Multi-Radio Dual Connectivity:

Multi-Radio Dual Connectivity (MR-DC) is a generalization of the Intra-E-UTRA Dual Connectivity (DC) described in TS 36.300 [2], where a multiple Rx/Tx capable UE may be configured to utilize resources provided by two different nodes connected via non-ideal backhaul, one providing NR access and the other one providing either E-UTRA or NR access. One node acts as the MN and the other as the SN. The MN and SN are connected via a network interface and at least the MN is connected to the core network.

MR-DC is designed based on the assumption of non-ideal backhaul between the different nodes but can also be used in case of ideal backhaul.

MR DC two type.

1. MR-DC EPC: EN-DC

2. MR DC 5GC: NGEN-DC , NE-DC & NR-DC

MR-DC EPC: E-UTRAN supports MR-DC via E-UTRA-NR Dual Connectivity (EN-DC), in which a UE is connected to one eNB that acts as an MN and one en-gNB that acts as an SN. The eNB is connected to the EPC via the S1 interface and to the en-gNB via the X2 interface. The en-gNB might also be connected to the EPC via the S1-U interface and other en-gNBs via the X2-U interface.

MR-DC with the 5GC :

E-UTRA-NR Dual Connectivity:
NG-RAN supports NG-RAN E-UTRA-NR Dual Connectivity (NGEN-DC), in which a UE is connected to one ng-eNB that acts as a MN and one gNB that acts as a SN. The ng-eNB is connected to the 5GC and the gNB is connected to the ng-eNB via the Xn interface.

E-UTRA-NR Dual Connectivity
NG-RAN supports NG-RAN E-UTRA-NR Dual Connectivity (NGEN-DC), in which a UE is connected to one ng-eNB that acts as a MN and one gNB that acts as a SN. The ng-eNB is connected to the 5GC and the gNB is connected to the ng-eNB via the Xn interface.

NR-NR Dual Connectivity
NG-RAN supports NR-NR Dual Connectivity (NR-DC), in which a UE is connected to one gNB that acts as a MN and another gNB that acts as a SN. The master gNB is connected to the 5GC via the NG interface and to the secondary gNB via the Xn interface. The secondary gNB might also be connected to the 5GC via the NG-U interface. In addition, NRDC can also be used when a UE is connected to two gNB-DUs, one serving the MCG and the other serving the SCG, connected to the same gNB-CU, acting both as a MN and as a SN

	EN-DC	NGEN-DC	NE-DC	NR-DC
Master node	eNB	eNB (ng-eNB)	gNB	gNB
Secondary node	gNB (en-gNB)	gNB	eNB (ng-eNB)	gNB
Interface used between MN and SN	X2	Xn	Xn	Xn
Core Network used	EPC	5GC	5GC	5GC
Control Plane connection between CN and RAN	S1-C interface between MN and MME	N2 interface between ng-eNB and AMF	N2 interface between gNB and AMF	N2 interface between gNB and AMF
User Plane connection between CN and RAN	S1-U interface between MN and S-GWand optionally between SN and S-GW	N3 interface between ng-eNB and UPF and optionally between gNB and UPF	N3 interface between gNB and UPF and optionally between ng-eNB and UPF	N3 interface between gNB acting as an MN and UPF and optionally between gNB acting as an SN and UPF