Hybrid FSO/RF networks: A review of practical constraints, applications and challenges

Abstract

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.