Enhanced Node B for New Radio (NR Secondary Node in EN-DC)

Description

The en-gNB is a logical radio access network node introduced in 3GPP Release 15 to enable the first phase of 5G deployment, known as Non-Standalone (NSA) architecture option 3/3a/3x. In this architecture, the en-gNB provides New Radio (NR) user plane and control plane protocol terminations towards the User Equipment (UE) while acting as a Secondary Node (SN) within an E-UTRA-NR Dual Connectivity (EN-DC) configuration. The master node in this arrangement is an eNB (LTE base station), which anchors the control plane connection to the core network (EPC) and manages the overall radio resource control (RRC) connection. The en-gNB connects to the EPC via the S1-U interface for user plane traffic and to the eNB via the X2 interface for inter-node coordination, including the X2-C control plane and X2-U user plane.

Architecturally, the en-gNB is defined across multiple 3GPP specifications. TS 37.340 provides the overarching multi-connectivity framework, defining how the en-gNB interacts with the master eNB through procedures such as Secondary Node Addition, Modification, and Release. TS 38.401 describes the overall NG-RAN architecture, while TS 38.473 specifies the F1AP protocol used when the en-gNB is split into a Central Unit (CU) and Distributed Unit (DU). TS 38.471 defines the E1AP protocol for the CU control plane and user plane separation. TS 36.420 covers the X2 interface between the eNB and en-gNB, and TS 28.540 addresses management aspects. TS 23.401 and 23.501 provide the core network architecture context for EPC and 5GC respectively.

The en-gNB supports NR radio interface capabilities including higher bandwidths (up to 100 MHz per carrier in FR1), massive MIMO antenna arrays, beamforming, and higher-order modulation schemes. It can operate in both Frequency Range 1 (FR1, sub-7 GHz) and Frequency Range 2 (FR2, mmWave). The en-gNB handles NR Physical Downlink Shared Channel (PDSCH) and Physical Uplink Shared Channel (PUSCH) transmissions, along with associated control channels such as PDCCH and PUCCH. It maintains its own NR cell configurations, including SSB (Synchronization Signal Block) transmissions for cell detection and measurement.

In the EN-DC bearer architecture, the en-gNB can host Secondary Cell Group (SCG) bearers, where user plane data is split at the master node and routed through the en-gNB, or it can host Split Bearers where data is split at the PDCP layer. The en-gNB supports both MCG (Master Cell Group) and SCG bearer types, with the SCG split bearer being the most common configuration for maximizing throughput. The en-gNB reports radio measurements to the master eNB, which makes handover and secondary node change decisions.

From a deployment perspective, the en-gNB was critical for early 5G launches because it allowed operators to leverage existing LTE core networks (EPC) and control plane infrastructure while adding NR capacity through the secondary node. This avoided the need for immediate 5G core (5GC) deployment. Over time, as Standalone (SA) architecture became available, the role of the en-gNB evolved, but it remains a fundamental component for NSA deployments and dual connectivity scenarios throughout Releases 15 through 20.

Purpose & Motivation

The en-gNB was created to solve a critical deployment challenge: how to introduce 5G NR technology rapidly without requiring a complete overhaul of existing LTE core network infrastructure. When 3GPP began standardizing 5G in Release 15, operators faced a dilemma — building a fully standalone 5G network with a new core (5GC) would take years and enormous capital investment, but delaying 5G deployment meant falling behind in the race for enhanced mobile broadband capacity. The en-gNB concept, embedded in the EN-DC architecture, provided an elegant solution by allowing NR base stations to operate as secondary nodes anchored to existing LTE infrastructure.

Before the en-gNB, there was no standardized mechanism for LTE and NR to interoperate at the radio access level. Previous dual connectivity solutions, such as those defined in Release 12 for LTE-LTE dual connectivity, only addressed homogeneous LTE deployments. The introduction of NR required a new framework that could handle the significant differences between LTE and NR radio interfaces, including different numerologies, frame structures, and beam management procedures. The en-gNB specification in TS 37.340 addressed these challenges by defining new X2 signaling procedures, measurement configurations, and bearer architectures specifically designed for EN-DC.

The en-gNB also solved the problem of control plane anchoring. By keeping the RRC connection anchored at the LTE master eNB, the en-gNB avoided the complexity of dual RRC termination and ensured that mobility management remained stable during the transition period. This design choice meant that UEs could benefit from NR's higher throughput for user plane data while maintaining the proven reliability of LTE control plane procedures. This approach significantly reduced the risk of connection failures during the early deployment phase when NR coverage was still being built out.