Interest in autonomous intelligent cruise control and collision-warning radar systems for in-vehicle applications continues to increase, and the reasons are obvious. Studies show that 60 percent of forward collisions can be avoided with an extra half-second of warning time; with 1 second of extra warning time, drivers can avoid roughly 90 percent of forward collisions. Automotive radar systems operating at 76.5 GHz are considered to be the best means of detecting objects close to a car.
In recent years various radar sensors have been proposed and applied in the intelligent vehicle systems for collision avoidance and cruise control on heavily loaded highways. Many proposals are based on frequency-modulated continuous-wave (FMCW) radar, in which the reflections are studied as the transmitter is continuously and linearly swept from a given lower frequency to a given higher frequency. FMCW radar works on the principle of the Doppler effect.
Assume, for example, the lower and upper frequencies of the sweep are 2,000 MHz and 5,000 MHz. When a 2,000-MHz signal is transmitted and hits an object, its reflection is bounced back to the antenna. By the time the signal reaches the antenna, the transmitted signal would have shifted to, say, 2005 MHz. The returning signal at 2,000 MHz and the transmitted signal at 2,005 MHz are fed into a mixer, yielding an intermediate frequency of 5 MHz. That intermediate frequency is directly proportional to the distance to the reflecting surface.
Thus, the radar system will include a "circulator" from the receiver-a mixer that combines transmitted and received signals to derive an intermediate frequency. RF bandpass filters are used between the circulator and the antenna. A power divider is used here to portion the output of the transmitter between the antenna and the receiver/mixer loop. Generally, 4/5 of the power goes to the circulator and 1/5 to the local oscillator. The output of the mixer is evaluated with an oscilloscope-counter combination or digital signal processor.
For automotive applications, a fully integrated FMCW radar front-end design requires small size and low-cost manufacturing, and it can be capable of acquiring the range and angular information of all obstacles in its field of view. A prototype FMCW system, built and marketed by Epsilon Lambda Electronics, operates in the 76- to 77-GHz frequency band. The system generates an FMCW waveform using a voltage-controlled oscillator (VCO) with an InP Gunn diode that is phase-locked to a harmonic of a dielectric resonator oscillator (DRO).
In Epsilon Lambda's implementation, the frequency of the mixed signal is compared with a low-frequency linear sweep reference to generate an error signal that corrects the VCO sweeping slope. A linearizer is used to stabilize the RF frequency and to improve system phase noise. The receiver uses a homodyne approach and includes a single balanced mixer receiver, I/Q mixer receiver and monopulse receiver options.
The integrated radar transceiver includes a planar array and monopulse antenna that is about as long and wide as a business card. The FM modulation technique of the radar front end may also be used to broadcast a digital radar system identifier. By periodically switching between two transmit frequencies according to an assigned code, the radar identifier can be broadcast as a part of the system modulation by considering two frequencies as binary code.
Epsilon Lambda produces three types of radar transceiver/antenna modules to determine range, relative velocity and the azimuth angle for multiple targets in automotive applications. A mechanically scanned patch antenna (Model ELFI71-1A), with a single balanced mixer and fixed beam, generates a single video output representing the target return from which the target range and velocity information can be derived. A mechanically scanned patch antenna with I/Q mixer and fixed beam (Model ELFI71-1B radar transceiver) compares two IF outputs with a quadrature phase relationship. That feature provides a means for discriminating the polarity of the beat signal independently from the range information. And, a fixed monopulse patch antenna with sum and delta channel mixer (Model ELFI71-1C radar transceiver) can generate the target angular information simultaneously with the range information without mechanical scanning.
Ordinarily, the range of radar is proportional to the fourth root of the transmitter power. Thus, doubling the range requires that the transmitted power be increased 16 times. That means that there often is a practical, economical limit to the amount of power that should be employed to increase the range of radar.
With 3.5-db/km signal attenuation, a radar system can get a target 200 meters away in clear weather-and 100 meters away in a heavy rain-assuming a 50-dB/km attenuation in a 150-mm/hr downpour. This also assumes that the target cross-section is 10 dBsm at 76.5 GHz, the transmitting power is higher than 10 dBm, the gain of the antenna is better than 27 dBi and the system noise figure is less than 18 dB.
The IF output frequencies are different in periods T and 2T for a target moving with a relative velocity, Vd. The frequency shift, fr, due to the time delay in range and Doppler frequency, fd, are:
fr = (2R/C)(Br/T)
fd = 2Vdf/C,
where Vd is the relative velocity of the target, C is the velocity of propagation, f is the operating frequency and R is the range of the target.
The range resolution, deltaR, of a radar transceiver depends on the sweeping bandwidth, Br. Since
R = (C/2Br)(frT),
at one period only one frequency is processed (frT= 1), so that the theoretical range resolution is:
Change in R = C/(2Br).
Therefore, to get 1-m range resolution, the theoretical FM bandwidth required is at least 150 MHz. The actual range resolution also depends on the target beat frequency width and receiver frequency resolution. Epsilon Lambda transceivers have an FM sweep range of 200 MHz.
The maximum range of a radar system depends on the radar equation characteristics. The change in R depends on the FM sweep bandwidth and the FFT beat signal sample spacing. The accuracy of the range measurement depends on the FM sweep linearity. Any nonlinearity of the FM signal will cause the calculated range of the target to be ambiguous. Normally, the FM sweep signal is not ideally linear. For a free-running VCO, the linearity is typically about 10 percent. Use of a linearizer with the VCO improves linearity dramatically-to between 0.03 and 0.5 percent.
A fully integrated FMCW forward-looking radar transceiver with flat antenna makes it possible to determine target range, relative velocity and the azimuth angle with or without mechanical scanning. The FM sweep linearity of this system is better than 0.5 percent, and the range resolution is better than 1 meter. With the monopulse option, the azimuth angle accuracy and resolution of the system are of 10th degrees. Compared with a 3-D mechanical scanning or lens transceiver/antenna, the volume of the transceiver is quite small, since the 2-D patch array antenna has been used. The simplicity and producibility of this kind of microstrip antenna also help to lower the cost of large scale production of the front end.