NR over NTN: Understanding the Transparent Payload Mode

In the pursuit of ubiquitous connectivity, Non-Terrestrial Networks (NTNs) have emerged as a groundbreaking solution, expanding the reach of cellular networks beyond terrestrial infrastructure. One of the most promising advancements in this field is integrating New Radio (NR) technology over NTN, promising unprecedented coverage and capabilities. This blog post explores a key deployment aspect of NR over NTN: the transparent payload mode.

We’ll delve into its significance, deployment options, and how it’s poised to reshape the future of wireless connectivity.

Understanding Deployment Options

NR over NTN introduces two deployment options: the transparent payload mode and the regenerative payload mode. In this blog, we’ll focus on the transparent payload mode, as highlighted by Release 17.

Transparent Payload Mode Architecture

The transparent payload mode is an architectural approach that leverages satellite technology to seamlessly extend the NR-Uu radio interface from the feeder link to the service link and vice versa. Here’s a closer look at its key components and functionalities:

Transparent Payload Mode Architecture (source: 3GPP TR 38.821-g00)

Satellite-based NTN Gateway (GW): At the heart of the transparent payload mode is the satellite-based NTN Gateway. This essential component provides the necessary functions to facilitate the transmission of signals from the NR-Uu interface. The GW serves as a bridge between the satellite and user equipment (UE), enabling the seamless transfer of data.

Feeder Link and Service Link: The feeder link connects the satellite-based NTN GW with the satellite, while the service link establishes a link between the satellite and the UE. The satellite repeats the NR-Uu radio interface signals from the feeder link to the service link and vice versa. This approach allows for a continuous flow of data between the terrestrial network and the user equipment.

NR-Uu Interface Signal Forwarding: The satellite-based NTN GW is equipped with the capabilities to forward the NR-Uu interface signals. This ensures that the communication between the UE and the terrestrial network remains uninterrupted despite the involvement of satellite links.

Satellite Types

Different satellite constellations cater to varying use cases:

• Low-Earth-Orbit (LEO): LEO satellites, positioned between 500 and 2000 km altitude, boast a round-trip time (RTT) of under 30 ms. The transparent payload mode harmonizes with LEO’s low latency, making it ideal for real-time applications.
  
• Medium-Earth-Orbit (MEO): MEO satellites, with their velocity and orbital period characteristics, provide an intermediate solution catering to a diverse range of applications.
• Geostationary-Earth-Orbit (GEO): GEO satellites orbit at 35,786 km altitude, resulting in an RTT of around 544 ms. The transparent payload mode accommodates this latency by mitigating latency fluctuations, thereby finding relevance even in the GEO architecture.

Location Determination in NTN

In the realm of NTNs, UE location is predominantly achieved through Global Navigation Satellite Systems (GNSS) or Assisted-GNSS (A-GNSS).

• Global Navigation Satellite System (GNSS): GNSS is a constellation of satellites that enable accurate location determination by transmitting signals to receivers on Earth. Popular systems include GPS (Global Positioning System), GLONASS, Galileo, and BeiDou. UE devices equipped with GNSS receivers can triangulate their position based on signals received from multiple satellites.
• Assisted-GNSS (A-GNSS): A-GNSS enhances GNSS accuracy by leveraging additional assistance data from cellular networks or other sources. This assistance data includes satellite ephemeris, almanac data, and other information, which aids in quicker satellite acquisition and improved location accuracy, especially in challenging environments like urban canyons.

The considerable separation between ground-based User Equipments (UEs) and the satellite in space has a significant effect on the link budget due to the introduction of substantial path attenuation. Moreover, this substantial distance between them gives rise to notable time delay or Round-Trip Time (RTT), a parameter that is influenced by both the transmission time and the elevation angle.

The UE needs to know its location and the satellite ephemeris. Knowing this, it can estimate the required Timing Advance value of the target gNB. Using that information, the UE can perform pre-compensation of delay and Doppler shift for all UL transmissions.

Service Continuity

NTN will primarily be used for use cases where the device is stationary. With a terrestrial network, we would not consider mobility aspects since the cell and the UE are stationary. However, in the case of NTN, it is different. 

Satellites positioned within Low-Earth Orbit (LEO) exhibit rapid motion compared to a stationary point on the Earth’s surface. This means the ephemeris data keeps changing frequently, and the UE must calculate the timing advance (TA) continuously. 

The satellite’s directed emission towards the Earth determines the region where it can provide user services. The operation of satellite beams can be classified into two scenarios:

• Moving-Beam Scenario: This scenario involves satellites with fixed beams, resulting in a shifting footprint on the Earth’s surface. In this case, the beam’s position moves concerning a static Earth location.
• Fixed-Beam Scenario: This scenario pertains to satellites equipped with adjustable or steerable beams. As the satellite orbits the Earth, these beams are adapted to maintain coverage over the same geographical expanse. In this scenario, the beams can be adjusted to cover the area as long as the satellite remains visible above the horizon from that particular region.

The fixed-beam scenario offers the maximum duration during which a User Equipment (UE) can remain within the same satellite’s coverage area. This period corresponds to the interval in which the satellite remains visible above the horizon from the UE’s location. This span typically ranges between seven to ten minutes.

In the next blog, we will talk about service continuity when the device is not stationary. The device can be either in idle or connected mode. During idle mode, the UE location is known at the Tracking Area level. While the UE is in connected mode, mobility is handled by handovers.

In terrestrial networks, handovers are triggered by the network based on measurements reported from the UE about the current cell and neighbor cell signal quality. However, the same cannot be applied to the Non-Terrestrial Networks. 

Conclusion

In the pursuit of a connected world that knows no boundaries, Non-Terrestrial Networks (NTNs) emerge as a beacon of promise. The potential to bridge the digital divide, provide seamless coverage, and revolutionize industries is tantalizing. However, it’s essential to recognize that the path to realizing these benefits is not without its challenges.

To truly harness the transformative power of NTN, a robust Quality Assurance (QA) process becomes paramount. This process is a litmus test, ensuring that the technology stands up to its promises. Through rigorous testing, validation, and refinement, we can untangle the complexities, address the challenges, and mold NTN into a reliable, efficient, and accessible solution.

Simnovus UE and O-RU Simulator will be with you in this process of ensuring NTN delivers what it is capable of.