Programming Chirp Parameters in TI Radar Devices

1 1 SWRA553 May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedProgram mingChirpParametersin TI RadarDevicesApplicationReportSWRA553 May 2017 ProgrammingChirpParametersin TI RadarDevicesVivekDhamABSTRACTT hisapplicationreportprovidesinformationo n how to selectthe rightchirpparametersin a fast FMCWR adardevicebasedon the end applicationand use case,and programthemoptimallyon TI s ChirpConfigurationon of the FrameStructureShowingThreeBurstsLoopedTw ice(startindexhavingan offsetof 2)..119 Exampleof AdvancedFrameConfigurationof Two Sub of Tables1 ExampleChirpConfigurationsfor a Functionof DFEM odeand May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedProgram mingChirpParametersin TI RadarDevicesTrademarksAll trademarksare the propertyof (FMCW)mmWaveradarsensorsare becomingincreasinglypopularfor multipleautomotiveand systemrequirementsand care-aboutsineachof theseapplicationscouldbe very ,rangeresolution,max velocityrequirement,sensorfield of view,datamemory,processorMIPS,and so forthare someof the aspectsthat needto be analyzedbasedon the end relationshipsbetweentheFMCW chirpconfigurationand systemperformanceparametershelpsin selectingthe s mmwaveradardevices(MMIC)

2 Providelargeflexibilityin configuringchirpparametersand also allowmultiplechirpconfigurationsin a timingparametersare accuratelycontrolledby thedigitaltimingengineand a programmingof chirpparametersand explainsthe varioussystemconsiderationsthat determinethe valuesfor ChirpConfigurationon SystemParametersIn linearFMCW radars,the transmit(TX)signalis a singletonewith its sweepin frequencyis commonlyreferredto as a Chirp . A set of thesechirpsforma Frame and this can be usedas the observationwindowfor the variousparametersof thechirpramp(like frequencyslope,sweepbandwidth,and so forth)impactthe depictsa singlechirpand the showsframestructurethatconsistsof a seriesof chirpsfollowedby represents FastFMCW modulation,whereeachchirpis typically10 s of s in TypicalFMCWC hirpFigure2.

3 TypicalFrameStructure 2Rx Txr4max32 CdetPtG G c NTRangebased on SNRf4kT NF SNRV S u u umaxmaxIFcRange2 S u ChirpConfigurationon SystemParameters3 SWRA553 May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedProgram mingChirpParametersin TI RadarDevicesThe followingsectionslist key systemperformanceparametersthat are typicallyconsideredin any radarapplicationand how the chirpconfigurationimpactseachone of RangeResolutionThe maximumand minimumdistanceoverwhicha radarsensorcan detectobjectsis an importantparameterfor a ,the rangeresolution(abilityto distinguishtwo nearbyobjects) applicationslike automotiveadaptivecruisecontrol(ACC),it s importantto be able to viewa far offobject(>150m).

4 Detectinga far-offobjectcan be limitedby eitherthe SNRof the receivedsignalor the IFbandwidthsupportedby the max rangerelationshipwith the IF bandwidthis shownin Equation1. TI s AWR1243radardeviceprovidesa large15 MHzbandwidth,allowingmoreflexibilityin the slopethat can be used,whichindirectlyhelpsincreasethe max velocityas will be seenlater.(1)IFmax MaximumIF bandwidthsupportedc Speedof lightS Slopeof the transmittedchirpNotethat the IFmaxis also dependenton the ADCsamplingfrequency(ADCsampling) a complex1xsamplingmode,the IF bandwidthis limitedto * (ADCsampling). In caseof complex2x and real samplingmodes,the IF bandwidthis limitedto * (ADCsampling)/2 . The maximumADCsamplingfrequencyin theTI s radardevicesis otheraspectthat couldlimit the max rangeis the signalto noiseratio(SNR)of signalreceivedby dependson: RF performanceof the Radardevice,like TX outputpower,RX noisefigure,as well as chirpparameterslike chirpdurationand numberof chirpsin the frame.

5 Antennaparameterslike the TX and RX antennagain in the directionof interest. Objectcharacteristicslike RadarCrossSection(RCS).RCSis a measureof the amountof decideshow detectablethe objectis with a radarsensor. MinimumSNRrequiredby the detectionalgorithmto detectan object.(2)Pt Tx outputpowerGRx,GTx RX and TX Antennagain RCSof the objectN Numberof chirpsTr ChirptimeNF Noisefigureof the receiverSNRdet MinimumSNRrequiredby the algorithumto detectan objectk BoltzmanconstantTdet Ambienttemperature 2sindSTOCU nambiguous max velocity2NT , , CUnambiguousmax velocity4T resolutioncRange2 B uImpactof ChirpConfigurationon May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedProgram mingChirpParametersin TI manyapplicationsit is importantto be able to resolvetwo closelyspacedobjectsas two separateobjects.

6 Ratherthandetectthemas smallestdistancebetweentwo objectsthat allowsthemto bedetectedas separateobjectsis referredto as primarilydependson the chirpsweepbandwidththat the radarsensorcan largerthe sweepbandwidth,the betterthe s radardevicessupporta 4 GHzsweepbandwidththat allowsa rangeresolutionof as lowas approximately4cm.(3)c Speedof lightB Sweepbandwidthof FMCW chirpBetterrangeresolutionalso helpsin detectingvery closeby objects,hence,improvinga the distance,the relativevelocityof the objectis anothercriticalparameterof FastFMCW modulatedradarsdependson the chirpcycletime,that is,the time differencebetweenthe startof two in turn dependson how fast thefrequencysweepcan be performedand the minimuminter-chirptime fasterthe MMICcan rampthe frequency,the higherthe s MMIC allowsa fast rampof 100 MHz/ s.

7 Alsothe closedloop PLL is designedto supporta very fast settlingofthe ,the time takenfor the VCOto jumpfromthe end of the rampfrequencytorestartthe next rampis very low and allowsfor a smalleridle time (as low as 2 sec).For minimumidletime computation,see Section5.(4)Tc TotalChirptime ,whichinlcudeschirptime+idletime Wavelengthof the signalusedThe actualmeasurablemax velocitycan be extendedbeyondthe unambiguousmax applications,like parkassist,you mightneedto separateout objectswith smallvelocitydifferences,forwhichgoodvel ocityresolutionis the transmitframeduration,that is, increasingthe numberof chirpsin a frameimprovesthe velocityresolution.(5)Nc Numberof Chirpin a ResolutionIn orderto locatethe objectin the 2D space,the angleof the objectis also requiredalongwith a radarsystem,the angleis estimatedby receivingthe reflectedsignalfromthe objectusingmultiplereceiversthat are spacedapartwith a distance d.

8 The signalarrivingat eachof the successivereceiversis delayedby d*sin( ) and this delay causesa phaseshift of. This phaseshiftbetweeneachof the receiversis usedto estimatethe angle( ) of the unambiguous angular range sin2d ChirpConfigurationon SystemParameters5 SWRA553 May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedProgram mingChirpParametersin TI RadarDevicesFigure3. AngleEstimationBasicsThe measurableunambiguousangularfield of viewfromthe MMIC perspectivedependson the spacingbetweenthe receivers(d).(6)d Spacingbetweenreceiverantennas WavelengthSo for the widestangularfield of view,the spacingof the receiverantennashouldbe /2 , theoreticallygiving 90 antennaspacing,the measurabledistanceat differentangleswouldalso dependon theantennagain peakgain at one angle(mostlyat 0 , that is,directlyfacingthe frontof the antenna)and thenthe gain wouldreduceas the exampleantennapatternwherethe gain at 90 angleis > 15 dB lowerthanwhatit is at 0 ExampleAntennaGainPatternRX TX180 Angular resolutiond N NcosO uuT SRX180 Angular resolution (deg)

9 D NcosO uuT SImpactof ChirpConfigurationon May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedProgram mingChirpParametersin TI angularfield of view,it mightalso be importantto resolvetwo objectsat closeby angles,that is, example,in an automotiveradaruse case,it wouldbe importantto detecttwo cars far off in two differentlanesratherthandetectthemas one singlecar. In general,theangularresolutionmeasurementd ependson the numberof largerthenumberof antennas,the betterthe resolution.(7) c Angleof interest,that is, the angleat whichthe objectsare presentNRx Numberof receiverantennasThe angularresolutioncan be moredetails,see MIMOR adar. If thereare multipletransmittersavailable,thenthe transmitantennascan be spacedin suchaway that eachof the transmitterspairedwith the set of receiverstogethercreatea example,if thereare 3 TX and 4 RX, thena MIMO radarsystemcan producethe equivalentangularresolutionof 12 virtualchannels.

10 (8)NTx Numberof transmitantennasFigure5. EffectiveRecevierChannelsin CommonApplications7 SWRA553 May 2017 SubmitDocumentationFeedbackCopyright 2017,TexasInstrumentsIncorporatedProgram mingChirpParametersin TI RadarDevices3 ChirpConfigurationsfor CommonApplicationsThe mostcommonapplicationsfor radarin the automotivecontextincludeshortrangeradar( typicallymountedin the corners)and mid or long-rangeradar(typicallyfrontfacing).Th is sectionshowschirpconfigurationsfor a 22mUSRR,45mSRR,125mMRRand a is importantto notethat theseare just genericexampleconfigurationsand it is possibleto changethe parametersbasedon the customer s use-casesas applicableto variousTI s mmwaveradardevicesare shownin ExampleChirpConfigurationsfor TypicalApplicationsParameterUnitsLRRMRRS RRUSRRMax umambiguousrelativevelocity(1) (complex) samplesper chirp500500250250 RangeFFT size-512512256256 Frametime (total) (active) (1)