Simultaneously Transmitting and Reflecting Reconfigurable Intelligent Surfaces Empowered Cooperative Rate Splitting with User Relaying

TL;DR

This paper addresses max-min fairness in downlink STAR RIS-assisted CRS by integrating six transmission modes formed from HD/FD CRS relaying and ES/MS/TS STAR RIS protocols. It develops a unified SCA-based AO algorithm to jointly optimize BS active beamforming, STAR RIS passive beamforming, common-rate allocation, and time allocation under power and energy constraints, along with a low-complexity closed-form RIS design. The results show significant fairness gains over conventional CRS and STAR RIS-aided MA schemes, with insights into when FD or HD relaying and ES/MS/TS RIS protocols are preferable, and demonstrate substantial computational savings from the low-complexity algorithm. The work provides practical guidelines for deploying STAR RIS-enabled CRS in future networks and establishes a foundation for scalable interference management in 6G-like systems.

Abstract

In this work, we unveil the advantages of synergizing cooperative rate splitting (CRS) with user relaying and simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR RIS). Specifically, we propose a novel STAR RIS-assisted CRS transmission framework, featuring six unique transmission modes that leverage various combination of the relaying protocols (including full duplex-FD and half duplex-HD) and the STAR RIS configuration protocols (including energy splitting-ES, mode switching-MS, and time splitting-TS). With the objective of maximizing the minimum user rate, we then propose a unified successive convex approximation (SCA)-based alternative optimization (AO) algorithm to jointly optimize the transmit active beamforming, common rate allocation, STAR RIS passive beamforming, as well as time allocation (for HD or TS protocols) subject to the transmit power constraint at the base station (BS) and the law of energy conservation at the STAR RIS. To alleviate the computational burden, we further propose a low-complexity algorithm that incorporates a closed-form passive beamforming design. Numerical results show that our proposed framework significantly enhances user fairness compared with conventional CRS schemes without STAR RIS or other STAR RIS empowered multiple access schemes. Moreover, the proposed low-complexity algorithm dramatically reduces the computational complexity while achieving very close performance to the AO method.

Figures (6)

• Figure 1: The transmission architecture of the proposed STAR RIS-assisted CRS.
• Figure 2: Six transmission modes of the proposed STAR RIS-assisted CRS.
• Figure 3: Max-min rate versus the number of STAR RIS elements $N$, $\mathrm{SNR}=20~\mathrm{dB}$, $K=4$, $N_t=4$.
• Figure 4: Max-min rate versus SNR, $N=50$, $K=4$, $N_t=4$.
• Figure 5: Max-min rate versus SNR, $N=50$, $K=4$, $N_t=4$.