Performance of Generalized QAM/FSO Systems With Pointing Misalignment and Phase Error Over Atmospheric Turbulence Channels

This article investigates the performance of free-space optical (FSO) systems using sub-carrier quadrature amplitude modulation (SC-QAM) signaling for broadband terrestrial applications. While previous studies ignored the influence of phase noise, we theoretically analyze and derive closed-form expressions of the system's average bit error rate (ABER) considering the combined effect of pointing misalignment, turbulence-induced fading, and phase error. The log-normal distribution is used to model the atmospheric turbulence in the weak regime while the gamma-gamma distribution is applied for modeling the turbulent atmosphere in the moderate and strong regimes. The phase error, which becomes complicated under the effect of atmospheric turbulence, is modeled by Tikhonov distribution. The numerical results reveal that the influence of the phase error on the system performance is dominant in the weak turbulence and high order modulation while the fading becomes more severe in the moderate-to-strong turbulence regime. The accuracy of analytical results is also validated by Monte-Carlo simulations and a good match can be confirmed.