Performance Analysis of Rate-Splitting Multiple Access in Coordinated Direct and Relay Transmission

Next generation internet-of-things (IoT) demands massive connectivity, reliability, scalability, quality of service (QoS), and high spectral efficiency. In order to serve a large number of devices with varying channel conditions, non-orthogonal multiple access (NOMA) has emerged as a potential solution. However, a major drawback of NOMA is that it suffers from loss of degrees of freedom which results in insufficient gain. To combat this, rate-splitting multiple access (RSMA) has been proposed in the literature. In this work, we introduce RSMA in a coordinated direct and relay transmission (CDRT) with two clusters of users to improve spectrum efficiency and system performance. In the proposed system, a base station (BS) communicates directly with users in cluster-1, and through decode and forward (DF) relay with users in cluster-2. A passive intelligent reflecting surface (IRS) is synergistically combined with a DF relay called the hybrid relay network (HRN) to provide beneficial multipath diversity. We derive the analytical expressions for outage probability and ergodic capacity of the HRN system. The analytical expressions are validated through simulation results. Further, we compare our proposed scheme with the existing NOMA-CDRT scheme and provide insights based on the analysis.