The self-interference problem of linear frequency modulated continuous wave (LFMCW) radar is a known issue that limits the radar's detection range. Analog adaptive interference cancellation (AIC) technique is effective to mitigate the self-interference problem. However, we find that the phase difference between the error signal and reference signal paths may significantly deteriorate the stability of the AIC system. Therefore, in this paper, we analyze the effect of phase difference on system stability through the mathematical modeling and simulation. We find that the system is stable when the phase difference is between -90 and 90 degrees, and diverges when it is between 90 and 270 degrees. Therefore, to avoid system instability, we propose to add a phase shifter in the reference signal path to compensate the phase difference. The experiment results show that compared with the traditional delay-based compensation method, our phase compensation based method can increase interference cancellation ratio (ICR) by 15 dB for a single-antenna system and 12 dB for a dual-antenna system.
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