Low-Complexity RSMA Approach for Enhanced Multi-User Decode-and-Forward Relay Systems

Rate-Splitting Multiple Access (RSMA) has emerged as a robust transmission strategy for multi-antenna wireless systems. This paper investigates the performance of RSMA in a downlink Decodeand-Forward (DF) relay network under imperfect Channel State Information (CSI) at both the transmitter and the relay. The system operates in two phases: in the first phase, the Base Station (BS) transmits signals to both BS Users (BUs) and the relay; in the second phase, the relay decodes and forwards the signals to Relay Users (RUs) located outside the BS coverage area. RSMA is employed for facilitating transmission from both the BS and the relay. To optimize the network performance, we derive a tractable lower bound for the ergodic sum-rate, which enables the power allocation coefficients of common and private streams in the RSMA structures to maximize the overall sum-rate in both phases. The simulation results demonstrate that the proposed power allocation algorithm, coupled with a low-complexity precoding design, significantly improves the sum-rate performance of DF relay RSMA networks compared to scenarios where RSMA is not utilized. Notably, RSMA outperforms Spatial Division Multiple Access (SDMA)-based benchmarks, achieving sum-rate gains of up to 81%. Furthermore, a three-user use-case scenario is examined, revealing that RSMA consistently surpasses Non-Orthogonal Multiple Access (NOMA) and Orthogonal Multiple Access (OMA)-based benchmarks, even in the presence of imperfect channel state information (CSI) at both the transmitter and the relay.