Non-line-of-sight methodology for high-speed wireless optical communication in highly turbid water

Abstract

Underwater wireless optical communication (UWOC) is an emerging technology for discovering, exploiting, and protecting various underwater resources. Due to the diverse water clarity conditions, light signals are expected to propagate in a fashion dominated by the line-of-sight (LOS) to non-line-of-sight (NLOS) regime. To fully benefit from the high capacity underwater internet that UWOC could offer, especially in the presence of turbid water, a system that obviates the stringent requirements for pointing, acquisition, and tracking (PAT) is required. Herein, we demonstrated a robust NLOS UWOC link fully relieving the requirement on PAT. Based on a system design consisting of an ultraviolet (UV) laser for enhanced light scattering and a high sensitivity photomultiplier tube (PMT), we established an NLOS link with a data rate of 85 Mbit/s and a transmission distance of 30 cm using on-off keying (OOK) in emulated highly turbid harbor water. Further, a data rate of 72 Mbit/s is still achieved when the alignment is totally lost, i.e., the pointing directions of the transmitter and receiver are parallel. A longer transmission distance up to 40 m is also envisaged. Our findings will pave the way for a practical, short-reach, NLOS UWOC link in realistic oceanic scenarios.