On weighted sum-rate maximization of full-duplex systems in presence of STAR-RIS

Abstract

Reflecting intelligent surface (RIS) is one of the key enabling technologies for beyond fifth generation wireless networks to further improve coverage, spectral- and energy-efficiency of wireless networks. Recently, a novel simultaneous transmission and reflection RIS (STAR-RIS) technology is introduced which enables more degrees-of-freedom in simultaneously serving users in reflection and transmission regions of RIS. This paper investigates a STAR-RIS assisted two-user full-duplex communication system, wherein multi-antenna base station (BS) and single-antenna users are subject to maximum power constraints. Firstly, weighted sum rates are computed for three well-known STAR-RIS protocols, e.g. energy splitting, mode selection, and time splitting. Then, for each of the cases, weighted sum rate optimization problem is investigated to find optimal resource allocations, i.e. base station's precoding and combining matrices, coefficients of STAR-RIS, and power allocations. The optimization problem is transferred into multiple convex sub-problems using equivalent weighted minimum mean-square-error forms. Also, by use of the successive convex approximation method, tunable parameters of the STAR-RIS are optimized. Although the original problem is a non-convex one, proposed iterative alternating technique achieves an acceptable sub-optimal performance. Finally, performance of the system is analysed and compared to some baselines to highlight superiority of the STAR-RIS compared to the conventional RIS schemes.