Sum rate maximization for hybrid relay-RIS-assisted MU-MISO systems: multiple access techniques

Abstract

Space Division Multiple Access (SDMA) is a pivotal multiple access technique in modern wireless communication systems. Additionally, Reconfigurable Intelligent Surfaces (RIS) is recognized as fundamental technology for next-generation wireless communications. This work investigates a multiuser downlink Multiple-Input Single-Output (MISO) system with an underloaded or critically loaded network. Here, a multiantenna base station (BS) communicates with multiple single-antenna users by leveraging a combination of a half-duplex decode-and-forward (DF) relay and a full-duplex RIS. In this study, the aim is to maximize the sum rate by joint design of active beamforming at the BS and the DF relay in addition to passive beamforming at the RIS under maximum power constraints, minimum SINR constraints at the relay, and unit-modulus constraint for the RIS elements. The complex design problem is addressed using Lagrangian Dual Transformation (LDT), Quadratic Transformation (QT), and Semidefinite Relaxation (SDR). Also, an alternating optimization algorithm is proposed. In the context of performance evaluation, benchmarks such as Non-Orthogonal Multiple Access (NOMA) and Orthogonal Multiple Access (OMA) are considered. Furthermore, Particle Swarm Optimization (PSO) is utilized for RIS phase profile optimization as a benchmark for the model-based proposed algorithm. This comparative analysis provides insights into the effectiveness of the proposed SDMA-enabled hybrid RIS-Relay communication system and the efficiency of the metaheuristic PSO algorithm for large sizes of the RIS.