A Comparative Study of Modulation Schemes and Mitigation Techniques for Gamma-Gamma FSO Channels

Abstract

FSO communication systems are proven to be an
effective substitute to RF technologies as FSO communication
technologies boast high data rates which are not subjected to
licensing and are not affected/spoiled by electromagnetic
interference. However, despite these positive aspects, FSO links
suffer from air turbulence which has strong detrimental effects on
links and cause irradiance fluctuations (scintillation) and extreme
fading. This has caused this paper to offer an in-depth performance
analysis of FSO systems operating on a Gamma-Gamma
turbulence channel and further explores the performance of
different turbulence mitigation schemes and modulation strategies.
This analysis uses OOK as a basis of comparison to other
modulation schemes like 4 PPM, DPSK, and DCO-OFDM and
explores the possible improvements Adaptive Optics (AO) and
Dynamic Beam Steering (DBS) systems may provide to link
reliability. This paper investigates the performance of DBS under
three different settings: best path selection, path steering with a
realistic motion model, and path steering with an alignment error.
The results of many Monte Carlo simulations indicate that when it
comes to turbulence, DPSK will be the best option because it adapts
to changes. This paper suggests that DCO-OFDM is not optimal
because it is the most adversely affected by channel fading. The
best way. This works better than Adaptive Optics, especially in
turbulent conditions. These results indicate that combining strong
modulation techniques like DPSK with spatial diversity methods
like DBS is essential for constructing dependable, high-speed freespace
optical networks, even in challenging atmospheric
conditions.