The NTN architecture is designed to bridge the digital divide, enabling seamless connectivity across geographies where traditional ground-based networks are either impractical or economically unfeasible. Key elements of NTN include satellites (transparent or regenerative) that serve as critical intermediaries for transmitting and managing data. Transparent satellites primarily relay signals without processing(often called as Bent Pipe), while regenerative satellites enhance network capabilities by processing data directly onboard.

The architecture offers multiple deployment scenarios, such as those leveraging geostationary orbits (GEO) and low earth orbits (LEO). These deployments support a range of use cases, including steerable beams for flexible coverage and fixed beams that move with the satellite to ensure consistent service. Within the 5G NTN framework, user equipment and relay nodes connect through varied configurations, such as bent-pipe satellite systems or satellite-based gNodeBs (gNB), to provide uninterrupted coverage across land, air, and sea. This integrated approach underscores the transformative potential of NTNs in delivering reliable, high-speed connectivity to even the most remote regions, revolutionizing communication in areas as diverse as aviation, maritime, and rural development.

• Payload Type
• Reference Scenarios
• Architecture Options
• Deployment Scenario

Payload Type

The NTN payload type refers to the technology and functional capabilities of the payload installed on a spaceborne or airborne platform, such as a satellite or high-altitude platform. The payload is responsible for transmitting, receiving, and possibly processing communication signals as part of the NTN infrastructure. There are two primary types of NTN payloads: non-regenerative (or "bent-pipe" or "transparent") and regenerative (or "non-transparent").

NOTE : The term 'payload' in this context refers to the payload of satellite (i.e, the NTN equipment in the statellite). don't get confused with the term 'payload' in a data packet

• Non-Regenerative Payload: Also referred to as the "bent-pipe payload" or "transparent mode," this type of payload acts as an analogue RF repeater. The spaceborne or airborne platform equipped with a non-regenerative payload has no on-board signal processing capabilities. Instead, it simply receives the uplink RF signal, changes the frequency carrier, filters the signal, amplifies it, and transmits it back down via the downlink. This type of payload is cost-effective and simpler to implement since it relies on ground-based processing systems for complex operations. However, its functionality is limited to signal relay without enhancing or managing the signal.
• Regenerative Payload: Also known as the "non-transparent mode," a regenerative payload adds on-board processing capabilities to the spaceborne or airborne platform. In addition to performing basic RF operations like filtering, frequency conversion, and amplification, a regenerative payload can also handle tasks such as demodulation, decoding, switching, routing, encoding, and modulation. Effectively, this type of payload includes base station-like functionality within the platform, enabling it to manage signals more independently. Regenerative payloads reduce the dependence on ground infrastructure and improve efficiency, as the processed data requires less bandwidth and can be routed more intelligently.

< TR 38.811 v15.4.0-Figure 4.6-1: NTN Beam patterns >

Reference Scenarios

Following two illustration describes two typical scenarios of Non-Terrestrial Networks (NTN), showcasing how they use satellites or Unmanned Aircraft Systems (UAS) to provide communication services to user equipment (UE) on the ground

< TR38.821 v16.2.0-Figure 4.1-1: Non-terrestrial network typical scenario based on transparent payload >

< TR38.821 v16.2.0-Figure 4.1-2: Non-terrestrial network typical scenario based on regenerative payload >

Let's break down the key elements:

Common Elements in Both Scenarios

• Sat-gateways: These act as the bridge between the NTN and existing ground-based networks (like the internet). They could be thought of as specialized ground stations.
• Feeder Link: This is the radio connection between the sat-gateway(s) and the satellite/UAS. It's how data gets to and from the satellite/UAS.
• Service Link: This is the radio connection between the user equipment (like your phone) and the satellite/UAS. This is how you send and receive data.
• Satellite/UAS Platform: This is the core of the NTN, providing the communication relay in the sky. It can have either a transparent or regenerative payload (more on that below).
• Beam Footprints: The satellite/UAS generates multiple beams to cover a specific service area. These beams have a footprint on the ground, often shaped like an ellipse.
• Field of View: This is the area on the ground that the satellite/UAS can "see" and provide service to, limited by its antenna design and minimum elevation angle.

Scenario 1: Transparent Payload

• In this setup, the satellite/UAS acts as a simple repeater. It receives signals, amplifies them, and transmits them back down. Think of it like a mirror reflecting light.
• Key Feature: The signal is not processed or altered in any way onboard the satellite/UAS.

Scenario 2: Regenerative Payload

• Here, the satellite/UAS has more advanced capabilities. It can demodulate, decode, and even route signals. It's like having a mini base station in space.
• Key Feature: This allows for more efficient use of bandwidth and potentially better performance.
• ISL (Inter-Satellite Links): This scenario often includes connections between satellites, enabling them to work together as a network. This is particularly useful for constellations of multiple satellites.

Key Differences and Considerations

• Complexity: Regenerative payloads are more complex and expensive than transparent payloads.
• Latency: Regenerative payloads can introduce slightly more latency due to the onboard processing.
• Coverage: The choice of payload and satellite/UAS type can impact the coverage area and service quality.
• Applications: Different payload types might be better suited for different applications (e.g., voice calls, internet access, data collection).

There can be various scenarios in terms of how non-terrestrial networks can connect users to the internet. It focuses on six different scenarios, each with unique characteristics. These scenarios consider factors like satellite orbits, how the signal is processed, and whether satellites communicate with each other.

The scenarios are divided into two main categories: those using transparent satellites and those using regenerative satellites. Transparent satellites simply relay the signal without any processing, while regenerative satellites have more advanced capabilities, similar to a ground-based base station.

Within each category, there are scenarios involving satellites in geostationary orbit (GEO) and low Earth orbit (LEO). GEO satellites stay in a fixed position relative to the ground, while LEO satellites move across the sky. The scenarios also consider whether the satellite beams are fixed or steerable, which affects how the coverage area changes over time.

< TR38.821 v16.2.0-Table 4.2-1: Reference scenarios >

< TR38.821 v16.2.0-Table 4.2-2: Reference scenario parameters >

Architecture Options

Four distinct Non-Terrestrial Network (NTN) architecture options are proposed and each demonstrating a different approach to integrating satellite or aerial platforms into 5G systems. These options explore variations in how user equipment (UEs) and relay nodes connect to the core network through these non-terrestrial elements. Some architectures utilize the satellite/aerial platform as a simple relay, transparently passing signals between ground stations and UEs or relay nodes. Others leverage the platform's processing capabilities by placing gNB functions onboard, enabling direct communication with UEs or relay nodes. This diversity in architectural design offers flexibility in deploying NTNs to extend 5G coverage and capacity, particularly in areas where terrestrial networks are challenging to implement

< TR38.811 v15.4.0-Table 4.7-1: 5G system elements mapping in NTN architecture >

Each of the options can be presented as illustrations as below.

< Based on TR38.811 v15.4.0 - 4.7 Non-Terrestrial Network architecture options>

Followings are brief description for each options : (NOTE : The titles for each option in this description is an arbitry title. 3GPP does not specify any title/name for each option)

• Option A: Simple Relay/Bent Pipe
• Imagine the satellite or high-altitude platform like a mirror reflecting a special kind of 5G signal ("Satellite friendly" NR signal). It simply bounces the signal between your phone and the ground station (gNB) without changing it.
• Option B:  Mini Base Station in the Sky
• Here, the satellite or platform is more sophisticated. It has some of the same equipment as a ground base station, allowing it to directly communicate with your phone. Think of it as a mini cell tower in space.
• Option C: Relay for Faraway Places
• This is similar to Option A, but instead of connecting directly to your phone, it connects to a relay station on the ground. This relay station then communicates with your phone. This is helpful for extending coverage to very remote areas.
• Option D: Advanced Relay for Faraway Places
• This combines the ideas of Option B and C. The satellite or platform has base station equipment and connects to a relay station on the ground. This offers both advanced processing and extended coverage.

Deployment Scenario

There are various different deployment scenario that we can think of. 5 different deployment options are defined in TR 38.811 as summarized below.

These deployments vary in terms of the platform's orbit (GEO or Non-GEO),  altitude (ranging from 600 km down to 8 km for UAS), and the frequency used for communication with user equipment (around 2 GHz or 20 GHz).

The table also outlines differences in beam pattern (fixed or moving), duplexing mode (FDD), channel bandwidth (up to 2 * 800 MHz), and supported NTN architecture options.  Furthermore, it specifies the type of terminal used in each deployment, whether it's a Very Small Aperture Terminal (VSAT) for relay nodes or a 3GPP class 3 UE for direct user access.

Each deployment option caters to different scenarios and use cases. For instance, D1 and D2 utilize GEO satellites for indirect access via relay nodes, while D3 and D4 employ Non-GEO satellites for direct user access. D5 focuses on UAS for low-latency services with both indoor and outdoor coverage. The table concludes by listing the main rationales and supported use cases for each deployment, ranging from enhanced mobile broadband (eMBB) to public safety and IoT services.

< TR38.811 v15.4.0 - Table 5.1-1: Reference Non-Terrestrial Network Deployment scenarios to be considered in the NR-NTN study >

3GPP Reference

• Non-Terrestrial Networks (NTN) - 3GPP Technology Article (2024)
• 3GPP TR 38.811 : Study on New Radio (NR) to support non-terrestrial networks
• 3GPP TR 38.821 :  Solutions for NR to support non-terrestrial networks (NTN)
• 3GPP TR 38.913 : Study on scenarios and requirements for next generation access technologies
• 3GPP TR 23.737 - Study on architecture aspects for using satellite access in 5G
• RP-193234 : Solutions for NR to support non-terrestrial networks (NTN)
• TS 22.261 : Service requirements for the 5G system; Stage 1 (Release 18)

Other References

• Non-terrestrial networks (NTN) - R&S
• Non-Terrestrial Network Advantages, Challenges, and Applications - Keysight
• 5G & Non-Terrestrial Networks - 5G Americas White Paper (2022)
• Orbit of a satellite Calculator
• LEO Small-Satellite Constellations for 5G and Beyond-5G Communications
• Satellite Communications in the New Space Era: A Survey and Future Challenges
• Assessing satellite-terrestrial integration opportunities in the 5G environment  
• Satellite and Terrestrial Network for 5G
• Application of 5G new radio for satellite links with low peak-to-average power ratios
• 5G from Space: An Overview of 3GPP Non-Terrestrial Networks