Simultaneously Transmitting and Reflecting (STAR)-RISs: A Coupled Phase-Shift Model

TL;DR

This paper introduces a coupled phase-shift model for STAR-RISs in communication systems, optimizing power consumption for NOMA and OMA, and demonstrating improved performance over independent phase-shift models through an efficient algorithm.

Contribution

It proposes a coupled phase-shift model for STAR-RISs, addressing practical implementation challenges and developing an efficient optimization algorithm for power minimization.

Findings

The coupled phase-shift model outperforms independent models in simulations.

The proposed algorithm achieves high-quality solutions with linear complexity.

Numerical results validate the effectiveness of the coupled model and optimization approach.

Abstract

A simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) aided communication system is investigated, where an access point sends information to two users located on each side of the STAR-RIS. Different from current works assuming that the phase-shift coefficients for transmission and reflection can be independently adjusted, which is non-trivial to realize for purely passive STAR-RISs, a coupled transmission and reflection phase-shift model is considered. Based on this model, a power consumption minimization problem is formulated for both non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA). In particular, the amplitude and phase-shift coefficients for transmission and reflection are jointly optimized, subject to the rate constraints of the users. To solve this non-convex problem, an efficient element-wise alternating optimization algorithm is developed to find a high-quality suboptimal solution, whose complexity scales only linearly with the number of STAR elements. Finally, numerical results are provided for both NOMA and OMA to validate the effectiveness of the proposed algorithm by comparing its performance with that of STAR-RISs using the independent phase-shift model and conventional reflecting/transmitting-only RISs.