This article provides an overview of current satellite topologies, their key enabling technologies and how satellite communication can supplement 5G networks.

Cellular macro and micro base station densification is already underway to support 5G, with complex radio techniques to support 5G data rates, capacity and coverage. The 3GPP release 16 has a release date of June, with release 17 expected during the second half of 2021. Improvements to V2X, industrial IoT, multi-SIM devices, reliability and low latency performance, access to unlicensed spectrum to 71 GHz, efficiency and interference and other features are expected to be fleshed out. An addition to the large list of 24 items discussed at the 3GPP meeting in Spain late last year, 5G new radio (NR) support from non-terrestrial access (NTN) technologies such as satellites and high altitude platforms are being defined. Satellite technology can be a contributing asset to the framework of 5G globally, due to the inherent benefits of the platform.

5G BACKHAUL

With a multitude of radio access technologies enabling 5G, backhaul has necessarily evolved, decomposing the baseband unit (BBU) and remote radio head found in LTE networks into separate functional blocks: a centralized unit (CU), distributed unit (DU) and radio unit (RU). Cooperative radio techniques such as carrier aggregation, downlink coordinated multi-point transmission/reception and MIMO make the most of the limited sub-6 GHz spectrum, while massive MIMO (mMIMO) improves network capacity and coverage at each cell site with high spectral efficiency. Solutions such as dense mmWave small cell deployments move up the spectrum to access large bandwidth. These various solutions enable 5G enhanced mobile broadband (eMBB), ultra-reliable and low latency communications (uRLLC) and massive machine type communications (mMTC), the 5G capabilities defined by the International Telecommunications Union (see Figure 1).

Figure 1 5G generic use cases. Source: ITU.

As shown in Figure 2, the current strategy for the 5G radio access network (RAN), termed the gNodeB (gNB), comprises a two-tier architecture with a distributed tier (DU) providing low latency, for factory automation and medical services, and a centralized tier (CU) handling the power hungry processing. The separation of the RU and DU exposes the common public radio interface (CPRI), which is enhanced for 5G and called the enhanced CPRI (eCPRI) interface. In some scenarios, the DU can be integrated with the RU to function as a small cell.

Figure 2 Generalized 5G backhaul architecture.

SATELLITE CONVERGENCE WITH 5G

The use of a satellite-terrestrial architecture to supplement the 5G RAN is being explored through several testbeds. The EU Horizon 2020 is a collaboration involving companies across Europe, with the goal to develop a “Satellite and Terrestrial Network for 5G.” Another project, funded by the European Space Agency, is the “Demonstrator for Satellite-Terrestrial Integration in the 5G Context (SATis5G).” SpaceX, OneWeb and Amazon are developing low earth orbit (LEO) constellations to provide connectivity anywhere on the globe. High throughput satellite (HTS) technology in geostationary orbit (GEO) is another player in the convergence of the satellite-terrestrial network with 5G, offering spot beam and multicast capabilities. The cellular standards organization 3GPP is also working on defining the role of satellite communications in 5G through studies of non-terrestrial networks with satellites in LEO, medium earth orbit (MEO) and GEO.¹

From the launch in 2004 of Anik F2, with a throughput of 4 Gbps, to the 2017 launch of EchoStar XIX, with a throughput of 200 Gbps, HTS technology has evolved dramatically. Soon, Tbps speeds will be feasible with Ka-Band transponders and optimization techniques to decrease the cost per bit. There is a goal for “plug and play” capability for the satellite network to support 5G through satellite network virtualization, allowance of the cellular network to control satellite radio resources, development of link aggregation for small cell connectivity, optimized security through key management and authentication between cellular and satellite access technologies and integration of the multicast benefits of satellite technologies.²

Figure 3 Satellite integration with the 5G network. Source: SaT5G-project.

As shown in Figure 3, the organizations involved in the H2020 project have identified four use cases for satellite integration with 5G networks, where the satellites connect to:

• Trunks and head-ends for terrestrial networks in isolated, hard-to-reach areas.
• User premises, providing direct service to underserved areas.
• Moving platforms.
• Hybrid architectures, complementing terrestrial capabilities with services such as multicasting.

Fixed Backhaul

Fixed satellite backhaul to base stations or individual small cells can support eMBB where no cost-effective terrestrial backhaul exists. Often, this is in underdeveloped and underserved regions of the globe with little cellular infrastructure and wireless access. In addition to eMBB, satellites can support mMTC for IoT applications such as smart farming.

Content to Premises

Bringing ubiquitous high speed coverage to homes across a country is difficult, let alone providing global coverage. A viable approach for hard-to-reach areas is a line-of-sight link from space, as installing fiber optic cables over the last mile is costly, time-consuming and may not be viable with the density of users. In 2017, around 10 million rural homes in the U.S. did not have access to broadband with download speeds of at least 25 Mbps.³ In these cases, routing fiber to a wireless access point on a tower is generally the best option, or broadband service may use microwave backhaul from the wireless internet service provider to a fiber connection. Often, residents outside of the limited coverage area have no connection. In these underserved or underdeveloped regions, a hybrid xDSL terrestrial and satellite broadband link can provide service for homes and offices using distributed small cells connecting to the satellite. Adding caching and storage capabilities to the cells can provide near-seamless connectivity. With phased array antennas, the cells can simultaneously receive multicast and unicast streams from several orbital locations.

Moving Platforms

Satellite links are practical for moving platforms, providing the capability to connect anywhere on the ground or in the air, regardless of the speed of the platform. Terrestrial infrastructure to do this must be relatively elaborate, with complex handoff capability to support moving platforms such as planes, ships, trucks, cars and railway cars. In many cases, hybrid multiplay support may be feasible, although more remote moving platforms such as planes and ships would likely require a fixed satellite backhaul. Sat5G envisions three major use cases for this application: 1) updating content for on-board systems, 2) broadband access and 3) business and technical data transfer for the moving platform company.

Complementary Services for Low Density Areas

This case entails using the satellite to provide live or on-demand multimedia content, offloading it from the ground 5G infrastructure in areas with low population densities and where service costs are high. Satellite multicasting sends the same data packet to multiple user terminals in a broad geographic area, which avoids taxing the capacity of the 5G infrastructure. The major limitation for this application is the high latency of GEO satellite links. To provide timely video sessions, caching optimization techniques with knowledge of the popularity of specific content by region, with online prefetching through the satellite, can be used.⁴