Intelligent Reflecting Surface-Aided LEO Satellite Communication: Cooperative Passive Beamforming and Distributed Channel Estimation

Abstract

Low-earth orbit (LEO) satellite communication plays an important role in assisting/complementing terrestrial communications by providing worldwide coverage, especially in harsh environments such as high seas, mountains, and deserts which are uncovered by terrestrial networks. Traditionally, the passive reflect-array with fixed phase shifts has been applied in satellite communications to compensate for the high path loss due to long propagation distance with low-cost directional beamforming; however, it is unable to flexibly adapt the beamforming direction to dynamic channel conditions. In view of this, we consider in this paper a new intelligent reflecting surface (IRS)-aided LEO satellite communication system, by utilizing the controllable phase shifts of massive passive reflecting elements to achieve flexible beamforming, which copes with the time-varying channel between the high-mobility satellite (SAT) and ground node (GN) cost-effectively. In particular, we propose a new architecture for IRSaided LEO satellite communication where IRSs are deployed at both sides of the SAT and GN, and study their cooperative passive beamforming (CPB) design over line-of-sight (LoS)-dominant singlereflection and double-reflection channels. Specifically, we jointly optimize the active transmit/receive beamforming at the SAT/GN as well as the CPB at two-sided IRSs to maximize the overall channel gain from the SAT to each GN. Interestingly, we show that under LoS channel conditions, the highdimensional SAT-GN channel can be decomposed into the outer product of two low-dimensional vectors.By exploiting the decomposed SAT-GN channel, we decouple the original beamforming optimization problem into two simpler subproblems corresponding to the SAT and GN sides, respectively, which are both solved in closed-form. Furthermore, we propose an efficient transmission protocol to conduct channel estimation and beam tracking, which only requires independent processing of the SAT and GN

Low Altitude Satellite Constellation for Futuristic Aerial-Ground Communications

“…An IRS consists of a large number of low-cost passive components, each of which may independently reflect the incoming signal with a programmable phase shift to achieve three-dimensional passive beamforming without transmitting radio-frequency chains [101,102]. Therefore, IRS installed on a UAV is an effective option for SATCOM and ground users [103][104][105][106].…”

Nonterrestrial Communications Assisted by Reconfigurable Intelligent Surfaces

“…To better compensate for the severe propagation path loss in a satellite communication system, Zheng et al [132] proposed to deploy RISs both at satellite and GU sides. The proposed two-sided RISs-assisted system outperforms the one-sided RIS-assisted system with only one RIS deployed at the GU side or at the satellite side and the system without the assistance of RIS.…”

“…In highfrequency communications, the propagation channel can experience an important variation due to only a few millimeters of movement. This is the reason why in RANTNs, real-time channel estimation is more challenging due to mobile aerial platforms and becomes even more difficult when ARISs instead of TRIS are considered [132], [167]. Achieving real-time reconfigurability in such various mobility conditions is still an open question that requires the development of appropriate tools.…”

IRS-Empowered 6G Networks: Deployment Strategies, Performance Optimization, and Future Research Directions

“…For instance, the authors in [125] jointly optimized the power allocation and the IRS's phase shift using a Mesh Adaptive Direct Search method to maximize the channel capacity of an IRSassisted GEO SatCom network. Similarly, an IRS-aided LEO SatCom architecture was proposed in [126], where the IRS elements are deployed on the LEO satellites and the ground nodes. Afterwards, the authors jointly optimized the active and passive beamforming on the LEO satellite and the ground nodes to maximize the channel capacity.…”