Robust contactless sensor for real-time vital signs monitoring

Abstract

Wireless sensing find many biomedical applications due to the benefit and convenience of having no contact with the patients. Low power radio waves, that are biologically safe, can monitor the patient’s medical conditions such as the breathing and heart beat rate. Electro-magnetic (EM) waves extract the small movements of the chest and heart via field disturbance phenomena. The patient’s breathing and heart beat modulate the radio waves for the vital signs to be monitored in real-time. Ultra-wideband (UWB) radar is an EM scattering technology that has been proven to discriminate targets with less than 30mm range resolution, which is smaller than the average size of a human heart. The UWB radar emits 200ps pulses that occupy a bandwidth from 3 to 10GHz. For linearly polarized radiation, the radar cross-section (RCS) of a moving target having complex shapes such as the thorax section of a human body causes regular fading. The scattering from a complex target rotates the radio wave vector to result in non-optimal signal reception due to polarization losses. The research focuses on the design of a circularly polarized (CP) UWB radar system for contactless vital signs monitoring of a patient. Circular polarization is introduced to address the fading RCS problem thus achieving a more robust system. The same CP UWB radar system with radio waves that penetrate through dielectric objects can be used for time-critical rescue operations in locating survivors buried under collapsed structures or in buildings engulfed in flames where visibility is hampered. Furthermore, by employing material characterization and microwave imaging techniques, it is possible to use the proposed system for other biomedical applications such as tumor localization and damaged tissue identification. The thesis is organized as follows. Firstly, the operation of UWB radar is presented and the research topic addressed. The pulse generator design suited for low pulse repetition rates is then introduced. The design of a decade bandwidth circularly polarized antenna array is described. The methodology and performance of the robust vital signs monitoring system is shown and compared with the linear polarized counterpart. A new method for material characterization using time domain RCS measurements is demonstrated. The future work is finally proposed.