5G NTN (Non-Terrestrial Network) Technology Terminology

The NTN (Non-Terrestrial Network) introduced by 3GPP in its standardization roadmap aims to achieve full 5G coverage and connectivity through satellites and airborne platforms. Key terminology includes:

1. NTN Definition: This is a wireless network technology approved by 3GPP, where access nodes are deployed on space-based or air-based platforms such as satellites or High Altitude Platform Stations (HAPS), rather than being fixed to ground infrastructure. NTN networks are typically used to extend coverage to areas where ground network deployment is impractical or economically unfeasible. From a 3GPP perspective, NTN is not an independent technology, but rather an extension of 5G (NR). NTN reuses and adapts NR protocols, parameters, and procedures as much as possible to support long propagation delays, high Doppler shifts, large cell sizes, and platform mobility.

2. NTN Platforms: This is the most basic classification of satellite orbits, which directly affects latency, coverage, and mobility; specifically including:

• GEO (Geostationary Orbit): GEO satellites are located at an altitude of approximately 35,786 kilometers and are stationary relative to the Earth. GEO (Geosynchronous Orbit) satellites have wide coverage but high round-trip delay, making them unsuitable for latency-sensitive services.
• MEO (Medium Earth Orbit): MEO satellites operate at altitudes between 2,000 and 20,000 kilometers, achieving a balance between coverage and latency; this is particularly emphasized in the current 3GPP NTN specifications.
• LEO (Low Earth Orbit): LEO satellites operate at altitudes between 300 and 2,000 kilometers. They offer low latency and high throughput, but move very quickly relative to the Earth, leading to frequent inter-satellite handovers and significant Doppler effects.
• VLEO (Very Low Earth Orbit): VLEO refers to experimental satellites designed to operate at altitudes below 300 kilometers. They are expected to achieve ultra-low latency but face significant atmospheric challenges.
• HAPS (High Altitude Platform Station): HAPS typically operate at altitudes between 20 and 50 kilometers. HAPS platforms include: solar-powered drones, balloons, and airships. High Altitude Platform Systems (HAPS) can act as NR base stations, relays, or coverage enhancers, and compared to satellites, they have quasi-static characteristics and significantly lower latency.

3. Wireless Access (Terminology)

• NTN gNB: This is a 5G (NR) base station specifically modified for non-terrestrial deployment. Depending on the architecture, the NTN gNB can be fully hosted on a satellite or HAPS, partially deployed in space and partially on the ground, or entirely ground-based with the satellite acting as a relay. The functional division between space and ground is a key design choice.
• Transparent Payload or Bent-Pipe Architecture: In a transparent payload or bent-pipe architecture, the satellite does not perform baseband processing. This architecture aims to simplify satellite design, but its operation is highly dependent on the availability of ground infrastructure and feeder links; the transmission payload performs the following functions:
• Receiving radio frequency signals from user equipment (UE)
• Performing frequency shifting and amplification
• Forwarding them to the ground base station (gNB) via the feeder link
• Regenerative Payload: Performs part or all of Layer 1 and Layer 2 processing on the satellite. In this model, the satellite itself carries the gNB functionality. This architecture reduces feeder link latency, improves scalability, and enables localized decision-making. However, regenerative payloads increase the complexity and cost of the satellite.

4. NTN Links

• Service Link: Specifically refers to the wireless connection between the user equipment (UE) and the NTN platform (satellite or high-altitude platform). It uses the NR air interface waveform suitable for large cell radii and extended timing advance. Diagram of 5G NTN service link, inter-satellite link, feeder link, and ground network integration.
• Feeder Link: This connects the satellite to the gateway ground station, which interfaces with the 5G core network. Feeder links typically operate at higher frequencies and require high-capacity backhaul links.
• Inter-Satellite Link (ISL): Supports direct communication between satellites, allowing data to be routed in space without direct involvement of ground stations. ISL enhances network resilience and reduces end-to-end latency.

5. Network Architecture

• Gateway Earth Station: The gateway earth station acts as the interface between the satellite system and the 5G core network. It connects the feeder link and plays a crucial role in mobility and session continuity. 5GC supporting NTN: From a protocol perspective, the 5G core network (5GC) remains largely unchanged. Enhancements primarily focus on: supporting long latency, handling large cells, and optimizing processing procedures for idle and connected modes.
• D2D NTN (Direct-to-Device): User equipment (UE) communicates directly with satellites/high-altitude platforms (HAPS) without intermediate ground access.
• Hybrid NTN-TN architecture: NTN complements the terrestrial network, used for fallback, offloading, or extending coverage.
• Relay-based NTN: Satellites or high-altitude platforms (HAPS) act as relay nodes between user equipment (UE) and the terrestrial network.