An Adaptive Transmission Protocol Enabled by The State Switching Strategy of Beyond-Diagonal RIS

Thanks to inter-element interconnections and flexible element arrangements, the beyond diagonal reconfigurable intelligent surface (BD-RIS) breaks through the limitation of traditional RIS to achieve enhanced performance and enlarged coverage. However, existing BD-RIS research assumes that BD-RIS is always turned ON to assist transmission, while, in some scenarios where the transmitter-receiver direct link exists and remains strong, the performance gains of BD-RIS may not justify its manipulation complexity. In order to smartly use BD-RIS, a novel adaptive transmission scheme is proposed in this paper. The proposed scheme first defines two working states of the BD-RIS, namely ON and OFF. Based on these two states, we design a new beam training protocol that enables BD-RIS to intelligently switch its working state according to real-time channel conditions. Furthermore, we construct a decision threshold as the decision basis for state switching to ensure the efficiency and reliability of protocol execution. Simulation results show that, without always turning ON the BD-RIS and performing sophisticated wave manipulation, the proposed protocol can guarantee successful transmission regardless of whether there is a strong transmitter-receiver link or not.