Here, we aim to cover the EN-DC (EUTRA-NR Dual connectivity) initial access case.
Bullet Point for EN-DC case.
1. LTE is the main anchor (master) node.
2. First UE do LTE attach as like normal legacy LTE system.
3. After LTE attach, n/w send the rrc reconfiguration message where it provide the detail of NR cell.
N/w may or may not configure the NR cell measurement before adding the NR cell. In case of blind addition, n/w directly add the NR cell w/o getting any measurement result whereas in other case, 1st n/w configure the NR measurement then UE do measurement for NR cell as IRAT measurement and accordingly send the MR result to n/w based on the MR reporting configuration. After getting the MR result, n/w decide to add the NR cell.
Below is the sample signalling between UE & n/w for the step 1-3
Sample decoding of UECapabilityEnquiry:
message
c1 : ueCapabilityEnquiry :
{
rrc-TransactionIdentifier 0,
criticalExtensions c1 :
ueCapabilityEnquiry-r8 :
{
ue-CapabilityRequest
{
eutra-nr,
nr
},
eutra-nr ==> Capability for this is defined under UE-MRDC-Capability container under UECapabilityInformation. There are many important things defined under this container, example - EUTRA & NR supported band combination.
Sample extracted from (UE-MRDC-Capability)
supportedBandCombinationList
{
{
bandList
{
eutra :
{
bandEUTRA 1,
ca-BandwidthClassDL-EUTRA a,
ca-BandwidthClassUL-EUTRA a
},
nr :
{
bandNR 78,
ca-BandwidthClassDL-NR a,
ca-BandwidthClassUL-NR a
}
},
nr ==> NR related capability is under UE-NR-Capability container.
After UE capability enquiry, there is RRC connection reconfiguration with LTE attach accept detail and measurement configuration of NR cell.
Below is sample decoding of "RRC connection reconfiguration" for measurement configuration and DRB configuration
{
measObjectId 2,
measObject measObjectNR-r15
:
{
carrierFreq-r15 643333,
rs-ConfigSSB-r15
{
measTimingConfig-r15
{
periodicityAndOffset-r15 sf20-r15 : 18,
ssb-Duration-r15
sf5
},
subcarrierSpacingSSB-r15 kHz30
},
quantityConfigSet-r15
1,
bandNR-r15 setup : 78
}
},
{
reportConfigId 3,
reportConfig
reportConfigInterRAT :
{
triggerType event :
{
eventId
eventB1-NR-r15 :
{
b1-ThresholdNR-r15 nr-RSRP-r15 : 46,
reportOnLeave-r15 FALSE
},
hysteresis 2,
timeToTrigger ms80
},
maxReportCells 4,
reportInterval ms120,
reportAmount infinity,
reportQuantityCellNR-r15
{
ss-rsrp TRUE,
ss-rsrq TRUE,
ss-sinr FALSE
}
}
},
{
measId 3,
measObjectId 2,
reportConfigId 3
},
Please note the marked DRB & EPS ID. Here MCG bearer (DRB ID - 3) is established with LTE-PDCP. MCG bearer can also be established with NR-PDCP also.
This is important, why will discuss during NR cell addition.
After this NR measurement configuration, UE send the MR report with NR cell result and finally based on this MR result n/w decide to add the NR cell.
In this RRC connection reconfiguration (with NR cell addition info), n/w can do either a standalone SCG bearer or split bearer. Will discuss Bearer type with detail in another section.
In this current context, i am taking example of split bearer.
Brief about split bearer: PDCP is common for both bearer (MCG & SCG) and RLC/MAC/PHY is separate for MCG & SCG bearer. In case of split bearer, PDCP is always NR.
During LTE attach, DRB is configured with LTE PDCP as marked highlighted (yellow) so now during NR cell addition this bearer is reconfigured with NR-PDCP. Refer below snippet for the same
Here existing DRB(3) is reconfigured with NR-PDCP
Below Snippet shows the Secondary cell Group (cellGroupID - 1), it also shows SCG bearer RLC entity is associated with the same DRB ID -3. So there after both MCG & SCG bearer is associated with the same PDCP entity (NR-PDCP) and hence this become case of split bearer.
4. In EN-DC case, all the required detail to access the NR cell are provided via dedicated signalling (rrc reconfig msg) only as compare to SA (Standalone Mode) where UE gets initial few basic information via MIB/SIB1 signaling.
Refer the below major top level IE's of RRC reconfiguration message used to add the NR cell when LTE is master.
 |
This is the Snippet of RRC Reconfiguration message used to add NR cell. NR cell configuration added under NR_Config_15 --> NR-SecondaryCellGroupConfig_r15
|
In above Picture, During NR cell addition major focused IE's are NR_Config_r15 and NR_SceondaryCellGroupConfig_r15.
Below figure depicted the expansion of
NR-SecondaryCellGroupConfig_r15
 |
Expansion of NR-SecondaryCellGroupConfig-r15 IE
|
There are three major IE for NR cell.
1. RadioBearerConfig
2. SecondaryCellGroup ==> This is our focus for the current article
3. MeasConfig
Below figure depicted the expansion of secondaryCellGroup IE further.
 |
| Secondary Cell Group IE structure |
Conclusion So far:
1. We have expanded major IE's which are used to add the NR cell in en-dc case.
2. How existing LTE RRC reconfiguration adapted to add the NR cell related stuff, generally we say it 5G container. 5G container has been fitted in exiting LTE RRC reconfig and then we can further expand this 5G container to see 5G related IE's
What Next: Discussion of major NR cell parameters
1. CellGroupID - The IE CellGroupId is used to identify a cell group. 0 identifies the master cell group. Other values identify secondary cell groups. In 3GPP rel - 15 of the specification only
values 0 and 1 are supported.
2. spCellConfig - sPcell is special cell for the defined group, in more convenient way, it is primary cell of the mentioned group (Pcell of MCG and PScell of SCG). PScell is primary cell of the Secondary Cell Group.
3. ServCellIndex: As depicted in the above figure, it is serving cell ID of PScell. Serving cell ID for MCG group is always 0. ServCellIndex is different from Pcell ID. For every NR cell (PScell & Scell) n/w gives both ServCellIndex and Pcell ID. ServCellIndex is just an identifier to bound different IE with a particular cell.
4. ReconfWithSync: This field is mandatory present in case of SpCell change, PSCell addition and security key change otherwise optionally present. Major IE under this is spcellConfigCommon.
5. ServingCellConfigCommon: Under this, all cell specific parameter for UE serving cell are defined. This IE contains all the information which a UE would typically acquire from SSB, MIB and SIB when accessing the cell in Idle state. Few important parameters ex - Primary cell ID, SSB ARFCN, sub carrier spacing, Reference Point A, Operating band of serving cell. In SA mode, all these information received via SSB, MIB and SIB etc. Here in case of EN-DC, all these cell specific parameters also conveyed via dedicate signalling only
