Radio frequency (RF) spectrum is highly occupied and adding further broadband channels to fulfill the exiting user requirements has become difficult. Optical free-space communication can be regarded a possible alternative as it offers several potential advantages such as reliable connectivity, secure link, higher data rates, and large bandwidth. Consequently, free-space optical (FSO) communication system which is the most dominant optical wireless technology has become more attractive in current era to deploy additional broadband channels, and it can support bandwidth-hungry services. FSO has appealing benefits such as easy deployment, inherited secure communication, higher data rate, non-interfering link, and licensed-free large spectrum. FSO communication links are also susceptible to several meteorological situations including smog, fog, scintillation, smoke, snow, and dust. A critical research question is to ensure connectivity under these adverse circumstances. FSO communication is severally hampered by link attenuation, atmospheric turbulence, and line-of-sight (LOS) demands. To overcome these challenges, relay nodes can be employed to improve coverage area and error rate of FSO communication system. However, relay nodes cannot overcome pointing errors in FSO communication due to various critical factors such as building sway. To enhance the availability of FSO systems, a redundant backup RF link to form a hybrid network is a viable solution. The coexistence of both RF and optical system is proposed to tackle challenges as mentioned before and attain benefits of both spectrums. A hybrid FSO/RF technology is promising as it can substantially enhance the availability and reliability than individual channels and offers unique solution to high-throughput wireless connectivity, comparable data rates, and insensitivity to weather conditions. This hybridization approach can help both channels to jointly recover the deficiencies of each technology and secure efficient data transmission with highly variable channel conditions. The immediate switching in hybrid FSO/RF systems can enable suboptimal usage of FSO network. In this study, we examine switching techniques, routing protocols, channel models, and modulation schemes. We outline several projects discussed in literature and examine various application scenarios. Finally, we discuss potential challenges, physical layer security issues and associated practical solutions.

射频 (RF) 频谱被高度占用，增加更多宽带信道以满足现有用户需求变得困难。光学自由空间通信可以被视为一种可能的替代方案，因为它提供了几个潜在的优势，例如可靠的连接、安全的链路、更高的数据速率和大带宽。因此，作为最主要的光无线技术的自由空间光 (FSO) 通信系统在当前时代对部署额外的宽带信道变得更具吸引力，它可以支持需要带宽的服务。FSO 具有吸引人的优势，例如易于部署、继承的安全通信、更高的数据速率、无干扰链路和免许可的大频谱。FSO 通信链路也容易受到多种气象情况的影响，包括烟雾、大雾、闪烁、烟雾、雪和灰尘。一个关键的研究问题是确保在这些不利情况下的连通性。FSO 通信受到链路衰减、大气湍流和视距 (LOS) 需求的影响。为了克服这些挑战，可以采用中继节点来提高 FSO 通信系统的覆盖范围和误码率。然而，由于建筑物摇摆等各种关键因素，中继节点无法克服 FSO 通信中的指向错误。为了提高 FSO 系统的可用性，冗余备份 RF 链路以形成混合网络是一种可行的解决方案。提出射频和光学系统共存以应对前面提到的挑战并获得两种频谱的好处。混合 FSO/RF 技术很有前景，因为它可以比单个通道显着提高可用性和可靠性，并为高吞吐量无线连接、可比数据速率和对天气条件不敏感提供独特的解决方案。这种混合方法可以帮助两个通道共同恢复每种技术的不足，并在高度可变的通道条件下确保高效的数据传输。混合 FSO/RF 系统中的即时切换可能导致 FSO 网络的次优使用。在这项研究中，我们研究了交换技术、路由协议、信道模型和调制方案。我们概述了文献中讨论的几个项目，并检查了各种应用场景。最后，我们讨论了潜在的挑战、物理层安全问题和相关的实际解决方案。