Sum-rate maximization in OFDM system with practical model reconfigurable intelligent surfaces

Reconfigurable Intelligent Surfaces (RISs) have emerged as a promising technology for enhancing wireless communication by reconfiguring the propagation environment. Most existing studies assume ideal RIS elements with zero electrical resistance, which limits their applicability in realistic settings. This study considers a practical RIS-aided wideband OFDM system with resistive reflecting elements (REs), where a single-antenna source communicates with a single-antenna destination. The goal is to maximize the transmission rate by jointly optimizing the subcarrier power allocation and RE phase shifts. An alternating optimization (AO) framework is proposed, in which each variable is updated iteratively, and the phase shifts are refined via gradient descent (GD). A joint optimization strategy was also considered for comparison. Furthermore, a low-complexity alternative based on solving a channel power maximization (CPM) problem is introduced to obtain suboptimal phase shifts. The numerical results show that the AO method achieves near-optimal performance with a significantly reduced computation time compared to the successive convex approximation (SCA). Although the GD-based CPM method incurs higher complexity than dimension-wise sinusoidal maximization (DSM), it yields superior rate performance and improved robustness against RE resistance variations, making it more suitable for practical RIS deployment.