Simplified 3GPP NTN Architecture: Understanding the 5G Non-Terrestrial Network

Introduction 3GPP NTN Architecture

Non-Terrestrial Networks (NTNs) are one of the prominent features introduced into the 5G ecosystem by the 3rd Generation Partnership Project (3GPP), particularly through Release 17.

While wireless communication is one of the most visible aspects of a cellular network, a complete cellular communication system consists of both wireless and wired communication links. NTN extends this traditional cellular architecture by introducing non-terrestrial platforms such as satellites and High-Altitude Platform Stations (HAPSs).

A simplified 3GPP NTN architecture helps us understand how a user device communicates through an NTN platform and ultimately connects to the terrestrial cellular core network and data networks such as the Internet.

3GPP NTN Architecture

What is a Simplified NTN Architecture?

A simplified NTN architecture consists of several important components that work together to provide communication services.

At a high level, the communication path can be represented as:

NTN UE → Service Link → NTN Payload → Feeder Link → NTN Gateway → gNB/eNB → Core Network → Data Network

The NTN architecture therefore combines:

• User Equipment

• NTN Payload

• NTN Platform

• NTN Gateway

• gNB/eNB

• Core Network

• Data Networks

Each component has a specific role in establishing communication between the user and the network.

NTN User Equipment (NTN UE)

The NTN User Equipment (UE) is the device used by the end user to communicate with the NTN system.

An NTN UE can take several forms depending on the application.

Typical examples include:

• Handheld smartphones

• IoT devices

• Very Small Aperture Terminal (VSAT) devices

The NTN UE communicates with the NTN payload using a radio interface protocol stack.

This radio interface can use different 3GPP-defined technologies depending on the deployment and application.

Examples include:

• 5G New Radio (NR)

• Narrowband IoT (NB-IoT)

• eMTC/LTE-M

  
  

Service Link or Access Link

The communication link between the NTN UE and the NTN payload is known as the:

Service Link, also referred to as the Access Link.

This is the radio communication link through which the user equipment accesses the NTN system.

For example, a smartphone communicating with a satellite-based NTN payload uses the service/access link to transmit and receive radio signals.

The radio interface used on this link can be based on technologies such as 5G NR, NB-IoT, or eMTC/LTE-M.

NTN Payload

The NTN payload is one of the most important elements of the simplified NTN architecture.

The payload resides on an NTN platform.

The NTN platform can be a satellite or an airborne platform.

Examples include:

Satellite Platforms

• GEO satellites

• MEO satellites

• LEO satellites

HAPS Platforms

• High-Altitude Platform Stations (HAPSs)

• For example, stationary aircraft operating in the stratosphere

The NTN payload provides the necessary communication functionality between the service link and the feeder link.

NTN Platforms

The NTN payload is physically located on an NTN platform.

The choice of platform has a significant impact on the characteristics of the network.

For example, NTN platforms can include satellites operating in different Earth orbits.

GEO

Geostationary Earth Orbit (GEO) satellites operate at approximately 35,786 km above Earth.

MEO

Medium Earth Orbit (MEO) satellites operate at significantly lower altitudes than GEO satellites, but much higher than LEO satellites.

LEO

Low Earth Orbit (LEO) satellites operate relatively close to Earth and are commonly used in satellite constellations.

HAPS

High-Altitude Platform Stations (HAPSs) operate in the stratosphere and represent an airborne approach to providing wide-area communication coverage.

Feeder Link

The NTN payload communicates with the NTN Gateway (NTN-GW) using an implementation-specific radio interface.

The communication link between the NTN payload and the NTN Gateway is called the:

Feeder Link

Therefore, the simplified communication path contains two important wireless links:

NTN UE ↔ NTN PayloadService / Access Link

and

NTN Payload ↔ NTN GatewayFeeder Link

These two links work together to connect the user equipment with the terrestrial network.

Transparent NTN Payload

The simplified architecture uses a transparent NTN payload.

A transparent payload does not perform technology-specific baseband processing.

Instead, it primarily performs the conversion of signals between the service link and the feeder link.

This is an important architectural concept.

The NTN payload essentially acts as a communication bridge between the UE-facing radio link and the gateway-facing feeder link.

The payload can perform functions such as:

• RF filtering

• Frequency conversion

• Amplification

• Signal transmission between service and feeder links

  
  

Downlink Operation

Let's understand how communication works in the downlink direction.

The downlink direction is:

gNB/eNB → NTN-GW → NTN Payload → NTN UE

The signal originating from the gNB/eNB reaches the NTN Gateway.

The NTN Gateway then communicates with the NTN payload through the feeder link.

The NTN payload receives the signal and performs functions such as:

• Radio Frequency (RF) filtering

• Frequency conversion

• Amplification

The resulting RF signal is then transmitted toward the NTN UE.

The UE processes the received signal using an appropriate 3GPP-defined radio interface, such as:

• 5G NR

• NB-IoT

• eMTC/LTE-M

Therefore, from the user's perspective, the NTN payload provides the RF connectivity required for the UE to communicate with the cellular network.

Uplink Operation

The communication process works in the opposite direction for the uplink.

The uplink path can be represented as:

NTN UE → NTN Payload → NTN-GW → gNB/eNB

The NTN UE transmits an RF signal using a suitable 3GPP-defined carrier frequency.

The NTN payload receives this RF signal from the UEs located within the cell.

After appropriate:

• Filtering

• Amplification

the NTN payload transmits the RF signal toward the NTN Gateway using a different carrier frequency.

The NTN Gateway then provides connectivity toward the gNB/eNB.

This allows the UE's uplink communication to reach the terrestrial cellular network.

NTN Gateway and gNB/eNB Interface

The interface between the NTN Gateway (NTN-GW) and the gNB/eNB is not specifically defined in the 3GPP specifications.

This provides flexibility in how the gateway and base station components are implemented.

The interface can be internal to the gNB/eNB.

In such an implementation, the NTN Gateway and gNB/eNB can be integrated into a single entity.

Alternatively, the interface can be external to the gNB/eNB.

In that case, the interface may be:

• Wired

• Wireless

The exact implementation therefore depends on the network architecture and deployment.

Connection to the Core Network

Similar to a traditional terrestrial network, the gNB/eNB connects to the cellular core network using 3GPP-defined interfaces.

However, the core network used depends on the type of NTN technology being deployed.

NR-NTN

For NR-NTN, the gNB connects to the 5G Core Network (5GC).

The simplified path is:

NTN UE → NTN Payload → NTN-GW → gNB → 5GC

IoT-NTN

For IoT-NTN, the eNB connects to the Evolved Packet Core (EPC).

The simplified path is:

IoT UE → NTN Payload → NTN-GW → eNB → EPC

This demonstrates how NTN can support different 3GPP radio technologies and corresponding core network architectures.

Connection to Data Networks

Once communication reaches the core network, the core network connects to external data networks.

One of the most common examples is the:

Internet

This aspect is similar to a traditional terrestrial network.

Therefore, an NTN user can ultimately access data networks through the cellular core network, just as users do through conventional terrestrial cellular networks.

The overall simplified communication path can therefore be visualized as:

User Equipment↓Service / Access Link↓NTN Payload↓Feeder Link↓NTN Gateway↓gNB/eNB↓5GC / EPC↓Internet / Data Network

5G NR, NB-IoT and eMTC/LTE-M in NTN

The NTN architecture can support different radio interface technologies.

5G NR

5G New Radio (NR) is used for NR-NTN deployments and connects through the 5G Core Network (5GC).

NB-IoT

Narrowband IoT (NB-IoT) is designed for IoT connectivity and can be used in NTN scenarios where low-power and wide-area IoT connectivity is required.

eMTC/LTE-M

eMTC/LTE-M is another cellular technology that can support IoT-related NTN applications.

The use of these technologies enables NTN to address different connectivity requirements rather than being limited to a single type of service.

NTN Architecture vs Traditional Terrestrial Network

There is an important difference between a traditional Terrestrial Network (TN) and an NTN.

In a conventional terrestrial network, the radio access network equipment is generally deployed on the ground.

In an NTN, a communication payload is deployed on an airborne or spaceborne platform.

However, the overall cellular system still contains both wireless and wired components.

The NTN therefore should not be viewed as a completely separate network. Instead, it can be integrated with the existing cellular ecosystem.

This allows NTN to complement terrestrial networks and extend connectivity into areas where terrestrial infrastructure may be difficult to deploy.

Why the Simplified NTN Architecture is Important

Understanding the simplified architecture is essential for anyone studying 5G NTN and future 6G networks.

It explains how the different network elements interact:

UE provides the user-side connection.

Service Link connects the UE with the NTN payload.

NTN Payload provides the non-terrestrial radio bridge.

Feeder Link connects the payload with the NTN Gateway.

NTN Gateway connects the non-terrestrial segment with the terrestrial network.

gNB/eNB provides the cellular radio access network functionality.

5GC/EPC provides the core network functions.

Data Network provides access to services such as the Internet.

Understanding these interfaces and communication paths is fundamental to understanding more advanced NTN architectures.

Conclusion

The simplified 3GPP NTN architecture demonstrates how non-terrestrial platforms can be integrated into cellular networks.

An NTN UE communicates with an NTN payload through the service or access link. The NTN payload is located on platforms such as LEO, MEO, GEO satellites or HAPSs.

The payload communicates with the NTN Gateway through the feeder link.

In the simplified architecture, the payload is transparent and primarily performs functions such as RF filtering, frequency conversion, and amplification, rather than technology-specific baseband processing.

The NTN Gateway then connects toward the gNB/eNB, while the cellular access network connects to the appropriate core network.

For NR-NTN, the gNB connects to the 5G Core (5GC), while for IoT-NTN, the eNB connects to the EPC.

Finally, the core network provides connectivity to data networks such as the Internet, making NTN an extension of the broader cellular communication ecosystem.

Understanding this simplified architecture provides a strong foundation for studying the more detailed NTN architectures, protocols, interfaces, mobility procedures, and 3GPP specifications used in 5G-Advanced and future 6G networks.

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