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On the performance of non-adaptive and adaptive optical wireless communications in atmospheric turbulence Hassan, Md. Zoheb


Optical wireless communication is an attractive solution to the "last mile" bottleneck problem with additional benefits of having rapid deployment, enhanced security, protocol transparency, and unlicensed transmission bandwidth. However, atmospheric turbulence induced signal fading is a major performance degrading factor for any outdoor optical wireless communication systems. Since the atmospheric turbulence induced fading varies slowly, adaptive transmission is an effective fading mitigation solution for an outdoor optical wireless communication system. In this thesis, we primarily focus on the performance analysis of optical wireless communication systems employing different adaptive transmission schemes. We first study the average symbol error rate for a nonadaptive subcarrier intensity modulated optical wireless communication systems employing general order rectangular quadrature amplitude modulation. We consider three different turbulence channel models, i.e., the Gamma-Gamma channel, the K-distributed channel, and the negative exponential channel with different levels of turbulence. Exact average symbol error rate expressions are derived using a series expansion of the modified Bessel function. In addition, detailed truncation error analysis and asymptotic error rate analysis are also presented. Numerical results demonstrate that our series solutions are highly accurate and efficient. Next we investigate a variable-rate, constant-power adaptive subcarrier intensity modulation employing M-ary phase shift keying and rectangular quadrature amplitude modulation for optical wireless communication over the Gamma-Gamma turbulence channels. The adaptive schemes offer efficient utilization of optical wireless communication channel capacity by adapting the modulation order according to the received signal-to-noise ratio and a pre-defined target bit-error rate requirement. Highly accurate series solutions are presented for the achievable spectral efficiency, average bit-error rate, and outage probability using a series expansion approach of the modified Bessel function. In addition, asymptotic bit-error rate and outage probability analyses are presented. Our asymptotic bit-error rate analysis shows that the diversity order of both non-adaptive and adaptive systems depends only on the smaller channel parameter of the Gamma- Gamma turbulence. Numerical results demonstrate high accuracy of our series solutions with a nite number of terms and an improved spectral efficiency achieved by the adaptive systems without increasing the transmitter power or sacrificing bit-error rate requirements. Finally, ergodic capacity is investigated for the optical wireless communications employing subcarrier intensity modulation with direct detection, and coherent systems with and without polarization multiplexing over the Gamma-Gamma turbulence channels. We consider three different adaptive transmission schemes: (i) variable-power, variable-rate adaptive transmission, (ii) complete channel inversion with fixed rate, and (iii) truncated channel inversion with fixed rate. For the considered systems, highly accurate series expressions for ergodic capacity are derived using a series expansion of the modified Bessel function and the Mellin transformation of the Gamma- Gamma random variable. Our asymptotic analysis reveals that the high SNR ergodic capacities of coherent, intensity modulated, and polarization multiplexing systems gain 0:33 bits/s/Hz, 0:66 bits/s/Hz, and 0:66 bits/s/Hz respectively with 1 dB increase of average transmitted optical power. Numerical results indicate that a polarization control error less than 10 [degree] has a little influence on the capacity performance of polarization multiplexing systems.

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