[论文解读] Stacked Intelligent Metasurfaces for Multiuser Downlink Beamforming in the Wave Domain

Jiancheng An, Marco Di Renzo+3 more|arXiv (Cornell University)|Sep 6, 2023

Antenna Design and Analysis被引用 7

一句话总结

该论文提出了一种用于波域多用户下行链路波束成形的堆叠智能超表面（SIM），以可重构波前调控替代传统的数字波束成形。通过联合优化多层超表面的功率分配与离散相移，SIM在消除高复杂度数字处理和DAC的同时，相较传统MISO系统实现了200%的和速率增益。

ABSTRACT

Intelligent metasurface has recently emerged as a promising technology that enables the customization of wireless environments by harnessing large numbers of low-cost reconfigurable scattering elements. However, prior studies have predominantly focused on single-layer metasurfaces, which have limitations in terms of wave-domain processing capabilities due to practical hardware limitations. In contrast, this paper introduces a novel stacked intelligent metasurface (SIM) design. Specifically, we investigate the integration of SIM into the downlink of a multiuser multiple-input single-output (MISO) communication system, where an SIM, consisting of a multilayer metasurface structure, is deployed at the base station (BS) to facilitate transmit beamforming in the electromagnetic wave domain. This eliminates the need for conventional digital beamforming and high-resolution digital-to-analog converters at the BS. To this end, an optimization problem is formulated to maximize the sum rate of all user equipments by jointly optimizing the transmit power allocation at the BS and the wave-based beamforming at the SIM, subject to constraints on the transmit power budget and discrete phase shifts. Furthermore, we propose a computationally efficient algorithm for solving the formulated joint optimization problem and elaborate on the potential benefits of employing SIM in wireless networks. Numerical results are illustrated to corroborate the effectiveness of the proposed SIM-enabled wave-based beamforming design and to evaluate the performance improvement achieved by the proposed algorithm compared to various benchmark schemes. It is demonstrated that considering the same number of transmit antennas, the proposed SIM-based system achieves about 200\% improvement in terms of sum rate compared to conventional MISO systems. The code for this paper is available at \url{https://github.com/JianchengAn}.

研究动机与目标

• 解决单层可重构智能表面（RIS）在多用户MIMO系统中波束成形精度有限与硬件复杂度高的问题。
• 通过用堆叠超表面实现基于波的计算，替代数字波束成形，实现高效能、高容量的下行链路通信。
• 联合优化多层超表面中的发射功率分配与离散相移，以在功率与硬件约束下最大化和速率。
• 在实际能量受限的无线网络中，验证SIM-based波域波束成形的可行性与性能增益。

提出的方法

• 在基站集成多层堆叠智能超表面（SIM）结构，直接在波域中整形电磁波。
• 将系统建模为带有SIM的多用户MISO下行链路，SIM通过可配置散射单元对入射信号施加离散相移。
• 构建联合优化问题，通过联合调节发射功率与SIM相移以在功率与相位量化约束下最大化和速率。
• 设计一种计算高效的算法，结合投影梯度上升与逐次优化，求解非凸优化问题。
• 利用通过复数导数规则推导出的闭式梯度表达式，基于信干噪比（SINR）灵敏度实现基于迭代的相移更新。
• 采用混合设计，各层共同参与波前整形，实现干扰消除与波束方向性增强。

实验结果

研究问题

• RQ1 堆叠智能超表面（SIM）架构是否能通过实现更复杂的波前整形，在多用户下行链路波束成形中优于单层RIS？
• RQ2 通过SIM实现的波域波束成形在多大程度上可减少对高分辨率数模转换器与数字波束成形硬件的需求？
• RQ3 超表面层数如何影响可实现的和速率与干扰抑制能力？
• RQ4 在SIM系统中，离散相移（如4位分辨率）与连续相位控制之间的性能差距有多大？
• RQ5 所提出的功率分配与相移联合优化方法相较于传统MISO波束成形，在频谱效率方面表现如何？

主要发现

• 所提出的SIM系统相较相同发射天线数的常规MISO系统，和速率提升约200%。
• 最优性能在七层堆叠超表面时达到，表明增加层数可增强干扰消除与波束方向性。
• 当每个相移器的量化位数超过四位时，离散SIM的性能几乎与连续相位理想情况无异。
• 联合优化算法显著优于基准方案，包括传统数字波束成形与固定相位RIS设计。
• 基于波的波束成形方法消除了基站对高复杂度数字波束成形器与高分辨率DAC的需求。
• 理论分析证实，可通过复数导数高效计算和速率对每个相移的梯度，从而实现实用的迭代优化。