Mutual Coupling (MC) is an unavoidable feature in Reconfigurable Intelligent Surfaces (RISs) with sub-wavelength inter-element spacing. Its inherent presence naturally leads to non-local RIS structures, which can be efficiently described via non-diagonal phase shift matrices. In this paper, we focus on optimizing MC in RIS-assisted multi-user MIMO wireless communication systems. We particularly formulate a novel problem to jointly optimize active and passive beamforming as well as MC in a physically consistent manner. To characterize MC, we deploy scattering parameters and propose a novel approach to optimize them through an offline optimization method, rather than optimizing MC on the fly. Our numerical results showcase that the system performance increases with the proposed MC optimization, and this improvement is achievable without the need for optimizing MC on-the-fly, which can be rather cumbersome.

翻译：互耦是亚波长间距可重构智能表面中不可避免的特性，其固有存在自然导致非局域RIS结构，此类结构可通过非对角相移矩阵进行有效描述。本文聚焦于优化RIS辅助多用户MIMO无线通信系统中的互耦问题，特别提出一种新型问题框架，以物理一致的方式联合优化有源/无源波束赋形及互耦。为表征互耦，我们引入散射参数并创新性地提出离线优化方法对其进行优化，而非实时动态调整。数值结果表明，所提出的互耦优化方案能提升系统性能，且该性能增益无需通过繁琐的实时互耦优化即可实现。