A 79GHz UWB Pulse-Compression Vehicular Radar in 90nm CMOS

Kai-Wen Tan∗, Chang-Ming Lai∗, Po-Hui Lu∗, Chia-Hou Tu∗, Jen-Ming Wu∗,Shawn S. H. Hsu∗,Guo-Wei Huang†, and Ta-Shun Chu∗

∗National Tsing Hua University, Hsinchu, Taiwan†National Nano Device Laboratories, Hsinchu, Taiwan

Abstract— A 79GHz UWB pulse-compression (PC) vehicu-lar radar system is presented. A PC waveform is a long pulsewith internal modulation. The energy of it can be increasedwithout raising the instantaneous peak power. Meanwhile,a PC waveform preserves an equal bandwidth and therange resolution associated with a single pulse waveform.In addition, the PC technique also allows multiple radarsoperating simultaneously based on the code division multipleaccessing (CDMA). In this work, the internal modulation ofthe PC waveform is binary phase shift keying (BPSK) witha 31-bit length pseudo noise (PN) sequence. The modulationrate is 1 Gb/s and the maximum pulse width is 31 nsec.The CMOS chip presents a self-contained radar system withthe transmitter, receiver, frequency synthesizer, and completetiming circuit fully integrated in a standard 90nm CMOStechnology.

Index Terms— CMOS, UWB, CDMA, pulse compression,radar, millimeter wave, direct conversion transceiver, pseudonoise code.

I. INTRODUCTION

Fully-integrated silicon-based vehicular radar deviceshave the potential to offer a low-cost solution for roadsafety enhancement [1], [2]. In general, radar systems canbe classified into continuous wave (CW) and impulse radio(IR) systems. Compared with CW radar systems, IR radarsystems have certain advantages in terms of less multi-patheffect and higher range resolution. In IR radar systems, thewaveform of propagation signal is usually a short pulsewhich has a large bandwidth but small energy. It resultsin high range resolution, but low detection probability in aradar system. Although, the overall energy into a decisiondevice in the receiver can be increased through integratingmultiple pulses.

A PC waveform (Fig. 1) is a long pulse with internalmodulation, it can hold an equal bandwidth and moreenergy which lead equal range resolution and higher detec-tion probability in comparison with a short pulse. It alsoallows multiple radars operating simultaneously throughthe CDMA technique. In this work, a 79GHz CMOSUWB radar employing binary phase-coded PC waveformis reported. With a large instantaneous bandwidth andtime-gated operation, a high range resolution within a largedetection range can be achieved.

Fig. 1. The waveform and spectrum of a single short pulse and a pulsecompression waveform.

Fig. 2. System-level block diagram of the proposed PC radar system.

II. SYSTEM ARCHITECTURE

The proposed CMOS UWB radar architecture is struc-tured as follows (Fig. 2). The transmitting path containsa binary phase coded PC waveform generator, an up-conversion mixer, and a driver amplifier (DA); the receiv-ing path is composed of a low noise amplifier (LNA),quadrature down-conversion mixers, and correlators; afrequency synthesizer generating the mm-wave carrierfor frequency up/down-conversion and a timing circuitrydiscriminating the distance of a target from entire rangebins.

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Fig. 3. Simplified PC radar system circuit schematics including transmitter, receiver, frequency synthesizer, and timing circuitry.

In the transmitter, a binary phase-coded PC waveformis generated via a trinary encoder. A trinary code hasthree levels for signal representation which are positive(+1), negative (−1), and zero (0). It provides binaryphase-coded PC waveforms with positive or negative levelsinside the pulse interval and zero level outside the pulseinterval. The reported trinary encoder comprises two 31-bitparallel differential shift registers with 1GHz clock and ahigh-speed combinational logic circuitry. One shift registercontrols the pulse time width and the other sets the binaryphase shift keying (BPSK) modulation. The codes for thepulse width and the BPSK modulation are loaded froma series-to-parallel interface. The output digital signals ofthese two shift registers are fed into a combinational logiccircuitry to produce a differential binary phase-coded PCsignal in the baseband. The next stage is a mixer which up-converts the baseband pulse to the mm-wave frequency. ADA buffers the waveform generator and delivers pulses toan antenna. The carrier frequency of the mm-wave binaryphase coded PC waveform is 79 GHz, the modulation rateis 1 Gb/s in baseband which corresponds to 15cm rangeresolution, the maximum pulse width is 31 nsec, the pulserepetition frequency is 5 MHz which corresponds to 30mmaximum detection range.

In the receiver, the LNA receives and amplifies thepulses which is reflected from a target from antenna.Following the LNA, quadrature mixers down-convert thereceived signals from the mm-wave band to the base-

band. There is a correlator in each quadrature path whichfunctions as a matched filter. It provides high signal-to-noise ratio (SNR) by corrupting the received noise andinterferences through cross correlating received signal witha template. The correlator consists of a programmablegain amplifier (PGA), a multiplier, a resettable integrator,and an analog output buffer. The quadrature amplitude ofthe baseband signals are adjusted by the PGAs based onthe programmable length of transmitting codes and thedistance of a target to enhance the dynamic range of thereceiver. The output signal of each PGA is multiplied withthe code template in a current-commutating mixer and theresulting signal is poured into an integrating capacitor.The codes of the template for the pulse width and theBPSK modulation are stored in two parallel high speedshift registers.

A 79GHz frequency synthesizer is implemented togenerate the mm-wave carrier signal for both transmitterand receiver. A range-gated timer which measures time offlight (TOF) of pulses It provides 400 range bins withinthe maximum detection range of 30 m.

III. CIRCUIT SCHEMATICS

The simplified circuit schematics of the reported radarsystem are shown in Fig. 3.

The transmitter starts with a trinary encoder whichproduces differential modulated signal in baseband. Fully-customized high-speed biphase flip flops are realized in

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TABLE ISUMMARY AND COMPARISON

the shift register of the trinary encoder. The double bal-anced current-commutating Gilbert cell is implementedfor the frequency up-converter to minimize the LO-to-RFfeedthrough.

The LNA is a five-stage single-ended common-sourceamplifier with slow-wave transmission-line matching net-works. The input matching network is accomplished byusing shunt and series inductors. The shunt inductor actsas an electrostatic discharge (ESD) protection device. Thefrequency down-converters utilize a similar design as thefrequency up-converter. The PGA is a two-stage Cherry-Hooper amplifier with a local feedback to increase itsbandwidth. An analog correlator is implemented to achievehigh-speed and wide-bandwidth operation. The correlatorin each quadrature path includes a current-commutatingmixer, a Gm-C integrator, and an analog output buffer.

The carrier of the PC waveforms is generated by aninteger-N phase locked loop (PLL) and a frequency tripler.The range-gated timer is mainly composed of a ring-oscillator-based PLL and a multiplexer.

IV. MEASUREMENT RESULTS

Table I summarizes the measured performance of thereported CMOS UWB PC radar chip and compares itwith the work reported in [2]. The transmitted spreadspectrum of a binary phase-coded PC waveform with a2GHz bandwidth and 77GHz carrier frequency is shownin Fig. 4(a). The modulation rate is 1 Gb/s in baseband,the code length of PN code is 31, and the pulse width is 31nsec. The loopback measurement results are shown in Fig.4(b). The transmitter is serially connected to the receiverthrough RF cables. By changing the transmitted timingof the PC signals with a step of 500 psec, the receivercan sense the loop-back signal at different range bins. The

Fig. 4. (a) The spread spectrum of a PC waveform. (b) The loopbackmeasurement results.

Fig. 5. Chip microphotograph of the mm-wave PC radar system.

black and gray curves represent the correlation coefficientsversus range bins when the transmitted signal is identicaland different to the template of receiver, respectively. Itshows that the reported PC radar can search range binsthrough time-gated mechanism and corrupt interferencesfrom other users. The chip microphotograph is shown inFig. 5.

V. CONCLUSION

A 79GHz single-chip PC radar in a 90nm CMOStechnology has been demonstrated. PC radars are immuneto multi-path effect and can achieve high range resolutionsimultaneously. Therefore, it is a promising candidate forvehicular radar applications.

VI. ACKNOWLEDGMENT

The authors thank National Science Council (NSC) andNational Chip Implementation Center (CIC) in Taiwan forchip support and implementation of this work, respectively.The authors also acknowledge Prof. H. Wang at GeorgiaTech, Prof. Y. J. Wang at NCTU, Dr. A. Goel at MTK, Dr.H.H. Hsieh at TSMC, and Mr. R. Chen at USC for theirnumerous suggestions.

REFERENCES

[1] T. Fukuda et al., ”A 26GHz short-range UWB vehicular-radar using2.5Gcps spread spectrum modulation,” in IEEE MTT − S Int.Microwave Symp Dig., pp. 1311-1314, June 2007.

[2] V. Jain et al., ”A single-chip dual-band 22-to-29 GHz/77-to-81 GHzBiCMOS transceiver for automotive radars,” in IEEE Int. Solid−State Circuits Conf. Dig. Tech. Papers, pp. 308-309, Feb.2009.

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