Nonlinear distortion mitigation in dynamic-range-limited optical wireless communication systems
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The objective of this dissertation is to mitigate the nonlinear distortion in dynamic-range-limited optical wireless communication (OWC) systems. The light emitting diode (LED) is the major source of nonlinearities. To drive the LED, the input electric signal must exceed the turn-on voltage of the device. On the other hand, the signal is also limited by the saturation point or maximum permissible value of the LED. As a result, the nonlinearity in an OWC system belongs to a family of dynamic-range-limited nonlinearities. To overcome nonlinearities, we can deal with either the signal or the system. One straightforward approach is to choose or modify input signals such that they are insensitive to the nonlinear distortions. Another approach is to compensate for the nonlinearities by predistortion or postdistortion. In the preliminary research, we analyze how to maximize the signal-to-noise-plus-distortion ratio (SNDR) of the nonlinearities in OWC systems. The result herein can serve as a guideline to design predistortion linearization of nonlinear devices like LEDs. Additionally, we study the constrained clipping method which can mitigate the distortions so that the clipping levels can be more aggressive than the simple clipping to further reduce the double-sided peak-to-average power ratios (PAPRs) for optical orthogonal frequency division multiplexing (OFDM) signals. In the follow-up research, we extend our work to multiple-input multiple-output (MIMO) OWC systems and bit-loaded OFDM systems. In MIMO OWC systems, we study the design of MIMO transceivers under the OWC-specific constraints. In bit-loaded OFDM systems, we consider constellation-wise distortion constraints into the design of PAPR reduction techniques.