On the Performances of $N$th Best User Selection Scheme in Multiuser Diversity Free-Space Optical Systems Over Exponentiated Weibull Turbulence Channels

The average bit error rate (ABER) and capacity of multiuser diversity freespace optical (FSO) systems with Nth best user selection scheme are investigated by considering atmospheric turbulence-induced fading. The atmospheric turbulence channel is modeled by the exponentiated Weibull distribution, and it can accurately predict the probability density function of the irradiance fluctuations in weak-to-strong turbulence regimes under all aperture averaging conditions. With the help of the Gauss-Laguerre quadrature rule, the analytical expression of ABER is derived and studied with different receiver aperture sizes, turbulence strength values, user numbers K, and orders N. Results show that, under moderate turbulence condition, the mitigation effect of aperture averaging for ABER degradation by decreasing K or increasing N is more significant than that of weak turbulence condition. The average capacity is then achieved on the basis of the second kind Stirling number and Meijer's G-function. The investigation on the average capacity shows that a higher capacity can be obtained in the strong turbulence regime than that in the weak turbulence regime, and this improvement is more significant with large receiver aperture size. Monte Carlo simulation and Romberg integration are first provided to verify the proposed analytical ABER and average capacity expressions, respectively. This paper benefits the design and development of multiuser FSO systems.