标题： 将元纤维引入堆叠智能超表面以进行MIMO通信：仅含两层的低复杂度设计 显示英文标题

标题： Introducing Meta-Fiber into Stacked Intelligent Metasurfaces for MIMO Communications: A Low-Complexity Design with only Two Layers

摘要： 叠加工智能超表面（SIMs），集成了多个可编程超表面层，最近作为一种先进的波域信号处理技术崭露头角。 SIMs受益于灵活的空间自由度（DoF），同时降低了对昂贵射频（RF）链路的需求。 然而，当前最先进的SIM设计面临诸如复杂的相移优化和多层导致的能量衰减等挑战。 为了解决这些问题，我们提出将元纤维引入SIMs，旨在减少层数并提高能量效率。 首先，我们介绍一种与传统多层结构具有相同灵活信号处理能力的元纤维连接的两层SIM，并解释其工作原理。 随后，我们制定并解决了最小化SIM信道与期望信道矩阵之间的均方误差（MSE）的优化问题。 具体而言，通过设计与发射-SIM和接收-SIM相关的元原子的相移，建立了一个具有并行子信道的无干扰系统。 为了降低计算复杂度，提出了交替优化（AO）算法每次迭代中每个相移的显式表达式。 我们表明所提出的算法适用于传统的多层SIM。 分析了信道容量边界和计算复杂度，以提供设计见解。 最后，给出了数值结果，表明与传统的七层SIM相比，所提出的带有元纤维的两层SIM在信道容量方面提高了25%以上，同时总元原子数量减少了59%。 显示英文摘要

摘要： Stacked intelligent metasurfaces (SIMs), which integrate multiple programmable metasurface layers, have recently emerged as a promising technology for advanced wave-domain signal processing. SIMs benefit from flexible spatial degree-of-freedom (DoF) while reducing the requirement for costly radio-frequency (RF) chains. However, current state-of-the-art SIM designs face challenges such as complex phase shift optimization and energy attenuation from multiple layers. To address these aspects, we propose incorporating meta-fibers into SIMs, with the aim of reducing the number of layers and enhancing the energy efficiency. First, we introduce a meta-fiber-connected 2-layer SIM that exhibits the same flexible signal processing capabilities as conventional multi-layer structures, and explains the operating principle. Subsequently, we formulate and solve the optimization problem of minimizing the mean square error (MSE) between the SIM channel and the desired channel matrices. Specifically, by designing the phase shifts of the meta-atoms associated with the transmitting-SIM and receiving-SIM, a non-interference system with parallel subchannels is established. In order to reduce the computational complexity, a closed-form expression for each phase shift at each iteration of an alternating optimization (AO) algorithm is proposed. We show that the proposed algorithm is applicable to conventional multi-layer SIMs. The channel capacity bound and computational complexity are analyzed to provide design insights. Finally, numerical results are illustrated, demonstrating that the proposed two-layer SIM with meta-fiber achieves over a 25% improvement in channel capacity while reducing the total number of meta-atoms by 59% as compared with a conventional seven-layer SIM.