Metasurface-Assisted Ultra-Wideband Radar for High-Sensitivity Non-Contact Respiratory Detection

Abstract

Non-contact respiration monitoring using impulse-radio ultra-wideband (IR-UWB) radar is attractive because of its low transmitted power, fine range resolution, and clutter-rejection capability; however, practical performance at meter-scale standoff is often limited by weak radar–thorax coupling and clutter-dominated echoes. This paper presents a wearable, flexible metasurface that passively enhances the chest-return signature without modifying the radar hardware or increasing transmit power. The proposed metasurface is implemented as a 6 × 6 array of subwavelength unit cells on a 1-mm-thick polyimide substrate and is optimized for low-GHz IR-UWB operation centered at 7.29 GHz under multilayer tissue loading. Full-wave simulations confirm strong impedance matching, low insertion loss, stable broadside field reinforcement, and bending robustness under practical conformal conditions. In addition, the simulated 1-g averaged specific absorption rate (SAR<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1g</sub>) is only 0.0013W/kg under 1Waccepted power, which is far below commonly adopted exposure limits. Experimental validation using a commercial XeThru X4M06 IR-UWB radar at a 150 cm standoff demonstrates that the metasurface-assisted configuration yields more stable respiration-rate tracking and a clearer, more periodic breathing waveform than the baseline case without metasurface. The improvement is maintained for both standing and supine postures under identical radar settings. These results indicate that conformal, fully passive metasurfaces can improve the robustness and practical usability of IR-UWB respiration sensing at meter-scale distances while preserving the inherent low-power and safety advantages of UWB radar.

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