MIMO Radar (Rev. A)

1 1 SWRA554A May 2017 RevisedJuly 2018 SubmitDocumentationFeedbackCopyright 2017 2018,TexasInstrumentsIncorporatedMIMOR adarApplicationReportSWRA554A May 2017 RevisedJuly 2018 MIMOR adarSandeepRaoABSTRACTMIMO radaris a key technologyin improvingthe angleresolution(spatialresolution)of basicprinciplesof the MIMO-radarand the brieflydiscusseswaysto implementMIMO-radaron the TI the the of Figures1 AngleEstimationUsingTwo RX RX TX and 8 RX 2-DimensionalMIMOA rray(WithAzimuthand ElevationEstimationCapability)..57 DifferentConfigurationsThatRealizethe ConfigureDevicefor ConfigureDevicefor trademarksare the propertyof r 1 FOVsin2d T r 1sin2 d & T S May 2017 RevisedJuly 2018 SubmitDocumentationFeedbackCopyright 2017 2018,TexasInstrumentsIncorporatedMIMOR adar1 IntroductionThe termsingle-input-multiple-output(SIMO)ra darrefersto a radardevicewith a singletransmit(TX)andmultiplereceive(RX) angleresolutionof a SIMO radardependson the numberof example,a devicewith four RX antennashas an angleresolutionof about30 , whileadevicewith eightRX antennashas an angleresolutionof about15.

2 Therefore,a directapproachtoimprovingthe angleresolutionrequiresincreasingthe numberof RX approachhas itslimitsbecauseeachadditionalRX antennarequiresa separateRX processingchainon the device(eachwith an LNA,mixer,IF filter,and ADC).Multiple-input-multiple-output(MIMO )refersto a radarwith multipleTX and multipleRX ,the angleresolutionof a MIMO radarwith NTXTX antennasand NRXRX antennascan bemadeequivalentto that of a SIMO radarwith NTX NRXRX MIMO radarthereforeprovidesa cost-effectiveway to improvethe angleresolutionof the applicationnoteservesas an introductionto the MIMO radarand equipsengineerswith sufficientinformationto designa MIMO radarapplicationusingthe mmWaveproductline fromTI.

3 Section2 is aquickoverviewof the basicsof lays out the foundationalprinciplesof sectionexplainshow multiplexingtransmissionsacrossTX antennascan discussesdifferentstrategiesfor multiplexingthe TX includesadiscussionon implementingthe MIMO Radar ,usingthe TI angleof arrivalof an objectrequiresat leasttwo RX showsa radarthathas one TX antennaand two RX antennasseparatedby a distance, AngleEstimationUsingTwoRX AntennasThe signalfromthe TX antennais reflectedfroman object(at an angle with regardto the Radar )and isreceivedat bothRX signalfromthe objectmusttravelan additionaldistanceof dsin( ) toreachthe secondRX correspondsto a phasedifferenceof = (2 / )dsin( ) betweenthesignalsreceivedat the two RX ,whenthe phasedifference, , is estimated,the angleof arrival, , can be computedusingEquation1.

4 (1)Becausethe phasedifference, , can be uniquelyestimatedonly in the range( , ), it followsbysubstituting = in Equation1, that the unambiguousfield of view(FOV)of the radaris as followsinEquation2.(2)Thus,the maximumFOVof Equation3 is achievedwith an interantennadistance,d = /2.(3)RES2 / NT May 2017 RevisedJuly 2018 SubmitDocumentationFeedbackCopyright 2017 2018,TexasInstrumentsIncorporatedMIMOR adarIn general,a radarhas NRX> two RX antennas,as shownin Figure2 for the caseof NRX= 4. The signalateachsubsequentantennahas an additionalphase-shiftof with respectto the ,a linearprogressionin the phaseof the signal(withreferenceto the first RX antenna)acrossthe N antennas(for example,[0 2 3 ] in Figure2) , can be reliablyestimatedbysamplingthe signalacrossthe NRXantennas,and performingan FFT (oftenreferredto as the angle-FFT)on this AngleEstimationUsingFourRX AntennasNOTE.

5 A typicalFMCW radarsignalprocessingchainalso includesa range-FFTand a Doppler-FFTthat are performedbeforethe the rangeand moreinformation,see numberof antennasresultsin an FFT with a sharperpeak,thus,improvingthe accuracyofangleestimationand enhancingthe showsthe angle-FFTfroma radardevicewith four and eightantennas(interantennadistanceof / 2), and two pointobjectsat = 10 and =+10 . The radardevicewith four antennascannotresolvethe two objects;however,the AngleResolutionImprovesWithIncreasingNum berof RX AntennasAppendixA discussesthat for an RX antennaarraywith N equispacedantennas(separatedby / 2 ), theangleresolutionis givenby Equation4.(4)Principleof the May 2017 RevisedJuly 2018 SubmitDocumentationFeedbackCopyright 2017 2018,TexasInstrumentsIncorporatedMIMOR adar3 Principleof the MIMOR adarBuildingon the discussionof Section2, let us say we wantto doublethe angleresolution(half res)capabilityof the radarin Figure2.

6 One way to doublethe angleresolutionis to doublethe numberof RXantennas(fromfour to eight),as shownin RadarWith1 TX and 8 RX AntennasUsingMIMO concepts,the sameresultcan be achievedwith just one additionalTX antenna,discussedasfollowsin referenceto Principleof MIMOR adarThe radarin Figure5 has two transmitantennas,TX1 and transmissionfromTX1 resultsin aphaseof [0 2 3 ] at the four RX antennas(withthe first RX antennaas a reference).BecausethesecondTX antenna(TX2)is placeda distanceof 4d fromTX1,any signalemanatingfromTX2 traversesan additionalpathof length4dsin( ) comparedto ,the signalat eachRX antennaseesan additionalphase-shiftof 4 (withregardto transmissionfromTX1).

7 The phaseof the signalat thefour RX antennas,due to a transmissionfromTX2,is [4 5 6 7 ]. Concatenatingthe phasesequencesat the four RX antennas,due to transmissionsfromTX1 and TX2,gets the sequence[0 2 3 4 5 6 7 ], whichis the samesequenceseenin Figure4 with one TX and eightRX can be saidthat the 2TX 4 RXantennaconfigurationof Figure4 synthesizesa virtualarrayof eightRX antennas(withone TX antennabeingimplied).To generalizethe previousdiscussion,with NTXand NRXantennas,userscan generate(withproperantennaplacement)a virtualantennaarrayof NTXX NRX. Thus,employingMIMO radartechniques,resultsin a multiplicativeincreasein the numberof (virtual)antennas,and correspondsto improvementin pmdenotesthe coordinatesof the mthTX antenna(m = 0, 1.)

8 NTX),and qndenotesthe coordinatesofthe nthRX antenna(n = 0, 1, 2, ..NRX),thenthe locationof the virtualantennascan be computedas pm+qn, for all possiblevaluesof m and n. For examplein Figure5, p1 = 0 and p2 = 4, and q1 = 0, q2 = 1, q3 =2, and q4 =3 (wherethe coordinatesare expressedin unitsof d, and the TX1 (respectivelyRX1)isassumedto be the originfor the TX (respectively,RX) the MIMOR adar5 SWRA554A May 2017 RevisedJuly 2018 SubmitDocumentationFeedbackCopyright 2017 2018,TexasInstrumentsIncorporatedMIMOR adarFigure6 showsthe principleof MIMO radarcan also be extendedto A 2-DimensionalMIMOA rray(WithAzimuthand ElevationEstimationCapability)Differentp hysicalantennaconfigurationscan be usedto realizethe ,wherethe physicalarraysin Fig.

9 (a) and Fig. (b) bothsynthesizethe samevirtualarrayof Fig. (c). In suchcases,easeof onboardplacementand routingmay dictatethe final DifferentConfigurationsThatRealizethe SameVirtualAntennaArrayMultiplexingStrat egiesfor the May 2017 RevisedJuly 2018 SubmitDocumentationFeedbackCopyright 2017 2018,TexasInstrumentsIncorporatedMIMOR adar4 MultiplexingStrategiesfor the MIMOR adarSection3 detailedhow the MIMO radarworksby havingthe sameset of RX antennasprocesssignalsfromtransmissionsb y multipleTX is importantto notethat the RX antennasmustbe able toseparatethe signalscorrespondingto differentTX antennas(for example,by havingdifferentTX antennastransmiton orthogonalchannels).

10 Thereare differentwaysto achievethis separation[3], and two suchtechniquesare discussedhere:time divisionmultiplexing(TDM)and binaryphasemodulation(BPM).Thesetechniqu esare describedas follows,in the contextof frequency -modulatedcontinuous-wave(FMCW) radars,thoughthe an introductionto FMCW radartechnology,see [5]. (TDM-MIMO)In TDM-MIMO[1], the orthogonalityis in severalblocks,with eachblockconsistingof NTXtime slotseachcorrespondingto transmissionby one of the NTXTX , for an FMCW radarwith NTX= 2, alternatetime slotsare dedicatedto TX1 and the mostsimpleway to separatesignalsfromthe multipleTX antennasand is a typicalprocessingschemefor TDM-MIMOFMCW Radar ,the 2D-FFT(range-DopplerFFT[5]) isperformedfor one radarwith NTX= 2and NRX= 4, wouldcompute4 2 = 8, and suchrange-Dopplermatricesas shownin Figure9.