An Element Selection Enhanced Hybrid Relay-RIS Assisted Communication System

In this letter, we propose a simple yet efficient element selection (ES) enhanced hybrid relay-reconfigurable intelligent surface (RIS) assisted communication system, where a subset of RIS elements is selected from the whole element set to connect with an additional power amplifier (PA) for realizing the active RIS. In contrast, the remanent elements reflect the signals passively without amplification serving as the passive RIS. To obtain the optimal active RIS element set, we propose to utilize a 0–1 diagonal ES matrix in the conventional RIS-assisted systems to express the connection relationship between the RIS elements and the PA. Then, we relax the 0–1 constraint and jointly optimize the ES matrix and RIS coefficients with an alternating optimization (AO) method. Due to the particular form of the ES matrix, the optimization problem can be transformed into a linear programming form, which indicates that the optimal solution of the ES matrix is the boundary of the constraint set, i.e., 0 or 1, which compensates for the influence of linear relaxation. The corresponding simulation results demonstrate that the proposed ES scheme was capable of outperforming the sub-connected scheme and the HR-RIS scheme without ES.