In many countries, 5G networks will be launched initially in a configuration referred to ‘Non-Standalone Architecture – Option 3’. This means that the mobile device connects to a 4G cell that has a co-located 5G cell which it can then include in the data transfer. This is why ‘option 3’ is also referred to as EN-DC (eUTRAN NR – Dual Connectivity). When I was recently asked how and where a 5G capable mobile devices signal their 5G capabilities to a 4G LTE cell during connection establishment, I could give a rough answer. However, I decided to look up the details in the 3GPP specifications and write them down here as I’m sure this was not the last time the question come up.
In 4G LTE, the UE signals its radio network capabilities on request by the network in a ‘ueCapabilityInformation’ message, e.g. during the attach procedure. There are various ‘containers’ inside that encapsulate the parameters for different radio technologies, e.g. LTE, UMTS, etc. The message and the parameters are specified in 3GPP TS 36.331. For EN-DC, two new containers have been added in 3GPP Release 15 which are described in 3GPP TS 38.331:
UE-NR-Capability UE-MRDC-Capability
The first one is for pure “5G NR” connectivity information (i.e. the 5G standalone architecture) while the UE-MRDC-Capability container contains information for dual-connectivity, such as for example the band combination list that tells the network which LTE and NR carriers can be combined to a single transmission channel.
So it’s quite straight forward, two new containers in an already existing LTE message!