Key points for 5G (Non Stand-Alone) - LTE eNB - 5G gNB dual connectivity (EN-DC)
1. Introduction
E-UTRAN New Radio - Dual Connectivity (EN-DC) is a technology that enables introduction of 5G services and data rates in a predominantly 4G network.
UEs supporting EN-DC can connect simultaneously to LTE Master Node eNB (MN-eNB) and 5G-NR Secondary Node gNB (SN-gNB).
This approach permits cellular providers to roll out 5G services without the expense of a full scale 5G Core Network.
5G gNBs can be introduced early in areas with high traffic congestion.
An EN-DC enabled UE first registers for service with the 4G EPC.
The UE also starts reporting measurements on 5G frequencies.
If the signal quality for the UE will support a 5G service, the LTE eNB communicates with the 5G-NR gNB to assign resources for a 5G bearer.
The 5G-NR resource assignment is then signalled to the UE via an LTE RRC Connection Reconfiguration message.
Once the RRC Connection Reconfiguration procedure is completed, the UE simultaneously connects to the 4G and 5G networks.
2. SIB-2 Indication
If the 4G network supports EN-DC, the first indication that it gives to the UE is through SIB-2.
The SIB2 broadcast from the MN-eNB signals the presence of 5G-NR PLMNs via the PLMN-InfoList-r15
3. DC-NR Enabled in Attach Request Message
Now it’s the job of UE to send indication to the network if it supports DC-NR.
This is done through Attach Request message that UE sends during 4G attach procedure, where an IE is sent stating the capability to support DC-NR.
The value of this IE should be 1 to enable.
4. Restrict DCNR
If the network restricts NR request of the UE, it sends an Information element “Restrict DCNR” supported in Attach Accept message.
5. UE capabilities
During DC-NR configuration, Network always send two UE capability Enquiry, first for legacy network capabilities and second for the EutraNR(NSA)/NR(SA) capabilities
Important Information Elements that goes in UE capability Information message for NR
a) Supported bandwidth by 5G bands: Check the second UE capability information sent by UE for 5G-nr and look for supported bandwidth information element for a specific band for both DL and UL.
b) MIMO support for the NR band:In first UE capability information, check for r-15 message with nr band information.
c) Modulation Type (256QAM, 64QAM etc): to check what type of modulation the NR band supports for both DL and UL.
d) Subcarrier spacing: This IE element determines the carrier spacing and the value for it will be-15Khz, 30Khz, 60 KHz or 120 Khz. Depending on the network support, this value changes. For LTE network, subcarrier spacing is 15Khz. For NR, it varies from 30 to 120 Khz. To check what Subcarrier spacing is supported by the network, second check UE capability Information message, under feature set DL/UL CC.
e) DC-NR support: To check if the UE is supporting EN-DC, look for below IE in first UE Capability Information message under r-15.
f) EUTRAN band combinations with NR Bands: To check all the NR band combinations with eutra bands, look into the second UE capability Information message. The decoded message for eutra-nr will contain all the band combinations.
6. RRC Connection Reconfiguration Message
This message is used by LTE network to configure measurement report to read NR cell, add/release NR for dual connectivity, while adding configure NR IE etc.
a) RRC Connection Reconfiguration message to enable NR cell Measurement Reporting:During Eutra attach procedure, eNB sends an RRC Connection Reconfiguration message that includes measurement objects for 5G NR frequencies.
This procedure is important for NR Addition.The message below are the information elements that initiates measurement reporting by UE for NR cell:
- Firstly, measObjectID is created for r-15 frequencies. You can see this IE under measConfig:
This message can also be used to observe NR cell carrier frequency, band information, subcarrier spacing etc - Secondly, Under ReportConfigToAddModList in the same message, configuration for measurement reporting are set under a reportConfig ID.
Configuration always initiates the measurement reporting based on an event that is set. - Thirdly, another MeasID is created to link reportConfig ID and measObject ID in the same RRC Connection Reconfig message:
b) Measurement Reporting for NR
After Measurement report configuration in RRC Connection Reconfiguration message, UE sends a measurement report containing NR cell RSRP values.
c) RRC Connection Reconfiguration message for NR Cell Addition
The 4G eNodeB sends an RRC Connection Reconfiguration to the UE. The message assigns 5G radio resources to the UE. This message can be seen just after measurement report for 5G NR is sent by the UE.
The UE after receiving the configuration message, if it agrees with the configuration, responds with a RRC Connection reconfiguration complete message.
LTE Events Summary
- Event A1: Serving cell becomes better than the threshold.
- Event A2: Serving cell becomes worse than the threshold
- Event A3: Neighbour cell becomes (including offset) better than the serving cell.
- Event A4: Neighbour cell becomes better than the threshold.
- Event A5: serving cell becomes worse than Thresh 1 & the neighbour cell becomes better than thresh 2
- Event A6: Neighbour Becomes offset better than Scell (for CA)
- Event C1: CSI-RS resource becomes better than threshold.
- Event C2: CSI-RS resource becomes offset better than reference CSI-RS resources
- Event V1: the channel busy ratio is above threshold for a V2X devices
- Event V2: the channel busy ratio is below a threshold for a V2X devices
NR Events Summary
- Event A1: Serving becomes better than threshold. For this measurement, consider the serving cell to be the NR Spcell or the NR Scell that is
- configured on the frequency indicated in the associated measObjectNR.
- Event A2: Serving becomes worse than threshold. For this measurement, consider the serving cell to be the NR SpCell or NR Scell that is
- configured on the frequency indicated in the associated measObjectNR.
- Event A3: Neighbour becomes offset better than Spcell.
- Event A4: Neighbour becomes better than threshold.
- Event A5: Spcell becomes worse than threshold1 & neighbour becomes better than threshold 2
- Event A6: Neighbour becomes offset better than Scell.