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NTN 11 min read

Non-Terrestrial Networks (NTNs): Overview, Challenges and 3GPP Standardization

By Umer Aftab Rana
Non-Terrestrial Networks (NTNs): Overview, Challenges and 3GPP Standardization
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1. Overview and Definition

Definition: NTNs refer to networks or segments of networks that operate through airborne or spaceborne vehicles to provide wireless communication.

Heterogeneous 3D Architecture: Unlike traditional quasi-bi-dimensional (2D) terrestrial networks (TNs), 6G envisions a three-dimensional (3D) heterogeneous architecture where terrestrial infrastructures are complemented by non-terrestrial stations operating at different altitudes.

Key Platforms: NTN platforms are broadly categorized into two segments

  • Space-Based Segment: Satellites operating in Low Earth Orbit (LEO) at altitudes of 300–1,500 km (with LEO conventionally bounded at 2,000 km; TR 38.821 uses 600 km and 1,200 km as reference altitudes), Medium Earth Orbit (MEO) at 7,000–25,000 km, and Geostationary/Geosynchronous Earth Orbit (GEO/GSO) at 35,786 km [TR 38.811, Table 4.2-1; TR 38.821, cl. 4.2]. Note that 35,786 km is the sub-satellite altitude; because of the slant geometry, the satellite-to-UE distance for GEO reaches roughly 40,600 km at low elevation angles, which is the figure used for the maximum link distance in the 3GPP reference scenarios [TR 38.821, cl. 4.2].
  • Airborne/Atmospheric Segment: High-Altitude Platform Stations (HAPS) such as airships, balloons, or planes operating at altitudes of 15 to 25 km (ITU Radio Regulations define the HAPS altitude band as 20–50 km; 3GPP reference scenarios in TR 38.811 use 8–50 km for the UAS/HAPS platform class), and Unmanned Aerial Vehicles (UAVs)/drones operating from a few hundred meters to several kilometers above ground level [TR 38.811, cl. 4.2; ITU-R RR Art. 1.66A].

2. Key Benefits and Use Cases

The integration of NTNs with terrestrial networks is driven by several major potential benefits. The service-level framing of these use cases originates in the SA1 studies and requirements [TR 22.822; TS 22.261, cl. 6.3 and Annex on satellite access] and the architecture studies [TR 23.737]:

  • Ubiquitous Global Coverage: NTNs extend connectivity to under-served, unserved, or remote areas where deploying terrestrial infrastructure is economically or physically challenging, such as oceans, deserts, farmlands, and mountainous regions , use case family "service continuity/ubiquity";
  • Network Resilience and Disaster Recovery: NTNs serve as highly resilient fallback communication systems when terrestrial networks are damaged by natural disasters (e.g., tsunamis, earthquakes)
  • Advanced Backhauling: Non-terrestrial terminals can wirelessly serve on-the-ground backhaul requests, saving terrestrial resources and avoiding the high costs of traditional fiber-like deployments. They can also complement terrestrial backhaul in dense regions to achieve load balancing.
  • Massive Internet of Things (IoT): NTNs offer global continuity of service for wide-area IoT applications (e.g., smart agriculture, cargo tracking, and environmental monitoring) where devices are scattered over vast areas lacking reliable TN coverage
  • Multicasting and Broadcasting: The wide geographical footprint and inherent broadcast nature of space/aerial platforms make them highly efficient for conveying multimedia, entertainment, and public safety content to a massive number of users simultaneously
  • Energy-Efficient Hybrid Multiplay: Space platforms like satellites can run on clean, renewable solar energy once in orbit, although a full energy accounting must also include launch and manufacturing footprint as well as satellite replacement cycles. On-demand aerial platforms like UAVs can implement smart duty cycle control to reduce the management costs of always-on fixed terrestrial infrastructures. (This is a research/deployment argument rather than a 3GPP-specified feature.)

3. Technical Challenges

Integrating NTNs into cellular networks introduces several severe physical and protocol-layer challenges. The authoritative 3GPP characterisation of these effects is in TR 38.811 (channel model and impairments) and TR 38.821 (solutions and reference parameters), with the IoT equivalent in TR 36.763:

  • Propagation Delay and Latency: The long distances between ground terminals and spaceborne nodes (up to roughly 40,600 km of slant range for GEO at low elevation) result in high round-trip times (RTT) — on the order of 541 ms for a GEO transparent-payload link in the 3GPP reference scenarios, versus roughly 25–41 ms for LEO at 600–1,200 km [TR 38.821, cl. 4.2 and Table 4.2-x reference parameters]. This can cause time-out domino effects on chained network functions in the terrestrial core network, which is why RAN and CN timers (RA response window, HARQ RTT, MAC timers, TAU timers) were extended
  • Doppler Shifts and Frequency Variations: LEO satellites move at extreme velocities — 7.56 km/s relative to Earth for the 600 km reference orbit used in TR 38.821, and higher still at lower altitudes such as 300 km. This high-speed mobility introduces severe Doppler shifts and trajectory-dependent frequency variations that require active pre-compensation, performed by the user equipment (UE) using its GNSS position together with broadcast satellite ephemeris [TS 38.331 (ephemeris and common TA signalling); TS 38.213 (UL timing/frequency pre-compensation)], and complemented by common pre-compensation applied on the network side [TR 38.821, cl. 7].
  • Path Loss and Atmospheric Effects: Signals traveling through the atmosphere experience severe free-space path loss, atmospheric gas absorption, rain/snow attenuation, and ionospheric/tropospheric scintillation [TR 38.811, cl. 6 (channel model), which builds on ITU-R P.618, P.676, P.681 and P.531].
  • Orbital State and Clock Uncertainty: Factors like gravitational forces, solar radiation pressure, and atmospheric drag disturb satellite trajectories, which is why ephemeris validity/expiry and periodic re-acquisition are specified [TS 38.331 (ephemeris validity duration); TR 38.821, cl. 7]. Additionally, the small oscillators typically carried by LEO satellites are less stable than the atomic clocks flown on navigation satellites in MEO (GNSS constellations), requiring complex bias modeling to prevent localization and synchronization errors. Note that commercial GEO and MEO communication satellites do not necessarily carry atomic clocks; the atomic-clock reference point is the GNSS fleet.
  • Co-Channel and Cross-Link Interference: Spectrum sharing or co-deployment of NTNs and TNs in adjacent or shared bands introduces severe interference [TR 38.863 (NTN RF and co-existence aspects); TR 38.821, cl. 6]. Co-channel interference covers downlink-to-downlink and uplink-to-uplink coupling between the terrestrial and non-terrestrial systems, while cross-link interference (CLI) refers specifically to downlink-to-uplink and uplink-to-downlink coupling between nodes.

4. 3GPP Standardization Roadmap

The 3rd Generation Partnership Project (3GPP) has systematically integrated NTNs into cellular standards across successive releases:

Release 14–16 (Preparatory/Study Phase): SA1 use-case and requirement studies for satellite access in 5G began in Release 14 and were consolidated in TR 22.822 ("Study on using satellite access in 5G"), feeding satellite requirements into TS 22.261. RAN then studied NTN channel models, deployment scenarios, and system requirements in TR 38.811 (Release 15) and identified candidate solutions for NR operation over NTNs in TR 38.821 (Release 16), with SA2 architecture aspects in TR 23.737 and regulatory/extraterritorial aspects in TR 22.926. This work was exploratory: no normative NTN radio features were specified in Release 16, and the Frequency Range 1 (FR1, below 7.125 GHz) focus was carried forward into the Release 17 work item rather than being standardized here.

Release 17 (Normative Introduction, frozen 2022 — WI: NR_NTN_solutions, IoT_NTN, 5GSAT_ARCH): This release marked the formal normative standardization of NTNs. It supported basic functionalities using a transparent payload (bent-pipe) architecture, operating in Frequency Division Duplex (FDD) mode [TS 38.300, cl. 16.14; TS 38.331; TS 38.321; TS 38.213]. Access was specified in FR1 only, in two ranges allocated to Mobile Satellite Services: n255 (L-band, 1626.5–1660.5 MHz UL / 1525–1559 MHz DL) and n256 (S-band, 1980–2010 MHz UL / 2170–2200 MHz DL), with UE and satellite-access-node RF/RRM requirements in TS 38.101-5 and TS 38.108 respectively. Ka-band appears in the Release 17 reference scenarios (TR 38.821) as the assumed feeder link between gateway and satellite, but was not standardized for the access link at this stage. Release 17 also specified support for Narrowband IoT (NB-IoT) and Enhanced Machine Type Communication (eMTC) over satellite (IoT-NTN), based on TR 36.763 and realised in the LTE specification series (TS 36.300, TS 36.331, TS 36.321), and covered LEO, MEO and GEO orbits, Earth-fixed and Earth-moving beams, and handheld, IoT and vehicle-mounted terminal types. Core-network and architecture support is in TS 23.501 (satellite access aspects).

Release 18 (5G-Advanced Enhancements, frozen 2024 — WI: NR_NTN_Ph2, IoT_NTN_enh, 5GSAT_ARCH_Ph2): Focused on physical and protocol enhancements, including:

  • Improving coverage for handheld terminals, in particular uplink coverage [TR 38.821 baseline; Rel-18 NR_NTN_Ph2 objectives, RP-213690].
  • Extending operations above 10 GHz to Ka-band for Very Small Aperture Terminals (VSATs), through the FR2-NTN bands n510, n511 and n512 (17.7–20.2 GHz DL / 27.5–30 GHz UL, all FDD), added to TS 38.101-5 and TS 38.108. A further FR1 band, n254 (1610–1626.5 MHz UL / 2483.5–2500 MHz DL), was also added.
  • Addressing mobility, cell re-selection, and service continuity between TN and NTN as well as NTN-to-NTN [TS 38.304; TS 38.331].
  • Introducing network-verified UE location to satisfy regulatory requirements, based on the study in TR 38.882 (accuracy target of 5–10 km for country discrimination and core-network selection, supporting emergency calls, lawful interception, public warning and charging), with the underlying requirement in TS 22.261 and the regulated-service list in TR 23.737 / TR 22.926.
  • For LTE-based NTN: mobility management, throughput, power-saving and discontinuous-coverage enhancements, including HARQ feedback disabling and neighbour-cell ephemeris signalling [TS 36.300; TS 36.331].

Release 19 (Regenerative Architecture and NTN Phase 3, frozen December 2025 — WI: NR_NTN_Ph3, IoT_NTN_Ph3 (UID 1020096), 5GSAT_Ph3 with security 5GSAT_Ph3_SEC (UID 1060065) and charging 5GSAT_Ph3-CH (UID 1070014)): Introduces regenerative payloads where a full base station (gNB, or a full eNB in the IoT-NTN case) is hosted onboard the satellite. Key features include:

  • Store & Forward (S&F) operations for delay-tolerant communications when satellite connectivity is intermittent, including feeder-link switchover. In Release 19 S&F is specified for IoT UEs only, not for NR broadly [IoT_NTN_Ph3].
  • UE-Satellite-UE communications, allowing direct user-plane routing in space to avoid ground-network latency. The Release 19 architecture work is captured in TR 23.700-29 ("Study on integration of satellite components in the 5G architecture, Phase 3"), covering NGSO constellations with and without inter-satellite links, with the feeder link assumed available at least for session establishment; security aspects are in TR 33.700-29 and management aspects in the SA5 study FS_NTN_OAM_Ph2.
  • Early steps toward operations for UEs without Global Navigation Satellite System (GNSS) capabilities, together with positioning enhancements for satellite-only access [SA1 Rel-19 satellite study feeding TS 22.261; RAN work under NR_NTN_Ph3].
  • Beyond the regenerative theme, Release 19 also delivers downlink coverage improvements, uplink capacity enhancements (including UE multiplexing via orthogonal cover codes and early data transmission), multicast/broadcast (MBS) support, RedCap/eRedCap device support, TDD mode for NB-IoT NTN, public warning support for NB-IoT, high-power UEs and sub-5 MHz (down to 3 MHz) channel bandwidths in FR1-NTN, and new bands including Ku-band (band-numbering clarification for FR1-NTN vs FR2-NTN in TS 38.108, cl. 5.1.1), plus system architecture, security and charging refinements.

Release 20 & 6G Vision (open, targeting ~2027 — incl. IoT_NTN_Ph4 and a GNSS-resilient NR-NTN study): Rel-20 initiates a one-year study on GNSS-resilient NR-NTN operation, assessing the impact of not relying on GNSS information for initial access and connected-mode procedures under temporary loss or degradation of GNSS, after which 3GPP will decide how to proceed. The IoT-NTN track (Phase 4) specifies radio improvements to support IMS voice calls over geosynchronous satellites for NB-IoT NTN — including semi-persistent scheduling for downlink and uplink voice packets, updated RRC connection setup and emergency call procedures, UE transmit power beyond the current PC1 limit up to 37 dBm, and extension of the GEO-designed IMS NB-IoT solutions to LEO scenarios without additional specifications. Rel-20 also launches the official 6G studies, which kicked off at the end of 2025 (SA1 6G requirements and the RAN 6G study, with the timeline confirmed at the June 2026 plenary), with normative 6G specifications expected from Release 21. 6G NTN aims to natively design a fully unified, multi-layered 3D network from "Day-1" — a harmonized radio design integrating TNs and NTNs, rather than a terrestrial standard adapted after the fact.

5. Enabling Technologies and Architectural Advancements

Payload Configurations (standardized)

  • Transparent (Bent-Pipe) Payload: The satellite acts as an analog RF repeater, amplifying and converting the frequency of the feeder link signal while preserving the waveform. The base station (gNB) remains on the ground. This is the Release 17/18 baseline [TR 38.821, cl. 4.2; TS 38.300, cl. 16.14].
  • Regenerative Payload: The satellite hosts onboard processing capabilities (demodulation, decoding, and routing) and acts as a base station (gNB) directly in space. Studied in TR 38.821 and standardized from Release 19 [NR_NTN_Ph3; TR 23.700-29]. It reduces RTT for gNB–UE procedures such as random access and HARQ, and enables inter-satellite routing and handover with minimal ground-core involvement, at the cost of higher payload complexity and power consumption.

Split gNB Architecture (CU/DU disaggregation)

To adapt the disaggregated RAN to NTNs, 3GPP supports splitting the gNB into a ground-based Central Unit (gNB-CU) and a satellite-based Distributed Unit (gNB-DU) using functional split option 2, which places RLC, MAC, and physical layer functions directly onboard the satellite while RRC, SDAP and PDCP remain in the ground-based CU [functional split options defined in TR 38.801, cl. 11; CU/DU architecture and F1 interface in TS 38.401; NTN application in TR 38.821, cl. 5 and TR 23.700-29]. (Note: this is disaggregated RAN in the CU/DU sense; the term "dRAN" conventionally denotes the classical all-in-one distributed cell site and should not be used for this split.)

Transport Layer Acceleration (operator practice, not 3GPP-specified)

Standard TCP is less effective over high-latency satellite links. Operators utilize acceleration techniques implemented in performance-enhancing proxies (PEPs), such as TCP spoofing (sending false TCP ACKs from a nearby entity to quickly ramp up the transmission rate) and TCP multiplexing (splitting a single session into multiple flows optimized for the non-terrestrial link) [IETF RFC 3135, "Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations"; see also RFC 2488 for TCP over satellite]. These techniques are deployment choices rather than 3GPP features, and their effectiveness is increasingly limited by end-to-end encryption in QUIC (RFC 9000) and TLS, which hides transport headers from intermediate proxies.

Research-Stage Enablers (not yet standardized in any release)

  • Reconfigurable Intelligent Surfaces (RIS): Integrating passive, active, or simultaneously transmitting and reflecting (STAR)-RIS on aerial platforms or ground structures helps steer beams, mitigate blockages, reduce transmission power, and manage co-channel interference. RIS remains a research and pre-standardization topic for 6G rather than a specified 5G-Advanced NTN feature; the nearest 3GPP touchpoints are the Rel-18 study on network-controlled repeaters (TR 38.867) and ETSI ISG RIS work.
  • Artificial Intelligence and Machine Learning (AI/ML): AI is proposed for dynamic resource allocation, predictive mobility management, semantic-aware data compression (reducing network overhead by transmitting high-value contextual data instead of raw data), and managing time-varying interference patterns. 3GPP has normative AI/ML work for the NR air interface (TR 38.843, Rel-18 study; Rel-19/20 normative work) and studies on AI/ML for NG-RAN (TR 37.817) and AI/ML for mobility in Rel-20, but semantic communication and most AI-driven NTN resource management remain research topics targeted at 6G.

References

3GPP Technical Reports (studies)

  • TR 22.822 — Study on using satellite access in 5G (Rel-16, SA1)
  • TR 22.926 — Guidelines for extraterritorial 5G systems (SA1)
  • TR 23.737 — Study on architecture aspects for using satellite access in 5G (SA2)
  • TR 23.700-27 — Study on 5G system with satellite backhaul (Rel-18, SA2)
  • TR 23.700-29 — Study on integration of satellite components in the 5G architecture, Phase 3 (Rel-19, SA2)
  • TR 33.700-29 — Study on security aspects of satellite access, Phase 2 (Rel-19, SA3)
  • TR 36.763 — Study on NB-IoT / eMTC support for Non-Terrestrial Networks (Rel-17, RAN)
  • TR 37.817 — Study on enhancement for data collection for NR and EN-DC (AI/ML for NG-RAN)
  • TR 38.801 — Study on new radio access technology: radio access architecture and interfaces (functional split options)
  • TR 38.811 — Study on New Radio (NR) to support non-terrestrial networks (Rel-15; channel model and scenarios)
  • TR 38.821 — Solutions for NR to support non-terrestrial networks (NTN) (Rel-16)
  • TR 38.843 — Study on artificial intelligence (AI)/machine learning (ML) for NR air interface (Rel-18)
  • TR 38.863 — Non-terrestrial networks (NTN) related RF and co-existence aspects (Rel-17)
  • TR 38.867 — Study on NR network-controlled repeaters (Rel-18)
  • TR 38.882 — Study on requirements and use cases for network-verified UE location for NTN in NR (Rel-18)

3GPP Technical Specifications (normative)

  • TS 22.261 — Service requirements for the 5G system; Stage 1 (satellite access requirements)
  • TS 23.501 — System architecture for the 5G System (5GS); satellite access aspects
  • TS 36.300 / TS 36.331 / TS 36.321 — E-UTRA overall description, RRC and MAC (IoT-NTN)
  • TS 38.101-5 — NR UE radio transmission and reception; Part 5: Satellite access RF (NTN bands n254, n255, n256, n510–n512)
  • TS 38.108 — NR Satellite Access Node radio transmission and reception (incl. FR1-NTN / FR2-NTN band-numbering, cl. 5.1.1)
  • TS 38.213 — NR physical layer procedures for control (UL timing/frequency pre-compensation)
  • TS 38.300 — NR overall description; Stage 2 (cl. 16.14, NTN)
  • TS 38.304 — NR UE procedures in idle mode and RRC inactive state (NTN cell re-selection)
  • TS 38.321 / TS 38.331 — NR MAC and RRC protocol specifications (ephemeris, common TA, extended timers)
  • TS 38.401 — NG-RAN architecture description (gNB-CU/gNB-DU split, F1 interface)