Enhancing Privacy in Radar-Based Vital Sign Monitoring Via Non-Linear FMCW Waveforms

Vital sign monitoring via a phased array radar transmitting continuous-wave (CW) or linear frequency-modulated continuous-wave (FMCW) offers contactless, low-cost, continuous monitoring of vital signs, i.e., human heart rate and breathing rate. In this paper, we demonstrate that, besides the authorized receiver of radar echoes, a passive eavesdropper could process the reflections of the CW or linear FMCW waveforms off the patient being monitored and decipher their vital signals, thus violating their privacy. We then propose an approach to protect vital sign privacy. Specifically, we propose the use of a nonlinear FMCW waveform that distorts the radar echoes, making it more difficult for the eavesdropper to extract the vital signs without knowledge of the specific parameters of the non-linear FMCW waveform. At the same time, the authorized receiver, with knowledge of those parameters, can easily extract the vital signs. Therefore, the proposed waveform can simultaneously implement vital sign monitoring and user privacy protection. Numerical experiments demonstrate the effectiveness of our proposed scheme.