Beyond Diagonal RIS: A New Frontier for 6G Internet of Things Networks

Reconfigurable intelligent surface (RIS) technology is emerging as a key enabler for upcoming sixth-generation (6G) and beyond wireless networks, shifting from passive radio environments to programmable wave propagation. While diagonal RIS (D-RIS) offers benefits, its restricted manipulation capability limits overall performance. This paper explores the novel concept of beyond-diagonal RIS (BD-RIS) for the Internet of Things (IoT), which introduces non-diagonal elements in the scattering matrix to achieve finer control of electromagnetic wavefronts. We first outline the drawbacks of conventional D-RIS and present representative BD-RIS architectures with their operating modes. We then emphasize the distinctive advantages of BD-RIS in supporting advanced 6G IoT applications. A case study on BD-RIS-enabled vehicle-to-vehicle (V2V) communication demonstrates potential performance improvements. Finally, we identify key challenges that remain and propose new research directions to advance energy-efficient, scalable, and reliable 6G IoT networks.