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In recent years, free space optical (FSO) communication has gainedsignificant importance owing to its unique features: large bandwidth, licensefree spectrum, high data rate, easy and quick deployability, less power and lowmass requirement. FSO communication uses optical carrier in the near infrared(IR) and visible band to establish either terrestrial links within the Earthsatmosphere or inter-satellite or deep space links or ground to satellite orsatellite to ground links. However, despite of great potential of FSOcommunication, its performance is limited by the adverse effects (viz.,absorption, scattering and turbulence) of the atmospheric channel. Out of thesethree effects, the atmospheric turbulence is a major challenge that may lead toserious degradation in the bit error rate (BER) performance of the system andmake the communication link infeasible. This paper presents a comprehensivesurvey on various challenges faced by FSO communication system for bothterrestrial and space links. It will provide details of various performancemitigation techniques in order to have high link availability and reliabilityof FSO system. The first part of the paper will focus on various types ofimpairments that poses a serious challenge to the performance of FSO system forboth terrestrial and space links. The latter part of the paper will provide thereader with an exhaustive review of various techniques used in FSO system bothat physical layer as well as at the upper layers (transport, network or linklayer) to combat the adverse effects of the atmosphere. Further, this surveyuniquely offers the current literature on FSO coding and modulation schemesusing various channel models and detection techniques. It also presents arecently developed technique in FSO system using orbital angular momentum tocombat the effect of atmospheric turbulence.

Free Space Optical Communication: Challenges and Mitigation Techniques