Abstract
This paper surveys the literature on point-to-point (P2P) links for integrated satellite-aerial networks, which are envisioned to be among the key enablers of the sixth-generation (6G) of wireless networks vision. The paper first outlines the unique characteristics of such integrated large-scale complex networks, often denoted by spatial networks, and focuses on two particular space-air infrastructures, namely, satellites networks and high-altitude platforms (HAPs). The paper then classifies the connecting P2P communications links as satellite-to-satellite links at the same layer (SSLL), satellite-to-satellite links at different layers (SSLD), and HAP-to-HAP links (HHL). The paper surveys each layer of such spatial networks separately, and highlights the possible natures of the connecting links (i.e., radio-frequency or free-space optics) with a dedicated survey of the existing link-budget results. The paper, afterwards, presents the prospective merit of realizing such an integrated satellite-HAP network towards providing broadband services in under-served and remote areas. Finally, the paper sheds light on several future research directions in the context of spatial networks, namely large-scale network optimization, intelligent offloading, smart platforms, energy efficiency, multiple access schemes, distributed spatial networks, and routing.
Highlights
Connectivity is the backbone of modern digital economy with over three billion people connected worldwide and more than 14 billion devices connected through the Internet core network
Unlike most of the aforementioned surveys that only focus on a single non-terrestrial network layer, i.e., either satellites or high-altitude platforms (HAPs), our current paper focuses on P2P links in a multilayered spatial network
Assessing the true benefits arising from integrating various single-layer networks at different altitudes remains, subject to several physical hurdles
Summary
Connectivity is the backbone of modern digital economy with over three billion people connected worldwide and more than 14 billion devices connected through the Internet core network. The links in such a multilayer network can be established using both radio-frequency (RF) waves and freespace optics (FSO), as discussed in details later in the paper. CDL’s limited spectrum constraints limit its data rate between 274 Mbps to 3 Gbps, which do not satisfy the demand for high-speed wireless links [7] [28] In this context, U.S Defense Advanced Research Projects Agency (DARPA) started a program called “Free-space Optical Experimental Network Experiment (FOENEX)” to develop links that can transmit data using FSO at a much higher speed. Such systems operate by switching to low-capacity RF links in bad weather conditions, or to high-capacity FSO links under perfect transceivers alignment and suitable weather conditions One such hybrid project is Integrated Aerial Communications (FaRIA-C) headed by DARPA [38].
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