If you go by 3GPP and common sense it can be expected that most operators having an LTE network today will launch 5G as what is referred to as 5G EN-DC (eUTRAN New Radio – Dual Connectivity), a.k.a. 5G NR ‘option 3’ in 3GPP circles. In other words, mobile devices will still camp on the existing 4G LTE eNB base stations and 5G gNB resources will be added when required. This makes sense in many cases as 5G will often be deployed on higher frequency bands and also not everywhere at first. Hence, the idea is to use the LTE network as a coverage layer and add 5G to the connection when available. Also, this has the advantage that no 5G core network (5GC) is required at first. But where do we go from here as 5G coverage gets more widespread and operators start using a 5G core network in addition to the existing 4G EPC?
In addition to ‘option 3’, which is the first version of 5G specified in 3GPP Release 15, 3GPP is the in the process in specifying additional options to connect 4G and 5G radio networks to 4G and 5G core networks. This slide set gives a good overview of the different options.
All options (except option 3) shown in the slide set only focus on how 5G capable mobile devices will connect to the network in the future. But what about ‘legacy’ 4G LTE devices? How will they connect to the network once the RAN is connected to a 5G core network?
From my point of view the answer is that in practice, network operators will use several 5G options to connect the radio and core networks simultaneously, i.e. the LTE eNBs and the 5G gNBs are each connected to the 4G LTE EPC core network and also to the 5G core network (5GC). This interesting whitepaper from the GTI shows how such a multi-option configuration could look like in practice on page 12 in figure 8.
By connecting the 4G radio network to the 4G and 5G core and also the 5G radio network to the 4G and 5G core, it will be possible to support several 5G connectivity flavors at the same time. Here are a few examples:
Support of Option 3, i.e. 4G is the coverage layer, 5G added in Dual Connectivity Mode: This is done via the connection of the 4G eNB to the 4G core and the 5G gNB connection to the 4G eNB.
Support of Option 4, i.e. 5G provides the coverage layer and 4G is added via Dual Connectivity for capacity and speed. In this scenario the mobile device (UE) must not only be 5G EN-DC (option 3) capable but must also support native 5G connectivity and connect to the 5G gNB directly. The 5G gNB is connected to the 5G core network which has a different signaling protocol than the 4G core network so the UE must be capable of communicating with the 5G core. Early ‘option 3-only devices might not support this and might not be able to support option 4 deployments. Note the ‘might’ in the sentence as this is just a wild guess at this point. This configuration makes sense when 5G is deployed on a low frequency band, e.g. the 700 MHz band in Europe. This way, 5G could provide the coverage layer and 4G could be added to increase data rates (e.g. 1800, 2600 MHz) when available.
Support of Option 7, i.e. the 4G eNB is also connected to the 5G core network and the 5G gNB is added in Dual Connectivity mode to improve speed. This makes sense in areas where the LTE radio access network continues to be used for coverage on a lower band and 5G for speed on a higher band. From a system perspective this makes a difference compared to option 3 because it makes it easier for the system to hand-over a UE between 4G and 5G cells. This is because the UE always communicates to the 5G core instead of switching between the EPC and the 5GC. This also means less NAS signaling and fewer data path changes in the network.
And, not to forget, there’s also option 2, i.e. the 5G gNB RAN is standalone and a mobile device communicates only to the gNB, there is no dual connectivity to 4G. To me, this option only makes sense if there is 5G coverage in low and high bands to provide both coverage and capacity so the network is no longer required to add LTE resources to a UE that is being served from a 5G gNB. From my point of view this scenario is even further in the future than the other options.
4G/5G handovers: Once 5G UEs support option 2 and 4, i.e. they camp on the 5G gNB rather than the 4G eNB, it will also be necessary to support 5G to 4G and perhaps also 4G to 5G handovers on mobile devices and also in the network while 4G network coverage exceeds 5G coverage.
The important point in all of this is the following: A network operator is not restricted to only deploying one 5G option in its live network. An operator might start with option 3 and then, after some time, adds a 5G core network. After that, more and more spectrum will be moved from 4G LTE to 5G NR over time as the number of 5G capable devices grows. At the same time mobile devices will evolve and will not only be able to talk to a 4G core network and thus be limited to option 3 but will also implement the 5G core network protocol stack. As a result, it will then be able to make use of the all options which are deployed in a network simultaneously.