This paper has proposed an experimental system for non-orthogonal multiple access (NOMA) wireless optical communication in challenging underwater turbulent environments, employing the gallium nitride (GaN)-based micro-LED array. This design of the GaN-based micro-LED array enables the independent transmission of signals from distinct data streams within the NOMA framework, facilitating direct optical power-domain superposition of NOMA signals. The experimental setup involves emulating oceanic turbulence channels, characterized by varying the level of scintillation intensity, to thoroughly investigate the bit error rate (BER) performance. The outcomes unequivocally demonstrate the superiority of our proposed NOMA scheme, as compared to conventional circuit-driven optical NOMA systems utilizing fixed LED array grouping, particularly in the presence of turbulent underwater channels. The proposed NOMA scheme exhibits consistently superior BER performance and maintains excellent linearity at the lower frequencies while effectively mitigating signal distortion at the higher frequencies.

In order to alleviate the impact of turbulence on the performance of underwater wireless optical communication (UWOC) in real time, and achieve high-speed real-time transmission and low cost and miniaturization of equipment, a 2×2 real-time multiple-input and multiple-output (MIMO) high-speed miniaturized UWOC system based on a field-programmable gate array (FPGA) and a high-power light-emitting diode (LED) array is designed in this Letter. In terms of multiplexing gain, the imaging MIMO spatial multiplexing and high-order modulation for the first time are combined and the real-time high-speed transmission of PAM-4 signal based on the LED array light source in 12 m underwater channel at 100 Mbps rate is implemented, which effectively improves the throughput of the UWOC system with a high-power commercial LED light source. In light of diversity gain, the system employs the diversity of repeated coding scheme to receive two identical non-return-to-zero on-off keying (NRZ-OOK) signals, which can compensate the fading or flickering sublinks in real time under the bubble-like simulated turbulence condition, and has high robustness. To our knowledge, this is the first instance of a high rate and long-distance implementation of a turbulence-resistant real-time MIMO miniaturized UWOC system based on FPGA and high-power LED arrays. With spatial diversity or spatial multiplexing capabilities, its low cost, integrity, and high robustness provide the system with important practical prospects.

This Letter investigates the performance of the two-way multi-hop system for underwater optical wireless communications. With the decode-and-forward (DF) relaying, the two-way multi-hop system is modeled, where the effects of absorption, scattering, and oceanic turbulence are all taken into account. An exact closed-form expression for outage probability is derived under the assumption that the oceanic turbulence obeys a log-normal distribution. Numerical results demonstrate the impacts of various parameters on the outage performance and indicate that the two-way multi-hop system significantly improves the performance in comparison to both the one-way multi-hop system and the two-way two-hop system.