Wave Propagation and Polarization Effects in RF Based Sensing and Communication for Smart Insoles

Wireless smart insoles are increasingly used for motion analysis, providing critical insights for healthcare, sports performance, and injury prevention. A key challenge is accurately measuring and wirelessly transmitting foot movement and placement data in real time, independently of inertial measurement units tied to biomechanical models, which only estimate gait parameters for a limited range of activities such as walking or running. Radio-frequency phase-based ranging presents a promising solution for a broader range of gait analysis applications, including irregular movement of athletes in sport or people with diseases like Parkinson’s. However, wireless system design must address two key challenges: (1) antenna robustness against the non-deterministic properties of the ground and the proximity of the body, and (2) polarization mismatch due to foot movement. This study is the first to numerically and experimentally investigate how foot motion and insole operating conditions affect wave propagation and wireless communication at 2.45 GHz. We analyze the impact of soil properties and antenna polarization, demonstrating that vertical polarization significantly improves signal transmission compared to horizontal polarization, with a gain of 6–10 dB at a height of 5cm. This improvement, driven by surface wave propagation, provides insights into the antenna design strategies for wireless insoles that can enhance the accuracy of phase-based ranging for foot distance measurement, as confirmed through simulations and experimental validation. These findings contribute to the energy-efficient, real-time ranging and communication enabled by optimized antenna designs, reducing power consumption and improving motion analysis for a wide range of users, including athletes requiring precise performance metrics and patients undergoing diagnostics or rehabilitation.