Geometrically Shaped 32QAM and Modified Binary Switching Coding Method in Underwater Visible Light Communication

Visible light communication (VLC) has emerged as a promising communication method in 6G, particularly in the domain of underwater communication. However, because of its intimate coupling to the light-emitting diodes (LED), the transmission rate is limited by the modulation bandwidth and the nonlinearity of the LED. With current device development, LED bandwidth is substantially lower than that of other optical communication devices, necessitating improved spectral efficiency and nonlinearity robustness to achieve high-speed transmission. In this paper, we propose a hexagonal constellation-based geometrically shaped (GS) 32QAM with carrierless amplitude and phase (CAP) modulation for the underwater VLC system. A comprehensive performance comparison of four GS modulation methods, including our proposed hexagonal-32QAM, normal 32QAM, square-32QAM and 32APSK is investigated in both theory and experiment. The hexagonal-32QAM with a modified binary switching coding (BSC) can obtain a Q factor gain of 0.44 dB, and a data rate improvement of 25 Mb/s compared with Gary coding based normal 32QAM. The experimentally results illustrate the feasibility of geometric shaping 32QAM in underwater visible light communication.