EIT-Based Modeling and Performance Analysis of Spherical Wave and Plane-Wave Diffraction Propagation in SIM-MIMO Systems

It is a challenge to accurately model the propagation with multilayer structures, such as stacked intelligent metasurfaces (SIMs) in electromagnetics research, as conventional models fail to capture dominant diffraction and interlayer coupling effects. To address this challenge, this article develops a physics-based propagation framework grounded in vector Rayleigh-Sommerfeld diffraction integrals. The framework is complemented by an equivalent spatial spectrum formulation based on plane-wave decomposition, which reframes diffraction as a spatial filtering process within linear system theory. For practical applicability, it also incorporates computationally efficient near- and far-field approximations governed by an adaptive phase-error-controlled boundary. To demonstrate its engineering relevance, the framework is applied in electromagnetic information theory (EIT). In an SIM-MIMO system, it establishes a direct link between the diffraction Green's function and the channel matrix. This connection enables the derivation of mutual information (MI) and reveals fundamental tradeoffs between physical parameters and channel capacity. In summary, the proposed approach not only establishes a rigorous link between electromagnetic modeling and system performance but also provides physics-based co-design guidelines that challenge conventional assumptions and enable the optimization of next-generation SIM-MIMO architectures.