Reconfigurable intelligent surface (RIS)-aided near-field communications are investigated. First, the necessity of investigating RIS-aided near-field communications and the advantages brought about by the unique spherical wave-based near-field propagation are discussed. Then, the family of patch array-based RISs and metasurface-based RISs is introduced along with respective near-field channel models. A pair of fundamental performance limits of RIS-aided near-field communications, namely, their power scaling law and effective degrees of freedom (EDOF), are analyzed for both patch array-based and metasurface-based RISs, which reveals the potential performance gains that can be achieved. Furthermore, associated near-field beam training and beamforming design issues are studied, where a two-stage hierarchical beam training approach and a low-complexity element-wise beamforming design are proposed for RIS-aided near-field communications. Finally, a suite of open research problems is highlighted for motivating future research.

中文翻译：

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6G 可重构智能地面辅助近场通信：机遇与挑战


研究了可重构智能表面（RIS）辅助的近场通信。首先，讨论了研究RIS辅助近场通信的必要性以及独特的基于球面波的近场传播所带来的优势。然后，介绍了基于贴片阵列的 RIS 和基于超表面的 RIS 系列以及各自的近场通道模型。针对基于贴片阵列和基于超表面的 RIS，分析了 RIS 辅助近场通信的一对基本性能限制，即功率缩放定律和有效自由度 (EDOF)，揭示了潜在的性能增益这是可以实现的。此外，还研究了相关的近场波束训练和波束成形设计问题，其中提出了用于 RIS 辅助近场通信的两阶段分层波束训练方法和低复杂度逐元素波束成形设计。最后，强调了一系列开放研究问题以激励未来的研究。