Reconfigurable intelligent surface has recently emerged as a promising technology for reshaping the wireless environment by leveraging massive low-cost passive elements. Prior works mainly focus on a single-layer metasurface that lacks the capability of suppressing inter-user interference. By contrast, we propose in this paper a stacked intelligent metasurfaces (SIM)-enabled transceiver design for multiuser multiple-input single-output downlink communications. Specifically, a SIM having a multilayer structure is deployed at the base station to perform the transmit beamforming directly in the electromagnetic wave domain. As a result, the conventional digital beamforming and high-resolution analog-to-digital converters as well as the excessive number of radio-frequency chains are fully removed, which sharply reduces the hardware cost and energy consumption, while substantially decreasing the precoding delay benefiting from the computation at the speed of light. To this end, we formulate an optimization problem for maximizing the sum rate of all users by jointly designing the transmit power allocated to different users and the wave-based beamforming. Finally, numerical results based on a customized alternating optimization algorithm corroborate the effectiveness of our SIM-enabled wave-based beamforming design as compared to various benchmark schemes. Most notably, the wave-based beamforming is capable of decreasing the precoding delay by eight orders of magnitude compared to its digital counterpart.

翻译：可重构智能表面近来通过利用海量低成本无源元件，已成为重塑无线环境的一项前景广阔的技术。现有研究主要集中在单层超表面上，这类超表面缺乏抑制用户间干扰的能力。相比之下，本文提出了一种基于堆叠智能超表面（SIM）的收发机设计，用于多用户多输入单输出下行链路通信。具体而言，在基站部署具有多层结构的SIM，直接在电磁波域中执行发射波束成形。因此，传统的数字波束成形、高分辨率模数转换器以及过多的射频链路被完全消除，大幅降低了硬件成本与能耗，同时得益于光速计算，有效减少了预编码延迟。为此，我们通过联合设计分配给不同用户的发射功率与基于波的波束成形，构建了一个最大化所有用户和速率的优化问题。最后，基于定制交替优化算法的数值结果验证了我们的SIM赋能波域波束成形设计相较于多种基准方案的有效性。最值得注意的是，与数字波束成形相比，波域波束成形能够将预编码延迟降低八个数量级。