  In this paper, the effect of transmitter beam size on the performance of free
space optical (FSO) communication has been determined experimentally.
Irradiance profile for varying turbulence strength is obtained using optical
turbulence generating (OTG) chamber inside laboratory environment. Based on the
results, an optimum beam size is investigated using the semi-analytical method.
Moreover, the combined effects of atmospheric scintillation and beam wander
induced pointing errors are considered in order to determine the optimum beam
size that minimizes the bit error rate (BER) of the system for a fixed
transmitter power and link length. The results show that the optimum beam size
increases with the increase in zenith angle but has negligible effect with the
increase in fade threshold level at low turbulence levels and has a marginal
effect at high turbulence levels. Finally, the obtained outcome is useful for
FSO system design and BER performance analysis.
