© 2021 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. On the Path to 6G: Embracing the Next Wave of Low Earth Orbit Satellite Access Xingqin Lin†, Stefan Cioni‡, Gilles Charbit§, Nicolas Chuberre⊥, Sven Hellsten†, and Jean-Francois Boutillon⊥ †Ericsson, ‡European Space Agency, §MediaTek, ⊥Thales Alenia Space Contact: [email protected] Abstract— Offering space-based Internet services with mega- constellations of low Earth orbit (LEO) satellites is a promising solution to connecting the unconnected. It can complement the coverage of terrestrial networks to help bridge the digital divide. However, there are challenges from operational obstacles to technical hurdles facing the development of LEO satellite access. This article provides an overview of state of the art in LEO satellite access, including the evolution of LEO satellite constellations and capabilities, critical technical challenges and solutions, standardization aspects from 5G evolution to 6G, and business considerations. We also identify several areas for future exploration to realize a tight integration of LEO satellite access with terrestrial networks in 6G. I. THE NEW SPACE RENAISSANCE Figure 1: Global coverage of a LEO constellation with hundreds of satellites at 600 km altitude. (The colors indicate the number of satellites in view for As the commercial fifth generation (5G) systems are being each point on Earth. Dark blue denotes only one satellite in view. The rolled out throughout the world, research focus starts to be lighter the color, the higher the number of satellites in view). gradually shifted towards the sixth generation (6G) systems that Several factors have been contributing to the renewed are anticipated to achieve another giant leap in communications interest in LEO satellite access. A key enabler is a significant [1]. We expect that the communication with mega- decrease in launch cost with the advent of disruptive launchers constellations of low Earth orbit (LEO) satellites will be one of reusing rocket parts, as provided, for example, by SpaceX. the connectivity’s new frontiers on the path to 6G, Meanwhile, the use of components off the shelves (COTS) and complementing terrestrial networks to provide limitless the adoptions of lean principles in satellite design and connectivity everywhere, thus helping to bridge the digital manufacturing allow for mass production with a faster divide [2]. manufacturing cycle at reduced costs. It has also become An LEO is an Earth-centered orbit with an altitude between commercially feasible to use advanced technologies in satellite 350 km and 2,000 km above sea level. Compared to medium communications such as multi-spot beam technologies and Earth orbit (MEO) or geosynchronous Earth orbit (GEO) sophisticated onboard digital processing. Another key driver is counterparts, LEO’s proximity to Earth results in lower latency a greater willingness to invest in LEO satellite access to help in LEO satellite access, less energy for launching, and less connect the unconnected, motivated by commercial potential, power for signal transmission from and to the satellites. economic development, and humanitarian considerations of However, LEO satellites at 600 km altitude travel at a speed of bridging the digital divide. around 7.8 km/s. The fast movement of LEO satellites and the The 3rd Generation Partnership Project (3GPP) has been relatively limited coverage area of an individual LEO satellite working on adapting 5G systems to support satellite require a large constellation of satellites to provide service communications [7][8]. Evolving 5G to support LEO satellite continuity across the targeted coverage area [3]. Figure 1 access is built on the inherent flexibility of the 5G systems. The illustrates the global coverage offered by an LEO constellation first design objective is to connect 5G handset devices by with hundreds of satellites at 600 km altitude. 3GPP-based satellite access networks in the sub-6 GHz Over the last several years, the world has witnessed a spectrum, so that 5G connectivity can be provided to the areas resurgent interest in space-based Internet services, particularly where terrestrial 5G networks are not available. The second with mega-constellations of LEO satellites such as SpaceX design objective is to provide broadband connectivity to more Starlink, Amazon Kuiper, and OneWeb [4]. These initiatives advanced devices, such as very small aperture terminal (VSAT) are ambitious, involving thousands and tens of thousands of or Earth station in motion (ESIM), especially in higher satellites and driven by non-traditional, cash-rich, and high-tech frequencies (e.g., Ku/Ka bands). Besides direct satellite access, players. The interest in LEO satellite access has run its course 3GPP has also been working on 5G satellite backhaul, which before. A series of proposals on providing connectivity from can facilitate the offering of cellular services in the areas where space surged in the 1990s, but most of them failed, and a few terrestrial backhaul means are not possible or prohibitively (e.g., Iridium [5] and Globalstar [6]) have had varying degrees expensive to build. of success. © 2021 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. equipped with four ISLs supporting communication with adjacent satellites at a data rate of 10 Mbps and high-speed processors for routing voice and data connections. For many reasons (both technically and commercially driven), the LEO systems launched in the 1990s were not as successful as expected because they only provided voice and data connectivity to a limited number of user equipments (UEs). But a wind of change is blowing thanks to the wide range of 5G use cases and new market opportunities. There has been a resurgence of interest in using mega-constellations of LEO satellites to provide connectivity from space [9]. Some new constellations such as OneWeb, Starlink, AST SpaceMobile, Figure 2: Ubiquitous 6G coverage provided by tightly integrating Amazon Kuiper, and TeleSat have planned to launch hundreds terrestrial networks with LEO satellite access. or thousands of space vehicles to provide global connectivity, 5G was initially designed targeting terrestrial complementing existing terrestrial network infrastructures. communications without considering LEO satellite access. We Some of the key information on these prominent LEO expect that there will be only moderate specification constellations is summarized in Table 1. We refer the interested enhancements to enable 5G to support LEO satellite access, readers to [10] for a more in-depth discussion on constellation leading to suboptimal performance. In addition, most of the design. advanced 5G features will likely be out of reach for LEO The design of these new mega-constellations is driven by the satellite access in the 5G era. The integration of 5G terrestrial target use case and market segment. In line with the available networks with LEO satellite access will be loose. In contrast, spectrum allocated to mobile satellite services below 6 GHz, an we anticipate that the integration will become much tighter in LEO satellite system will mainly focus on the Internet of Things 6G, providing seamless mobility between terrestrial and LEO (IoT) devices and services with low-medium data rates directly satellite access networks. As illustrated in Figure 2, the tight to handheld terminals. As far as satellite spectrum in higher integration of terrestrial networks with LEO satellite access will frequencies is concerned (e.g., Ku/Ka bands), the new LEO be essential to achieve global coverage in the 6G era. Figure 2 satellite constellations will target broadband connectivity to also illustrates the overall LEO satellite access network more advanced devices, such as VSAT or ESIM. architecture, which consists of feeder link connecting gateway and satellite, service link connecting satellite and terminal, and B. LEO satellite capabilities inter-satellite link (ISL) connecting one satellite to another. LEO satellite payload should be sized according to the The excitement about space-based Internet services is often average target traffic, but the available platform power is often clouded by myths and confusion. There are challenges, from a constraint. This calls for efficient power recharging technology to business model, facing the development of LEO capabilities when the traffic is low (e.g., switching off most satellite access with mega-constellations. The objective of this operational functions over poles and oceans to maximize the article is to provide an overview of state of the art in critical recharging phase with solar panels). The adoption of ISLs can aspects of LEO satellite access to shed light on the research and reduce the overall number of gateways on the ground, minimize development in this area on the path to 6G. In this article, we the infrastructure and operational