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.

I. Introduction

Microwave passive sensing makes use of electromagnetic waves illuminated by a third-party transmitter to detect a target [1]. By comparing the direct signal from the transmitter and the reflected signal from the target of interest, the motion of the target can be detected. With the rapid growth of the Internet of Things (IoT) and satellites internet, wireless signals are ubiquitously present in the ambient air. It enables the possibility of microwave passive sensing at most places on our planet. Since microwave passive sensing does not transmit its own radio frequency (RF) signals, it inherently reduces cost and power compared with conventional radar sensors. Moreover, it provides a solution to address the interference with the wireless communication signals and improves the spectrum efficiency. The fast-growing wireless communication technologies require more and more radio spectrum and have started to affect conventional radars because of the potential interference. For example, major U.S. airlines recently warned that the new C-band 5G service could potentially interfere with the altimeter and cause a significant number of aircraft to be unusable. The automotive and consumer electronics industries have also been looking for interference mitigation approaches due to the increasing number of cars and appliances that rely on wireless devices operating at the same frequency for communication or sensing purposes. Therefore, microwave passive sensing technology has tremendous opportunities, and it has gained attention in recent decades.