There are three possible MR-DC with the 5GC architectures as follow:  1. 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.

E-UTRAN supports MR-DC via E-UTRA-NR Dual Connectivity (EN-DC), also called MR-DC with EPC, in which a UE is connected to one eNB that acts as a MN and one en-gNB that acts as a 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.

E-UTRAN supports Dual Connectivity (DC) operation whereby a multiple Rx/Tx UE in RRC_CONNECTED is configured to utilise radio resources provided by two distinct schedulers, located in two eNBs connected via a non-ideal backhaul over the X2 interface

The Xn User plane (Xn-U) interface is defined between two NG-RAN nodes.  The transport network layer is built on IP transport and GTP-U is used on top of UDP/IP to carry the user plane PDUs.

The NG user plane interface (NG-U) is defined between the NG-RAN node and the UPF. The transport network layer is built on IP transport and GTP-U is used on top of UDP/IP to carry the user plane PDUs between the NG-RAN node and the UPF.

NG-U provides non-guaranteed delivery of user plane PDUs between the NG-RAN node and the UPF.

gNB-CU-Control Plane (gNB-CU-CP): a logical node hosting the RRC and the control plane part of the PDCP protocol of the gNB-CU for an en-gNB or a gNB. The gNB-CU-CP terminates the E1 interface connected with the gNB-CU-UP and the F1-C interface connected with the gNB-DU.

The different sublayers used in  Radio Protocol Architecture are: Medium Access Control (MAC), Radio Link Control (RLC), Packet Data Convergence Protocol (PDCP) and Service Data Adaptation Protocol (SDAP), Radio Resource Control (RRC) and Non-Access Stratum (NAS). Where:

-The physical layer offers to the MAC sublayer transport channels;

gNB Central Unit (gNB-CU): a logical node hosting RRC, SDAP and PDCP protocols of the gNB or RRC and PDCP protocols of the en-gNB that controls the operation of one or more gNB-DUs. The gNB-CU terminates the F1 interface connected with the gNB-DU. 

The gNB and ng-eNB host the following functions:

-Functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling);

MATLAB has introduced its 5G Toolbox, which provides standards compliant waveforms and reference examples for modeling, simulation, and verification of the physical layer of 3GPP 5G New Radio (NR) communications systems. 5G Toolbox can be used  to design algorithms and predict end-to-end link performance of systems that conform to the 5G Release 15 standard.