Cell-Free Massive MIMO SWIPT with Beyond Diagonal Reconfigurable Intelligent Surfaces

This paper investigates the integration of beyond-diagonal reconfigurableintelligent surfaces (BD-RISs) into cell-free massive multiple-inputmultiple-output (CF-mMIMO) systems, focusing on applications involvingsimultaneous wireless information and power transfer (SWIPT). The systemsupports concurrently two user groups: information users (IUs) and energy users(EUs). A BD-RIS is employed to enhance the wireless power transfer (WPT)directed towards the EUs. To comprehensively evaluate the system's performance,we present an analytical framework for the spectral efficiency (SE) of IUs andthe average harvested energy (HE) of EUs in the presence of spatial correlationamong the BD-RIS elements and for a non-linear energy harvesting circuit. Ourfindings offer important insights into the transformative potential of BD-RIS,setting the stage for the development of more efficient and effective SWIPTnetworks. Finally, incorporating a heuristic scattering matrix design at theBD-RIS results in a substantial improvement compared to the scenario withrandom scattering matrix design.