Demonstration of a Real-Time Multi-Rate Adaptive DCO-OFDM Underwater Wireless Optical Communication System with a Maximum Water Attenuation of 54.8 dB

As underwater platforms become increasingly networked and autonomous, reliable and physically secure links are emerging as critical engineering infrastructure for ocean observation, offshore energy, and maritime security. Underwater wireless optical communication (UWOC) addresses the bandwidth–latency gap left by acoustic systems, yet most existing implementations operate at fixed rates and cannot adapt to dynamic underwater channels. This study presents a real-time multi-rate adaptive bidirectional (time-divided) UWOC system implemented on field-programmable gate arrays (FPGAs). A DC-biased optical orthogonal frequency-division multiplexing (DCO-OFDM) physical layer with four switchable formats (BPSK, 4-QAM, 8-QAM, and 16-QAM) is combined with a pilot-free blind SNR estimation algorithm exploiting virtual carriers and an ACK/NAK stop-and-wait automatic repeat request (ARQ) protocol. The system autonomously adjusts its physical rate between 10 and 100 Mbps across a maximum single-pass water attenuation of 54.8 dB. The narrow line-of-sight beam geometry and tight receiver field of view further offer inherent spatial filtering against eavesdropping, positioning the system as a candidate secure multi-modal component of underwater communication networks. Outdoor-pool experiments with an optical attenuation coefficient of 1.37 dB/m, certified by the China Electronic Product Reliability and Environmental Testing Research Institute (CEPREI, the Fifth Electronics Research Institute of MIIT), demonstrated a mean BER ≤ 1.8 × 10−6 in all four modes, with every individual certified run below the 10−5; criterion (worst single run: 5.4 × 10−6). These results establish a real-time hardware proof of concept validated in an outdoor pool, constituting a step toward field-deployable adaptive underwater optical links in marine engineering scenarios—AUV docking, subsea infrastructure inspection, and underwater sensor networks—and provide a verified platform on which physical-layer security extensions can be built.