Studies On OOK Modulation And Demodulation System For Underwater Optical Communication

Underwater wireless communication is critical to human development andutilization of the ocean marine resources, both in commercial and military fields,the underwater wireless communication has played an extremely important role. Atpresent, the underwater wireless communication is mainly divided into threecategories: the underwater long wave radio communication which is mainly used bythe military, the underwater wireless acoustic communication and the underwaterwireless optical communication. Compared with the other two means ofcommunication, the underwater wireless optical communication has the advantagesas following. First of all, the speed of optical communication is high, and thecommunication rate of light waves can reach Gbps order of magnitude inunderwater. Secondly, the high bandwidth and high frequency of light waves makeit have stronger bearing capacity. Thirdly, the optical communication is very safeand secret. Optical communication is especially capable of resisting disturbance ofelectromagnetic wave and radiation. In addition, the laser have a good sense ofdirection, when the signal is intercepted, the receiver can finds the break accidentquickly. Finally, the underwater wireless optical communications has otheradvantages, such as small volume, low power consumption and low price.In this paper, the OOK modulation and demodulation system for underwateroptical communication are studied, it is summarized as following:(1) The three popular methods of underwater wireless communication areintroduced, and the strengths and weaknesses of these three methods are discussed.The status and development trend of underwater wireless optical communication areintroduced.(2) The optical properties of seawater is analyzed. Moreover, the absorptionand scattering of the contents of seawater at visible wavelength are studied in detail.Link model and optical attenuation formula for underwater channel communicationare established, and the transmission distances at different wavelengths arecalculated according to different sea water models. (3) The OOK, LPPM and DPIM modulation methods are analyzed and theirperformance is compared. And the OOK modulation system is selected as themethod for underwater wireless communication in our research.(4) The principle of underwater optical communication system is stated. Theselection of emitting light source, photo detector and the design of modulation/demodulation circuit module, signal processing module, system software and awaterproof device are achieved. Moreover, the assembling of the whole system iscompleted.(5) The available of the underwater wireless optical communications system isverified through the water tank experiment. And the underwater wireless opticalcommunications experiment is achieved successfully in Weihai offshore turbidwaters (attenuation coefficient of0.5152m-1) by using our designed OOKmodulation system. When communication rate is9600bps and the receiving moduleaccess load is34kΩ, the maximum transmission distance was up to24m (achieveerror-free communication within20m). Moreover, when the receiving moduleaccess load is70kΩ and the communication rate is9600bps, the maximumtransmission distance was up to27m. And the bit error rate of our underwaterwireless optical communication system is less than4103.