E-UTRA NR Dual Connectivity with MCG using E-UTRA and SCG using NR

Description

E-UTRA NR Dual Connectivity (EN-DC) is a specific dual connectivity configuration defined by 3GPP where the User Equipment (UE) is concurrently connected to two different radio access technologies: LTE (E-UTRA) and 5G New Radio (NR). In this architecture, the LTE base station (eNodeB) acts as the Master Node (MN), forming the Master Cell Group (MCG). The 5G NR base station (gNB) acts as the Secondary Node (SN), forming the Secondary Cell Group (SCG). The UE maintains a single control plane connection to the LTE Master Node via the MCG. The core network connection is anchored in the Evolved Packet Core (EPC), not the 5G Core (5GC), which classifies EN-DC as a Non-Standalone (NSA) 5G deployment mode.

How it works involves coordinated operation between the eNodeB (MN) and the gNB (SN). The LTE eNodeB is the control plane anchor, handling all Radio Resource Control (RRC) signaling, mobility management, and connection to the EPC (specifically the MME and S-GW). The NR gNB is primarily responsible for providing additional user plane capacity. Data can be split at the PDCP layer (located at the MN) or at the core network (S-GW). The MN's PDCP layer can route data packets to its own RLC layer (for transmission over LTE) or to the SN's RLC layer (for transmission over NR) via the X2 interface (enhanced as X2-C and X2-U). This requires tight synchronization and coordination between the two nodes.

Key components include the UE supporting both LTE and NR radios, the LTE eNodeB (Master eNB or MeNB), the NR gNB (Secondary gNB or SgNB), and the EPC. The critical interfaces are the LTE-Uu interface between UE and eNodeB, the NR-Uu interface between UE and gNB, and the X2 interface between the eNodeB and gNB for control (X2-C) and user plane (X2-U) coordination. The role of EN-DC in the network was to serve as the primary early deployment path for 5G, allowing operators to leverage their dense LTE infrastructure to provide wide-area 5G coverage and high data rates without requiring immediate investment in a full 5G core network, accelerating time-to-market for 5G services.

Purpose & Motivation

EN-DC was created to solve the problem of how to introduce and deploy 5G New Radio technology rapidly and cost-effectively before the 5G Core network was fully standardized and deployed. The primary motivation was to enable operators to offer enhanced mobile broadband (eMBB) services with very high data rates using 5G NR spectrum, while relying on the mature, ubiquitous, and stable LTE network for control plane functions and coverage anchoring.

Historically, it addressed the limitations of a pure "greenfield" 5G Standalone (SA) deployment, which would have required simultaneous rollout of new radio and a new core network, a massive and slow capital investment. EN-DC, as a Non-Standalone architecture, allowed a phased approach. It leveraged the existing LTE infrastructure as a reliable control plane and coverage layer, overlaying 5G NR capacity only in targeted areas (e.g., dense urban hotspots, stadiums) where the high throughput was most needed.

It solved key technical and business challenges: It provided a clear migration path, reduced initial deployment risk and cost, and allowed for early device ecosystem development focused on data-centric use cases. By anchoring to the EPC, it also ensured backward compatibility and service continuity for voice (VoLTE) and other LTE services. EN-DC was the cornerstone of the first wave of commercial 5G deployments globally, bridging the gap between 4G and full 5G Standalone systems.