5G NTN (Use Cases, Architectures, Challenges)
- Venkateshu Kamarthi
- Sep 22, 2025
- 6 min read
What is 5G NTN
5G NTN(Non‑Terrestrial Networks) is the 3GPP‑standardized use of satellites and high‑altitude platforms as part of the 5G system to extend coverage, resilience, broadcast, and backhaul; it is needed to reach areas that terrestrial networks cannot economically or physically cover and it complements terrestrial networks through seamless integration, multi‑connectivity, and shared 5G Core architecture.
Why NTN is needed
Ubiquitous coverage: Extends service to remote, maritime, and aeronautical environments and low‑density areas where towers or fiber are impractical.
Resilience and disaster recovery: Provides backup connectivity when terrestrial sites are damaged or congested, maintaining basic voice/data and public alerts.
Broadcast/multicast and IoT: Efficiently delivers wide‑area content and firmware updates and connects massive IoT devices in hard‑to‑reach places.
How NTN complements terrestrial networks
Seamless integration: NTN connects through NTN gateways to the 5G Core using the same NR procedures so devices and services can roam or use both networks transparently.
Hybrid operation: Use cases include satellite backhaul for isolated terrestrial cells, moving‑platform networks on ships/aircraft, and direct‑to‑device fallbacks for messaging and voice.
Division of strengths: Terrestrial excels at ultra‑low latency/high capacity in dense areas, while NTN excels at reach and resilience; together they form a unified 3D network fabric.
Examples
Rural broadband: A village small cell uses NTN feeder links for backhaul to the 5GC where laying fiber is uneconomical.
Direct‑to‑device messaging: Smartphones attach to an S/L‑band LEO beam for emergency texts during a storm when towers are down.
Aviation/maritime: A shipboard LAN or aircraft picocell connects over Ka‑band to the 5GC, providing consistent passenger service end‑to‑end
NTN Types
Satellite communication systems is a sustainable solution for providing connectivity to remote or rural areas unserved or under-served by terrestrial communications systems. The Satellite communications coverage is already global, composed of geostationary orbit GEO satellites, medium Earth orbit (MEO), Low Earth Orbit (LEO), and Highly Elliptical Orbiting (HEO) constellations. On top of the satellite systems, under the umbrella of non-terrestrial networks, High Altitude Platform Systems (HAPS) have been added to provide non terrestrial but local service coverage [3GPP_TR38.821].
LEO (Low Earth Orbit, ~160–2,000 km): Constellations of fast‑moving satellites that pass overhead every minutes; lowest propagation delay and best handheld link budgets, but strongest Doppler and frequent beam/satellite handovers. Ideal for direct‑to‑device messaging/voice, IoT, and backhaul where low latency matters.
MEO (Medium Earth Orbit, ~2,000–20,000 km): Fewer satellites than LEO with wider footprints; latency and Doppler between LEO and GEO; often used for navigation and some broadband. Useful when fewer space assets are preferred and moderate latency is acceptable.
GEO (Geostationary, ~35,786 km): Satellite appears fixed over one region with enormous coverage and stable beams; highest latency but simpler mobility and broad broadcast/multicast. Suits wide‑area content distribution, control channels, and resilient backhaul where delay tolerance is higher.
HEO (Perigee: under 1,000 km altitude, Apogee: above 35,756 km). Highly Elliptical Orbit (HEO), also known as a highly eccentric orbit, is an elliptical satellite orbit around Earth characterized by a large difference between its closest point (perigee) and farthest point (apogee). This means the satellite moves much faster when near Earth and much slower when far away.
HAPS (High‑Altitude Platform Systems, stratospheric ~18–22 km): Solar aircraft/balloons that form “towers in the sky”; near‑terrestrial latency, earth‑fixed cells, and flexible repositioning, but weather/energy constraints. Good for rapid restoration, rural infill, and event capacity.
Satellite RAN architecture
The NG-RAN in below figure encompasses a set of SAN connected to the 5GC through the NG Air Interface. A SAN provides the NR User Plane (UP) and Control Plane (CP) terminations towards an NTN-enabled UE, which can access the NTN services through the payload via the service link, and it includes a transparent NTN payload on-board the NTN platform, a gateway, and gNB functions. It shall be mentioned that a single gNB might serve multiple NTN payloads and a single NTN payload might be served by multiple gNBs on-ground, depending on the specific NTN system design. The NTN payload transparently forwards the radio protocols that are received from the UE (connected via the service link) to the NTN gateway (via the feeder link) and vice versa.
Core components
UE and NR‑Uu: Standard NR device and air‑interface on the service link to the satellite; 3GPP focuses radio procedures on this access leg.
NTN payload and platform: The satellite (LEO/MEO/GEO or HAPS) hosts either a simple analog repeater (transparent) or onboard baseband/RF for RAN functions (regenerative).
NTN gateway (NTN‑GW): Ground station that terminates the feeder link to the satellite, anchoring connectivity to the 5G Core and often hosting gNB elements in transparent mode.
gNB and 5GC: In transparent mode the full gNB is on the ground; in regenerative mode part or all of the gNB is onboard and interfaces to the 5GC through the gateway.
Links and interfaces
Service link: UE↔satellite using NR‑Uu; subject to high Doppler and varying propagation delay depending on orbit.
Feeder link: Satellite↔gateway carrying either NR‑Uu (transparent relay of the waveform) or NG/F1 over the Satellite Radio Interface (regenerative)
Deployment modes
Transparent payload (bent‑pipe)
What it is: The satellite repeats the NR‑Uu signal without baseband processing; the complete 5G RAN (CU/DU/RU) is on the ground at or behind the NTN‑Gateway.
Interfaces carried: The waveform itself (NR‑Uu) is effectively relayed over the feeder link; no RAN protocol termination in space.
Pros/cons: Easiest to deploy and upgrade, leverages ground compute and O&M; inherits longer lower‑layer RTT and feeder‑link dependence for mobility and scheduling.
Example: LEO satellite relays S‑band NR‑Uu between a handset and a ground gNB co‑located with the gateway.
Non-Transparent or Regenerative payload (onboard processing)
What it is: The satellite regenerates and processes signals and can host parts of the 5G-RAN; NG or F1 ride the feeder link as “NG over SRI” or “F1 over SRI,” and ISLs can connect satellites.
Interfaces carried: NR‑Uu on the service link; NG or F1 over the feeder link, enabling onboard DU/RU or full gNB and optionally on‑satellite UPF.
Pros/cons: Shorter PHY/MAC loop, better use of feeder spectrum, space‑based mobility via ISLs; higher SWaP complexity and satellite upgrade cycles.
NTN spectrum
3GPP Release 17 introduced two dedicated NR‑NTN FR1 bands for mobile‑satellite service—n255 in L‑band and n256 in S‑band—both using FDD; later releases add more (e.g., n254 in FR1 and Ka‑band n510/n511/n512), but Rel‑17’s baseline is n255/n256.
Release 17 NTN bands
n255 (L‑band, MSS): UL 1626.5–1660.5 MHz; DL 1525–1559 MHz; duplex FDD; intended for handheld/direct‑to‑device with lower path loss and better penetration.
n256 (S‑band, MSS): UL 1980–2010 MHz; DL 2170–2200 MHz; duplex FDD; complements n255 with wider ecosystem and contiguous spectrum near terrestrial IMT.
Band characteristics and channelization
FR1 operation with NR subcarrier spacings like 15/30/60 kHz and UE channel bandwidths up to 20 MHz in Rel‑17, with discussion of wider options later.
NR‑ARFCN mapping and RF requirements for NTN FR1 bands are specified in TS 38.101‑5 Rel‑17, including maximum transmission bandwidth configurations per SCS.
Context and evolution beyond Rel‑17
3GPP’s NTN overview confirms Rel‑17 defined n255/n256 for handhelds with GNSS‑assisted timing/Doppler handling; Rel‑18 adds n254 (UL 1610–1626.5 MHz, DL 2483.5–2500 MHz) and Ka‑band VSAT options n510/n511/n512.
Industry summaries list the same FR1 and Ka‑band additions while noting Rel‑17 is limited to below 2.7 GHz for NTN bands.
Key technical challenges
Long and variable latency: LEO introduces multi‑ms one‑way delays and time‑varying RTT, which stress HARQ, RLC/PDCP timers, scheduler loops, and real‑time control interfaces.
Strong Doppler and dynamics: High relative velocities cause large, time‑variant frequency offsets and beam motion, requiring pre‑compensation, robust synchronization, and Doppler‑aware scheduling and measurements.
Moving beams and mobility: Frequent handovers due to satellite/beam motion drive high signaling overhead; smarter triggers and predictive/AI‑assisted handover are being explored.
Fronthaul/functional splits: Classical RU‑DU fronthaul (eCPRI) budgets don’t fit ISL delays; practical splits favor co‑locating RU+DU in space and using delay‑tolerant F1 to a CU, limiting hops across ISLs.
Feeder link constraints: Limited feeder capacity and switching events constrain the number of onboard DUs and drive trade‑offs between beam count, split options, and gateway topology.
Synchronization and timing: Maintaining phase/time across moving space nodes and gateways with feeder link switches and ISLs complicates precise sync for OFDM and MIMO.
Power, compute, and thermal: Satellites have tight SWaP budgets; running PHY/MAC/DU or full gNB requires efficient silicon, dynamic function placement, and thermal design.
Protocol and interface adaptation: O-RAN and 3GPP interfaces expect low terrestrial latency; NTN needs resilience to delay/jitter, session persistence across satellite transitions, and distributed RIC placement.
RF propagation and polarization: Large cells, variable elevation, Faraday rotation, and atmospheric effects at Ku/Ka complicate link budgets, beamforming, and polarization management.
Security and reliability: Open interfaces plus wide attack surfaces (jamming/spoofing), and intermittent GW visibility require robust security and routing strategies.
References
"5G Non-Terrestrial Networks Technologies, Standards, and System Design" By
Alessandro Vanelli-Coralli, Nicolas Chuberre, Gino Masini, Alessandro Guidotti,
Mohamed El Jaafari
https://www.sharetechnote.com/html/NTN/NTN_Architecture.html
3GPP TR38.821