  Free space optical (FSO) communication has been receiving increasing
attention in recent years with its ability to achieve ultra-high data rates
over unlicensed optical spectrum. A major performance limiting factor in FSO
systems is atmospheric turbulence which severely degrades the system
performance. To address this issue, multiple transmit and/or receive apertures
can be employed, and the performance can be improved via diversity gain. In
this paper, we investigate the bit error rate (BER) performance of FSO systems
with transmit diversity or receive diversity with equal gain combining (EGC)
over atmospheric turbulence channels described by the Double Generalized Gamma
(Double GG) distribution. The Double GG distribution, recently proposed,
generalizes many existing turbulence models in a closed-form expression and
covers all turbulence conditions. Since the distribution function of a sum of
Double GG random variables (RVs) appears in BER expression, we first derive a
closed-form upper bound for the distribution of the sum of Double GG
distributed RVs. A novel union upper bound for the average BER as well as
corresponding asymptotic expression is then derived and evaluated in terms of
Meijer's G-functions.
