Reconfigurable intelligent surface has recently emerged as a promising technology for shaping the wireless environment by leveraging massive low-cost reconfigurable elements. Prior works mainly focus on a single-layer metasurface that lacks the capability of suppressing multiuser interference. By contrast, we propose a stacked intelligent metasurface (SIM)-enabled transceiver design for multiuser multiple-input single-output downlink communications. Specifically, the SIM is endowed with a multilayer structure and is deployed at the base station to perform transmit beamforming directly in the electromagnetic wave domain. As a result, an SIM-enabled transceiver overcomes the need for digital beamforming and operates with low-resolution digital-to-analog converters and a moderate number of radio-frequency chains, which significantly reduces the hardware cost and energy consumption, while substantially decreasing the precoding delay benefiting from the processing performed in the wave domain. To leverage the benefits of SIM-enabled transceivers, we formulate an optimization problem for maximizing the sum rate of all the users by jointly designing the transmit power allocated to them and the analog beamforming in the wave domain. Numerical results based on a customized alternating optimization algorithm corroborate the effectiveness of the proposed SIM-enabled analog beamforming design as compared with various benchmark schemes. Most notably, the proposed analog beamforming scheme is capable of substantially decreasing the precoding delay compared to its digital counterpart.

翻译：可重构智能超表面近期通过利用大规模低成本可重构单元，成为塑造无线环境的一项有前景技术。现有研究主要关注缺乏多用户干扰抑制能力的单层超表面。相比之下，我们提出了一种面向多用户多输入单输出下行链路通信的堆叠智能超表面（SIM）收发机设计方案。具体而言，SIM具有多层结构，部署在基站端直接在电磁波域执行发射波束成形。因此，基于SIM的收发机无需数字波束成形，可采用低分辨率数模转换器和中等数量的射频链路工作，在显著降低硬件成本与能耗的同时，得益于波域处理特性大幅减少预编码时延。为发挥SIM收发机的优势，我们构建了一个优化问题，通过联合设计分配给用户的发射功率与波域模拟波束成形来最大化所有用户的总速率。基于定制化交替优化算法的数值结果验证了所提SIM模拟波束成形方案相较于多种基准方案的有效性。尤为重要的是，与数字波束成形方案相比，所提模拟波束成形方案能够显著降低预编码时延。