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Enhancing Underwater Optical Wireless Communication in Spatio-Temporal Environments

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Thesis under embargo until 4th February 2027(24.87 MB)

Date

2025

Authors

Govinda Waduge, Tharuka

Supervisor

Seet, Boon-Chong
Vopel, Kay

Item type

Thesis

Degree name

Doctor of Philosophy

Journal Title

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Volume Title

Publisher

Auckland University of Technology

Abstract

Underwater optical wireless communication (UOWC) is an emerging field of technology. It has significance towards realising the vision of 6G communication, of a hyper-connected world, being the only underwater wireless technology that has been demonstrated to perform over several GHz of bandwidth, and at very high data rates at practically useful distances. As such, UOWCs may become paramount for meeting the speed and capacity demand, as marine industries shift towards the use of unmanned and/or autonomous vehicles. Nevertheless, the current research body and commercially available devices seemed to indicate that UOWCs may not yet be achieving their full potential, particularly due to lapses in the understanding of the spatio-temporal nature of the marine environment and applying corresponding mitigatory measures. Therefore, firstly, in this work, the factors affecting the received-optical-power of receivers with photodiode arrays, were explored in different, homogeneous, Jerlov water types. The results highlighted the importance of designing UOWCs being considerate of the medium-induced attenuation and geometric spreading. This included choices such as the modulation scheme, and the geometric shape of the receiver. However, marine environments are often inhomogeneous and exist in layers. Furthermore, out in the open, UOWCs cannot be isolated from solar interference. To address this, the depth-spectral distribution profiles for sunlight have been characterised in the different types of clear, and coastal, stratified waters. Thereafter, a subsequent analysis showed how utilising a mix of both short (blue) and long (yellow) transmission wavelengths along the water column, may enhance the link quality under sunlight. This property along the water column may have significant implications towards underwater networks, which may need to transmit at different wavelengths based on the factors such as the link location and depth, the output power, the beam shape, link orientation, etc. Additionally, there was a gap in understanding the inherent optical properties of the most turbid, stratified water profile. This was addressed by analysing in-situ data, collected in the Hauraki Gulf, New Zealand, and deriving the respective coefficients. The findings showed that the conventional means of deriving the inherent optical properties of turbid, coastal seawater using the Chl-a concentration, may not be accurate. This seemed also the case for the water in the boundary layer, for which an alternative method was proposed, using the profiles of solar irradiance and turbidity. A subsequent analysis showed that a reflector-aided, non-line-of-sight communication may perhaps enhance UOWCs in the boundary layer, up to about 39.7%. Based on these findings, a novel modulation scheme was proposed based on spread-spectrum modulation and differential signalling. The scheme was examined for performance by simulation and in a scaled-experiment inside a fish-tank. The results seemed very positive in clear water environments; also, where solar interference was the greatest. The marine environments were emulated using seawater, scattering agent, and for the first time in UOWC research – live algae. The experiment also raised additional insights and concerns towards the use of dichroic and birefringent optical filters in turbid, scattering environments. This finding may have implications towards realising all UOWC where the transmission colour is a communication factor, especially in coastal water. Thus, employing the recommendations made in this Thesis may help to enhance UOWCs that transpire through water bodies with spatio-temporal distributions of constituents, severe ambient sunlight, and the highly turbid and attenuating bottom water.

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Permanent link

http://hdl.handle.net/10292/20579

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Doctoral Theses