ETSI TS 137 340 V18.7.0 (2026-03)

Universal Mobile Telecommunications System (UMTS); LTE; 5G; Evolved Universal Terrestrial Radio Access (E-UTRA) and NR; Multi-connectivity; Overall Description; Stage-2 (3GPP TS 37.340 version 18.7.0 Release 18)

RTS/TSGR-0237340vi70

General Information

• Status: Not Published
• Technical Committee: 3GPP RAN 2 - Radio layer 2 specification and Radio layer 3 RR specification
• Current Stage: 8 - Draft receipt by ETSI Secretariat
• Completion Date: 12-Mar-2026

Standards Content (Sample)

ETSI TS 137 340 V18.7.0 (2026-04)

TECHNICAL SPECIFICATION
Universal Mobile Telecommunications System (UMTS);
LTE;
5G;
Evolved Universal Terrestrial Radio Access (E-UTRA) and NR;
Multi-connectivity;
Overall Description;
Stage-2
(3GPP TS 37.340 version 18.7.0 Release 18)

3GPP TS 37.340 version 18.7.0 Release 18 1 ETSI TS 137 340 V18.7.0 (2026-04)

Reference
RTS/TSGR-0237340vi70
Keywords
5G,LTE,UMTS
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ETSI
3GPP TS 37.340 version 18.7.0 Release 18 2 ETSI TS 137 340 V18.7.0 (2026-04)
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ETSI
3GPP TS 37.340 version 18.7.0 Release 18 3 ETSI TS 137 340 V18.7.0 (2026-04)
Contents
Intellectual Property Rights . 2
Legal Notice . 2
Modal verbs terminology . 2
Foreword . 6
1 Scope . 7
2 References . 7
3 Definitions, symbols and abbreviations . 8
3.1 Definitions . 8
3.2 Abbreviations . 10
4 Multi-Radio Dual Connectivity . 10
4.1 General . 10
4.1.1 Common MR-DC principles . 10
4.1.2 MR-DC with the EPC . 11
4.1.3 MR-DC with the 5GC . 11
4.1.3.1 E-UTRA-NR Dual Connectivity . 11
4.1.3.2 NR-E-UTRA Dual Connectivity . 11
4.1.3.3 NR-NR Dual Connectivity . 11
4.2 Radio Protocol Architecture . 11
4.2.1 Control Plane . 11
4.2.2 User Plane . 12
4.3 Network interfaces . 14
4.3.1 Control Plane . 14
4.3.1.1 Common MR-DC principles . 14
4.3.1.2 MR-DC with EPC . 15
4.3.1.3 MR-DC with 5GC . 15
4.3.2 User Plane . 15
4.3.2.1 Common MR-DC principles . 15
4.3.2.2 MR-DC with EPC . 16
4.3.2.3 MR-DC with 5GC . 16
5 Layer 1 related aspects . 16
6 Layer 2 related aspects . 17
6.1 MAC Sublayer . 17
6.2 RLC Sublayer . 17
6.3 PDCP Sublayer . 17
6.4 SDAP Sublayer . 18
6.5 BAP Sublayer . 18
7 RRC related aspects. 18
7.1 System information handling . 18
7.2 Measurements . 18
7.3 UE capability coordination . 20
7.4 Handling of combined MN/SN RRC messages. 21
7.5 SRB3 . 21
7.6 Split SRB . 22
7.7 SCG/MCG failure handling . 22
7.8 UE identities . 23
7.9 Inter-node Resource Coordination . 23
7.10 UE assistance information . 24
7.11 F1-C transfer over E-UTRA . 24
7.12 F1-C Traffic Transfer in NR-DC . 24
7.13 Activation and Deactivation of SCG . 25
7.14 RLM/BFD relaxation . 25
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8 Bearer handling aspects . 26
8.1 QoS aspects . 26
8.2 Bearer type selection . 27
8.3 Bearer type change . 28
8.4 User data forwarding . 29
9 Security related aspects . 29
10 Multi-Connectivity operation related aspects . 30
10.1 General . 30
10.2 Secondary Node Addition . 31
10.2.1 EN-DC . 31
10.2.2 MR-DC with 5GC . 34
10.2.3 Conditional PSCell Addition . 39
10.3 Secondary Node Modification (MN/SN initiated) . 39
10.3.1 EN-DC . 39
10.3.2 MR-DC with 5GC . 46
10.4 Secondary Node Release (MN/SN initiated) . 54
10.4.1 EN-DC . 54
10.4.2 MR-DC with 5GC . 56
10.5 Secondary Node Change (MN/SN initiated) . 59
10.5.1 EN-DC . 59
10.5.2 MR-DC with 5GC . 67
10.6 PSCell change . 75
10.7 Inter-Master Node handover with/without Secondary Node change . 76
10.7.1 EN-DC . 76
10.7.2 MR-DC with 5GC . 78
10.8 Master Node to eNB/gNB Change . 80
10.8.1 EN-DC . 80
10.8.2 MR-DC with 5GC . 82
10.9 eNB/gNB to Master Node change . 84
10.9.1 EN-DC . 84
10.9.2 MR-DC with 5GC . 85
10.10 RRC Transfer . 86
10.10.1 EN-DC . 86
10.10.2 MR-DC with 5GC . 88
10.11 Secondary RAT data volume reporting . 91
10.11.1 EN-DC . 91
10.11.2 MR-DC with 5GC . 92
10.12 Activity Notification. 93
10.12.1 EN-DC . 93
10.12.2 MR-DC with 5GC . 96
10.13 Notification Control Indication . 99
10.13.1 EN-DC . 99
10.13.2 MR-DC with 5GC . 99
10.14 PDU Session Split at UPF . 99
10.14.1 PDU Session Split at UPF during PDU session resource setup . 99
10.14.2 PDU Session Split at UPF during PDU session resource modify (5GC initiated) . 100
10.14.3 PDU Session Split at UPF (RAN initiated QoS flows offloading from MN to SN) . 101
10.14.4 PDU Session Split at UPF (RAN initiates QoS flows offloading from SN to MN) . 102
10.15 F1-C Traffic Transfer . 103
10.16 Support of inter-system handover involving EN-DC or MR-DC with 5GC . 104
10.16.1 General . 104
10.16.2 Inter-system handover from EPS to 5GS with the Secondary Node used as target . 104
10.16.3 Inter-system handover from 5GS to EPS with the Source Node used as target Secondary Node . 105
10.17 Inter-Master Node RRC Resume without Secondary Node change . 106
10.17.1 MR-DC with 5GC . 106
10.18 Self-optimisation for PSCell addition/change . 108
10.18.1 General . 108
10.18.2 PSCell change failure. 108
10.18.3 Conditional PSCell addition or change failure. 109
10.18.4 Successful PSCell Addition/Change Report . 110
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10.18.5 RA Report retrieval . 110
10.19 Conditional Handover with Secondary Node . 111
10.19.1 EN-DC . 111
10.19.2 MR-DC with 5GC . 114
10.19.3 CHO with candidate SCG(s) . 117
10.20 Subsequent Conditional PSCell Addition or Change . 118
11 Service related aspects. 129
11.1 Roaming and Access Restrictions . 129
11.2 Support of Network Sharing . 129
11.3 ARPI/SPID Handling from MN . 129
12 X2/Xn Interface related aspects . 129
13 Other aspects . 129
13.1 Interference avoidance for in-device coexistence . 129
13.2 Sidelink . 129
13.3 SCG UE history information . 130
13.4 Application Layer Measurement Collection . 130
13.4.1 Overview . 130
13.4.2 SRB5 . 130
13.4.3 QoE Measurement Configuration . 130
13.4.3.1 QoE Measurement Collection Activation and Reporting in NR-DC . 130
13.4.3.2 RAN Overload Handling . 132
13.4.3.3 Support for RAN visible QoE measurements and reporting in NR-DC . 132
13.4.4 QoE Measurement Continuity for Mobility . 132
Annex A (informative): Layer 2 handling for bearer type change . 134
Annex B (informative): Supported MR-DC Handover Scenarios . 136
Annex C (informative): Change history . 137
History . 143

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3GPP TS 37.340 version 18.7.0 Release 18 6 ETSI TS 137 340 V18.7.0 (2026-04)
Foreword
This Technical Specification has been produced by the 3rd Generation Partnership Project (3GPP).
The contents of the present document are subject to continuing work within the TSG and may change following formal
TSG approval. Should the TSG modify the contents of the present document, it will be re-released by the TSG with an
identifying change of release date and an increase in version number as follows:
Version x.y.z
where:
x the first digit:
1 presented to TSG for information;
2 presented to TSG for approval;
3 or greater indicates TSG approved document under change control.
Y the second digit is incremented for all changes of substance, i.e. technical enhancements, corrections,
updates, etc.
Z the third digit is incremented when editorial only changes have been incorporated in the document.
ETSI
3GPP TS 37.340 version 18.7.0 Release 18 7 ETSI TS 137 340 V18.7.0 (2026-04)
1 Scope
The present document provides an overview of the multi-connectivity operation using E-UTRA and NR radio access
technologies. Details of the network and radio interface protocols are specified in companion specifications of the 36
and 38 series.
2 References
The following documents contain provisions which, through reference in this text, constitute provisions of the present
document.
- References are either specific (identified by date of publication, edition number, version number, etc.) or
non-specific.
- For a specific reference, subsequent revisions do not apply.
- For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including
a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same
Release as the present document.
[1] 3GPP TR 21.905: "Vocabulary for 3GPP Specifications".
[2] 3GPP TS 36.300: "Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal
Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2".
[3] 3GPP TS 38.300: "NR; NR and NG-RAN Overall description; Stage 2".
[4] 3GPP TS 38.331: "NR; Radio Resource Control (RRC) protocol specification".
[5] 3GPP TS 38.423: "NG-RAN; Xn application protocol (XnAP)".
[6] 3GPP TS 38.425: "NG-RAN; NR user plane protocol".
[7] 3GPP TS 38.401: "NG-RAN; Architecture description".
[8] 3GPP TS 38.133: "NG-RAN; Requirements for support of radio resource management".
[9] 3GPP TS 36.423: "Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2
Application Protocol (X2AP)".
[10] 3GPP TS 36.331: "Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource
Control (RRC); Protocol specification".
[11] 3GPP TS 23.501: "System Architecture for the 5G System; Stage 2".
[12] 3GPP TS 38.101-1: "User Equipment (UE) radio transmission and reception; Part 1: Range 1
Standalone".
[13] 3GPP TS 38.101-2: "User Equipment (UE) radio transmission and reception; Part 2: Range 2
Standalone".
[14] 3GPP TS 38.101-3: "User Equipment (UE) radio transmission and reception; Part 3: Range 1 and
Range 2 Interworking operation with other radios".
[15] 3GPP TS 36.323: "Evolved Universal Terrestrial Radio Access (E-UTRA); Packet Data
Convergence Protocol (PDCP) specification".
[16] 3GPP TS 38.323: "NR; Packet Data Convergence Protocol (PDCP) specification".
[17] 3GPP TS 38.340: "Backhaul Adaptation Protocol (BAP) specification".
[18] 3GPP TS 23.287: "Architecture enhancements for 5G System (5GS) to support Vehicle-to-
Everything (V2X) services ".
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[19] 3GPP TS 23.285: "Architecture enhancements for V2X services".
[20] 3GPP TS 23.502: "Procedures for the 5G System; Stage 2".
[21] 3GPP TS 38.213: "NR; Physical layer procedures".
[22] 3GPP TS 24.301: "Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage
3".
[23] 3GPP TS 38.473: "F1 application protocol (F1AP)".
[24] 3GPP TS 23.304: "Proximity based Services (ProSe) in the 5G System (5GS)".
[25] 3GPP TS 23.586: "Technical Specification Group Services and System Aspects; Architectural
Enhancements to support Ranging based services and Sidelink Positioning".
3 Definitions, symbols and abbreviations
3.1 Definitions
For the purposes of the present document, the terms and definitions given in TR 21.905 [1] and the following apply. A
term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905 [1]
and TS 36.300 [2].
Child node: IAB-DU's or IAB-donor-DU's next hop neighbour IAB-node.
Complete candidate configuration: one type of a candidate configuration as defined in TS 38.331 [4].
Conditional PSCell Addition: a PSCell addition procedure that is executed only when PSCell addition execution
condition is met.
Conditional PSCell Change: a PSCell change procedure that is executed only when PSCell change execution
condition is met.
En-gNB: node providing NR user plane and control plane protocol terminations towards the UE, and acting as
Secondary Node in EN-DC.
Fast MCG link recovery: in MR-DC, an RRC procedure where the UE sends an MCG Failure Information message to
the MN via the SCG upon the detection of a radio link failure on the MCG.
IAB-donor: gNB that provides network access to UEs via a network of backhaul and access links.
IAB-MT: IAB-node function that terminates the Uu interface to the parent node using the procedures and behaviours
specified for UEs unless stated otherwise.
IAB-node: RAN node that supports NR access links to UEs and NR backhaul links to parent nodes and child nodes.
The IAB-node does not support backhauling via E-UTRA.
Master Cell Group: in MR-DC, a group of serving cells associated with the Master Node, comprising of the SpCell
(PCell) and optionally one or more SCells.
Master node: in MR-DC, the radio access node that provides the control plane connection to the core network. It may
be a Master eNB (in EN-DC), a Master ng-eNB (in NGEN-DC) or a Master gNB (in NR-DC and NE-DC).
MCG bearer: in MR-DC, a radio bearer with an RLC bearer (or two RLC bearers, in case of CA packet duplication in
an E-UTRAN cell group, or up to four RLC bearers in case of CA packet duplication in a NR cell group) only in the
MCG.
MN terminated bearer: in MR-DC, a radio bearer for which PDCP is located in the MN.
MCG SRB: in MR-DC, a direct SRB between the MN and the UE.
Multi-Radio Dual Connectivity: Dual Connectivity between E-UTRA and NR nodes, or between two NR nodes.
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Ng-eNB: as defined in TS 38.300 [3].
NR sidelink communication: AS functionality enabling at least V2X Communication as defined in TS 23.287 [18] and
ProSe Communication (including ProSe UE-to-Network Relay and non-Relay communication) as defined in TS 23.304
[24], between two or more nearby UEs, using NR technology but not traversing any network node.
NR sidelink discovery: AS functionality enabling ProSe non-Relay Discovery and ProSe UE-to-Network Relay
discovery for Proximity based Services as defined in TS 23.304 [24] between two or more nearby UEs, using NR
technology but not traversing any network node.
Parent node: IAB-MT's next hop neighbour node; the parent node can be IAB-node or IAB-donor-DU.
PCell: SpCell of a master cell group.
PSCell: SpCell of a secondary cell group.
Ranging/Sidelink Positioning: AS functionality enabling ranging-based services and sidelink positioning as defined in
TS 23.586 [25].
RLC bearer: RLC and MAC logical channel configuration of a radio bearer in one cell group.
Secondary Cell Group: in MR-DC, a group of serving cells associated with the Secondary Node, comprising of the
SpCell (PSCell) and optionally one or more SCells.
Secondary node: in MR-DC, the radio access node, with no control plane connection to the core network, providing
additional resources to the UE. It may be an en-gNB (in EN-DC), a Secondary ng-eNB (in NE-DC) or a Secondary
gNB (in NR-DC and NGEN-DC).
SCG bearer: in MR-DC, a radio bearer with an RLC bearer (or two RLC bearers, in case of CA packet duplication in
an E-UTRAN cell group, or up to four RLC bearers in case of CA packet duplication in a NR cell group) only in the
SCG.
SN terminated bearer: in MR-DC, a radio bearer for which PDCP is located in the SN.
SpCell: primary cell of a master or secondary cell group.
SRB3: in EN-DC, NGEN-DC and NR-DC, a direct SRB between the SN and the UE.
SRB5: in NR-DC, a direct SRB between the SN and the UE dedicated for sending application layer measurement report
information.
Split bearer: in MR-DC, a radio bearer with RLC bearers both in MCG and SCG.
Split PDU Session (or PDU Session split): a PDU Session whose QoS Flows are served by more than one SDAP
entities in the NG-RAN.
Split SRB: in MR-DC, a SRB between the MN and the UE with RLC bearers both in MCG and SCG.
Subsequent Conditional PSCell Addition or Change (subsequent CPAC): a conditional PSCell addition or change
procedure that is executed based on pre-configured subsequent CPAC configuration which is not implicitly released by
the UE upon (conditional or non-conditional) PSCell addition, (conditional or non-conditional) PSCell change,
(conditional or non-conditional) PCell change or SCG release.
User plane resource configuration: in MR-DC with 5GC, encompasses radio network resources and radio access
resources related to either one or more PDU sessions, one or more QoS flows, one or more DRBs, or any combination
thereof.
V2X sidelink communication: AS functionality enabling V2X Communication as defined in TS 23.285 [19], between
nearby UEs, using E-UTRA technology but not traversing any network node.
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3.2 Abbreviations
For the purposes of the present document, the abbreviations given in TR 21.905 [1] and the following apply. An
abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in
TR 21.905 [1], TS 36.300 [2] and TS 38.300 [3].
BFD Beam Failure Detection
CHO Conditional Handover
CLI Cross Link Interference
CPA Conditional PSCell Addition
CPAC Conditional PSCell Addition or Change
CPC Conditional PSCell Change
DAPS Dual Active Protocol Stack
DC Intra-E-UTRA Dual Connectivity
DCP DCI with CRC scrambled by PS-RNTI
EN-DC E-UTRA-NR Dual Connectivity
IAB Integrated Access and Backhaul
IDC In-Device Coexistence
LTM L1/L2 Triggered Mobility
MCG Master Cell Group
MN Master Node
MR-DC Multi-Radio Dual Connectivity
MUSIM Multi-Universal Subscriber Identity Module
NE-DC NR-E-UTRA Dual Connectivity
NGEN-DC NG-RAN E-UTRA-NR Dual Connectivity
NR-DC NR-NR Dual Connectivity
QMC QoE Measurement Collection
QoE Quality of Experience
RLM Radio Link Monitoring
SCG Secondary Cell Group
SMTC SS/PBCH block Measurement Timing Configuration
SN Secondary Node
SPR Successful PSCell Addition/Change Report
V2X Vehicle-to-Everything
4 Multi-Radio Dual Connectivity
4.1 General
4.1.1 Common MR-DC principles
Multi-Radio Dual Connectivity (MR-DC) is a generalization of the Intra-E-UTRA Dual Connectivity (DC) described in
TS 36.300 [2], where a multiple Rx/Tx capable UE may be configured to utilise resources provided by two different
nodes connected via non-ideal backhaul, one providing NR access and the other one providing either E-UTRA or NR
access. One node acts as the MN and the other as the SN. The MN and SN are connected via a network interface and at
least the MN is connected to the core network.
The MN and/or the SN can be operated with shared spectrum channel access.
All functions specified for a UE may be used for an IAB-MT unless otherwise stated. Similar as specified for UE, the
IAB-MT can access the network using either one network node or using two different nodes with EN-DC and NR-DC
architectures. In EN-DC, the backhauling traffic over the E-UTRA radio interface is not supported.
NOTE 1: MR-DC is designed based on the assumption of non-ideal backhaul between the different nodes but can
also be used in case of ideal backhaul.
NOTE 2: All MR-DC normative text and procedures in this version of the specification show the aggregated node
case. The details about non-aggregated node for MR-DC operation are described in TS 38.401 [7].
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4.1.2 MR-DC with the EPC
E-UTRAN supports MR-DC via E-UTRA-NR Dual Connectivity (EN-DC), in which a UE is connected to one eNB
that acts as a MN and one en-gNB that acts as a SN. The eNB is connected to the EPC via the S1 interface and to the
en-gNB via the X2 interface. The en-gNB might also be connected to the EPC via the S1-U interface and other en-gNBs
via the X2-U interface.
The EN-DC architecture is illustrated in Figure 4.1.2-1 below.
S
S
1 1
- -
U U
S
S
1 1
1 1
S
S
X
2 2
X
Figure 4.1.2-1: EN-DC Overall Architecture
4.1.3 MR-DC with the 5GC
4.1.3.1 E-UTRA-NR Dual Connectivity
NG-RAN supports NG-RAN E-UTRA-NR Dual Connectivity (NGEN-DC), in which a UE is connected to one ng-eNB
that acts as a MN and one gNB that acts as a SN.
4.1.3.2 NR-E-UTRA Dual Connectivity
NG-RAN supports NR-E-UTRA Dual Connectivity (NE-DC), in which a UE is connected to one gNB that acts as a
MN and one ng-eNB that acts as a SN.
4.1.3.3 NR-NR Dual Connectivity
NG-RAN supports NR-NR Dual Connectivity (NR-DC), in which a UE is connected to one gNB that acts as a MN and
another gNB that acts as a SN. In addition, NR-DC can also be used when a UE is connected to a single gNB, acting
both as a MN and as a SN, and configuring both MCG and SCG.
4.2 Radio Protocol Architecture
4.2.1 Control Plane
In MR-DC, the UE has a single RRC state, based on the MN RRC and a single C-plane connection towards the Core
Network. Figure 4.2.1-1 illustrates the Control plane architecture for MR-DC. Each radio node has its own RRC entity
(E-UTRA version if the node is an eNB or NR version if the node is a gNB) which can generate RRC PDUs to be sent
to the UE.
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RRC PDUs generated by the SN can be transported via the MN to the UE. The MN always sends the initial SN RRC
configuration via MCG SRB (SRB1), but subsequent reconfigurations may be transported via MN or SN. When
transporting RRC PDU from the SN, the MN does not modify the UE configuration provided by the SN.
In E-UTRA connected to EPC, at initial connection establishment SRB1 uses E-UTRA PDCP. If the UE supports EN-
DC, regardless whether EN-DC is configured or not, after initial connection establishment, MCG SRBs (SRB1 and
SRB2) can be configured by the network to use either E-UTRA PDCP or NR PDCP (either SRB1 and SRB2 are both
configured with E-UTRA PDCP, or they are both configured with NR PDCP). Change from E-UTRA PDCP to NR
PDCP (or vice-versa) is supported via a handover procedure (reconfiguration with mobility) or, for the initial change of
SRB1 from E-UTRA PDCP to NR PDCP, with a reconfiguration without mobility before the initial security activation.
If the SN is a gNB (i.e. for EN-DC, NGEN-DC and NR-DC), the UE can be configured to establish a SRB with the SN
(SRB3) to enable RRC PDUs for the SN to be sent directly between the UE and the SN. RRC PDUs for the SN can only
be transported directly to the UE for SN RRC reconfiguration not requiring any coordination with the MN.
Measurement reporting for mobility within the SN can be done directly from the UE to the SN if SRB3 is configured.
In NR-DC, the UE can be configured to establish a SRB with the SN (SRB5) to enable RRC messages which include
application layer measurement report information to be sent directly between the UE and the SN. The application
measurement report ca
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