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Non-terrestrial communications have emerged as a key enabler for seamlessconnectivity in the upcoming generation networks. This kind of network cansupport high data rate communications among aerial platforms (i.e., unmannedaerial vehicles (UAVs), high-altitude platforms (HAPs), and satellites) andcellular networks, achieving anywhere and anytime connections. However, thereare many practical implementation limitations, especially overload powerconsumption, high probability of blockage, and dynamic propagation environment.Fortunately, the recent technology reconfigurable intelligent surface (RIS) isexpected to be one of the most cost-efficient solutions to address such issues.RIS with low-cost elements can bypass blockages and create multipleline-of-sight (LoS) links, and provide controllable communication channels. Inthis paper, we present a comprehensive literature review on the RIS-assistednon-terrestrial networks (RANTNs). Firstly, the framework of the RANTNs isintroduced with detailed discussion about distinct properties of RIS in NTNsand the two 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), andRIS-assisted air-to-air (A2A) communications. In combination withnext-generation communication technologies, the advanced technologies in RANTNsare discussed. Then we overview the literature related to RANTNs from theperspectives of performance analysis and optimization, followed by the widelyused methodologies. Finally, open challenges and future research direction inthe context of the RANTNs are highlighted.

Non-terrestrial Communications Assisted by Reconfigurable Intelligent Surfaces