NR-NR Dual Connectivity (NR-DC) is a 5G connectivity option where the 5G device connects to the 5GC (5G Core) using NR (New Radio) only, but with both a MN (Master Node) and a SN (Secondary Node). NG-RAN supports NR-DC, in which a 5G device is connected to one gNB that acts as a Master gNB (MgNB) and another gNB that acts as a Secondary gNB (SgNB).
In NR-DC configuration, The control plane from the 5GC (N2 interface) is terminated in the MN and the MN has a control plane connection to the 5G device. The MN (MgNB) and the SN (SgNB) are interconnected via Xn-C. The SN terminates the user plane from the 5GC (N3 interface) and has a user plane connection to the 5G device, both directly and via the MN using the Xn-U interface. Additionally, the 5G device may simultaneously have a user plane connection to the 5GC (5G Core) directly via the master node, for bearers which cannot be terminated on the SN.
NR-DC can also be used when a UE is connected to two gNB-DUs, one serving the MCG (Master Cell Group) and the other serving the SCG (Secondary Cell Group), connected to the same gNB-CU, acting both as a MN and as a SN.
For instance, UEs operating in NR standalone mode with 5GC (5G Core) can use the high band (mm wave/FR2) for data transfer for the higher throughput due to more available bandwidth , and also setup the simultaneous voice calls in the mid-band (FR1) and low-band (FR1).
› The Node in FR2 can act as MN (master gNB) and the Node in FR1 can act as SN (secondary gNB).
› VoNR call can be Setup and release in the master gNB (FR1) and split bearers are kept during ongoing VoNR call.
Here I am trying to depict a NR-DC configuration where MN works in FR1 and supports VoNR and SN works in FR2 and supports data transfer using bearer split.
The A4 measurement for NR-DC SN (Secondary gNB) addition is triggered when the following signalling message received at MN (Master gNB):
Initial Context Setup
Incoming handover to the MN
PDU session resource setup or modify
If UE is NR-DC capable and the NR-DC is operable on the master node so the MN configures the UE to start A4 measurements for the NR-DC setup.