Types of Beams in Non-Terrestrial Networks (NTN)
In a Non-Terrestrial Network (NTN), different types of beams can be used to provide radio coverage over the Earth's surface. According to the 3GPP approach, three major beam types are considered:
Earth-Fixed Beams
Quasi-Earth-Fixed Beams
Earth-Moving Beams
These beam types are important because they determine how radio coverage changes geographically over time and have a direct impact on mobility management, registration area management, RF planning, network processing, and overall 5G NTN system design.
A given NTN payload uses one of these beam types depending on its capabilities and configuration.
These beam types can also be referred to as different types of NTN cells or types of service links.
1. Earth-Fixed Beams
An Earth-fixed beam continuously covers a fixed geographical region on the Earth's surface.
For example, an NTN platform such as a GSO/GEO satellite or a HAPS can provide radio coverage to the same geographical area at different time instants:
t₁, t₂, …, tₙ, and so on.
The platform remains stationary relative to the Earth's surface and creates a fixed beam toward the same geographical region.
Therefore, the coverage area of an Earth-fixed beam does not change with time.
Concept of Earth-Fixed Coverage
Consider a geographical area X.
At:
t₁ → Area X is covered
t₂ → Area X is covered
t₃ → Area X is covered
tₙ → Area X is still covered
The same beam continues to serve the same geographical region.
This makes Earth-fixed beams relatively straightforward from a network-management perspective.
2. Advantages of Earth-Fixed Beams
Earth-fixed beams can significantly simplify the operation and design of a 5G NTN system.
Important areas affected include:
Mobility management
Registration area management
Radio-frequency (RF) planning
RF network design
Network processing requirements
Since the geographical coverage area remains fixed, the network does not need to continuously adapt to changing beam locations in the same way as it does with moving beams.
Earth-fixed beams can therefore reduce the overall processing requirements for:
User devices
Radio networks
Core networks
This makes Earth-fixed beams particularly attractive for NTN platforms capable of maintaining a stationary coverage pattern relative to Earth.
3. Platforms Supporting Earth-Fixed Beams
Examples of NTN platforms that can provide Earth-fixed beams include:
GSO/GEO satellites
HAPS platforms
A GEO satellite remains effectively stationary relative to an observer on Earth because of its geostationary orbit.
Similarly, a HAPS can maintain its position relative to a particular geographical region.
As a result, both platforms can maintain a relatively fixed coverage area.
4. Quasi-Earth-Fixed Beams
The second type of NTN beam is the quasi-Earth-fixed beam.
Unlike an Earth-fixed beam, a quasi-Earth-fixed beam does not remain associated with exactly the same beam from the same platform indefinitely.
Instead, different beams successively cover the same geographical area as the NTN platform moves.
Consider a geographical area X.
During the period:
t₁ → t₂
a quasi-Earth-fixed beam covers area X.
Then, during:
t₂ → t₃
the beam moves on and covers another geographical area, Y.
At the same time, another incoming quasi-Earth-fixed beam begins covering area X.
This enables continuous service to the geographical area even though the individual beam providing the service changes.
5. Beam Handover in Quasi-Earth-Fixed Systems
An important characteristic of quasi-Earth-fixed beams is the overlap between outgoing and incoming beams.
For example, during the transition period around t₁ to t₂, an outgoing beam and an incoming beam overlap.
This overlap allows user devices to be transferred from one beam to another.
The basic process can be represented as:
Outgoing Beam → Overlap Period → Incoming Beam
The overlap is important because it provides an opportunity for the user device to transition between beams while maintaining service continuity.
6. Beam Steering with NGSO Satellites
Non-Geostationary Orbit (NGSO) satellites, such as MEO and LEO satellites, are continuously moving relative to the Earth's surface.
However, these satellites may have beam-steering capabilities.
Beam steering allows the satellite to continue illuminating approximately the same geographical area even while the satellite itself moves from one location to another along its orbit.
This creates a quasi-Earth-fixed coverage behavior.
Instead of allowing the coverage area to move directly with the satellite, the beam can be steered to maintain coverage over a particular geographical region for a certain period.
7. Continuous Coverage Using Quasi-Earth-Fixed Beams
For continuous coverage of a particular geographical region, an incoming quasi-Earth-fixed beam replaces an outgoing quasi-Earth-fixed beam.
For example:
Beam A → Geographic Area X → Beam B → Geographic Area X
The first beam eventually moves away from area X, while another beam takes over coverage of the same area.
This approach allows the geographical region to remain continuously covered even though the individual satellite beams providing that coverage change over time.
8. Design Complexity of Quasi-Earth-Fixed Beams
Although quasi-Earth-fixed beams can provide continuous coverage, they introduce additional complexity compared with terrestrial networks and Earth-fixed beam systems.
They require significant changes or additional considerations in:
Mobility management
Registration area management
RF planning
RF design
Beam management
User-device processing
Radio-network processing
Core-network processing
Because the beam serving a geographical area can change over time, the network must account for these dynamic changes.
Consequently, quasi-Earth-fixed beams increase the overall processing requirements for both the user device and the radio and core networks.
9. Earth-Moving Beams
The third beam type is the Earth-moving beam.
An Earth-moving beam does not attempt to continuously illuminate the same geographical area.
Instead, the geographical area covered by the beam changes as the NTN platform moves.
For example, consider a beam at three consecutive time instants:
t₁ → Geographic Area X
t₂ → Geographic Area Y
t₃ → Geographic Area Z
Therefore, the beam itself moves across the Earth's surface.
The coverage area changes continuously as the NTN platform moves.
10. Earth-Moving Beams and NGSO Satellites
Earth-moving beams can be used by NGSO satellites, including:
MEO satellites
LEO satellites
In this case, the satellite's movement causes different geographical regions to be illuminated at different points in time.
Unlike a quasi-Earth-fixed beam, the beam does not necessarily attempt to remain focused on the same geographical region.
The coverage area therefore moves naturally with the satellite.
11. Maintaining Continuous Coverage with Earth-Moving Beams
Although an individual Earth-moving beam moves across the Earth's surface, continuous coverage of a particular region can still be achieved by using multiple beams and/or satellites.
As one Earth-moving beam stops covering a geographical area, another Earth-moving beam begins covering that area.
For example, consider the coverage transition around t₂:
NGSO Satellite 1 covers geographical area Y
Another NGSO satellite covers geographical area X′
Together, the beams provide continuous and contiguous coverage across the required region
Therefore, similar to quasi-Earth-fixed beams, continuous coverage requires an incoming Earth-moving beam to replace an outgoing Earth-moving beam.
12. Complexity of Earth-Moving Beams
Earth-moving beams introduce significant operational and design complexity into a 5G NTN system.
Compared with a terrestrial network, additional considerations are required for:
Mobility management
Registration area management
RF planning
RF design
Beam transitions
Network processing
User-device processing
Because the geographical coverage associated with the beam changes continuously, the network must manage these changes efficiently.
Earth-moving beams therefore increase the overall processing requirements for:
User devices
Radio networks
Core networks
13. Comparison of the Three Beam Types
The fundamental difference between the three beam types is how the coverage area changes with time.
Characteristic | Earth-Fixed Beam | Quasi-Earth-Fixed Beam | Earth-Moving Beam |
Coverage behavior | Fixed geographical area | Attempts to maintain coverage of a geographical area through successive beams | Coverage moves across Earth |
Coverage changes with time | No | Beam changes while area can remain covered | Yes |
Typical platform | GSO/GEO, HAPS | NGSO with beam steering | NGSO satellites |
Beam steering | Not required in the same way | Important capability | Not necessarily used to maintain same area |
Beam transition | Generally simpler | Incoming beam replaces outgoing beam | Incoming beam replaces outgoing beam |
Mobility complexity | Lower | Higher | Higher |
Registration management | Simpler | More complex | More complex |
RF planning | Simpler | More complex | More complex |
Processing requirements | Lower | Higher | Higher |
14. Earth-Fixed vs Quasi-Earth-Fixed vs Earth-Moving
The three beam concepts can be understood through a simple geographical example.
Earth-Fixed
The beam remains over Area X:
X → X → X → X
The same geographical area remains covered.
Quasi-Earth-Fixed
Different beams successively cover Area X:
Beam A → X → Beam B → X → Beam C → X
The geographical area remains approximately the same, but the beam providing the service changes.
Earth-Moving
The beam itself moves across different geographical areas:
X → Y → Z → …
The geographical coverage of the individual beam changes continuously.
This distinction is fundamental to understanding NTN mobility and beam management.
15. Impact on 5G NTN Design
The choice of beam type has a direct influence on the design and operation of a 5G NTN.
Earth-Fixed Beams
Because the coverage region remains fixed, they simplify:
Mobility management
Registration area management
RF planning
RF design
Network processing
Quasi-Earth-Fixed Beams
These require additional mechanisms because different beams successively cover the same geographical region.
Therefore, they increase:
Mobility-management complexity
Registration-management complexity
RF planning complexity
Processing requirements
Earth-Moving Beams
These introduce similar challenges but with continuously changing geographical coverage.
The network must account for the movement of coverage across the Earth's surface and coordinate successive beams to maintain service.
16. Ideal Coverage vs Real-World NTN Coverage
The conceptual illustrations of Earth-fixed, quasi-Earth-fixed, and Earth-moving beams generally show continuous and contiguous coverage.
However, such ideal coverage may not always be achievable in practical NTN deployments.
Several factors can prevent perfectly continuous and contiguous coverage, including:
An inadequate number of NGSO satellites
Limitations in achievable beam patterns on the Earth's surface
RF planning constraints
RF design limitations
Satellite constellation configuration
Other practical system limitations
Therefore, the theoretical illustrations should be understood as simplified representations of beam behavior rather than a guarantee of perfect real-world coverage.
Conclusion
Beam configuration is a fundamental aspect of Non-Terrestrial Network design. The three major beam types—Earth-fixed, quasi-Earth-fixed, and Earth-moving beams—provide different approaches to delivering radio coverage from NTN platforms.
Earth-fixed beams continuously cover the same geographical region and can significantly simplify mobility management, registration area management, RF planning, and network processing. GSO/GEO satellites and HAPS are examples of platforms that can provide this type of coverage.
Quasi-Earth-fixed beams use successive beams to maintain coverage over approximately the same geographical region. NGSO satellites such as MEO and LEO satellites can use beam steering to achieve this behavior. An incoming beam replaces an outgoing beam, with overlap supporting the transfer of user devices.
Earth-moving beams, on the other hand, move their coverage across the Earth's surface as the NTN platform moves. Multiple beams and satellites can work together so that an incoming beam replaces an outgoing beam and maintains coverage continuity.
The selection and management of these beam types have a direct impact on 5G NTN mobility, registration, RF planning, processing requirements, and overall system architecture.
Finally, while theoretical models often illustrate continuous and contiguous coverage, practical NTN deployments may face limitations caused by the number of available NGSO satellites, achievable beam patterns, and RF planning and design constraints.
Understanding these three beam types is therefore essential for anyone working with 5G NTN and future integrated terrestrial–non-terrestrial networks.
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