A Wi-Fi Frequency Band Passive Biomedical Doppler Radar Sensor

In the past decades, dramatic investments and growth in wireless communication have occupied more and more radio frequency (RF) spectrum and created a shortage in the available radio spectrum. Hence, spectrum efficiency is a critical issue at the forefront of the research. Compared with conventional radar sensors, which rely on a transmitter and a receiver to detect a target, microwave passive sensing does not require a dedicated transmitter. Therefore, microwave passive sensing can have lower power and lower cost while saving the RF spectrum that is becoming limited in the modern wireless era. Since RF signals exist ubiquitously, microwave passive sensing technology has tremendous opportunities. This article proposes a passive biomedical Doppler radar sensor that operates at the Wi-Fi 2.4-GHz band to detect physiological motions of human subjects with a third-party 2.4-GHz transmitter. The proposed passive radar does not require a reference signal. It has a simple architecture with an LNA and a customized single-input diode-based mixer to maximize the down-conversion gain. Experiment results demonstrated that the proposed passive radar receiver has a voltage gain of 53.9 dB from the antenna output to the mixer output, and it can measure the respiration rate and the heart rate of a human subject successfully. The measured respiration rate and heart rate match the results from a fingertip pulse monitor and a chest band respiration monitor, respectively. Gesture detection was also demonstrated with the proposed passive radar.