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UBC Theses and Dissertations

Free-space optical communication systems using on-off keying in atmospheric turbulence Yang, Luanxia


In this thesis, we focus on the bit-error rate (BER) performance improvements for free-space optical (FSO) communication links operating over atmospheric turbulence channels using on-off keying (OOK). Laser beams employed in these links are subject to scintillation, during their propagation through atmospheric channels, and this can lead to significant BER performance degradation. Such systems can suffer from irreducible error floors that result from the use of demodulation with fixed and unoptimized detection thresholds. The resulting error floors are analyzed for the general case of low and high state offsets (i.e., nonzero extinction ratios). To improve the BER performance, there are three techniques developed in this thesis. The first technique employs electrical signal-to-noise ratio (SNR) optimized detection. The system uses the electrical SNRs to implement adaptive detection thresholds and eliminate the error floors. The system can accommodate operation with nonzero extinction ratios, as it uses the method of moments and maximum likelihood estimation techniques to estimate the low and high state offsets and electrical SNR. The second technique employs source information transformation. Using source information transformation can also eliminate error floors, and it can detect the OOK signal without knowledge of the instantaneous channel state information and probability density function of the turbulence model. It is shown that source information transformation can achieve comparable performance to the idealized adaptive detection system, with greatly reduced implementation complexity. The third technique employs convolutional code. Using convolutional code can mitigate the effects of turbulence induced fading. The BER performance is analyzed for FSO systems using convolutional code and OOK. Through our analysis, it is shown that using convolutional code can improve the BER performance of an FSO system significantly.

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