LineofSight(LOS)MIMO

PhilHorkin,AF7GYphil@horkin.net

BSEE1975

CommunicaIonsSystemEngineeronSatelliteandTerrestrialSystemsformorethan40years

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Tradi9onalMIMORequiresAMul9pathEnvironment

•  Mul9pleInputMul9pleOutput(MIMO)systemsaresimilartotradi9onallinksexceptthattheyusemul9pleantennasandsignalprocessinginamul9pathenvironment

•  Byexploi9ngthemul9pathenvironmentthechannelcapacitycanbeincreasedforthesamespectrumandthusenablingmul9pleindependentdatastreamsonthesamechannel

•  A2x2MIMOisshown,butotherssuchas3x3arealsopossible

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Conven9onalMIMOCon9nued

•  Digitalsignalprocessingisusedtode-correlateandremoveinforma9onineachstreamfromotherscrea9ngindependence

•  Asspectrumisascarceandalimitedcommodity,thistechniquepermitsgreateruseofthisresourcetoincreasecapacity

•  AcceptanceofthistechnologyhasresultedinwidespreadusebyLTE(cellular/smartphones),WI-FIhotspotsandothernetworkequipment

•  Butwhataboutenvironmentswithoutmul9pathtoimpairpropaga9on–Isthereawaytousethesameconcept–  ForexamplesatelliteoraircraUtogroundlinksormountaintoptomountaintoplineofsight(LOS)repeaters

•  Thispresenta9onwillfocusonLOSapplica9ons

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LineofSightMIMO

•  Thereareotherscenariossuchasmountaintoptomountaintopwheretherearenomul9pathimpairmentstopropaga9on

•  Ifasimilartechniquecouldbefoundtoincreasechannelcapacitythenthesamebenefitsmightberealized,removingmutualinterference

•  InLOScondi9ons,eachreceivingantennacollectsafullunperturbedsampleofeachtransmiXedstreammakingtheapplica9onofthismethodmoredifficultandlimited

•  Weneedtofindawaytode-correlatethesignal(removemutualinterference)andthuspermiZngthesepara9onofthesignalsatthereceiver

•  Thispaperwillshowhowthiscanbedone,whatthelimita9onsareandhowchannelcapacitymaybemul9plied

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LineofSightMIMOGeometry

•  Toenablesepara9onofsignals,aspecificgeometrybetweentransmitandreceivepathsmustbemaintainedtoensureafixedphasedifferencebetweensetsofsignals

•  Forthistalk,Iwillfocusonthe2x2MIMOforsimplicity

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Assump9ons/Defini9ons:•  UniformLinearArraysatTx&Rx

–  N≡TxAntennas(dtsepara9on)≡2–  M≡Antennas(drsepara9on)≡2–  V≡Maximum(MandN)≡2–  dt ≡Separa9onoftransmitantennas–  dr ≡Separa9onofreceiveantennas

•  R=distancebetweentransmitandreceiveantennas(R>>dt,dr)

𝑑↓𝑟 𝑑↓𝑡 = 𝜆𝑅/𝑉 

ForopImalinter-antennaarrays

GeometryCon9nued

•  Thisgeometrycreatesa90ophasedifferencebetweenpaths

•  Thisdifferencewillbeexploited•  IftheleUsiderepresentsthe

transmiZngsta9ons,thenbothreceiverscapturesignalsfromeachtransmiXer

•  Atransversalfilterorspace-9meequalizercanseparatethesignals

•  ThisisoUencalledatappeddelayline

•  Signalprocessingisdoneinthedigitaldomain

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S1

S2

S1+S2

S1+S2

SignalProcessingStructure

•  RecallthatbothantennasreceiveS1andS2equallywell•  Tappeddelaylineprocessorsareusedtoenableoneoutputto

passS1whilenotchingoutS2andviceversa•  Adigitalsignalprocessorisusedtosetthedelaysandweights

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Prac9calApplica9onof2x2MIMO

•  Antennasepara9oniskeytomakingthiswork•  Theexampleshowstherequiredantennasepara9onatbothends

for5and10GHztransmissionsystemsoverarangeof50miles

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ReceiverAnten

naSep

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eters

TransmiXerAntennaSepara9on,metersdr10GHz dr5GHz

IandQSignalsandConstella9onDiagrams

•  DigitalsignalprocessingoUenisdonewithI(in-phase)andQ(quadrature)signalcomponentstoformcomplexsignals

•  SDRformsIandQsamplesearlyinthemodulatortoenablearbitrarysignalformatstobecreatedsuchasSSBorPSK63

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In-phaseaxis

Quadratureaxis

Qvalue

Ivalue •  ThefiguretotheleUshowsIandQcomponentsofasignal

•  Thisfigureshowshowa2-bitwordismappedto4possiblesymbolvaluestobetransmiXedbasedontheirIandQcomponents

TransmissionSchemesakaModula9onDensity

•  DataiscomposedofdigitalwordstobetransmiXedonesymbolata9me

•  EachsymbolcanrepresentNbitstoincreasethethroughput•  CommonschemesrangefromBPSKto1024QAM

1bits/sym2bits/sym4bits/sym6bits/symbol 8bits/symbol

•  Increasingthemodula9ondensityrequiresincreasingenergyperbitandbecomesmoresuscep9bletonoiseinthedemodula9onprocessbutcommunicatesmoreinforma9onperunit9me

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256QAM

BPSK QPSK

Demodula9onIssues

•  Thefigureshowstheeffectnoisehasatdifferentsignaltonoiselevelsfor16QAM(similartohistogramofeachdatavalue)

•  Inaddi9on,sincethepointsgetclosertogetherwithincreasingmodula9ondensity,moregoodbitsarerequiredintheADCanddemodula9onprocess,andthisoUenlimitswhatcanbedone

•  256QAMisreallyefficienttogetlotsofdataacross(89mesBPSK)butrequiresabout7goodbitsvs.4

•  Acleaverwaytoincreasecapacitywhilereducingmodula9oncomplexityusesdualpolariza9on

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Polariza9onImprovement

•  Eachantennamaytransmiteitherasingleordualpolarizedsignal•  Ifdualpolarizedsignalsareusedwithsufficientpolariza9on

isola9on,thechannelcapacityispossibleforthesamebandwidth•  Acleavertrickistostartwithdatawordsoftwicethelengthand

transmithalfoneachpolariza9on•  Thispermitsasimplermodula9onconstella9ontobeusedon

eachwhiles9llachievingthethroughputofthelargerconstella9on•  Forexamplea256-QAM8-bitwordpersymbolcanbetransmiXed

as2-dualpolarized16-QAMsignals•  Wes9llachievethenetthroughputof8bitspersymbolusingonly

2streamsof4bitspersymbol•  Applyingthisschemeto2x2MIMOhasgreatu9lityindata

throughput

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PuZngItAllTogether•  Let’scomparea2x2MIMOvs.SISO(SingleInputSingleOutput)•  Foreachcomparison,thesametotaltransmiXerpowerisused

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Ata15dBSNRaSISOsystemcanachieve5bitspersecondperHzofbandwidthcomparedtoMIMOthatachieves10UsingdualpolarizaIonallowseachtooperatewithsimplerprocessing

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SNR,dB

ComparisonofTransmissionSystems

SISO 2x2MIMO

256QAMequivalent

64QAMequivalent

16QAMequivalent

QPSKequivalent

1024QAMequivalent

Thiseffec9velyallowsustotransmit10Mbpsina1MHzchannel

IwouldliketothanktheMicroHAMSfortheopportunitytopresentthispaperatthe2016MicroHAMSDigitalConference

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IwanttothankJohnPetrich,W7FUforhiscommentsonthispresentaIon

QUESTIONS?

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•  Prac9calconsidera9ons?•  Viablefrequencyranges?•  Signalprocessinghardware?•  Bandwidthlimita9ons?•  Constella9onlimita9ons?•  Polariza9onimperfec9ons?•  Others?