In the previous part on this topic I’ve been looking at why it is unlikely that we will see 5G NR Option 3 networks directly transitioning to a 5G-only Option 2 architecture. So how could intermediate steps look like?
5G Core Network As The Basis For Evolution
The next step in any kind of 5G network evolution will be to put a 5G core network (5GC) in place. That doesn’t mean that the 4G EPC can be switched-off at the same time, it must be kept running alongside the 5GC to serve LTE-only mobile devices or 5G/LTE devices that only support Option-3. At this point it should be noted that super-legacy CS and PS core for 2G/3G network coverage will also still be present in the network to cater for old 2G/3G only devices, GSM/GPRS M2M traffic or domestic subscribers and international roamers that can’t use VoLTE for voice services. But at least it has now become possible to virtualize most network components so I can imagine that those legacy cores will just run as virtual instances on general compute and routing platforms rather than on dedicated hardware platforms as in the past. It will be quite some administrative overhead to operate three different core networks but I don’t see much of an alternative here for at least a decade to come.
Option 4 with NR in a Low Band as Anchor
Once the decision to roll out a 5GC has been made, the question will then be how to combine the existing LTE and NR coverage. If NR coverage has been rolled out in a low frequency band between 600-800 MHz band and additional NR and LTE resources are available in higher frequency bands, it would be natural to use the NR cell in the low frequency band as the anchor of a connection, as it has the longest range, and add/drop other LTE and NR resources depending on their signal strength. This is what is referred to as NR Option 4 and requires that the mobile device implements the new 5G Non-Access Stratum (NAS) protocol to talk to the 5GC. In addition, devices also have to implement the new 5G Access Stratum (AS) protocol to talk directly to a 5G NR cell as anchor.
Option 7 with LTE in a Low Band as Anchor
If low band spectrum is scarce in an area and should thus be fully utilized for LTE to also allow LTE-only mobile devices to communicate with reasonable speeds, the alternative approach is to enhance its software to allow 5GC capable devices to use the new 5G NAS protocol over the LTE cell to connect to the 5GC instead of to the EPC. This way, LTE-only devices and Option 3 legacy-5G devices would use this enhanced LTE cell as before to connect to the EPC while newer 5G devices would use the new 5G NAS protocol to talk to the 5GC. This requires an upgrade of the LTE base station software and is referred to as Option 7. Option 3 and Option 7 devices can communicate with the (evolved, enhanced?) eLTE eNB, albeit with different NAS protocols and each can be assigned the same additional LTE and NR spectrum resources.
A Mix of Option 3, 4 and 7 in the Network
In practice this makes it likely that most networks will implement a mix of NR Option 3, 4 and/or 7 depending on the geographic area. Option 3 for legacy-5G devices, Option 4 with low-band NR in rural areas with enough capacity to also have a low-band LTE cell available for legacy devices and Option 7 perhaps in urban areas.
Pure Option 2, Only In the Very Long Term?
So where does that leave room for Option 2, i.e. all air interface component carriers being NR carriers? I think it’s either in the very short term to launch a network which is then perhaps updated to add LTE spectrum to a connection for additional capacity. This would in effect transform the Option 2 network into an Option 4 or 7 configuration. In most cases I would expect to see Option 2 only in the very long term when most spectrum has been migrated to NR in a larger area and adding the little remaining LTE spectrum to a connection brings few benefits. In other words, areas in which a pure Option 2 network is operated, i.e. in which an NR anchor cell would not add LTE spectrum to a connection will be very rare at best for a very long time to come.
Handovers and VoLTE
Two final thoughts for today on all the different options: When a device is connected to the 5G core and leaves the 5G coverage area it needs to make a handover to the EPC and the 4G NAS protocol. One could also speculate, however, that by this time all LTE eNodeBs in the network have been upgraded with a link to the 5GC and thus enables mobile devices to use the 5G NAS protocol to communicate with the 5GC rather than with the EPC.
One thing that is for sure, however, is that the IMS core for VoLTE needs to be upgraded at the same time the 5GC core is used for mobile broadband connections. Today, the IMS has standardized interfaces to the EPC and to mobile devices to enforce quality of service. Similar interfaces need to be put in place between the IMS and the 5GC as well, which, compared to all other things, will fortunately be a relatively easy thing to do.
Outlook: Spectrum Sharing
So much for the different options. In the next part on this topic I’ll have a look at yet another transition method from LTE to NR which is referred to ‘spectrum sharing’ which has only recently entered the public discussion.