Multiuser MIMO-OFDM Communications Assisted by an Active RIS Using Practical Broadband Model

Being capable of realizing smart and controllable channel environments, the reconfigurable intelligent surface (RIS) shows great potential to improve both the spectral and energy efficiencies. Due to the multiplicative fading effect of reflective RIS links, the capacity gain induced by passive RIS (PRIS) is limited in communication scenarios with blocked or very weak direct links. This work introduces an active RIS (ARIS) to a multiuser multi-input multi-output (MIMO) broadband communication system with orthogonal frequency division multiplexing (OFDM), aiming at enhancing the transmission rate. Specifically, a sum rate optimization problem is formulated based on a practical broadband reflection model of the ARIS. An algorithm for addressing a suboptimal subcarrier allocation, the optimal precoding matrix of the base station (BS), and the suboptimal reflecting coefficients of ARIS is proposed. Specifically, a channel attenuation coefficient-based Hungarian algorithm is used to fastly address a suboptimal subcarrier allocation. The optimal precoding matrix of the BS is presented explicitly, where water-filling form solution is given for two special cases. The weighted minimum mean square error method associated with a penalty method is used to find a suboptimal reflecting coefficients of the ARIS, which well handle the coefficient coupling across subcarriers inherent in the practical broadband RIS. Numerical results validate the convergence and effectiveness of the proposed algorithm. It is shown that by using the proposed algorithm, the sum rate of the system can be enhanced significantly.