Reconfigurable intelligent surfaces (RISs) enable programmable control of wireless propagation. Beyond environmental deployments, integrating metasurfaces at the antenna front end allows direct manipulation of the radiated electromagnetic field and enables wave-domain signal processing. In this context, stacked intelligent metasurfaces (SIMs) have recently been proposed as an advanced architecture in which multiple programmable metasurface layers interact through wave propagation, enabling richer and more flexible electromagnetic transformations than conventional single-layer designs. By leveraging cascaded wave-matter interactions at the transmitter or receiver front end, SIMs substantially expand the design space of programmable wireless systems. This survey provides a comprehensive overview of SIMs technologies from the electromagnetic processing perspective, covering their physical principles, modeling frameworks, hardware realizations, and emerging architectural designs. We review existing modeling approaches based on cascaded operators, multiport impedance formulations, and network parameter representations, and discuss their implications for scalable optimization and system design. The survey further examines key communication functionalities enabled by front-end metasurface processing, including communication performance optimization, near-field and wideband transmission, learning-driven control, integrated sensing and communications, and emerging architectures such as cell-free and non-terrestrial networks. Finally, we identify open research problems related to physical modeling, scalability, hardware-algorithm co-design, and network integration, and outline promising directions toward realizing SIM-based antenna front ends as fully programmable electromagnetic processors for future sixth-generation (6G) wireless systems.

翻译：可重构智能超表面（RIS）实现了对无线传播环境的可编程调控。除了在环境中部署外，将超表面集成于天线前端可直接调控辐射电磁场，实现波域信号处理。在此背景下，堆叠智能超表面（SIM）作为一种先进架构被提出，其通过多个可编程超表面层间的波传播相互作用，实现了比传统单层设计更丰富、更灵活的电磁变换。通过在发射端或接收端前端利用级联的波-物质相互作用，SIM显著拓展了可编程无线系统的设计空间。本综述从电磁处理视角对SIM技术进行全面概述，涵盖其物理原理、建模框架、硬件实现及新兴架构设计。我们回顾了基于级联算子、多端口阻抗公式和网络参数表示的现有建模方法，并讨论了它们对可扩展优化与系统设计的意义。本文进一步探讨了通过前端超表面处理实现的关键通信功能，包括通信性能优化、近场与宽带传输、学习驱动的控制、集成感知与通信，以及无蜂窝网络与非地面网络等新兴架构。最后，我们指出了与物理建模、可扩展性、硬件-算法协同设计及网络集成相关的开放研究问题，并展望了将基于SIM的天线前端实现为未来第六代（6G）无线系统全可编程电磁处理器的潜在发展方向。