Reconfigurable Intelligent Surfaces for 6G Non-Terrestrial Networks: Assisting Connectivity from the Sky

Sixth-generation (6G) non-terrestrial networks (NTNs) are advanced wireless communication systems that operate beyond traditional terrestrial networks. These networks utilize various technologies and platforms to provide flexible, enhanced connectivity and coverage. When operating at high frequency, ground user terminals require low-directional antennas, which experience poor link budgets from satellites and thus drive the quest for novel solutions. Reconfigurable Intelligent Surfaces (RISs) have recently emerged as a promising technology for 6G and beyond cellular systems. This article studies the potential of RIS-in-tegrated NTNs to revolutionize next-generation connectivity. First, it discusses the fundamentals of RIS technology. Secondly, it delves into reporting the recent advances in RIS-integrated NTNs. Subsequently, it presents a novel framework based on the current state-of-the-art for IRS-integrated NTNs with classical single connected diagonal RIS and fully connected beyond diagonal RIS architectures. Finally, the article highlights open challenges and future research directions to revolutionize the realm of RIS-integrated NTNs.