Non-Terrestrial Networks, the Next Big Thing in 5G
Since the invention of mobile phones, cellular networks have relied on terrestrial infrastructures. For the first time in history, 5G technology can potentially migrate parts of the mobile network infrastructure to space through non-terrestrial networks (NTNs).
Satellites will be able to transmit and process data in place of terrestrial base stations. NTNs will significantly improve mobile network coverage and connection continuity compared to conventional terrestrial network architecture and provide a disaster-proof solution for emergency communications. This article will walk through the technical challenges of NTN, explore the features coming with 3rd Generation Partnership Project (3GPP) Release 17 and future expansions, and discuss some of the new use cases enabled by NTN.
NTN architectures defined by 3GPP
First, for satellites to participate in mobile network operations, there must be a direct connection between satellites and ground-based gateways. A terrestrial network can access its core network through aerial nodes. In this case, satellites act as jump stations from one ground station to another, forming a bent-pipe link in between UE and satellite gateway. There is no data processing in the satellites for this architecture. Therefore, 3GPP Release 17 has defined this architecture as the transparent NTN radio access network (RAN) architecture. It is the most straightforward architecture to achieve NTN functions for most use cases. Release 17 only supports this transparent architecture.
There are two other architectures defined in Release 17, but they are not supported yet. One of these architectures has a full 5G base station or gNB built into the satellite. The other architecture only has a distributed unit (DU) built into the satellite. These two architectures allow data processing in satellites, reducing propagation delay in half compared to the transparent architecture. However, because these two architectures require unique components, they are more challenging to implement.
Understanding the different types of orbits is an essential part of understanding NTN. One type is called a geostationary orbit, or GEO. This type of satellite rotates at the same angular speed as the earth. The satellite stands still to a reference point on the ground. GEO has a unique distance relative to the ground, which is 35,786km. The main benefit of using GEO orbits is that a single satellite can provide continuous coverage for a large area.
Non-geostationary (NGSO) satellites include low-earth orbits (LEO, 300 to 2,000km), medium-earth orbits (MEO, 7,000 to 25,000km), and highly elliptical orbits (HEO 1,000 to 35,756km). These satellites are not still with respect to the earth.
The altitude of LEO satellites is much lower than that of GEO orbits, regardless of the satellite architecture. As a result, the propagation delay of GEO satellites is about 20 times greater than that of LEO orbits. Most commercial satellite applications today use LEO orbits to reduce latency. However, many more satellites are required to provide the same size of coverage and coverage continuity as one GEO satellite.
Key technical challenges
Due to the long distance of signal transmission from space to the ground, the wireless signal suffers from significant signal attenuation. This leads to the first major challenge faced by NTN: propagation delay. Propagation delay is greater as the satellite altitude increases. As mentioned earlier, regenerative satellite architectures can cut propagation delays in half compared to transparent satellite architecture. The best case is a 6ms propagation delay, which falls within the 5G latency tolerance range.
Doppler shift is another challenge with NTN. The radiating signal source is stationary in conventional mobile network base stations. It does not have wavelength shifts relative to the receivers. However, satellites are moving at high-speed relative to the observer on the ground. The received signal could have a higher or lower frequency, depending on which direction the satellite is moving.
The Doppler shift decreases as the satellite altitude increases. Therefore, LEO satellites suffer from the worst Doppler shift. Currently, like propagation delay, it is not possible to remove the Doppler shift completely. Unlike the propagation delay, sending a reference signal can help the user equipment (UE) adjust the frequency shift.
NTN features support by 3GPP
NTN technology is still at an early stage of development. For it to achieve its full potential, the mobile industry must work closely with the satellite industry to develop the features and standards. Release 17 is the first 3GPP release that includes support for NTN. As mentioned earlier, 3GPP Release 17 defines three RAN architectures for NTN but only supports the transparent architecture currently. That includes implicit support for high-altitude platform station (HAPS), LEO scenarios, GEO scenarios, and all NGSO scenarios with circular orbit at an altitude lower than 600km. Future 3GPP releases will extend support to other architectures and other orbit scenarios.
To compensate for the Doppler shift, Release 17 only supports time advance (TA) estimation through UEs with a global navigation satellite system (GNSS). It means that UEs can only rely on the information they get from GNSS to compute timing and frequency and then adjust the doppler shift. The accuracy of this method can be very low, but it does not require users to upgrade their devices because GNSS is a common function in today’s smart devices. In the future, the network should be able to send reference signals to UEs. UEs will be able to use both GNSS and reference signals to improve the estimation accuracy. This feature will also enhance frequency synchronisation between UEs and the network.
Use cases enabled by NTN
The most significant advantage of NTN is the opportunity to provide global coverage. NTN can provide continuous connections to cover the surface of the earth. From users' standpoint, UEs should have a stable network connection if they can see the sky. That means being able to access mobile networks while in remote regions, rural communities, and on ships crossing the ocean. NTN can be extremely helpful during emergencies in areas where technical failures, power outages, floods, earthquakes, and wars can disrupt terrestrial networks. These use cases were never possible in the past.
Current terrestrial networks also have difficulty providing continuous connections when users move very fast, such as when UEs are in a bullet train or a car driving on a highway. The reason for the unstable connection is the constant handovers between terrestrial base stations. NTN can provide more stable connections for these use cases because of the broad coverage of each satellite.
Today, cross-ocean ships rely heavily on satellites for communication purposes but cannot access mobile networks when they are out to sea. With NTN technology, these ships or UEs on these ships will be able to access the 5G network through satellites. Integrating these technologies will significantly improve connection speed, increase service types, and reduce costs for these ships.
An exciting new technology
NTN needs the mobile industry to work closely with the satellite industry from both a technical and business standpoint. This process will take time. The good news is that 3GPP Release 17 has already started supporting NTN features. More features will be introduced and supported in the future. UE manufacturers have already begun working on devices that have satellite capabilities. Ultimately, satellites and terrestrial networks will work together from 5G onward.