Radar-assisted wireless sensor communications using frequency-modulated reconfigurable intelligent surfaces

Short-range continuous-wave (CW) radar sensing is increasingly adopted in Internet of Things (IoT) applications including localization, gesture recognition, and environmental monitoring across a wide range of indoor and industrial environments, motivating compact architectures that integrate perception with embedded data delivery. This paper extends this functionality to low-data-rate IoT communications by introducing a radar-assisted reconfigurable intelligent surface (RIS) sensing framework based on frequency-coded backscatter, in which the RIS acts as a sensing node. A 24-GHz CW radar illuminates a compact programmable RIS tag whose effective radar cross section (RCS) is periodically modulated to embed data into the backscattered signal. The RIS is composed of four unit cells, each loaded with PIN diodes that act as switching elements. Modulation is achieved by controlling the biasing of the diodes. Data is encoded through discrete RIS modulation frequencies and recovered from symmetric radar baseband sidebands, shifting information away from DC clutter and providing robustness to common Doppler-induced frequency offsets. The sensing node does not perform any active RF transmission and relies solely on the low-frequency electronic control of the RIS. Indoor experiments demonstrate real-time sensing with a symbol duration of 250 ms, error-free frame reception for static or moving tags up to 10 m and 4 m, respectively, and an average packet error rate (PER) below 0.1% during prolonged operation.