The plethora of wirelessly connected devices, whose deployment density is expected to largely increase in the upcoming sixth Generation (6G) of wireless networks, will naturally necessitate substantial advances in multiple access schemes. Reconfigurable Intelligent Surfaces (RISs) constitute a candidate 6G technology capable to offer dynamic over-the-air signal propagation programmability, which can be optimized for efficient non-orthogonal access of a multitude of devices. In this paper, we study the downlink of a wideband communication system comprising multiple multi-antenna Base Stations (BSs), each wishing to serve an associated single-antenna user via the assistance of a Beyond Diagonal (BD) and frequency-selective RIS. Under the assumption that each BS performs Orthogonal Frequency Division Multiplexing (OFDM) transmissions and exclusively controls a distinct RIS, we focus on the sum-rate maximization problem and present a distributed joint design of the linear precoders at the BSs as well as the tunable capacitances and the switch selection matrices at the multiple BD RISs. The formulated non-convex design optimization problem is solved via successive concave approximation necessitating minimal cooperation among the BSs. Our extensive simulation results showcase the performance superiority of the proposed cooperative scheme over non-cooperation benchmarks, indicating the performance gains with BD RISs via the presented optimized frequency selective operation for various scenarios.

翻译：无线连接设备的激增，预计在即将到来的第六代（6G）无线网络中其部署密度将大幅增加，这自然需要多元接入方案的实质性进展。可重构智能表面（RIS）是一项候选的6G技术，能够提供动态的空中信号传播可编程性，可被优化以实现大量设备的高效非正交接入。本文研究了一个包含多个多天线基站（BS）的宽带通信系统的下行链路，每个基站希望通过一个超对角（BD）且频率选择性的RIS辅助服务其关联的单天线用户。假设每个基站执行正交频分复用（OFDM）传输并独立控制一个不同的RIS，我们聚焦于和速率最大化问题，并提出了一种分布式联合设计方案，用于优化各基站处的线性预编码器，以及多个BD RIS处的可调电容和开关选择矩阵。所构建的非凸设计优化问题通过逐次凹逼近方法求解，仅需基站间的最小化协作。我们广泛的仿真结果展示了所提出的协作方案相对于非协作基准的性能优越性，并通过所提出的优化频率选择性操作，为多种场景揭示了BD RIS带来的性能增益。