Description
The Secondary Cell Group (SCG) is a fundamental concept in 3GPP's dual connectivity (DC) and multi-connectivity frameworks, introduced from Release 12 onward. In a DC scenario, a user equipment (UE) is simultaneously connected to two nodes: a Master Node (MN) managing the Master Cell Group (MCG) and a Secondary Node (SN) managing the SCG. The SCG comprises one or more secondary cells (SCells) provided by the SN, which can be of the same or a different radio access technology (RAT) as the MCG—for example, LTE MCG with NR SCG in EN-DC (E-UTRA-NR Dual Connectivity). The SCG adds extra radio resources, increasing overall bandwidth and improving data throughput, reliability, and mobility robustness.
Architecturally, the SCG is controlled by the SN, which handles radio resource management (RRM) for its cells, including scheduling, bearer split configuration, and mobility within the SCG. The MN retains control of the MCG and coordinates overall UE connectivity, managing signaling like RRC connection and handover. Data flows can be split at various points: bearers may be terminated at the MN (MCG bearers), at the SN (SCG bearers), or split across both (split bearers). The SCG uses interfaces such as X2 (between eNBs in LTE) or Xn (between gNBs in NR) for coordination with the MCG. Key procedures involve SCG addition, modification, and release, triggered based on measurement reports and network policies to optimize performance.
The SCG operates with specific physical and protocol layer aspects: SCells within the SCG can be activated/deactivated dynamically to save power, and they support carrier aggregation (CA) principles. In NR-based SCGs, features like bandwidth parts (BWP) and flexible numerology are applicable. The SCG enhances network performance by enabling load balancing, reducing interruption times during handovers, and supporting high-demand use cases like enhanced mobile broadband (eMBB). It is integral to 5G non-standalone (NSA) deployments, where LTE anchors control while NR SCG provides high-speed data. As multi-connectivity evolves, the SCG concept extends to multi-RAT scenarios, forming the basis for advanced aggregation techniques in 5G-Advanced and beyond.
Purpose & Motivation
The SCG was created to address the growing demand for higher data rates and more reliable connections, which single connectivity or carrier aggregation within one node could not fully meet. Prior to Release 12, LTE Advanced relied on carrier aggregation (CA) within a single eNB, limited by available spectrum and site constraints. Dual connectivity with SCG allows aggregation of resources from geographically separate base stations, increasing total bandwidth and providing macro-diversity gains. This solved issues like cell-edge performance degradation and capacity bottlenecks, especially in heterogeneous networks with small cells.
With the transition to 5G, the SCG became crucial for smooth migration, enabling LTE-NR interworking in non-standalone mode. It allowed operators to leverage existing LTE infrastructure for coverage and control while adding NR SCGs for enhanced data capabilities, addressing the challenge of deploying 5G without a full core network overhaul. The SCG also supports service continuity and ultra-reliable low-latency communication (URLLC) by enabling redundant paths. Its development was motivated by the need for flexible, efficient multi-connectivity solutions to support diverse 5G use cases and network evolution.
Classification
Release Timeline
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (91 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the specification introduced clarifications and corrections for the SCG function, including the handling of SRB duplication in the SCG and security resources for split and SCG bearers at the SgNB. It provided corrections on procedures following an SCG failure, reconfiguration of the SCG part of DRBs, and the pre-condition for SCG reconfiguration with sync. The release also addressed terminology and field corrections, such as for `scg-ChangeFailure` and the `scg-RB-Config` in CG-Config.
- Corrections to deletion of SCG Keys TS 33.401CR0647
- Clarify that both split bearers and SCG bearer may need security resources at the SgNB TS 33.401CR0660
- Correction on the terminology scg-ChangeFailure TS 36.331CR3807
- Addition of SRB duplication in SCG TS 36.331CR3813
- Clarification on mobility of UE configured with SN terminated DRB without SCG TS 36.331CR4035
- Correction on 36.331 for reconfiguration of SCG part of DRBs in NE-DC TS 36.331CR4080
+ 6 more changes
In Release 16, key enhancements for the SCG function included the introduction of an NR SCG release procedure specifically for UE power saving and the support for resuming the SCG configuration during the RRC Connection Resume procedure. Furthermore, the release provided corrections and clarifications for handling SCG during overheating assistance scenarios and for storing the SCG configuration in the UE's inactive state context.
- Resuming SCG in RRC Resume TS 36.423CR1391
- Introduction of NR SCG Release for Power Saving TS 36.423CR1511
- Resuming SCG in RRC Resume TS 37.340CR0176
- Resuming SCG in RRC Resume TS 38.423CR0259
- Introduction of NR SCG Release for Power Saving TS 38.423CR0405
- Miscellaneous corrections on overheating assistance information for NR SCG TS 36.331CR4489
+ 20 more changes
In Release 17, key enhancements for the SCG function focused on enabling and coordinating Conditional Handover (CHO) with an SCG configuration, ensuring the SCG could be maintained during the CHO execution. This included introducing specific UE capabilities for this feature and defining necessary coordination with intra-SN SCG reconfiguration procedures. The release also provided corrections and clarifications on SCG state reporting, activation status, and timer handling to improve reliability.
- SCG BL CR to TS 36.423 TS 36.423CR1609
- SCG BL CR to TS 38.401 TS 38.401CR0176
- SCG BL CR to TS 38.423 TS 38.423CR0633
- Support of CHO with SCG configuration - 36331 [CHOwithDCkept] TS 36.331CR4823
- Enabling CHO with SCG configuration [CHOwithDCkept] TS 36.423CR1590
- Support of CHO with SCG configuration - 37340 [CHOwithDCkept] TS 37.340CR0329
+ 31 more changes
In Release 18, key enhancements for the SCG function focused on Conditional Handover (CHO) integration and operational refinements. The release introduced CHO with SCG(s) and procedures for its cancellation, while also clarifying bearer management for this scenario. Additionally, it brought corrections and clarifications to areas like SCG state handling, UE history information, and HARQ buffer flushing during SCG deactivation.
- SCPAC: Updates to Security for Selective SCG Activation TS 33.501CR1970
- Introduction of CHO with SCG(s) TS 38.423CR1090
- Handover Cancel in CHO with SCG(s) TS 37.340CR0390
- Clarification of the bearer menagement in case of CHO with SCG TS 37.340CR0394
- Correction on HARQ buffer flush at SCG deactivation TS 38.321CR1657
- Addition of new UE capability for CHO with SCG configuration test case TS 38.508CR0806
+ 4 more changes
In Release 19, the work on the Secondary Cell Group (SCG) function focused on corrections and clarifications to specific procedures. These included corrections for Mobility Robustness Optimization (MRO) during SCG failure and for candidate SCG in Conditional Handover (CHO) scenarios. Further corrections were made to the inter-SN SCG Link Traffic Management (LTM) procedure and the associated TA (Timing Advance) information transfer.
- Correction on MRO for SCG failure in EN-DC TS 36.300CR1450
- Correction on SN initiated inter-SN SCG LTM procedure TS 37.340CR0428
- Correction on MRO for S-CPAC and CHO with candidate SCG for 37.340 TS 37.340CR0429
- Correction on TA Information Transfer inter-SN SCG LTM TS 38.423CR1699
- Correction on UE Based TA Measurement ID assignment for inter-CU (SCG) LTM TS 38.423CR1700
- Correction on the execution of SCG LTM TS 38.331CR5558
Explore further
Broader topics and technologies where SCG plays a role.
Defining Specifications
3GPP specifications that define or reference SCG, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.725 vg20 | Study on URLLC Architecture Enhancements | Rel-16 |
| TS 32.425 vj00 | E-UTRAN Performance Measurements | Rel-19 |
| TS 33.401 vj10 | EPS Security Architecture | Rel-19 |
| TS 33.501 vk00 | 5G Security Architecture and Procedures | Rel-20 |
| TS 33.825 vg01 | Security for 5G URLLC Services | Rel-16 |
| TS 36.101 vj30 | LTE UE Radio Transmission & Reception Requirements | Rel-19 |
| TS 36.211 vj10 | LTE Physical Layer Specification | Rel-19 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.321 vj00 | E-UTRA MAC Protocol Specification | Rel-19 |
| TS 36.323 vj00 | PDCP Protocol Specification | Rel-19 |
| TS 36.331 vj00 | LTE RRC Protocol Specification | Rel-19 |
| TS 36.401 vj00 | E-UTRAN Overall Architecture Description | Rel-19 |
| TS 36.410 vj00 | S1 Interface: General Aspects and Principles | Rel-19 |
| TS 36.413 vj10 | S1 Application Protocol (S1AP) | Rel-19 |
| TS 36.423 vj10 | X2 Application Protocol (X2AP) Specification | Rel-19 |
| TS 36.424 vj00 | X2 Interface User Plane Transport Protocols | Rel-19 |
| TS 36.842 vc00 | Small Cell Enhancements for LTE Higher Layers | Rel-12 |
| TS 36.875 vd10 | Dual Connectivity Extension Requirements | Rel-13 |
| TS 37.340 vj00 | Multi-Connectivity Operation Overview | Rel-19 |
| TS 37.483 vj10 | E1 Application Protocol (E1AP) | Rel-19 |
| TS 38.101 vj31 | NR User Equipment Radio Transmissions | Rel-19 |
| TS 38.133 vj20 | 5G UE Radio Requirements for RRC_IDLE Mobility | Rel-19 |
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TS 38.306 vj00 | NR UE Radio Access Capability Parameters | Rel-19 |
| TS 38.321 vj00 | NR MAC Protocol Specification | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |
| TS 38.401 vj10 | NG-RAN Architecture Specification | Rel-19 |
| TS 38.413 vj10 | NG Application Protocol (NGAP) | Rel-19 |
| TS 38.423 vj10 | Xn Application Protocol (XnAP) specification | Rel-19 |
| TS 38.463 vj00 | E1 Application Protocol (E1AP) | Rel-19 |
| TS 38.508 vj11 | 5G NR UE Radio Transmission & Reception | Rel-19 |
| TS 38.521 vj20 | NR Physical Layer UE Conformance Testing | Rel-19 |
| TS 38.522 vj11 | UE Conformance Test Applicability Statement | Rel-19 |
| TS 38.523 vj20 | 5G NR UE Conformance Testing: Idle/Inactive | Rel-19 |
| TS 38.755 vj10 | NR FR1 DL Fragmented Carriers Study | Rel-19 |
| TS 38.793 vj00 | Simultaneous Rx/Tx Band Combinations TR | Rel-19 |
| TR 38.804 ve00 | Study on New Radio Access Technology; Radio Interface Protocol Aspects | Rel-14 |
| TR 38.839 vh00 | Simultaneous Rx/Tx band combinations | Rel-17 |
| TS 38.863 vj10 | NR NTN RF and Co-existence Spec | Rel-19 |
| TR 38.881 vi00 | Technical Report on Lower MSD for Inter-band CA/EN-DC/DC | Rel-18 |
| TR 38.894 vi00 | Technical Report | Rel-18 |
| TR 38.912 vj00 | Study on New Radio Access Technology | Rel-19 |
