Combining SIL tag and IL receiver for concurrent vital sign and position sensing

This work proposes a low-complexity wireless system that operates in the 2.4 GHz ISM band for concurrent vital sign and position sensing. The prototype consists of two main parts. One is the self-injection-locked (SIL) tag, which produces a frequency-modulated (FM) signal. The other is the group of injection-locked (IL) receivers that are synchronized due to injection locking by the FM signal. In an experiment, a tagged subject is standing still, breathing normally, and surrounded by the receivers. The FM signal that is emitted by the tag contains the Doppler shift due to subject´s cardiopulmonary motion. The synchronized receivers are responsible for receiving the signal to extract the vital sign and position information. Basically, vital signs including respiration and heart rates are obtained from the extracted cardiopulmonary signals, whereas information about the position of the subject is extracted from the phase difference of the received FM signals between the different receivers.