A Risk-Averse Outage Probability Minimization Method for RIS-Aided RSMA Systems

This letter presents a novel method for optimizing the outage performance of Reconfigurable Intelligent Surfaces (RIS)-assisted Rate-Splitting Multiple Access (RSMA) systems. The objective is to minimize the maximum outage probability for all messages to improve fairness. A new formulation based on minimizing the Entropic Value-at-Risk (EVaR) of the perceived minimum rate among all messages in the system is proposed to simplify the objective. We show that this formulation is equivalent to minimizing an upper bound on the maximum outage probability, then we transform it into a tractable smooth problem by devising a twice-differentiable approximation to it. In addition to its lower complexity, our method is shown by numerical results to outperform two benchmarks, namely, Discrete Exhaustive Search (DES) and Sequential Quadratic Programming (SQP)-based search methods.