Design, Channel Modeling and Simulation of LED-Based Short-Range Underwater Optical Communication Systems

The number and complexity of subsea applications is constantly increasing, as are the demands on data traffic. Fast underwater optical wireless communication (UOWC) represents a solution for shorter distances. This thesis focuses on the adaption of this technology to the challenging conditions in the channel.

Natural water features a narrow spectral window of least attenuation in the blue to green regime. In conjunction with the spectral properties of the LEDs emission and the detectors responsivity, this results in a threefold relationship. In order to enable an energy-efficient system design in coastal waters and to bypass the green-gap in LEDs, the use of converted-green LEDs is proposed and investigated. The effect and the constraints of optical filtering to suppress natural ambient light is examined, a model is developed, and simulations for optimization are carried out. Since the signal-to-noise ratio is very important in an UOWC system, in-depth noise analyses are performed, comprising different detector types and various operating conditions. LEDs can be used as photodetectors, and this is examined for single-color power LEDs in terms of a potential dual-use in UOWC. An unexpected high responsivity and a distinct bandpass characteristic is discovered, and a potential for low-cost applications is clearly identified. A comprehensive survey of housing and port technologies is given and the optical path is analyzed. The total characteristics of assemblies in various configurations are determined by underwater goniometric measurements and compared to calculations. The exact prediction of the link budget relies on individual system attenuation coefficients ksys, but these exhibit many dependencies. In order to generate first spectral curve sets of these coefficients, extensive Monte Carlo simulations of the underwater light field are conducted. Additionally, a method for experimental determination of ksys is developed and field tests are conducted.


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