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As the most important means of transportation for people, cars have a greater impact on their daily travel and lifestyle than anything else. Since the birth of the world's first three wheeled car in 1886, people have been carrying the dream of autonomous driving and have been committed to developing and innovating automotive technology.
Although true autonomous vehicles still need to wait for some time, Advanced Driver Assistance Systems (ADAS), which actively protect driving safety, are gradually maturing and becoming popular. ADAS mainly uses various sensors installed on the vehicle to collect data, perceive the surrounding environment at any time during the driving process, collect data, recognize, detect and track static and dynamic objects, and combine navigation map data for system calculation and analysis, so as to make the driver aware of possible dangers in advance and effectively increase the comfort and safety of car driving. At present, ADAS sensors that perceive the environment include cameras, ultrasonic sensors, and millimeter wave radars. Of course, autonomous vehicle also need on-board laser radar.
Lidar has always been favored by mainstream autonomous drivers for its ability to achieve 3D perception of the surrounding environment. However, whether it is LiDAR, cameras, or ultrasonic sensors, they are all easily affected by harsh weather conditions, which can lead to performance degradation or even failure (adverse weather conditions are often the main cause of high accident rates), thus all have "fatal" defects! At this time, millimeter wave radar has become one of the indispensable core sensors for automotive ADAS, thanks to its absolute advantage of penetrating dust, fog, rain, and snow, not being affected by adverse weather conditions, and its only super strong ability to work "24/7"!
millimeter wave radar
1. Millimeter wave radar - operates 24/7
Millimeter wave radar, as the name suggests, is a radar that operates in the millimeter wave frequency band. Millimeter Wave (MMW) refers to electromagnetic waves with a length of 1-10mm, corresponding to a frequency range of 30-300GHz. As shown in Figure 2, millimeter waves are located in the wavelength range where microwave and far-infrared waves overlap, so millimeter waves have both the advantages of these two spectra and their own unique properties. The theory and technology of millimeter waves are respectively the extension of microwaves to high frequencies and the development of light waves to low frequencies.
According to the theory of wave propagation, the higher the frequency, the shorter the wavelength, the higher the resolution, and the stronger the penetration ability. However, the greater the loss during propagation, the shorter the transmission distance; Relatively speaking, the lower the frequency, the longer the wavelength, the stronger the diffraction ability, and the farther the transmission distance. So compared to microwaves, millimeter waves have higher resolution, better directionality, stronger anti-interference ability, and better detection performance. Compared with infrared, millimeter waves have less atmospheric attenuation, better penetration through smoke and dust, and are less affected by weather. These characteristics determine that Millimeter wave radar has the ability to operate 24/7.
2. Frequency band division of atmospheric window and millimeter wave radar
Usually, water vapor and oxygen in the atmosphere have an absorption effect on electromagnetic waves. Currently, the vast majority of millimeter wave application research focuses on several "atmospheric window" frequencies and three "attenuation peak" frequencies. The so-called 'atmospheric window' refers to the high transmittance bands where electromagnetic waves are less reflected, absorbed, and scattered through the atmosphere. As shown in Figure 3, we can see that the "atmospheric window" where millimeter wave propagation is less attenuated is mainly concentrated around the 35GHz, 45GHz, 94GHz, 140GHz, and 220GHz frequency bands. There is a maximum attenuation peak near the 60GHz, 120GHz, and 180GHz frequency bands. Generally speaking, the "atmospheric window" frequency band is more suitable for point-to-point communication and has been adopted by low altitude air to ground missiles and ground-based radars, while the "attenuation peak" frequency band is preferred by covert networks and systems with multi-channel diversity to meet the requirements of network security factors.
At present, the frequency bands of vehicle mounted radar in major countries are mainly concentrated in three frequency bands: 24GHz, 60GHz, and 77GHz. Table 1 shows the frequency division of vehicle mounted radar in major countries. Among them, the wavelength of 24GHz is 1.25cm (although the wavelength of 24GHz is 1.25cm, it is still referred to as millimeter wave in the industry), 60GHz is 5mm, and the wavelength of 77GHz is even shorter, only 3.9mm. As mentioned earlier, the higher the frequency, the shorter the wavelength, and the higher the resolution and accuracy. So, higher precision 77GHz radar is striving to become the mainstream sensor in the automotive field.
3. Industrial layout of millimeter wave radar
The United States, Europe, and Japan are leading in the research of in vehicle radar technology. More and more companies and suppliers are investing in the development of automotive radar systems, device development, and algorithm research. From the perspective of the industrial layout of millimeter wave radar, the system is currently controlled by overseas giants such as Continental, Bosch, Hella, Delphi, Autoliv, etc. The core components are mainly monopolized by Infineon, Texas Instruments, STMicroelectronics, and Analog Devices. Compared to foreign companies, vehicle mounted millimeter wave radar is still in its infancy in China. In terms of 24GHz radar, a few domestic enterprises have already achieved research and development results, and market-oriented products are about to be launched; However, in the field of 77GHz millimeter wave radar, it is still in its early stages, and only a very small number of domestic enterprises can achieve the prototype stage of 77GHz radar. The industrialization process still needs to be breakthrough. However, in recent years, domestic innovation and entrepreneurship companies have gradually grown, such as Xingyidao Technology, Huayu Automotive, Hayan Technology, Zhibo Technology, Sensitech, Haomi Wave Technology, Yixing Transistor, Qingneng Huabo, Sijie Microelectronics, Jiatran Microelectronics, etc., and have achieved breakthroughs in some core technologies. We believe that breaking the monopoly of foreign enterprises is just around the corner!
4. The ranging and velocity measurement principles of millimeter wave radar
Radar is the transliteration of the English word RADAR, derived from the abbreviation of Radio Detection and Ranging, meaning "radio detection and ranging", which uses radio methods to discover targets and determine their spatial positions. This also reveals that the most important task of radar is to detect the distance, velocity, and direction of target objects.
The principle of millimeter wave radar ranging is very simple, which is to emit radio waves (millimeter waves), then receive the echo, and measure the position data and relative distance of the target based on the time difference between transmission and reception. According to the propagation speed of electromagnetic waves, the distance formula for the target can be determined as: s=ct/2, where s is the target distance, t is the time from the electromagnetic wave being emitted from the radar to receiving the target echo, and c is the speed of light.
Millimeter wave radar speed measurement is based on the principle of Doppler Effect. The so-called Doppler effect refers to the difference in frequency between the vibration received by the observer and the frequency emitted by the vibration source when the vibration sources such as sound, light, and radio waves move at a relative velocity v. Because this phenomenon was first discovered by Austrian scientist Doppler, it is called the Doppler effect. That is to say, when the emitted electromagnetic wave moves relative to the detected target, the frequency of the echo will be different from that of the emitted wave. When the target approaches the radar antenna, the reflected signal frequency will be higher than the transmitted signal frequency; On the contrary, when the target moves away from the antenna, the reflected signal frequency will be lower than the transmitted signal frequency, as shown in Figure 5. The frequency change formed by the Doppler effect is called Doppler shift, which is proportional to the relative velocity v and inversely proportional to the frequency of vibration. In this way, by detecting this frequency difference, the relative speed of the target relative to the radar can be measured, that is, the relative speed between the target and the radar. The distance to the target can be measured based on the time difference between the transmitted pulse and the received pulse.
ADAS
5. The main applications of millimeter wave radar in automotive ADAS
For vehicle safety, the most important criterion is the relative distance and relative speed information between two vehicles. Especially when driving at high speeds, if the distance between two vehicles is too close, it is easy to cause a car following accident. With excellent ranging and speed measurement capabilities, millimeter wave radar is widely used in automotive ADAS such as self adjusting cruise control (ACC), forward collision warning (FCW), blind spot detection (BSD), assisted parking (PA), and lane changing assistance (LCA).
Usually, in order to meet the detection needs of different distance ranges, a car will be equipped with multiple short-range, medium range, and long-range millimeter wave radars. The 24GHz radar system mainly achieves close range detection (SRR), while the 77GHz radar system mainly achieves medium to long range detection (LRR). Different Millimeter wave radar have their own functions, playing different roles in the front, body, and rear of the vehicle.
6. Other applications of millimeter wave radar
Millimeter wave radar plays a very important role not only in automotive ADAS applications, but also in unmanned aerial vehicles, security, intelligent transportation, industry, and military fields.
Drones: Their main applications are reflected in two aspects: height control and obstacle avoidance.
Security: mainly used for security surveillance in important areas.
Smart transportation: mainly used for vehicle detection, traffic volume investigation, traffic incident detection, traffic guidance, speeding monitoring, electronic checkpoints, electronic police, and traffic light control.
Industry: mainly used for industrial level gauges, forklifts, heavy-duty bulldozers, safe construction near high-voltage power towers, production safety monitoring, etc.
Military: mainly used in radar detection, missile guidance, satellite remote sensing, electronic countermeasures, etc.
7. Development of Smart Millimeter Wave Radar
Millimeter wave radar, as one of the most core sensors in automotive ADAS, currently has the biggest "drawback" of being unable to identify pedestrians and accurately model surrounding obstacles due to its low resolution. High resolution intelligent radar sensors are crucial for achieving advanced autonomous driving. So some millimeter wave radar companies are focusing on developing imaging technology for radar.
In order to open the eyes of radar, various companies have shown their talents and boldly innovated using different technologies. Among them, the most outstanding ones are the new generation imaging radar product WARLORD from Metawave International and the Ultres system from Arbe Robotics. The former adopts a new type of super data antenna, which can emit highly directional electromagnetic beams that can be controlled. At the same time, an AI engine is embedded in the radar product to achieve the discovery, recognition, tracking, and classification of objects; The latter's radar scheme is based on mathematical algorithms for synthetic aperture radar (SAR) imaging technology. SAR imaging technology refers to the use of wide bandwidth transmission signals to achieve high range resolution and relative motion equivalent long synthetic arrays to achieve high azimuth resolution. Although these imaging technologies still have some areas to be improved, they have made good breakthroughs, and I believe they will play an important role in the upcoming L4 and L5 autonomous vehicle.
8. Prospects of Millimeter Wave Radar
Although the widespread use of autonomous vehicles on the road is still far away, in addition to technological and cost factors, there are still relevant legal and ethical issues that need to be resolved. However, as a "primary form" of autonomous driving, ADAS can already allow us to experience the joy of future autonomous vehicles in specific environments! Various types and levels of autonomous driving technology will present a common development, covering different market demands and business models.
Millimeter wave radar, cameras, LiDAR and other sensors have their own advantages and disadvantages. In order to ensure safety always comes first, the fusion of multiple sensors is the trend, which also provides necessary technological reserves for the implementation of higher-level autonomous driving solutions. With the continuous improvement of innovative intelligent 3D imaging radar technology, it can even be hoped that millimeter wave radar can partially replace expensive laser radar. In short, whether it is the current ADAS, higher-level autonomous driving, or even the ultimate unmanned driving, Millimeter wave radar, as the only sensor that can work 24/7, will be an indispensable environmental perception sensor.