Performance Comparison of OFDM based FSO Communication System under Log-normal and Gamma-Gamma Distribution

Abstract

Recently, researches on Orthogonal Frequency Division Multiplexing (OFDM) based Free Space Optics (FSO) communication system gain interest since it combines the advantages of both techniques by mitigating the multipath propagation effects to improve the transmission data rate. The performance of such systems depends on base band modulation used, number of subcarriers, turbulence modeling, nature of laser beam and so on. In this paper, a performance comparison is carried out considering three different levels (weak, medium and strong) of atmospheric turbulence with log-normal and Gamma- Gamma scintillation model for plane, spherical and Gaussian beams. The Gamma-Gamma distribution analysis shows that for all kinds of laser beams, Bit Error Rate (BER) is high for weak turbulence which decreases further for medium and strong. The only difference is that the BER variation for Gamma-Gamma distribution with Gaussian beam is more prominent than for plane beam. The decrease in BER is clearly observed at higher SNR value. This kind of improvement in BER is observed with log-normal PDF at low SNR with weak turbulence. Thus we can say that weak atmospheric turbulence can be modeled by log-normal PDF and medium to strong condition can be modeled by Gamma-Gamma PDF.