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In this paper, we propose an adaptive beam that adapts its divergence angleaccording to the receiver aperture diameter and the communication distance toimprove the received power and ease the alignment between the communicatingoptical transceivers in a free-space optical communications (FSOC) system forhigh-speed trains (HSTs). We compare the received power, signal-to-noise ratio,bit error rate, and the maximum communication distance of the proposed adaptivebeam with a beam that uses a fixed divergence angle of 1 mrad. The proposedadaptive beam yields a higher received power with an increase of 33 dB inaverage over the fixed-divergence beam under varying visibility conditions anddistance. Moreover, the proposed adaptive divergence angle extends thecommunication distance of a FSOC system for HSTs to about three times underdifferent visibility conditions as compared to a fixed divergence beam. We alsopropose a new ground transceiver placement that places the ground transceiversof a FSOC system for HSTs on gantries placed above the train passage instead ofplacing them next to track. The proposed transceiver placement provides areceived-power increase of 3.8 dB in average over the conventional placement ofground-station transceivers next to the track.

Beam with Adaptive Divergence Angle in Free-Space Optical Communications for High-Speed Trains