A new approach to satellite communication: Harnessing the power of reconfigurable intelligent surfaces

Abstract

It is widely accepted that user-centric and ubiquitous connectivity, which are desired by both end users and operators for the 6th generation (6G) and beyond communication technologies, can be achieved through the unique orchestration of terrestrial and non-terrestrial networks (NTNs) in next-generation communication systems. This vision is also described by the 3rd Generation Partnership Project (3GPP) in Technical Report (TR) 38.821 for the operation of New Radio (NR) in NTNs. According to the definition by the 3GPP, an NTN basically consists of unmanned aerial vehicles, high-altitude platform stations (HAPS) systems, and dense satellite deployments. Low-Earth orbit (LEO) satellites and HAPS systems are considered to be the key enablers for NTNs due to their unique features, which include longer operating times and wider coverage areas. The most important pillars of non-terrestrial networks are ultra-dense satellite constellations. Although satellite networks are considered a prominent solution, many challenging open issues remain to be addressed. The most prominent ones are the size, weight, and power (SWaP) constraints, high path loss, and energy efficiency. As known, multi-antenna technologies are used to mitigate high path loss by taking advantage of its beamforming capacity. However, the hardware and signal processing units of multi-antenna systems are quite complex and costly. These costs are much higher in satellite networks. Recently, it was shown that a passive antenna solution with reconfigurable smart surfaces can reduce these costs and help increase communication performance. In this regard, we propose the use of reconfigurable intelligent surface (RIS) to improve coordination between these networks given that RISs perfectly match SWaP restrictions of operating in satellite networks as a main focus of this thesis. A comprehensive framework of RIS-assisted non-terrestrial and interplanetary communications is presented that pinpoints challenges, use cases, and open issues. Furthermore, the performance of RIS-assisted NTNs under environmental effects, such as solar scintillation and satellite drag, is discussed in light of simulation results. First, we propose a novel architecture involving the use of RIS units to mitigate the path loss associated with long transmission distances. These RIS units can be placed on satellite reflectarrays, and, when used in broadcasting and beamforming, it can provide significant gains in signal transmission. This study shows that RIS-assisted satellites can provide a severe improvement in downlink and achievable uplink rates for terrestrial networks. Although RIS has the potential to increase efficiency and perform complex signal processing over the transmission environment instead of transceivers, RIS needs information on the cascaded channel in order to adjust the phase of the incident signal. Consequently, channel estimation is an essential part of RIS-assisted communications. A study presented in the thesis evaluates the pilot signal as a graph. It incorporates this information into the graph attention networks (GATs) to track the phase relation through pilot signaling. The proposed GAT-based channel estimation method investigates the performance of the direct-to-satellite (DtS) networks for different RIS configurations to solve the challenging channel estimation problem. It is shown that the proposed GAT demonstrates a higher performance with increased robustness under changing conditions and has lower computational complexity compared to conventional deep learning (DL) methods. Moreover, based on the proposed method, bit error rate (BER) performance is investigated for RIS designs with discrete and non-uniform phase shifts under channel estimation. One of the findings in this study is that the channel models of the operating environment and the performance of the channel estimation method must be considered during RIS design to exploit performance improvement as far as possible. We show that RIS can improve energy efficiency in ground-to-satellite com munications. To complete the puzzle of overall satellite communications, we investigate RIS-assisted inter-satellite communication performance in terms of BER and achievable rate as well since broadband inter-satellite communication is one of the key elements of satellite communication systems that orchestrate massive satellite swarms in cooperation. Thanks to technological advancements in microelectronics and micro-systems, the terahertz (THz) band has emerged as a strong candidate for inter-satellite links (ISLs) due to its promise of wideband communication. In particular, multi-antenna systems can improve the system performance along with the wideband supported by the THz band. However, multi-antenna systems should be considered due to their SWaP constraints. On the other hand, as a state-of-the-art multi-antenna technology, RIS is able to relax SWaP constraints because of its passive component-based structures. However, as similar reflection characteristic throughout the wideband is challenging to meet, it is possible to observe beam misalignment. In the thesis, we first provide an assessment of the use of the THz band for ISLs and quantify the impact of misalignment fading on error performance. Then, to compensate for the high path loss associated with high carrier frequencies, and to further improve the signal-to-noise ratio (SNR), we propose using RISs mounted on neighboring satellites to enable signal propagation. Based on a mathematical analysis of the problem, we present the error rate expressions for RIS-assisted ISLs with misalignment fading. Also, numerical results show that RIS can leverage the error rate performance and achievable capacity of THz ISLs as long as a proper antenna alignment is satisfied. As the misalignment error seems one of the challenges on the path toward practical RIS-assisted NTN, the acquisition of a reliable direction of arrival (DoA) estimation becomes more of an issue in achieving promised improvements in RIS-assisted communication systems. For that reason, we address DoA estimation problem in RIS-assisted communication systems in the thesis. For this aim, we use a single-channel intelligent surface whose physical layer compression is achieved using a coded-aperture technique, probing the spectrum of far-field sources that are incident on the aperture using a set of spatiotemporally incoherent modes. This information is then encoded and compressed into the channel of the coded-aperture. The coded-aperture is based on a metasurface antenna design and it works as a receiver, exhibiting a single-channel and replacing the conventional multi-channel raster scan-based solutions for DoA estimation. The GAT network enables the compressive DoA estimation framework to learn the DoA information directly from the measurements acquired using the coded-aperture. This step eliminates the need for an additional reconstruction step and significantly simplifies the processing layer to achieve DoA estimation. We show that the presented GAT integrated single-pixel radar framework can retrieve high-fidelity DoA information even under relatively low signal-to-noise ratio (SNR) levels. Along with above work, in this thesis we analyse the performance of the main communication pillars of an end-to-end RIS-assisted satellite communication system and focus on the development of solutions to open problems that are essential in practical application.