  Passive human sensing using wireless signals has attracted increasing
attention due to its superiorities of non-contact and robustness in various
lighting conditions. However, when multiple human individuals are present,
their reflected signals could be intertwined in the time, frequency and spatial
domains, making it challenging to separate them. To address this issue, this
paper proposes a novel system for multiperson detection and monitoring of vital
signs (i.e., respiration and heartbeat) with the assistance of
space-time-coding (STC) reconfigurable intelligent metasurfaces (RISs).
Specifically, the proposed system scans the area of interest (AoI) for human
detection by using the harmonic beams generated by the STC RIS. Simultaneously,
frequencyorthogonal beams are assigned to each detected person for accurate
estimation of their respiration rate (RR) and heartbeat rate (HR). Furthermore,
to efficiently extract the respiration signal and the much weaker heartbeat
signal, we propose an improved variational mode decomposition (VMD) algorithm
to accurately decompose the complex reflected signals into a smaller number of
intrinsic mode functions (IMFs). We build a prototype to validate the proposed
multiperson detection and vital-sign monitoring system. Experimental results
demonstrate that the proposed system can simultaneously monitor the vital signs
of up to four persons. The errors of RR and HR estimation using the improved
VMD algorithm are below 1 RPM (respiration per minute) and 5 BPM (beats per
minute), respectively. Further analysis reveals that the flexible beam
controlling mechanism empowered by the STC RIS can reduce the noise reflected
from other irrelative objects on the physical layer, and improve the
signal-to-noise ratio of echoes from the human chest.
