A Survey on Hybrid Free Space Optical and Radio Frequency Systems: Classification, Progress, Observations, and Challenges

Data rate, security, spectral efficiency, energy efficiency and reliability represent the main requirements in most of communication systems. Different wireless technologies can achieve different levels of these requirements, where a specific wireless technology cannot individually attain the best performance in all of these system requirements. Free-Space Optical (FSO) and Radio Frequency (RF) communications are fundamental and popular wireless technologies. Although FSO systems can achieve high data rates, maintain high security levels and minimize energy consumption, they suffer from low reliability in some weather conditions (such as fog and haze), require Line-of-Sight (LoS) connections, and are limited to point-to-point communication systems. On the other hand, RF technology has been shown as a reliable system under diverse weather conditions, allows for users and data multiplexing and represents an efficient option for non-LoS communication scenarios. However, compared to FSO, the achievable data rate and security level by RF are limited. To this end, combining both technologies (i.e., FSO and RF) has been widely nominated as an efficient hybrid communication system that exploits the advantages of both technologies in different scenarios and conditions. In the literature, a wide set of hybrid FSO/RF systems has been presented to accomplish different objectives including improving throughput, boosting reliability, maintaining connectivity and enhancing security. This survey paper provides a comprehensive overview of hybrid FSO/RF systems, presenting a classification framework of the different hybrid setups proposed in the literature, and identifying the main design objectives of hybrid FSO/RF systems. The survey includes technical revisions of related literature, followed by a set of the observations made and another set of challenges and open issues in the field of hybrid FSO/RF systems.