Quasi-orbital angular momentum for microwave communications
Creators: name: FOUDA, Reham
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400601
subject: Antennas and propagation
classificationCode: 400608
subject: Wireless communication systems and technologies (incl. microwave and millimetrewave)

description: The increasing demands of current and future communications, have made it imperative on the telecommunications industry, and research community, to continue extending on their innovative capabilities and knowledge. With most of society's communications occupying the microwave domain, the issues of spectral congestion, channel capacity limitations and security, remain a top priority.<br><br>

Orbital Angular Momentum (OAM), as a fundamental property of all electromagnetic (EM) fields was first discovered, and studied by the physics community in the 1990s. These studies were mainly within the optical range of the spectrum, and only recently has OAM been introduced to the microwave domain for communications. OAM is associated with a helical phase profile to the radiation pattern, where its number of 2_ rotations, are dependent on the OAM mode number. Electromagnetic microwaves carrying different OAM modes, are said to be orthogonal to one another, allowing multiple channels on the same frequency. Hence; a promising candidate for addressing spectral congestion. It also has natural anti-interference and anti-jam capabilities, due to their theoretically infinite mode diversity.<br><br>

The realisation of OAM within the microwave domain, however, has its own unique challenges, seperate to those in optics. These challenges include, but not limited to, significant radiation pattern divergence, the need of multiple, or large apertures to appreciate OAM's diverse mode capabilities, and limitations on aperture circumference. This inevitably deems it difficult to realise a practical OAM communications system, exploiting OAM's mode diversities and capabilities for transforming microwave communications. Realising the capabilities of OAM however, and potential in innovating the telecommunications industry, the author looked to address some of those limitations within the microwave frequency domain in this thesis. The three main original contributions to knowledge, made to further the BoK on OAM in microwaves, will be discussed and presented in this thesis, are summarized below.<br><br>
The first original contribution to knowledge, by the author, investigates the capability of inducing OAM with Quasi-Circular Apertures. The proposed Quasi-Circular Array Antennas are in the form of partial circular arrays, in contrast to full conventional circular arrays used to generate OAM carrying radiation patterns. These Quasi-Circular Array Antenna apertures proved that OAM can be generated by only a partial circular aperture (significantly smaller array apertures), whilst generating Quasi-OAM radiation patterns with more gain, and directivity. <br><br>
The second original contribution to knowledge, by the author, investigates the theoretical limitations on the circumference of partial apertures in relation to the resulting OAM mode power. To further reduce the aperture size of the Quasi-Circular Arrays for compactness and practicality in OAM-based communication systems, the author proposes a method to augment the OAM mode power, whilst using a Quasi-Circular Array Antenna aperture of circumference, less than the theoretical limit of π.<br><br>
The third original contribution contribution to knowledge, by the author, investigates the robustness of Quasi-OAM radiation patterns in comparison to conventional OAM, using Bit- Error-Rate (BER) measurements to benchmark the performance of each aperture. The proposed Quasi-Circular Array Antenna aperture is then tested for performance in the presence and absence of OAM mode cross-talk. This is to verify the Quasi-OAM mode orthogonality using the proposed Quasi-Circular Array Antenna aperture, for OAM-based communications. This resulted in a proposed Quasi-OAM based communication system, where the robustness of Quasi-OAM radiation patterns proved to be better than conventional OAM radiation patterns in the absence of mode cross-talk, and very similar in the presence of cross-talk. <br><br>
In contrast to conventional OAM-based communication systems in the microwave domain, the proposed Quasi-OAM based communication system, Quasi-Circular Array Antennas, and the ability to augment the OAM power with much smaller apertures, is envisioned to innovate current OAM-based communication systems. This is whilst exploiting the additional benefits of Quasi-Circular Array Antenna apertures and Quasi-OAM radiation patterns, alongside the existing benefits of OAM, as demonstrated in this thesis. These benefits are also envisioned to extend outside the telecommunications domain, where OAM is currently being explored.