Multiperson Detection and Vital-Sign Sensing Empowered by Space-Time-Coding Reconfigurable Intelligent Surfaces
Xinyu Li, Jian Wei You, Ze Gu, Qian Ma, Jingyuan Zhang, Long Chen, Tie Jun Cui
Abstract
Passive wireless human sensing has attracted increasing attention due to its non-contact nature and robustness under different lighting conditions. This paper proposes a system for multiperson detection and vital-sign (i.e., respiration and heartbeat) monitoring using space-time-coding (STC) reconfigurable intelligent surfaces (RISs). Specifically, the proposed system scans the area of interest (AoI) for human detection by using the STC RIS-generated harmonic beams. Simultaneously, specific beams are assigned to the detected persons for vital-sign sensing. Furthermore, we propose an improved variational mode decomposition (VMD) algorithm to process the reflected echo signal and estimate respiration and heartbeat rates of the detected persons. We build a prototype to validate the proposed system. Experimental results demonstrate that the system can simultaneously monitor respiration and heartbeat of up to four persons, with average errors below 1 RPM (respiration per minute) and 5 BPM (beats per minute). Further analysis reveals that the flexible beam control enabled by the STC RIS can reduce the noise reflected by irrelative objects, thus improving the signal-to-noise ratio of echoes from the human chest. This work offers a contact-free care to round-the-clock monitor vital signs of multiple persons, which are critical indicators for assessing general health state and identifying various diseases.