# OMG, millimeter wave is not simple! Millimeter wave radar bearing principle + advantages

Dec 1, 2022

1. Basic principles of automotive millimeter-wave radar

Millimeter wave refers to electromagnetic waves with a wavelength of 1-10mm, which has a large bandwidth, high resolution, and small size of antenna components, which can adapt to harsh environments. The millimeter-wave radar for vehicles usually adopts the FMCW (frequency modulated continuous wave) radar system with simple structure and low cost, which is suitable for short-range detection. The radar antenna sends out a series of continuous frequency-modulated millimeter waves. The frequency changes with time according to the law of the modulation voltage. It is generally a continuous triangular wave. The transmitted and received signals are shown in Figure 2. The solid line in the figure is the transmitted signal, and the dotted line is the reflected signal of relatively stationary and relatively moving objects. The reflection is the same as the transmitted signal waveform, but differs by a delay time td.

td=2R/C(1)

In the formula, R is the target distance, and c is the speed of light.

The frequency difference between the transmitted signal and the reflected signal at a certain moment is the intermediate frequency fb of the mixing output. The frequency shift of the reflected signal from a relatively moving object due to the Doppler effect. The intermediate frequency frequencies output at the rising edge and falling edge of the triangular wave are fb+ and fb- respectively. The following formula holds: Thereby, the distance R and the relative movement speed v of the target vehicle are obtained. It can be seen from equations (2) and (3) that the determination of the intermediate frequency fb of the millimeter-wave radar signal is the key to finding R and V. The determination of fb is to perform spectrum analysis on the frequency difference between the transmitted and reflected signals. Signal spectrum analysis mainly includes FFT method and non-FFT method. The so-called FFT method is to perform Fourier transform on the analyzed signal, transform it from the time domain to the frequency domain, analyze it in the frequency domain, and then convert it back to the time domain through inverse Fourier transform if necessary. . The non-FFT method obtains the frequency parameters of the signal through other methods, such as the maximum entropy method, the MUSIC method, and the like. Considering the complexity, real-time and stability of the method, for automotive radar, the FFT method should be the first choice for spectrum analysis. This method is relatively mature, easy to implement, and has strong real-time performance, and is suitable for real-time processing of signals under vehicle operating conditions. Figure 2 FMCW transmit and echo signals

2. Structure of automotive millimeter-wave radar Figure 3 shows the structure of a linear frequency modulation radar (LFCW) automotive millimeter-wave radar, including an antenna, a transceiver module, a signal processing module, and an alarm module. The RF transceiver front-end is the core component of the radar system. A lot of in-depth research on the front-end has been carried out at home and abroad, and great progress has been made. Front ends of various structures have been developed, mainly including waveguide front ends, microstrip front ends, and monolithic integration of front ends. The RF front-end developed in China is mainly a waveguide structure front-end. A typical RF front end mainly includes antenna, linear VCO, amplifier, balanced mixer part. The IF signal output by the front-end mixing is sent to the post-stage data processing part through IF amplification. The basic goal of the data processing part is to eliminate unnecessary signals (such as clutter) and interference signals, and to process the mixed signals amplified by the intermediate frequency to extract information such as target distance and speed from the signal spectrum.

3. The principle of millimeter wave radar azimuth measurement

In the field of automotive active safety, automotive millimeter-wave radar sensor is one of the core components, of which 77GHZ millimeter-wave radar is an indispensable key component in smart cars. It can quickly perceive surrounding objects within a range of 0-200 meters in all-weather scenarios. Sensing devices for distance, speed, azimuth and other information. So how does it calculate the position, velocity and direction of the monitored target?

1. Location

The millimeter-wave radar emits directional millimeter-waves in the corresponding band through the transmitting antenna. When the millimeter-wave encounters an obstacle target, it is reflected back, and the reflected millimeter-wave is received through the receiving antenna. According to the millimeter wave band, calculate the time of the millimeter wave on the way & TImes; speed of light ÷ 2 by the formula, and then combine the driving speed of the preceding vehicle and the driving speed of the vehicle, you can know the distance between the millimeter wave radar (the vehicle) and the target The relative distance, and the location of the target is also known.

2. Speed

In addition, according to the Doppler effect, the frequency change of the millimeter-wave radar and the relative speed of the vehicle and the tracked target are closely related. The relative speed of motion compared to the car. Therefore, it is shown that when the sensor issues a safe distance alarm, if the vehicle continues to accelerate, or the front monitoring target decelerates, or the front monitoring target is stationary, the frequency of the millimeter wave reflected echo will become higher and higher, and vice versa. The frequency is getting lower and lower.

3. Azimuth

Regarding the measurement of the azimuth angle of the monitored target, the detection principle of the millimeter radar is: after the millimeter wave is transmitted through the transmitting antenna of the millimeter wave radar, it encounters the monitored object, reflects it back, and passes through the parallel receiving antenna of the millimeter wave radar. The azimuth angle of the monitored target can be calculated by the phase difference of the millimeter wave reflected from the same monitoring target. The schematic diagram is as follows:

The azimuth angle αAZ is the geometric distance d between the millimeter-wave radar receiving antenna RX1 and the receiving antenna RX2, and the phase difference b of the reflected echoes received by the two millimeter-wave radar antennas, and then the azimuth angle αAZ is calculated by the trigonometric function. value, so that the azimuth of the monitored target can be known.

The monitoring of position, speed and azimuth angle is what millimeter-wave radar is good at. Combined with the strong anti-jamming ability of millimeter-wave radar, it can work stably all day and all day. Therefore, millimeter-wave radar is selected as the core sensing technology of automobiles.

With the popularity of autonomous driving, lidar has been sought after as never before because of its advantages of high precision, large amount of information, and no visible light interference. But we can notice that the current mainstream autonomous driving solutions have not completely abandoned millimeter-wave radar. What is the reason for this?

The first is the weather we all know. The wavelength of laser is much smaller than that of millimeter-wave radar (nm vs mm), so it is not a joke that the haze causes the lidar to fail. For the same reason, the detection distance of millimeter-wave radar can easily exceed 200 meters, while the current performance of lidar generally does not exceed 150 meters, so for scenarios such as highway following, millimeter-wave radar can do better.

Secondly, millimeter-wave radar is cheap. As a mature product, the current price of millimeter-wave radar is about 1.5 thousand, while the price of lidar is still calculated in ten thousand. And because the amount of data obtained by lidar is far more than that of millimeter-wave radar, a higher-performance processor is required to process the data, and a higher-performance processor also means higher prices. So for engineers, in simple scenarios, millimeter-wave radar is still the best choice.

The above is the relevant content of “millimeter wave” brought by this editor. Through this article, I hope everyone has a clear understanding of the azimuth angle measurement of millimeter wave radar and its advantages. If you like this article, you may wish to continue to pay attention to our website. The editor will bring more exciting content later. Finally, thank you very much for reading, have a nice day!