A Survey on Non-Geostationary Satellite Systems: The Communication Perspective Hayder Al-Hraishawi, Member, IEEE, Houcine Chougrani, Steven Kisseleff, Member, IEEE, Eva Lagunas, Senior Member, IEEE, and Symeon Chatzinotas, Senior Member, IEEE Abstract—Non-geostationary (NGSO) satellites are envisioned (NGSO) satellites to satisfy the burgeoning demand for global to support various new communication applications from count- broadband, high-speed, heterogeneous, ultra-reliable and low less industries. NGSO systems are known for a number of key latency communications. For instance, the emerging NGSO features such as lower propagation delay, smaller size, and lower signal losses in comparison to the conventional geostationary satellites and mega constellations such as SES O3b, OneWeb, (GSO) satellites, which will enable latency-critical applications Telesat, and Starlink have a system capacity reaching the to be provided through satellites. NGSO promises a dramatic terabits-per-second level [3]. boost in communication speed and energy efficiency, and thus, In the last few years, the notion of utilizing large constel- tackling the main inhibiting factors of commercializing GSO lations of LEO satellites to provide reliable, low-latency and satellites for broader utilizations. However, there are still many NGSO deployment challenges to be addressed to ensure seamless high-speed Internet from space has re-gained popularity and integration not only with GSO systems but also with terrestrial experienced a tremendous growth. This trend is rather surpris- networks. These unprecedented challenges are discussed in this ing given the unfortunate faring of past NGSO constellations, paper, including coexistence with GSO systems in terms of but it appears that both technological and business momentums spectrum access and regulatory issues, satellite constellation and are favorable with impressive achievements from SpaceX, architecture designs, resource management problems, and user equipment requirements. Beyond this, the promised improve- SES O3B, and OneWeb. In fact, between 2014 and 2016, a ments of NGSO systems have motivated this survey to provide the new wave of proposals for large LEO constellations emerged state-of-the-art NGSO research focusing on the communication with the target of providing global broadband services [4]. prospects, including physical layer and radio access technologies Specifically, the number of satellites were launched into space along with the networking aspects and the overall system features has dramatically increased according to the recent satellite and architectures. We also outline a set of innovative research directions and new opportunities for future NGSO research. database released by the Union of Concerned Scientists (UCS) Index Terms—Non-Geostationary (NGSO) satellite constel- [5]. This database has listed more than 3,000 operational lations, satellite communications, space information networks, satellites currently in orbit around Earth with huge difference space-based Internet providers, spacecraft. between the number of geostationary (GSO) and NGSO satel- lites in favor of the latter as depicted in Fig. 1. I. INTRODUCTION Satellites have a distinctive ability of covering wide geo- graphical areas through a minimum amount of infrastructure 1400 GSO on the ground, which qualifies them to be an appealing NGSO solution to fulfill the growing diversified applications and 1200 services either as a stand-alone system, or as an integrated satellite-terrestrial network [1]. Currently, the field of satellite 1000 communications is drawing increasing attention in the global telecommunications market as several network operators start 800 using satellites in backhauling infrastructures for connectivity 600 and for 5G system integration [2]. Recently, due to the swift rise of NewSpace industries that are developing small satellites 400 with new low-cost launchers, a large number of satellite op- Number of Launched Satellites erators are planning to launch thousands of non-geostationary 200 arXiv:2107.05312v1 [eess.SP] 12 Jul 2021 H. Al-Hraishawi, H. Chougrani, S. Kisseleff, E. Lagunas, and S. 0 Chatzinotas are with the Interdisciplinary Centre for Security, Relia- 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 bility and Trust (SnT), University of Luxembourg, Luxembourg. E- Year mails: fhayder.al-hraishawi, houcine.chougrani, steven.kisseleff, eva.lagunas, [email protected]. This research was funded in whole by the Luxembourg National Re- Fig. 1. Comparison between GSO and NGSO in terms of the number of search Fund (FNR) in the frameworks of the FNR-CORE project ”Mega- launched satellites per year. LEO: Self-Organised Lower Earth Orbit Mega-Constellations” (Grant no. C20/IS/14767486). For the purpose of open access, the authors have applied a Creative Commons Attribution 4.0 International (CC BY 4.0) license to any NGSO satellites on a geocentric orbit include the low earth Author Accepted Manuscript version arising from this submission. orbit (LEO), medium earth orbit (MEO) and highly elliptical 1 orbit (HEO) satellites, which are orbiting constantly at a lower A. Prior Related Surveys altitude than that of GSO satellites, and thus, their link losses and latency due to signal propagation are lower [6]. These Over the last few years, a number of excellent surveys and intrinsic features of NGSO systems besides the high capacities, tutorials pertained to satellite communications appeared in the large footprints, and fast deployment, offer an interesting literature, [16]–[29], to report and study the technical develop- set of advantages for the high-speed interactive broadband ments and challenges, including satellite network architectures, services [7]. Furthermore, the most newly developments in attributes and applications of lower orbit satellites, satellite- NGSO systems empower satellites to manage narrow steerable terrestrial systems integration, and small satellite systems. In beams covering a relatively broad area, which facilities the the following, research scope and contributions of the relevant use of smaller and lower cost equipment at the user terminals surveys will be briefly presented. Afterwards, a comparison [8]. Specifically, the offered capacities by NGSO satellites between these surveys and our work in this paper will be can be further increased through utilizing high frequencies summarized at a glance in Table I in order to point out the along with employing throughput enhancement techniques distinctive contribution of each survey. such as spectrum sharing, cooperative gateway diversity, user The survey in [16] has captured the recent technical ad- clustering and interference mitigation, and multiple antenna vances in scientific, industrial and standardization analyses communications [9]. in the domain of satellite communications with presenting the important research directions for satellite communication Furthermore, satellite systems have been contributing to applications and use cases such as new constellation types, deliver telecommunication services in a wide range of sectors on-board processing capabilities, non-terrestrial networks and such as aeronautical, maritime, military, rescue and disaster space-based data collection and processing. A review of the relief. Beyond this, NGSO systems are envisaged to be an state-of-the-art research progress of satellite communications appealing solution for future non-terrestrial networks (NTN) covering land mobile satellite (LMS) communication net- to meet the demanding 6G system requirements in terms of works, hybrid satellite-terrestrial relay networks, and satellite- both large throughput and global connectivity [10]. Towards terrestrial integrated networks is provided in [17] under the this direction, the third generation partnership project (3GPP) framework of physical-layer security. The potentials and chal- standards group has been codifying the use of satellite commu- lenges of satellite-based IoT architecture have been also stud- nication networks to integrate space and terrestrial communi- ied in [17], along with popularized performance metrics in cation networks in order to support future wireless ecosystems order to evaluate system security. Moreover, the limitations of [11], [12]. Moreover, by harnessing satellites geographical land mobile satellite (LMS) systems in terms of connectivity, independence, wireless connectivity can be extended to the stability, and reliability are studied in [18], where the LMS underserved and unserved areas, where NGSO systems can be is considered as a satellite-based communication system that an efficient solution for viable deployments of 5G networks. can serve ground users in different areas. LMS systems NGSO satellite capabilities of ubiquitous coverage and con- are overviewed based on satellite orbits, operating frequency nectivity can also be leveraged for provisioning resiliency and bands, and signal propagation along with highlighting some continuity of 5G services to the mobile platforms such as on- future research challenges. board aircraft, high-speed trains, sea-going vessels, and land- The work in [19] surveys the research efforts for implement- based vehicles that are beyond the reach of a cell site [13]. ing inter-satellite communication for small satellite systems, by reviewing various constellation design parameters within In addition to the NGSO satellite unique capabilities in the first three layers of OSI model, i.e., physical, data link, and providing global coverage,