Reconfigurable Intelligent Surface (Ris)-Enhanced Two-Way Ofdm Communications

In this paper, we focus on the reconfigurable intelligent surface (RIS)-enhanced two-way device-to-device (D2D) multi-pair orthogonal-frequency-division-multiplexing (OFDM) communication systems. Specifically, we maximize the minimum bidirectional weighted sum-rate by jointly optimizing the sub-band allocation, the power allocation, and the discrete phase shift (PS) design of the reflecting elements at the RIS. To tackle the main difficulty of the non-convex PS design, we firstly formulate a semi-definite relaxation problem and further devise a low-complexity solution for the PS design by leveraging the projected sub-gradient method. We demonstrate the desirable performance gain for the proposed designs through numerical results, where the number of reflecting elements at the RIS and the deployment location of RIS are shown to play a key role in enhancing the performance gain.