Free Space Optical Networks: Applications, Challenges and Research Directions

Free Space Optics (FSO) is an optical communication technique which disseminates light in free space such as air, vacuum or celestial space, to wirelessly transfer data. FSO communication offers high data rates up to 2.5 Gbps over a range of 100 m to a few kilometers and has the potential to replace the traditional means of communications in a wide variety of applications. Despite advantages such as high bandwidth and unlicensed spectrum, its widespread use has been inhibited by factors including interference and atmospheric turbulences. Many of these challenges have been addressed by researchers who have proposed modulation techniques, channel coding, diversity, hybrid systems and physical implementation of hardware. There is a need for a comprehensive review of contemporary research in FSO in order to examine the significant contributions and to identify open challenges and potential directions for future research endeavors. This paper presents a state-of-the-art review of the use of FSO in important indoor and outdoor applications and identifies some of the open research problems. We indicate the following open research problems: (a) the future FSO systems would offer both illumination and communication which necessitates modulation and coding techniques for a better dimming control along with efficient data communication, (b) security threats at the physical layer are a major concern to be addressed, (c) FSO transceiver design for future 5G systems faces a challenge of optimizing bit-error-rate and signal-to-noise ratio at the receiver, and (d) cost-effective hybrid RF/FSO solutions are needed to combine the benefits of RF and FSO.