  In recent years, free space optical (FSO) communication has gained
significant importance owing to its unique features: large bandwidth, license
free spectrum, high data rate, easy and quick deployability, less power and low
mass requirement. FSO communication uses optical carrier in the near infrared
(IR) and visible band to establish either terrestrial links within the Earths
atmosphere or inter-satellite or deep space links or ground to satellite or
satellite to ground links. However, despite of great potential of FSO
communication, its performance is limited by the adverse effects (viz.,
absorption, scattering and turbulence) of the atmospheric channel. Out of these
three effects, the atmospheric turbulence is a major challenge that may lead to
serious degradation in the bit error rate (BER) performance of the system and
make the communication link infeasible. This paper presents a comprehensive
survey on various challenges faced by FSO communication system for both
terrestrial and space links. It will provide details of various performance
mitigation techniques in order to have high link availability and reliability
of FSO system. The first part of the paper will focus on various types of
impairments that poses a serious challenge to the performance of FSO system for
both terrestrial and space links. The latter part of the paper will provide the
reader with an exhaustive review of various techniques used in FSO system both
at physical layer as well as at the upper layers (transport, network or link
layer) to combat the adverse effects of the atmosphere. Further, this survey
uniquely offers the current literature on FSO coding and modulation schemes
using various channel models and detection techniques. It also presents a
recently developed technique in FSO system using orbital angular momentum to
combat the effect of atmospheric turbulence.
