Cross-Frequency Sensing in Bistatic ISAC Systems

Due to discrepancies in clock oscillators between transmitter and receiver, bistatic sensing in integrated sensing and communications (ISAC) systems suffers from the clock asynchronism issue. Previous research has demonstrated that these offsets can be effectively mitigated through cross-antenna techniques. However, such techniques may not always be preferred due to the side effects of restricted spatial degrees of freedom or complicated signal models. In this paper, we introduce cross-frequency techniques to address the clock asynchrony issue for line-of-sight (LOS) dominant bistatic sensing. We begin by uncovering the rotational invariance properly inherent in a time-frequency-domain signal matrix that is typically constructed for bistatic sensing. We then propose two novel methods, i.e., cross-frequency cross-correlation and cross-frequency signal ratio, to effectively suppress clock asynchronism without compromising spatial sensing ability. Furthermore, we evaluate the performance of these methods using key metrics, including the maximum unambiguous delay, delay resolution, and analytical target SNRs. Extensive simulation and experimental results are provided, verifying the effectiveness of the proposed methods and their superiority versus efficiency over prior art in sensing accuracy.