Forward and Backward Scattering Models in Near-Field Radio Frequency Sensing

Recent advances in microwave technology have enabled touchless wearable sensors for vital-sign monitoring. Near-field radio frequency (NFRF) sensors improve electromagnetic (EM) energy coupling to capture motion of internal organs and deep tissues through clothing and coverings. Conventional NFRF sensors, which rely on backscattering (BS), are prone to signal distortion from near-field nonlinearities, direct-path saturation, and multipath interference. This work systematically evaluates forward scattering (FS) and BS using the Mie theory, full-wave CST simulations with realistic torso models, and experimental validation on phantoms and human cardiograms. FS demonstrates higher SNR and stable, monophasic waveform behavior across geometric changes, supporting robust physiological feature extraction and motivating hybrid forward–backward near-field sensing.