Year

2023

Degree Name

Doctor of Philosophy

Department

School of Electrical, Computer and Telecommunications Engineering

Abstract

Satellite mega constellations in Low Earth Orbit are becoming increasingly popular. Starlink (by Space X) and OneWeb are two but recent examples of the ongoing race to provide seamless global connectivity. Having thousands or and potentially tens of thousands of small satellites orbiting the Earth could enable the migration of terrestrial communication and internet-based services to outer space. One of these communication services is the fifth generation (5G) communication technology. The appearance of 5G in the terrestrial infrastructure happened in 2019 but there is an increasing trend to consider how 5G can be enabled from space. This trend is supported by the recent initiatives of 3GPP and the European Space Agency on Non-Terrestrial Networks (NTN) for 5G Systems and Satellites for 5G (S45G) respectively. As a matter of fact, the progress of Low Earth Orbit (LEO) mega constellations and 5G technologies is simultaneous and will be interleaved under the new Satellite-Terrestrial (S-T) hybrid paradigm. 5G enabled LEO constellations will complement the existing terrestrial cellular infrastructure as they offer many benefits. For example, coverage of remote areas, adaptability to diverse traffic, seamless handovers, global coverage and line of sight communication links which are crucial in the mm-Wave 5G frequencies. A promising platform that could accommodate such a system is a class of nanosatellites called CubeSats. CubeSats are another emerging technology and are known for their small size, low cost, ease of deployment and their suitability for mega constellations in LEO. One of the main challenges of CubeSats with 5G capabilities will be faced during the design and integration of a suitable antenna system to achieve high data rate downlink communications. In particular, a hybrid antenna system must be devised that provides high gain, increased throughput, must work in mm-Wave bands and can adapt to the high traffic demands of 5G. In this thesis, we propose a Reflectarray antenna system to meet the aforementioned needs. Reflectarrays are a hybrid of reflector and phased array antennas. Reflectarrays have an abundant of potentials such as high gain, beam shaping, beam scanning, reconfigurability and multi-beam characteristics. On the other hand, reflectarrays present certain limitations regarding their bandwidth, polarization diversity and cost. Moreover, the technology of reconfigurable reflectarrays at mmWave started in 2012, hence limited knowledge and implementations are available. Reflectarrays have been proposed for a variety of satellite applications mainly based on Geostationary Orbit while limited work exists on designs of CubeSats equipped with this type of antenna. Due to the aforementioned research gaps, this work studies reflectarrays as candidates for future 5G enabled CubeSat constellations with a focus on architectures that can achieve high gain that can be maintained over a wide frequency band. To that end, in this thesis, CubeSat missions are first surveyed from an antenna perspective in order to understand the requirements that each mission type poses to the antenna system of CubeSats. Based on this survey, an antenna selection guide is designed, which is used to select a suitable antenna type for 5G enabled CubeSats. It was found that for CubeSat missions, such as 5G from Space, where high data rates are important, reflectarrays that operate in X and Ka-bands are a suitable antenna solution. On the other hand, a limiting performance metric of relfectarrays is their gain-bandwidth which indicates the frequency range that a reflectarray can maintain a stable and high gain. Hence, in this thesis, wideband reflectarrays are studied and categorized according to four wideband phase tuning mechanisms, namely, multi resonance, angular rotation, true time or phase delay lines and subwavelength periodicity approach. The bandwidth is studied both from a unit cell and reflectarray system perspective with emphasis on the gain-bandwidth performance. From this study, several design guidelines are formed that can be followed to design low profile and low cost reflectarrays suitable for 5G enabled CubeSats. Based on these guidelines four wideband reflectarray unit cells operating in X and Ka-band frequencies are designed and optimized. The unit cells utilize the multi-resonance and subwavelength periodicity strategies. From these unit cells, the two with the best bandwidth performance are selected and are used to design and optimize a high-gain X-band reflectarray and a high-gain Ka-band reflectarray, surpassing the performance of existing state-of-the-art wideband reflectarrays in terms of the 1-dB gain bandwidth. Moreover, considering the real estate limitations of CubeSats and small satellite platforms, these two aforementioned reflectarrays are integrated into a single aperture. Consequently, a dual-band reflectarray, achieving a wide 1-dB gain bandwidth at both X and Ka-bands, is designed. Notably, the presented reflectarray is the first dual-band wideband reflectarray proposed for CubeSat applications in the current literature.

FoR codes (2008)

0906 ELECTRICAL AND ELECTRONIC ENGINEERING

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.