I. Introduction
The pursuit of enhanced spectral efficiency continues to shape the evolution of communication systems. Over the past few decades, wireless communication has experienced remarkable advancements propelled by various technologies, including multiple-input multiple-output (MIMO), orthogonal frequency division multiplexing, and millimeter-wave solutions. However, these technologies have primarily focused on elevating data rates through spectrum resource optimization and multiplexing, often without treating the channel as a malleable element in the design process. Traditional physical-layer communication systems typically optimize system performance based on specific channel conditions to achieve higher spectral efficiency. In recent years, research on metasurfaces has made substantial strides, leading to the emergence of reconfigurable intelligent surfaces (RIS) that can dynamically manipulate communication environments. This introduces the channel as a new dimension for design, holding the potential to enhance spectral efficiency while minimizing energy consumption and hardware costs [2], [3], [4], [5], [6], [7].