Wireless RF Vital Signal Sensor Using Autoregressive Spectral Estimation Method

This letter describes a compact and novel wireless vital signal sensor at 2.4 GHz that can detect heartbeat and respiration signals. The oscillator circuit incorporates a planar resonator, which functions as a series feedback element as well as a near-field radiator. The periodic movement of the body during aerobic exercise causes an input impedance variation of the radiator within the near-field range. This variation results in a corresponding change in the oscillation frequency, and this change has been utilized for detection of human vital signals. In addition, a surface acoustic wave (SAW) filter and power detector have been used to increase the system sensitivity and to transform the frequency variation into a voltage waveform. Also, the autoregressive spectral estimation method is applied to achieve precise vital sign detection. The experimental results show that the proposed sensor placed 20 mm away from a human body can detect the vital signals very accurately.

I. Introduction

The Increasing demand for the detection of vital signals through wireless systems has led to considerable attention being focused on the role of a radio frequency (RF) bio-sensor for monitoring vital signals, especially, for example, in cardiopulmonary monitoring of the sleep apnea syndrome and for patients whose severe burns preclude the attachment of electrodes or leads to the body [1]. Many recent research efforts and studies on wireless vital signal sensing have focused on the Doppler radar principle [2], [3]. Even though excellent performance has been achieved with a wireless vital signal sensor based on Doppler radar theory, the existing Doppler radar approaches suffer from cofrequency interference. Moreover, they are incapable of detecting vital signals if more than one person is in the measurable range. Although an ultrawideband (UWB) pulse Doppler radar can detect multiple subjects and their range information, it is accompanied by a significant system complexity and cost. Accordingly, the Doppler radar has reached the limit of its practical use in the medical area with conventional approaches. Also, the related research and studies on the development of electrocardiography (ECG) using wet or dry electrodes have been completed [4]. The uses of these approaches are limited by the inconvenience and discomfort of wet adhesive electrodes in direct contact with the body. As an alternative, noncontact capacitive electrodes have started becoming much more commonplace [5], [6] because they do not require an ohmic connection to the body and are insensitive to skin conditions. However, this approach utilizes a static capacitance variation. Therefore, the measurement range is limited to only a few millimeters.