  Communication using orbital angular momentum (OAM) modes has recently
received a considerable interest in free space optical (FSO) communications.
Propagating OAM modes through free space may be subject to atmospheric
turbulence (AT) distortions that cause signal attenuation and crosstalk which
degrades the system capacity and increases the error probability. In this
paper, we propose to enhance the OAM FSO communications in terms of bit error
rate and spectral efficiency, for different levels of AT regimes. The
performance gain is achieved by introducing orthogonal frequency division
multiplexing (OFDM) with index modulation technique to the OAM FSO system.
