Average symbol error rate of free-space optical communications using SC-QAM over strong atmospheric turbulence 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 strong atmospheric turbulence channels together with pointing errors. We take into account atmospheric attenuation, atmospheric turbulence and pointing errors. In order to model atmospheric turbulence, we employ a gamma-gamma distribution for strong turbulent condition. Meanwhile, we consider pointing errors by using a fading model where the influence of beam width, detector size and jitter variance is considered. Critically, we use a combination of these models to analyze the combined effect of atmospheric turbulence and pointing error to FSO/SC-QAM systems. Moreover, we derive the closed-form expression 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. In addition, the simulation results show that the closed-form expression can provide a precision in evaluating ASER performance of such systems.