Electronic Theses and Dissertations (Masters)
Permanent URI for this collectionhttps://hdl.handle.net/10539/37969
Browse
Search Results
Item Modelling OAM Crosstalk with Neural Networks: Impact of Tip/tilt and Lateral Displacement(University of the Witwatersrand, Johannesburg, 2024) Makoni, Steven Gamuchirai; Cheng, LingThis research focuses on a critical challenge within Free Space Optical ( FSO) commu- nication systems, specifically those utilizing Mode Division Multiplexing (MDM) with Orbital Angular Momentum ( OAM ) modes of a limited transmission range. Despite these systems’ potential to significantly enhance spectral efficiency and transmission capacity, their effectiveness is hindered by the limited range caused by atmospheric turbulence-induced aberrations. Atmospheric turbulence and mis- alignments distort the optical wavefront, causing degradation in orthogonal spatial modes and resulting in power spreading into adjacent modes, known as crosstalk in MDM systems. This research presents a simple neural network model for estimating OAM crosstalk in FSO systems, specifically focusing on atmospheric turbulence-induced aberrations. Firstly, we generated datasets through simulation and experimentation for validation purposes. We then develop and evaluate the neural network model, assessing its accuracy under various turbulence aberrations. The simple neural network, trained solely on tip/tilt and displacement inputs and without retraining, accurately estimated OAM spectra using approximated inputs in turbulent condi- tions, closely matching experimentally measured spectra. Despite the presence of turbulent aberrations, the model showed a minimal decrease in the coefficient of determination, indicating its ability to generalize well to unseen measurements. Our findings indicate that a simple neural network trained solely on tilt and displacement inputs can accurately estimate OAM crosstalk amidst many turbulence aberrations for ℓ ∈ [-5, 5] as a proof of concept. This implies that simple detectors such as cameras can be used to implement or optimize digital signal processing for error detection and correction utilizing the knowledge of crosstalk, offering promising avenues for improving system efficiency and quality of service for MDM systems. In summary, this research leveraged neural networks to model OAM crosstalk induced by misalignments and turbulence. The model’s ability to estimate OAM crosstalk due to misalignments and atmospheric turbulence shows potential for use in real-time predictive systems. With further refinement, neural network models could indicate the evolution of OAM crosstalk in FSO communications that em- ploy OAM multiplexing schemes in atmospheric turbulence. The demonstrated efficacy of the neural network model positions it as a valuable tool for enhancing the robustness of FSO communications employing higher-order OAM modes.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 The techno-economic impact of a high penetration of embedded generators on South African, Brazilian, Australian and Ugandan distribution networks: A comparative review(University of the Witwatersrand, Johannesburg, 2024) Rakgalakane, Motladitseba Dorcas; Jandrell, IanOwing to current electricity capacity shortages and rising electricity prices in South Africa, customers are opting for self-generation to mitigate the effects of load shedding and offset their electricity bills. In June 2021, the South African government removed the licensing requirement for private generation to encourage the uptake of self- generation, close capacity shortages and promote investment in private generation. While the increase in private generation is seen by the electricity industry as a positive step towards meeting energy supply demands, there are concerns about the impact that high numbers of embedded generation facilities will have on the distributors, i.e., their networks and revenues. The aim of this study was to conduct a review of the technical, economic and regulatory impact of a high number of embedded generators on distributors and their networks. The impact in South Africa is compared with the impact in Brazil, Australia and Uganda. The research study seeks to identify some of the success strategies implemented by these countries to address challenges associated with private embedded generation, and to provide recommendations for South Africa. South Africa compares well with Brazil and Australia in terms of electricity access and installed generation capacity vs population; however, in terms of embedded generation, particularly from variable renewable energy sources, South Africa’s penetration levels are still lower than those of Brazil and Australia, although higher than those of Uganda. The review highlights that the impact of embedded generation is largely driven by technical, economic and regulatory policy changes. The absence of a clear market structure or market direction, enabling legislation and policies, regulatory tools (such as national rules for integration or compensation and unbundled tariffs for some customer categories) make it difficult to minimise the negative effects of a high penetration of embedded generation and to capitalise on potential positive effects. In Brazil and Australia, the success of renewable energy embedded generation is largely a result of clear policy and regulations, which lead and drive positive changes in their electricity industries. Recommendations are made for legislation, policy and regulation changes to support embedded generation, the creation of a clear market structure, and the publication of national guidelines for embedded generation management. In addition, tariffing mechanisms should be reviewed to ensure a fair distribution of costs.