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 IRS-aided 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 single-reflection 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 high-dimensional 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 in a distributed manner, thus substantially reducing the implementation complexity. Simulation results validate the performance advantages of the proposed IRS-aided LEO satellite communication system with two-sided cooperative IRSs, as compared to various baseline schemes such as the conventional reflect-array and one-sided IRS.

中文翻译：

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智能反射地面辅助低轨卫星通信：协作无源波束形成和分布式信道估计


低地球轨道（LEO）卫星通信通过提供全球覆盖，在辅助/补充地面通信方面发挥着重要作用，特别是在地面网络无法覆盖的公海、山区和沙漠等恶劣环境中。传统上，固定相移的无源反射阵列已应用于卫星通信，以低成本定向波束形成来补偿长传播距离造成的高路径损耗；然而，它无法灵活地使波束成形方向适应动态信道条件。鉴于此，我们在本文中考虑了一种新型智能反射面（IRS）辅助的LEO卫星通信系统，通过利用大量无源反射元件的可控相移来实现灵活的波束形成，以应对之间的时变信道高机动性卫星（SAT）和地面节点（GN）具有成本效益。特别是，我们提出了一种 IRS 辅助的 LEO 卫星通信的新架构，其中 IRS 部署在 SAT 和 GN 的两侧，并研究了它们在视距 (LoS) 主导的单卫星上的协作无源波束形成 (CPB) 设计。 -反射和双反射通道。具体来说，我们联合优化 SAT/GN 处的主动发射/接收波束成形以及两侧 IRS 处的 CPB，以最大化从 SAT 到每个 GN 的总体信道增益。有趣的是，我们表明在LoS信道条件下，高维SAT-GN信道可以分解为两个低维向量的外积。 通过利用分解的 SAT-GN 信道，我们将原始波束成形优化问题解耦为分别对应于 SAT 和 GN 侧的两个更简单的子问题，这两个子问题都以封闭形式求解。此外，我们提出了一种有效的传输协议来进行信道估计和波束跟踪，只需要以分布式方式独立处理SAT和GN，从而大大降低了实现复杂度。仿真结果验证了与传统反射阵列和单侧 IRS 等各种基线方案相比，所提出的具有两侧协作 IRS 的 IRS 辅助 LEO 卫星通信系统的性能优势。