An introduction to LTE Smart base station antennas

Mobility Network EngineeringFebruary 2017

Dr. Mohamed Nadder Hamdy

White paper

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ContentsIntroduction 4

PhasedArrayAntennas 4

Antennaarraypatterns 4

Electricaldowntilt 5

SwitchedBeamAntennas 5

Butlermatrix 5

Luneburglens 5

Adaptivearrayantennas(AAA) 6

Planar arrays 6

CommScope TTTT series 6

Columnpatternandmutualcoupling 6

Scanninganglelimitations 7

Radiatingpatterns 7

Calibrationport 8

Eigenbeamforming(precodedMIMO) 8

Precoding 8

Antenna ports 9

LTE MIMO schemes 10

PracticalimplementationwiththeTTTT 11

Multiple-cellbeamforming 12

MassiveMIMObeamforming 12

Conclusion 13

References 14

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ListoffiguresFigure1,Beamsandnullssteering 4

Figure2,Andrewpatentedantennaelementexamples 4

Figure3,Basestationpanelantenna 4

Figure4,Arrayfactor 4

Figure5,Antennaelementsphaseshifts 5

Figure6,AndrewPatentU.S.6924776B2 5

Figure7,RFpathandradiationpatternofafour-portButlerMatrixantenna 5

Figure8,Luneburglensconstruction 5

Figure9,CommScopeTTTT65AP-1XRantenna 6

Figure10,ElementspacingandH-BW 6

Figure 11, Max scan angle versus column spacing 7

Figure12,Broadcastbeamspatterns 7

Figure13,CalibrationportonaCommScopeantenna 8

Figure14,Calibrationboard 8

Figure15,Sprint/SAMSUNG8T8Rimplementation(U.S.) 8

Figure16,DLphysicalchannelprocessing 8

Figure17,PrecodedMIMO 8

Figure18,Twohalf-wavelengthseparateddipolesapplyingsinglelayercodebook 9

Figure 19, Mapping antenna ports to physical antennas 9

Figure20,TX-RXmodes 10

Figure21,MIMOmodesinLTE-advancedselectedonthebasisofUESINRandspeed 11

Figure22,Beamformingwithfour-columnantenna 11

Figure23,Downlinkperformanceofmultiple-cellbeamformingii 12

Figure24,TheLundUniversitymassiveMIMOtestbed(LuMaMi) 12

Figure25,SpectralefficiencysimulationformassivebeamformingFDDversusTDDii. 13

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IntroductionWe are in the midst of yet another technology upheaval in which everyday objects—from clothing to cars—are being converted into smart devices. Smart phones and smart TVs give rise to smart homes and intelligent buildings, which evolve and expand into smart grids, smartcitiesandsmartgovernments.It’snosurprise,then,thatthetraditionalbasestationantennaisfollowingasimilarpath.

Thispaperprovidesabriefoverviewoftheevolutionofsmartantennasandtheirsometimes-confusingrelationshiptoactiveantennas, beamforming, precoding and MIMO transmission modes.

Unliketraditionalantennasthatcouldtransmitandreceiveonlyusingafixedradiationpatternregardlessoftheenvironment,smartantennasareadaptivebeamformers.Theycandynamicallyadjustthemainbeamandnullsdirectionsintheirpatternsaccordingtochannelconditionsandthelocationsofconnectedusers.

Forexample,usingdigitalsignalprocessing(DSP)techniques,smartantennasareabletoestimatethedirectionofarrival(DoA)foraconnecteddeviceandshapetheradiatingpatternsaccordingly.Tobetterunderstandhowthesetechniqueswork,weneedtotakeacloser look at some of the technologies that make them possible:

• Principles of phased antenna arrays

• Beam steering using switched beam antennas

• Beamformingwithadaptivearrayantennas

• Precoding and Eigen beamforming systems

Figure1,BeamsandNullsSteering

Smart Antennas possess “Super Powers” to dynamically adjust their main beams and nulls directions

Figure2,Andrewpatentedantennaelementsexamples

Figure3,Basestationpanelantenna

Figure4,Arrayfactor

Phased array antennasAphasedarrayantennaappliesphasecontrol(ortimedelay)ateachradiatingelement,soitsbeamcanbescannedtodifferentanglesinspace.Hereishowit’sconstructed.

Antennaarraypatterns

Atypicalbasestationpanelantennaconsistsofanumberofradiatingantennaelements(AE).SomeexamplescanbeseeninFigure2.

TherearedifferenttypesofAEs,withwireandapertureelementsbeing the most common. Examples of wire AEs include dipole and monopole elements, while aperture AEs include slot elements. Some designsincorporatecombinationsofbothtypesandcanalsobebuiltover printed circuit boards or micro strip patches. Every AE has a radiationpattern,usuallyreferredtoasanelement pattern, whose characteristicsaredeterminedbythedesignoftheelement.

WhenmultipleAEsaremountedinalinealongasharedreflector,theresultisapanelantennawithlineararrays(Figure3).

WhileeachAEinalineararrayhasitsownradiationpattern,theRFeffectoftheentirearraydependsonthespacing, phase shifts and amplitude variation between its elements. Together, these three variables are used to describe the array factor pattern. By combining thearrayfactorpatternandtheelementpattern,wecandeterminetheoverallfar-fieldradiationpatternofthepanelantenna.

The RF signal phases and amplitudes feeding the AEs can be controlledinordertoshapethearrayfactorpattern.Thiseventuallydirectsandshapesthefinalbeam.

Antenna element

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Luneburg lens

TheLuneburglenswasfirstproposedbyDartmouthCollegeprofessorRudolfKarlLüneburgin1944.Luneburglensesaretypicallycomposed of layered structures of discrete concentric shells that formasteppedrefractiveindexprofile.Thedifferentrefractiveindices decrease radially from the center to the outer surface.

TheLuneburglenscanbeutilizedforelectromagneticradiations,rangingfromvisiblelighttoradiowaves.Thelens’svariablerefractiveindices align and direct the radiated waves from its surface in one direction,asshowninFigure8.SimilartotheButlermatrix,thedirectionoftheantennabeamcanbescannedbymovingtheradiatingelementalongthelens’scircumference.

Butler matrix

The Butler matrix,firstdescribedbyJesseButlerandRalphLowein1961,useshybridcombinersandphaseshifters.

AsshowninFigure7,allfourinputports(1R,2L,2Rand1L)aremappedtoallfourAEs(#1,#2,#3,#4)butwithdifferentphaseshiftcombinations.Asaresult,eachinputportisabletogenerateadifferentbeamdirection.

Moreover, when an input signal is switched across all input ports—suchthatoneportisconnectedatatime—andtheproperswitchpositionisselected,thebeamcanbeelectricallysteeredtowardspecifictrafficareas.

ElectricaldowntiltElectricaldowntilting—aprimitivebeamformingtechniqueforverticalpatterns—hasnowbecomeastandardfeatureinalmostallbasestationantennas(BSAs).Aswiththephasedarrayantenna,electricaldowntiltworksbyadjustingthephaseshifters,whichaltersthepanel’sarrayfactor.ThepolarplotinFigure5illustratestheeffectphaseshiftinghasonthedirectionofthemainlobe.

Theantenna’s“electrical”downtiltisactuallyaccomplishedmechanically,asthephaseshiftersareadjustedeithermanuallyonthe antenna itself or remotely using a controlled stepper dc motor. Figure6showsadualpolarizedantennawithelectricaltilt.ThephaseshifterscanbeclearlyseenconnectedtotheAntennaElements’feeding network.

Phase shift Phase shift

Figure5,Antennaelementsphaseshifts

Figure6,AndrewPatentU.S.6924776B2

Figure7:RFpathandradiationpatternofafour-portButlermatrixantenna

Switched beam antennas Theswitchedbeamantennarepresentsthenextstepintheevolutiontoward the smart antenna. It consists mainly of an antenna array and simplebeamformingnetworkcapableofgeneratinganumberofdiscretepredefinedfixedbeamdirections.Thesystemdynamicallyselectsthebestbeamforspatialselectivitybasedonchannelconditions.Twoofthemorepopularswitchedbeamantennadesignsinclude the Butler matrix and Luneburg lens.

RadiatingElement

Figure8,Luneburglensconstruction

RafractiveIndices

Cartesianplot Polarplot

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Adaptivearrayantennas(AAA)Planar arrays

WehaveseenhowbeamformingtechniquesrelyonthealterationoftheRFsignalphasesandamplitudesthatarefedacrosstheinternalantennaradiatingelements.Forlineararrayantennas,verticalbeamsteering(e-tilting)isadirectapplicationofthisconcept.However,toenablehorizontalandverticalbeamsteering,atwo-dimensionalrectangulararrayisneeded.Thisisknownasaplanararray,akeycomponentoftoday’sadaptivearrayantennas(AAA).

Adaptivearrayantennasconsistofcolumnsofaplanararraythatworktogethertoformasteerablebeamthatisshapedforimprovedside lobesi.AdaptivebeamformingusesdigitalsignalprocessingtechnologytoidentifytheRFsignal’sdirectionofarrival(DoA)toaUserEquipment(UE)—andgenerateadirectionalbeamtowardsit ii.

CommScope TTTT series

Formany,thecommonbeliefhasbeenthatanysuccessfuladaptiveantennaarraymustinvolveactiveantennasorantennaswithintegratedradios.However,itisactuallypossibletoachievehorizontalelectricalbeamsteeringoverregularantennaswiththesupportofexternalbasestationradiosiii. The CommScope TTTT antenna series is a good example of this capability.

Horizontal beam steering is realizable with mutli-column antennas and external radios

Figure9,CommScopeTTTT65AP-1XRantenna

TherearecurrentlytwoTTTTseriesmodelsinCommScope’sportfolio.

Model Number HorizontalBeamwidth Gain(dBi) VerticalBeamwidth Frequency(MHz)/ No. of PortS

TTT65AP-1XR 65° 17.8 4.8° 2490-2690/8

TTTT90AP-1XR 90° 17.2 4.7° 2490-2690/8

ColumnpatternandmutualcouplingItshouldbenotedthatthehorizontalbeamwidth(H-BW)listedinTTTTseriesdatasheetsisnotreflectiveoftheentirepanel.Instead,itshowsthepatternshapeforoneoftheantenna’sfouridenticalcolumns.Suchcolumnpatternshapeisdependentonanumberofphysicalfeatures.Theseincluderadiatingelementsdesign,reflectorwidths and, most importantly, intercolumn spacing.

Becauseanarray’sbehaviorisdominatedbythemutualcouplingbetween its various elements, the elements generally behave very differentlyinanarraythanwhenisolatediv.Forexample,theH-BWlistedonthedatasheetisinverselyaffectedbycolumnspacing:a65-degreeH-BWtypicallyrequires0.65λspacingbetweencolumns,whereasa90-degreeH-BWwilluse0.5λspacing.Figure10showsthiseffectfora7x9elementdipolearray(λ/2dipoles,λ/4aboveground.)ElementspacingsaredenotedasDx and Dy.

Figure10,ElementspacingandH-BWv

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ScanninganglelimitationsA linear array with its peak at ϴo can also have other peak values, depending on its spacing dx.Theseadditionalunwantedpeaksarecalled “grating lobes.” To avoid these lobes, it is important to observe therelationshipbetweenthecolumnpatternhorizontalhalf-powerbeamwidth(H-HPBW)andthemaximumscanningangle.Forelementspacing dx(0.5λ<dx<λ),thearrayfactorwillhaveonlyonesinglemajorlobe,andgrating-lobemaximawillnotoccurfor -90°<ϴo<+90°aslongasvi

|sinθ0 | > –1<where S = dx/ λs

1

Figure11,Maxscanangleversuscolumnspacing

RadiatingpatternsAnadaptivearraybeamformingantennahasthreemainradiatingpatterntypes:

I.Singlecolumnpattern

Thispatterntype,describedindatasheets,usesunweightedinputs and is typically used for uplink channels.

II.Broadcastpattern

This wider, nonsteerable beamisintendedtocovertheentirecellareaandiscreatedbyfeedingthefourcolumnswithdifferentamplitudes and phases. By reducing the amplitude, the broadcast patternexhibitsagainreduction,alsoknownas“weightingloss.”Thebroadcast is usually used for downlink control channels.

III.Servicebeampattern

When all columns are fed together with a uniform phase progression, the result is a narrow beam that can be steered horizontally. In a servicebeampattern,theamplitudesmightbeunchanged—ortaperedforsidelobesuppressionwithaddedweightinglosses.Thistypeofradiatingpatternisusedmainlyfordownlinkdatatraffic.

Togeneratethesepatterns,theRANsupplierradiosmustbeabletoapplydifferentamplitudeandphaseshiftsacrosstheantennaports.For example, for broadcast or service steering modes, the radios might applythebelow-phaseandamplitudedifferenceacrosstheantennas’fourarrays(columns).SeeTable1.

AsshowninFigure12,byusingspecificamplitudeandphasesettings,theTTTT65APandTTTT90AParecapableofgenerating65-degreeH-BWbroadcastpatterns.Ironically,the90-degreemodelexperienceslessweightinglossforproducinga65-degreebroadcastpattern.

Figure12,Broadcastbeamspatterns

Beam mode Column 1 Column 2 Column 3 Column4

65°BroadcastPhaseΦ 0 115 100 0

Amplitude 0.81 1 0.73 0.6

90° BroadcastPhaseΦ 0 130 110 0

Amplitude 0 130 110 0

BeamSteering@0°PhaseΦ 0 0 0 0

Amplitude 1 1 1 1

BeamSteering@30°PhaseΦ 0 100 200 300

Amplitude 1 1 1 1

Table1,Beamsteeringsettings

Plottingthisrelationshipfordifferentcolumnspacings,asinFigure11,itiscleartheconditionisalwaystruefor0.5 λ(S=0.5)columnspacing,which leads to the ±90-degree scanning angle range. For max scanning angles of ±32,±45and±60degrees,thespacingis0.65 λ, 0.59λand0.53λ,respectively.

Therefore, 0.5 λ intercolumn spacing corresponds to the 90-degree columnH-BWwith±90-degree antenna scanning angle range. Similarly 0.65 λ corresponds to 65-degreecolumnH-BWwith±32-degree antenna scanning angle range.

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CalibrationportWehaveseenhowthepatternsofanAAAaredependentonitsports’phaseandamplitudevariationsaslistedinTable1.Thesesettingscanbeaccuratelyappliedfromtheradio’sside,butthisdoesnotnecessarilyguarantee these exact values will arrive at the antenna ports. This is especiallytruegiventhevariancesinlengthoftheconnectionjumpersbeingused.Thecalibrationport,showninFigure13,isdesignedtocompensateforanyvariance.Intheexamplebelow,allsub-arraysarecoupled to a single CAL port.

Figure13,CalibrationportonaCommScopeantenna

InsidetheCommScopeTTTTantennas,acalibrationboard (Figure14)builtwithWilkinsonPowerdividersmaintainsacouplinglevelof-26decibelstowardthecalibrationport.Thecouplinglevelisabalancebetweenpowerlossesandmeaningfulcalibrationlevelforaccuratecalibration.

Figure14,Calibrationboard

OEMradiosmustalsobeabletosupportthecalibrationfunctionality.ThisisdonebydesigningtheRRUwithadedicatedCALport(Figure15)forconnectiontothecorrespondingCALportontheantenna.Thisensuresaccuratephaseandamplitudesettings,regardlessofthefeeders’lengths.TheOEMradiosshownalsohaveeightantennaportstosupportthefourx-polcolumnsantennas.

Figure15,Sprint/SAMSUNG8T8Rimplementation(U.S.)

Eigenbeamforming(precodedMIMO)Inadditiontothetraditionalbeamformingcapabilitiesofphasedarrayantennasbeamforming,LTEenablesanotherbeamformingeffectresultingfromchannelprecoding.TheeffectiscommonlyknownasEigen beamforming.

Figure 16 shows DL codeword processing by LTE physical layer.

Figure16,DLphysicalchannelprocessing

Figure17,PrecodedMIMO

Precoding

InaMIMOsystem,thefinalreceivedsignalisaresultofdestructiveandconstructivemultipathreflections.Whenprecodingisadded,thetransmitter—apprisedofcurrentchannelconditions—caneffectivelycombine the independent data streams prior to transmission in order toequalizesignalreceptionacrossthemultiplereceiveantenna vii.Inotherwords,theprecoderreverseschanneleffectsfromthetransmitterside.

Codeword:a transport block after adding error protection as CRC, channel coding and rate matching. One or two code words, CW0 and CW1, may be used depending on the channel conditions and use case. Layer: represents the number of independent data streams that can be simultaneously transmitted. (e.g., MIMO streams).

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Codebook indexNumberofLayersυ

1 2

0

1

2

3

11

21√ 0

110

21√

11

11

21

21

j1

21√

11

21√

j j1 1

j1

21√

11

21√ 0

110

21√

11

11

21

21

j1

21√

11

21√

j j1 1

j1

21√

11

21√ 0

110

21√

11

11

21

21

j1

21√

11

21√

j j1 1

j1

21√

11

21√ 0

110

21√

11

11

21

21

j1

21√

11

21√

j j1 1

j1

21√

11

21√ 0

110

21√

11

11

21

21

j1

21√

11

21√

j j1 1

j1

21√

11

21√ 0

110

21√

11

11

21

21

j1

21√

11

21√

j j1 1

j1

21√

Table2,Codebookfortransmissionon2antennaports

MIMOprecodingreliesonthetransmitterbeingabletoacquirecertaininformationfromtheuserequipment(UE)reportsregardingthe channel. For instance, in the open-loop modes, the UE sends minimalinformationtotheeNodeB.Thisincludes:

• Rank indicator (RI): Number of layers that can be supported under thecurrentchannelconditionsandmodulationscheme(e.g.,single-layerSISOortwo-layer2x2MIMO)

• Channelqualityindicator(CQI):Channelconditionsstatusunderthe current transmission mode.

In closed-loop modes, the UE must also provide a precoding matrix indicator (PMI)inordertoestimatetheoptimumprecodingmatrixforthecurrentchannelconditions.

LTEspecifiesanumberofpredefinedcodebooks,knownatbothtransmitterandreceiversidesasprecodingmatrices.Table2showsanexamplefortwo-antenna-portprecodingcodebooks,usingsingleor dual layers.

Codebook index 0, the simplest precoding matrix, maps each layer to a single antenna, without any coupling to other antennas.

y (0) (i) = x (0) (i) + x (i) (i)21

21

y (1) (i) = x (0) (i) — x (i) (i)21

21

y (0) (i) = x (0) (i) 2

1√

y (1) (i) = x (i) (i) 2

1√

y (0) (i) = x (0) (i) + x (i) (i)21

21

y (1) (i) = x (0) (i) — x (i) (i)21

21

y (0) (i) = x (0) (i) 2

1√

y (1) (i) = x (i) (i) 2

1√Codebook index 1 provides a linear combination of the sums and

differencesofthetwoinputlayers,respectively.

As the precoding matrices apply amplitude and/or phase shifts to the input signal, the result is the implicit Eigen beamformingeffectontheantennas’patterns,asshowninFigure18.

Antenna portsIntheLTE3GPPstandards,theterm“antennaports”oftencreatesconfusion. These ports, in fact, do not correspond to actual physical antennas, but are considered logical. As detailed in Table 3, LTE antenna ports have standard numbers that are mapped to the transmission of referencesignals(RS)specifictotheUE.TheseUE-specificRSsignalsare used for beamforming modes and must be supported by TDD UEs. FDDUEsmayprovideoptionalsupport.

Reference Signal Antenna Ports

CellspecificRS

0

0, 1

0, 1, 2, 3

UEspecificRS5,7,8

5,7-14

MBSFN RS 4

MBSFN RS 6

CSI RS 15-22

DemodulationRS 107-110

Table3,Referencesignalsmappingtoantennaports-3GPPTS36.211V12

Signalsfrommultipleantennaportscanbetransmittedonasingletransmit antenna; conversely, a single antenna port can also be spreadacrossmultipletransmitantennasix. In either case, the BTS is responsible for mapping logical antenna ports to physical ones.

Figure19,Mappingantennaportstophysicalantennas.x

Figure18,Twohalf-wavelengthseparateddipolesapplyingsingle-layercodebookviii

11

21√ 0

110

21√

11

11

21

21

j1

21√

11

21√

j j1 1

j1

21√

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LTE MIMO schemesLTEdefinesanumberofitsso-calledtransmissionmodes(TM).Dependingonchannelconditions,theTMcanutilizeSISO(singleantenna),MISO(TXdiversity),MIMO(spatialmultiplexing)and beamforming,underclosed-oropen-looptransmissionmodes.

Table4,LTETransmissionmodes

Figure20,TX-RXmodes

Table4showsanexampleofLTEtransmissionmodesforPDCCHandPDSCHconfiguredbyC-RNTI(cell—radionetworktemporaryidentifier)xi.

Trans. Mode Mapping Loop status Transmission scheme of PDSCH corresponding to PDCCH 3GPPRelease

TM1 SISO Open Single-antennaport,port0 Rel.8

TM2 MISO Open Transmit diversity Rel.8

TM3 MIMO Open Large delay Cyclic Delay Diversity (CDD) Rel.8

TM4 MIMO Closed Closed-loopspatialmultiplexing Rel.8

TM5 MIMO Closed Multi-userMIMO Rel.8

TM6 MIMO Closed Spatialmultiplexingusingsingletransmission layer Rel.8

TM7 MIMO Closed Single-antennaport,port5 Beamforming,Rel.8

TM8 MIMO Dual layer transmission, port 7and8 Beamforming,Rel.9

TM9 MIMO Upto8layertransmission,ports7-14 Beamforming, Rel.10

TM10 MIMO Upto8layertransmission,ports7-14

MultipleCell Beamforming, Rel.11

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AsshowninFigure21,theeNodeBappliesdifferentMIMOmodestodifferentUEs,accordingtotheirsignalquality,speedandotherfactors.

Figure21,MIMOmodesinLTE-advancedselectedonthebasisofUESINRandspeedxii

PracticalimplementationwiththeTTTTIntroducedearlier,CommScope’sTTTTantennaseriesfeatureseightexternalports(excludingtheCALport)andfourcross-polarizedcolumnsinside.Thisconfigurationsupportsawiderangeoftransmission modes and beamforming schemes.

Having eight external ports enables the antennas to be deployed in 8T8RTDDmode(eightbranchtransmitandeightbranchreceive).

Asbeamformingreliesoncolumnswithmutualcouplingcorrelation,itrequirescolumnsofsimilarpolarizations.ReferringtoFigure22,thefourcross-polarizedcolumnscanbethoughtofasfourco-polcolumnsonports2,4,6and8(red)andfourcross-polcolumnsonports1,3,5and7(blue).Inotherwords,twobeamformingantennas(BF1andBF2)separatedbypolarizationdiversity.

Dependingontheconfigurationoftheradio,theTTTTcanthensupportbeamformingTM8,9and10,listedinTable4,withMIMOschemesrangingfrom2xto8xormulti-userMIMO.However,ifconfiguredasinFigure22—twocross-polarizedbeamformers,eachwith four columns—we can see how the antenna can support 2x MIMOwithbeamforming.Table5providesasummaryofuseful tipsinselectingantennasforbeamformingandMIMOmodes. FormoredetailsonchoosingtherightantennaforLTEapplications,seeCommScope’swhitepaper,Base station antenna selection for LTE networks xiii.

TheTTTTcansupportbeamformingTM8,9and10,withMIMOschemesrangingfrom2xto8xormulti-userMIMO.

Figure22,Beamformingwithfour-columnantenna

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Optimum application Downlink(TM)

Single-column

45°HBWDense site spacing, hightrafficareas

MIMO(2and3optimal; 4and6possible)65°HBW

All sites, all speeds. Bestall-around

85°HBW Rural sites, coverage challenges

Two-column

0.7λcolumnspacingCorrelated/beamforming; cell edge DL throughput

MIMO,BF(2,3,4,5and6)

1λcolumnspacingDecorrelated/multi-layer;DLcellpeakthroughput; UL cell edge throughput

MIMO(2and3optimal; 4and6possible)

Four-column0.5λcolumnspacing Correlated/beamforming;DLcelledgethroughput

BF,MIMO(8optimal;3,4,5, 6,7and9possible)

0.65λcolumnspacing Bestcolumnpattern/uplinkcelledgethroughputBF,MIMO(8and9optimal;3,4,5,

6and7possible)

Table5,Antennaselection

Multiple-cellbeamformingInlocationslackingadominantservingcell,itispreferabletomitigateintercell interference using coordinated transmission. Coordinated beamformingacrossmultiplecellshasledtoanewtransmissionmodeknownasmultiple-cellbeamforming(MC-BF).Theconcepthasbeenstudiedin3GPPascoordinatedmultipoint(CoMP)transmission/reception.FirststandardizedinLTE3GPPRel.11asTM10(seeTable4,LTEtransmission),itisalsosometimesreferredtoasnetworkMIMO,distributedMIMO,multiple-cellMIMO,etc.ii

Theideaistohavejointlydesignedbeamformingweightsfrommultiplecells,suchthatthecontributingcellstransmittheirsignalstowardusersconstructively.MC-BFcanbeclassifiedasjoint or coordinated. InjointMC-BF,twoormorecellsjointlyandconstructivelytransmitsignalstoasingleUEusingthesametime-frequencyresources.CoordinatedMC-BFcoordinatesschedulingandbeamformingacrossdifferentcellssotheyarealignedtoreduceinterference.ii

Figure22showssimulationresultsforspectralefficiencyimprovementswithcoordinatedMC-BFcomparedto2x2MIMO.

Figure23,Downlinkperformanceofmultiple-cellbeamforming ii

Massive MIMO beamformingMassiveMIMOhasrecentlygainedtheindustry’sattentionasoneofthekeytechnologiesforachieving5Gperformancetargets.Theinitialversionofamassivebeamformingschemein3GPP—thatis,full-dimensionMIMO(FDMIMO)—isbeingstandardizedinRelease13 ii.

The technology involves the use of a large excess of service antennas overactiveterminals.Whenarrangedinlargearrays,massiveMIMOallows narrower, focused and highly directed beams. This improves spectralandenergyefficiencies,leadingtohighercapacities.

Figure24,TheLundUniversitymassiveMIMOtestbed(LuMaMi)

AsmassiveMIMOreliesonspatialmultiplexingandprecoding,thebasestationneedssufficientknowledgeoftheuplinkanddownlinkchannels.Foruplinkinformation,terminalscansimplysendpilotsignals—basedonwhichthebasestationestimatesthechannelresponses to each of its terminals.

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Figure25,SpectralefficiencysimulationformassivebeamformingFDDversusTDD ii.

ConclusionSmartantennasusuallyrefertoadaptivebeamformingantennas.Thetechnologyrequirestwo-dimensional(planar)arraysofradiatingelements,andconnectedradioswithsignalprocessingtechniquesthatenablesteeringthebeamsandnullsinverticaland horizontaldomains.

Usingtransmissionmodes8,9and10,LTEsupportsclassicalbeamforming.Itcanalsoenableanotherimplicitbeamformingeffectby taking advantage of channel precoding.

Today,2x2MIMOLTE-TDDhorizontalbeamformingcanberealizedusingexternalradiosattachedtofourcross-polarizedantennacolumns(eightports).Theconfigurationcanalsosupportmulti-userMIMOoperation.

In the future, massive and full dimension MIMO—using excessive transmittersandradiatingantennaelementsarrangedinmassivearrays—will allow narrower, focused and highly directed beams. This willdramaticallyimprovethespectralandenergyefficienciesneededintomorrow’s5Gsystems.

Acquiringdownlinkchannelconditions,however,ismorechallenging.LTEstandardsrequirethebasestationtransmitpilotwaveformsthattheterminalscanusetoestimatechannelstatusinformation(CSI).TheCSIisquantizedandrelayedbacktothebasestationasCQI,PMIandRIinformation.Thenumberofpilotwaveformsandchannelresponseseachterminalmustestimateisproportionaltothenumberof antennasxiv.Thiseventuallyaddstremendouscomputationaloverheads, considering the massive MIMO antenna numbers.

UsingLTE-TDD,versusFDD,andchannelreciprocity,DLchannelconditionscanbeestimatedwithfarlesscomplexity.Asaresult,TDDmaybeamorepromisingcandidatefor5Gsystemswithlarge-scaleantennaarrays.ItshouldbenotedthatmassiveMIMOwithLTE-FDDis also possible, but comes with higher complexity and cost.

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References

i ChuckPowell,TechnicalAnalysis:Beamformingvs.MIMOAntennas,June2014.

ii ShanzhiChen,ShaohuiSun,QiubinGao,andXinSu,AdaptiveBeamforming inTDD-BasedMobileCommunicationSystems:StateoftheArtand5GResearchDirections,IEEEWirelessCommunications,December2016.

iii MohamedNadder,Antennamythsforbasestationantennas,CommScopewhitepaper,January2016.

iv RobertJ.Mailloux,PhasedArrayAntennaHandbook,secondedition,ArtechHouse.

v Butler,J.,andR.Lowe,‘‘BeamformingMatrixSimplifiesDesignofElectronicallyScannedAntennas,’’Elect.Design,Vol.9,April12,1961,pp.170–173.

vi TheodoreC.Cheston,RadarHandbook,Chapter7,McGrawHill,1990.

vii RandallT.Becker,AgilentTechnologies,PrecodingandSpatiallyMultiplexedMIMO in3GPPLong-TermEvolution,HighFrequencyElectronics.

viii4GAmericas,MIMOandsmartantennasformobilebroadbandsystems,June2013.

ix http://rfmw.em.keysight.com/wireless/helpfiles/89600B/webhelp/subsystems/lte/content/lte_antenna_paths_ports_explanation.htm

x RohdeandSchwarz,ContinuinginthedirectionofLTE-Advanced:thelatesttestsignalsforLTE Rel. 9.

xi 3GPPTS36.213V13.1.1,Physicallayerprocedures.

xii KevinLinehanandJuliusRobson,Whitepaper:“WhatbasestationantennaconfigurationisbestforLTE-Advanced?”

xiii IvyY.Kelly,MartinZimmerman,RayButlerandYiZheng,BasestationantennaselectionforLTEnetworks,CommScopewhitepaper,May2015.

xiv ErikG.Larsson,ISY,LinköpingUniversitySweden.OveEdforsandFredrikTufvesson,LundUniversity,Sweden.ThomasL.Marzetta,BellLabs,Alcatel-Lucent,UnitedStates,MassiveMIMOforNextGenerationWirelessSystems,IEEECommunicationsMagazine, February2014.

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