Distributed Beamforming Design for LEO Satellite-to-Phone Communications Enhancement
Creators: name: Xu, Zhuoao
Publisher: name: University of Surrey
Year: 2024
Subjects: Distributed beamforming, LEO, Satellite-to-Phone, Link Budget, Simultaneously Transmitting and Reflecting Surface

description: Benefiting from reduced rocket launch costs and advanced launch technologies, Low Earth Orbit (LEO) satellite constellations are experiencing a resurgence. The development of LEO satellite networks holds significant importance as they offer global communication coverage unaffected by terrestrial conditions. They are particularly valuable for remote areas, oceans, deserts, mountains, and even in extreme natural disaster scenarios such as earthquakes and tsunamis, where they remain largely unaffected, providing timely assistance to those in need. Furthermore, compared to higher orbit satellites such as geostationary satellites, the lower orbit of LEO satellites reduces communication latency and path loss. Additionally, advancements in communication technology and satellite production techniques have contributed to the renewed attention on LEO mega-constellations after two decades of relative obscurity.&lt;br&gt;The primary objective of this study is to investigate the facilitation of distributed beamforming technology for direct LEO satellite-to-smartphone communication. A major obstacle in direct LEO satellite-to-smartphone communication is the insufficient communication link budget, resulting in support only for narrowband communication services such as short messaging. To enhance the link budget, Starlink employs ground terminals as relays to receive signals from LEO satellites and forward them to user terminals, while the AST group utilises large antenna arrays at the satellite end. The approach adopted in this thesis is to leverage the collaborative operation of multiple LEO satellites using distributed beamforming technology to enhance the communication link budget. The advantage of this distributed network architecture is its ability to fully utilise large-scale constellations, providing a flexible and resilient network structure.&lt;br&gt;We first validate that beams from two distant transmitters can still be superimposed in phase at a distant point, resulting in signal enhancement at the receiver. We then apply this concept to LEO satellite networks, where electromagnetic (EM) waves from multiple satellites arrive at the common receiver simultaneously with the same phase and frequency.&lt;br&gt;Furthermore, we analyse how variations in the structural parameters of the distributed satellite array, such as the number of satellites, their positions, and their trajectories, affect the ground coverage pattern. Additionally, the enhancement of the link budget through distributed beamforming is significantly influenced by satellite time synchronisation conditions, including phase alignment, frequency synchronisation, and time calibration. Therefore, we also conduct a detailed analysis of the degradation effects when there are deviations in phase, frequency, and time.&lt;br&gt;Simulation results of the ground coverage patterns reveal that the adoption of distributed beamforming technology inevitably leads to uneven coverage. To improve the continuity and stability of this network connection, we propose a novel EM surface capable of achieving reflection on one side and transmission on the other. This asymmetric structure, with independent control over transmission and reflection, enhances energy efficiency.