1 5G New Radio Evolution Meets Satellite Communications: Opportunities, Challenges, and Solutions Xingqin Lin, Björn Hofström, Eric Wang, Gino Masini, Helka-Liina Maattanen, Henrik Rydén, Jonas Sedin, Magnus Stattin, Olof Liberg, Sebastian Euler, Siva Muruganathan, Stefan Eriksson G., and Talha Khan Ericsson Contact: [email protected] the development cycle and the costs of satellite manufacturing Abstract— The 3rd generation partnership project (3GPP) and launching processes have been dramatically reduced [5]. completed the first global 5th generation (5G) new radio (NR) A major driver of the success of terrestrial mobile networks standard in its Release 15, paving the way for making 5G a over the past few decades has been the international commercial reality. So, what is next in NR evolution to further standardization effort yielding the benefits of significant expand the 5G ecosystem? Enabling 5G NR to support satellite economies of scale. The 3rd generation partnership project communications is one direction under exploration in 3GPP. There has been a resurgence of interest in providing connectivity (3GPP) has been the dominating standardization development from space, stimulated by technology advancement and demand body of several generations of mobile technology. The for ubiquitous connectivity services. The on-going evolution of 5G international standardization effort helps ensure compatibility standards provides a unique opportunity to revisit satellite among vendors and reduce network operation and device costs. communications. In this article, we provide an overview of use In contrast, the interoperability between different satellite cases and a primer on satellite communications. We identify key solution vendors has been difficult and the availability of technical challenges faced by 5G NR evolution for satellite devices is limited, leading to an overall fragmented satellite communications and give some preliminary ideas for how to communications market up to date [6]. overcome them. The satellite industry has realized the need to embrace standardization, and furthermore to join forces with the mobile Index Terms—5G, New Radio, Satellite Communications, Non- Terrestrial Networks industry in 3GPP. The on-going evolution of 5G standards provides a unique opportunity to revisit satellite I. INTRODUCTION communications. The satellite industry has been active in the 3GPP 5G standards process. Thus far, 3GPP has completed a The 5th generation (5G) wireless access technology, known few study items on 5G evolution for satellite communications as new radio (NR), features spectrum flexibility, ultra-lean [7][8] and is currently conducting further studies. In addition to design, forward compatibility, low latency support, and the efforts in industry, there have been contributions to the field advanced antenna technologies [1]. Built on the first release of of satellite communications from academia [9] [10]. We refer NR (Release 15), the evolution of NR will bring additional interested readers to the recent IEEE JSAC special issue for capabilities to provide better performance and address new more theoretical works on satellite communications [11]. application areas. Meanwhile, we are witnessing a resurgent The objective of this article is to investigate the opportunities interest in providing connectivity from space. In the past few and challenges associated with adapting 5G NR for satellite years, there has been a surge of proposals about using large communications. A similar effort has been made for adapting constellations of low earth orbit (LEO) satellites, such as the 4th generation (4G) wireless access technology – long-term OneWeb [2] and SpaceX [3], to provide broadband access, in evolution (LTE) – for satellite communications [12]. There are addition to the existing satellite communications systems such a few other works such as [13] and [14] that touched upon as Iridium. It is anticipated that the integration of satellite satellite communications for 5G, while our article further communications into 5G will facilitate anything, anytime, address NR specific aspects. Another related work [15] anywhere connectivity in the 5G era and beyond. examined the impact of satellite channel characteristics on NR The ambition of providing connectivity from space is not physical and medium access control layers, but the analysis was new. A series of satellite communications projects (e.g., Iridium based on NR specifications released in December 2017, which and Globalstar) were planned in the 1990’s but with limited only support non-standalone NR deployment and have been success, partly due to the fast growth of terrestrial networks that significantly updated by 3GPP in 2018. were more economically appealing. A resurgence of interest in providing connectivity from space started around 2014, II. USE CASES OF SATELLITE COMMUNICATIONS stimulated by technology advancement and demand for ubiquitous connectivity services. The advancement of Satellite access networks have been playing a microelectronics following Moore’s law has paved the way for complementary role in the communications ecosystem. Despite using advanced technologies in satellite communications such the wide deployment of terrestrial mobile networks, there are as multi-spot beam technologies, onboard digital processing, unserved or underserved areas around the globe due to and advanced modulation and coding schemes [4]. Meanwhile, 2 Figure 1: An illustration of satellite communications system architecture economic rationales. For example, providing coverage in rural A. Satellite Orbits or remote areas has been challenging in many countries because the investment cost may not justify the expected revenue. In The height of a satellite orbit, which is the distance of the contrast, a single communication satellite can cover a large satellite from the earth’s surface, determines the satellite orbital geographic area, and thus it might be economically appealing speed around the earth. The satellite’s orbit also depends on the to use satellite communications to augment terrestrial networks orbital eccentricity and inclination in addition to the height. to provide connectivity in rural and remote areas. In urban There are three main types of satellites based on the orbital areas, high-throughput satellites communications systems may heights. help offload traffic in terrestrial networks. Another potential Geosynchronous/geostationary earth orbit (GEO): A alternative is to use satellites for backhauling, fostering the geosynchronous satellite has an orbital height of 35,786 km, rollout of 5G services with potentially reduced costs in rural and and its orbit matches the earth’s rotation with an orbital period remote areas. of 24 hours. From a ground observer’s perspective, the satellite The large satellite coverage can also benefit communication appears fixed at a single longitude, though it may drift north and scenarios with airborne and maritime platforms (onboard south. If the orbital eccentricity and inclination are both zero, aircrafts or vessels), while being attractive in certain machine- the satellite orbits over the equator in a circular orbit. The to-machine and telemetry applications. Additionally, satellites resulting orbit is geostationary, and the satellite appears to be in are resilient to natural disasters on earth, making satellite a fixed position to an observer on the ground. communications key for emergency services in case that the Medium earth orbit (MEO): The orbital height of MEO terrestrial network infrastructures are degraded. satellites ranges from 2,000 km above the earth’s surface up to Service continuity is closely related to ubiquitous the height of a GEO satellite. The associated orbital period connectivity. When a user equipment (UE) enters an unserved ranges from about 2 to 24 hours. or underserved area, the connectivity service may be disrupted. Low earth orbit (LEO): The orbital height of LEO satellites Integrating satellite communications into terrestrial networks to is mainly from 400 km to 2,000 km above the earth surface. fill the coverage holes can enable smoother service continuity. LEO satellites move fast and rotate around the earth every 1.5 Satellite communications are well positioned for to 2 hours. Since a LEO satellite quickly changes its position broadcasting/multicasting data and media to a broad audience relative to an observer on the ground, the satellite is only visible spread over a large geographical area. While television to the observer for a few minutes. broadcasting has undoubtedly been the main satellite service in B. Satellite Communications System Architecture this area, there are other use cases. For instance, mobile Generally, a satellite communications system consists of the operators and Internet service providers can also utilize satellite following components [7], with an illustration given in Figure communications to multicast content to the network edge to facilitate content caching for local distribution. 1. • Satellite: Spaceborne platform including spacecraft bus for satellite operation (power, thermal control, altitude III. PRELIMINARIES OF SATELLITE COMMUNICATIONS control, etc.) and communication payload (antennas and transponders); In this section, we describe the basics of satellite • Terminal: UE used by an end-user to communicate with communications. the network; • Gateway: Ground station for connecting a satellite to other parts of the network; 3 (a) (b) Figure 2: Varying coverage in satellite