ETSI TS 137 340 V15.7.0 (2019-10)

TECHNICAL SPECIFICATION
Universal Mobile Telecommunications System (UMTS);
LTE;
5G;
NR;
Multi-connectivity;
Overall description;
Stage-2
(3GPP TS 37.340 version 15.7.0 Release 15)

3GPP TS 37.340 version 15.7.0 Release 15 1 ETSI TS 137 340 V15.7.0 (2019-10)

Reference
RTS/TSGR-0237340vf70
Keywords
5G,LTE,UMTS
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3GPP TS 37.340 version 15.7.0 Release 15 2 ETSI TS 137 340 V15.7.0 (2019-10)
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3GPP TS 37.340 version 15.7.0 Release 15 3 ETSI TS 137 340 V15.7.0 (2019-10)
Contents
Intellectual Property Rights . 2
Legal Notice . 2
Modal verbs terminology . 2
Foreword . 5
1 Scope . 6
2 References . 6
3 Definitions, symbols and abbreviations . 7
3.1 Definitions . 7
3.2 Abbreviations . 8
4 Multi-Radio Dual Connectivity . 8
4.1 General . 8
4.1.1 Common MR-DC principles . 8
4.1.2 MR-DC with the EPC . 8
4.1.3 MR-DC with the 5GC . 9
4.1.3.1 E-UTRA-NR Dual Connectivity . 9
4.1.3.2 NR-E-UTRA Dual Connectivity . 9
4.1.3.3 NR-NR Dual Connectivity . 9
4.2 Radio Protocol Architecture . 9
4.2.1 Control Plane . 9
4.2.2 User Plane . 10
4.3 Network interfaces . 12
4.3.1 Control Plane . 12
4.3.1.1 Common MR-DC principles . 12
4.3.1.2 MR-DC with EPC . 13
4.3.1.3 MR-DC with 5GC . 13
4.3.2 User Plane . 13
4.3.2.1 Common MR-DC principles . 13
4.3.2.2 MR-DC with EPC . 14
4.3.2.3 MR-DC with 5GC . 14
5 Layer 1 related aspects . 14
6 Layer 2 related aspects . 14
6.1 MAC Sublayer . 14
6.2 RLC Sublayer . 15
6.3 PDCP Sublayer . 15
6.4 SDAP Sublayer . 15
7 RRC related aspects. 15
7.1 System information handling . 15
7.2 Measurements . 15
7.3 UE capability coordination . 17
7.4 Handling of combined MN/SN RRC messages. 17
7.5 SRB3 . 17
7.6 Split SRB . 18
7.7 SCG/MCG failure handling . 18
7.8 UE identities . 19
7.9 Inter-node Resource Coordination . 19
8 Bearer handling aspects . 19
8.1 QoS aspects . 19
8.2 Bearer type selection . 20
8.3 Bearer type change . 21
8.4 User data forwarding . 21
9 Security related aspects . 22
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10 Multi-Connectivity operation related aspects . 22
10.1 General . 22
10.2 Secondary Node Addition . 22
10.2.1 EN-DC . 22
10.2.2 MR-DC with 5GC . 24
10.3 Secondary Node Modification (MN/SN initiated) . 26
10.3.1 EN-DC . 26
10.3.2 MR-DC with 5GC . 30
10.4 Secondary Node Release (MN/SN initiated) . 34
10.4.1 EN-DC . 34
10.4.2 MR-DC with 5GC . 36
10.5 Secondary Node Change (MN/SN initiated) . 37
10.5.1 EN-DC . 37
10.5.2 MR-DC with 5GC . 40
10.6 PSCell change . 43
10.7 Inter-Master Node handover with/without Secondary Node change . 43
10.7.1 EN-DC . 43
10.7.2 MR-DC with 5GC . 45
10.8 Master Node to eNB/gNB Change . 47
10.8.1 EN-DC . 47
10.8.2 MR-DC with 5GC . 49
10.9 eNB/gNB to Master Node change . 50
10.9.1 EN-DC . 50
10.9.2 MR-DC with 5GC . 51
10.10 RRC Transfer . 52
10.10.1 EN-DC . 52
10.10.2 MR-DC with 5GC . 53
10.11 Secondary RAT data volume reporting . 55
10.11.1 EN-DC . 55
10.11.2 MR-DC with 5GC . 56
10.12 Activity Notification. 56
10.12.1 EN-DC . 56
10.12.2 MR-DC with 5GC . 57
10.13 Notification Control Indication . 59
10.13.1 EN-DC . 59
10.13.2 MR-DC with 5GC . 59
10.14 PDU Session Split at UPF . 59
10.14.1 PDU Session Split at UPF during PDU session resource setup . 59
10.14.2 PDU Session Split at UPF during PDU session resource modify (5GC initiated) . 60
10.14.3 PDU Session Split at UPF (RAN initiated QoS flows offloading from MN to SN) . 61
11 Service related aspects. 64
11.1 Roaming and Access Restrictions . 64
11.2 Support of Network Sharing . 64
12 X2/Xn Interface related aspects . 64
13 Other aspects . 64
13.1 Interference avoidance for in-device coexistence . 64
Annex A (informative): Layer 2 handling for bearer type change . 65
Annex B (informative): Supported MR-DC Handover Scenarios . 67
Annex C (informative): Change history . 68
History . 71

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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.
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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".
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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].
En-gNB: node providing NR user plane and control plane protocol terminations towards the UE, and acting as
Secondary Node in EN-DC.
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)
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.
Ng-eNB: as defined in TS 38.300 [3].
PCell: SpCell of a master cell group.
PSCell: SpCell of a secondary cell group.
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)
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.
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.
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.
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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] and TS 36.300 [2].
DC Intra-E-UTRA Dual Connectivity
EN-DC E-UTRA-NR Dual Connectivity
MCG Master Cell Group
MN Master Node
MR-DC Multi-Radio Dual Connectivity
NE-DC NR-E-UTRA Dual Connectivity
NGEN-DC NG-RAN E-UTRA-NR Dual Connectivity
NR-DC NR-NR Dual Connectivity
SCG Secondary Cell Group
SN Secondary Node
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.
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].
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.
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S
S
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. The ng-eNB is connected to the 5GC and the gNB is connected to the
ng-eNB via the Xn interface.
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. The gNB is connected to 5GC and the ng-eNB is connected to the gNB via the
Xn interface.
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. The master gNB is connected to the 5GC via the NG interface and to the secondary gNB
via the Xn interface. The secondary gNB might also be connected to the 5GC via the NG-U interface. In addition, NR-
DC can also be used when a UE is connected to two gNB-DUs, one serving the MCG and the other serving the SCG,
connected to the same gNB-CU, acting both as a MN and as a SN.
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.
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
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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.
Split SRB is supported for all MR-DC options, allowing duplication of RRC PDUs generated by the MN, via the direct
path and via the SN. Split SRB uses NR PDCP. This version of the specification does not support the duplication of
RRC PDUs generated by the SN via the MN and SN paths.
In EN-DC, the SCG configuration is kept in the UE during suspension. The UE releases the SCG configuration (but not
the radio bearer configuration) during resumption initiation.
In MR-DC with 5GC, the UE stores the PDCP/SDAP configuration when moving to RRC Inactive but it releases the
SCG configuration.
Figure 4.2.1-1: Control plane architecture for EN-DC (left) and MR-DC with 5GC (right).
4.2.2 User Plane
In MR-DC, from a UE perspective, three bearer types exist: MCG bearer, SCG bearer and split bearer. These three
bearer types are depicted in Figure 4.2.2-1 for MR-DC with EPC (EN-DC) and in Figure 4.2.2-2 for MR-DC with 5GC
(NGEN-DC, NE-DC and NR-DC).
For EN-DC, the network can configure either E-UTRA PDCP or NR PDCP for MN terminated MCG bearers while NR
PDCP is always used for all other bearers.
In MR-DC with 5GC, NR PDCP is always used for all bearer types. In NGEN-DC, E-UTRA RLC/MAC is used in the
MN while NR RLC/MAC is used in the SN. In NE-DC, NR RLC/MAC is used in the MN while E-UTRA RLC/MAC
is used in the SN. In NR-DC, NR RLC/MAC is used in both MN and SN.
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Figure 4.2.2-1: Radio Protocol Architecture for MCG, SCG and split bearers from a UE perspective in
MR-DC with EPC (EN-DC)
QoS
Flows
SDAP
MCG
Split SCG
Bearer
Bearer Bearer
NR PDCP NR PDCP NR PDCP
MN RLC MN RLC SN RLC SN RLC
MN MAC SN MAC
UE
Figure 4.2.2-2: Radio Protocol Architecture for MCG, SCG and split bearers from a UE perspective in
MR-DC with 5GC (NGEN-DC, NE-DC and NR-DC).
From a network perspective, each bearer (MCG, SCG and split bearer) can be terminated either in MN or in SN.
Network side protocol termination options are shown in Figure 4.2.2-3 for MR-DC with EPC (EN-DC) and in Figure
4.2.2-4 for MR-DC with 5GC (NGEN-DC, NE-DC and NR-DC).
NOTE 1: Even if only SCG bearers are configured for a UE, for SRB1 and SRB2 the logical channels are always
configured at least in the MCG, i.e. this is still an MR-DC configuration and a PCell always exists.
NOTE 2: If only MCG bearers are configured for a UE, i.e. there is no SCG, this is still considered an MR-DC
configuration, as long as at least one of the bearers is terminated in the SN.
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Figure 4.2.2-3: Network side protocol termination options for MCG, SCG and split bearers in MR-DC
with EPC (EN-DC).
QoS QoS
Flows
Flows
SDAP SDAP
Split
MCG SCG SCG
Split MCG
Bearer
Bearer Bearer
Bearer
Bearer Bearer
NR PDCP NR PDCP NR PDCP NR PDCP NR PDCP NR PDCP
Xn
MN MN
SN SN
MN RLC MN RLC SN RLC SN RLC
RLC RLC RLC RLC
MN MAC SN MAC
MN SN
Figure 4.2.2-4: Network side protocol termination options for MCG, SCG and split bearers in MR-DC
with 5GC (NGEN-DC, NE-DC and NR-DC).
4.3 Network interfaces
4.3.1 Control Plane
4.3.1.1 Common MR-DC principles
In MR-DC, there is an interface between the MN and the SN for control plane signalling and coordination. For each
MR-DC UE, there is also one control plane connection between the MN and a corresponding CN entity. The MN and
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the SN involved in MR-DC for a certain UE control their radio resources and are primarily responsible for allocating
radio resources of their cells.
Figure 4.3.1.1-1 shows C-plane connectivity of MN and SN involved in MR-DC for a certain UE.

Figure 4.3.1.1-1: C-Plane connectivity for EN-DC (left) and MR-DC with 5GC (right).
4.3.1.2 MR-DC with EPC
In MR-DC with EPC (EN-DC), the involved core network entity is the MME. S1-MME is terminated in MN and the
MN and the SN are interconnected via X2-C.
4.3.1.3 MR-DC with 5GC
In MR-DC with 5GC (NGEN-DC, NE-DC and NR-DC), the involved core network entity is the AMF. NG-C is
terminated in the MN and the MN and the SN are interconnected via Xn-C.
4.3.2 User Plane
4.3.2.1 Common MR-DC principles
There are different U-plane connectivity options of the MN and SN involved in MR-DC for a certain UE, as shown in
Figure 4.3.2.1-1. The U-plane connectivity depends on the bearer option configured:
- For MN terminated bearers, the user plane connection to the CN entity is terminated in the MN;
- For SN terminated bearers, the user plane connection to the CN entity is terminated in the SN;
- The transport of user plane data over the Uu either involves MCG or SCG radio resources or both:
- For MCG bearers, only MCG radio resources are involved;
- For SCG bearers, only SCG radio resources are involved;
- For split bearers, both MCG and SCG radio resources are involved.
- For split bearers, MN terminated SCG bearers and SN terminated MCG bearers, PDCP data is transferred
between the MN and the SN via the MN-SN user plane interface.
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Figure 4.3.2.1-1: U-Plane connectivity for EN-DC (left) and MR-DC with 5GC (right).
4.3.2.2 MR-DC with EPC
For MR-DC with EPC (EN-DC), X2-U interface is the user plane interface between MN and SN, and S1-U is the user
plane interface between the MN, the SN or both and the S-GW.
4.3.2.3 MR-DC with 5GC
For MR-DC with 5GC (NGEN-DC, NE-DC and inter-gNB NR-DC), Xn-U interface is the user plane interface between
MN and SN, and NG-U is the user plane interface between the MN, the SN or both and the UPF.
5 Layer 1 related aspects
In MR-DC, two or more Component Carriers (CCs) may be aggregated over two cell groups. A UE may simultaneously
receive or transmit on multiple CCs depending on its capabilities. The maximum number of configured CCs for a UE is
32 for DL and UL. Depending on UE's capabilities, up to 31 CCs can be configured for an E-UTRA cell group when
the NR cell group is configured. For the NR cell group, the maximum number of configured CCs for a UE is 16 for DL
and 16 for UL.
A gNB may configure the same Physical Cell ID (PCI) to more than one NR cell it serves. To avoid PCI confusion for
MR-DC, NR PCIs should be allocated in a way that an NR cell is uniquely identifiable by a PCell identifier. This PCell
is in the coverage area of an NR cell included in the MR-DC operation. In addition, NR PCIs should only be re-used in
NR cells on the same SSB frequency sufficiently distant from each other. X2-C/Xn-C signalling supports
disambiguation of NR PCIs by including the CGI of the PCell in respective X2AP/XnAP messages (e.g. SGNB
ADDITION REQUEST/S-NODE ADDITION REQUEST) and by providing neighbour cell relationship via non-UE
associated signaling (e.g. via the Xn Setup procedure or the NG-RAN node Configuration Update procedure).
6 Layer 2 related aspects
6.1 MAC Sublayer
In MR-DC, the UE is configured with two MAC entities: one MAC entity for the MCG and one MAC entity for the
SCG. The serving cells of the MCG other than the PCell can only be activated/deactivated by the MAC Control
Element received on MCG, and the serving cells of the SCG other than PSCell can only be activated/ deactivated by the
MAC Control Element received on SCG. The MAC entity applies the bitmap for the associated cells of either MCG or
SCG. PSCell in SCG is always activated like the PCell (i.e. deactivation timer is not applied to PSCell). With the
exception of PUCCH SCell, one deactivation timer is configured per SCell by RRC.
In MR-DC, semi-persistent scheduling (SPS) resources can be configured on both PCell and PSCell.
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In MR-DC, the BSR configuration, triggering and reporting are independently performed per cell group. For split
bearers, the PDCP data is considered in BSR in the cell group(s) configured by RRC.
In MR-DC, separate DRX configurations are provided for MCG and SCG.
6.2 RLC Sublayer
Both RLC AM and UM can be configured for MR-DC, for all bearer types (MCG, SCG and split bearers).
6.3 PDCP Sublayer
In EN-DC, CA duplication (see [3]) can be applied in the MN and in the SN, but MCG bearer CA duplication can be
configured only in combination with E-UTRAN PDCP and MCG bearer CA duplication can be configured only if DC
duplication is not configured for any split bearer.
In NGEN-DC, CA duplication can only be configured for SCG bearer. In NE-DC, CA duplication can only be
configured for MCG bearer. In NR-DC, CA duplication can be configured for both MCG and SCG bearers.
In MR-DC, RoHC (as described in TS 36.323 [15] and TS 38.323 [16]) can be configured for all the bearer types.
6.4 SDAP Sublayer
In MR-DC with 5GC, the network may host up to two SDAP protocol entities for each individual PDU session, one for
MN and another one for SN (see clause 8.1). The UE is configured with one SDAP protocol entity per PDU session.
7 RRC related aspects
7.1 System information handling
In MR-DC, the SN is not required to broadcast system information other than for radio frame timing and SFN. System
information for initial configuration is provided to the UE by dedicated RRC signalling via the MN. The UE acquires, at
least, radio frame timing and SFN of SCG from the PSS/SSS and MIB (if the SN is an eNB) / NR-PSS/SSS and PBCH
(if the SN is a gNB) of the PSCell.
Additionally, upon change of the relevant system information of a configured SCell, the network releases and
subsequently adds the concerned SCell (with updated system information), via one or more RRC reconfiguration
messages sent on SRB1 or SRB3, if configured.
7.2 Measurements
If the measurement is configured to the UE in preparation for the Secondary Node Addition procedure described in
clause 10.2, the Master node should configure the measurement to the UE.
In case of the intra-secondary node mobility described in clause 10.3, the SN should configure the measurement to the
UE in coordination with the MN, if required.
The Secondary Node Change procedure described in clause 10.5 can be triggered by both the MN (only for inter-
frequency secondary node change) and the SN. For secondary node changes triggered by the SN, the RRM
measurement configuration is maintained by the SN which also processes the measurement reporting, without providing
the measurement results to the MN.
Measurements can be configured independently by the MN and by the SN (intra-RAT measurements on serving and
non-serving frequencies). The MN indicates the maximum number of frequency layers and measurement identities that
can be used in the SN to ensure that UE capabilities are not exceeded. If MN and SN both configure measurements on
the same carrier frequency then the configurations need to be consistent (if the network wants to ensure these are
considered as a single measurement layer). Each node (MN and SN) can configure independently a threshold for the
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SpCell quality. In (NG)EN-DC scenario, when the PCell quality is above the threshold configured by the MN, the UE is
still required to perform inter-RAT measurements configured by the MN on the SN RAT (while it's not required to
perform intra-RAT measurements); when the PSCell quality is above the threshold configured by the SN, the UE is not
required to perform measurements confi
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