Study of All-Optical FSO Relay based Systems under the Influence of the Atmospheric Turbulence Channel

Nor, Norhanis Aida Mohd (2018) Study of All-Optical FSO Relay based Systems under the Influence of the Atmospheric Turbulence Channel. Doctoral thesis, Northumbria University.

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Abstract

Free space optical (FSO) communications is an emerging high-speed data rate, high-bandwidth, and license-free access solution for a number of applications including the “last mile” access networks. However, the reliability and availability of FSO systems are affected by a number of atmospheric phenomena such as fog, rain, haze, smoke, and turbulence. Interestingly, the severity of such random degradations is highly related to the transmission distance, thus resulting in link deterioration and ultimately complete link failure. In this thesis, an all-optical FSO relay-assisted system technique is adopted to mitigate the destructive effects due to distance dependent atmospheric turbulence-induced fading. In this scheme, relays are incorporated to the direct link between the transmitter and the receiver nodes in order to reduce the turbulence induced path loss per link, thus extending the link span and ensuring higher link availability (i.e., 99.999%) as well as improving the overall system performance. Two all-optical relaying schemes are proposed and investigated, namely all-optical amplify-and-forward (AOAF) FSO relay and all-optical regenerate-and-forward (AORF) FSO relay assisted systems. In the AOAF FSO system, the performance analysis of triple-hop AOAF FSO communications system is investigated under the impact of non-homogeneous atmospheric turbulence condition. We present novel experimental results for the single, dual, and triple-hop AOAF FSO systems under several turbulence configurations and regimes. We also provide a mathematical framework for the end-to-end signal-to-noise ratio (SNR) and the bit error rate (BER) performance and confirm that the derived analytical results reasonably match with the experimental results especially at relatively high SNR. The evaluated BER performances under different turbulence regimes, modelled by the Gamma-Gamma distribution model show that the considered relay-assisted FSO system offers a significant performance improvement for weak to strong turbulence regimes, even without the knowledge of the channel state information. For instance, at a target BER of 10^-5 the proposed triple-hop FSO scheme offers ~5 dB and ~4 dB of SNR gains compared to direct transmission for turbulence strengths

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