Effect of Pointing Errors on BER Performance of Multidimensional LDPC-Coded OAM Modulation with Direct Detection over Turbulent FSO Channels

In this invited paper, we study orbital angular momentum (OAM) - based signal transmission over free-space optical (FSO) link. After low-density parity-check (LDPC) encoding and multidimensional mapping, intensity modulation is implemented, and OAM multiplexing is performed. Signal is transmitted over FSO channel with atmospheric turbulence, pointing errors, and atmospheric loss. At the receiver end, signal is firstly demultiplexed by OAM demultiplexer, detected by photodiode, and further decoded by applying the multidimensional posterior-probability demapper and LDPC decoder. The bit-error rate (BER) performances are estimated by Monte Carlo simulations. The numerical results show that even when there is a strong misalignment between transmitter and receiver apertures, BER of the system can be reduced by applying appropriate quasi-cyclic LDPC codes. We show that BER values under 10-6 can be achieved for some typical values of electrical energy per information bit - to - noise spectral density ratio even when turbulence is strong.