5G Stuff: Control Plane protocol stack architecture for DC between NR and LTE

Below figure describes the control plane protocol stack entities. Here RRC/PDCP/MAC/PHY are terminated at UE and gNB side and NAS is terminated at NextGen CP.

For DC between LTE and NR, any RAT can be Master node and other can be Secondary node. Secondary Node owns  its radio resources and its responsibility to assign them to cells. To support this, some kind of co-ordination between Master and Secondary nodes are expected.

UE has a single RRC state machine and is based on Master node RAT. Depends on the network implementation, UE can be communicating with both Nodes or only to Master node.

When C-Plane established only with Master node
       In this case, RRC PDUs and Inter node PDUs from Secondary Node are sent to Master Node and these are embedded by Master Node along with its RRC PDUs when transported to UE. 

When C-Plane established with both nodes
       In this case, SRB can be configured at UE with SCG  so that RRC PDUs can be sent to the UE directly from the Secondary Node. No co-ordination with Master node is needed.

Below one shows the control plane for DC between NR and LTE, when either LTE being the Master node or NR being the Master node along with UEs' RRC State. This structure is largely based on Option C1 as mentioned in 3GPP TR 36842 V12.0.0