Space-Time Coding for Orbital Angular Momentum Multiplexed Free-Space Optical Systems

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 intermodal crosstalk and power disparities between OAM modes. In this paper, we are interested in multiple-input multiple-output (MIMO) coherent FSO communication systems using OAM multiplexing. We propose space-time (ST) coding at the transmitter to enhance the bit error rate (BER) performance against atmospheric turbulence. Through numerical simulations, we show performance improvement thanks to ST coded schemes for different MIMO dimensions. Furthermore, we derive an analytical expression for the error probability upper bound of the ST coded OAM FSO channel affected by atmospheric turbulence. The theoretical error probability is compared with Monte Carlo simulations and a good agreement is observed.