Abstract
This paper presents a W-band continuous-wave (CW) Doppler radar sensor for non-contact measurement of human respiration and heartbeat. The very short wavelength of the W-band signal allows a high-precision detection of the displacement of the chest surface by the heartbeat as well as respiration. The CW signal at 94 GHz is transmitted through a high-gain horn antenna to the human chest at a distance of 1 m. The phase-modulated reflection signal is down-converted to the baseband by the quadrature mixer with an excellent amplitude and phase matches between I and Q channels, which makes the IQ mismatch correction in the digital domain unnecessary. The baseband I and Q data are digitized using data acquisition (DAQ) board. The arctangent demodulation with automatic phase unwrapping is applied to the low-pass filtered I and Q data to effectively solve the null point problem. A slow-varying DC component is rejected in the demodulated signal by the trend removal algorithm. Then, the respiration signal with a frequency of 0.27 Hz and a displacement of ~6.1 mm is retrieved by applying a low-pass filter. Finally, the respiration signal is removed by the band-pass filter and the heartbeat signal is extracted, showing a frequency of 1.35 Hz and a displacement of ~0.26 mm. The extracted respiration and heartbeat rates are very close to the manual measurement results. The demonstrated W-band CW radar sensors can be easily applied to find the angular location of the human body by using a phased array under a compact size.
Highlights
Continuous-wave (CW) radar sensors based on Doppler effect have been widely used to remotely detect human vital signs such as respiration and heartbeat rates [1,2]
Together with the trend and peak removal, low-pass and band-pass filtering allows the accurate extraction of very small displacement by the heartbeat from very large respiration signals, thanks to the very short wavelength of W-band frequency
This work demonstrates that the millimeter-wave Doppler radar sensor can be used for the accurate detection of the human heartbeat as well as respiration
Summary
Continuous-wave (CW) radar sensors based on Doppler effect have been widely used to remotely detect human vital signs such as respiration and heartbeat rates [1,2]. Most of the reported CW radar sensors for vital sign detection are implemented using low microwave frequency such as 915 MHz, 2.45 GHz, and 5.8 GHz, at which the electromagnetic waves exhibit low attenuation in human tissues as well as in free space [1,3]. As the carrier frequency of the radar sensors increases, the phase of the reflected signal is more sensitively modulated by the displacement due to a short wavelength, allowing higher accuracy in the detection of vital signs [2]. We applied the W-band frequency (94 GHz) for non-contact detection of human respiration and heartbeat For this purpose, the W-band CW radar sensor was implemented in waveguide-based modules using horn antenna, quadrature mixer module, low noise amplifier (LNA).
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