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.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (54 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the NR-NR Dual Connectivity (NR-DC) function was newly introduced, defining an architecture where a UE is connected to one gNB as a Master Node and another gNB as a Secondary Node. This release specified foundational procedures for NR-DC, including security context initialization with K_SN derivation, user plane security activation decisions split between the MN and SN, and support for UE capability reporting and synchronization. It also introduced specific configurations for measurements, PDCCH monitoring, and PUCCH within the NR-DC framework.
- Introduction of late drop NGEN-DC, NE-DC and NR-DC TS 38.331CR0916
- Introduction of UE capability for NR-DC with SFN synchronization between PCell and PSCell TS 38.331CR1265
- Security Procedures for Dual Connectivity TS 33.501CR0185
- Clarification for UP security in dual connectivity TS 33.501CR0516
- CR on 37.340 for s-Measure in NR-DC and NE-DC TS 37.340CR0118
- Capability report and coordination in NR-DC TS 37.340CR0148
+ 9 more changes
In Release 16, the NR-DC (NR-NR Dual Connectivity) function was enhanced to support operation within the same frequency range and introduced cross-carrier scheduling with different numerologies. The release also specified detailed procedures for power control, PRACH handling, and UE capability filtering for NR-DC, while providing corrections and clarifications for configurations like SMTC for PSCell addition and PDCP duplication capability. Furthermore, it defined security context initialization and key derivation (K_SN) procedures specific to the dual connectivity architecture where a UE is connected to a master and a secondary gNB.
- Introduction of NR-DC in same Frequency Range and of Cross-carrier Scheduling with Different Numerologies TS 38.213CR0077
- NR-DC Cell Group capability filtering TS 38.331CR2703
- CR on support of NR-DC within the same gNB-DU TS 37.340CR0246
- Corrections on NR-DC and on Cross-carrier Scheduling with Different Numerologies TS 38.213CR0086
- Corrections on NR-DC and on Cross-carrier Scheduling with Different Numerologies TS 38.213CR0111
- CR on Power Control for NR-DC TS 38.213CR0128
+ 14 more changes
In Release 17, NR-DC enhancements introduced specific procedures for CSI-RS resource coordination and refined power control parameters for FR1-FR1 operation. The release also provided clarifications and corrections for functionalities like inter-frequency measurements without gaps and multi-slot PDCCH monitoring in mixed capability scenarios. Furthermore, it defined the generation of the KIAB key for NR-DC security context initialization and detailed the distinct security decision-making roles for the Master Node and Secondary Node regarding user plane integrity protection.
- KIAB generation for NR-DC scenario TS 33.501CR1242
- Introduction of further multi-RAT dual-connectivity enhancements TS 37.340CR0309
- Introduction of further multi-RAT dual-connectivity enhancements TS 38.331CR2954
- Corrections on further Multi-RAT Dual-Connectivity enhancements TS 38.213CR0331
- Correction on multi-slot PDCCH monitoring in NR-DC and CA scenarios with mixed capability types TS 38.213CR0342
- CSI-RS resource coordination in NR-DC TS 38.331CR3991
+ 3 more changes
In Release 18, the enhancements for NR-DC specifically introduced support for QoE (Quality of Experience) measurements and reporting, requiring RAN-visible procedures. Furthermore, the release defined refinements for handling radio and RLC bearers during LTM (Layer Traffic Management) cell switch execution and introduced Inter-node Coordination mechanisms for managing the aggregated bandwidth.
- Introduction of QoE enhancement for NR-DC TS 37.340CR0372
- CR for RAN visible QoE measurements and reporting in NR-DC TS 37.340CR0383
- Correction on 37.340 for stage-2 description of QoE in NR-DC TS 37.340CR0386
- Handling of radio and RLC bearers at LTM cell switch execution in NR-DC TS 38.331CR5383
- Correction of IAB F1-C transfer in NR-DC TS 37.340CR0409
- Introduction of Inter-node Coordination on the Aggregated Bandwidth for the NR-DC (r18) TS 38.331CR4736
+ 2 more changes
In Release 19, the NR-DC function introduced support for applying Artificial Intelligence/Machine Learning (AIML) to enhance its operation. Furthermore, the release included specific corrections to the security context initialization procedures for selective Secondary Cell Group (SCG) Change (SCPAC) to prevent key-stream reuse, as detailed in clause 6.10.2 of the security specification.
Explore further
Broader topics and technologies where NR-DC plays a role.
Defining Specifications
3GPP specifications that define or reference NR-DC, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 33.501 vk00 | 5G Security Architecture and Procedures | Rel-20 |
| TS 33.825 vg01 | Security for 5G URLLC Services | Rel-16 |
| TS 37.340 vj00 | Multi-Connectivity Operation Overview | Rel-19 |
| TS 37.571 vj00 | UE Conformance for Positioning | Rel-19 |
| TS 38.133 vj20 | 5G UE Radio Requirements for RRC_IDLE Mobility | Rel-19 |
| TS 38.171 vj10 | 5G A-GNSS UE Positioning Requirements | Rel-19 |
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |
| TS 38.523 vj20 | 5G NR UE Conformance Testing: Idle/Inactive | Rel-19 |
| TR 38.846 vi10 | Technical Report | Rel-18 |