  Non-terrestrial 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 paper, we present a comprehensive literature review on the RIS-assisted
non-terrestrial networks (RANTNs). Firstly, the framework of the RANTNs is
introduced with detailed discussion about distinct properties of RIS in NTNs
and 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), 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.
