Analysis and Optimization of Beyond Diagonal-RIS Assisted RSMA Multi-User MIMO Network

The beyond-diagonal reconfigurable intelligent surface (BD-RIS) represents the next advancement in reconfigurable intelligent surfaces (RIS), offering enhanced flexibility by supporting multiple modes and architectures. This study examines the spectral efficiency (SE) of a single-cell RSMA-based multi-user MIMO system assisted by two sector BD-RIS. The scattering matrix of BD-RIS and the power allocation between common and private precoders primarily determine the SE. We employ a two-step optimization process to determine these parameters. First, the scattering matrix is optimized by maximizing the total channel gain of the user equipment (UEs). Using the optimized scattering matrix, the power allocation for the common and private streams is then calculated by maximizing the SE of the system through a fractional programming technique, which gives a closed-form solution. The SE is computed using the optimized parameters and is numerically plotted to evaluate the performance of the system. The system is extensively analyzed across various precoding techniques, numbers of UEs, and BD-RIS elements, and is also compared to space division multiple access (SDMA). Numerical results show that BD-RIS outperforms both the traditional RIS and the scenario without RIS.