Statistical characteristics and performance evaluation of FSO links with misalignment fading influenced by correlated sways

In this paper, we analyze the performance of a free space optical (FSO) link under the influence of attenuation loss, misalignment fading affected by correlation between the horizontal and vertical sways of an optical beam, and atmospheric turbulence-induced fading. In practice, the horizontal and vertical displacements of the signal that leads to misalignment may not necessarily be uncorrelated. Hence, we consider a generalized misalignment fading model for the analysis. Further, the atmospheric turbulence is characterised by the Gamma-Gamma fading. We investigate the statistical characteristics of the end-to-end instantaneous signal power to noise power ratio (SNR), and derive expression in closed form for the probability density function (pdf) and cumulative distribution function (cdf) of the SNR measured end-to-end, under given channel conditions. We develop mathematical analysis for the exact outage behaviour of the link and its asymptotic behaviour at high SNR regime. The exact and high SNR error performance of the system is also evaluated considering q-PSK and q-QAM modulated FSO link. We also discuss a special case of a binary phase shift keyed (BPSK) modulated FSO link and evaluate its bit error rate (BER) performance. Further, the exact average channel capacity is investigated for the FSO link and its asymptotic analysis at higher SNR is also presented. The obtained mathematical results are also illustrated and verified by the numerical plots.