Stacked intelligent metasurfaces (SIMs) have emerged as a powerful paradigm for wave-domain signal processing, enabling fine-grained control over electromagnetic (EM) propagation in next-generation wireless systems. However, conventional multi-layer SIMs often suffer from excessive structural complexity, high computational overhead, and significant power attenuation across layers, limiting their performance. In this paper, we first characterize SIMs from the perspectives of functionality, application, and layer configuration, revealing the inherent trade-offs between signal processing flexibility and power efficiency. Then, two representative 2-layer architectures, the meta-fiber-connected SIM (MF-SIM) and the flexible intelligent layered metasurface (FILM), are introduced, each advocating a distinct 2-layer SIM design philosophy. Moreover, we identify several open challenges in topology optimization for MF-SIM, shape control for FILM, and hybrid 2-layer architectures. Finally, case studies considering 2-layer MF-SIM and FILM assisted point-to-point multiple-input multiple-output (MIMO) and multi-user communication systems validate that properly designed 2-layer SIMs can significantly reduce power loss and optimization burden while maintaining good signal processing performance, offering a promising pathway toward practical SIM-enabled 6G systems.

翻译：堆叠智能超表面（SIM）已成为波域信号处理的一种强大范式，能够在下一代无线系统中实现对电磁（EM）传播的精细控制。然而，传统的多层SIM通常存在结构复杂度过高、计算开销大以及层间功率衰减显著等问题，限制了其性能。本文首先从功能、应用和层配置的角度对SIM进行表征，揭示了信号处理灵活性与功率效率之间固有的权衡关系。接着，介绍了两种具有代表性的双层架构：超光纤连接SIM（MF-SIM）和柔性智能分层超表面（FILM），各自体现了不同的双层SIM设计理念。此外，我们指出了MF-SIM的拓扑优化、FILM的形状控制以及混合双层架构中存在的若干开放挑战。最后，通过考虑双层MF-SIM和FILM辅助的点对点多输入多输出（MIMO）以及多用户通信系统的案例研究验证，合理设计的双层SIM能够在保持良好信号处理性能的同时，显著降低功率损耗和优化负担，为实际支持SIM的6G系统提供了一条有前景的技术路径。