Optimization of Rate-Splitting Multiple Access in Beyond Diagonal RIS-Assisted URLLC Systems

Abstract

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