Intermodulation-Based Nonlinear Smart Health Sensing of Human Vital Signs and Location

Abstract

This paper discusses the use of a nonlinear sensing technology based on radio frequency (RF) intermodulation response to track both the vital signs and location of human subjects. Smart health sensing was realized through the use of a wearable nonlinear tag and an intermodulation-based nonlinear sensor operating in both Doppler and frequency shift keying (FSK) modes. The Doppler mode was used to detect the heartbeat and breathing of the target subject while human subject localization was achieved in the FSK mode. One of the key advantages of this nonlinear smart sensor system was clutter rejection. This system identified the signal reflected from the wearable nonlinear tag and suppressed undesired signals and interferences that were reflected from other objects. The wearable tags used for the experiments were passive, hence they did not require any battery or power supply for their operation. Since the respiration signal is typically stronger than the heartbeat signal, the nonlinear detection setup was designed such that the respiratory signal receives less gain to avoid its sidelobes and harmonics from interfering heartbeat signal detection. This enhanced the heartbeat signal quality so that the cardiac activity could be easily tracked. Four types of experiments were performed on multiple subjects to demonstrate the advantages of this intermodulation-based nonlinear smart health sensing system. Previously, 2nd order harmonics were utilized for target localization and vital sign monitoring. However, these 2nd order harmonics suffer from high path loss and licensing issues. In this paper, target localization and smart health sensing were realized using 3rd order intermodulation with less path loss and no licensing issues compared with its harmonic counterparts. The experiment performed in nonlinear FSK mode was able to detect and locate the source of motion with high accuracy. Similarly, vital signs were recorded in the nonlinear Doppler mode. The design effectively made the amplitude of the heartbeat signal component more prominent, so that the sidelobes and harmonics of respiration do not suppress heartbeat signal.