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Failures in 5G Implementation – Part 1
In the whitepaper, we discussed the different deployment modes in 5G. We also discussed the importance of the 5G NSA and SA deployment modes. The ENDC mode requires coordination between the existing LTE nodes and new 5G nodes (e.g. gNB), a new radio is introduced, and several new messages have been introduced along with many modifications to existing messages. Consequently, one faces many failures in 5G implementation during the early days. In the conformance and feature testing, most often one faces configuration issues from the network side and the UE side. In this blog, some of those failures in 5G implementation are explored which one faces while testing in the ENDC mode.
Attach Failure Cause #1: Issues in UE and Network Capabilities
In the 5G NSA mode, the master cell group (MCG) is LTE eNB and the secondary cell group (SCG) is en-gNB. If the network supports ENDC, then in the SIB2 message, UE can find PLMN-Info-r15 value as true. After the cell selection is complete, UE sends the Attach request to the network. In this message, the UE additional security capability IE is present for 5G, and the DCNR bit in UE network capability IE is set. If this bit is not set, the network does not send the ENDC configuration. These are the basic things one needs to check for all the devices.
Then eNodeB asks the UE for EUTRA-NR (ENDC) UE capabilities information. The UE reports that it supports EUTRA-NR radio access technology. EUTRA-NR specific capabilities are specified in UE-MRDC-capability containers. During some specific feature testing, one should make sure that UE supports the particular feature in the UE-MRDC-capability container.
Attach Failure Cause #2: Incorrect Measurement Configuration and Report
In a real-time scenario, it is expected that the MCG (eNB) adds the SCG only when the UE provides a good result for it in the measurement report. When it comes to measurement, it is important what kind of measurement events should be used. Since it is inter-frequency/inter-RAT, there has to be a measurement gap.
The UE initially connects with the LTE network and the SCG is added using two different methods.
- Adding NR cell based on B1 based measurement event
- Adding collocated NR cells like CA without a measurement event
During the validation phase, the UE may not generate measurement report information to the network for SCG. In that case, it is important to check the RRC reconfiguration message for the B1 event whether the configuration is right or wrong for IEs like b1-threshold-r15, hysteresis, time to trigger, maxReportCells, report interval and report amount. And also check if the measurement gap (GP value) configuration is correct for the IE named gpOffset. A lot of new gap patterns have been introduced for NR measurement. Below are some snapshots from Simnovator, the UE simulator, where the important IEs are highlighted with valid and invalid values.
Fig: Important IEs in the RRC Connection Reconfiguration Message
Fig: Invalid IEs in the RRC Connection Reconfiguration Message
For the conducted mode setup, ensure that there is no power loss or cable loss while performing the testing. In OTA mode, due to the bad RF conditions, UE may not send measurement reports.
Attach Failure Cause #3: Issues in RACH procedure with NR cell
Once the UE reports the measurement value to the network, the network sends the CG-configuration value in the RRC reconfiguration message to add SCG. However, due to missing neighbour cell configuration at the eNB, it may not send the CG-configuration for SCG.
In the CG-config message, a UE can find cellGroupID, RLC and MAC configuration, PHR and BSR configuration, servCellindex, physCellid, DownlinkConfigCommon, initialDownlinkBWP, initialUplinkBWP, RACH configuration, RLF timersAndConstants, slot configuration, DRB-ToAddMod IEs, securityConfig, and more than one RLC information.
UE detects the SSB (PSS, SSS, and PBCH) of NR gNB. Once it successfully detects SSB of NR gNB, it performs the PRACH procedure to PSCell of the SN (NR gNB). A UE acquires all the information required for the RACH procedure from the RRC Connection Reconfiguration message instead of SIB in the case of ENDC.
However, the above may not happen if a UE doesn’t read the SSB properly either due to UE build issues, or maybe due to the wrong network configuration of SSB.
Apart from these, one can face other failures in 5G implementation like security parameter issues, RACH failures, and SCG failures that will be discussed in the next blog. Considering the massive scope of 5G networks and the wide range of failures that may be encountered, it is vital to focus on unique test and measurement solutions. Simnovus UE Simulator (Simnovator) and NW Emulator (Callbox) offer a simplified and cost-effective approach to 5G testing that will significantly help operators, manufacturers, and equipment suppliers to overcome these failures and validate their configurations, software, and hardware for superior performance.