School of Electrical & Information Engineering (ETDs)
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Item Characterisation of Standard Telecommunication Fibre Cables for Cost-Effective Free Space Optical Communication(University of the Witwatersrand, Johannesburg, 2024) Iga, Fortune Kayala; Cox, Mitchell A.In an era marked by an escalating demand for high-speed internet connectivity, optical communication plays a crucial role in meeting these needs. Free Space Optical (FSO) communication, which involves the wireless transmission of optical signals through the atmosphere, holds promise for extending existing fibre optic networks and connecting individuals beyond current coverage areas. Despite the potential, commercial FSO systems remain prohibitively expensive. A cost-effective FSO system can be achieved by utilising small form-factor pluggable (SFP) transceiver modules. These budget-friendly devices offer powerful transmit lasers and highly sensitive receiving photodiodes. To utilise these devices, optical signals are collimated out of a transmitting fibre into the atmosphere and coupled back into a receiving fibre. However, further investigation is needed to determine the optimal fibre cables for transmitting and receiving optical signals across the atmosphere to maximise received optical power and achieve efficient FSO communication. This study aims to characterise the light coupling performance of standard telecommunication fibre cables, with a focus on the optical power transmitted from and received by the fibre cable under atmospheric conditions. The methodology employed for characterising the power transmitted by the fibre cables involves mea- suring the optical power in the fundamental Gaussian mode. This mode optimises transmission through the atmosphere by minimising beam divergence. Subse- quently, light coupling from free-space is characterised by measuring the optical power coupled into the different fibre cables under non-ideal conditions, including misalignment and atmospheric turbulence. The findings of this research show notable correlations between the physical attributes of the fibre cables, namely refractive index profile, core size and numerical aperture, and their transmission and reception performances. The comprehensive characterisations of the standard fibre cables presented in this study provide insights into their suitability for distinct roles within a low-cost FSO system.Item Peak-to-average power ratio reduction in optical-OFDM systems using lexicographical permutations(University of the Witwatersrand, Johannesburg, 2023) Niwareeba, Roland; Cox, Mitchell A.; Cheng, LingThe work presented in this thesis extends and contributes to the research in reducing the high Peak-to-Average Power Ratio (PAPR) in optical-Orthogonal Frequency Division Multiplexing (OFDM) systems using probabilistic-based and hybrid techniques. Whereas the high PAPR problem has been extensively studied and a number of solutions provided for the conventional Radio Frequency (RF)-OFDM systems, there are only a few solutions proposed specifically for PAPR reduction in optical-OFDM systems. Although the probabilistic-based techniques such as Conventional Selected Mapping (CSLM) and Data Position Permutation (DPP) result into significant PAPR reduction performance with negligible Bit Error Rate (BER) degradation, the resulting increase in both hardware and computational complexity as a result of a large number of Inverse Fast Fourier Transform (IFFT) operations that have to be performed to generate the candidate signals is still a major drawback. In order to reduce the complexity, in this research, two techniques which are applied in opticalOFDM systems are proposed. The first technique is the hybrid method composed of a modified CSLM and µ-law companding techniques called Low Complexity Hybrid Selected Mapping (LCHSLM). The proposed method achieves almost 50% reduction in complexity compared to CSLM with less BER degradation. The second technique based on lexicographical permutations called Lexicographical Symbol Position Permutation (LSPP) works by dividing the optical-OFDM symbol into a number of sub-blocks and performing lexicographical permutations to obtain the candidate signals after the IFFT operations. In the proposed LSPP, all the candidate permutation sequences are not obtained at once unlike in the DPP where the number of candidate permutation sequences increases at a factorial rate of growth as the number of sub-blocks increases resulting in a more complex system. Additionally, the research proposes an algorithm where a threshold PAPR value is introduced and the candidate signals are generated until a candidate with a PAPR value less or equal to the threshold is obtained. The results show that the complexity in terms of IFFT operations can be reduced substantially depending on the selected threshold and the number of candidate signals. Furthermore, the research introduces a new algorithm based on the global gain (net gain) to determine the most suitable number of permutation candidate sequences to achieve a reasonable PAPR reduction performance without increasing the time and hardware complexity to levels that the systems cannot tolerate.