Abstract:
Performance Investigation of FSO Communication System Under Sub
Tropical Weather Condition of Bangladesh
Use of optical signal for primitive communication is very historic. Signaling communication
by means of fire beacons and smoke dates back to 800 -150 BC. However, FSO communication
system had seen its mature application since 1960s. Today FSO communication competes with
radio due to its high bandwidth, easy installation, low initial cost, versatile applications, etc.
However, its signal degradation due to environment is inherent and higher than that associated
with radio signal. This paper has defined FSO channel models based on MPSK and LPPM
modulation schemes while analyzing their performances under the variation of modulation
order and environmental factors including the local rain condition. Meijer G function based
close-form expressions and MATLAB-based simulation for performance evaluation have been
used. The study analysis has confirmed that FSO links degrade heavily under turbulence,
misalignment and local rain condition. The study finds that the SNR degradation due to change
of turbulence condition is 20 dB for MPSK and 20-27 dB LPPM links at BER value of 10-4.
The strong misalignment causes a degradation of 20 dB for MPSK and 18 dB for LPPM
schemes. Lastly, the heavy rain conditions of Bangladesh cause a loss of 15-20 dB for MPSK
and 14-25 dB for LPPM schemes. The higher order FSO links provide better spectral efficiency
with disadvantage of SNR penalty for MPSK, but with the advantage of SNR gain for LPPM
schemes. The penalty of MPSK schemes becomes as high as 13 dB when switched from BPSK
to 16-PSK link. Conversely, the gain of LPPM schemes becomes 15 dB when switched from
2-PPM to 16-PPM link. Both MPSK-SIM and LPPM based FSO links have their merits and
demerits relative to each other. Higer order MPSK is more beneficial for band-width efficiency
and high data-rate applications. But it is more sensitive to phase noise and atmospheric
turbulence. Besides, its system is more complex due to the need of phase tracking. On the other
hand, higher order LPPM is better power efficient and more advantageous in turbulent
environment due to its strong resilience against amplitude jitters, although it has sensitivities
to timing jitters. Its system has some complexities due to slot and timing synchronization
requirement. Considering Bangladesh environment with unfavorable conditions, both schemes
are subject to link degradation and even link break due to extreme conditions. The high-speed
high-order FSO link with either modulation scheme fits better for short distance application in
Bangladesh environment. Even mitigation techniques can be applied to improve SNR
performance. The link needs to be adaptive with switching features from high order to low
order for maintaining the link continuity.
