Antenna Selection for Enhancing Privacy in Radar-Based Vital Sign Monitoring Systems

Radar-based vital sign monitoring (VSM) systems have become valuable for non-contact health monitoring by detecting physiological activities, such as respiration and heartbeat, remotely. However, the conventional phased array used in VSM is vulnerable to privacy breaches, as an eavesdropper can extract sensitive vital sign information by analyzing the reflected radar signals. In this paper, we propose a novel approach to protect privacy in radar-based VSM by modifying the radar transmitter hardware, specifically by strategically selecting the transmit antennas from the available antennas in the transmit array. By dynamically selecting which antennas connect or disconnect to the radio frequency chain, the transmitter introduces additional phase noise to the radar echoes, generating false frequencies in the power spectrum of the extracted phases at the eavesdropper's receiver. The antenna activation pattern is designed to maximize the variance of the phases introduced by antenna selection, which effectively makes the false frequencies dominate the spectrum, obscuring the actual vital sign frequencies. Meanwhile, the authorized receiver, having knowledge of the antenna selection pattern, can compensate for the phase noise and accurately extract the vital signs. Numerical experiments are conducted to validate the effectiveness of the proposed approach in enhancing privacy while maintaining vital sign monitoring.