Flexible Intelligent Layered Metasurfaces for Downlink Multi-user MISO Communications
Hong Niu, Jiancheng An, Chau Yuen
TL;DR
This work tackles the power-inefficiency of deep-stack stacked intelligent metasurfaces (SIMs) by introducing a flexible two-layer FILM architecture that uses shape-controllable metasurfaces to reduce layer count without sacrificing processing capability. An alternating-optimization framework combines closed-form phase-shift updates with gradient-descent shape optimization to achieve channel fitting, steering the end-to-end channel toward an effective identity, HP ≈ αI_K. Theoretical bounds on sum-rate and a complexity analysis are provided, and simulations at 28 GHz show FILM yielding over 200% higher sum-rate and more than 7 dB BER gains compared with a conventional seven-layer SIM, while maintaining robust performance under loss. These results highlight FILM as a practical, hardware-efficient approach for high-capacity MU-MISO systems with flexible metasurfaces.
Abstract
Stacked intelligent metasurfaces (SIMs) have recently gained attention as a paradigm for wave-domain signal processing with reduced reliance on costly radio-frequency (RF) chains. However, conventional SIMs rely on uniform inter-layer spacing and require deep stacking to ensure processing capability, resulting in severe power attenuation in practice. To address this issue, we propose a flexible intelligent layered metasurface (FILM) architecture consisting of two shape-controllable flexible metasurface layers. By replacing rigid metasurfaces with flexible ones in both layers, the transmission coefficient matrix can be dynamically adjusted, significantly decreasing the number of required layers while maintaining signal processing performance. Firstly, we develop a two-layer FILM-assisted multi-user multiple-input single-output (MU-MISO) system, wherein we formulate a channel fitting problem aimed at reducing the difference between the FILM-induced and target channels. Then, we solve this non-convex problem by employing an alternating optimization (AO) method, featuring closed-form phase shift updates and a gradient descent-based shape optimization. Furthermore, we analyze the upper bound on sum-rate and the complexity of computation to provide insights into design trade-offs. Finally, simulation results demonstrated that the proposed transmissive FILM architecture achieves over 200\% improvement in sum-rate and more than 7 dB bit-error rate (BER) gain compared to the conventional seven-layer SIMs.