Phase Noise Distortion Compensation in the FMCW Radar Based on Oppositely Modulated Double Chirps

In coherent continuous-wave (CW) radar systems, such as frequency-modulated CW (FMCW) radar systems, phase noise significantly limits both the maximum detectable measurement range and the measurement precision for targets at long range. Approaches that minimize the amount of phase noise during signal generation have so far only led to a marginal improvement of radar systems, and are therefore unsuitable for targets over long range. This paper presents a fundamentally different approach based on full phase noise compensation, achieved by simultaneously emitting two FMCW chirps in opposite directions and then offsetting the beat frequencies. This approach was simulated and verified through measurement using an FMCW primary radar system up to the sensitivity limit of the measurement setup. The metric used was the range precision. Targets were created over ranges of 76-880 m with an analog fiber optical link. The experimental verification results showed an improvement in range precision by a factor of 43. This oppositely frequency-modulated continuous-wave (OFMCW) approach can enable high precision and low-noise measurements for future radar systems.

I. Introduction

CW radar systems, in particular FMCW radar systems, are preferred to pulse radars for achieving high measurement precision over target ranges of up to several hundred meters. The advantage of these systems lies in their efficient signal compression and correlation, which result in a high signal-to-noise ratio (SNR). However, in classical FMCW radar systems, where only one chirp is transmitted at a time, the phase noise decorrelates with increasing target ranges, which limits the achievable precision of such systems. This was impressively demonstrated in [1]. In this respect, phase-locked loop (PLL)-based systems operated in FMCW mode, as shown in [2], could have significantly higher phase noise than previously assumed. Previous approaches have aimed to reduce the influence of phase noise by using lower phase noise frequency synthesizers, but this has lead to marginal improvements. In this work, an already in [3] patented approach is investigated and experimentally verified, in which the influence of phase noise in the estimation of beat frequencies is completely compensated using a similar reference signal. For the first time, this approach makes the precision as well as the accuracy of a radar measurement with respect to phase noise independent of the target range.