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1. Definition of millimeter wave radar sensor
Radar is the abbreviation of Radio detection and ranging. The commonly used automotive-grade radar frequencies are 24GHz and 77GHz, and the corresponding wavelengths are 12mm and 4mm. Why use these two frequencies instead of other frequencies of millimeter waves? Don't think about it, it's not because of any special technical reasons, it's purely because of insufficient spectrum resources.
Each frequency band of electromagnetic waves will be allocated to specific equipment or industries according to regulations to prevent mutual interference between equipment signals. This allocation work is formulated by the International Telecommunication Union_Baidu Encyclopedia (baidu.com). The current frequency band allocation is very tight. It is not difficult to understand if you think about the competition among countries for 5G frequency bands in the past few years. 24GHZ and 77GHz are the frequency bands specially allocated by the International Telecommunication Union for automotive radars.
Millimeter wave radar sensor technology is relatively mature, especially at the military level. The current constraints on automotive-grade millimeter wave radars are mainly the balance between performance, volume and cost.
2. Basic Principles of Radar
The principle of radar is very simple and easy to understand: emit electromagnetic waves to the target -> receive reflected echo signals -> measure the time, phase and other signals of the echo -> calculate the distance, speed and angle information of the target. Its measurement indicators are nothing more than range and resolution.
2.1 Antenna
According to the mechanism of electromagnetic waves generated by alternating current, radar first needs a device that can generate high-frequency alternating current. This part uses a voltage-controlled oscillator (VCO). The output oscillation frequency will change with the input voltage, and then the high-frequency current is converted into electromagnetic waves emitted into space using the transmitting antenna. The receiving antenna and the transmitting antenna are reciprocal, and the antenna can also convert electromagnetic waves in space into alternating electrical signals.
2.2 Distance Measurement: Pulse and FMCW
Pulse Radar
The distance measurement method is easy to think of. The radar emits a pulse and measures the "time of flight—ToF" from the pulse signal from emission to return. This method is very common in laser radar.
Frequency Modulated Continuous Wave (FMCW)
Of course, in reality, automotive radar does not use this method, but uses "Frequency Modulated Continuous Wave (FMCW)". The frequency of the electromagnetic wave emitted by the radar changes linearly (note: it is a linear change in frequency), so its echo signal also changes linearly. By measuring the difference between the current transmitting frequency and the receiving frequency, the distance of the detected object can be obtained.
millimeter wave radar sensor
2.3 Speed measurement: Doppler effect
The radar's measurement of the speed of a moving object is based on the Doppler principle (Christian Johann Doppler, Austria, 1842). In short: when the observer moves toward the wave source, the wave is compressed and the observed frequency becomes higher; when the observer moves away from the wave source, the wave is stretched and the observed frequency becomes lower. Recall the content of high school class: when the train is coming towards you, the whistle is sharp; when the train is moving away from you, the whistle is low.
2.4 Angle measurement: big pot cover vs MIMO
An important indicator for measuring angle measurement is angular resolution, that is, the size of the angle between the two smallest discernible targets. For example, if the radar lateral angular resolution is 1°, it means that within a range of 100m, the radar's "pixel" size is 1.74m. If there are two adjacent 0.5m targets at this time, the radar cannot identify them.
Autonomous driving requires the highest possible angular resolution, what should we do? A simple idea is to make the emitted electromagnetic wave beam as narrow as possible and highly directional. At this time, the radar angular resolution is the beam width of the electromagnetic wave, and the field of view within a certain range can be achieved with the scanning mechanism. This is the principle of laser radar.
However, this is extremely difficult for ordinary radars. The radiation waveform of a simple point source antenna is close to a spherical wave. We need to design the antenna to reduce its lobe width (large antenna gain and long propagation distance). The beam width of a general antenna is: , where D is the antenna aperture. The larger the physical size of the antenna, the stronger the radar beam directivity. And the antenna cannot achieve highly consistent directivity like a laser.
Schematic diagram of directional antenna radiation, the larger the aperture, the better the directivity
It is obviously impossible for a car to drive with a large pot cover scanning radar. In fact, automotive radars often use patch antennas (CMOS technology), which are cheaper and smaller, and their beam widths are generally between 30-60°. The solution of measuring angles by limiting the beam width is naturally bankrupt.
In a word,millimeter wave radar sensor technology is relatively mature, especially at the military level.