operator providing dual-mode HSPA/LTE femtocells to households which use significant quantities of mobile broadband data. Using conservative assumptions regarding the take-up and use of LTE devices, the study showed thatthe femtocelloperator isable to realizea substantial returnon their LTE/HSPA femtocell service investment across a wide range of mobile broadband customers.

Figure5:NetworkcostsavingsfromdeployingfemtocellsaspartofanoperatorsLTEnetwork rolloutstrategy(femtocellsallowslowerdeploymentofthemacronetwork)2

In the second study, SRG examined the case of an operator deploying a new LTE network. The operator provides each new subscriber with a fully-subsidized LTE femtocell in order to guarantee great performance and customer experience indoors. This ensures a high quality LTE experienceintheplacewheremostdataisaccessed,andsupportsaggressiveLTEadoptionratesbyfixingoneofthemostchallengingissuesinnewnetworkdeploymentpoorindoorcoverageand data speeds. The study indicated that the operator could fully fund a mass deployment of femtocells by temporarily delaying the deployment of just 5.5% of its macro cell sites7(seeFigure5).

4.2 Indoorcoverageissues

Studies have shown that a large percentage of calls and an even larger percentage of mobile data sessions are initiated indoors8. It will be a major challenge to meet the traffic and bandwidth

2TheBusinessCaseforFemtocellsintheMobileBroadbandEra,FemtoForumwhitepaper,March2010,availablefromwww.femtoforum.org7 Acknowledgingthatinmanycountriesoperatorsarecommittedtoacertainpercentagepopulationcoverageunderthetermsoftheirspectrum licence. 8 SeeforexampleInformaTelecoms&Media,MobileBroadbandAccessatHome,Aug08



demand using outdoor macro network coverage alone. Addressing indoor capacity through macrocells alone may be very costly in many scenarios. The number of macrocell sites needed to providedeep indoorcoverageand/orhighdata-rateserviceswill increaseveryrapidly.However,many operators may not have the opportunity or possibility to add new sites.

Withmobilebroadband,residentialandenterprisecoveragebecomescriticaltothebusinesscase.TheissueisheightenedforoperatorsdeployingLTEathigherfrequencies(e.g.2.6GHz)whichdonotpenetratebuildingsaswellaslowerfrequencies(e.g.700MHz),buttheissueislikelytooccurforallfrequencies.

Whileitischallengingtoprovidecoverageandcapacityindoorsfromamacrocellnetwork,servinguserswithalocalindoorfemtocellhasanumberofsignificantadvantages,including:

l Enhancedindoorpropagationenvironment:

Thebuildingprovidessiteshieldingfrommacrocells.Through-wallattenuationthatnormallyreduces signal strength to the handset now provides shielding which reduces interference with and from the macro network.

Thisisabenefitoffemtocellswhichisoftenoverlooked:whilethefemtocellimprovesservicetotheusers,thesiteshieldingbenefitsallnetworkusers,includingthosenotconnectedtothefemtocell.

l Richangularmultipath,maximisinggainsavailablefromMIMO

The benefits from MIMO come from having several independent paths between the transmitterandreceiver.TheclutteredenvironmentindoorswithmanywallsandreflectionsprovidesthemultiplepathsneededtomakeMIMOeffective(seeFigure6).

l GreaterLTEefficiency

FemtocellsallowLTEtoworkatitshighestmodulationratesandspectralefficiency,allowingtheairinterfacetodeliverhigh-speedservicestoalargenumberofhigh-usagecustomers.

Figure6:IndoorEnvironmentGoodforMIMO&OFDMA



4.3 ShannonsandCoopersLaws

TheShannonlimitofacommunicationschannel isthetheoreticalmaximuminformationtransferrateofthechannelforaparticularnoiselevel.TheperformanceofLTEasanextgenerationsystemisclosetoShannonslimit(seeFigure7).Inmacrocellsoperatedatsignificantload,theoperatingpoint is typically near the lower throughput levels in the graph.

Thismeansmorecellswithtighterinterferencecontrolswillbeneededtoprovidequalitycoverageandservice.Thiscanshifttheoperatingpointtowardsmuchhighersignalquality(SINR)levelsandrealise higher throughputs as a result. This is supported by a detailed Femto Forum study9 which demonstratesthattheadditionoffemtocellstothenetworkallowsfemtocelluserstoconsistentlyreceivemuchclosertotheheadlineLTEdataratesthanthoseconnectedtomacrocells,evenwhenthe femtocells are used in the same spectrum as the macrocells. Femtocell users also do not have to contend for resources with as many users as those on the macrocells.

Paul Jacobs, the CEOofQualcommhas said10 ,the improvement ofwireless links that enhanceuserthroughputs isreaching its limit... thereisanothermethod:denselydeployingbasestationstoshortenthedistancebetweenbasestationsandmobileterminalsAccordingtotheresultsofourresearch,thiseffortwillpossiblyresultineighttimeshigherthroughputperuser.Inretrospect,aneight-timesimprovementisequivalenttothatbroughtbythecellphonesshiftfromanalogto digital.Thisisreallyanexcitingfact.

Figure7:LTEapproachesShannonsLimit,whilefemtocellscancontrolinterferenceto substantiallyincreasethroughputovermacrocells

9 InterferencemanagementinOFDMAfemtocells,FemtoForumwhitepaper,March2010,Availablefromwww.femtoforum.org10 http://techon.nikkeibp.co.jp/english/NEWS_EN/20080905/157548/



Inaddition,CoopersLawstatesthatwirelesscapacitydoublesevery30months.Since1950capacity has increasedmore than amillion fold:much of this has come from bettermodulation, bettercodingandusingmorefrequenciesbutthedominantfactorisusingsmallercells(seeFigure8). Femtocells continue and extend this trend to allow cells to be even smaller while still being cost-effectivewhenservingonlyafewusers.

Figure8:UsingsmallercellshasmadethedominantcontributiontoCoopersLaw11

11 http://www.arraycomm.com/serve.php?page=Cooper



5. UsesofFemtocellsinDifferentEnvironments

Femtocellshaveinitiallytargetedtheconsumermarket,providingdedicatedcoverageathomeorinsmalloffices.Nevertheless,femtocellstandardsandeconomiesofscalecanalsodeliver cost-effectivedeploymentsinlargerofficesandinhigh-trafficindoorandoutdoorlocations.

Toaddresstheaboveapplicationscenarios,femtocellsarebeingdevelopedfor:

l Residential/SOHOuseforindoorprivatedeployment l Enterprise use for indoor public or private deployment l Outdooruseforpublicaccessdeployment

Figure9:TypesofFemtocellDeployments

With 3G femtocells being deployed and LTE arriving, multi-standard femtocells are under development.Thedualmode3G/LTEfemtocellpropositionisinterestingasitreducescosts,avoidstheneedforseparateboxesanddoesntrequirealluserstoadoptLTEatonce12.

Asmentionedearlier,femtocellscandeliverhigherdataratesthanthemacronetwork,especiallytoindoorusers.Thisallows3G/HSPA/HSPA+femtocellstopotentiallydeliveraserviceexperienceindoorscomparabletotheLTEexperienceoutdoorsinthewidernetwork.Assuch,anexisting3Gnetworkoperatormaywishtoconsiderhow3GfemtocellscancontinuetobeusedtoenhancethedeploymentofanLTEoverlayonanexistingHSPAnetwork.

LTE femtocells will benefit from the SON features available in LTE Release 8 and in the further releasesofLTE,especiallyfromthedataprovidedbythehandsettohelpthesystembuildandadaptitsconfigurationfilesinanautomaticway.

5.1 FemtocellsforResidential/SOHOUse

Femtocells forresidential/SOHOusearedeployedbytheenduserwithoutrequiringanymanualconfiguration.TheyareconnectedtothenetworkofthemobileoperatorthroughxDSL,cableor

12ThestudycarriedoutbySignalsResearchGroup(reference2)includesanalysisofthebusinesscasefordeployingdualmodefemtocells.



fibrealreadypresentinthehouse,apartmentorofficepremises.Thefixedbroadbandlinkmayplaceaconstraintonthemaximumdatarateavailabletotheenduser.

Residential femtocells are typically deployed in a Closed Access mode (allowing use only by authorized handsets approved by the femtocell owner), supporting mobility with the surroundingpubliclayers(e.g.2G/3G/LTEmacrocellsandmicrocells,whichrequirenore-planningto accommodatethefemtocells).

The number of simultaneous users supported is typically between 4 and 8. The femtocell accesspointsaremanagedthroughasystembasedonTR-06913protocols,possiblysharedwiththemanagementoftheDSLlines,connectedtothemanagementsystemoftheradioaccessnetwork.

5.2 FemtocellsforEnterpriseUse

FemtocellsforenterpriseusemaybedeployedbyITstafffromwithintheenterprisewithoutanyadditional specialist radio skills. More complex deployments may be carried out by a systems integratorcontractedbytheenterprise,orbythemobileoperator.ThefemtocellsareconnectedtothemobileoperatorsnetworkthroughtheIPlinkalreadyusedforthedatatrafficoftheenterprise(includingVoIP).InordertoenablecorporateuserstoaccessservicesontheenterpriseLANdirectly,localbreakoutofpacketdatatrafficwillbeanoption.Thisalsopreventstheoperatorscorenetworkfrombeingaffectedbyalargevolumeofdatatrafficfromtheenterprise.

Femtocells forenterpriseuseare typicallydeployed inanOpenAccess (orHybridAccess)mode(allowingusebyanyhandsetonthenetwork),supportingmobilitybetweenfemtocellsbelongingto theenterpriseandthesurroundingpublic layers (2G/3G/LTE)whichrequirenore-planning to accommodatefemtocells.LTEfemtocellsmaymakeuseofLTEscapabilitytoofferdifferentlevelsofservicetodifferenttypesofenterprisetraffic,(including,forexample.guaranteedbitrateandbesteffortsserviceclasses).

The number of simultaneous users supported on each femtocell is typically between 8 and 32. Managementofthefemtocellsissharedbytheoperator(formobilenetworkrelatedparametersusingaTR-069system)andtheenterprise(e.g.tocontroluseraccesstoenterpriseapplicationsviathefemtocell).

5.3 FemtocellsforOutdoorUse

Femtocells foroutdooruseareexpectedtooperateaspubliccellsandaretherefore likely tobe deployed by the operator. They require dedicated high speed backhaul links with sufficient bandwidth to support the high throughput made possible by LTE.

The number of simultaneous users supported on each femtocell is typically between 32 and 64. Themainobjectiveofthesefemtocellsistoprovideextracoverageandcapacitybyoffloadingslow movinganddataintensiveusersfromthemacro/microlayers.Outdoorfemtocellsaremanagedbythe mobile operator.

13 BroadbandForum,http://broadband-forum.org/



6. LTEFemtocellStandards,Architectures&Services

LTEisthefirstcellulartechnologywhichwillbeabletotakefulladvantageoffemtocellstandardsfrom theoutset, as thefirstnetworkdeployments takeplace. LTE femtocells aredesigned tofitseamlesslyintotheEvolvedPacketCore(EPC)infrastructureusingthesameinterfacesdefinedforthemacrocells.ThesameEPCcansupportacombinationoffemtocellsandmacrocells.

TheremainderofthissectionprovidesanupdateonthestatusofLTEfemtostandards,describesthearchitecturesavailableforfemtocelldeployment,anddiscussespotentialservicesthatcanbedeployed.

6.1 Standardization

Theindustryactivityonfemtocells,inparticulartheFemtoForumsactivityonWCDMAfemtocells,resultedinhavingthefemtocellconceptconsideredearlyinthestandardizationprocessofLTE,bothbyNGMN in thedefinitionof requirementsandby3GPP in thedevelopmentof standards fromRelease8.WiththestandardizationofLTE, it ispossibletouse3GPPRelease9forthedefinitionof standards-compliant LTE femtocells.Many solutionsdefined forWCDMA femtocells arebeing reusedforLTEfemtocells,forexampleformanagementandsecurity,whichshouldresultinreducedtimetomarketandinreducedcosts.

6.2 ArchitecturalAspects14

SinceLTE/SAE isbasedonaflatall-IParchitecture, thearchitectureand interfacesare the sameforfemtocellsasformacrocells.LTEfemtocells (HomeeNodeBs)requirenonewinterfacestobe definedandnochangesarerequiredtoEPCelements.

There are multiple possible architectures for connecting LTE femtocells to the core network.A new optional element, called the Home eNodeB Gateway (HeNB GW), is defined to provide aggregationofmultipleHomeeNodeBsinthecorenetwork,asshowninFigure10.LTEfemtocells arefullycompatiblewiththeall-IPflatEPCarchitecture,resulting intheHomeeNodeBGateway beingoptional.TheHeNBGWaggregatesS1interfaces(S1-MMEandS1-U),potentiallyimprovingthe scalability of the core network in regard to femtocells.

TherearetwovariationsoftheHeNBGWdefined,withoneaggregatingthecontrolplaneonlywhiletheotheraggregatesboththecontrolandbearer.Figure10showsthearchitecturewheretheHeNBGWaggregatesboththecontrolandbearerplanetrafficandsendsthecontroltraffictotheMMEandthebearertraffictotheS-GW.

14 Notethatthissectiondescribes3GPPsHeNBarchitecture,whichimplieslowpowerfemtocellsforresidentialandsmallbusinessapplications.LargeenterpriseusemaybebasedonHeNBoronhigherpowerfemtocellsandmayintroduce,infuture3GPPReleases,morecomplexrequirementsresultinginadifferentarchitecture.



Figure10:HeNBGWaggregatesboththecontrolandbearerplanetraffic(source:3GPP)

Figure11showsthevariantofthearchitecturewheretheHeNBGWonlyaggregatesthecontrolplanetrafficfrommultipleHeNBstotheMME.

Figure11:HeNBGWaggregatescontrolplanetrafficonly(source:3GPP)

Finally, as stated earlier, the HeNB GW is an optional element and the HeNB can be directly connectedtotheMMEandS-GW(seeFigure12),assumingthattheMMEandS-GWhavesufficientcapacity to support large numbers of femtocell S1 interfaces.



Figure12:LTEfemtocellswithoutHeNBGW(source:3GPP)

6.3 ServiceAspects

Femtocells will enable a wide variety of services in the femtozone15,including:

l Great home coverage with greater capacity l Femtozonetariffs(cheaperuseofmobileservicesviathefemtocell) l Premiumservicequality(fasterdata,highercallquality) l Unlimiteddataservices,ortiereddataservices l Compellingnewfemtozoneservices,includingmultimediaandconvergenceservices

Figure13:EnablingFemtocellCustomerPropositions

Femtocells canactasaportal to in-building servicesandautomation femtozoneservices.This includes high-bandwidth connected-home services, and services that require presence, context, andlocation.

15 Theregionwithinwhichfemtocell-specifictariffsandservicesareavailable.



Femtocellscanofferawiderangeofnewservices.Examplesrelevanttothehomeinclude:

l LegacymobileserviceswithhigherQualityofExperience(QoE),higherspeedservices,andreduced latency.

l Access to services available today on personal computers and other home devices. l Newservicesenabledbythecombinationoftelecomandmultimediacapabilities fromthe

integrationofthefemtocellinthedigitalhome. l Mobile and interactive TV services, includingmobility and integrationwithHighDefinition

IPTV services enabled via the femtocell. l Upgradingcontentbasedonfemtocellcoverageanddevicetype,e.g.,highdefinitiondynamic

picturesonthemobileterminalformovieselection. l Home multimedia networking with device continuity, e.g., between PCs, TV, etc. These

applicationsmayincludemachine-to-machineserviceswithinthehomeorbusiness,e.g.,localappliances and web cameras.

l FixedmobileconvergenceintegrationofHDTVandhomemediaservices.

Figure14:EnablingNext-GenerationFemtozoneServices



7. TechnicalDeploymentConsiderations&Challenges

7.1 InterferenceManagement

DetailedstudiesofinterferencemanagementforLTEhavebeencarriedoutintheFemtoForum9. Several scenarios were identified as benign, while others can be handled through interference mitigationtechniques.

The fundamental interference scenarios are the same for LTE femtocells as for 3G WCDMA femtocells,forexample:

l Femtotomacrointerference(bothuplinkanddownlink) l Macrotofemtointerference(bothuplinkanddownlink) l Femtotofemtointerference(bothuplinkanddownlink)

Theextentoftheinterferenceissuedependsonseveralfactorsincluding:

l Co-channeloperationversusadjacentchanneloperation l ClosedAccessfemtocellsversusOpenAccess

Somescenarios(e.g.femtodownlinktomacro,femtouplinktomacro,femtodownlinktonearbyfemtoandfemtouplinktonearbyfemto)couldpotentiallyhavesignificantimpactonperformance.However, it has been found that interferencemitigation techniques can provide acceptable and robust performance.

SomeoftheinterferencemitigationtechniquesapplicabletoLTEareinlinewiththosedevelopedfor3GWCDMAfemtodeployments,suchasadaptivefemtopowercontrolbasedondistanceormacrosignalmeasurements at the femtocell on thedownlink. Enhancements such asmacroUE-awarepowercontroloffemtonodeshavealsobeenidentifiedbytheFemtoForum9.

Having the flexibility of frequency-domain resourcemanagement and coordination, LTE offers anewdimensionforadditionalinterferencemitigationtechniques.Forexample,enhancedfrequency selective scheduling, and semi-static Fractional Frequency Reuse and dynamic resource coordination have been explored9. 3GPP study reports 36.921 and 36.922 evaluate the ICIC mechanisms which could be implemented for support of Rel9 LTE femtocells.

These interferencemitigation techniques are particularly useful for dynamically allocating radio resource or spectrum within a cell or between multiple cells (e.g. between femtocells in an enterprisedeployment)astheloadprofilevariesintheRAN.

Overall, the Femto Forum study9 found that LTE femtocells can coexist with macrocells in thesamechannels,giventheuseofappropriateinterferencemitigationtechniques,eveninthemost challenging situations for interference. This is important, given that wide bandwidths may be needed for LTE, and the limited spectrum which may be available to operators in some frequencybands.

9 InterferencemanagementinOFDMAfemtocells,FemtoForumwhitepaper,March2010,Availablefromwww.femtoforum.org



7.2 Spectrum

LTEsupportsmanyfrequencybands,currentlyrangingfrom700MHzto2.6GHz.Femtocellscanuse any and all of these standardized bands and in the process can increase their usefulness to the operatoranduser.Dedicatedspectrumbandsforfemtocellsarenotrequired.Someofthespectrumandregulatoryopportunitiespresentedbyfemtocellsinclude:

l Opening up spectrum bands: conventional macrocell deployments exhibit very different economics at high frequencies, particularly bands above1GHz, so suchbands are usuallyseen as being appropriate for delivering high capacity but not as a main means of delivering coverage. Femtocell performance, however, is almost independent of spectrumband. This enables femtocells to be used economically in higher frequencies, but also to operate efficiently in lower frequencybands. In thesebands, femtocells can reduce their coverageareatodeliverhighcapacitywithoutcreatingexcessiveinterference.Theirself-tuningnaturealsoallowsthemtotolerateinterferencefromawiderangeofsources,suchasothercellsanddigital TV.

l Re-using existing bands: femtocells offer the possibility of making use of small elementsofspectrumwhichmaynotbeheavilyusedtoday,suchasTDDsub-bandsandevenguardbandsinsomecircumstances.Theycanalsofacilitateprogressivemigrationofbandsbetween technologies, so that for example bands used for 2G traffic can gain an underlay of LTE femtocellsbeforethosebandsarecompletelyclearedofexistingtraffic.

l Spectrum and economic efficiencies: the dense spectrum reuse and high interference protectioninherentinfemtocellsdeliververyhighcapacityfromagivenspectrumbandwidth,delivering veryhigh spectrumefficiencywhich is a goal for operators and regulators alike. Likewise, the ability to deliver deep indoor coverage atmuch lower cost than amacrocell network enables economic efficiencies which are in the interests of the end-customer, operator and regulator alike.

l Innovation and competition: femtocell deployments can support new approaches to enablinganinnovativeandcompetitivemarketformobilebroadbandservices.Forexample, inside-outdeploymentsoffemtocellsasthestartingpointforanewnetworkcanenablenewandexistingoperatorstoprovideLTEserviceswithinsmallerspectrumbandwidthsthanwouldbe viable for macrocells. They also enable the development of new services and enhanced customer experience with a lower entry barrier than is conventionally possible, thereby encouragingandenablinginnovationinthecustomerserviceoffered.

7.3 Backhaul

In the following, we focus on residential applications, giving a brief picture of fixed networks evolution and backhaul speeds. Backhaul availability and bandwidth is a critical component of femtocelldeployments.Thefollowingaresomeconsiderationsrelatedtobackhaul:

l Throughout the world, progressive evolution to faster residential broadband speeds is predicted,withmajorgrowthintheperiodforLTEintroduction(seeFigure15).

l TheexistingclassicalbroadbandmarketisexpectedtomigratetoVDSLandFTTH,including: l Multichannel,interactive,andHDIPTV l InternetTV/Video l Homemultimedianetworkingcapabilities



l According to IDATE, high bandwidth fixed network deployments are forecast to accelerate: there will be about 30 million subscribers (FTTH) in Japan by 2013 (ADSL subscribersdecreasingto7.5million)and,by2015,morethan21millioninEurope(4.5millioninFrance)andstill90millionADSLsubscribers.

l AccordingtotheFibertotheHomeCouncil16,asofSeptember2009thenumberofU.S.homespassed by FTTH networks exceeded 17million, withmore than 5million of these homes connected. The FCCs National Broadband Plan recommends that at least 100 million additional U.S. homes have affordable access to broadband with speeds of at least 100 megabits per second by the year 202017.

Furthermore,increasingamountsofdatatrafficareexpectedtoremainwithinthehomeLAN(see6.3),whichisnotlimitedbythebackhaulbandwidthavailable.Inthiscase,availabilityofthefullairinterfacespeedsofLTEtothehandsetinthehomewillbeanadvantage,evenifthefixedbroadbandconnectionhaslowerbandwidth.

Figure15:BroadbandConnectionsandLTESubscribers

16 http://www.ftthcouncil.org/en17 http://www.broadband.gov/



8. Conclusions

Torealizeitsfullpotential,LTEbenefitsfromtheinclusionoffemtocellsfromtheoutset:

l TherecentlypublishedSignalsResearchGroup/FemtoForumstudy18 suggests there is a good business case for femtocells in LTE.

l Effectiveinterferencemitigationtechniqueshavebeenidentifiedindetailedstudies,enablingcapacityandperformancebenefits.

l StandardshavebeenagreedfortheintegrationandmanagementoffemtocellsinLTE. l Femtocellshelpenablenewserviceopportunities.

LTEcanrevolutionisemobilebroadbandandfemtocellscanplayaroleinhelpingittodeliveritspotential.ByadoptingfemtocellsoperatorscanrolloutamuchbetterperformingLTEnetworkthantheycouldwithmacrobasestationsaloneandatalowercostandwithlessrisk.Allthesefactors are crucial in the current uncertain economic environment.SimonSaunders,ChairmanoftheFemtoForum.

18 TheBusinessCaseforFemtocellsintheMobileBroadbandEra,FemtoForumwhitepaper,March2010,Availablefromwww.femtoforum.org

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Why Lte Needs Femto Cells