On the orbital angular momentum based modulation/demodulation scheme for free space optical communications

A modulation/demodulation scheme based on orbital angular momentum is designed to mitigate the effect of atmospheric turbulence for free space optical communications. The constellation of transmitting signals is designed as a super-cube structure with Gray coding. To maximize the average signal to interference and noise ratio of the receiving signal, the optimum set of OAM modes is designed. The decision threshold is calculated based on the distribution of the receiving signal. Simulation results show that the proposed scheme can achieve lower bit error rate (BER) less than 10^-6 at the optical-signal-to-noise-ratio (OSNR) of 30dB over the channel with the atmospheric turbulence intensity of 10^-15 m^-2/3.