The Design And Implementation Of LED-based High-speed Underwater Optical Communication System

Wireless communications technology has undergone a massive growth in recenthistory. Building upon almost a century of terrestrial radio links, new applications ofwireless links and networks have enjoyed large success in military, commercial, andscientific fields. The successes of these new applications are owed mostly to decadesof study in radio-frequency (RF) propagation. However, wireless links have beenconfined to above water applications, as RF energy is highly attenuated by seawater.To implement wireless communications underwater, other methods must be explored.Wireless underwater communication is mainly divided into three types, includingunderwater optical communication, underwater acoustic communication and theunderwater electromagnetic wave communication. These three methods have theirown characteristics and applications. Among these three methods, underwater opticalcommunications are being considered for a variety of applications in littoral waters.Although the communications range can be as short as a few meters in turbid water,the bandwidth can be fairly high (significantly higher than with acoustic methods),operation can be covert, and applications such as short range communications amongplatforms can be feasible. Such platforms include Unmanned Underwater Vehicles(UUV), submarines, ships, buoys, docking stations, and divers.In this thesis, optical communication using LEDs is presented as an improvementover acoustic modems for scenarios where high speed, but only moderate forscenarios where high speed, but only moderate distances, is required and lower power,less complex communication systems are desired. A super bright blue LED basedtransmitter system and a blue enhanced photodiode based receiver system weredeveloped tested with the goal of transmitting data at rates of1Mbps over distances of at least10meters. Test results in underwater showed the successful transmission oflarge data files over a distance of13meters and at transmission rates of at least3Mbps. With an improved test environment, even better performance may be possible.Specific work completed as follows:1Explore the underwater wireless communication methods, including underwateracoustic communication technology, the introduction of the radio wave technology.Analysis underwater light propagation characteristics, including light absorption andscattering, as well as some other factors affect the optical attenuation of thepropagation generated. Through analyzing the advantages and disadvantages of theunderwater communication channel, it provides a theoretical basis for underwaterwireless optical communication system design.2Use the Binary On-Off Keying principle to design an optical transmitter system,comparing the performance of light-emitting diodes and laser diodes, and select thelight emitting diode as transmitter light source. Select FET as an LED driver, transmitdata to the circuit system through the USB-TTL converter, finally complete and testthe transmitter system.3Design the optical receiver system, select the photodiode as a photodetector,convert the current signal carried by the photodiode into a voltage signal through thecurrent-voltage converter, and then convert the negative voltage signal to a positivevoltage and amplify it through the inverting voltage amplifier, the data signal can beconverted into a voltage level suited for TTL-USB cable by the comparator. Completeand test the receiver system, finally complete the whole underwater opticalcommunication system, and achieve the goal of at least3Mbps data transmissionwithin a range of13meters.