Analysis of the radar channel and its variability in vital-sign sensing applications

In millimeter-wave (mmWave) radar-based vital-sign sensing, the radar sensor usually operates in the radiative near field of the target, which is the human body. Therefore, in contrast to far-field conditions, scattering cannot be quantified by a target-inherent radar cross section (RCS) as it strongly depends on the relative position and directivity of the radar antenna. To investigate the resulting consequences for the application, this article examines the radar channel transfer function, i.e., the ratio between the power waves feeding and being received by the antenna, with particular focus on its variability due to changes of the relative antenna position. The analysis is supported by a physical-optics-based simulation model tailored to practical circumstances. Measurements at 24 GHz and 61 GHz using antennas with different directivities and a human-like mannequin as a target confirm the simulative predictions. The radar channel suffers from significant variations, e.g., displacing the antenna by less than 1mm can cause a 30-dB receive power drop. A statistical analysis comprehensively evaluates the radar channel variability and clarifies the challenges for the application that stem from target near-field effects.