Performance evaluation of FSO MIMO links in Gamma-Gamma fading with pointing errors

In this paper, we study the equal gain combining (EGC) scheme in free space optical (FSO) multiple-input multiple-output (MIMO) communication systems over the Gamma-Gamma fading channel with pointing errors. The probability density function (PDF) of the Gamma-Gamma fading with pointing errors contains the Meijer-G function, which makes the error analysis of the considered system very difficult (almost undoable). We propose a new power series based representation for the PDF of the Gamma-Gamma fading with pointing errors. The average bit error rate (BER) for the EGC scheme is derived by using the proposed series based expression of the PDF. We also derive the asymptotic BER for the EGC scheme. The effect of pointing errors over the performance of the scheme is analyzed under different scenarios and it is observed that the pointing errors significantly degrade the diversity of the FSO MIMO system; but under strong atmospheric turbulence pointing errors can be neglected.