Nonterrestrial communications have emerged as a key enabler for seamless connectivity in the upcoming generation networks. This kind of network can support high data rate communications among aerial platforms (i.e., unmanned aerial vehicles (UAVs), high-altitude platforms (HAPs), and satellites) and cellular networks, achieving anywhere and anytime connections. However, there are many practical implementation limitations, especially overload power consumption, high probability of blockage, and dynamic propagation environment. Fortunately, the recent technology reconfigurable intelligent surface (RIS) is expected to be one of the most cost-efficient solutions to address such issues. RIS with low-cost elements can bypass blockages and create multiple line-of-sight (LoS) links and provide controllable communication channels. In this article, we present a comprehensive literature review on the RIS-assisted nonterrestrial networks (RANTNs). First, the framework of the RANTNs is introduced with detailed discussion about distinct properties of RIS in NTNs and the two deployment types of RIS, that is, terrestrial RISs (TRISs), and aerial RISs (ARISs), and the classification of RANTNs, including RIS-assisted air-to-ground (A2G)/ground-to-air (G2A), ARIS-assisted ground-to-ground (G2G), and RIS-assisted air-to-air (A2A) communications. In combination with next-generation communication technologies, the advanced technologies in RANTNs are discussed. Then, we overview the literature related to RANTNs from the perspectives of performance analysis and optimization, followed by the widely used methodologies. Finally, open challenges and future research direction in the context of the RANTNs are highlighted.

中文翻译：

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可重构智能表面辅助的非地面通信


非地面通信已成为下一代网络无缝连接的关键推动因素。这种网络可以支持空中平台（即无人机（UAV）、高空平台（HAP）和卫星）和蜂窝网络之间的高数据速率通信，实现随时随地的连接。然而，实际实现存在许多限制，特别是过载功耗、高阻塞概率和动态传播环境。幸运的是，最新的可重构智能表面（RIS）技术有望成为解决此类问题的最具成本效益的解决方案之一。具有低成本元件的 RIS 可以绕过障碍并创建多个视距 (LoS) 链路并提供可控的通信通道。在本文中，我们对 RIS 辅助非地面网络 (RANTN) 进行了全面的文献综述。首先，介绍了 RANTN 的框架，详细讨论了 NTN 中 RIS 的不同属性以及 RIS 的两种部署类型，即地面 RIS（TRIS）和空中 RIS（ARIS），以及 RANTN 的分类，包括RIS 辅助的空对地 (A2G)/地对空 (G2A)、ARIS 辅助的地对地 (G2G) 和 RIS 辅助的空对空 (A2A) 通信。结合下一代通信技术，讨论了RANTN中的先进技术。然后，我们从性能分析和优化的角度概述了与 RANTN 相关的文献，然后介绍了广泛使用的方法。最后，强调了 RANTN 背景下的开放挑战和未来研究方向。