mmWave Radar Sensor ModulesWhat is mmWave radar? Understanding the mechanism and features of mmWave radar from the basics

The frequencies which are available in mmWave radar lie within the mmWave band, which encompasses radio waves with a wavelength ranging from 1 mm to 10 mm and frequencies between 30 GHz and 300 GHz. In terms of features, these signals provide strong directivity and directionality, which makes it easy to emit radiation narrowed to a specific range, and possess characteristics that minimize interference with other types of radio waves.

A radar sensor is a sensing device that emits radio waves at a target and receives the reflected waves to primarily detect the distance, angle, and velocity information of the target.

Due to the extremely short wavelength of the radio waves that it uses, mmWave radar can perform detailed observations as well as distinguish and detect multiple objects. Furthermore, they are capable of demonstrating stable performance even in inclement weather and accurate detection in low-visibility environments. In addition, recent advances in semiconductor technologies are expected to make mmWave radar more compact and inexpensive, making them suitable for introduction in a wider range of fields.
As a result, mmWave radar is currently attracting particular attention as a sensing technology, because it is environmentally resistant, offers high-precision detection capabilities, and is undergoing accelerated adoption thanks to technology innovations.

The wavelength of radio waves in the mmWave band is extremely short and ranges from 1 mm to several millimeters. Because it can secure a wider bandwidth, mmWave radar, which uses mmWaves, can distinguish differences in distance and position to a finer degree and also separately detect multiple objects.

Radio waves in the mmWave band can propagate on a larger scale than fine particles such as rain, fog, and dust. As a result, the effects of scattering and absorption by these fine particles are comparatively small, and the radio wave attenuation can be reduced. Another feature of the mmWave frequency band is that the absorption by water molecules in rain drops and fog is often lower than visible light.

Typical infrared sensors are limited primarily to temperature change and presence detection, but mmWaves can measure distance and speed.

Because ultrasonic sensors use sound waves, the reflections are easily disturbed by the effects of environmental noise, and they are primarily used for short-range applications. However, mmWave radar is capable of long-distance detection.

Cameras are dependent on the light source conditions, and their performance can significantly decrease due to darkness, backlighting, and fog, etc. However, mmWave radar uses radio waves and can function even in dark places or at night.

LiDAR uses laser light, which makes it susceptible to rain, fog, and dust, and causes the detection performance to decrease. However, mmWaves have longer wavelengths, which makes them less susceptible to such effects.

Radar works by emitting radio waves at a target, receiving the radio waves that return from the target as reflected waves, and measuring information about the target from those reflected waves.

mmWave radar uses FMCW and pulse methods to detect distance, angle, and speed.

The “FM” in FMCW method stands for “Frequency Modulation,” which changes the frequency of the transmission signal over time. A signal with a frequency that increases or decreases over time is called a “chirp signal.” The CW (Continuous Wave) method uses continuous waves and continuously emits radio waves for a set period of time. In this method, the reflected wave, which is the same as the transmitted signal that bounces back from the target, becomes a signal with a frequency that changes over time, which makes it possible to calculate the speed and distance from the target from the frequency difference between the transmitted and reflected waves. Continuous Wave is used in a broader range of applications as a method that can detect speed.

The pulse method emits a short, pulse-like transmission signal and uses the time it takes to receive the reflected waves that bounce back from the target to calculate the distance, etc., to the target. This is a simple detection method that enables a comparatively simple circuit configuration and is often used when high precision is not required.

The following is the typical configuration of an FMCW mmWave radar.
The transmitting side consists of a synthesizer that generates the millimeter wave radio waves to be transmitted and a transmission antenna that emits the transmission waves (chirp signal). The receiving side has a receiving antenna to receive the reflected waves, a mixer that multiplies the received and transmitted waves to create a frequency difference, an A/D (Analog-Digital) converter that converts the mixed signal to a digital signal, and a DSP (Digital Signal Processing) processor, which is a circuit that processes digital signals.

The time required to receive the reflected wave after emitting the transmitted wave varies depending on the distance to the target.
When calculating the frequency difference between the transmitted wave (chirp signal), which has a frequency that increases or decreases over time, and the wave received as the reflected wave, that frequency difference varies based on the distance to the target.
Therefore, when digitizing the IF (Intermediate Frequency) signal formed through mixing and performing a Fourier transform using DSP, that frequency difference is obtained as a frequency spectrum, and the distance information can be calculated from that result.

When receiving the reflected waves after emitting the transmitted waves (chirp signal), a phase difference occurs when comparing the received waves that arrive at multiple receiving antennas. This is because the distance to the target differs between each of the receiving antennas. The angle information can be calculated from this phase difference and the distance differences.

The speed is detected by receiving the reflected waves after emitting the transmitted waves (chirp signal) and using the phase of the IF (Intermediate Frequency) signal formed through mixing. When an object is moving at a speed, the phase differences between the IF signals in the pre- and post-movement chirp signals are due to the time difference before and after the movement, so the time difference of the moving object can be determined from the phase difference, and the speed information can be calculated as a result.

Because the radio waves that can be used for mmWave radar have an extremely broad frequency band and little spatial attenuation, they can be emitted over long distances. Therefore, the distance resolution needed to distinguish between objects is extremely small, which makes long-distance detection possible, so high-performance detection is one advantage of mmWave radar.

The mmWave radio waves have an extremely high degree of directionality and low diffusion compared to infrared and ultrasonic waves, which makes them less susceptible to being influenced by the environment. Therefore, the impact of changes in light illuminance due to weather or time variations is small, and the effect on detection performance is also small, which is an advantage.

For simple distance measurements, using existing infrared and ultrasonic sensors makes circuit design easier and less expensive. If the penetration of radio waves is used, it increases the flexibility of the cover that shields the front of the sensor. However, the difficulty of handling high-frequency signals in the mmWave band also increases. High-performance detection is also possible with the laser and camera methods. However, when the system cost is considered, mmWave radar offers a sensor that makes it easier to balance low cost with high detection performance.