On Group Connected Transmissive Beyond Diagonal RIS for MIMO Systems

Reconfigurable intelligent surfaces (RIS) have emerged as an important technology for next-generation wireless networks by intelligently manipulating the wireless propagation environment. Beyond Diagonal RIS (BD-RIS) extends the traditional RIS architecture by allowing non-diagonal reflection matrices, enabling more flexible signal manipulation. Transmissive RIS (T-RIS), on the other hand, facilitates the transmission of signals through the metasurfaces. In this paper, we propose a novel design called transmissive BD-RIS (T-BD-RIS), which integrates the functionalities of BD-RIS and T-RIS to enhance the user data rate. We design an algorithm for the group connected (GC) configuration, which jointly optimizes the beamforming at the base station, the T-BD-RIS transmission matrix, and the receive combiner at the user side. The fully connected (FC) and single connected (SC) cases are special instances of the proposed generic GC design, obtained by an appropriate choice of the number of groups. We evaluate the performance of various schemes, demonstrating the potential of the proposed approach.