Evaluation of occlusion immunities of free-space orbital angular momentum shift keying

Orbital angular momentum (OAM) provides a new high-dimensional degree-of-freedom for lasers enabling high-dimensional shift keying, thus showing huge potential in free-space optical data-transmissions. Ensuring the accuracy of data transmission is of concern in any communication system. However, in the free-space OAM shift keying link, interferences from environmental occlusions will broaden the OAM spectrum and thus affect the correctness of information received. In this paper, the occlusion immunities of two commonly used OAM shift keying strategies-OAM-encoding-symbols and OAM-encoding-bits-are evaluated under different occlusion scenarios. We introduce radial, azimuthal, and random occlusions into the link and then analyze the bit error rate (BER) of various OAM shift keying strategies. The results illustrate that the strategy of OAM-encoding-symbols exhibits stronger occlusion immunity with zero BER than OAM-encoding-bits. Moreover, proof-of-concept experiments are done to transmit grayscale images under occlusions. The final decoding results show identical data-transmission performance with simulations.