Research On Coding And Modulation And Demodulation Technology Of Blue-green Laser Satellite-submarine Communication

The blue-green laser has been concerned and was widely used in submarinecommunication systems since it was recognized as the "window" for underwatercommunications. This paper took the satellite-submarine communications systems as thebackground, the channel coding, modulation and demodulation and other key technologies inwireless optical communication systems were studied.Firstly, the wireless optical communication channel in the satellite-submarinecommunications was researched, including air and seawater channel. The light transmissioncharacteristics were analyzed. Atmospheric molecular absorption to light is relatively smallercomparing the scattering and even negligible, while the scattering of suspended particles tolight was approximated as Mie distribution; seawater’s absorption to light is serious, but theattenuation to blue-green laser band is smaller, the suspended particles’ scatter to light wasequivalent to Mie distribution, the total attenuation of seawater can be approximated asexponential distributed. Then a Monte Carlo model was established to simulate photontransport process, the light spot and the curve of pulse delay spread was obtained when thelight transported in different weather and different water environments.Secondly, the Turbo code was selected as channel coding techniques in wireless opticalcommunication systems and studied deeply. Take the PCCC structure as an example, thecoding of Turbo codes was discussed and the decoding algorithm was derived in detail.Subsequently, the improved the decoding algorithms in recent years were summarized, andparallel decoding structure was improved to reduce the decoding complexity and improve thedecoding performance, and another decoding structure—converse coding decoder(CC-DEC)was proposed relying on time-frequency demodulation system, which is the inversetransformation of coding process. The proposed decoding structure is greatly simple andreduced time-delay. Turbo code design was competed by Matlab software, and variousparameters to the performance of Turbo codes were simulated, then the appropriateparameters for optical communication were found. The CC-DEC’s performances were alsosimulated and the result proved that it can achieve the simple decoding function.In addition, the time-frequency keying modulation (TFSK) was studied deeply. TFSKwas used in blue-green laser submarine communications because of its good characteristics of resistance to frequency selective fading. However, the traditional TFSK demodulationrequires precise local reference signal, which limited its development. In this article, thesideband correlation algorithm (SCA) was used in TFSK demodulation process, which madeuseful signals were detected even the SNR was negative, and finally T-TFSK system wasformed combining with Turbo code. The TFSK and T-TFSK systems were both simulated,the result proved the T-TFSK to be not only feasible but also effective, and have obviousadvantages.