[A Brief Analysis of the Concept and Working Principle of Millimeter-Wave Radar Sensors] Whether it's lidar, cameras, or ultrasonic sensors, they are all susceptible to performance degradation or even failure due to harsh weather conditions, thus possessing a "fatal" flaw! However, millimeter-wave radar, with its absolute advantages of penetrating dust, fog, rain, and snow, and being unaffected by harsh weather, and its unique ability to work "all day and all night," has become one of the indispensable core sensors for automotive ADAS! In this article, we'll take a "close look" at millimeter-wave radar to understand its concept and working principle.
Technology changes lives, but safety always comes first.
As the most important means of transportation, automobiles have a profound impact on people's daily travel and lifestyles. Since the birth of the world's first three-wheeled automobile in 1886, people have harbored the dream of driverless cars and have been committed to developing and innovating automotive technology. We are now delighted to see some driverless concept cars emerge, but the fatal accident involving an Uber self-driving car hitting a pedestrian this March cast a shadow over the booming development of driverless technology. It also made people realize that while technology brings convenience and speed, we must always remember that "safety always comes first!"
Figure 1. The world's first three-wheeled car and the Smart self-driving concept car.
According to MEMS Consulting, while truly driverless cars are still some time away, Advanced Driver Assistance Systems (ADAS), which actively protect driving safety, are gradually maturing and becoming more widespread. ADAS primarily utilizes various sensors installed on the vehicle to collect data, constantly perceiving the surrounding environment during driving, collecting data, identifying, detecting, and tracking static and dynamic objects, and combining this with navigation map data for system calculations and analysis. This allows the driver to anticipate potential dangers, effectively increasing driving comfort and safety. Currently, ADAS sensors for environmental perception include cameras, ultrasonic sensors, and millimeter-wave radar. Of course, autonomous vehicles also require onboard LiDAR. LiDAR has long been favored by mainstream autonomous driving proponents due to its ability to achieve 3D perception of the surrounding environment. However, whether it's LiDAR, cameras, or ultrasonic sensors, they are all susceptible to performance degradation or even failure due to adverse weather conditions (adverse weather conditions are often a major cause of accidents), thus possessing a "fatal" flaw! In such situations, millimeter-wave radar, with its absolute advantages of penetrating dust, fog, rain, and snow, and being unaffected by severe weather, and its unique ability to work "all day and all night," has become one of the indispensable core sensors for automotive ADAS! Below, we'll take a "close look" at millimeter-wave radar to understand its concept and working principle.
Millimeter-wave radar – operates in all weather and all time
Millimeter-wave radar, as the name suggests, is radar that operates in the millimeter-wave frequency band. Millimeter waves (MMW) refer to electromagnetic waves with a length of 1-10 mm, corresponding to a frequency range of 30-300 GHz. As shown in Figure 2, millimeter waves lie in the wavelength range where microwaves and far-infrared waves overlap, thus possessing the advantages of both wavelengths while also having their own unique properties. The theory and technology of millimeter waves represent the extension of microwaves to higher frequencies and the development of light waves to lower frequencies.
Figure 2 Electromagnetic spectrum
According to wave propagation theory, higher frequency and shorter wavelength result in higher resolution and stronger penetration, but also greater propagation losses and shorter transmission distances. Conversely, lower frequency and longer wavelength result in stronger diffraction capabilities and longer transmission distances. Therefore, compared to microwaves, millimeter waves offer higher resolution, better directivity, stronger anti-interference capabilities, and better detection performance. Compared to infrared, millimeter waves experience less atmospheric attenuation, have better penetration through smoke and dust, and are less affected by weather conditions. These characteristics determine that millimeter-wave radar has all-weather, all-day operational capabilities.
Atmospheric windows and frequency band allocation for millimeter-wave radar
Water vapor and oxygen in the atmosphere typically absorb electromagnetic waves. Currently, most millimeter-wave application research focuses on a few "atmospheric windows" and three "attenuation peaks." An "atmospheric window" refers to a band of electromagnetic waves with high transmittance where they are less reflected, absorbed, and scattered by the atmosphere. As shown in Figure 3, the "atmospheric windows" where millimeter-wave propagation experiences less attenuation are mainly concentrated around the 35GHz, 45GHz, 94GHz, 140GHz, and 220GHz bands. Attenuation reaches its maximum near the 60GHz, 120GHz, and 180GHz bands, forming the "attenuation peaks." Generally speaking, the "atmospheric window" bands are more suitable for point-to-point communication and have been adopted by low-altitude air-to-ground missiles and ground-based radars, while the "attenuation peak" bands are preferentially selected by multi-path diversity covert networks and systems to meet network security requirements.
Figure 3. Millimeter wave at different frequency bands
Atmospheric attenuation trend chart. Currently, the frequency bands of automotive radar in major countries are mainly concentrated in three bands: 24GHz, 60GHz, and 77GHz, as shown in Table 1. 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 and the shorter the wavelength, the higher the resolution and accuracy. Therefore, the more accurate 77GHz radar is striving to become the mainstream sensor in the automotive field.
Table 1. Frequency Allocation of Vehicle-Mounted Radar in Major Countries
Industrial layout of millimeter-wave radar
The United States, Europe, and Japan are leading the way in automotive radar technology research. More and more companies and suppliers are now investing in automotive radar system development, component development, and algorithm research. Looking at the industry landscape of millimeter-wave radar, the system is currently controlled by overseas giants such as Continental, Bosch, Hella, Delphi, and Autoliv, while core components are mainly monopolized by Infineon, Texas Instruments (TI), STMicroelectronics (ST), and Analog Devices (ADI). Compared to foreign companies, automotive millimeter-wave radar in China is still in its early stages. In the 24GHz radar field, a few domestic companies have achieved research and development results, and commercial products are about to be launched; however, in the 77GHz millimeter-wave radar field, it is still in its initial stage, with only a very few domestic companies able to reach the prototype stage, and industrialization still requires breakthroughs. However, in recent years, the number of domestic innovative and entrepreneurial companies has gradually increased, such as Xingyidao Technology, Huayu Automotive, Falcon Eye Technology, Zhibo Technology, Senstech, Haomibo Technology, Yixing Semiconductor, Qingneng Huabo, Sijie Microelectronics, and Gateland Microelectronics, and has achieved breakthroughs in some core technologies. It is believed that breaking the monopoly of foreign companies is just around the corner!
Figure 4. Panoramic view of the global millimeter-wave radar industry chain
The ranging and velocity measurement principles of millimeter-wave radar
Radar, a transliteration of the English word RADAR, is an abbreviation of Radio Detection and Ranging, meaning "radio detection and ranging." It uses radio waves to detect targets and determine their spatial location, revealing that radar's most important task is to detect the distance, speed, and direction of target objects. The principle of millimeter-wave radar ranging is simple: it transmits radio waves (millimeter waves), receives the echo, and measures the target's position and relative distance based on the time difference between transmission and reception. Based on the speed of electromagnetic wave propagation, the formula for determining the target distance is: s = ct/2, where s is the target distance, t is the time from the radar's transmission to the reception of the target's echo, and c is the speed of light. Millimeter-wave radar velocity measurement is based on the Doppler effect. The Doppler effect states that when a vibration source such as sound, light, or radio waves moves relative to an observer at a speed v, the frequency of the vibration received by the observer differs from the frequency emitted by the vibration source. This phenomenon was first discovered by the Austrian scientist Doppler, hence the name Doppler effect. In other words, when there is relative movement between the emitted electromagnetic wave and the detected target, the frequency of the echo will differ from the frequency of the emitted wave. When the target approaches the radar antenna, the frequency of the reflected signal will be higher than the frequency of the emitted signal; conversely, when the target moves away from the antenna, the frequency of the reflected signal will be lower than the frequency of the emitted signal, as shown in Figure 5. The frequency change caused by the Doppler effect is called the Doppler shift, which is directly proportional to the relative velocity v and inversely proportional to the frequency of vibration. Thus, by detecting this frequency difference, the target's speed relative to the radar, i.e., the relative speed between the target and the radar, can be measured. Based on the time difference between the emitted and received pulses, the target's distance can be determined.
Figure 5 Doppler effect
The primary application of millimeter-wave radar in automotive ADAS (Advanced Driver Assistance Systems) is crucial for vehicle safety. The most important basis for judgment is the relative distance and speed between two vehicles, especially at high speeds, where close proximity can easily lead to rear-end collisions. Thanks to its excellent distance and speed measurement capabilities, millimeter-wave radar is widely used in automotive ADAS systems such as Adaptive Cruise Control (ACC), Forward Collision Warning (FCW), Blind Spot Detection (BSD), Parking Assist (PA), and Lane Change Assist (LCA). Typically, to meet the detection needs of different distance ranges, a single vehicle will be equipped with multiple short-range, medium-range, and long-range millimeter-wave radars. The 24GHz radar system primarily achieves short-range detection (SRR), while the 77GHz radar system primarily achieves medium-to-long-range detection (LRR). Different millimeter-wave radars perform different functions, acting in front of, behind, and behind the vehicle.
Figure 6. Main applications of millimeter-wave radar in automotive ADAS.
Other applications of millimeter-wave radar
Millimeter-wave radar plays a crucial role not only in automotive ADAS applications but also in drones, security, intelligent transportation, industry, and the military. • Drones: Primarily used for altitude hold and obstacle avoidance. • Security: Primarily used for security surveillance in critical areas. • Intelligent Transportation: Primarily used for vehicle detection, traffic volume surveys, traffic incident detection, traffic guidance, speeding detection, electronic checkpoints, electronic police systems, and traffic light control. • Industry: Primarily used for industrial level gauges, excavators, heavy bulldozers, safe construction near high-voltage power line towers, and production safety monitoring. • Military: Primarily used for radar detection, missile guidance, satellite remote sensing, and electronic warfare.
Smart millimeter-wave radar development
Millimeter-wave radar, as one of the core sensors in automotive ADAS, currently suffers from a major drawback: its low resolution makes it unable to identify pedestrians and accurately model surrounding obstacles. High-resolution intelligent radar sensors are crucial for achieving advanced autonomous driving. Therefore, some millimeter-wave radar companies are focusing on developing radar imaging technology. To "open the eyes" of radar, various companies are employing different technologies and bold innovations. Among the more prominent examples are Metawave's next-generation imaging radar product WARLORD and Arbe Robotics' Ultres system. The former uses a novel metamaterial antenna capable of emitting a controllable, highly directional electromagnetic beam, and embeds an AI engine into its radar product to achieve object detection, identification, tracking, and classification. The latter's radar solution is based on synthetic aperture radar (SAR) imaging technology, which utilizes a large bandwidth transmitted signal to achieve high resolution in the range direction and a relatively motion equivalent long synthetic array to achieve high resolution in the azimuth direction. Although these imaging technologies still have some areas for improvement, they have made significant breakthroughs and are expected to play an important role in Level 4 and Level 5 autonomous vehicles in the near future.
Figure 7. Intelligent radar achieves imaging.
Conclusion
While widespread deployment of driverless cars is still a long way off, due to technological and cost factors, as well as unresolved legal and ethical issues, ADAS (Advanced Driver Assistance Systems), as a "basic form" of autonomous driving, already allows us to experience the fun of future driverless vehicles in specific environments! Various types and levels of autonomous driving technologies will develop together, each covering different market demands and business models. Millimeter-wave radar, cameras, and LiDAR each have their advantages and disadvantages. To ensure safety is always the top priority, multi-sensor fusion is the inevitable trend, providing the necessary technological reserves for realizing higher-level autonomous driving solutions. With the continuous improvement of innovative intelligent 3D imaging radar technology, it's even possible to hope that millimeter-wave radar can partially replace expensive LiDAR. In short, whether it's the current ADAS, higher-level autonomous driving, or even the ultimate driverless car, millimeter-wave radar, as the only sensor capable of working around the clock, will be an indispensable environmental perception sensor, safeguarding our travel safety!
