Performance analysis of FSO system with different modulation schemes over gamma-gamma turbulence channel

Free-space optical communications (FSO) suffers from irradiance fluctuation caused by atmospheric turbulence, which results in optical power loss and consequently decreased signal-to-noise ratio (SNR). The error performances of the FSO based on On-Off Keying (OOK), Differential Phase Shift Keying (DPSK), and Binary Phase Shift Keying (BPSK) schemes in a turbulent atmosphere are presented. The received irradiance after propagating the atmosphere is modeled using the gamma-gamma distribution to evaluate the system error performance in turbulence regimes from weak to strong. The results show that, to obtain a BER of 10-6 at weak turbulence regime, ~15 dB and ~18 dB SNRs are required for BPSK and DPSK, respectively. However, for OOK with a fixed threshold of 0.5 under the same channel condition, OOK reaches an error floor greater than 10-3. The values of SNR required to achieve the same BER increase as the turbulence strength increase to moderate and strong regimes.