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Failures in 5G Implementation – Part 2
In the last blog, we discussed the failures in 5G implementation encountered while validating devices or base stations in the ENDC mode. We summed up issues that can arise due to incompatibility in the UE and Network capabilities, incorrect measurement configuration and report exchanged and problems in the CG-config sent by the base station to the UE. In continuation of that, in this article, we will discuss the security parameter issues, RACH parameter issues, and SCG failure issues.
Failure Cause #4: Security Configuration
Even if the UE reads the RRC reconfiguration message properly, the UE wouldn’t attempt RACH on NR cell if the UE does not support all RRC parameters configured by the network e.g. security parameter. If NIA0 is disabled at the gNB for regulatory requirements and the gNB receives the UE 5G security capabilities to contain only NIA0 for integrity protection algorithms, the gNB shall reject the emergency attach request from the UE.
Failure Cause #5: RACH Parameters
During testing, if the RACH parameter is not configured properly, UE will not attempt RACH on NR cells. For example, if one configures prach-configindex for short preamble length but with that configures Prach-Root Sequence Index for larger preamble length l839 instead of short preamble length l139.
In some cases, one can face issues where the UE sends Msg1 to the network but the network does not receive Msg1 from the UE side. This can happen due to multiple reasons e.g. due to the bad radio condition at the network or UE uplink power is not sufficient to transmit msg1. There could be a lot of CRC failures on the downlink, resulting in the UE to not decode the message from the base station correctly. Improper radio apparatus in between the UE and the base station or incorrect configuration of radio power at the UE or gNB can be the cause of such issues.
Most of PRACH fail because the UE sends preambles repetitively but doesn’t receive Random Access Response from PScell. In some cases, the target NR cells fail to give the RAPID (Random Access Preamble Identity) to UE or RACH fails due to mismatch of RAPID (Network build issue).
Failure Cause #6: SCG Failures
Other types of failures in 5G implementation can also be encountered. NR PScell addition happens after the UE receives the RRC Connection Reconfiguration message from the LTE network. In the case of the NSA mode, devices send SCG failure on the LTE network only. When this happens, the device sends SCG Failure Information message with various failure causes as given below. This is based on 38.331 126.96.36.199 v15.4 or higher.
Let’s take a few examples:
Suppose UE is in connected mode with MN and SN but not UL synchronized (timeAlignmentTimer expired before RRC Inactivity timer). Either UE wants to transfer any UL data, or the network has data to be sent as DL data or UE wants to send DL HARQ feedback to the network. In all the cases, the UE initiates the RACH procedure and if RACH gets failed on NR cell and RLF occurs, UE triggers the SCG failure with the cause randomAccessProblem.
Now let us take another example of maximum RLC retransmission. Let’s say for the secondary cell group, the RLC retransmission reaches the maximum threshold value set by the network. In such a case, the UE triggers SCG failure to master node with the cause rlc-MaxNumRetx. If a similar thing happens at the master node(LTE) RLC layer, a re-establishment procedure is triggered.
In short, if problems occur on the MN node (in case of ENDC, MCG is LTE), the UE triggers the Re-establishment procedure. However, if the problem occurs on the SN node (in case of ENDC, SCG is NR), the UE triggers the SCG failure with proper failure type.
In these two blogs, we briefly mentioned the different kinds of failures in 5G implementation which one can encounter during the initial deployment. In future blogs, we will cover more such issues. In this long and packed journey with a target of delivering live 5G services, we at Simnovus provide a helping hand to NEMs and mobile network operators to address the issues they face by offering cost-effective test and measurement solutions.