On the Performance of LTE-Advanced MIMO: How to Set and Reach Beyond 4G TargetsEeva Lhetkangas, Kari Pajukoski, Esa Tiirola Radio Research Nokia Siemens Networks Oulu, Finland Jyri Hmlinen, Zhong Zheng Department of Communications and Networking Aalto University School of Electrical Engineering Aalto, Finland AbstractStandardizationof3GPPLTE-Advanced(LTE-A), alsocalledas4G,iscontinuingstrongly.Yet,inparallelthe discussiononperformancetargetsforsystemsbeyond4Ghave been initiated in industry.The aim of this paper is to investigate thelinklevelperformanceofLTE-Adownlink(DL)whenlarge antennaarrayMIMOisemployed.Thelinklevelsimulation results show very high user throughputs when employing e.g. 8x8 MIMO.Yet,thecomparisonbetweensimulationresultsand known spectral efficiency bounds show that LTE-A performance is still quite far away from theoretical limits. Results also indicate that realistic beyond 4G peak rate target on link level can be set closeto10Gbpsprovidedthat8x8MIMOand256QAMare applied,andtheoperationbandwidthcanbedoubledfrom current 100MHz. The spatial correlation in both transmitter and receiver ends will reduce the performance.KeywordsBeyond4G;LTE-Advanced;MIMO;256QAM; spatial channel correlation; link performance I. INTRODUCTION ECENT3rdGenerationPartnershipProject(3GPP)system standards will provide a major increase in user data rates. According to the original requirements of the Long Term Evolution(LTE)Release8,thedownlinkpeakdata ratetarget wassetto100MbpsandinthemostrecentversionofLTE, LTE-Advanced(referred asLTE-Aandalsosometimescalled as 4G), target rate is up to 1 Gbps for stationary users [1] [2].While LTE-A development strongly continues within 3GPP Release11standardizationefforts,thediscussiononbeyond 4G(B4G)havebeenalsoopenedrecently[3][4].Inparallel withtheraceforeverincreasingpeakdataratesalsohigher system level spectral efficiency is of great importance for B4G. Hence,inadditionto10Gpbspeakrateaimistoreachhigh area throughput on geographically limited areas through dense network deployments. Ifmultiple-input-multiple-output(MIMO)isnotapplied, thelinklevelperformanceof currentLTEis closetothe limit setbyShannonlaw[5]whichshowsthepowerofcurrently used orthogonal frequency division multiple access (OFDMA) methodology.Onsystemlevelthemostpromisingwayto reach higher spectral efficiency than today is the deployment of smallcellsandusageoflargeantennaarraysenablingupto eight stream MIMO. Moreover, besides MIMO the high signal tointerferenceandnoiseratio(SINR)insmallcellssupports the usage of high order modulation schemes like 64QAM.The target of this paper is to examine the spectral efficiency boundsofLTE-Adownlinkbysimulatingthespectral efficiencyaccordingtothemostrecentstandardsandby comparingtheresultsagainsttheanalyticalperformance bounds.In[5],suchacomparisontoShannonboundisdone forLTEdownlinkinSISO,SIMOand2x2MIMOscenarios. In[6],[7]performanceboundsforOFDMbasedMIMO wirelesssystemsarepresentedandcomparedtoLTElink simulationresultsin4x4MIMOscenario.Inthispaperwe extenddownlinklinklevelsimulationsandanalysistocover LTE-Awithrankindexesupto8.Wecomparesimulation results to theoretical spectral efficiency bounds and analyze the mostcriticalcausesofperformancelosses.Inaddition,we examinethepotentialofLTE-Aasabasisforbeyond4G system.Forthatpurposeweanalyzetheconditionsunder whichthe10Gbpspeakdataratecanbeachieved.Study includes usage of 256QAM modulation that has not previously beenexaminedinthiscontext.Wealsostudyhowspatial channelcorrelationaffectstotheLTE-Amaximumspectral efficiency.Thepaperisorganizedasfollows:InSectionIIwegivea shortmotivationforthisstudyandpresenttheused performance bounds. In Section III we give an overview of the used link simulation conditions and parameters and present the simulationresultsandanalysis.SectionIVconcludesthis paper. II.MEANS TO INCREASE THROUGHPUT IN B4G SYSTEMS When setting link level targets for B4G systems we need to consider at least the following aspects:Increasingtheoperationbandwidthisthemost straightforwardwaytoincreasethelinklevelthroughput. Yet, improving the spectral efficiency should be one of the mainB4Gtargetsbecauseotherwisechangerequestfor existing physical layer specifications is irrelevant. Fromcommunicationtheoryperspectivethenaturalway toincreasespectralefficiencyistointroduceadditional MIMObranchesandtoapplyhigherordermodulation schemes. Since practical implementation limits the number R European Wireless 2012, April 18-20, 2012, Poznan, Poland ISBN 978-3-8008-3426-9 VDE VERLAG GMBHofantennasincurrentlydeployedfrequencycarrierswe haveassumedupto8receiveantennasinterminalsthat arepreferablylaptops.Duetosmallcelldeploymenthigh ordermodulationmaybecomefeasibleand256QAMhas been also seen as an attractive alternative.Thepracticalradiosystemalwaysrequiressignaling overhead.Asabaselineassumptionwehaveused20% controloverheadthatweexpecttoenableupto8stream MIMOdesigns.Thisisactuallyquitechallengingdesign target since reference signal overhead is rapidly increasing with additional MIMO streams. A.Maximum Spectral Efficiency Let us consider a simple pre-study on how to reach 10 Gbps for B4G. In Fig. 1 we have throughput for different MIMO and modulationconfigurationsasafunctionofoperation bandwidth when error free reception takes place. We note that aim of Fig. 1 is just to show the limits that cannot be exceeded byincreasingthetransmissionpower.Apracticalquestionto beconsideredinthenextsectionswillbewhetherwecan operate close to the limits of Fig. 1 within a practically feasible signal to noise (SNR) region. Figure 1. Options for reaching 10 Gbps with 20% control overhead FromFig.1wefindthat10Gbpscan,inprinciple,be reachedforexamplewith6MIMOstreamsand64QAM assuming350MHzbandwidth.Moreambitiousapproach would be to assume 200 MHz bandwidth and 8x8 MIMO with 256QAMsupport.This200MHzscenariorequires50b/s/Hz spectralefficiencywhichisinlinewiththepreviousB4G discussion[4].Thus,inordertosetanotableperformance challengeonphysicallayerperformanceweconcentrateinto this scenario. Let us consider maximum spectral efficiency (SE) for LTE-A under different modulation and control overhead constraints. If control overhead ratio is denoted by Oc, rank of the MIMO is RMIMOandappliedmodulationcarriesbsbitspersymbolwith maximum coding rate, then we have SE = (1 0c) NSP NRB NSC Ns bs RMIM0Bw, whereNSF,NRB,NSCandNsarethenumbersofsubframesper second,ResourceBlocks(RBs),OFDMAsubcarriersand OFDMAsymbolsineachsubframe,respectively.InLTEthe numberofsubframesis1000persecondand12subcarries composetheresourceblock.In200MHzbandwidththe numberofRBsis1000ifweassumeastraightforward extension for LTE operation band.Normal cyclic prefix length of14OFDMAsymbolspersubframeistakenintoaccountin the calculations. Maximumspectralefficienciesfordifferentmodulation schemes and control overheads are presented in TABLE I. We have limited the discussion to 8 stream MIMO. In second case ofTABLEIwehavetakenintoaccounttheoverheadfrom8 demodulation reference signals (DRS) required for 8x8 MIMO while in third case we have also added overhead from 1 control symbol(7.143%)and2cellspecificreferencesignal(CRS) sequences(3.57%overheadperCRS).The256QAMspectral efficiencyhasbeenconsideredincasewherethereiscontrol overheadconsistingofcontrolsymbolandDRSs(total 21.43%).InthiscasenooverheadfromCRSistakeninto account, which would be feasible scenario if 3GPP Release 11 extensioncarrierreducingoreliminatinglegacycontrol signalingorCRSisused.Wenotethatcodingrates30dB range is entered. Furthermore, based on European Wireless 2012, April 18-20, 2012, Poznan, Poland ISBN 978-3-8008-3426-9 VDE VERLAG GMBHthe results, round 50dB SNR is needed before achieving around 8GbpsdataratesonLTE-Adownlinkwhenassuming 256QAM modulation, 8x8 MIMO and 200 MHz bandwidth. From the analytical spectral efficiency boundswe find that the spectral efficiency lost due to linear receiver increases with numberofantennasandchannelrank.Thiseffectisfurther demonstratedinFig.5,wherethepercentageoflostspectral efficiencyresultingfromusageoflinearMMSEreceiveris plottedfordifferentnumberofantennaswhenSNRis20dB, 30dBand40dB.Yet,performancelossduetolinearMMSE receiver with perfect channel knowledge is not as large as loss duetoothercausessuchascontrolandRSoverheadand modulation restrictions. Figure 5. Share of spectral efficiency consumed in linear receiver as a function of maximum rank Figure 6. Shares of LTE-A (8x8 MIMO) data and performance loss components as a function of SNR. InFig.6wehavesummarizedtheLTE-Aspectral efficiencyandperformancelossesintermsofsharesofmain performancelosscauses(scenario2).ThecontrolandRS overhead form the largest cause of performance loss, followed byguardandCPoverhead.RSoverheadissmaller inthelow SNRrangesincewithpoorchannelconditionssmallerranks areusedandthusfewerMIMOreferencesignalsareneeded. RS overhead then increases as a function of used rank index up torank3,fromwhichonwarditstaysconstant.Withlarge SNRvalues(from20-30dBonwards),therestrictionsofthe used coding and modulation set (no larger modulation schemes than64QAMareincludedintheset)becomesevidentasa noticeablyincreasingBICMoverheadarea.Linearreceiver (MMSE)overheadintheFig.6illustratesthelosscausedby theusedlinearMMSEreceiverwiththeLTE-Aprecoding codebook.Theoverheadcausedbythereceivercouldbe reducedbyselectingmoreefficientreceiverorbyincreasing the precoding codebook. Spectralefficiencyincaseoflowtransmitterantenna correlation(=0.3)andhighreceiverantennacorrelation (=0.9)ispresentedinFig.7.Itisnoticedthatthespectral efficienciesaremuchworsethanincorrespondingscenarios without spatial correlation. Like it can be seen from Fig. 7a, it requiresover100dBSNRtoobtainthemaximumreachable spectralefficiencyin4x4MIMOscenario.Evenmuchhigher SNRwouldbeneededtoreachthemaximumin8x8MIMO scenario (Fig. 7b). a)b)Figure 7. Spectral efficiency (bits/s/Hz) bounds of low Tx & high Rx channel correlation: a) scenario 4a (4x4 MIMO), b) scenario 5 (8x8 MIMO) European Wireless 2012, April 18-20, 2012, Poznan, Poland ISBN 978-3-8008-3426-9 VDE VERLAG GMBH ConsiderOLMIandCLMIcurvesinFig.7.Itiscleary visiblethatthethroughputevenwithreallyhighSNRvalues (such as 50dB) is still far away from the level that was reached withoutspatialcorrelation.Itcanbealsonoticedthatthe spectral efficiency lost due to MMSE receiver with high ranks ismuchlargerincaseofspatialcorrelationcomparedtothe samescenariowithoutcorrelation.However,whenrank adaptationisinuseandchannelcorrelationrestrictstherank selection to small ranks, linear MMSE receiver does not cause large performance loss. It can be also seen from the form of the achieveableBICMcurvesthatappliedrankindexesinthis scenario are low, on level 2-3. Since rank selection is restricted bythechannelcorrelation,themaximumthroughputisquite thesamefor4x4and8x8MIMOcases.Thusthereisnoreal benefitfromhighnumberofMIMOantennasincaseofhigh spatialcorrelation.However,sincetherankindexincreases slowlyasafunctionofSNR,thehighestordermodulationin the adaptive MCS set plays more important role when channels arecorrelated.ItcanbeseenfromFig.7thatextensionto 256QAM improves the maximum spectral efficiency even with relatively low SNRs, starting approximately from 10dB. It can be concluded that extending the MCS set to even higher order modulations increases the maximum spectral efficiency even in therangeofpracticallyfeasibleSNRvalues.Thelargerthe spatial correlation, the lower the SNR area where improvement from higher order modulation is obtained. This is also visible in Fig. 8 where we present the BICM bound results of scenario 4 withmaximummodulationof64QAMand256QAM. Figure 8. BICM spectral efficiency (bits/s/Hz) bounds of scenario 4 IV.CONCLUSION Inthispaper,wehavepresentedlinklevelsimulation resultsfordifferentLTE-AMIMOscenarios.Wehave compared the results to theoreticalperformanceboundsandto beyond4Gthroughputtargetof10Gbpsassuming approximately200MHzbandwidth.Resultsshowthat maximum LTE-A spectral efficiency is quite far away from thetheoreticalShannoncapacitylimit.ThecontrolandRS overheadisthebiggestconsumerofthewastedspectral efficiency,followedthenbyguardbandsandcyclicprefix overhead.LinearMMSEreceiverdoesnotseemtocause remarkablelosstothespectralefficiencyeveninhighspatial correlationscenario.Inaddition,effectofextending modulationschemeto256QAMwasinvestigated.Itcanbe statedthatincaseofhighspatialcorrelationincreasing modulationto256QAMorevenfurtherimprovesthespectral efficiencyalreadywithrelativelylowSNRvalues(~10dB onwards). We have also examined the effect of spatial channel correlation that clearly restricts the rank usage and therefore the obtained maximum spectral efficiency. ACKNOWLEDGEMENTS This work was supported partly by NETS2020 project. REFERENCES [1]3GPPTR36.913v.9.0.0,RequirementsforFurtherAdvancementsforEvolvedUniversalTerrestrialRadioAccess(E-UTRA).Available: www.3gpp.org. [2]H. Holma and A. Toskala, LTE for UMTS Evolution to LTE-Advanced, 2nd ed., Wiley, 2011. [3]B. Raaf, W. Zirwas, K. J. Friederichs, E. Tiirola, M. Laitila, P. Marsch, R.Wichman,VisionforBeyond4GBroadbandRadioSystems,IEEE PIMRC WDN workshop, 2011. [4]S.Liu,etal,A25Gb/s(/km2)UrbanWirelessNetworkBeyondIMT-Advanced, IEEE Communications Magazine, February 2011. [5]P.Mogensen,et.al.,LTECapacityComparedtotheShannonBound, IEEE VTC, April 2007. [6]M.Rupp,S.CabanandC.Mehlfuhrer,TheShannonlimitinmobile cellular systems: How far off are we? 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