Effect of Channel Aging on Beyond Diagonal Reconfigurable Intelligent Surfaces

Reconfigurable intelligent surface (RIS) has appeared as a revolutionary candidate technology for future sixth-generation (6G) networks, but most works have relied on single-connected reflective RISs, which are mathematically described by diagonal phase shift matrices. In this work, based on the recently presented research on beyond diagonal (BD) RIS unifying different RIS models and architectures towards enhanced advantages such as a greater coverage, we study the impact of channel aging due to user equipment (UE) movement. Especially, we evaluate how channel aging diminishes the system performance of multiple sector BD-RIS systems. Through a robust design, concerning the average sum-rate maximisation problem, we jointly design the BD-matrix and transmit precoder under the channel aging conditions. Numerical results show how channel aging affects performance with respect to fundamental system parameters and shed light on how general aging can be compensated.