E-UTRA NR Dual Connectivity with E-UTRAN connected to 5GC

Description

NGEN-DC, which stands for E-UTRA NR Dual Connectivity with E-UTRAN connected to 5GC, is a specific dual connectivity (DC) architecture defined by 3GPP. In this configuration, a User Equipment (UE) maintains simultaneous connections with two different radio access nodes: a Master Node (MN) which is an LTE eNodeB (E-UTRA), and a Secondary Node (SN) which is a 5G NR gNB. The critical architectural aspect of NGEN-DC is that the Master eNodeB is connected to the 5G Core Network (5GC) via the NG interface, not the legacy Evolved Packet Core (EPC). This distinguishes it from EN-DC, where the eNodeB connects to the EPC. The LTE node provides the control plane anchor (via the NR RRC connection is conveyed via the LTE node), while both nodes can deliver user plane data to the UE, aggregating radio resources for higher throughput and reliability.

How NGEN-DC works involves sophisticated coordination between the Master eNodeB and the Secondary gNB. The UE initially connects to the LTE network, which is configured to support connectivity to the 5GC (this is known as an LTE anchor in 5GC). When conditions are favorable (e.g., strong NR signal available), the Master eNodeB can initiate the addition of a Secondary gNB for the UE. This process involves signaling between the eNodeB and gNB over the Xn interface (the inter-base station interface in 5G). The Master Node retains control of the UE's Radio Resource Control (RRC) connection and manages the connection to the 5GC's Access and Mobility Management Function (AMF). The Secondary Node provides additional radio resources (a secondary cell group, SCG) for data transmission. Data can be split at the PDCP layer (option 3 or 3a/3x architectures), where the Master Node's PDCP layer handles routing of packets to either its own RLC layer (for transmission over LTE) or to the Secondary Node's RLC layer (for transmission over NR).

Key components of NGEN-DC include the UE supporting both E-UTRA and NR radios, the LTE eNodeB acting as the Master Node (MN), the NR gNB acting as the Secondary Node (SN), and the 5G Core Network. The interfaces involved are the NG interface between the MN and the 5GC, the Xn interface between the MN and SN for coordination, and the Uu radio interface for both LTE and NR. The UE must support the necessary protocol stack, including dual PDCP entities for split bearer operation. The role of NGEN-DC in the network is to provide a crucial migration path for operators deploying 5G. It allows them to introduce 5G NR coverage in hotspots or new spectrum bands while relying on the ubiquitous LTE network for wide-area coverage and control plane reliability. This provides users with an early 5G data rate boost in supported areas while ensuring seamless mobility and service continuity on the LTE layer.

Purpose & Motivation

NGEN-DC was created to facilitate a flexible and efficient migration from 4G LTE networks to full 5G standalone (SA) networks. Before 5G SA, the initial 5G deployments used Non-Standalone (NSA) architecture, specifically EN-DC, where the NR gNB was an add-on to an LTE network connected to the EPC. While EN-DC provided a 5G speed boost, it did not allow access to new 5GC capabilities like network slicing or advanced QoS. NGEN-DC solves this by allowing the LTE network itself to connect to the new 5GC, enabling operators to deploy and trial the 5G core while still using their extensive LTE radio assets as the primary coverage layer.

The problem it addresses is the 'chicken and egg' scenario of 5G deployment: building a nationwide 5G NR coverage layer is time-consuming and expensive. NGEN-DC allows operators to launch 5G services (via the 5GC) much faster by leveraging their existing LTE radio access network (RAN) as the master and coverage layer. It provides a middle ground between pure NSA (EN-DC with EPC) and pure SA (NR connected to 5GC). This enables subscribers to benefit from new 5G core services and potentially higher data rates via NR secondary cells, even in areas where 5G NR coverage is sparse or non-existent.

Historically, it addresses the limitation of EN-DC, which tied 5G radio innovation to the legacy EPC. NGEN-DC decouples this by connecting the LTE RAN to the 5GC, making the LTE network '5GC-aware'. This was a strategic step in 3GPP Release 15, allowing for a more gradual transition where the core network could be modernized independently of achieving full NR coverage. It motivated operators to begin deploying 5GC infrastructure and testing new services without waiting for NR build-out to be complete, thereby accelerating the overall 5G ecosystem development.