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:
The Satellite Constellation Owner is ‘The Network Operator’
The easiest scenario is a standalone LTE or 5G NTN network, i.e. the satellite constellation owner or the owner of the NTN payloads that fly on satellites of another company is the network operator. In case of moving LEO or MEO satellites, this would make the owner a global network operator. In other words, such a network would have a single Mobile Country Code (MCC) and Mobile Network Code (MNC) all around the world and the network operator would operate its own mobile core network.
MCC and MNC are actually mis-nomers for a global NTN network, but those are the 3GPP terms used to identify a network operator since the beginning of time. Even today, ‘global’ MCC/MNCs exist for network operators offering global services and roam on national networks for the purpose. MCC 901 is assigned to identify global services and the MNC identifies the company behind it instead of a particular national network operator. I guess a similar setup could be used for NTN network operators.
As NTN payloads only offer bent-pipe services as per 3GPP Release 17, each NTN payload must have a ground station in sight. This means that the network will not function in areas without a ground station. A typical example of a (non-NTN) satellite constellation that works like this today is Globalstar. Excluded in this part of the 3GPP specifications are more advanced satellite systems that can forward user data between satellites, which make the service independent of ground station visibility of every satellite in the constellation. Iridium is an example of such a (non-NTN) deployment today.
Multiple Core Networks – A MOCN Approach?
The 3GPP architecture extensions for NTN also describe an operation mode where the RAN, i.e. the satellite constellation and the eNodeBs/gNodeBs at the ground station could connect to several core networks of different parties. Let’s make a practical example: National network operators from 3 countries that share a border with each other would like to extend their terrestrial services with a satellite component for devices in their respective country. A ground station with eNodeBs/gNodeBs that communicates with satellites flying over these three countries could then be connected to the three core networks of the three national network operators. Depending on which country a particular satellite beam (i.e. cell) covers, a corresponding MCC/MNC could be broadcast in that beam. In beams (i.e. cells) that offer coverage across borders, several MCC/MNC could be broadcast. This is referred to as MOCN (Multi-Operator Core Network) and already used in terrestrial networks. Have a look here for details.
How To Switch Between Terrestrial And Satellite?
So if the terrestrial network and the satellite network use the same MCC/MNC in a country, how could one ensure the device remains in the terrestrial network while available and only changes to satellite once it runs out of options? That seems pretty simple: Priorities! Even today, different LTE/5G terrestrial frequency bands are configured in the system information broadcast with different priorities. This way, mobile devices can be steered, for example, to the highest frequency band it receives and thus preserve lower frequency bands for indoor coverage use cases. With priorities, the satellite frequency could be made the lowest priority, so a UE would only use it if no terrestrial channels can be found. Easy!
Extending MOCN into space, if that is ever done, has a number of interesting consequences. Let’s say operator 1 of country 1 and operator 2 of country 2 share an international border. Both operators have their core network connected to the NTN network, i.e. they have an S1 link from their MME or an N1 link to their AMF to the ground station. How would the ground station know which device to connect to which core network? The first half of the answer is easy: The mobile device includes the information which core network it wants to connect to during the RRC connection establishment. But a beam (i.e. cell) could have large country overlaps, so using only the UEs MCC/MNC might not be enough.
Mobile Devices Require GPS
A feature that might come in handy in such a cross border scenario is the requirement that an NTN capable mobile device needs to be able to determine its location via GPS/Galileo/Glonas/Baidu etc. This is necessary in a first instance, as the mobile device is required to pre-compensate for the delay its radio signal has to the satellite. The only way it can do this is by knowing its own position and the position of the satellite. Knowing the position of the satellite is easy, as air interface system information broadcast (SIB) messages have been extended to include the satellite’s (i.e. NTN payload’s) orbit information, also referred to as ephemeris information. For this information to be useful, the mobile device requires the exact time, which it learns from the GPS satellites in addition to its own location. The mobile device includes its location during connection establishment, an NTN extension to the standard, which can then be used by the network to determine in which country the device is currently located, which in turn can then be used to determine to which core network to connect the device to, or to refuse service in case satellite services are not allowed in a country.
EHPLMN – Another Interesting Steering Possibility
Let’s make the setup even more complicated: In addition to the MCC/MNC of the two (or more) network operators, a global MCC/MNC could also be broadcast, so devices could roam into this (virtual) network when they can’t get service from their home network operator. Or perhaps devices from network operator 3 in country 3 could roam via the core network of operator 2 while in operator 2’s country, if prices are cheaper than via the global MCC network. Yes, at this point, the world starts spinning (pun intended).
But be that as it may, connecting a ground station to several core networks, potentially in different countries, and using 3GPP mechanisms that have already been specified a long time ago for terrestrial use could make a lot setup options and business models possible. It will be interesting to see if and which setups will see the light of day in the years to come.
Let’s make our heads spin even more but in a different direction: This paper from 5G Americas speculates about the use of Equivalent Home PLMN (HPLMN) priorities to steer a UE between terrestrial and satellite networks:
“Optionally for the SNO [Satellite Network Operator], it could choose to broadcast the MNO’s [Mobile Network Operator's] PLMN as Equivalent Home PLMN (EHPLMN) from its satellite network, while defining its HPLMN as a lower priority PLMN than the MNO’s PLMN ID in the EHPLMN List. Per 3GPP standard (23.122 and 31.10228), the SNO subscriber will treat the MNO’s network as higher priority network to camp, so the SNO subscriber can access the MNO’s network to benefit from the greater speed and lower latency and only use the SNO’s satellite network when it’s outside of the MNO’s terrestrial network coverage.”
So much for today, but we are not done yet, 3GPP TS 36.300 and 38.300 have a lot more to offer. So stay tuned for the next part.