NR E-UTRA Dual Connectivity

Description

NR E-UTRA Dual Connectivity (NE-DC) is a specific dual connectivity (DC) architecture standardized by 3GPP, defined under the umbrella of MR-DC (Multi-RAT Dual Connectivity). In NE-DC, the user equipment (UE) is configured with two distinct cell groups: a Master Cell Group (MCG) and a Secondary Cell Group (SCG). The MCG is provided by a master node (MN) operating the 5G New Radio (NR) radio access technology. The SCG is provided by a secondary node (SN) operating the 4G Evolved Universal Terrestrial Radio Access (E-UTRA or LTE) technology. The master node terminates the control plane connection to the core network, which in this architecture is the 5G Core (5GC). The MN is responsible for the initial access, connection establishment, and mobility management. The secondary node provides additional radio resources for the user plane, enhancing throughput. The UE maintains a single Radio Resource Control (RRC) connection with the master node (NR), while the secondary node is managed via RRC messages that are transported via the master node. The user plane data can be split and aggregated at various points, such as the Packet Data Convergence Protocol (PDCP) layer in the master node (for MCG split bearer) or in the secondary node (for SCG split bearer). This requires tight inter-node coordination via the Xn interface (between gNB and eNB) to manage radio resources, handovers, and bearer management. NE-DC is a foundational element for non-standalone (NSA) 5G deployment option 3, where the 5G NR layer is anchored by the 5GC but leverages existing LTE infrastructure for coverage and capacity.

Purpose & Motivation

NE-DC was introduced to facilitate the smooth introduction of 5G NR services by leveraging the extensive, existing deployment of 4G LTE networks. It solves the problem of limited initial 5G NR coverage by allowing a device to use a 5G NR connection as the primary anchor for control and high-speed data, while simultaneously using a robust LTE connection for additional data capacity and to maintain service continuity at the cell edge. This approach allows mobile network operators to launch 5G services more quickly and cost-effectively, as they can utilize their LTE assets as a complementary layer. Historically, dual connectivity concepts were first introduced in LTE-Advanced (LTE-A) with intra-LTE DC (where both cell groups are LTE). NE-DC extends this principle to multi-RAT scenarios, specifically targeting the 5G migration path. It addresses the limitations of a pure standalone 5G rollout, which would require ubiquitous NR coverage from day one to guarantee service, by providing a practical intermediate step that delivers enhanced user experience through aggregated NR and LTE resources.