NR-NR Dual Connectivity (NR-DC)

NR-DC is the variant when the UE is connected to one gNB that acts as a Master Node and one gNB that acts as a Secondary Node. The Master Node is connected to 5GC while the Secondary Node is connected to Master Node via the Xn interface.

Typically NR-DC is deployed between FR1-FR2 considering two bands configuration, UE support is required before the network can configure the UE with NR-DC Configuration.
In the NR-DC Configuration, the IE CellGroupId is used to identify a cell group. Value 0 identifies the master cell group(MCG), Value, Other values identify secondary cell groups(SCG). In the Rel-15 version of the specification only values 0 and 1 are supported.

Multi Radio Dual Connectivity (MR-DC) Protocol architecture(1)

The network can ask UE to report if it is NR-DC capable by setting the flag “includeNR-DC” in the IE UE-CapabilityRequestFilterCommon in UE Capability Inquiry.

UE-CapabilityRequestFilterCommon ::=  SEQUENCE {
mrdc-Request  SEQUENCE {
omitEN-DC                   ENUMERATED {true} OPTIONAL,  -- Need N
includeNR-DC                ENUMERATED {true} OPTIONAL,  -- Need N
includeNE-DC                ENUMERATED {true} OPTIONAL   -- Need N
} OPTIONAL,                                              -- Need N
...,

includeNR-DC: Only if this field is present, the UE supporting NR-DC shall indicate support for NR-DC in-band combinations and include feature set combinations that are applicable to NR-DC.

Signalling Radio Bearers in NR-DC

As shown in the above Image, A UE in NR-DC mode can be configured with Data Radio bearers and Signalling radio bearers on both MN and SN nodes.

Signaling between UE and secondary node is done via SRB3 using DCCH logical channel. SRB3 is also used when UE is in (NG)EN-DC mode. The network can also configure Split-SRB where SRB can be used between the Master node and Secondary node. When UE is configured with NR-DC mrdc-SecondaryCellGroup in RRC Reconfiguration message is set to nr-SCG

Radio Bearers in NR-DC

Radio Link Monitoring in NR-DC :

When UE is in NR-DC Mode, UE Can be configured to monitor Radio Link for both Pcell and Pscell. The UE should be able to monitor the downlink radio link quality based on the reference signal configured as RLM-RS resource(s) in order to detect the downlink radio link quality of the PCell and PSCell.
The network can configure all SSBs, or all CSI-RSs, or a mix of SSBs and CSI-RSs as RLM-RS resources.
UE is not expected to perform RLM outside the active DL BWP, it is expected to only perform inside the active DL BWP.

Measurements in NR-DC

The network can configure Measurement Configuration for both Pcell and Pscell when UE is in NR-DC Connected mode.
When UE is in NR-DC, the UE may receive two independent measConfig from network:

  • a measConfig, associated with MCG, that is included in the RRCReconfiguration message received via SRB1; and
  • a measConfig, associated with SCG, that is included in the RRCReconfiguration message received via SRB3, or, alternatively, included within a RRCReconfiguration message embedded in a RRCReconfiguration message received via SRB1.

When a UE is in NR-DC mode with SRB3 Configured and if a measurement report is triggered for the MeasConfig associated with the SCG, UE sends the MeasurementReport message via SRB3. If SRB3 is not configured and if a measurement report is triggered for the MeasConfig associated with the SCG, UE sends MeasurementReport message via SRB1 embedded in NR RRC message ULInformationTransferMRDC of the Pcell.

A UE in NR-DC Mode configured with SRB3 detects MCG Failure then it sends MCGFailureInformation message t embedded in NR RRC message ULInformationTransferMRDC via SRB3 on the secondary node

RRCReconfiguration-IE’s

mrdc-SecondaryCellGroup
Includes an RRC message for SCG configuration in NR-DC or NE-DC. For NR-DC (nr-SCG), mrdc-SecondaryCellGroup contains the RRCReconfiguration message as generated (entirely) by SN gNB. In this version of the specification, the RRC message can only include fields secondaryCellGroup, otherConfig, conditionalReconfiguration and measConfig.

radioBearerConfig2
Configuration of Radio Bearers (DRBs, SRBs) including SDAP/PDCP. This IE can only be used if the UE supports NR-DC or NE-DC.

secondaryCellGroup
Configuration of secondary cell group ((NG)EN-DC or NR-DC).

scg-Response
In case of NR-DC (nr-SCG-Response), this field includes the RRCReconfigurationComplete message. In case of NE-DC (eutra-SCG-Response), this field includes the E-UTRA RRCConnectionReconfigurationComplete message

RRCResume-IEs

mrdc-SecondaryCellGroup
Includes an RRC message for SCG configuration in NR-DC or NE-DC.
For NR-DC (nr-SCG), mrdc-SecondaryCellGroup contains the RRCReconfiguration message as generated (entirely) by SN gNB.

radioBearerConfig2
Configuration of Radio Bearers (DRBs, SRBs) including SDAP/PDCP. This IE can only be used if the UE supports NR-DC or NE-DC.

What is NR-DC Mode?

NR-NR dual connectivity which involves two gNB acting as Master gNB and Secondary gNB is referred to as “NR-NR dual connectivity” or “NR-DC”. NR-DC in Rel-15 only includes the scenarios where all serving cells in MCG in FR1 and all serving cells in SCG in FR2.