The design, simulation, and pattern synthesis of novel reflectarrays

The main focus of this thesis is on the development of both a comprehensive understanding and a thorough computational routine of the reflectarray metasurfaces designs, with focuses on a liquid crystal-based reconfigurable reflectarray metasurface and on the phase-retrieval/optimisation techniques for reflectarray-based pattern synthesis. A dielectric-based polarisation converting reflectarray metasurface is also presented, with the advantage of having a thinner profile over the traditional quartz-based half-wave plates. In the introductory sections, a thorough review of the state-of-the-art metasurfaces is presented, with a focus on applications to high-frequency wireless communications, as the motivation of this PhD is on the development of technologies that would facilitate the wireless communication challenges for the 5G-and-beyond frequency spectrum. In this section, a review of array antennas, including phased arrays, reflectarrays, transmitarrays, as well as metasurfaces utilising Mie-resonance, plasmonic resonance, geometric phase (Pancharatnam-Berry phase) and photoconductive material is presented. The following chapter on the theoretical background ensures the understanding of the fundamental mechanisms that will be applied to the study of reflectarray metasurfaces and optimisation routines. The high-frequency propagation associated with beyond-5G wireless communications brings many challenges to the current standards: some of the biggest problems are the much greater path loss and heavy non-line-of-sight signal attenuation. Traditionally, this has been dealt with in phased arrays. However, in the introduction part of this thesis, I show that this becomes impractical due to the requirements of enormous array sizes and expensive high-frequency phase shifters. Therefore, in our research, we have focused on reconfigurable reflectarrays as an intermediate solution to alleviate the tough propagation challenges faced by beyond-5G wireless communications. The reconfigurable reflectarray can either be designed to reflect off from a nearby mobile cell site to enhance the signal strength for non-line-of-sight areas, or it can include an integrated source to function independently, reducing the losses associated with power amplifiers and complex circuitries associated with the enormous array sizes. This thesis aims to produce a high-frequency tailored reconfigurable reflectarray design, which combines the conceptual advantages from state of-the-art lumped-element-based and liquid crystal-based reflectarrays. As shown in the literature review section, most recent researches on lumpedelement- based reconfigurable reflectarrays are designed for the sub 40 GHz frequencies; with higher frequencies, the intrinsic losses associated with lumped-elements such as PIN diodes make them unsuitable choices. On the other hand, liquid crystals have been used as a tunable material for different radio-frequency applications; however, most state-of-the-art designs of liquid crystal-based reflectarrays do not incorporate individual biasing control for maximum beam-control. There are also challenges faced with individually-biased reconfigurable reflectarrays. Traditionally, phased arrays can perform single beam-scanning or multiple beam-scanning with the control of multiple sub-arrays. We intend to achieve more complex beam-functionalities (such as vortex, null, and magnitude-specific beams) within the domain of manipulating one individual array. This is already possible with optimisation algorithms such as the genetic algorithm. However, traditional optimisers such as the genetic algorithm and particle swarm optimisation are far too slow to be implemented in an “online” mode, where the algorithm runs onboard the reflectarray to give low-latency solutions. The “online” optimisation mode would be very beneficial as it would reduce the channel occupation from the transmission of configuration information and thus increase channel capacity. In this thesis, I aim to develop an individually biased liquid crystal-based reconfigurable reflectarray for >100 GHz frequencies. I also aim to develop an algorithm that is sufficiently quick to have the potential to be practically utilised as an onboard pattern synthesis optimisation method. Additionally, using the same design principles, I have designed an all-dielectric-based reflectarray metasurface that acts as a polarisation-converting quarter-wave plate, which is much thinner than traditional quartz-based quarter-wave plates. In the Research Results and Publications chapter, a complete procedure for the design of LC-based reconfigurable and dielectric-based nonreconfigurable reflectarray metasurfaces is presented, where much of the content comes from the author’s own publications[52, 78, 50, 51]. This thesis provides details on the computational tools/programs used, cross-platform routines development with CST Studio Suite, MATLAB and VBA, and the pattern synthesis algorithm, whereby a genetic algorithm is employed for the global optimisation, and an improved Gerchberg-Saxton algorithm is developed and adapted to the application of faster local optimisation for the pattern synthesis. For the all-dielectric reflectarray metasurface, the further functionality of polarisation conversion (linear to circular and circular to linear) is demonstrated on top of the beam-manipulation capabilities of the reflectarrays. The reflectarray metasurfaces can be designed to beamform, beamsteer, beamsplit/multibeam, as well as achieve novel beam profiles such as the vortex profile. Originally, the idea was to completely focus on the liquid crystal-based study and to develop a down-scaled 28 GHz proof-of-concept, for which partial work had already begun (the simulation, optimisation and initial planning on the construction with collaborators from other departments); however, due to the pandemic and numerous other uncontrollable factors, this was later discarded and replaced by remaining on and extending upon the computational studies, to further understand and improve the pattern synthesis algorithms and the other types of phase-change metasurfaces.