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, 2 nd order harmonics were utilized for target localization and vital sign monitoring. However, these 2 nd order harmonics suffer from high path loss and licensing issues. In this paper, target localization and smart health sensing were realized using 3 rd 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.

Wearable Belt With Built-In Textile Electrodes for Cardio—Respiratory Monitoring

“…By using the electromagnetic radiation in the microwave region, it is possible to monitor cardio-respiratory activity without any contact with the subject under examination and, consequently, to carry out non-invasive monitoring. The systems proposed in the literature for the remote monitoring of breath activity are mainly based on Doppler [6,7], ultra wideband (UWB) [8,9] and FMCW radar techniques [10,11]. Their major advantage is that, without the need for any cable or electrode, it is possible to locate the patient inside the room, and then, to measure his/her respiratory rate and heartbeat.…”

Sensing of Life Activities at the Human-Microwave Frontier

“…Furthermore, an intermodulation sensing in frequency shift keying (FSK) mode was developed for localization of human subjects based on life activities [35]. Another feature of this nonlinear detection setup was that the heartbeat signal component received more gain than the respiration signal, so that the sidelobes and harmonics of respiration do not interfere with heartbeat signal.…”

A Review on Low-Cost Microwave Doppler Radar Systems for Structural Health Monitoring

“…However, as the motion amplitude of the bridge decreased to less than 2 mm, the accuracy on the radar measurements deteriorated, as shown in Figure 3b (segment 2). In contrast to the active transponder mode, which might be costly and is more power demanding than the passive backscattering mode, recent advancements on radar technol- In contrast to the active transponder mode, which might be costly and is more power demanding than the passive backscattering mode, recent advancements on radar technologies leveraging nonlinear tags show alternative solutions for clutter rejection and target discrimination [65][66][67][68]. Although further investigations are required to demonstrate the feasibility of displacement reconstruction through radar detection aided by nonlinear tags, promising results on range tracking and physiological motion monitoring have paved the way for future SHM applications [65].…”

“…Although further investigations are required to demonstrate the feasibility of displacement reconstruction through radar detection aided by nonlinear tags, promising results on range tracking and physiological motion monitoring have paved the way for future SHM applications [65]. Recent works on passive nonlinear tags, which inherit the advantages of passive operation, low cost, and simple hardware, also demonstrated clutter rejection capabilities [66][67][68].…”