Finite-SNR diversity gain analysis of FSO systems over gamma-gamma fading channels with pointing errors

Abstract

Free space optical (FSO) communication systems are subject to turbulence-induced fading as a result of fluctuations in the refractive index. Gamma-Gamma (GG) distribution is commonly used in the literature to characterize moderate-to-strong turbulence conditions. The performance of FSO systems is further degraded by pointing error-induced fading as a result of building sways. While the asymptotic analysis presented in earlier works is important to understand the maximum diversity gains of FSO systems attainable over fading channels, such gains might not be available in practical range of signal-to-noise ratio (SNR). Furthermore, different systems with the same asymptotic slope may have different slopes in the range of finite SNR values. In this paper, we consider a multiple-input multiple-output (MIMO) FSO system with intensity modulation and direct detection (IM/DD). We derive approximate expression for the finite-SNR diversity order over GG turbulence channels in the presence of pointing errors. We also investigate convergence speed to full diversity. We provide numerical results to demonstrate the accuracy of our derivations and corroborate on our analytical findings.