Phase Correction in Asynchronous FMCW Radar Systems for Accurate Noncontact Cardiopulmonary Monitoring

Interferometric Frequency-Modulated Continuous-Wave (FMCW) radar can wirelessly detect small displacement motions such as the cardiopulmonary signals through the phase-based range-tracking (PBRT) technique. However, in asynchronous radar systems, there exist phase shifts between sequential pulsed chirps due to the unsynchronized sampling, which could distort the detected displacement motion. This paper presents a phase correction technique for automatic compensation of the unwanted phase shifts. It calibrates the digitized beat signals that are collected based on the synchronized sawtooth signals to achieve phase synchronization across pulsed chirps. The working principle and theory are introduced in details. Experiments were carried out by using a 79 GHz FMCW radar to validate the proposed technique. The results demonstrate that the demodulation errors due to the system asynchronization have been dramatically reduced, e.g. 0.4 % error for a 4 cm displacement motion.