This paper proposes a general optimization framework for rate splitting multiple access (RSMA) in beyond diagonal (BD) reconfigurable intelligent surface (RIS) assisted ultra-reliable low-latency communications (URLLC) systems. This framework can provide a suboptimal solution for a large family of optimization problems in which the objective and/or constraints are linear functions of the rates and/or energy efficiency (EE) of users. Using this framework, we show that RSMA and RIS can be mutually beneficial tools when the system is overloaded, i.e., when the number of users per cell is higher than the number of base station (BS) antennas. Additionally, we show that the benefits of RSMA increase when the packets are shorter and/or the reliability constraint is more stringent. Furthermore, we show that the RSMA benefits increase with the number of users per cell and decrease with the number of BS antennas. Finally, we show that RIS (either diagonal or BD) can highly improve the system performance, and BD-RIS outperforms regular RIS.

翻译：本文提出了一种通用的优化框架，用于超可靠低延迟通信（URLLC）系统中超越对角线可重构智能表面（BD-RIS）辅助的速率分割多址接入（RSMA）。该框架可为目标函数和/或约束条件为用户速率和/或能效（EE）的线性函数的一大类优化问题提供次优解。利用该框架，我们证明当系统过载时（即每小区用户数高于基站天线数），RSMA与RIS可互为增强工具。此外，研究显示RSMA的优势在数据包长度更短和/或可靠性约束更严格时更为显著。进一步表明，RSMA的收益随每小区用户数增加而提升，随基站天线数增加而降低。最后，我们证实RIS（无论是常规对角型还是超越对角型）均能大幅提升系统性能，且BD-RIS优于常规RIS。