A Robust and Widely Applicable Compensation Method for Quadrature Imbalance of Doppler Radar

Quadrature Doppler radar is widely utilized in micro-motion monitoring due to its advantages of noncontact, miniaturization, and excellent environmental adaptability. However, the amplitude and phase imbalance between the quadrature channels seriously affects the reliability of signal monitoring, and the existing calibration methods are limited by the amplitude modulation and multipath reflection effect and are sensitive to noise. In this article, a robust and widely applicable compensation method for the quadrature imbalance of Doppler radar termed finite impulse response differentiator-Hilbert transform (FIRD-HT) is proposed. The approach leverages the low-pass FIR differentiator to remove dc offset and suppress high-frequency noise and leverages the HT to calculate the quadrature imbalance coefficients. The Gram–Schmidt (GS) orthogonalization is further employed to calibrate baseband signals. Through strict mathematical derivation, it is proved that the FIRD-HT is not interfered with by the amplitude modulation and multipath reflection, which allows it to be applicable to a wide range of scenarios. Finally, simulation and experiments demonstrate that the FIRD-HT has greatly improved robustness and applicability compared with other state-of-the-art methods. The errors of the FIRD-HT are only 1/4 of the ellipse fitting method and 1/2 of the spectrum method at low signal-to-noise ratio (SNR). In close-range experiments, the errors of FIRD-HT are less than 1%, but other methods are not applicable due to amplitude modulation and multipath reflection.