Description
NR-NR Dual Connectivity (NR-DC) is an advanced multi-connectivity scheme in 5G where a user equipment (UE) is concurrently connected to two distinct NR cell groups: a Master Node (MN) and a Secondary Node (SN). These nodes can be gNBs (Next Generation NodeBs) operating in different frequency bands (e.g., mid-band and mmWave), and they are coordinated to serve the UE simultaneously. The architecture involves split bearer configurations where data radio bearers (DRBs) can be terminated at either the MN or SN, or split across both using the Packet Data Convergence Protocol (PDCP) duplication or splitting. The MN manages the control plane connection via Radio Resource Control (RRC), while the SN provides additional user plane resources, with coordination over the Xn interface between the nodes.
How NR-DC works involves several key procedures. During setup, the MN decides to add an SN based on measurement reports from the UE, triggering an SN Addition procedure via Xn signaling. The UE then establishes a secondary cell group (SCG) with the SN, configuring secondary cells (SCells) for data transmission. Data flow can be managed through MCG (Master Cell Group) bearers, SCG bearers, or split bearers, depending on the deployment. For split bearers, the PDCP layer at the MN or SN handles packet duplication or distribution to the RLC layers in both nodes, enhancing throughput and reliability. Synchronization and coordination between MN and SN are critical to avoid interference and ensure seamless handovers, with mechanisms like power control and scheduling coordination.
NR-DC's role in the network is to maximize resource utilization and service quality. It enables higher peak data rates by aggregating bandwidth from multiple carriers, often across different bands, which is especially useful in 5G where spectrum is fragmented. It improves reliability through packet duplication, meeting URLLC requirements for critical applications. Additionally, NR-DC enhances mobility by allowing the UE to maintain connectivity with one node while transitioning to another, reducing interruption times. This is vital in dense or heterogeneous networks with mixed macro and small cell deployments, ensuring consistent performance as users move.
Purpose & Motivation
NR-DC was developed to address the challenges of achieving ultra-high data rates, reliable connectivity, and efficient spectrum use in 5G networks. Previous technologies like LTE-NR Dual Connectivity (EN-DC) allowed aggregation between 4G and 5G but were limited by LTE's capabilities. As networks evolved to standalone 5G, there was a need for a native NR solution to fully exploit 5G's advanced features across multiple frequency bands. The purpose of NR-DC is to enable operators to leverage their diverse spectrum portfolios (e.g., combining low-band for coverage with high-band for capacity) without being constrained by legacy systems.
Historically, dual connectivity was introduced in LTE-Advanced to aggregate resources from multiple base stations, but it was primarily intra-LTE. With 5G's broader spectrum range and new use cases, NR-DC solves problems like fragmented spectrum utilization by allowing simultaneous use of non-contiguous bands. It also addresses mobility issues in dense networks by providing anchor points for seamless handovers. The motivation stems from the demand for enhanced mobile broadband (eMBB) and ultra-reliable communications, where single-node connectivity may be insufficient. NR-DC facilitates network densification and carrier aggregation in a pure 5G environment, driving performance improvements and supporting innovative services like augmented reality and industrial automation.
Key Features
- Simultaneous connection to a Master Node (MN) and Secondary Node (SN) in NR
- Support for split bearers with PDCP duplication or splitting for enhanced throughput and reliability
- Coordination over the Xn interface between gNBs for resource management
- Flexible bearer configurations (MCG, SCG, and split bearers)
- Enhanced mobility with reduced interruption during handovers between nodes
- Utilization of diverse frequency bands (e.g., FR1 and FR2) for optimal spectrum use
Evolution Across Releases
Introduced NR-DC as part of the initial 5G standalone (SA) specifications, defining the basic architecture with MN and SN roles, Xn interface procedures, and bearer types. It enabled dual connectivity between two NR nodes, focusing on eMBB enhancements and foundational mobility support.
Enhanced NR-DC with improvements for ultra-reliable low-latency communications (URLLC), including tighter synchronization and packet duplication mechanisms. It also introduced support for integrated access and backhaul (IAB) nodes in dual connectivity scenarios, expanding deployment flexibility.
Further optimized NR-DC for power efficiency and reduced capability (RedCap) devices, enabling dual connectivity for IoT applications. It also improved coordination for multi-band operations and sidelink integration, supporting advanced V2X use cases.
Extended NR-DC in 5G-Advanced with AI/ML-driven node selection and resource allocation, enhancing performance for extended reality (XR) services. It also introduced enhancements for non-terrestrial network (NTN) integration in dual connectivity setups.
Continued evolution with support for ambient IoT and network-controlled repeaters in NR-DC architectures, aiming to improve coverage and capacity in heterogeneous networks. It focuses on sustainability and intelligent connectivity management.
Expected to further refine NR-DC for 6G preparatory work, including advanced multi-connectivity schemes and integrated sensing capabilities. It will build on 5G-Advanced to support emerging spectrum and network topologies.
Defining Specifications
| Specification | Title |
|---|---|
| TS 33.501 | 3GPP TR 33.501 |
| TS 33.825 | 3GPP TR 33.825 |
| TS 37.340 | 3GPP TR 37.340 |
| TS 37.571 | 3GPP TR 37.571 |
| TS 38.133 | 3GPP TR 38.133 |
| TS 38.171 | 3GPP TR 38.171 |
| TS 38.213 | 3GPP TR 38.213 |
| TS 38.331 | 3GPP TR 38.331 |
| TS 38.523 | 3GPP TR 38.523 |
| TS 38.846 | 3GPP TR 38.846 |