“Vital signs are a set of medical parameters indicative of a person’s health and bodily functioning, which provide possible clues to disease and a tendency to recover or worsen. 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 physically active may have a higher body temperature, respiratory rate, and heart rate.
Vital signs are a set of medical parameters indicative of a person’s health and bodily functioning, which provide possible clues to disease and a tendency to recover or worsen. 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 physically active may have a higher body temperature, respiratory rate, and heart rate.
Millimeter-wave (mmWave) radar emits electromagnetic waves, and anything in its path reflects the signal back. By capturing and processing reflected signals, radar systems can determine the distance, velocity and angle of objects. Millimeter-wave radar can provide millimeter-level accuracy in object distance detection, making it an ideal sensing technology for human biological signals. 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 article will discuss how mmWave radar can be used to monitor vital sign signals such as BR and HR.
What do breathing rate and heart rate mean?
Generally, the vital sign parameters of healthy people are shown in the following table (1):
Table 1: Vital Signs of Healthy People
As mentioned earlier, the values of vital signs may vary with age, gender, fitness level, and physical or mental activity at the time of measurement. A comprehensive analysis of these parameters (HR and BR) helps healthcare practitioners assess the health and stress levels of the observed. The table below shows the resting heart rate for different age groups.
Table 2: Resting heart rate in different age groups (Source: Wikipedia)
The graph below (Fig. 1) shows the variation in heart rate under different physical or mental input conditions at the time of measurement.
Figure 1: Heart rate as a function of an individual’s health, stress and medical conditions (Source: AAAI)
Knowing heart rate and breathing rate can quickly diagnose some life-threatening conditions; such as Obstructive Sleep Apnea Syndrome (OSAS) and Sudden Infant Death Syndrome (SIDS). OSAS patients have prolonged pauses in breathing during sleep, while SIDS refers to babies who may be obstructed breathing due to sleeping on their stomachs or obstructions by foreign objects. Other breathing-related diseases include dyspnea and chronic obstructive pulmonary disease. See the diagrams below for breathing patterns in various situations.
Figure 2: Breathing Pattern (Source: Clinicalgate)
Studies have shown that people with a high resting heart rate have a higher risk of heart-related diseases, while those with a low resting heart rate may need a permanent pacemaker in the future.
The monitoring of respiratory rate and heart rate in patients with these diseases could potentially save their lives.
Contact and non-contact vital signs measurement
Most of the existing measuring instruments are of the contact type. They need to be attached to the patient in order to be measured and monitored. This is not very convenient for patients who require continuous monitoring for long periods of time. And, in the current COVID-19 pandemic, contactless vital signs 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 health care workers. Therefore, remote, non-contact instruments are our immediate need.
Millimeter wave radar
Millimeter-wave radar, as the name suggests, is a radar technology that utilizes radio frequency microwaves with a wavelength of 10mm to 1mm and a frequency of 30-300 GHz. The radar spectrum is 60-64 GHz for industrial applications and 76-81 GHz for automotive applications. Since the wavelength of the signal at these frequencies is shorter, the size of the radar antenna is also smaller. The small size of radar, coupled with advanced antenna technologies such as Antenna-on-Package (AoP) and Antenna-on-PCB (AoPCB), enables mmWave radars to be widely used in automotive navigation, building automation, healthcare and industrial applications.
The focus of this article is on frequency modulated continuous wave (FMCW) radar. FMCW radars continuously transmit frequency-modulated signals to measure the distance, angle, and velocity of target objects, while traditional pulsed radar systems send short pulses at regular intervals. For FMCW radars, the frequency of the signal increases linearly with time, and this signal is called a chirp (Figure 3).
Figure 3: Chirp in the time domain
An FMCW radar system sends a chirp signal and captures the signal reflected by objects in its path. Figure 4 is a simplified block diagram of the main components of an FMCW radar system.
Figure 4: FMCW radar system block diagram (Source: TI)
The “mixer” is used to mix the receive (RX) and transmit (TX) signals to produce an intermediate frequency (IF) signal. The output of the mixer consists of two signals, the sum of the Rx and Tx chirp frequencies and the difference in frequency. There is also a low-pass filter to limit the signal, allowing only the difference in frequency to pass.
Figure 5 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 that depends on how long it takes the signal to return to the radar. For each reflected chirp, there is a corresponding IF frequency.
Figure 5: Frequency Domain Representation of Tx and Rx Chirps and IF Frequency
Analysis of the IF signal spectrum shows 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 reflected chirps change. Since its wavelength is about 3.5mm, any small change will result in a large phase change. Small frequency changes are not easy to detect, while large phase changes are easy to detect. Therefore, in FMCW radar, the phase information is used to detect the velocity of the object. To determine object velocity, multiple chirps are used, the phase difference between successive reflected chirps is recorded, and velocity is calculated from this.
How does mmWave radar detect vital signs?
The advantage of short wavelengths is high precision. Millimeter-wave radars at frequencies of 60 or 77 GHz (corresponding to wavelengths in the 4 mm range) are able to detect movements as short as less than 1 mm.
Figure 6 shows a mmWave radar transmitting chirps to the patient’s chest region. The reflected signal is phase modulated due to the motion of the chest. Modulation covers all components of motion, including those caused by heartbeat and breathing. The radar transmits multiple chirps at predetermined time intervals. A distance fast Fourier transform (FFT) is performed on each pulse, and a distance bin corresponding to the position of a person’s chest is selected. Each chirp records the phase of the signal in that selected distance bin. From this, the phase change and thus the velocity are calculated. The obtained velocity still includes all motion components. The various components can be resolved by performing a spectral analysis of the obtained velocity by performing a Doppler FFT.
Figure 6: Heart Rate (HR) and Respiratory Rate (BR) Detection Settings
Figure 7 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 one minute is the heart rate and breathing rate.
Figure 7: HR and BR detection algorithms
Challenges for Vital Signs Monitoring with Millimeter-Wave Radar
The use of millimeter wave technology for vital signs monitoring is still in development. 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 components in the human body are also different. The industry’s ongoing research into reflected signal changes will hopefully yield results to enable more precise measurements by radar.
The main applications of millimeter-wave radar are concentrated in the defense, automotive and industrial fields. However, its recent advances in the healthcare industry are also of great importance. Higher accuracy, high-speed signal processing capabilities, enhanced distance detection, and integration of radar into small form factor chipsets will likely greatly advance healthcare applications such as patient activity monitoring, vital signs monitoring, and more. Additionally, millimeter-wave radar will likely be used to measure sleepiness, stress levels, and human mood, with significant implications for driver monitoring system development in healthcare and automotive applications.