DirectDemodNet: An End-to-End Neural Demodulator for Polarization-Diverse Underwater Visible Light Communication

We demonstrate an underwater visible light communication system using a circularly polarized 520 nm laser transmitter, 32APSK modulation, and a polarization-diverse dual-aperture receiver. An end-to-end post-equalization network, DirectDemodNet, directly maps dual-polarization received waveforms to 32APSK symbol logits, replacing conventional Least Mean Square (LMS) + Volterra equalization. By combining waveform-difference features, dual-scale dilated temporal convolutions, and multi-period positional encoding, DirectDemodNet improves nonlinear compensation and branch fusion. Extensive evaluations are conducted across data rates from 7.5 to 13.75 Gbps over a 1.2 m static underwater channel. Experiments show that DirectDemodNet broadens the forward error correction compliant operating range and provides a maximum net transmission rate gain of 4.095 Gbps over LMS + Volterra at the 7% Hard-decision Forward Error Correction (HD-FEC) threshold.