### Motivation and Contribution  
For a long time, the benefits of FSO technology have been well-known. Recent researches and developments in FSO enabling technologies on the other hand, made it easier to make use of these benefits. As a result, many new FSO-related research publications have  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **FSO Links** & **RF Links** \\ \hline Typical Data Rate & 100 Mbps to \(\sim\)Gbps & Less than 100 Mbps \\ Channel Security & High & Low \\ Component Dimension & Small & Large \\ Networking Architecture & Scalable & Non-scalable \\ Source of Signal & Atmospheric turbulence, & Multipath fading, rain, and \\ Degradation & observation, pointing error & user interferences \\ Link Distance & Short-range & Long-range \\ Superiority & Unlicensed Band & Non line of sight \\ \hline \hline \end{tabular}
\end{table}
Table 1: Properties of terrestrial FSO and RF access links communications.  
Figure 1: System block diagram of a hybrid RF/FSO system.  
recently been released. Given that most FSO technology classification attempts occurred in the late 1990s, we feel that existing FSO technology classifications are outmoded [17; 29; 44].  
The majority of the previous categorization attempts focused solely on reviewing and differentiating FSO systems, with little regard for the establishment of new/future FSO links. As a result, fitting some emerging and future configuration classes into existing single-level classification schemes may be challenging, if not impossible. As a result, numerous survey studies must incorporate new classes, causing the overall classification scheme to become inconsistent and non-systematic in its expansion. Consider the quasi (multi-spot) diffuse system [45; 46], which, despite its similarity to diffuse systems [17; 30; 47; 48], has been propagated as a different class. Furthermore, many recent advancements in FSO have resulted in inconsistencies, and in some cases, contradictions, between multiple classifications and definitions, such as in their naming conventions and operating principles. The terms line-of-sight (LOS), directed LOS, non-directed LOS and point-to-point (P2P), for example, all refer to the same FSO link configuration [17; 29; 30; 44; 47; 49].  
FSO systems are an excellent solution for existing projects to enhance their performance as a full-duplex FSO system can achieve 10 Gbps of data rate. In addition, FSO communication systems can be used in different applications for last-mile access. By comparing the FSO system with existing systems, we can conclude that the FSO system outperforms all of them, as explained in Section 3.  
This article offers a comprehensive examination of hybrid FSO communication systems, which make use of a variety of approaches and links to accomplish the best possible levels of performance. In recent years, there has been an increase in the instances in which these technologies have been utilized.  
The organization of this article is broken down as follows; Section 2 covers the essential conditions, in addition to covering fundamental concepts, Section 3 analyzes the performance of FSO in comparison to other communication methods for 5G backhauling, Section 4 explains multiplexing approach, Section 5 discusses communication networks that make use of relay nodes, Section 6 provides an overview of the various hybrid FSO systems, followed by Section 7 which explores projects that exclusively utilized FSO links, and ultimately Section 8 goes into details on models for the fourth-generation (4G), 5G, and 6G communication systems.