In this paper, the performance of a Multiple-Input/Single-Output (MISO) Free-Space Optical (FSO) systems is studied through an analytical approach which more accurately models the atmospheric channel. The diversity gain is evaluated based on measures determined by up to second-order statistics of the received power. The numerical results demonstrate the effects of diffraction and spatial correlation of turbulence-induced fading on the performance of multi-beam diversity systems. These effects have been mostly neglected in the literature by applying far-field assumptions. The authors' results can thus be useful in the design of practical multi-beam FSO systems especially when a compact design is desired.