Intelligent metasurface has recently emerged as a promising technology that enables the customization of wireless environments by harnessing large numbers of inexpensive configurable scattering elements. However, prior studies have predominantly focused on single-layer metasurfaces, which have limitations in terms of the number of beam patterns they can steer accurately due to practical hardware restrictions. 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 a 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, we formulate an optimization problem that aims 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 both the transmit power budget and discrete phase shift constraints. Furthermore, we propose a computationally efficient algorithm for solving this joint optimization problem and elaborate on the potential benefits of employing SIM in wireless networks. Finally, the numerical results corroborate the effectiveness of the proposed SIM-enabled wave-based beamforming design and 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.

翻译：智能超表面最近作为一种有前景的技术出现，它通过利用大量低成本可配置散射单元实现对无线环境的定制。然而，先前研究主要集中于单层超表面，由于实际硬件限制，其在能精确调控的波束模式数量上存在局限性。与此不同，本文引入了一种新颖的堆叠智能超表面（SIM）设计。具体而言，我们研究了将SIM集成到多用户多输入单输出（MISO）通信系统下行链路中的方案，其中由多层超表面结构组成的SIM部署在基站（BS）处，以促进电磁波域的发射波束成形。这消除了在基站中使用传统数字波束成形和高分辨率数模转换器的需求。为此，我们构建了一个优化问题，旨在通过联合优化基站的发射功率分配和SIM的基于波的波束成形来最大化所有用户设备的总速率，同时满足发射功率预算和离散相移约束。此外，我们提出了一种计算高效的算法来解决这一联合优化问题，并详细阐述了在无线网络中采用SIM的潜在优势。最后，数值结果证实了所提出的基于SIM的波域波束成形设计的有效性，并评估了所提算法相较于各种基准方案实现的性能提升。研究表明，在相同发射天线数量下，所提出的基于SIM的系统相较于传统MISO系统，总速率实现了约200%的提升。