Millimeter-wave radar, as a highly accurate and reliable sensing method, is now widely used in fields such as human body detection, vital sign monitoring, and behavioral analysis. The core principle of millimeter-wave detection is to utilize radio waves in the 1GHz to 300GHz frequency range. By detecting subtle changes in the echoes generated by the human body, millimeter-wave radar can non-contactly extract information such as the body's position, movement speed, subtle changes, and vital signs.

I. Why does millimeter-wave radar stand out among numerous sensing technologies?

1. Millimeter-wave radar has high resolution. Its high frequency and short wavelength mean that it can capture narrower beams of light of the same size, resulting in very high angular resolution. Furthermore, millimeter-wave radar has a very large bandwidth (several GHz), resulting in extremely high distance resolution, typically achieving centimeter-level accuracy. 2. Millimeter waves have extremely high penetration, easily penetrating common obstructions such as tables, chairs, wood, plastic, and some thin walls. However, they are highly reflective of human skin, making it easy to detect people hidden behind obstacles and even when wearing clothing.
3. Due to their extremely high resolution, millimeter waves are highly sensitive to subtle movements, easily detecting displacement changes at the millimeter or even micron level. Therefore, they are widely used in medical monitoring, such as respiratory and heart rate monitoring.
4. Millimeter waves are highly environmentally adaptable. Unlike optical sensors, they are not restricted by external lighting conditions and can operate unaffected by harsh environments such as strong sunlight, rain, and fog.

II. Core Detection Principles

Millimeter-wave radar detects human bodies based on the following two main physical principles:

1. Frequency-modulated continuous wave (FMCW) ranging and velocity measurement principles

Currently, mainstream millimeter-wave human detection radars on the market all use the FMCW system. The radar transmits a continuous wave signal (chirp signal) whose frequency increases linearly with time. When this signal collides with a human body, it is reflected by the body and then received by the receiving antenna. Because there is a frequency difference between the transmitted and received echo signals, this is also called beat frequency detection.
Distance Detection: The beat frequency is proportional to the target distance. By analyzing the beat frequency signal using a Fast Fourier Transform (FFT), the distance between the target and the detection source can be accurately calculated.
Speed Detection: When a person moves relative to the radar, the echo signal produces a Doppler shift proportional to the relative radial velocity. By analyzing the phase changes between multiple consecutive frequency spectra, the person's speed can be accurately measured.

2. Micro-Doppler Effect and Phase Detection

The micro-Doppler effect is a key requirement for millimeter-wave radar applications in vital sign monitoring and behavior recognition. Human life activities involve subtle movements, not completely static, and these subtle movements generate periodic echoes. Breathing causes centimeter-scale periodic displacements in the chest cavity, while heartbeats produce millimeter-scale microvibrations. These subtle periodic variations produce unique sidebands on the main Doppler shift frequency, known as micro-Doppler signatures. By analyzing the echo signal over a long period of time and at high resolution, these signatures can be extracted, allowing vital signs such as respiration rate and heart rate to be separated.

III. Typical Application Scenarios

1. Smart Home: Elderly fall detection, sleep quality monitoring, respiration and heartbeat detection, gesture monitoring;
2. Security Monitoring: Intrusion alarms, people counting;
3. Healthcare: Contactless continuous vital sign monitoring, comprehensive sleep and breathing screening;
4. In-vehicle Sensing: In-vehicle liveness detection, driver status monitoring;
5. Human-computer interaction: Gesture recognition, etc. IV. Current Limitations
1. Cost: High-performance millimeter-wave chips are still more expensive than ultrasonic and infrared chips.
2. Power Consumption: Millimeter-wave radar still consumes a bit too much power.
3. Interference Resistance: Strong reflections from multiple targets, metal objects, and other sources can cause false triggering.
4. Algorithm Dependency: High-precision detection relies heavily on advanced algorithms.
5. Regulatory Restrictions: Transmit power and frequency bands must strictly comply with radio regulations in each country.