NTN – Part 6 – Satellite Mobility

In the previous episodes, I’ve gone into some of the details of the Non-Terrestrial Network (NTN) extensions specified in 3GPP Rel. 17 36.300 and 38.300. One main aspect I haven’t addressed so far is mobility, so let’s have a look at that today! Actually, mobility in NTN is even more fun than in terrestrial networks because it’s not only the mobile device but also ‘the network’ that can potentially move from one place to another!

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NTN – Part 5 – Why Does the UE Need a GPS Fix?

And again a seamless continuation from part 4 on 3GPP’s Non-Terrestrial Networks specification extension in Release 17 in which I’d like to come back to a particular requirement: The mobile device has to be aware of its location, which means it has to get a GPS fix before it can contact the network via a satellite. This is an interesting and quite ‘uncommon’ requirement compared to how satellite communication works today. 3GPP describes this as follows in TS 36.300:

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NTN – Part 4 – Satellite and Mobile Operator Mix and Match

This part is the seamless continuation of part 3 on the 3GPP TS 36.300 and TS 38.300 extension for Non-Terrestrial Networks, i.e. satellite communication. So let’s jump back straight in. Before we talk more about specific architecture options, here are two more terms used in the specification that are important to understand:

As discussed in the previous posts, the satellite acts as a ‘bent-pipe’ repeater in the 3GPP specifications for satellite communication, and the eNodeB / gNodeB base station is on the ground. This means that instead of a direct link between a mobile device and the base station, there are now two legs in the connection: First, there’s the radio link between the mobile device and the satellite, which is referred to in the specifications as the ‘service link‘. And then, there’s the link between the satellite and the base station on the ground, which is referred to as the ‘feeder link‘. As the satellite is ‘only’ a repeater from an overall system point of view, both links carry the same radio signal. However, it is possible that the service link and feeder link use different frequency bands, i.e. the repeater in space (the NTN payload, see previous post) must be able to change the carrier wave frequency of the signal.

That being said, lets have a look at the different NTN network architecture options specified by 3GPP:

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NTN – Part 3 – 3GPP Rel. 17 – System Architecture for Satellite Services

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).

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NTN Part 2 – NB-IoT over Satellite

In the previous post on the topic, I’ve been looking at how 3GPP has studied and then extended the LTE and 5G NR air interface so it can be used over a satellite link. For transmitting data to and from smartphones with omnidirectional antennas over satellites, 3GPP has extended the LTE Narrowband Internet of Things (NB-IoT) air interface. But why a narrowband technology for Internet of Things applications, all the hype is around satellite connectivity for smartphones these days!? Thanks to the experience gained with my Garmin InReach Mini 2, I think I have a pretty good idea why. So let’s have a look at the limitations of a satellite link to small mobile devices and which nifty features that are part of terrestrial NB-IoT can also be very useful for this type of satellite communication.

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NTN Part 1 – 3GPP Rel. 17 – Non-Terrestrial Networks

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:

Continue reading NTN Part 1 – 3GPP Rel. 17 – Non-Terrestrial Networks