In part 1 of this series, I’ve taken a high level look at how the LTE and 5G air interface specifications have been extended in 3GPP Release 17 to accommodate for weaker signals, longer delays and patchy coverage typically experienced over satellite. In part 2, I’ve chased the question why the LTE NB-IoT and CAT-M in particular have been extended and which features they have that are also very useful if a smartphone or other mobile device communicates via a satellite. In this part, I’ll now have a closer look how the overall 3GPP LTE and 5G System Architecture has been enhanced in 3GPP Release 17 for use with Non-Terrestrial Networks (NTN).
Perhaps this should have been the first part of this article series but this is the order in which I researched the topic. 3GPP has specified the overall LTE system architecture in Technical Specification TS 36.300 and the 5G NR system architecture in TS 38.300. These two documents have now been extended in Release 17 to also contain information how LTE and 5G networks could incorporate satellite constellations for signal transmission. For the LTE version, have a look at chapters 4.3, 23.21 and Annex P.
It’s important to note that the satellite is seen as a ‘bent-pipe’ system, i.e. the satellite ‘only’ acts as a signal repeater in both directions. The eNodeB or gNodeb (5G) remains on the ground. The satellite is thus an element that sits on the air interface between the mobile device and the traditional base station.
O.k., so the satellite is a signal repeater, but it’s a bit of a special one. One thing that makes it special is that it is very far away from the mobile devices. And, in case of non-geostationary orbit (NGSO) satellites, the repeaters are moving relative to the ground. In other words, in some scenarios the coverage provided by ‘base stations’, as seen from mobile devices, are not fixed but actually moving. While I would typically refer to the coverage area of a single antenna as a ‘cell’ in earth based cellular networks, 3GPP refers to the coverage area of a cell that is projected onto the earth from a satellite as a ‘beam‘. The reason for this might be that the coverage area of a satellite is so large that from a capacity point of view it doesn’t make sense to only have a single antenna and hence a single cell over the complete area. Instead, modern satellites use antenna arrays and project independent bi-directional coverage ‘beams’ towards earth. This significantly increases overall capacity. And to make things even more exciting, 3GPP sees thee different options for how a ‘beam’ is projected towards the earth:
- Earth Fixed Beams: That’s the easy one, a geostationary satellite projects many beams towards the earth and each beam always covers the same geographical area. In other words: One beam is a cell that always covers the same area, i.e. this config is the same as in a terrestrial network.
- Earth Moving Beams: Low Earth Orbit (LEO) or Medium Earth Orbit (MEO) satellites move relative to the ground. Coverage is also provided by many coverage beams, but the beams keep moving over the earth’s surface. In other words, the cell is moving while the mobile device could be stationary. This kind of turns the system upside down. For the mobile device, this means that even if it is stationary, there will be frequent cell changes, i.e. handovers. More about this later.
- Quasi Earth Fixed Beams: Ok, here it gets really interesting. Again, we have LEO or MEO satellites that move over the earth. But instead of fixed beams, their antenna arrays compensate for the satellite’s movement, and from a mobile point of view, the beam it receives is fixed and does not move. The catch is that rather sooner than later, the satellite will go beyond the horizon experienced by a device so it can no longer keep up that beam. In that case, the beam (i.e. the cell) will either vanish, or the next satellite appearing above another part of the horizon will provide the quasi-fixed beam for this location. Handover in space!
So which option(s) will be used in practice? For geo-stationary satellites, fixed beams is the obvious option. For LEO and MEO satellites, I have no idea. If you know more how this is done in today’s non-3GPP based constellations such as Globalstar or Iridium (moving or quasi-fixed beams), please let me know.
The 3GPP spec doesn’t talk about ‘satellites’ but about ‘NTN payloads’. That sounds strange at first but there are two reasons for this. First: NTN (Non-Terrestrial Networks) could also use high altitude platforms such as high flying balloons instead of satellites, even though that seems to be an unlikely variant from today’s point of view. And second, a satellite is often multi-purpose and carries several ‘payloads’. Let’s take the current Iridium satellites as an example. Apart from providing Iridium voice and data services, the Iridium satellites carry another ‘payload’ which tracks ADS-B signals of planes. Search for ‘satellite payload’ on the web and you will find many other examples of satellites that carry different kinds of communication equipment, each being referred to as payload. Coming back to the beginning of the paragraph, the NTN payload is thus the part of the satellite that provides the ‘bent-pipe’ part for extending 3GPP LTE and 5G networks to and from space.
I’ll leave it at this for today’s post, but this is obviously just the tip of the iceberg of what is described in Release 17 of TS 36.300 and 38.300. I’ll pick up from here in the next posts and will discuss service and feeder links, the necessity of GPS for NTN operation, RRC mobility management, link switchover, required core network enhancements, multi-country operation, global roaming and many other things.