Distributed Irregular RIS-Assisted NOMA Systems: User Clustering and Topology Optimisation

This paper introduces movable antenna technology into the Reconfigurable Intelligent Surface (RIS)-assisted Non-Orthogonal Multiple Access (NOMA) communication system to explore the role of spatial domain freedom in enhancing communication system performance. It proposes a distributed irregular RIS-assisted NOMA system integrating user clustering with topology optimization. The irregular RIS topology is optimized to increase the communication freedom for NOMA users, thereby improving system performance based on channel differences. In this distributed RIS-assisted NOMA scenario, the improved spectral clustering algorithm (B-SPC) is first used to cluster the non-uniformly distributed NOMA users (UCs). Subsequently , irregular RIS-assisted communication is tailored for each UC based on fairness principles. The tabu search algorithm (TS) is then employed, utilizing the NOMA user grouping mechanism, to dynamically optimize the topology of the irregular RIS, with the goal of enhancing the channel differences between users within the group and improving decoding performance through successive interference cancellation. Additionally, the power allocation for users and the beamforming of the reflecting surface are jointly optimized to maximize the system's total spectral efficiency. The simulation results demonstrate that combining the B-SPC algorithm with TS to optimize the irregular RIS topology enhances convergence and clustering performance, improves the uniformity of the UCs and ensures fairness in user service. Furthermore, it effectively manages user channel conditions, increasing channel diversity and significantly boosting the overall spectral efficiency of the distributed irregular RIS-assisted NOMA system.