Adaptive Beam Divergence for Expanding Range of Link Distance in FSO With Moving Nodes Toward 6G

Free space optics (FSO) provides high-capacity optical wireless communications. To maintain an FSO link with a moving node, a beam should be steered toward it. As the FSO link distance gets shorter, it becomes difficult to maintain the link. This is because the beam radius has not largely expanded yet at shorter transmission distances and the received optical power decreases as the receiver moves away from the center of the beam. In this letter, we address an adaptive beam divergence control method to expand the range of the FSO link distance within which the FSO link can be maintained. Theoretical evaluation results show our method expanded the minimum FSO link distance to 5 m in assumed use cases, though it was at least 20 m with a fixed beam divergence. Furthermore, we confirmed the range of the beam divergence angle and the divergence changing interval in our method were feasible.