A Real-Time, Full-Duplex System for Underwater Wireless Optical Communication: Hardware Structure and Optical Link Model

Underwater wireless optical communication (UWOC) has been widely considered a supplement to traditional underwater acoustic communication. A real-time UWOC video delivery system was developed in a laboratory water tank based on a field-programmable gate array (FPGA) with binary frequency shift keying (2FSK) modulation. The system achieved full-duplex communication by using the transmission control protocol (TCP) and forward error correction (FEC). A high-power 445 nm light-emitting diode (LED) array was adopted to enhance the transmitted optical power and increase the transmission link distance. We present an underwater optical channel model that considers the effects of both geometry and channel loss, especially considering the impact of the refractive index of the optical medium and the non-line-of-sight (NLOS) links formed by water surface reflection. MATLAB was used to simulate this channel model and predict the received optical power distribution on the receiving plane. Additionally, we propose improved calculation methods for the consumed electrical power and transmitted optical power of the LED array. We also investigate the relationship between the optimum avalanche gain of an avalanche photodiode (APD) and the signal-to-noise ratio (SNR). This full-duplex system achieved a 1 Mbps data transmission rate at an SNR of 10.1 dB and a distance of 10 m for an underwater link. In addition, when the optical power of the LED array is enhanced, the link range is predicted to be 14.5 m with an attenuation coefficient of 0.056 /m.