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

Abstract

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 lightemitting 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. INDEX TERMS Binary frequency shift keying, full-duplex, FPGA, high-power LED array, optical link model, Reed-Solomon code, underwater wireless optical communication. I. INTRODUCTION The ocean is the cradle of life. Approximately 71% of the Earth's surface is covered by the ocean. The vast marine resources on Earth are indispensable for many aspects of life. It is necessary to exploit and utilize those resources with underwater wireless communication (UWC) technology, which has considerable potential in facilitating the use of underwater vehicles, devices, observatories, and sensors. Underwater wired communication uses fiber optic or copper cable, which is expensive, inflexible, and vulnerable to marine life, making it largely infeasible for use in underwater mobile systems. Acoustic waves, radio frequency (RF) waves, and optical waves are three primary physical information carriers for underwater wireless information transmission [1]-[3]. Acoustic waves involve mechanical waves with relatively little attenuation underwater (0.1-4 dB/km), and thus, they can cover long distances up to dozens of kilometers. However, acoustic waves have a low propagation speed (1500 m/s) and limited bandwidth (kHz), which leads to a multipath phenomenon, large time latency, and bulky antennas [2]. These characteristics hinder the application of acoustic waves in real-time and bandwidth-intensive scenarios. RF waves are another carrier that can provide a high data rate (Mbps), high bandwidth (MHz), and high speed...

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