1. Introduction

Millimeter-wave automotive radars withelectrically switched beam antennas have beendeveloped as forward-looking sensors for adaptivecruise control (ACC) and other systems1).DaimlerChrysler has already been offering the‘Distronic’ system, which is an ACC system withthe millimeter-wave automotive radar, since June19992). Other passenger vehicle manufacturers alsowill admittedly put the same systems on the marketwithin a few years. However, for collisionavoidance system and a stop & go system used in

urban areas, further reduction of the detection lossesis indispensable.

The improvement of an azimuthal angularresolution of the radar sensor is presumablyparticularly effective for the reduction of thedetection losses. The resolution of less than 2degrees has to be acquired empirically. Moreover,size is restricted by limitation of the installation area,and a simple structure that leads to cost reduction isalso required. An automotive radar attainingresolution of less than 2 degrees under theseconditions has never been reported as yet.

This paper proposes a millimeter-wave automotive

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Abstract

This paper proposes a millimeter-wave holographic radar with a simple structure for automotive applica-tions. The simplicity can be realized by switching both transmitting and receiving antennas. Also, a superresolution technique is introduced for the detection of angular positions in the proposed radar. The radar hasaccomplished an azimuthal angular resolution of less than 2 degrees and an azimuthal field of view (FoV) ofmore than 20 degrees. Simultaneously, the radar is capable of fulfilling the conditions that the width of theradar sensor must be less than 100 millimeters due to the limitation of the installation area.

Proposal for Holographic Radar with Antenna Switching

Yoshikazu Asano, Shigeki Oshima, Tomohisa Harada, Masaru Ogawa

ResearchReport

Keywords Automotive radar, Millimeter wave, Holographic radar, Super resolution, Antenna switching

Special Issue Millimeter-Wave Radar for Automotive Applications

radar that can accomplish high resolution of lessthan 2 degrees and that can fulfill the aboveconditions. Furthermore, the validity of theproposed radar is discussed on the basis ofexperimental results.

2. Solution for improvement of azimuthalangular resolution

A solution for obtaining azimuthal angularresolution of less than 2 degrees is discussed underthe condition that the width of the radar sensor mustbe less than 100 millimeters. This value isreasonable as a width of an onboard sensor.

A super resolution technique3) is introduced to finda solution. Figure 1 shows the fundamentalholographic radar for applying the super resolutiontechnique to the detection of the angular position.The resolution depends on the number of thereceiving antennas (receivers). Figure 2 shows thevariation of the resolution with the number ofantennas in Fig. 1. The variation is estimated withMARPET (Millimeter-wave Automotive RadarPerformance Evaluation Tool), which is a simulationsoftware developed by TOYOTA CRDL, Inc.4). Asan estimation, the beam width of every receivingantenna and every interval of the receiving antennasare assumed to be 26 degrees and 1.5 wavelengths,respectively. Also, ESPRIT5) is used as a superresolution technique here. Figure 2 illustrates thatthe resolution of less than 2 degrees can be obtainedwith nine receiving antennas. Then, the width of theradar sensor certainly becomes less than 100millimeters including the transmitting antenna.

From the above discussion, holographic radarpossessing nine sets of receiving antennas andreceivers, together with the super resolutiontechnique, has the capability to accomplishazimuthal angular resolution of less than 2 degrees.Moreover, the radar can fulfill the condition that thewidth of the radar sensor must be less than 100millimeters. For the configuration of the radar,however, the large number of components is anobstacle to cost reduction. Therefore, it isindispensable to design a novel holographic radarwith a simple structure.

3. Proposal of millimeter-wave holographicradar with simple structure

The necessity for holographic radar with nine setsof antennas and receivers has been described in theprevious chapter. This chapter proposes a novelholographic radar with a simple structure.

Figure 3 shows the structure of the novelholographic radar, that is, holographic radar withantenna switching. In this radar, the switching of thetransmitting and receiving the antennas enables a bigdecrease in the number of the antennas and thereceivers as compared with the fundamentalholographic radar. They are essentially equivalent indetecting the target direction, although the receivedsignal corresponding to each receiving antenna isobtained in the time division manner. The radaroperating in the time division manner is presumably

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Fig. 1 Configuration of fundamental holographic radarfor applying super resolution technique todetection of the angular position.

Fig. 2 Variation of angular resolution with number ofreceiving antennas. Interval of receivingantennas and beam width of receiving antennaare assumed to be 1.5 wavelengths and 26degrees, respectively.

applicable to automotive uses because automobilesgenerally move slower than airplanes.

Figure 4 illustrates the phase differences amongthe transmitted signals and the received ones. In thisfigure, the target is located far away in the left sidefrom the front by the angle θ. In addition, the phasedifferences among the received signals in thefundamental radar are illustrated. The fundamentalradar possesses one transmitting antenna and ninereceiving antennas arranged linearly with intervalsof d. Then, the phase ϕ0(n) of the signal receivedwith the nth antenna is given by Eq.(1) assumingthat ϕ0(1) is 0.

ϕ0(n) = 2π(n-1)d sinθ / λ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅(1)where λ is the wavelength. The phase of the signalreceived by each antenna varies with the direction ofthe target, and also with the phase changes for theposition of each receiving antenna. On the basis ofsuch variations in the received signal, the directionof the target can be detected.

On the other hand, the novel radar has threetransmitting antennas with intervals of 3d and threereceiving ones with interval of d. These antennasare arranged linearly. Then, the phase ϕ(nt, nr) ofthe signal obtained by the ntth transmitting antennaand the nrth receiving antenna is given by Eq.(2)assuming that ϕ(1, 1) is 0.

ϕ(nt , nr) = 2π{3(nt - 1) + (nr - 1)}d sinθ / λ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅(2)

Equation (2) shows that nine different phasevalues can be given according to the combinationsof three transmitting antennas and three receiving

antennas. Moreover, the following equation isderived from Eqs.(1) and (2).

ϕ0(3(nt - 1) + nr) = ϕ(nt , nr) ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅(3)Consequently, Eq.(3) represents that the phase

value of each receiving antenna in the fundamentalradar can be obtained with a certain combination ofthe transmitting antennas and the receiving antennasin the novel one. Therefore, they are shown to beequivalent in detecting the target direction.Moreover, the structure of the novel radar is said tobe simple, which is advantageous to cost reduction.

4. Discussion of the validity of novel holographicradar

In the previous chapter, holographic radar with aquite simple structure has been proposed. This

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Fig. 3 Configuration of novel holographic radar withswitching of transmitting and receiving antennas.

Fig. 4 Phase differences among transmitted andreceived signals in novel holographic radar (a)and among received signals in fundamentalholographic radar (b).

chapter discusses the validity of this radar throughexperiments. The azimuthal angular resolution ofless than 2 degrees is confirmed.

4. 1 Experimental radar for evaluating performance

The experimental radar sensor has been developedto evaluate the performance of the proposed radar.The principal specifications of the developed radarsensor are shown in Table 1. The block diagram isshown in Fig. 5. A frequency-modulated continuouswave (FMCW) is transmitted with three antennas inthe time division manner. Figure 6 shows theswitching signals of the transmitting and receivingantennas for a period of about 4.5msec correspondingto the ascending phase of the FM signal. When theswitching signal is high, the respective antenna isselected. For the period of the transmission from acertain selected antenna, three receiving antennas areswitched in order. The transmitting and receiving

antennas are switched successively with therespective fixed switching periods 2.4µsec and0.4µsec. Thus, the received signal is obtained in thetime division manner. The base-band signalobtained in the receiver is digitized. On the basis ofthe digitized signal fed to the signal processor unit,the range, the relative velocity and the azimuthaldirection of each target are obtained with the signalprocessing including the fast Fourier transform andthe super resolution technique.

4. 2 Experiments to evaluate angularresolution and FoV

Figure 7 shows the experimental result of theazimuthal angular resolution. In this figure, thearrangement of standard reflectors used in theexperiment is also depicted. The azimuthal directionfor each reflector is precisely detected in the regionthat the set angular difference between two reflectorsis more than 1.6 degrees. This result confirms thatthe azimuthal angular resolution of less than 2degrees has been accomplished. Also, Fig. 8 showsthe experimental result of the azimuthal FoV. Thereflector set within the azimuthal region of 15degrees in both right and left sides can be detectedwith an accuracy of less than 0.2 degrees.Consequently, the developed radar accomplishes lessthan 2 degrees in azimuthal angular resolution andmore than 20 degrees in azimuthal field of viewsimultaneously.

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Table 1 Principal specifications of developedexperimental radar sensor.

Fig. 5 Basic block diagram of developed holographicradar.

Fig. 6 Sequences of antenna switching signals forperiod of about 4.5msec corresponding toascending phase of FM signal. Switchingperiods of transmitting and receiving antennasare 2.4µsec and 0.4µsec, respectively.

5. Conclusion

The novel holographic radar, that is, holographicradar with antenna switching, has been proposed.The switching of the transmitting and receivingantennas enables a big decrease in the number of theantennas and the receivers as compared withfundamental holographic radar. No difference canbe seen between the radar operation for detecting thetarget direction, except for the time-divisionreception of the signal corresponding to eachreceiving antenna. The novel radar possessing threetransmitting antennas and three receiving ones,together with the super resolution technique, has thecapability to accomplish azimuthal angularresolution of less than 2 degrees. Simultaneously,the radar is capable of fulfilling the conditions thatthe width of the radar sensor must be less than 100millimeters due to the limitation of the installation

area and that the radar must be realized with asimple structure for cost reduction.

Further improvements of the azimuthal angularresolution and the azimuthal FoV require futureinvestigations for realizing advanced intelligent-vehicle systems, such as collision avoidance systemand a stop & go system.

References

1) Wenger, J. : "Automotive MM-Wave Radar: Statusand Trends in System Design and Technology", IEEColloq. on Automo. Radar and Nav. Techn. (1998)

2) Meinel, H, H. : "Automotive Millimeter wave Radar,Status, Trends and Producibility", Tech. Dig. of 2000Top. Symp. on Millimeter Waves (2000), 5

3) Johnson, R. L. and Miner, G. E. : "Comparison ofSuperresolution Algorithms for Radio DirectionFinding", IEEE Trans. on Aerospace and ElectronicSystems, AES22-4 (1986), 432

4) Asano, Y., Oshima,S. and Nishikawa, K. :"Estimation of Method of Automotive Radar Signalfor MARPET", Proc. of IEEE Veh. Technol. Conf.,(1997), 1912

5) Roy, R. and Kailath, T. : "ESPRIT – Estimation ofSignal Parameters Via Rotational InvarianceTechniques", IEEE Trans. on Acoust., Speech, SignalProc., 37-7(1989), 984

( Report received on April 23, 2002 )

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Fig. 7 Experimental result of azimuthal angularresolution.

Fig. 8 Experimental result of azimuthal FoV.

Yoshikazu AsanoYear of birth : 1957Division : Research-Domain 21Research fields : Radar SystemAcademic degree : Dr. Eng.Academic society : Inst. Electron. Inf.

Commun. Eng.

Shigeki OshimaYear of birth : 1955Division : Research-Domain 21Research fields : Radar System

SetAngularDifference

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R&D Review of Toyota CRDL Vol. 37 No. 2

Tomohisa HaradaYear of birth : 1959Division : Research-Domain 21Research fields : Mobile Communication

SystemsAcademic society : Inst. Electron. Inf.

Commun. Eng.2000 Excellent SIG Notes Awardfrom IPSJ Mobile Comput. andWireless Commun. Soc.

Masaru OgawaYear of birth : 1967Division : Research-Domain 21Research fields : Radar SystemAcademic society : Inst. Electron. Inf.

Commun. Eng.