Abstract—This paper presents a high performance 77GHzFMCW radar sensor for automotive applications. Powerfulautomotive radar systems are currently under development forvarious applications. Radar sensor based comfort systems likeAdaptive Cruise Control (ACC) are already available on themarket. The main objective from a radar sensor point of view isto detect all targets inside the observation area and estimatetarget range and relative velocity simultaneously with a highupdate rate.

FMCW radar sensors have the advantages of very high rangeresolution but a serious task occurs in multiple target situations tosuppress so-called ghost targets. In classical FMCW waveformsthis has been solved in using multiple chirp signals with differentslope. But in this case a long measurement time (approximately50ms) is needed which is a contradiction to a high update rate.Therefore, in this paper a new waveform design is presentedwhich has all advantages of FMCW radars but needs anextremely low measurement time (10ms) in a radar sensor with 3different antenna beams.

Index Terms—Automotive radar, waveform design, FMCW

I. INTRODUCTION

7 GHz radars are already on the market as Adaptive CruiseControl (ACC) systems for high performance automotive

applications. The most important requirement for these radarsis the simultaneous target range and velocity measurementwith high resolution and accuracy even in multi-targetsituations. The well-known waveforms need a relatively longmeasurement time (50-100ms). Future developments will bemore concentrated on safety applications like CollisionAvoidance (CA) or Autonomous Driving (AD). In this case therequirements for reliability (extreme low false alarm rate) andreaction time (extreme short measurement time) are muchhigher compared with ACC systems. To give a general idea ofthe most important requirements for automotive radar systemsthe maximum range for automotive radars is 200m, the rangeresolution is 1m and the velocity resolution is 2.5km/h,respectively.

To meet all these system requirements specific waveformdesign techniques must be considered. For ACC systems both

radar types of classical pulse waveform with ultra short pulselength (10 ns) or alternatively continuous wave (CW) transmitsignal with a bandwidth of 150 MHz are considered. The mainadvantage of CW radar systems in comparison with classicalpulse waveforms is the low measurement time and lowcomputation complexity for a fixed high range resolutionsystem requirement. Two classes of CW waveforms are wellknown in literature: The linear frequency modulated (LFM)and the frequency shift keying (FSK) CW waveform types.FSK uses at least two different discrete transmit frequencies(see Fig. 1).

CPI 0 CPI 1 Chirp 0 Chirp 1(a) (b))(tfT

Stepf

)(tfT

Sweepf

ttCPIT CPIT20

ChirpT ChirpT20

Fig. 1. Two CW waveform principles: (a) FSK modulation, (b) LFMmodulation.

This paper describes a new waveform design for automotiveapplications based on CW transmit signals which lead to anextreme short measurement time. The main idea of this newwaveform is based on a combination between LFM and FSKCW waveforms in an intertwined technique. Unambiguousrange and velocity measurement with high resolution andaccuracy can be required in this case even in multi-targetsituations. After an introduction into FSK and LFM waveformdesign techniques in section II and III the combined andintertwined waveform will be described in detail in section IV.

II. PURE FSK MODULATION PRINCIPLE

Pure FSK modulation as shown in Fig. 1 (a) uses twodiscrete frequencies Af and Bf (so-called two frequencymeasurement) [1] in the transmit signal. Each frequency istransmitted inside a so-called coherent processing interval(CPI) of length CPIT (e. g. ms 5=CPIT ). Using a homodyne

Waveform Design Principles for AutomotiveRadar Systems

Hermann Rohling, Marc-Michael MeineckeTechnical University of Hamburg-Harburg/ Germany

Department of TelecommunicationsEissendorfer Strasse 40, D-21073 Hamburg

phone: +49-40-42878-3028, fax: +49-40-42878-2281e-mail: rohling@tu-harburg.de

7

receiver the echo signal is down converted by theinstantaneous frequency into base band and sampled N times.The frequency step ABStep fff −= is small and will be

chosen in dependence of the maximum unambiguous targetrange. The time-discrete receive signal is Fourier transformedin each CPI of length CPIT and targets will be detected byan amplitude threshold (CFAR). Due to the small frequencystep in the transmit signal a single target will be detected at thesame Doppler frequency position in the adjacent CPI’s butwith different phase information on the two spectral peaks.The phase difference AB ϕϕϕ −=∆ in the complex spectra

is the basis for the target range R estimation. The relationbetween the target distance and phase difference is given bythe following equation

StepfcR

⋅∆⋅−=

πϕ

4. (1)

To achieve an unambiguous maximum range measurementof 150 m a frequency step of MHz 1=Stepf is necessary. In

this case the target resolution only depends on the CPI length

CPIT . The technically simple VCO modulation is anadditional advantage of this waveform. But this FSKwaveform does not allow any target resolution in the rangedirection, which is an important disadvantage of thismeasurement technique. Especially in automotive trafficenvironment more than a single fixed target will occursimultaneously inside an antenna beam. These fixed targetscan not be resolved by a FSK waveform.

III. PURE LINEAR FREQUENCY MODULATIONPRINCIPLE

Radars which apply pure linear frequency modulationtechnique (LFM) modulate the transmit frequency with atriangular waveform [STO92]. The oscillator sweep is givenby Sweepf . A typical value for the bandwidth is

MHz 150=Sweepf to achieve a range resolution of

m 12

=⋅

=∆SweepfcR . In general, a single sweep of the

LFM waveform gives an ambiguous measurement in range Rand relative velocity v. The down converted receive signal issampled and Fourier transformed inside a single CPI. If aspectral peak is detected in the Fourier spectrum at index κ(normalized integer frequency) the ambiguities in target rangeand velocity can be described in a R - v -diagram by thefollowing equation

κκ +∆

=∆

⇔∆

−∆

=R

Rv

vR

Rv

v, (2)

where v∆ describes the velocity resolution resulting from the

CPI duration ChirpT ( m/s 8.02

=⋅

=∆ChirpT

v λ, λ is the

wavelength of 4 mm @ 77 GHz and ms 5.2=ChirpT ).

ν

R

Target 1 Target 2

detected peaks for

Fig. 2. : example R-v-diagram for two targets measured with up chirp.

Fig. 2 shows an example inside the R- v -diagram for a twotarget situation measured with a single chirp. Each line drawnin Fig. 2 corresponds to a measured spectral line at index κindicating all solutions for equation 2 and all possiblecombinations of target Range R and velocity .

For reason of resulting range-velocity ambiguities furthermeasurements with different chirp gradients in the waveformare necessary to achieve an unambiguous range-velocitymeasurement even in multi-target situations. The well knownup-/ down-chirp principle as it is depicted in Fig. 1 (b) isdescribed in detail in [6]. LFM waveforms can be used even inmulti-target environments, but the extended measurement timeis an important drawback of this LFM technique.

t

f(t)

R

v

Tchirp

(a) (b)

Fig. 3. : (a) waveform for use in multi target situations and (b) correspondingexample R-v -diagram for a two target situation and the related intersectionpoints

In multi target situations a waveform as shown in Fig. 3 (a)is used which consists of 4 different chirp signals. In general,the frequencies modulation will be different in each of the 4chirps. In each chirp signal all targets are detected which stillfulfil equation (2). The detected spectral lines from all 4 chirpsignals can be drawn in a single R- v -diagram (Fig 3 (b))where the gradient of a single line is dependent on the chirpsweep rate. In multiple target situations many intersectionsbetween lines of different and adjacent chirps appear as theexample in Fig 3 (b) shows. If such an intersection pointoccurs which has no physical representation of a reflectionobject it is called a ghost target. A real target is represented byan intersection point between all considered 4 lines.

IV. CONCEPT OF COMBINED FSK AND LFMWAVEFORMS

The combination of FSK and LFM waveform designprinciple offers the possibility of an unambiguous target rangeand velocity measurement simultaneously. The transmitwaveform consist in this case of two linear frequencymodulated up-chirp signals (the intertwined signal sequencesare called A and B). The two chirp signals will be transmittedin an intertwined sequence (ABABAB...), where the stepwisefrequency modulated sequence A is used as a reference signalwhile the second up-chirp signal is shifted in frequency with

Shiftf . The received signal is down converted into base band

and directly sampled at the end of each frequency step. Thecombined and intertwined waveform concept is depicted inFig. 4.

A B A

B A B

t 0

) ( t f T

B T f , A T f , Shift f 1 −

= N f

f Sweep Incr

Sweep f

Chirp T Fig. 4. Combined FSK-LFMCW waveform principle.

Each signal sequence A or B will be processed separately byusing the Fourier transform and CFAR target detectiontechniques. A single target with specific range and velocitywill be detected in both sequences at the same integer index

BA κκκ == in the FFT-output signal of the two processedspectra. In each signal sequence A or B the same target rangeand velocity ambiguities will occur as described in Equation 2.But the measured phases Aϕ and Bϕ of the two (complex)spectral peaks are different and include the fine target rangeand velocity information which can be used for ambiguityresolution. Due to the coherent measurement technique insequence A and B the phase difference AB ϕϕϕ −=∆ canbe evaluated for target range and velocity estimation. Themeasured phase difference ϕ∆ can be described analyticallyby the following equation:

cf

Rv

vN

Shift⋅⋅−∆

⋅−

=∆ ππϕ 41

, (3)

where N is the number of frequency steps (or receivesignal samples) in each transmit signal sequence A and B. Inthis first step ϕ∆ is ambiguous but it is possible to resolvethese ambiguities by combining the two measurement resultsof Equation 2 and 3. The intersection point of the twomeasurement results is shown in Fig. 5 in a graphical way. Theanalysis leads to an unambiguous target range 0R and relative

velocity 0v :

RfNcNRcR

Shift ∆⋅⋅−⋅−⋅−∆⋅−⋅∆⋅=

)1(4)1(

0κπϕ

π(4)

RfNcRfcvNv

Shift

Shift

∆⋅⋅−⋅−⋅∆⋅⋅−∆⋅

⋅∆⋅−=)1(4

4)1(0

κπϕπ

(5)

This new intertwined waveform shows that unambiguoustarget range and velocity measurements are possible even inmulti-target environment. An important advantage is the shortmeasurement and processing time.

frequ

ency

measure

ment

TargetDetection!

phasemeasurement

0

v

0v

0R R

>Shiftf

>Sweepf

Fig. 5. Graphical resolution principle of ambiguous frequency and phasemeasurements.

V. SYSTEM EXAMPLE

In this section a waveform design based on the newintertwined signal is developed as an example for automotiveapplications. The signal bandwidth is MHz 150=Sweepf to

fulfill the range resolution requirement of 1 m. The stepwisefrequency modulation is split into 256=N separate bursts

of kHz 588255

MHz 150 ==Incrf each. The measurement

time inside a single burst A or B is assumed to be 5 µsresulting in a chirp duration of the intertwined signal of

ms 56.2=ChirpT which results in a velocity resolution of

km/h 7.22

=⋅

=∆ChirpT

v λ.

The important waveform parameter Shiftf is optimized on

the basis of high range and velocity accuracy. The highestaccuracy occurs if the intersection point in the R - v -diagramresults from two orthogonal lines as it is depicted in Fig. 7. Forthis reason the frequency shift between the signal sequences A

and B is kHz 29421 −=⋅−= IncrShift ff (the related

waveform is shown in Fig. 6). In this specific case Equation 4and 5 turn into

2210 κϕ

π−∆⋅−=

∆N

RR

, (6)

2210 κϕ

π+∆⋅−=

∆N

vv

. (7)

AB

AB

AB

t

Sweepf

)(tfT

BTf ,

ATf ,Shiftf

Incrf

Fig. 6. Combined FSK-LFM waveform with optimized frequency shift.

frequency measurement Target

Detection!

phase measurement

R 0 R

0 v

v

90°

Fig. 7. R - v -diagram for the combined waveform with optimized frequencyshift.

VI. EXPERIMENTAL CAR

The new waveform has been tested in our experimental car inrealistic street situations. The following picture shows the testcar which is equipped with a 77 GHz radar sensor, a smartbrake buster and a throttle control system for automaticdriving. The radar sensor detects all targets inside theobservation area and measures target range and velocitysimultaneously. The relevant target is selected by signalprocessing and the car control system is activated by thisinformation. In this case the car follows the detected relevantobject with controlled distance.

The experimental car was used for radar tests in more than40000 km on public streets. Many important experiences havebeen gained since this time. The implemented sensor is a 77GHz FMCW radar with a separated transmit and receiveantenna. The target azimuth angle is measured by the receivesignal in 3 adjacent and overlapping beams. The new radarwaveform shows very good results and especially extremeshort measurement time.

Fig. 8. Experimental car of the Technical University Hamburg Harburgequipped with a 77GHz far range radar sensor.

VII. CONCLUSION

The proposed intertwined CW waveforms show highperformance in range and velocity measurement accuracy. Themain advantage is the short measurement time in comparisonto classical LFM waveforms while the resolution and accuracyperformance is unchanged. Compared with a pure FSKwaveform the intertwined waveform allows resolution invelocity and range simultaneously. The properties of the newintertwined CW waveform technique are quite promising. Thisconcept is a good basis for high performance automotive radarsystems with different safety applications (e.g. pre crash)which require ultra short measurement and processing time.

REFERENCES[1] Artis, Jean-Paul; Henrio, Jean-François: "Automotive Radar

Development Methodology" International Conference on RadarSystems, Brest/ France, 1999.

[2] Klotz, Michael; Rohling, Hermann: "A high range resolution radarsystem network for parking aid applications" International Conferenceon Radar Systems, Brest/ France, 1999.

[3] Klotz, Michael; Rohling, Hermann: "24 GHz Radar Sensors forAutomotive Applications" International Conference on Microwaves andRadar, MIKON2000, Wroçław/ Poland, 2000.

[4] Levanon, N; “Multifrequency Complementary Phase-Coded RadarSignals” German Radar Symposium, GRS 2000, October 2000, Berlin,pp 431-436

[5] Rohling, Hermann; Meinecke, Marc-Michael; Mende, Ralph: "A77 GHz Automotive Radar System for AICC Applications" InternationalConference on Microwaves and Radar, MIKON98, Workshop, Kraków/Poland, 1998.

[6] Rohling, Hermann; Meinecke, Marc-Michael; Klotz, Michael; Mende,Ralph: "Experiences with an Experimental Car controlled by a 77 GHzRadar Sensor" International Radar Symposium, IRS98, München, 1998.

[7] Stove, A. G.: "Linear FMCW radar techniques" IEE Proceedings-F, Vol.139, No. 5, Oct. 1992