Joint Design for Simultaneously Transmitting And Reflecting (STAR) RIS Assisted NOMA Systems

TL;DR

This paper introduces a novel STAR-RIS assisted NOMA system that extends coverage and enhances signal manipulation, proposing an iterative optimization algorithm to maximize sum rate and demonstrating superior performance over traditional RIS systems.

Contribution

The paper proposes a new STAR-RIS assisted NOMA system with a joint optimization framework and an efficient decoding order scheme, advancing beyond traditional RIS methods.

Findings

The proposed algorithm effectively maximizes sum rate in STAR-RIS-NOMA systems.

Simulation results show STAR-RIS-NOMA outperforms conventional RIS-NOMA and RIS-OMA.

The joint optimization improves signal coverage and system capacity.

Abstract

Different from traditional reflection-only reconfigurable intelligent surfaces (RISs), simultaneously transmitting and reflecting RISs (STAR-RISs) represent a novel technology, which extends the half-space coverage to full-space coverage by simultaneously transmitting and reflecting incident signals. STAR-RISs provide new degrees-of-freedom (DoF) for manipulating signal propagation. Motivated by the above, a novel STAR-RIS assisted non-orthogonal multiple access (NOMA) (STAR-RIS-NOMA) system is proposed in this paper. Our objective is to maximize the achievable sum rate by jointly optimizing the decoding order, power allocation coefficients, active beamforming, and transmission and reflection beamforming. However, the formulated problem is non-convex with intricately coupled variables. To tackle this challenge, a suboptimal two-layer iterative algorithm is proposed. Specifically, in the inner-layer iteration, for a given decoding order, the power allocation coefficients, active beamforming, transmission and reflection beamforming are optimized alternatingly. For the outer-layer iteration, the decoding order of NOMA users in each cluster is updated with the solutions obtained from the inner-layer iteration. Moreover, an efficient decoding order determination scheme is proposed based on the equivalent-combined channel gains. Simulation results are provided to demonstrate that the proposed STAR-RIS-NOMA system, aided by our proposed algorithm, outperforms conventional RIS-NOMA and RIS assisted orthogonal multiple access (RIS-OMA) systems.