This paper considers a downlink Reconfigurable Intelligent Surface (RIS)-assisted Joint Communication and Sensing (JCAS) system within a physically-consistent setting, accounting for the effect of mutual coupling between RIS elements arising due to sub-element spacing. The system features a multiple-input multiple-output (MIMO) terrestrial base station (BS) and explores both monostatic and bistatic radar configurations to enable joint communication and sensing. In the monostatic configuration, both the transmitter and receiver are at the same location, while the bistatic configuration separates the transmitter and receiver spatially. System performance is evaluated using Fisher Information (FI) to quantify sensing accuracy and Mutual Information (MI) to measure communication efficiency. To achieve an optimal balance between communication and sensing, the RIS reflective coefficients and BS transmit beamforming are jointly optimized by maximizing a weighted sum of FI and MI. A novel solution approach is proposed for a single-user, single-object scenario, leveraging the mutual coupling model to enhance system realism. The impact of self-interference on sensing performance is also investigated through signal quantization. Numerical results reveal a fundamental trade-off between FI and MI and demonstrate that incorporating mutual coupling within a physically-consistent framework significantly improves both communication and sensing performance compared to conventional RIS-assisted JCAS models. Additionally, the analysis highlights how the choice of monostatic versus bistatic radar configuration affects system performance, offering valuable insights for the design of RIS-assisted JCAS systems.

翻译：本文在物理一致性的框架下，研究了下行可重构智能表面辅助的联合通信与感知系统，其中考虑了由RIS单元间距引起的单元间互耦效应。该系统采用多输入多输出地面基站，并探索了单站与双站雷达配置以实现通信与感知的联合功能。在单站配置中，发射机与接收机位于同一位置；而在双站配置中，发射机与接收机在空间上分离。系统性能采用费歇尔信息量来量化感知精度，并用互信息量来衡量通信效率。为实现通信与感知性能的最优平衡，通过最大化FI与MI的加权和，对RIS反射系数与基站发射波束成形进行联合优化。针对单用户单目标场景，本文提出了一种新颖的求解方法，利用互耦模型提升系统建模的真实性。同时，通过信号量化方法研究了自干扰对感知性能的影响。数值结果揭示了FI与MI之间的本质权衡，并证明在物理一致性框架中纳入互耦效应，相较于传统RIS辅助的JCAS模型，能显著提升通信与感知性能。此外，分析还阐明了单站与双站雷达配置的选择如何影响系统性能，为RIS辅助JCAS系统的设计提供了重要参考。