  Free space optical (FSO) communications using orbital angular momentum (OAM)
modes has recently received a considerable interest. Propagating OAM modes
through free space may be subject to atmospheric turbulence (AT) distortions
that cause intermodal crosstalk and power disparities between OAM modes. In
this article, we are interested in 2x2 multiple-input multiple-output (MIMO)
FSO coherent communication systems using OAM. We propose a selection criterion
for OAM modes to minimize the impact of AT. To further improve the obtained
performance, we propose a space-time (ST) coding scheme at the transmitter.
Through numerical simulations of the error probability, we show that the
penalty from AT is completely absorbed for weak AT and considerable coding
gains are obtained in the strong AT regime.
