  Vital sign monitoring plays a critical role in tracking the physiological
state of people and enabling various health-related applications (e.g.,
recommending a change of lifestyle, examining the risk of diseases).
Traditional approaches rely on hospitalization or body-attached instruments,
which are costly and intrusive. Therefore, researchers have been exploring
contact-less vital sign monitoring with radio frequency signals in recent
years. Early studies with continuous wave radars/WiFi devices work on detecting
vital signs of a single individual, but it still remains challenging to
simultaneously monitor vital signs of multiple subjects, especially those who
locate in proximity. In this paper, we design and implement a time-division
multiplexing (TDM) phased-MIMO radar sensing scheme for high-precision vital
sign monitoring of multiple people. Our phased-MIMO radar can steer the mmWave
beam towards different directions with a micro-second delay, which enables
capturing the vital signs of multiple individuals at the same radial distance
to the radar. Furthermore, we develop a TDM-MIMO technique to fully utilize all
transmitting antenna (TX)-receiving antenna (RX) pairs, thereby significantly
boosting the signal-to-noise ratio. Based on the designed TDM phased-MIMO
radar, we develop a system to automatically localize multiple human subjects
and estimate their vital signs. Extensive evaluations show that under
two-subject scenarios, our system can achieve an error of less than 1 beat per
minute (BPM) and 3 BPM for breathing rate (BR) and heartbeat rate (HR)
estimations, respectively, at a subject-to-radar distance of $1.6~m$. The
minimal subject-to-subject angle separation is $40{\deg}$, corresponding to a
close distance of $0.5~m$ between two subjects, which outperforms the
state-of-the-art.
