For the next-generation wireless back-haul network, free space optical (FSO) communications are considered in non-terrestrial networks. The fading issues for the atmospheric turbulence and the misalignment become important to achieve high received power for seamless and high data rate communications. The spatial diversity technique could be the solution to mitigate these fading issues in FSO systems with a few meters of distance between transmitters larger than the coherence length. However, the distant arrangement of the transmitters causes additional alignment errors in the misalignment detection process in the pointing, acquisition, and tracking (PAT) systems, which increases the pointing loss. Therefore, the increased pointing errors should be considered to obtain desired diversity gain. In this work, we develop a statistical misalignment model due to multiple beam transmissions and analyze the transmission performance for the spatial diversity based vertical FSO links. The proposed misalignment model is investigated by the log-normal atmospheric fading channels, the distant arrangement of transmitters, and the centroid algorithm in the PAT systems. The increased alignment error in the misalignment process for multiple beam transmissions is experimentally demonstrated. The spatial diversity based FSO systems require larger beam width to compensate for the increased pointing error. The simulation results show that the system optimization with the misalignment model can increase achievable diversity gain as the number of channels increases. The proposed scheme provides an enhanced link budget to design seamless FSO based mobile back-haul networks.
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