mmWave radar for vital signs monitoring

Vital signs are a set of medical parameters that indicate the status of health and body functions of a person. They give clues to possible diseases and trends of recovery or deterioration. There are four primary vital signs: body temperature (BT), blood pressure (BP), breath rate (BR) and heart rate (HR). Vital signs vary from person to person based on age, gender, weight and fitness level. These signs may also vary based on the physical or mental engagements of a person in a given situation. For instance, someone engaged in a physical activity can show high body temperature, breath rate and heart rate.

Millimeter wave (mmWave) radars transmit electromagnetic waves and any objects in the path reflect the signals back. By capturing and processing the reflected signals, a radar system can determine the range, velocity and angle of the objects. The potential of mmWave radar to provide millimeter level precision in object range detection makes it an ideal technology for sensing human bio-signals. In addition, mmWave technology brings in an advantage of contactless, continuous surveillance of a patient, making it more convenient for the person and the user.

In this article we discuss how mmWave radar can be used to monitor vital signs such as BR and HR.

What do BR and HR Vital Signs indicate?

Typically, vitals of a healthy person are as given in the table below (1):

Table 1: Vitals of a Healthy Person

These values, as mentioned earlier, may vary according to age, gender, fitness level and physical or mental activity at the time of measurement. A combined analysis of these parameters (HR and BR) help a health care professional to assess the health and stress levels of a person under observation. Resting heart rate of people at various age group is shown in the table below.

Table 2: Age-wise Resting Heart Rate (Source: https://en.wikipedia.org/wiki/Heart_rate#Resting_heart_rate)

Figure 1 below show variation in HR based on the physical or mental engagement of the person at the time of measurement.

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Figure 1: Variation of Heart Rate based on individual’s fitness, stress and medical states (Source: https://www.aaai.org/ocs/index.php/AAAI/AAAI18/paper/view/16967/15916)

HR and BR enable quick diagnosis of certain medical conditions that are fatal; for example, obstructive sleep apnea syndrome (OSAS) and sudden infant death syndrome (SIDS). In OSAS, patients pause breathing for long duration during sleep and in case of SIDS, infant’s breath is blocked either by lying down on the face or due to material obstructions. Dyspnea and chronic obstructive pulmonary disease are other breath related conditions. See figure below to understand the breath pattern in various conditions.

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Figure 2: Breath Pattern (Source: https://clinicalgate.com/chest-inspection-palpation-and-percussion/)

Studies indicate that individuals with high resting heart rate are at higher risk of heart related problems. And individuals with low resting heart rate may have the need for a permanent pacemaker implantation in future.

Monitoring breath rate and heart rate of patients with above conditions could potentially save lives.

Contact and contactless based measurement of vital signs

Most of the existing measurement devices are contact based instruments. They need to be attached to the patient’s body to measure and monitor. This is not always convenient for patients who need to be monitored continuously over an extended period of time. For instance, consider the ongoing COVID-19 pandemic situation, where contactless vital monitoring devices may become more relevant as they help minimize the spread of virus through touchpoints and contacts. This ensures better safety for healthcare professionals. Hence remote, contactless based instruments are the need of the hour.

mmWave radar

As the name suggests these are radar technologies that make use of RF waves with wavelengths from 10mm to 1mm with 30 to 300Gz frequency. The spectrum allocated for radars in industrial applications is 60 to 64Ghz and for automotive applications is 76 to 81GHz. Since the wavelength of signals at these frequencies is shorter, the radar antennae are smaller in size. The small size of these radars combined with the advancement in antenna technologies such as Antenna on Package (AoP) and Antenna on PCB (AoPCB) enabled its widespread use in car navigation, building automation, health care & industrial application.

In this article we focus on frequency modulated continuous wave (FMCW) radars.  FMCW radars continuously transmit a frequency-modulated signal to measure the range as well as angle and velocity of a target object. An FMCW radar differs from traditional pulsed-radar systems, which transmit short pulses periodically. In case of FMCW radars, the frequency of signals increases linearly with time. This type of signal is called a chirp (Figure 3).

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Figure 3: Chirp in time domain. (Source: Author)

An FMCW radar system transmits a chirp signal and captures the signals reflected by objects in its path. Figure 4 represents a simplified block diagram of the main components of an FMCW radar.

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Figure 4: FMCW Radar Block diagram (Source: TI.com)

A “mixer” combines the RX and TX signals to produce an intermediate frequency (IF) signal. The mixer output has both signals that are sum and difference in the frequencies of the Rx and Tx chirps. A low pass filter is used to allow only the signal with difference in frequencies to pass through.

Figure 5 shows the transmitted and received chirps in frequency domain. If there are multiple objects at different ranges, there will be multiple reflected chirps, each with a delay based on the time taken to travel back to the radar. For each reflected chirp there will a corresponding IF tone.

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Figure 5: Frequency domain representation of TX and Rx Chirps and the IF frequency tones (Source: TI.com)

On analyzing the frequency spectrum of the IF signal, each peak in the spectrum corresponds to one or more detected object and the frequency corresponds to the object’s range.

If the object moves towards or away from the radar, due to doppler effect, the frequency and phase of the reflected chirp changes. Since the wavelength is in the order of 3.5 mm, a small change results in large phase change. It is easy to detect large change in phase compared to a small change in frequency. Thus, in FMCW radars, phase information is used to detect velocity of the object. To determine objects velocity, multiple chirps are used. The difference in phase between successive reflected chirps are recorded and the velocity is calculated with it.

How mmWave radar detects vital signs?

An advantage of short wavelengths is the high accuracy. An mmWave radar operating at 60 or 77GHz (with a corresponding wavelength in the range of 4 mm), will have the ability to detect movements that are as short as a fraction of a millimeter.

Figure 6 shows an mmWave radar transmitting chirps towards the patient’s chest region. The reflected signal is phase modulated due to the movement of the chest. The modulation has all components of movement including the movements due to heartbeat and breathing. The radar transmits multiple chirps at a predefined interval. In each chirp, range FFT is done and the range bin corresponding to the location of the person’s chest is selected.  The phase of the signal in this selected range bin is noted for every chirp. From these, the change in phase is computed, which gives the velocity. The obtained velocity still includes components of all movements. A spectral analysis of this obtained velocity helps to resolve various components. This is achieved by doing doppler FFT.

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Figure 6: HR and BR detection setup. (Source: Author)

Figure 7 shows the HR and BR detection algorithm. An adult’s heartbeat frequency is between 0.8 and 2Hz, while the frequency of breath is in the range of 0.1 to 0.5Hz. From the doppler FFT, the velocity components at frequencies of heartbeat and breath rate are selected and plotted against time. The number of peaks in one minute for each of these frequencies provide the heart rate and breath rate of the person.

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Figure 7: HR and BR detection algorithm. (Source: Author)

Challenges in mmWave Radar based vital signs monitoring

Vital signs monitoring using mmWave technology is still under development. One of the major challenges is the variation of reflected signals across people. The reflection depends on the skin type, tissue, and its composition. The water content level in the body and various chemical composition differ. The ongoing studies on the variation of reflected signals is expected to yield results and achieve more accurate measurements by the radars.

Conclusion

The major focus of mmWave radar have been centered around defense, automotive and industrial applications. However, the recent advancements in the mmWave technologies is finding significance in the healthcare industry as well. The higher accuracy, high-speed signal processing capabilities, enhanced range detection and the integration of radar into an ultra-compact chipset are expected to greatly enable healthcare applications such as patient activity monitoring, vital signs monitoring, etc. In addition, mmWave radar could potentially be used to measure drowsiness, stress levels and human emotions of a person – which has high significance from healthcare perspective and in developing driver monitoring systems in automotive application.

References

1. Texas Instruments 68xx Vital Signs
2. Remote Monitoring of Human Vital Signs Using mm-Wave FMCW Radar
3. DeepHeart: Semi-Supervised Sequence Learning for Cardiovascular Risk Prediction