© The Institution of Engineering and Technology
Recently, double generalised gamma (GG) distribution has been proposed as a generic model, to describe the irradiance fluctuations in freespace optical (FSO) communication systems. In this study, first the authors derive an approximate analytical expression for the distribution of the sum of independent but not necessarily identically distributed double GG random variables (RVs), considering the effects of the turbulence and pointing errors. Using wellknown Beaulieu series, the authors describe the probability density function (pdf) and the cumulative distribution function (cdf) of the sum of double GG RVs in the form of a convergent series. The authors derive novel, exact closedform expressions for the coefficients of the series in terms of the Meijer's Gfunction, which makes it possible to calculate the pdf and cdf of the sum of double GG variates under different turbulence conditions. In the next step, based on the presented results, the authors derive approximate analytical expression for the outage probability of the system and investigate the outage performance of the FSO link with equalgain combining (EGC) receivers over double GG fading channels with pointing errors. Numerical results are further demonstrated to confirm the analytical results and also to show the good accuracy of the derived expressions.
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