Analytic and simulative comparison of turbulent FSO system with different modulation techniques

In this paper, we characterize the performance of Intensity Modulation/Direct Detection (IM/DD) and coherent detection modulation techniques in free-space optical (FSO) communication system in terms of bit error rate (BER). The system of interest includes M-ary Pulse Position Modulation (M-PPM) in correspondence to IM/DD and M-ary Phase Shift Keying (M-PSK) and M-ary Quadrature Amplitude Modulation (M-QAM) with respect to coherent detection. The system is subjected to weak turbulence regime of log-normal channel with scintillation index less than 0.75. Coherent techniques involve the amount of signal pulses transmitted in a particular time as the major component of their generation, except IM/DD which is a result of photon intensity, i.e. the number of photons forming a pulse. It is worth mentioning that in previous studies, the performance of IM/DD techniques had been evaluated in terms of photon-count fluctuation statistics and thus the impact of fading strength of channel on signal intensity was not discussed anywhere. In this work, an objective equivalence has been achieved between these two kinds of detection by deriving the BER expression of M-PPM in terms of probability density function of signal intensity, while considering fading strength, scintillation index, thermal noise and outage probability as system parameters for numerical analysis.