Performance of FSO systems employing SC-QAM over atmospheric turbulence channels and pointing errors

In this paper, we theoretically analyze the performance of free-space optical (FSO) communication systems using subcarrier intensity quadrature amplitude modulation (SC-QAM) over atmospheric turbulence channels in the presence of pointing errors. Unlike previous studies, we take into account both atmospheric turbulence and the pointing error effect. In order to model atmospheric turbulence, we employ a log-normal distribution for weak-to-moderate turbulent condition and a gamma-gamma distribution for strong turbulent condition. Meanwhile, we consider the pointing error effect by using a fading model where the influence of beam width, detector size and jitter variance is considered. More critically, we use a combination of these models to analyze the combined effect of atmospheric turbulence and pointing error to SC-QAM/FSO systems. Finally, we derive analytical expressions for evaluating the average symbol error rate (ASER) performance of such systems. Numerical results present the impact of pointing error on the performance of SC-QAM/FSO systems and how we use proper values of aperture size and beam width to improve the performance of such systems.