Eit-Based Electromagnetic Diffraction Propagation Characteristics of RIS-MIMO Transmitters

Reconfigurable Intelligent Surfaces (RIS) can directly modulate information into electromagnetic waves by using a data source to control the electromagnetic responses of the RIS units. With far fewer RF chains and modest area requirements, RIS is a promising candidate for antenna arrays in future holographic MIMO and extra-large MIMO systems. However, the physical structure of the RIS unit aperture causes diffraction, a phenomenon that was ignored in existing studies, making the adopted propagation models no longer applicable, especially in high-frequency or large-scale systems. In this paper, based on electromagnetic information theory (EIT), we present vector diffraction equations to accurately model the electromagnetic propagation characteristics of a RIS-MIMO transmitter. To accommodate near-field and far-field applications, we propose approximate propagation models respectively. This approach also serves as a theoretical method to determine the boundaries between different fields, helping researchers in selecting the appropriate propagation model based on phase precision requirements and computational complexity tolerance. Using these models, the mutual information of the RIS-MIMO transmitter is derived to reveal the upper bound of its information transmission capacity based on random field theory.