Survey on Beyond Diagonal RIS-Enabled 6G Wireless Networks: Fundamentals, Recent Advances, and Challenges

Beyond diagonal reconfigurable intelligent surfaces (BD-RIS) are a remarkable new technology for sixth-generation (6G) wireless networks. They give users more control over wireless propagation conditions than traditional diagonal RIS (D-RIS). This survey gives a full picture of BD-RIS, including its designs, how it works, and how it can be modelled mathematically. It also talks about the benefits of its performance. We look at BD-RIS categories, such as single-connected, fully-connected, group-connected, tree-connected, forest-connected, band-connected, cluster-connected, and dynamically-connected architectures, as well as their reflective, transmissive, hybrid, and multi-sector operating modes. This article looks at recent progress in BD-RIS-enabled 6G networks, emphasising on important aspects including channel estimation (CE), optimising sum-rate and spectrum efficiency, improving energy efficiency, received power and channel gain, security, and hardware prototypes and testbeds. It also talks about how BD-RIS works with other new 6G technologies. The survey points out some of the main problems with BD-RIS research, such as the restrictions of hardware design, adaptive CE, and the consequences of non-ideal hardware. Proposed future research objectives for BD-RIS include the incorporation of artificial intelligence and machine learning (AI/ML), the simultaneous optimisation of communication and sensing, and the improvement of physical layer security (PLS). This study concludes by emphasising BD-RIS’s disruptive potential to reshape 6G wireless networks, providing significant insights and lessons for future research and development.