Non-terrestrial networks (NTNs) present a plethora of connection possibilities in emerging 5G networks. These connectivity solutions range from satellite-based communications via airborne stations to scenarios that consider air-to-ground communications and uncrewed aerial vehicles (UAVs) flight control. These scenarios are also intended to be dynamic, taking advantage of the benefits of satellites in different orbital planes and with different coverage footprints. This article will present a technological overview of these different networks.
LAYING THE GROUNDWORK
To begin addressing the challenges of incorporating NTNs, 3GPP launched a Release 15 study, identified as TR 38.811, that addresses channel models and deployment scenarios. There is also a follow-up Release 16 study, TR 38.821, that proposes solutions for adapting 5G New Radio (NR) to support NTN. The main objective of these studies is to identify a feature set that enables NTN within the 5G system while minimizing the impact on the existing 5G system. The aim is to enable NTN without waiting until the final network environment is perfect. Release 17 addresses the technical specification of NTN. These specifications affect all the layers within a 5G system, ranging from the physical layer via the protocol stack to the network architecture enhancements. Considering this as the basic technology framework, Release 18 will continue to foster the NTN evolution with additional spectrum, protocol layer enhancements and service proliferation.
The major motivation to foster NTN communications is the desire to provide ubiquitous connections all over the globe. According to publicly available market statistics, wireless communications technologies covered more than 80 percent of the world’s population, but less than 40 percent of the world’s landmass in 2020. NTN satellite-based communications are well-suited to tackle this challenge and focus on worldwide ubiquitous coverage in maritime, remote and polar areas.
The first 5G NTN deployments will focus on ubiquitous connectivity and coverage. They will do this by separating the technology into NR-based NTN (NR-NTN) and IoT-based NTN (IoT-NTN). NR-NTN can be considered the enhanced mobile broadband (eMBB) equivalent of terrestrial 5G, enabling satellite-based connectivity focusing mainly on coverage and outdoor applications.
IoT-NTN is the extension of terrestrial IoT technologies like NB-IoT, LTE-M or 5G RedCap, in the future, but using satellite resources for connectivity. Satellite networks will have performance constraints, so NTN 5G will not compete with terrestrial 5G. Rather, 5G NTN will complement terrestrial 5G systems to provide connectivity in underserved regions as a means toward the goal of ubiquitous connectivity. The diagram in Figure 1 shows an overview of the role satellites may play in this goal of global connectivity.
NTN USE CASES
Two major thrusts have become apparent in the current evolution of NTN. The first is enhancing 5G NR specifications and hardware to allow NTN communications to be incorporated within the entire 5G network. NR-NTN focuses on providing eMBB services via airborne stations or satellites and this represents the long-term evolution of 5G into the sky. In Phase 1, the focus is on basic internet connectivity to provide voice, web browsing and text messaging services. These services will use sub-6 GHz spectrum and operate primarily on handheld devices. Phase 2 and beyond envision VSAT user equipment (UE) with enhanced RX capability using higher frequency ranges and offering much higher data rates. In the first phase, the architecture will be a transparent payload, but Release 19 is assumed to incorporate a regenerative payload architecture that will enable the NTN system to support fixed satellite services (FSS), broadcast satellite services (BSS) and mobile satellite services (MSS) as a transition to the capabilities envisioned in Phase 2 and beyond.
As described earlier, a major focus of NTN is ubiquitous coverage. However, 3GPP NTN focuses on more than underserved area coverage. At a higher level, these four use cases are categorized as follows in TR 22.822:
Service continuity: Providing radio access technology (RAT) coverage where it is not feasible with terrestrial networks like maritime or remote areas. TR 22.822 supports service continuity between land-based 5G access and satellite-based access networks owned by the same operator or by operator agreements.
Service ubiquity: Motivated by mission-critical communications and aims at permanent system availability, especially for public protection disaster relief use cases leading to outage or destruction of terrestrial network architectures. System availability can be resumed and obtained in a short time using NTN connections.
Service scalability: Follows the general aspect of traffic management strategies. Enhancements of traffic steering like the offloading of traffic from terrestrial to non-terrestrial communications provide better system efficiency, especially when considering the wide NTN gNB coverage range.
5G system backhaul services: Represent situations where UE are still connected to terrestrial RATs but the NTN connection serves as a backhaul connection to the core network.
The second major thrust involves incorporating the terrestrial IoT network into NTN. IoT-NTN proposes the adaptation of NB-IoT or enhanced machine-type communication (eMTC) for NTN connections. This implies reducing device and satellite complexity, along with accepting a lack of or reduced quality of service support. IoT-NTN communications will be on a best-effort approach, like latency-tolerant applications, but energy efficiency and power saving will play a pivotal role compared to NR-NTN. Release 17 prioritizes standalone deployment, applying a transparent bent pipe satellite architecture and assuming the UE possesses GNSS capabilities (not simultaneous operation) to pre-compensate time and frequency.1 Figure 2 shows a conceptual drawing of the 5G NR and IoT use cases for NTN networks.