mmWave Radar Monitoring

How to use mmWave Radar monitoring?

1. Vital Signs mmWave Radar Monitoring

Vital signs are a set of medical parameters that indicate a person’s state of health and bodily function, provide clues to possible disease, and tendencies toward recovery or deterioration. There are four main vital signs: body temperature (BT), blood pressure (BP), respiratory rate (BR) and heart rate (HR).

These parameters vary by age, gender, weight and fitness level. Under certain conditions, these parameters may also vary due to a person’s physical or mental activity. For example, a person who is engaging in physical activity may have a high body temperature, respiratory rate, and heart rate.

2. mmWave Radar Monitoring Technology

Millimeter-wave (mmWave) radar emits electromagnetic waves, and any object in its path reflects the signal back. By capturing and processing the reflected signals, radar systems can determine the distance, velocity and angle of objects.

mmWave radar can provide millimeter-level accuracy in object distance detection, making it an ideal sensing technology for human biosignals. In addition, millimeter wave technology can also perform non-contact continuous monitoring of patients, so it is more convenient for both doctors and patients.

This page is discussing how mmWave radar can be used to monitor vital sign signals such as BR and HR.

What do respiratory rate and heart rate mean?

Generally, the vital sign parameters of a healthy person are shown in the table below:

3. Vital Signs of Healthy People

As mentioned earlier, the value of vital signs may vary with age, sex, fitness level, and physical or mental activity at the time of measurement. The combined analysis of these parameters (HR and BR) helps the healthcare provider to assess the health and stress level of the observed person. The table below shows resting heart rates for different age groups.

Static heart rate of different age groups.

The variation of heart rate  is under different physical or mental exertion conditions at the time of measurement.

Heart rate changes based on individual health, stress and medical conditions

Knowing heart and breathing rates can quickly diagnose some fatal conditions; such as Obstructive Sleep Apnea Syndrome (OSAS) and Sudden Infant Death Syndrome (SIDS). Patients with OSAS experience prolonged pauses in breathing during sleep, while SIDS refers to infants who may have obstructed breathing due to sleeping on their stomach or obstruction by a foreign object. Other breathing-related conditions include dyspnea and chronic obstructive pulmonary disease. See the diagram below for breathing patterns in various situations.

Breathing Pattern

People with a high resting heart rate have a higher risk of heart-related disease, while those with a low resting heart rate are at risk of needing a permanent pacemaker in the future, studies have shown.

Monitoring the respiratory rate and heart rate of patients suffering from the above diseases will potentially save their lives.

4. Contact and Non-contact Vital Sign  of  mmWave Radar Monitoring

Most of the existing measuring instruments are contact type. They need to be attached to the patient for measurement and monitoring. This is not very convenient for patients who need continuous monitoring for a long time. And, in the current COVID-19 pandemic, non-contact vital sign monitoring equipment may become even more important as it will help minimize the spread of the virus through touchpoints and contacts, better ensure the safety of healthcare personnel. Therefore, remote, non-contact instrumentation is our immediate need.

5. mmWave Radar Frequency

Millimeter wave radar, as the name suggests, is a radar technology that uses radio frequency microwaves with a wavelength of 10mm to 1mm and a frequency of 30-300 GHz. The radar spectrum for industrial applications is 60-64 GHz and for automotive applications is 76-81 GHz. Due to the shorter wavelength of the signal at these frequencies, the size of the radar antenna is also smaller. The small size of radar, coupled with advanced antenna technology, such as antenna on package (AoP) and PCB antenna (AoPCB), millimeter wave radar can be widely used in medical insurance.

6. Frequency Modulated Continuous Wave (FMCW) Radar

This article focuses on frequency modulated continuous wave (FMCW) radar. FMCW radar continuously transmits a frequency-modulated signal to measure the range, angle and velocity of a target object, while conventional pulsed radar systems transmit short pulses at regular intervals. For FMCW radar, the frequency of the signal increases linearly with time, and this signal is called a chirp.

Chirps in the time domain

An FMCW radar system sends a chirp signal and captures the signal reflected from objects in its path. Simplified block diagram of the main components of an FMCW radar system.

Block Diagram of FMCW Radar System

Among them, the “mixer” is used to mix the receiving end (RX) and transmitting end (TX) signals to generate intermediate frequency (IF) signals. The output of the mixer consists of two signals, the sum and frequency difference of the Rx and Tx chirp frequencies. There is also a low pass filter to limit the signal, allowing only the difference in frequency to pass.

The folloeinh figure shows the transmitted and received chirps in the frequency domain. If there are multiple objects at different distances, there will be multiple reflected chirps, each with a delay depending on how long it takes for the signal to return to the radar. For each reflected chirp, there will be a corresponding IF frequency.

Frequency Domain Representation of Tx and Rx Chirps and IF Frequency

Analyzing the spectrum of the IF signal reveals that each peak in the spectrum corresponds to one or more detected targets, while the frequency corresponds to the distance to the target.

According to the Doppler effect, as an object moves toward or away from the radar, the frequency and phase of the chirp it reflects changes. Since its wavelength is about 3.5 mm, any small change will result in a large phase change. Small frequency changes are not easy to detect, while large phase changes are easily detected. Therefore, in FMCW radar, the phase information is used to detect the velocity of the object.

To determine an object’s velocity, multiple chirps are used, the phase difference between successively reflected chirps is recorded, and the velocity is calculated from this.

7. How does mmWave Radar Monitoring to detect Vital Signs?

The advantage of short wavelengths is high precision. Millimeter-wave radar with a frequency of 60 or 77 GHz (corresponding to wavelengths in the range of 4 mm) is able to detect movements as short as less than 1 mm.

The folloeinh image shows a mmWave radar transmitting chirps into a patient’s chest area. Due to the movement of the chest, the reflected signal is phase modulated. Modulation covers all components of motion, including those caused by heartbeat and respiration.

The radar sends multiple chirps at predetermined intervals. Each pulse is subjected to a range Fast Fourier Transform (FFT), and the range bin corresponding to the position of the person’s chest is selected. Each chirp registers the phase of the signal in that selected range bin.

From this, the phase change and thus the velocity are calculated. The obtained velocity still includes all motion components. Spectral analysis of the obtained velocities by performing a Doppler FFT allows the various components to be resolved.

Heart rate (HR) and respiratory rate (BR) detection settings

The following image shows the HR and BR detection algorithms. The heart rate of an adult is between 0.8 and 2 Hz, and the breathing rate is between 0.1 and 0.5 Hz. In the Doppler FFT, the velocity components of the heartbeat and respiratory rates are selected and plotted against time. The number of peaks each frequency produces in a minute is the heart rate and respiratory rate.

HR and BR detection algorithm

8. Challenges of Vital Signs Monitoring with mmWave Radar

Vital sign monitoring using mmWave technology is still evolving. One of its main challenges is the difference in reflected signals between different people. Reflexes depend on skin type, tissue and its composition. The water content and various chemical compositions in the human body are also different. The industry’s ongoing research into changes in reflected signals will hopefully yield results to enable more precise measurements by radar.

9. Medical Applications of  mmWave Radar Monitoring

Recent advances in mmWave radar in the healthcare industry are also significant. Higher accuracy, high-speed signal processing capabilities, enhanced distance detection, and integration of radar into small form factor chipsets will likely greatly promote the development of healthcare applications such as patient activity monitoring, vital sign monitoring, etc.

In addition, mmWave radar will potentially be used to measure sleepiness, stress levels and human emotions, which has great implications for the development of healthcare monitoring systems.

Factors to pay attention to in the application:

• mmWave radar can penetrate certain materials such as plastics, cotton fabrics, clothing, veneer, glass, clothing, plasterboard, or materials with low density.
• mmWave radar is not affected by environmental conditions such as rain, fog, dust, and snow.
• mmWave radar cannot penetrate the human body, liquid, high-density metal, load-bearing walls, tiles, etc. to form an excellent micro-movement of the human body, changes in the chest cavity, changes in the abdomen, and accurate monitoring of the speed and angle of movement.
• The sensitivity of the mmWave radar wave is very high, and it will be interfered by shaking non-human movements, such as shaking metal, pets, and animals. Factors such as scene mode, sensitivity, and human activity range need to be adjusted to avoid interference.

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