1. Basic Analysis of Millimeter Wave Radar

1.1 What is millimeter wave radar

1) Radar working in millimeter wave band. The working frequency band is generally 30GHz~300GHz, with the wavelength of 1~10mm, which is between microwave and centimeter wave. It has some advantages of microwave radar and photoelectric radar;

2) Compared with the centimeter wave radar, millimeter wave radar has the characteristics of small size, easy integration and high spatial resolution.

3) The working frequency of vehicle mounted millimeter wave radar is generally 24GHz and 77GHz;

1.2 Basic structure of millimeter wave radar

Hardware core: MMIC chip and antenna PCB board. Taking FMCW vehicle mounted radar system as an example, it mainly includes: antenna, transceiver module, signal processing module;

Schematic Diagram of Basic Structure of Millimeter Wave Radar

1) Front end monolithic microwave integrated circuit (MMIC) (suppliers: Infineon, Freescale, Xiamen Yixing and Nanjing Miller;) It includes many functional circuits, such as low noise amplifier (LNA), power amplifier, mixer, and even transceiver system;

Features: low circuit loss, low noise, wide frequency band, large dynamic range, large power, high additional efficiency, strong electromagnetic radiation resistance, etc;

2) High frequency PCB board of radar antenna: The mainstream scheme of millimeter wave radar antenna is microstrip array, that is, the high-frequency PCB board is integrated on the common PCB board to realize the antenna function. It is necessary to maintain sufficient signal strength of the antenna in a small integration space.

1.3 Basic working principle of millimeter wave radar

1) High frequency circuit is used to generate electromagnetic wave with specific modulation frequency (FMCW), send and receive electromagnetic wave reflected from the target through antenna, and calculate various parameters of the target through sending and receiving electromagnetic wave parameters.

2) It can measure range, velocity and azimuth of multiple targets at the same time; Velocity measurement is based on Doppler effect, while azimuth measurement (including horizontal angle and vertical angle) is realized by antenna array.

Schematic diagram of basic working principle of millimeter wave radar

1.4 Operating system of millimeter wave radar

1) Working system: According to different radiation electromagnetic wave modes, millimeter wave radar mainly has two working systems: pulse system and continuous wave system. Continuous wave can be divided into FSK (frequency shift keying), PSK (phase shift keying), CW (constant frequency continuous wave), FMCW (frequency modulated continuous wave) and other modes.

Different operating systems of millimeter wave radar

2) Different modulation forms of FMCW FM CW radar:

a. Sine wave modulation b, sawtooth wave modulation c, triangle wave modulation

The hardware composition of radars with different FM modes is basically the same, with only a small number of circuit modules, circuit parameters and signal processing algorithms being different; For the measurement of a single static object, the sawtooth modulation mode can meet the requirements; For moving objects, triangular wave modulation is often used;

A Brief Introduction to the Principles of Ranging, Lateral Velocity and Azimuth Measurement of 1.5 Millimeter Wave Radar

Distance measurement: (TOF) The target distance is obtained by detecting the flight (round trip) time of the millimeter wave by continuously sending the millimeter wave signal to the target, and then receiving the millimeter wave returned from the object with the sensor.

Velocity measurement: according to the Doppler effect, the moving speed of the target relative to the radar can be obtained by calculating the frequency change of the radar wave returned to the receiving antenna. In short, the relative speed is proportional to the frequency change.

Azimuth measurement: the azimuth of the target is calculated by the phase difference of the radar wave reflected from the same target received by the parallel receiving antenna;

2. Analysis of 4D millimeter wave radar

2.1 What is 3D radar:

1) Its signal antenna is only arranged in two-dimensional direction, so its target detection has only two-dimensional horizontal coordinates (x, y) and no height information (z); In addition, the velocity information of the object detected by the Doppler effect. Output: X/Y/V

2) At present, the vehicle mounted millimeter wave radars used in mass production are all 3D radars;

2.2 4D radar: (Supplier: Aoku)

1) Antennas are arranged in both horizontal and vertical directions, so it can additionally detect the height of objects, which is called 4D; Output: output X, Y, Z coordinates and velocity vector;

2) It can detect moving objects at different heights and levels

2.3 Difficulties and problems encountered in the development of 4D radar:

1) On millimeter wave radars with volume requirements, the close arrangement of antennas in vertical and horizontal directions will cause serious signal interference to each other, which needs to be solved by algorithms developed through long-term experience accumulation;

2) With the increase of radar signal reception, the performance requirements for analog to digital converter (ADC) will increase;

3) The reliability and real-time performance of signal processing algorithms need to be guaranteed. The traditional millimeter wave radar ECU may not be competent for processing large-scale point clouds.

4) The demand for data storage will increase, and additional storage units are needed.

3. Comparative Analysis of 24GHz and 77GHz Millimeter Wave Radar

3.1 24GHz band

1) The frequency band from 24.0GHz to 24.25GHz is narrow band (NB), with a bandwidth of 250MHz. It is commonly used in industry, science and medicine. The 24GHz band also includes an ultra wideband (UWB) with a bandwidth of 5GHz.

Schematic Diagram of 24GHz Millimeter Wave Radar Application Frequency Band

2) Among short-range radars, the NB and UWB radars in the 24 GHz band have been applied to traditional automotive sensors. Usually, NB radar can complete simple applications such as blind spot detection, but in most cases, including ultra short distance, due to the need for high frequency resolution, UWB radar is required.

3) However, due to the spectrum rules and standards formulated by ETSI and FCC, the UWB band will soon be phased out. After January 1, 2022, the UWB band will not be used in Europe and the United States. Only the narrowband ISM band can be used for a long time.

3.2 77GHz band

1) The 76-77GHz frequency band can be used for remote vehicle mounted radar, and has the advantage of equivalent isotropic radiation power (EIRP), which can be used for front-end remote radar, such as adaptive cruise control.

Schematic diagram of application frequency band of 77GHz millimeter wave radar

2) 77-81GHz short range radar (SRR) band is a new band; This band has recently gained significant appeal in terms of global regulation and industry adoption.

3) This frequency band can provide a wide scanning bandwidth up to 4 GHz, which is very suitable for applications requiring high range resolution (HRR).

3.3 Performance comparison of on-board 24GHz and 77GHz millimeter wave radars:

1) With different frequencies, the wavelength of the 24GHz millimeter wave radar is greater than 10cm, which is strictly a centimeter wave radar;

2) Compared with 24GHz, 77GHz can simultaneously meet the requirements of high transmission power and wide working bandwidth, which enable it to achieve long-distance detection and high range resolution at the same time;

3) Compared with 24GHz, 77GHz has significant advantages in object resolution, velocity measurement and ranging accuracy;

4) Compared with 24GHz, 77GHz radar is smaller in size, and its wavelength is less than one third of 24GHz. Therefore, the area of the transceiver antenna is greatly reduced, and the overall radar size is effectively reduced;

4. Interpretation of velocity resolution, range resolution, angular accuracy and range accuracy of millimeter wave radar

4.1 Range resolution: (for two or more targets)

In the radar image, when two targets are located at the same azimuth, but the distance is different from that of the radar, the minimum distance between them is the range resolution. The range resolution of radar is determined by the pulse width, that is, the desired range resolution can be achieved by reducing the pulse width, which requires a large bandwidth.

4.2 Range accuracy: (used to describe the accuracy of radar range parameter estimation for a single target)

The ranging accuracy is determined by the SNR of the echo signal.

4.3 Angular resolution: (the azimuth resolution of radar generally refers to the azimuth resolution)

The ability of radar to distinguish adjacent targets in angle is usually measured by the smallest resolvable angle. The angle resolution of the radar depends on the operating wavelength of the radar λ And antenna aperture L, about λ/ (2L)。

For example, an azimuth resolution of 1.6 ° means that when two objects need to be at least 1.6 ° apart in space, they can be distinguished from each other by radar in horizontal angle. If the distance between two objects is less than 1.6 °, the two objects will coincide in the angular direction;

4.4 Angular accuracy: (used to describe the accuracy of radar azimuth estimation of a single target?)

Specific analysis will be supplemented later....

5. Interpretation of target characteristics of millimeter wave radar

5.1 Target detection capability has unique advantages:

a. The detection range and speed of longitudinal target are strong;

b. It can realize remote sensing and detection;

c. High precision measurement can be made for both static and dynamic targets;

5.2 Disadvantages of current millimeter wave radar target detection:

1) Unable to image, unable to recognize image color;

2) Low sensitivity to horizontal targets, for example, poor detection effect on crossing vehicles;

3) The pedestrian reflection wave is weak, the pedestrian resolution is not high, and the detection distance is short;

4) Poor detection effect for high objects and small objects;

5.3 Advantages of millimeter wave radar compared with camera and laser radar:

1) All weather and all day working characteristics - no matter day or night, it is not limited by weather conditions and can work normally even in rainy and snowy days;

2) Strong environmental adaptability, can still work normally in bad weather environment - strong penetration, rain, fog, dust, etc. have little interference to millimeter wave radar;

3) Strong speed measurement and distance measurement capability;