Common path-based mobile free-space optical terminal with adaptive beamforming function for Gbps out-of-band full-duplex connectivity to UAVs

This paper presents simple and compact mobile free-space optical (FSO) terminals based on a common optical path with an adaptive beamforming function to provide an enhanced Gbps out-of-band full-duplex connection between a ground control system and an unmanned aerial vehicle (UAV). The mobile FSO terminals were carefully designed considering the UAV mobility and payload and the trade-off relationship between the geometric loss and volume of the FSO terminal for a given aperture diameter. Useful parameter information, such as the geometric loss, volume, and aperture diameter are presented in detail. As the novel common optical path structure-based mobile FSO terminal has a simple structure design and less optical components, it can effectively reduce the size, weight, and power consumption for UAV mounting. The beamforming function within the proposed system improves the optical tracking efficiency between FSO terminals by varying the divergence angle (from 0 to 8.84 mrad) according to the data link length. With the help of the beamforming function, the mobile FSO terminals showed significantly improved connectivity compared to the terminals employing collimated beams and fixed angle beams. Consequently, the proposed mobile FSO terminals provide reliable 1.25-Gbps out-of-band full-duplex error-free (with a bit error rate of <10−12) connectivity. This study also presents an analysis of the geometric loss and volume corresponding to the size of the aperture diameter of the mobile FSO terminals for realizing high data rate out-of-band full-duplex FSO communication for flight platforms.