From Multi-Port Models to Cascade Structures: Optimization of Active Unilateral Stacked Intelligent Metasurfaces

TL;DR

This paper introduces a multi-port S-parameter framework for analyzing and optimizing stacked intelligent metasurfaces with unilateral active interconnections, enabling efficient design and improved spectral efficiency.

Contribution

It develops a recursive cascade model for SIMs with active interconnections and proposes a gradient-based optimization algorithm that reduces computational complexity.

Findings

Numerical simulations show trade-offs between inter-layer spacing, active gain, and size.

The proposed model accurately predicts electromagnetic behavior of SIMs.

Optimization improves channel diagonalization and spectral efficiency.

Abstract

This paper develops a multi-port S-parameter framework for the analysis and optimization of stacked intelligent metasurfaces (SIMs) with unilateral active interconnections. By modeling each unit cell as a non-reciprocal two-port network, the resulting SIM exhibits a feed-forward structure that enables a recursive, cascade-like representation of the end-to-end transfer function while preserving electromagnetic accuracy. Based on this model, we derive an efficient gradient-based optimization algorithm with reduced computational complexity compared to conventional reciprocal SIM architectures. Numerical results, obtained from full-wave simulations, illustrate the trade-offs among inter-layer spacing, active gain, and SIM size in terms of channel diagonalization and achievable spectral efficiency.