A Backscatter Field Model for Near-Field RF Sensing

Backscatter radio has been widely used in remote and wearable sensing for body motion and vital signs. When a moving or shape-changing dielectric is placed in the near-field region of a pair of transmitter (Tx) and receiver (Rx) antennas, the Tx signal propagating to the dielectric will be backscattered and modulated by the motion as a time-varying complex phasor signal at the Rx. In this article, we proposed a semianalytical model based on the electromagnetic field perturbation of the electric dipole (ED) mode of a dielectric sphere. Our proposed model analyzed how the changes in object shape, antenna pair position, and carrier frequency affect the backscattered field. To validate our proposed field model, we designed a torso phantom and performed experiments with an antenna pair at eight different front positions as well as four different sub-1-GHz carrier frequencies. Our model-generated signals had an average of 0.98 cross-correlations with the experimental recording from the six central positions and four frequencies. Furthermore, we added scaling factors to represent the antenna performance for magnitude and phase matching. After performing a least-squares fitting, our scaling model achieved similar fitting errors as the ground-truth measurement variations with average adjusted $R$ -squared scores of 0.86 and 0.89 for different sensing positions and carrier frequencies.

Automated summary

This article proposes a semianalytical model based on the electric dipole mode of a dielectric sphere to analyze the effects of object shape, antenna position, and carrier frequency on the backscattered field. Experimental validation shows that the model accurately predicts the field behavior, and the antenna performance is adjusted using scaling factors for matching purposes.