Heartbeat Detection Using Static RIS-Assisted mmWave OFDM System: A Channel State Information Approach with Software-Defined Radios

This paper presents a novel vital-sign monitoring system for heartbeat detection by incorporating a static reconfigurable intelligent surface within a millimeter-wave (mmWave) OFDM communication framework. The system begins by generating an OFDM waveform, which is transmitted using the software-defined radio in combination with the mmWave frequency up/down-converters and the beamforming modules. The transceiver is designed as a double-conversion architecture. The OFDM signal with carrier frequency of 4.7 GHz is upconverted to 28 GHz and directed to the RIS through beamforming. The RIS, characterized by a gain of 29.1 dBi with a feeding antenna at 0.6 m, reflects the signal toward a stationary subject. A 10-second measurement interval is analyzed by estimating the channel state information, followed by Fourier transformation to extract the heartbeat frequency. Validation is performed by comparing the measured heartbeat rate with results from an Apple Watch, achieving a mean error of 3.6 bpm (4.06%). Furthermore, the communication performance of the system is evaluated, demonstrating an error vector magnitude (EVM) below 1.5 % and a signal-to-noise ratio (SNR) of 37 dB. The proposed system showcases the potential of RIS-assisted mmWave communication for integrated communication and health monitoring.