Multi-Harmonics Amplitude Analysis via Space-Time Coding Enabled Spatial-Spectral Mapping for Target Angle Estimation and Vital Sign Monitoring

This work presents a novel space-time-coding (STC)-enabled radar sensor that leverages spatial-spectral mapping for target angle estimation and vital sign monitoring (VSM). The implemented prototype integrates a 1-to-5 unequal Wilkinson power divider and five patch antennas with direct antenna modulation (DAM), digitally controlled via an FPGA. By manipulating received signals into multiple harmonics as a function of incident angle, the STC array enables multi-harmonic amplitude analysis for angular localization. Experimental validation demonstrates accurate DOA estimation across an angular range of –70<inline-formula><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> to +70<inline-formula><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula>, within the acceptable <inline-formula><tex-math notation="LaTeX">$\pm 7.5^{\circ }$</tex-math></inline-formula> tolerance defined by the physical width of the human torso (33–41 cm), at a distance of 1.2 m. Furthermore, vital sign information extracted from the harmonic corresponding to the estimated angle range exhibits strong agreement with the ground truth. Unlike conventional DOA techniques, the proposed method operates using a single-channel over-the-air signal and does not require complex signal processing algorithms, offering a low-complexity and scalable solution for indoor biomedical and smart home applications.