Power Efficiency Analysis of Spatial Diversity Based Vertical FSO Links With Pointing Error in Multiple Beam Transmissions

In this paper, we analyzed the diversity performance with increased pointing error caused by multiple beam transmissions for spatial diversity based vertical free space optical (FSO) back-haul links. For power-limited FSO systems, the spatial diversity technique can be employed to mitigate the atmospheric fading issues in vertical FSO links. However, the arrangement of multiple transmitters to achieve independent channels can cause additional alignment errors in the misalignment detection process of pointing, acquisition, and tracking (PAT) systems, as demonstrated by the experiment results. The increase of misalignment makes the pointing loss severe to reduce the achievable diversity gain, which results in insufficient received power to satisfy the system requirements. The system optimization with the statistical misalignment model can mitigate the effect of increased pointing error and improve the diversity gain as the number of channels increases. The proposed scheme provides an enhanced link margin for designing seamless FSO wireless back-haul networks for next-generation wireless communication systems.