Beyond Diagonal RISs: Architectures and Operating Modes

Reconfigurable intelligent surface (RIS) is expected to be a key technology in 6G to enhance wireless systems by efficiently and cost-effectively manipulating the propagation environment. In conventional RIS, each RIS element is independently controlled by a tunable load disconnected from the other elements. Thus, conventional RIS results in a diagonal scattering matrix, also known as a phase shift matrix, which has limited passive beamforming capabilities. To enhance the flexibility of RIS, beyond diagonal RIS (BD-RIS) has been introduced as a generalization of conventional RIS, in which the scattering matrix is not restricted to being diagonal. In this chapter, we review the emerging concept of BD-RIS, showing its promising gains in terms of performance and coverage over conventional RIS. We first model BD-RIS-aided systems and characterize the RIS scattering matrix as a function of the RIS circuit topology. Based on the presented model, we analyze the performance and circuit complexity of five BD-RIS architectures, including conventional RIS. In addition, we show that BD-RIS can enable full-space coverage by working in the so-called hybrid reflective and transmissive mode and multi-sector mode. Finally, we outline the future research directions for BD-RIS.