In this paper, a wideband cell-free multi-stream multi-user Multiple-Input Multiple-Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) system is considered operating within a smart wireless environment enabled by multiple Beyond Diagonal Reconfigurable Intelligent Surfaces (BDRISs). A novel decentralized active and passive beamforming framework, robust to imperfect channel state availability and with minimal cooperation among the system's multiple Base Stations (BSs) for deciding the final configurations of the shared BDRISs, is proposed, which aims to substantially reduce the overhead inherent in centralized solutions necessitating a central processing unit of high computational power. By considering a Dynamic Group-Connected (DGC) BDRIS architecture with frequency-selective responses per unit element, we formulate the system's sum-rate maximization problem with respect to the tunable capacitances and permutation matrices of the BDRISs as well as the precoding matrices of the BSs, which is solved via successive concave approximation and alternating projections as well as consensus-based updates for the BDRISs' design. Through extensive simulation results, it is showcased that the proposed robust decentralized cooperative approach with diverse BDRIS architectures outperforms non-cooperation benchmarks. It is also demonstrated that the considered DGC BDRIS architecture is able to provide sum-rate performance gains sufficiently close to the more complex fully-connected BDRIS structure.

翻译：本文研究了一种在由多个超对角可重构智能表面（BDRIS）赋能的智能无线环境中运行的宽带无蜂窝多流多用户多输入多输出（MIMO）正交频分复用（OFDM）系统。我们提出了一种新颖的去中心化主动与被动波束成形框架，该框架对不完善的信道状态信息具有鲁棒性，且系统内多个基站（BS）在确定共享BDRIS最终配置时仅需最小程度的协作，旨在显著降低集中式解决方案中因需要高计算能力中央处理单元而产生的固有开销。通过考虑一种具有单元频率选择性响应的动态组连接（DGC）BDRIS架构，我们以BDRIS的可调电容与置换矩阵以及基站的预编码矩阵为变量，构建了系统总速率最大化问题，并通过逐次凹逼近、交替投影以及基于共识的BDRIS设计更新算法进行求解。大量仿真结果表明，所提出的鲁棒去中心化协作方法结合多种BDRIS架构，其性能优于非协作基准方案。同时证明，所考虑的DGC BDRIS架构能够提供与更复杂的全连接BDRIS结构足够接近的总速率性能增益。