Secondary Cell Group

In Release 15, the SCG function was fundamentally introduced as part of the new EN-DC (E-UTRA-NR Dual Connectivity) architecture, enabling a UE to connect to a Secondary gNB (SgNB) for additional user plane capacity via SCG bearers. This release specified key procedures like the Secondary NG-RAN node Addition and the handling of security algorithms and keys between the UE and the SgNB. It also introduced support for features such as NR multiple frequency bands and inter-RAT handover from EN-DC, requiring corresponding updates to the X2AP and S1AP interfaces.

• EN-DC impacts to LTE MAC TS 36.321CR1196
• Support for unlicensed access as secondary RAT in S1AP TS 36.413CR1563
• X2AP corrections for agreed EN-DC BL CR TS 36.423CR1050
• X2AP CR for support of NR Multiple frequency band in EN-DC TS 36.423CR1125
• Introduction of EN-DC TS 36.424CR0027
• Clarification on LTE Overheating mechanism in EN-DC TS 36.300CR1155

+ 126 more changes

In Release 16, enhancements for the SCG function included introducing an NR SCG Release mechanism for UE power saving and enabling SCG resumption during the RRC Resume procedure. The release also expanded support for inter-RAT handover from NR to EN-DC and introduced new measurement reporting for Secondary Node Addition. Furthermore, capabilities like MDT support for EN-DC and the introduction of a secondary DRX group were added.

• Add measurements related to Secondary Node Addition for E-UTRA-NR Dual Connectivity TS 32.425CR0184
• Support of inter-RAT handover from NR to EN-DC in TS 36.331 TS 36.331CR4232
• Introduce of alternative cell reselection priority for EN-DC TS 36.331CR4229
• MDT support for EN-DC TS 36.413CR1747
• Support of WUS Group TS 36.413CR1762
• Resuming SCG in RRC Resume TS 36.423CR1391

+ 113 more changes

In Release 17, key enhancements for the SCG function included the support of Conditional Handover (CHO) with an SCG configuration, allowing the dual connectivity setup to be preserved during a handover preparation. Furthermore, a new UE capability was introduced for Cross-PUCCH SCell Group CSI reporting, improving feedback efficiency in multi-connectivity scenarios. The release also provided corrections and clarifications for user plane handling and architecture in EN-DC contexts, such as data forwarding during SA to EN-DC handovers.

• 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

+ 102 more changes

In Release 18, key enhancements for the SCG function included the introduction of Conditional Handover (CHO) with SCG(s) and specific procedure clarifications for bearer management and handover cancellation within this CHO with SCG framework. The release also introduced a new UE capability for intra-band EN-DC channel spacing and provided signaling support for intra-band non-collocated EN-DC deployments. Furthermore, it brought security updates for Selective SCG Activation (SCPAC), including procedures for Secondary Node key update.

• SCPAC: Updates to Security for Selective SCG Activation TS 33.501CR1970
• Lower MSD capability for EN-DC TS 36.331CR4991
• Corrections and Updates to UE capabilities for RAN1 feature group 55-6 TS 38.306CR1132
• Introduction of new capability for intra-band EN-DC channel spacing [Intra-Band_EN-DC_Channelspacing] TS 38.306CR1174
• Signaling support for intra-band non-collocated NR-CA, EN-DC TS 38.331CR4396
• Corrections and Updates to UE capabilities for RAN1 feature group 55-6 TS 38.331CR4862

+ 51 more changes

In Release 19, key enhancements for the SCG function included the introduction of signaling support for new intra-band non-collocated receiver types in EN-DC and NR-CA deployments, alongside the addition of Secondary Cell Measurement Skipping specifically for NR ATG (Air-to-Ground). The release also focused on corrections and refinements to existing procedures, such as Mobility Robustness Optimization (MRO) for SCG failure and the execution of inter-SN SCG Link Traffic Management (LTM) procedures.

• Introduction of Secondary Cell Measurement Skipping for NR ATG TS 38.331CR5477
• Introduction of signaling support for intra-band non-collocated EN-DC/NR-CA deployment Phase 2: new receiver type(s) TS 38.331CR5479
• 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
• CR for Autonomous updates of the UE-specific TA or common TA in an OCC group in NR NTN TS 38.213CR0763

+ 14 more changes