Demonstration of 15 M 7.33 Gb/s 450 nm Underwater Wireless Optical Discrete Multitone Transmission Using Post Nonlinear Equalization
In this paper, we experimentally demonstrate an underwater wireless optical communication (UWOC) system using a 450 nm gallium nitride (GaN) laser and adaptive bit power loading discrete multitone (DMT) To enhance the system capacity, a post nonlinear equalizer based on the simplified Volterra series is employed at the receiver to mitigate the nonlinear impairments of the UWOC system. By combining the adaptive bit power loading with nonlinear equalization, 7.33 Gb/s DMT modulated UWOC under 15 m tap water is achieved at a bit error rate below the 7% hard decision forward error correction (FEC) limit 3.8 x 10 3. The electrical signal bandwidth is 1.25 GHz, which corresponds to an electrical spectrum efficiency of ~6 bit/s/Hz. The capacity distance product reaches 109.95 Gb/s m in a single channel UWOC system with tap water. Compared with the linear equalization case, the system capacity at the FEC limit for 15 m underwater transmission is improved by ~18% with the nonlinear equalization. Furthermore, the impact of turbidity on the performance of UWOC system is investigated by measuring the signal to noise ratio (SNR) under different suspension concentrations of Al(OH)3 and Mg(OH)2. The results show that significant SNR gains >3 dB for transmission distance up to 11 m) can be obtained by the nonlinear equalization over a wide range of water turbidity levels representing "clear ocean," "coastal ocean," and "harbor water," which demonstrates the robustness of the proposed scheme in various ocean environments.
Journal of Lightwave Technology