In this paper, practical utilization of multiple distributed reconfigurable intelligent surfaces (RISs), which are able to conduct group-specific operations, for multi-group multicasting systems is investigated. To tackle the inter-group interference issue in the multi-group multicasting systems, the block diagonalization (BD)-based beamforming is considered first. Without any inter-group interference after the BD operation, the multiple distributed RISs are operated to maximize the minimum rate for each group. Since the computational complexity of the BD-based beamforming can be too high, a multicasting tailored zero-forcing (MTZF) beamforming technique is proposed to efficiently suppress the inter-group interference, and the novel design for the multiple RISs that makes up for the inevitable loss of MTZF beamforming is also described. Effective closed-form solutions for the loss minimizing RIS operations are obtained with basic linear operations, making the proposed MTZF beamforming-based RIS design highly practical. Numerical results show that the BD-based approach has ability to achieve high sum-rate, but it is useful only when the base station deploys large antenna arrays. Even with the small number of antennas, the MTZF beamforming-based approach outperforms the other schemes in terms of the sum-rate while the technique requires low computational complexity. The results also prove that the proposed techniques can work with the minimum rate requirement for each group.

翻译：本文研究了实际利用多个分布式可重构智能反射面（RIS）进行组特定操作的多组多播系统。为解决多组多播系统中的组间干扰问题，首先考虑基于块对角化（BD）的波束赋形方法。在BD操作消除所有组间干扰后，通过优化多个分布式RIS来最大化每组的最小速率。由于基于BD的波束赋形计算复杂度过高，提出了一种面向多播的迫零（MTZF）波束赋形技术以高效抑制组间干扰，并描述了弥补MTZF波束赋形不可避免损耗的新型多RIS设计方案。通过基本线性操作获得了最小化损耗的RIS闭式解，使所提基于MTZF波束赋形的RIS设计方案具有高度实用性。数值结果表明：基于BD的方法能实现高总速率，但仅适用于基站部署大规模天线阵列的场景。即使天线数量较少，基于MTZF波束赋形的方法在总速率方面仍优于其他方案，且所需计算复杂度较低。结果还证明，所提技术能在满足每组最小速率要求的条件下正常工作。