Improving the Performance of Underwater Wireless Optical Communications by Pointing Adjustable Beam Arrays

Considering the limited bandwidth of underwater wireless communications with radio frequency and acoustic waves and the constant increasing need for underwater high-speed data transmission, underwater wireless optical communication (UWOC) has been regarded as an attractive and viable alternative. However, UWOCs suffer from degradation attributed to underwater attenuation, oceanic turbulence, and pointing errors in oceanic channel. Therefore, performance enhancement technique plays a key role in the design and deployment of a UWOC system. In this article, we propose to improve the performance of a UWOC by transmitting with pointing adjustable beam arrays (PABA). In a PABA, we can adjust pointing directions of every single beam by reflectors. To decrease the path loss and turbulence-induced scintillations and weaken the influence of pointing errors in UWOCs, we make the one central beam and several surrounding beams in a PABA point toward the center of a receiver aperture, and make the remaining surrounding beams in a PABA point toward several target points located symmetrically on a ring with an adjustable radius around the center of a receiver aperture. This adjustable radius is set according to the average pointing errors in every specific UWOC link. To estimate the average pointing errors more accurate, we calculate pointing errors in view of both beam wander caused by oceanic turbulence and the misaligned displacements resulted by the changes in the position and attitude of an underwater platform. Finally, we validate the effect of PABA on improving the performance of UWOCs by simulations.