NR-NR Dual Connectivity (NR-DC) Explained

What is NR-NR Dual Connectivity (NR-DC)?

NR-DC is a variant of Dual Connectivity where the UE is connected to two gNBs:

• One gNB acts as a Master Node (MN).
• One gNB acts as a Secondary Node (SN).
• The Master Node is connected to the 5GC.
• The Secondary Node is connected to the Master Node via the Xn interface.

Typically, NR-DC is deployed between FR1 (Sub-6 GHz) and FR2 (mmWave) bands.

Prerequisite: UE support for NR-DC is required before the network can configure the UE with NR-DC Configuration.

In this configuration, the IE CellGroupId is used to identify a cell group:

• Value 0: Identifies the Master Cell Group (MCG).
• Other Values: Identify Secondary Cell Groups (SCG).
• Note: In Rel-15 version of the specification, only values 0 and 1 are supported.

Multi Radio Dual Connectivity (MR-DC) Protocol Architecture

The network can ask the 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

A UE in NR-DC mode can be configured with Data Radio Bearers (DRBs) and Signalling Radio Bearers (SRBs) on both MN and SN nodes.

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

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.
• Constraint: 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 the network:

1. A measConfig associated with MCG, included in the RRCReconfiguration message received via SRB1.
2. A measConfig associated with SCG, included in the RRCReconfiguration message received via SRB3, or embedded in an RRCReconfiguration message received via SRB1.

Reporting:

• With SRB3: If SRB3 is configured and a measurement report is triggered for the SCG, UE sends the MeasurementReport message via SRB3.
• Without SRB3: If SRB3 is not configured, UE sends MeasurementReport message via SRB1 embedded in the NR RRC message ULInformationTransferMRDC of the PCell.

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

Key IEs for NR-DC

RRCReconfiguration IEs

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

RRCResume IEs

• mrdc-SecondaryCellGroup: Includes an RRC message for SCG configuration in NR-DC or NE-DC.
• radioBearerConfig2: Configuration of Radio Bearers (DRBs, SRBs) including SDAP/PDCP.

Summary

NR-NR Dual Connectivity (NR-DC) involves two gNBs acting as Master gNB and Secondary gNB.

• Rel-15 Scope: NR-DC in Rel-15 only includes the scenarios where all serving cells in MCG are in FR1 and all serving cells in SCG are in FR2.