Challenges of NTN

I personally am so eager to see this realized because one of those areas shown above is my dream place -:), but there would be many challenges to be overcome. Followings are a short list of those challenges. (NOTE : To foresee on those challenges, I think it would be helpful to investigate on the challenges that the satellite communication systems being depolyed and tested now (as of Feb 2021) and SpaceX Starlink can be a good example since there are so much informations are available. Even though it is not easy to find the details on challenges on the fancy looking system, it would be more helpful as an engineer to look for the challenges and see how those challenges gets resolved).

< The places NTN is meant to reach - and every one of them is a challenge >

• List of Challenges
• Sorting the List : Which of These Are Permanent ?
• The Physics, in Numbers
• What 3GPP Actually Did About These
• What Has Changed Since This Page Was Written
• Challenges That Have Grown Since 2021

List of Challenges

What follows is a deliberately mixed list. It runs from things that are fixed by physics and can never be improved, through engineering problems that are hard but ultimately a matter of money and iteration, to questions that are not technical at all - what it costs to launch, whether anyone will pay for the service, and whether a regulator will allow it. They are set down here in the order they occur to an engineer thinking about the problem rather than sorted into neat categories, because that is honestly how they present themselves.

It is worth reading the list as a snapshot of the open questions rather than as a verdict. Some of these have since been answered by the specification, some have been overtaken by what operators actually built, and a few have quietly become harder than they looked. The sections after the list revisit them with that hindsight - first sorting them by whether they can improve at all, then putting numbers on the ones driven by physics, and finally comparing what was expected against what has happened since.

Latency : One obvious challenge that everybody can easily guess would be the long latency caused by pure physics. There is long distance between ground station/user terminal and the satellite. The speed of light is finite. So it would take a considerable amount of time (delay) for the radio wave to reach the user device. Of course, this latency would very depending on the altitude of the satellite. If the satellite is deployed on GEO(Geostationary Orbit), the delay would be huge. If they are deployed in LEO(Low Earth Orbit), the delay would be low (in some case it can be even shorter than what you experience with WiFi delivered by ground connection), but it would require huge cost to cover the wide area with LEO like StarLink.

Demand on Ground Stations/Integration with terrestrial core : Even though we can put the radio part (e.g, gNB, Relay etc) up in the space, the core network is (should be) on the ground. It mean that the satellite or airborne platform should be connected to ground station at some point. The question is how far it can get connected to the ground station. Obviously there would be a certain limit in distances in which a satellite can reach a ground station. As a result, we would need a lot of ground station as well as satellite if we want to cover wide area. In some cases, it will be very challenging to set a ground station. For example, if we need a satellite covering far in the oscien, how can we setup a ground station that can cover those satellites ? One possible solution for this situation is to make some satellites to get access to the core network indirectly via another sattelite instead of ground station. In order to do this, we would need to make relay networks or mesh networks among the satellites. Of course, theoretically this is possible and most of satellite communication systems put this into account from the design phase. But it would not be easy for implementation in reality (NOTE : This is one of the challenges that Starlink faces at early phase and as of Feb 2021 the communication between satellites is not supported)

Antenna Technology : Intuitively and by experience, we would easily guess that Antenna technology would ge a critical component for this kind of communication system. First how small we can make it ? We may tolerate the necessity of using a large sized dish antenna (i mean large size in comparison to mobile phone antenna), but everybody would like to have a smaller dishes in various reasons. Next, how to change the direction of antenna radiation pattern pointing in line of sight to a satellite ? Two options are considered for this. The first option is to put some mechanical component so that it can change the direction of the dish mechanically (like the dishes used in Starlink system). But it would be difficult to change the direction of the antenna fast enough to handle mobility situation. The second option would be to use phase array antenna by which we can change the direction of radiation pattern electronically. Technically this would be much better than the mechanical method, but cost would be a big issue.

User Terminal Alignment: For end-users, especially non-technical ones, aligning terminals to ensure optimal connectivity can be a significant challenge.

Large Doppler Shift :  Usually sattelites and airborne platform in this communication system moves very fast whereas most of user terminal is stationary or moves slowly. It implies there would be huge differences in terms of relative speed between satellites and user terminals. In turn, it mean there would be large doppler shift experienced by the reciever.

Large Delay Spread : In this kind of communication system, the physical distance between the transmitter and the reciever tend to be very far (e.g, ranging from several hundreds of kilometers(LEO) to 36,000 kilometers (GEO)). This would lead to large delay spread.

Competition to other technolgies  : Simply put, in terms of business point of view, would it be lucrative enough to invest huge money to compete against existing satellite communication system like Starlink or Kuiper ? (NOTE : There is a company called ASTspaceMobile that claim to provide SpaceMobile service with just a few hundreds LEO satellites for regular mobile phone. But almost no technical details are known as of now (Mar 2021)).

Handover complexity - Handovers between satellite beams/cells and between satellites and terrestrial networks could be complex and require tight coordination. Fast moving LEO satellites complicate this further. Handling handover may be easier (so advantage) in NTN comparing to conventional satellite protocol, but it still be more difficult comparing to terrestrial cellular technology.

Network Management and Traffic Optimization: Managing the traffic over a satellite network is more complex due to variable conditions and the mobility of the satellites themselves.

Power limitations - Satellites and aerial platforms have restrictions on power, antenna sizes, etc which can limit data rates and capacity. Efficient waveforms and modulation are needed.

Scalability: As the number of connected devices continues to grow, NTN systems must be scalable without incurring prohibitive costs or overly complex network management.

On-board Processing Capabilities: Satellites need to have sufficient processing power to handle complex operations, which can be a constraint given the limitations on size, weight, and power.

Launch costs - Getting satellites into orbit is still expensive. Larger constellations could require dozens or even tens of thousand of launches.

Orbit management - Avoiding collisions and managing constellation orbits takes planning, especially with large numbers of satellites. Debris is also concern when the collision happens or when the sattelites got out of life.

User terminal cost - For widespread consumer adoption, low-cost yet high-performance user terminals are needed. Striking the right balance on capabilities could be difficult.

Regulatory hurdles - Getting regulatory approval to launch large constellations and operate seamlessly across many countries can be challenging and time consuming.  

Business case - The large upfront capital costs make the business case difficult, especially when competing with expanding terrestrial 5G networks. Return on investment is a question.

Sorting the List : Which of These Are Permanent ?

The list above was written as a flat set of concerns, and read that way it can feel uniformly discouraging. It is much more useful sorted, because the items do not all have the same character. Some are consequences of physics and will never improve. Some are engineering problems that money and time have already moved a long way. And some are not technical at all.

NOTE : The most useful thing this sorting reveals is that 3GPP can only ever address the first two families. A specification cannot make light faster or a launch cheaper. What Rel-17 onwards actually did was take the physics family and make it survivable - which is why so much of the NTN specification is about compensation, offsets and validity timers rather than about new radio capability.

The Physics, in Numbers

Three items on the list above - latency, Doppler and delay - are stated qualitatively. They are worth pinning to actual figures, because the numbers are what drove every design decision described elsewhere on this site.

Those last two rows are the ones that create work rather than merely inconvenience. A LEO satellite is visible for about twelve minutes, so a cell has to be handed over every few minutes even for a UE that never moves. And because the delay is changing continuously, a timing advance value is not a setting but a trajectory - which is why the network broadcasts not just a common timing advance but its first and second derivatives.

NOTE : One term in the list above is worth a careful word, because the confusion behind it is extremely common. Delay spread in radio engineering normally means the spread of arrival times of multipath copies of the same signal - the thing that sets the cyclic prefix length. An NTN channel is strongly line-of-sight dominated with very little scattering, so its multipath delay spread is actually smaller than a typical terrestrial urban channel, not larger. What is genuinely large in NTN is the absolute propagation delay and the differential delay between UEs at different points in the same cell. Those are what the specification had to attack, and they are different quantities from delay spread. See NTN Timing Advance and NTN RACH.

What 3GPP Actually Did About These

When this page was written the challenges were open questions. Most of the technical ones now have concrete answers in the specification, and it is useful to see the list mapped onto the mechanisms that resolve it.

NOTE : Notice a pattern in the middle column. Almost every answer takes the same shape - move the predictable part of the problem out of the closed loop and compute it in advance. That single idea, made possible by requiring GNSS in the UE, is what turned NTN from a research topic into a specification. Nearly everything else follows from it.

What Has Changed Since This Page Was Written

Parts of the list above carry dates from early 2021, and several of them have simply been overtaken by events. Rather than delete the original observations - they are a useful record of what the open questions looked like at the time - here is what has happened since.

NOTE : The entries above are summaries of publicly reported developments rather than statements from any specification, and the direct-to-device landscape in particular moves quickly. The demonstration and service links collected on the What is it ? page are a better guide to the current state than any prose summary here.

Challenges That Have Grown Since 2021

The list above is mostly a list of things that have got better. In fairness, the balance has shifted the other way in several areas, and a challenges page written today would give these more prominence than the original list did.

• Orbital congestion and conjunction management : the original "Orbit management" bullet anticipated this, but understated the scale. With tens of thousands of active satellites either deployed or filed for, collision-avoidance manoeuvres have become routine operational overhead rather than rare events, and the long-term debris question is now a mainstream policy topic rather than a specialist one.
• Astronomy impact : large constellations affect both optical astronomy, through reflected sunlight in long exposures, and radio astronomy, through emissions near protected bands. Neither appeared on the original list and both are now a standing item in constellation licensing discussions.
• Spectrum coexistence for direct-to-device : this is the genuinely new regulatory problem. D2C transmits terrestrial mobile spectrum from orbit, and a satellite beam does not stop at a national border the way a cell site does. Coordinating that against neighbouring countries' use of the same band is a harder regulatory question than the original "Regulatory hurdles" bullet envisaged, and it is an active topic in ITU work.
• Satellite lifetime and re-entry : short-lived LEO satellites mean a constellation is in a permanent state of replacement, so launch cadence has to be sustained indefinitely rather than being a one-off deployment cost. The atmospheric effects of large numbers of re-entering satellites are an emerging area of study.
• The link budget has not moved : this deserves emphasis because it is easy to lose in the excitement about D2C. Serving an ordinary handset with a 0 dBi antenna and 200 mW of transmit power from hundreds of kilometres away is close to the edge of what is physically possible. That is why direct-to-device started with messaging rather than broadband, and why the data rates in the specification's own requirement tables are modest. No amount of standardisation changes the link budget.

Related Pages on this Site

• NTN - What is it ? - the overview, and the collected demonstration links
• Why NTN ? - the motivation these challenges are weighed against
• NTN Architecture - payload types, ISL, and the ground station question
• NTN Timing Advance - how the latency and delay-variation challenges are actually handled
• NTN Frequency Compensation - the Doppler challenge in detail
• NTN RACH - where the differential delay challenge bites hardest
• NTN Requirement - the data rates and delays the system is actually targeted at
• NTN Spectrum - the bands, and why terminal type drives the choice

3GPP Reference

• Non-Terrestrial Networks (NTN) - 3GPP Technology Article (2024)
• 3GPP TR 38.811 : Study on New Radio (NR) to support non-terrestrial networks - where the physics of the challenge list is characterised
• 3GPP TR 38.821 : Solutions for NR to support non-terrestrial networks (NTN) - the solution study behind most of the answers above
• 3GPP TS 38.300 : NR and NG-RAN Overall Description - clause 16.14, the normative NTN behaviour
• 3GPP TS 38.101-5 : NR; UE radio transmission and reception; Part 5: Satellite access - the RF reality behind the link budget point
• 3GPP TR 36.763 : Study on NB-IoT / eMTC support for Non-Terrestrial Networks - the IoT-NTN counterpart

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)
• LEO Small-Satellite Constellations for 5G and Beyond-5G Communications
• Satellite Communications in the New Space Era: A Survey and Future Challenges
• 5G from Space: An Overview of 3GPP Non-Terrestrial Networks
• AST SpaceMobile