NLoS Radar Sensing for Breathing Rate Estimation in Indoor Environments: An Experimental Study

Radar plays a key role in Vital Sign Monitoring (VSM) for applications such as smart homes and in-cabin security. In this context, dense environments with Non-Line-of-Sight (NLoS) propagation are common. The combined use of radar systems and dedicated passive reflectors or Reconfigurable Intelligent Surfaces (RIS) offers a promising solution for improving NLoS. In this paper, we explore an NLoS indoor sensing scenario with a metal plate reflector and demonstrate the feasibility of metasurface-aided FMCW radar for vital sign detection. We employ a millimeter-wave FMCW radar operating at 60-64 GHz in conjunction with a dummy target that mimics the chest movement of a 2–5-year-old child. Using this setup, we demonstrate precise breathing rate estimation in LoS and NLoS scenarios. Additionally, we show NLoS measurements of an adult subject to demonstrate feasibility in real-world scenarios where non-periodic body movements are involved.