Polarimetric Assessment Methodology for Doppler Radar Respiratory Measurements

Physiological radar measurements are used to extract biomedical motion data from body scattered radio signals. These often-faint reflected signals undergo both Doppler-phase and polarization alterations. Polarimetric analysis of physiological radar measurements can provide insight into the nature of the scattering mechanism and lead to the development of methods for enhancing the signal-to-noise ratio (SNR) for such systems and potentially expand the range of biomedical data which can be extracted. This article presents a comprehensive polarimetric study of physiological Doppler radar signatures and reports on an empirical study intended to examine polarimetric scattering associated with human respiration. A compact dual-polarization antenna was employed in a full-polarimetric configuration and a dual-receive configuration, enabling precise polarimetric measurements. Simulation and experimental results with robotic respiratory phantoms and human subjects demonstrated consistent levels of cross-polarized scattering across configurations and revealed a complex nature for cross-polarized signal generation from robotic targets and human subjects. Cross-polarized amplitudes of up to 15.6% of co-polarized amplitudes were observed for robotic phantoms, while a wide range was observed for human subjects with most at 70% or less, while in some cases amplitudes as high as 198% of co-polarized amplitudes were observed. The results indicate that a significant, yet difficult to predict, amount of polarization rotation occurs from both vertical and horizontal transmit polarizations, and that for either incident polarization there is no clear preference for co-polarized received signals. Thus, there is significant potential for improved SNR and signature content via polarimetric radar systems which can transmit and/or receive signals with complementary polarizations.