596IEICE TRANS. ELECTRON., VOL.E104–C, NO.10 OCTOBER 2021

PAPER Special Section on Microwave and Millimeter-Wave Technologies

High-Density Implementation Techniques for Long-Range RadarUsing Horn and Lens Antennas

Akira KITAYAMA†a), Nonmember, Akira KURIYAMA†, Hideyuki NAGAISHI†,and Hiroshi KURODA††, Members

SUMMARY Long-range radars (LRRs) for higher level autonomousdriving (AD) will require more antennas than simple driving assistance.The point at issue here is 50–60% of the LRR module area is used for an-tennas. To miniaturize LRR modules, we use horn and lens antenna withhighly efficient gain. In this paper, we propose two high-density imple-mentation techniques for radio-frequency (RF) front-end using horn andlens antennas. In the first technique, the gap between antennas was elim-inated by taking advantage of the high isolation performance of horn andlens antennas. In the second technique, the RF front-end including micro-strip-lines, monolithic microwave integrated circuits, and peripheral partsis placed in the valley area of each horn. We fabricated a prototype LRRoperating at 77 GHz with only one printed circuit board (PCB). To detectvehicles horizontally and vertically, this LRR has a minimum antenna con-figuration of one Tx antenna and four Rx antennas placed in 2×2 array,and 30 mm thickness. Evaluation results revealed that vehicles could bedetected up to 320 m away and that the horizontal and vertical angle errorwas less than +/− 0.2 degrees, which is equivalent to the vehicle widthover 280 m. Thus, horn and lens antennas implemented using the proposedtechniques are very suitable for higher level AD LRRs.key words: 77 GHz long-range radar, horn and lens antenna, antennaisolation, direction of arrival estimation, compact implementation

1. Introduction

New technologies to improve detection performance ofsensors such as cameras, light detection and ranging (Li-DAR), millimeter-wave radar, sonar, and fusion technol-ogy combining these sensors are being actively developedfor advanced driver assistance systems (ADASs) and au-tonomous driving (AD) systems [1]–[6]. Among these sen-sors, millimeter-wave radar is robust against weather andnighttime environmental changes, so it has been appliedto applications such as auto cruise control (ACC) and au-tonomous emergency braking (AEB) [7]–[10]. Long-rangeradar (LRR) for front-side vehicle monitoring is one of themost basic and important sensors to avoid collisions. LRRsfor ADAS can detect vehicles 200 m ahead with a field ofview (FOV) of 18 degrees and angular estimation accuracyof approximately 0.3 degrees. Figure 1 (a) shows the ba-

Manuscript received October 25, 2020.Manuscript revised February 1, 2021.Manuscript publicized March 12, 2021.†The authors are with Research & Development Group, Center

for Technology Innovation - Electronics, Hitachi, Ltd., Kokubunji-shi, 185–8601 Japan.††The author is with Sensing System Design Dept., AD/ADAS

Business Unit, Hitachi Astemo, Ltd., Hitachinaka-shi, 312–8503Japan.

a) E-mail: akira.kitayama.er@hitachi.comDOI: 10.1587/transele.2021MMP0006

Fig. 1 System configuration of LRR system

sic configuration of LRRs for ADAS, which has a simple“RF front-end”. In this paper, radio-frequency (RF) front-end is defined as follows: monolithic microwave integratedcircuits (MMICs), transmitter (Tx) and receiver (Rx) anten-nas, micro strip lines (MSLs) for connecting MMICs andantennas, and peripheral parts for operating MMICs.

On the other hand, the LRRs for higher levelADAS/AD systems will require high performance detectionin various use cases such as detection of pedestrians andvehicles over 100 m ahead, road edge detection, and longdistance detection in bad weather conditions. In terms ofperformance specifications for meeting these requirements,the LRRs for higher level ADAS/AD will need to detect ve-hicles over 300 m ahead with an FOV of 30 degrees. Therequirements to improve detection distance and angular es-timation accuracy are as follows:

(a) Improve signal-to-noise ratio (SNR) of Rx signals(b) High isolation between Tx and Rx channels and be-

tween Rx and Rx channels(c) Increase the number of Tx and Rx channels.

To improve the SNR, firstly, it is necessary to increase thereceived power Pr. According to the radar equation, the Pr

is as follows:

Pr =PtGtGrσλ

2

(4π)3R4(1)

Copyright c© 2021 The Institute of Electronics, Information and Communication Engineers

KITAYAMA et al.: HIGH-DENSITY IMPLEMENTATION TECHNIQUES FOR LONG-RANGE RADAR USING HORN AND LENS ANTENNAS597

Fig. 2 General configuration example of LRR PCB configuration

where Pt is the power fed to the Tx antennas, Pr is the Rxpower, Gt is the Tx antenna gain, Gr is the Rx antenna gain,λ is the wavelength in free space, σ is the radar cross sec-tion of the detected target, and R is the detection distance.In order to improve the detection distance from 200 m to300 m without changing radar operating parameters and thedetection targets, it is necessary to increase antenna gains(Gt and Gr) by about 7.0 dB in total. That means increasingthe effective area of the antenna by about 5.1 times. In ac-tuality, improving the output power and noise performanceof MMICs also improves SNR [11]–[15], but in any case,LRRs require highly efficient antennas.

Additionally, to reduce the coupling, the antennas andthe MSLs of each channel need to be physically separatedfrom adjacent channels. Furthermore, as shown in Fig. 1 (b),to increase the numbers of Tx and Rx channels for higherperformance and functionality, the area of the RF front-endneeds to be expanded. It is no exaggeration to say that theLRR size is determined by the RF front-end area. As aresult, higher performance LRRs are expected to be largerthan LRRs for simple ADASs. However, the space insidethe front bumper of a car is very narrow, so higher perfor-mance LRRs should be the same size as or not much largerthan the LRRs for simple ADASs. Therefore, a technique isrequired to implement RF front-end with high density andcompactness. To satisfy these requirements, we focus on thefeatures of a horn and lens antenna developed in a previousstudy [16]–[18]. This antenna has high gain, high isolation,and high implementation flexibility.

This paper proceeds as follows: In Sect. 2, two RFfront-end implementation techniques using horn and lensantennas are proposed. In Sect. 3, a fabricated prototypeLRR using the horn and lens antennas is described. Sec-tion 4 evaluates detection performance by installing the pro-

totype LRR in the front bumper of a vehicle. Finally, theconclusion is given in Sect. 5.

2. RF Front-End Implementation Techniques

2.1 Module Size and Cost Reduction in LRR

This section explains the size and cost of the LRRs forhigher level ADAS/AD, as opposed to simple ADASs. Forexample, higher level ADAS/AD require detection of heightinformation such as for falling objects and road signs, ob-ject size detection, and 300-m ultra-long-distance detection.Technology to realize these functions has been developed,such as multi-input multi-output (MIMO) technology andadvanced signal processing technology [19]–[22]. But thesetechnologies require an increase in the size, number of an-tennas and the 2D-array placement of antennas. Therefore,the size and cost of the LRR module are expected to in-crease.

The structure of LRRs for simple ADASs is shown inFig. 2 (a). They have two printed circuit boards (PCBs), gen-erally. RF front-end is implemented on PCB1, whose mate-rial is expensive and low dielectric loss at a high frequencyband such as 76 to 81 GHz. The antenna patterns and theMSLs are implemented on the top side of PCB1, and theMMIC on the bottom side of PCB1. The micro-control-unit(MCU), the Power IC, and the interface (I/F) connector areimplemented on PCB2, whose material is low cost such asFR4.

In contrast, the structure of LRRs for higher levelADAS/AD have more antennas as shown in Fig. 2 (b). Theyhave the same thickness about 30 mm or less as Fig. 2 (a),but have larger vertical and horizontal sizes, since the num-ber of antennas has increased. If the structure gives priority

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Fig. 3 Schematics of horn and lens antennas, MSL, and MMIC

to reducing the height using only one PCB, the thicknessof LRR is very thin, but the vertical and horizontal sizesare too large as shown in Fig. 2 (c), due to increase the areafor MMIC, transmission lines, and peripheral parts. Thisstructure eliminates the need for a connector and reduces thenumber of PCBs, which is advantageous in term of manu-facturing cost and quality assurance.

Rather than reducing the height of the LRR, we thoughtit was more valuable to make the vertical and horizontalsizes compact and use only one PCB to reduce costs. Theheight is 30 mm, and the PCB is only one, so it can be saidthat it is a hybrid of Fig. 2 (b) and (c). To achieve this, inthe next section, two high-density antenna implementationtechniques for horn and lens antennas are described.

2.2 High-Density Implementation Techniques for LRRUsing Horn and Lens Antennas

First, only an overview of the horn and lens antenna designis given here, because its details are described in a previ-ous paper [16]–[18]. A horn and lens antenna consists of aone-patch antenna, pyramidal horn, and dielectric lens. Asshown in Fig. 3 (a), the one-patch antenna WA × LA and ref-erence ground pattern WG × LG are designed on the PCB.A pyramidal horn made of aluminum is coupled to the ref-erence ground on the PCB to increase the antenna gain andreduce reflections at the input horn aperture Wi×Li shown inFig. 3 (b) to (d). The size of the output horn aperture Wo×Lo

is the antenna efficiency area. A dielectric lens was inte-grated onto the output aperture of the pyramidal horn to ob-tain a narrow beam width of millimeter-wave radiation.

The horn and lens antenna has higher radiation ef-ficiency and isolation than conventional antennas such asseries-fed antenna and comb-line antennas [23]–[28]. Forexample, the radiation efficiency of the horn and lens an-tenna is about 1.6 times higher than that of a series-fed an-tenna [16]. Therefore, to perform equivalently to a series-fed antenna, a 3-Tx and 4-Rx LRR using horn and lens an-tennas (Fig. 4 (b)) requires about 60% area of one using con-

Fig. 4 Proposed high-density implementation techniques, example forRF front-end implementation with 3 Tx and 4 Rx antennas, MSL, MMIC,and peripheral parts (top view).

ventional antennas (Fig. 4 (a)). The areas of the antennas,RF front-end, and PCB are as follows:

AANT = n(AANT EFF + AANT GAP) (2)

ARF = nAANT + mAPart (3)

APCB = ARF + Aother (4)

where n is the number of Tx and Rx channels, m is the num-ber of MMICs, AANT EFF is the antenna efficiency area, andAANT GAP is the gap between each antenna. APart is the areaof the MMIC, MSL, and peripheral parts such as the resis-tor, capacitor, and crystal for operating the MMIC. Aother

consists of areas such as the area for the screw for fixingthe PCB to the chassis, ground (GND) pattern, and the areafor the I/F connector, etc.. The structures of Fig. 4 (a) and(b) have AANT GAP to keep isolation from each antenna andaround GND pattern [29], [30].

In addition to the high gain performance of horn andlens antennas, we also focus on the high isolation perfor-mance [18]. There is little coupling between one-patch an-tennas because they are electrically isolated by the surround-ing input aperture of the horn. Furthermore, at the outputaperture area of the horn, there is also little coupling be-tween waveguides. For the first antenna implementationtechnique, we propose placing antennas without any gaps(AANT GAP = 0) such as in Fig. 4 (c).

Moreover, this implementation method allows the an-tenna aperture area to be placed above the PCB. For thesecond implementation technique, the antenna aperture area

KITAYAMA et al.: HIGH-DENSITY IMPLEMENTATION TECHNIQUES FOR LONG-RANGE RADAR USING HORN AND LENS ANTENNAS599

Fig. 5 Estimation results of RF front-end area for both antennas

Table 1 Estimation area of RF front-end (3 Tx & 4 Rx)

can overlap another area such as in Fig. 4 (d). Thus,the AANT on the PCB becomes AANT HI (Wi × Li) fromAANT HO (Wo × Lo), and the ARF is as follows:

ARF =

{nAANTHO

(nAANTHO >nAANTHI+mAPart

)nAANTHI+mAPart (otherwise)

(5)

Next, the effect of the high-density implementationtechniques using the horn and lens antennas will be de-scribed. To simplify the discussion, the antenna area is setto the values used in the previous study [18]: Wo × Lo is22 × 9 mm2, Wi × Li is 2 × 4 mm2, and the area of MSL is5×10 mm2 per channel. The area of MMIC is 10×10 mm2,and the area of peripheral parts is 20× 10 mm2. One MMIChas three Tx channels and four Rx channels, so, for exam-ple, if eight Rx channels are needed, two MMICs will berequired. Figure 5 shows the results of estimating the PCBarea for each number of antennas under the above assump-tions and Eqs. (4)(5).

Additionally, the RF front-end area using series-fed an-tennas was also estimated as shown in Table 1 under thefollowing assumptions and Eqs. (3)(4). The values of pa-rameters related to ARF are set as follows. AANT is 22 × 9 ×1.67 mm2, because the aperture efficiency of the series-fedantenna is about 0.6 times that of the horn and lens an-tenna [18]. Suppose AANT GAP is 0.2 times AANT .

In the case of one MMIC, using the horn and lens an-tennas implemented with the high-density techniques willreduce the area of RF front-end ARF by 46.8% comparedwith using series-fed antennas. In higher performance LRRsfor ADAS/AD, the number of MMICs is expected to in-crease up to 2 to 4, the number of antennas is expected toincrease up to 8 to 10 or more, and the antenna area is ex-pected to increase to improve SNR.

Finally, the relationship between the gap and isolationis calculated by electro-magnetic simulation which is doneusing ANSYS HFSST M software. For the physical charac-teristics parameters of the 3D-model of antennas as shownin Fig. 6, were referred to our previous research [18], and thegap “g” with the adjacent antenna is variable. According to

Fig. 6 Simulation model of conventinal antenna method (2 channels)

Fig. 7 Simulation results of isolation between adjacent antennas

the simulation results shown in Fig. 7, isolation of series-fedantenna are improved by increasing the gap, but the gap of1.5 mm is required to achieve the 30 dB of isolation requiredfor a general radar. On the other hand, the isolation of thehorn and lens antenna is 40 dB or more regardless of thegap. It shows that sufficient isolation can be obtained evenif they are adjacent vertically and horizontally (Tx-Rx andRx-Rx) as shown in Fig. 4 (d).

Therefore, the horn and lens antennas implemented byusing the two proposed techniques are more suitable fordownsizing the LRR module.

3. Fabricated Prototype LRR

A new design of a compact prototype LRR composed ofhorn and lens antennas is presented in this section. Ta-ble 2 shows the target performance of the prototype radar. Itshould be able to detect vehicles up to 300 m ahead and esti-mate antenna gain, angles vertically and horizontally. Sinceour experience shows that the 2-way antenna gain requiredfor 200 m detection is about 34 dBi, the target value for300 m detection is 41 dBi, which is 7 dB added to it. Thehorizontal and vertical FOVs are 18 and 5 degrees, respec-tively. The important point in this design is module size, andthese performances were realized with the minimum neces-sary antenna configuration. One Tx and four Rx antennaswere placed in a 2×2 array. The architecture has a one-chip MMIC with a modulation signal generator, transceiver,and analog-to-digital converter (ADC), and an MCU withan MMIC controller function and signal processing func-tion. Figure 8 shows the system architecture of the prototypeLRR. This radar uses fast-chirp-modulation (FCM), whoseparameters are designed to achieve the maximum detectionrange of 300 m as shown in Table 3.

Next, the design of the horn and lens antenna is ex-plained. The antenna aperture area is expressed as follow:

600IEICE TRANS. ELECTRON., VOL.E104–C, NO.10 OCTOBER 2021

AANT = Lo ×Wo =Gλ2

4πη(6)

where η is aperture efficiency, G is antenna gain, respec-tively. The η of horn and lens antenna is about 33% [16]–[18], and Gt + Gr is 41 dBi or more, so the Tx and Rx

Table 2 Target performance of prototype LRR

Fig. 8 Architecture of prototype LRR (1 Tx & 4 Rx)

Table 3 Modulation design of prototype LRR

Fig. 9 Prototype LRR configuration

horn antennas have 44 × 18 mm2 and 22 × 9 mm2 outputaperture areas, respectively, and both have an input aper-ture size Li × Wi of 2 × 4 mm2. To make the height ofTx and Rx horn 15 mm or less, two 22 × 18 mm2 Tx an-tennas used a low height technique in which two one-patchantennas were connected to differential output terminals ofthe MMIC. Each Rx one-patch antenna was connected todifferential input terminals of the MMIC through a balun.The sizes of the one-patch antenna LA ×WA and the groundpattern LG × WG were 1.06 × 1.38 mm2 and 1.5 × 3 mm2,respectively. The diameter and focus distance of the lenswere 22 and 16 mm, respectively. The material parame-ters of the horn and lens used aluminum and polyphenylenesulfide (PPS), respectively. When the prototype LRR wasdesigned, a dielectric constant of 4.2 and tan δ of 0.01 wereused as loss factors for the lens. They were derived from themeasurement results for PPS.

The overall configuration of the prototype radar isshown in Fig. 9. (a) shows the lens part, (b) shows the hornpart with module housing, (c) shows the top side view ofPCB, (d) shows the bottom side view of PCB, and (e) showsthe perspective view of prototype LRR. The size of the PCBis 37×51.2 mm2, and the top side implementation consists ofan MMIC, MSL, one-patch antenna, peripheral parts, screw,and I/F connector. The bottom side implementation consistsof an MCU, Power IC, I/F, and peripheral parts. The ma-terial of the top side PCB layer was Rogers RO3003. Adielectric constant of 3.2 and tan δ of 0.004 at 77 GHz wereused for parameters of the PCB model for design.

In the following, the design of the RF front-end imple-mentation is explained. To implement horn and lens anten-nas in a compact area while taking advantage of their goodcharacteristics, Tx and Rx antennas and Rx and Rx anten-nas are placed with zero spacing such as in Fig. 9 (b). Toimprove the implementation efficiency which is the ratio ofRF front-end area to module area, MSLs, MMICs, and pe-ripheral parts are placed in the valley of horn antennas suchas in Fig. 9 (c). The implementation area of each componentin the above design is summarized in Table 4. Thus, hornand lens antennas are useful for downsizing LRRs. In thenext section, the basic detection performance of this proto-

KITAYAMA et al.: HIGH-DENSITY IMPLEMENTATION TECHNIQUES FOR LONG-RANGE RADAR USING HORN AND LENS ANTENNAS601

Table 4 Area of each RF front-end implementation for prototype radar

type LRR is evaluated.

4. Prototype LRR Performance

4.1 Basic Characteristics of Prototype LRR

This section presents the basic performance evaluation re-sults of the prototype LRR. As shown in Fig. 10, the Rxsignal data from the 20 dBsm reflector placed 10 m fromprototype LRR was signal-processed by the MCU to outputthe signal power, noise, and direction of arrival. The sig-nal power is calculated by Fast Fourier Transform (FFT),and the direction of arrival is calculated by a mono-pulsemethod. The measurement was done by rotating the proto-type LRR 1 degree at a time. When the vertical characteris-tics were measured, the prototype radar was rotated 90 de-grees. Two-way antenna gain characteristics estimated fromradar Eq. (1) and received power are shown in Fig. 11 (a).The two-way antenna gain is 43.7 dBi, and the beam widthof −6 dB gain is 18 degrees in the horizontal direction and 5degrees in the vertical direction, so the target performancesin Table 2 was satisfied. In addition, the horizontal andvertical direction of arrival (DoA) estimation accuracies inFig. 11 (b) and (c) show that the DoA estimation error was+/− 0.2 degrees or less in both directions. It can be said thatthis measurement has high reproducibility because the vari-ation of the measurement results for three times is +/− 0.03or less. Therefore, the isolation performance of total RFfront-end is sufficient to obtain a high accuracy angular es-timation. From the above results, the basic performance ofthe prototype radar using horn and lens antennas is sufficientfor LRR in higher level AD system.

4.2 Detection Performance of Prototype LRR Imple-mented in Vehicle

Figure 12 shows a photograph of the prototype radar in-stalled in a vehicle’s front bumper. As shown in Fig. 13, thedetection performance of the prototype radar was evaluatedwhen the radar-installed vehicle moved at 20 km/h and thetarget vehicle moved at 50 km/h. The positions of target andobservation vehicle are measured by GPS station using real-time-kinematic (RTK) method with an accuracy of +/− 5cm or less. To confirm the variability of the measurement,it was repeated 3 times. This is because the positional re-lationship and relative orientation of the target and observa-tion vehicle may always vary. In Fig. 14, the horizontal axisis time in all graphs, and the vertical axis is front distance(YR) in (a), horizontal distance (XR) in (b), and height in(c).

Fig. 10 Measurement set up for basic performance of prototype LRR

Fig. 11 Measurement results of antenna characteristics of prototypeLRR

Fig. 12 Photograph of prototype LRR installed in vehicle bumper

First, detection distance performance is discussed us-ing Fig. 14 (a). Even though distance ambiguity has an in-fluence at distances over 300 m due to the restriction of the

602IEICE TRANS. ELECTRON., VOL.E104–C, NO.10 OCTOBER 2021

Fig. 13 Evaluation condition of detection performance of prototype LRR

Fig. 14 Detection results of prototype LRR implemented vehicle

modulation condition, the target vehicle could be detectedup to 320 m.

Next, the DoA estimation accuracy is described below.Figure 15 shows the difference between the DoA estima-tion results (Fig. 14 (b)(c)) and the GPS detection position,which is the position of the target vehicle with respect tothe observation vehicle. As shown in the result of measure-ments, the horizontal DoA estimation error is 0.5 degrees ormore up to 100 m. This is probably because the target ve-hicle width of 2 m corresponds to +/− 0.5 degrees at 100m. Since the reflection from the vehicle is not from a pointas shown in Fig. 10 but from an uneven surface, the posi-tion of the high-intensity reflection point varies dependingon the inclination of the target vehicle. At 280 m or more,the vehicle width can be approximated to a point of +/− 0.2degrees or less, so the horizontal DoA estimation error is+/− 0.2 degrees or less. The variation in the measurementresults for three times is within 2 m of the vehicle width, sothe target DoA estimation error in Table 2 was satisfied.

Fig. 15 Detection results of DoA estimation error by vehicle distance

Table 5 Benchmark basic performance of LRR

Finally, because performances and functions of the pro-totype and current LRRs are different, they cannot be simplyand fairly compared. However, detecting the target vehicleat over 300 m with the minimum configuration and havingan accuracy within +/− 0.2 degrees are remarkable results,although there is no angle separation resolution because themono-pulse angular estimation method was used as shownin Table 5.

As described in Sect. 2.2, higher performance LRRs areexpected to require 2 to 4 MMICs and more than 10 anten-nas. If the number of MMICs and antennas is quadrupledfrom those of LRRs for simple ADASs (i.e., to 4 MMICs,9 Tx antennas, and 16 Rx antennas) and if the module sizealso quadruples, the volume of higher performance LRRswill be 324 cm3, which is equivalent to the LRRs for sim-ple ADASs. Therefore, it can be said that using horn andlens antennas is one of the best ways to reduce the mod-ule size in higher performance LRRs, which are expected tohave a large antenna area and many antennas. By combininghorn and lens antennas with the proposed high-density im-plementation techniques, the most efficient implementation

KITAYAMA et al.: HIGH-DENSITY IMPLEMENTATION TECHNIQUES FOR LONG-RANGE RADAR USING HORN AND LENS ANTENNAS603

can be realized.

5. Conclusion

Toward both high performance and downsizing of LRRs forhigher level ADAS/AD, two high-density RF front-end im-plementation techniques were proposed. We focused onthe horn and lens antenna, which has a high radiation ef-ficiency per area, high isolation characteristic between eachantenna, and high compatibility with peripheral componentimplementation, and evaluated a prototype radar using hornand lens antennas implemented with the two proposed tech-niques.

First, one technique eliminated the gaps between Txand Rx antennas and between Rx and Rx antennas by us-ing horn and lens antennas. In addition, the other techniqueoverlapped the area of the horn and lens antennas with thearea of MSLs, MMICs, and peripheral components.

Second, the prototype LRR was fabricated by applyingthe above high-density implementation techniques. The Txantenna has 1 channel and is 44×18 mm2, Rx antennas have4 channels and are 22 × 9 mm2 placed in a 2 × 2 array, thePCB is 37 × 51.2 mm2, and the volume of the radar moduleis 45 × 60 × 30 mm3.

From results of evaluating the basic characteristics, theantenna beam width of Tx and Rx two-way antenna gainwas 18 degrees horizontally and 5 degrees vertically and theangle estimation accuracy was +/− 0.2 degrees.

The prototype LRR was installed in a vehicle, andits preceding vehicle detection performance was evaluated.The maximum detection distance was 320 m, and angle es-timation accuracy was +/− 0.5 degrees up to 100 m and+/− 0.2 degrees over 280 m.

The above discussion and measurement results demon-strate the usefulness of LRRs using the horn and lensantennas implemented with high-density techniques forlonger detection distance, higher isolation characteristics,and smaller module size. In the future, we will prototype anultra-compact LRR with an increased number of antennasfor higher level AD/ADAS function and evaluate its detec-tion performance.

Acknowledgments

We thank K. Ouchi, Hitachi Automotive Systems, Ltd., forhis support in fabricating the prototype radar module. Wealso thank M. Kudo, Hitachi Automotive Systems, Ltd.,for providing insights on the requirements and technologytrends related to autonomous driving systems.

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Akira Kitayama received B.S. and M.S.degrees in engineering from Kyoto Institute ofTechnology, Kyoto, Japan, in 2005 and 2007.In 2007, he joined the Central Research Labo-ratory, Hitachi Ltd., Tokyo, Japan, where he hasbeen engaged in R & D of RF circuits, transmis-sion lines that transfer signals with GHz-orderfrequency, and signal processing for automotiveradars. He is a researcher with Hitachi, Ltd., Re-search & Development Group, Center for Tech-nology Innovation - Electronics, Tokyo, Japan.

Akira Kuriyama received B.S. andM.S. degrees in physics from Osaka University,Osaka, Japan, in 1997 and 1999, respectively,and received a Dr. Eng. Degree from the TokyoInstitute of Technology in 2020. In 1999, hejoined the Central Research Laboratory, HitachiLtd., Tokyo, Japan, where he has been engagedin research and development of high-power am-plifiers for mobile terminals and RF elementsfor millimeter-wave radars. He is a Senior Re-searcher with Hitachi, Ltd., Research & Devel-

opment Group, Center for Technology Innovation - Electronics, Tokyo,Japan.

Hideyuki Nagaishi graduated fromShimabara Technical High School, Nagasaki,Japan. In 1987, he joined the Central ResearchLaboratory, Hitachi Ltd., Tokyo, Japan, wherehe has been engaged in R&D of large-scale in-tegrated circuits with Josephson devices and RFcircuits. He is currently engaged in R&D of an-tennas and RF circuits for radars. He is a Re-searcher with Hitachi, Ltd., Research & Devel-opment Group, Center for Technology Innova-tion - Electronics, Tokyo, Japan.

Hiroshi Kuroda received a B.S. degreein Electrical Engineering II from Kyoto Univer-sity in 1984 and a Dr. Eng. degree from the To-kyo Institute of Technology in 2005. In 1984,he joined the Hitachi Research Laboratory, Hi-tachi Ltd., Ibaraki, Japan, where he was engagedin R&D of automotive navigation systems andmillimeter-wave radar. He is currently engagedin the development of millimeter-wave radar anda sensor fusion system for AD and ADAS sys-tems. He is a Senior Engineer with Hitachi

Astemo, Ltd., Sensing System Design Dept., AD/ADAS Business Unit,Powertrain & Safety Systems Business Division, Ibaraki, Japan.