In recent months, satellite services for smartphones have been hyped a lot. While surprising for many, including me, this didn’t come out of nowhere, and a lot of companies have worked on this topic for quite some time. 3GPP has also picked up the topic a few years ago with study items in 3GPP Release 15 and 16. Now, with Release 17, the LTE and 5G NR air interfaces have been extended for use over satellites. The term for this in 3GPP: Non-Terrestrial Networks (NTN). So let’s have a look at what has actually been specified:
Both the LTE and 5G NR air interfaces have been designed and optimized to transfer data over relatively short distances of a few kilometers or a few tens of kilometers in extraordinary circumstances. When communicating with Low Earth Orbit (LEO) satellites, distances that need to be covered are well beyond 800 km, and communicating with geostationary satellites requires data to be sent over a distance of 35.000+ km. With NTN, 3GPP adds a few bits and pieces to use the LTE and 5G NR air interface to and from satellites. In fact, 3GPP has looked at two very different scenarios:
Stationary Mobile Terminals with Parabolic Antennas
The first scenario describes how to use the LTE/5G NR air interface with stationary terminals and a parabolic antenna for high data rate applications. Starlink, Amazon’s Kuiper and OneWeb are examples of this, even though I suppose all of them use a proprietary protocol and data transmission method. I can’t be sure however, as there is very little technical documentation available.
Mobile Satellite Services
Secondly, 3GPP has looked at how data could be sent to and from mobile devices with small and non-directional antennas. For this scenario, even the most optimistic people in 3GPP don’t see high speed Internet access to be delivered to mobile devices from space anytime soon. As the 5G air interface is currently only used for high speed applications, 3GPP has also taken a step back and extended the LTE specification for NTN, particularly for the two bandwidth and speed reduced flavors: LTE NB-IoT and LTE CAT-M.
NTN in 3GPP
For both scenarios, i.e. high data rate transmission to and from devices with a large dish antenna and low data rate transmissions to and from small mobile devices with built-in antennas, the air interface has only been extended very slightly to address the following differences between short range and long range communication:
Timing Advance: During the random access procedure, the network tells the mobile device how much to time-shift uplink transmissions compared to the observed downlink transmissions to stay in sync. The current parameter only covers a range a few kilometers and hence an addition is necessary to signal larger delays, i.e. longer distances.
HARQ over larger distances: This is a LTE/5G mechanism used to detect and retransmit faulty data blocks that also works with specific timing constraints. Due to longer signal delays, this mechanism needs to be adapted.
Satellite Location Information: For mobile devices to estimate when satellites might become available for communication, details on the satellite constellation and location of all satellites is very helpful. This is known as ‘ephemeris information’, which is added to periodic system information broadcasts.
And that’s pretty much it. ShareTechnote has a great overview of how the air interface protocol for LTE, NB-IoT, CAT-M1 and 5G NR has been extended, so have a look for the details. Tip: Use the web browser’s search function to highlight all occurrences of ‘r17’. Here are some examples of how LTE and NB-IoT were extended:
- SIB-31 and SIB-32 have been added to the list of System Information Broadcast messages which contain information such as ephemeris data and other satellite specific variables. The presence of SIB-31 is indicated in SIB-1.
- In SIB-2, an optional information element has been added for NTN timing advance related information.
- NTN specific parameters have been added in the UECapabilitiesInformation message, so the UE can signal its NTN capabilities to the network.
On the 5G NR side, SIB-19 has been introduced to broadcast NTN configuration information to ground based devices and similar mechanisms like for LTE have likely been specified for the timing advance and the HARQ mechanism to account for the larger delay.
Bent Pipe or eNB/gNBs in Space
Another interesting resource I came across on the web is this article on Microwaves and RF. The article describes that 3GPP has looked at two different ways of using satellites. The first approach, which the article says is what 3GPP Rel. 17 mainly uses, is ‘NTN transparent mode’. Here, the satellite acts as a repeater in both directions. The eNB or gNB, i.e. the traditional LTE/5G NR radio network elements are installed on the ground. Such a setup is also referred to as a ‘bent-pipe’ approach. A current (non-3GPP) satellite constellation that uses this approach is Globalstar.
The second approach is for the satellite to act as an eNB or gNB in space. While more complex, the advantage of such an approach is data can be forwarded between satellites when a ground station is not in view. This approach is used, for example, by Iridium since the 1990’s, and only 3 ground stations are in operation around the globe. Satellites communicating with each other is not science-fiction, but science fact for a long time now.
The article also gives some details on the frequency ranges discussed in 3GPP for NTN which are perhaps globally available: The first range of 30 MHz is between 1980 to 2010 in the uplink direction and 2170 to 2200 MHz in the downlink direction. And then there’s 34 MHz between 1626.5 and 1660.5 MHz for the uplink and between 1525 MHz to 1559 MHz for the downlink. Again a comparison: Iridium uses a 10 MHz chunk in the 1.6 GHz range, so the general frequency ranges that are looked at by 3GPP have been in use for satellite communication for a long time and hence, characteristics are well known.
Compared to what is used by terrestrial LTE and 5G networks, two times 30 MHz is not much. I guess which spectrum demand below 6 GHz being so high for terrestrial services, it’s unlikely that more could be allocated to satellite services, which will of course limit single user speeds and overall network capacity.
And finally, 3GPP has also looked at the feasibility of using spectrum at 20 and 30 GHz for NTN. I don’t know how well that would work to and from small mobile devices, perhaps this spectrum could mostly be used for devices with parabolic antennas? Just a guess, if you know more, please consider leaving a comment.
The question I am asking myself is why there is such a hype now? Or in other words: How has the cost equation changed since satellite constellations like Iridium and Globalstar have been launched in the 1990s, which, by the way, are still in use today for IoT-like applications? I guess that apart from the tremendous advances that have been made on computing power as well on electronic and RF components in the last three decades, shooting satellites to space has also become a lot cheaper. Starlink has well over 2000 satellites in orbit today, and more are launched in the order of once a week, 60 of them on a single rocket. Back in the 1990s, that was about as much science fiction as the gigabit datarates terrestrial LTE and 5G networks can deliver today.
Keep in Mind: It’s ‘Narrow-Band’ to Mobile Devices!
Before I close, I think its important to set things into perspective again. Despite all advances, we are not talking about individual datarates of gigabits or even megabits per second to and from mobile devices with small antennas and satellites. This is why 3GPP has focused on LTE NB-IoT and LTE-M for NTN and in the next post on this topic, I’ll have a look again why those narrowband technologies as they are today are so suitable not only for low power and low data rate scenarios with terrestrial networks, but also for satellite communication.