A Review on FSO/RF based Satellite-Aerial-Ground Communication Systems

Thanks to its unique and fundamental characteristics, Free Space Optic (FSO) has emerged as a significant choice for both fronthaul and backhaul links in satellite-aerial-ground (SAG) communication systems. These features have rendered FSO a key player in recent and future network systems and use more feasible in the first-hop of communication channel i.e., from satellite-High Altitude Platform (HAP). However, ensuring a dependable FSO connection to subsequent HAP-Ground Station (GS) network becomes challenging due to the adverse impact of weather and atmospheric conditions. An optimal solution is implementing a hybrid FSO/RF approach for the second leg of HAP-to-GS communication. In this configuration, the Radio Frequency (RF) link acts as a contingency connection in the event of FSO link breakdown. However, a notable limitation of hybrid FSO and RF links lies in the constrained spectral capacity of the RF link, especially when the FSO link encounters obstruction due to cloud cover in the second hop. In order to address this challenge, a solution has been devised involving the deployment of an additional unmanned aerial vehicle (UAV) to diversify the path of FSO link from HAP-UAV-GS, thus preventing cloud blockages while preserving the high speed functionality of the FSO link. This approach incorporates a mirror array constructed using reconfigurable intelligent surface (RIS) technology, an emerging and adaptable solution. The RIS-UAV channel model takes into consideration turbulence-induced and pointing errors due to hovering, offering a comprehensive strategy to overcome the issues arising from adverse weather conditions. This work begins by presenting a taxonomy of single-hop, dual-hop, and multi-hop FSO/RF-based SAG networks. It then proceeds to provide an assessment, discussion, and review of prior research endeavors related to FSO/RF fronthaul and backhaul links. Furthermore, it identifies current open research problems and challenges, while presenting potential research directions for in-depth analysis and investigation.