4.5 Access Analysis
4.5.2 RRC Connection Establishment Failures
4.5.2.1 Procedure for Troubleshooting an RRC Connection Establishment Problem
An RRC connection establishment failure can be processed by using the UE signaling procedure and STS signaling trace tool, as shown in Figure 4-70.
Figure 4-70 Procedure for Troubleshooting an RRC Connection Establishment Problem
4.5.2.2 Cause Analysis UE sent RRC
Call signaling interruption because the call is originated from a poorly covered cell with weak signals
Uplink RACH problem
Paging failure during the TAU
Cell reselection parameter misconfiguration: The call is not originated from the best cell due to cell reselection time delay.
RS power and power allocation parameter misconfiguration
Traffic congestion
Equipment fault
It is highly likely that an RRC connection establishment failure may occur due to the following factors:
Weak signals in the downlink
Uplink RACH problem
Cell reselection parameter misconfiguration
Equipment fault
4.5.2.3 Solutions to Highly Probable RRC Connection Establishment Problems
To solve these highly probable problems, ZTE recommends the following solutions:
Perform the RF optimization to solve an undershooting or overshooting problem.
Optimize the TA edges to reduce unnecessary location update. If possible, it is best to include the TA edges in a sparsely populated area.
To ensure that the UE can reselect a preferable cell for originating the call, optimize the cell reselection parameters of the problematic cell.
Modify such random access and power allocation parameters as PRACH, PCCH, PDCCH, PDSCH, and Msg3 power offset, whenever necessary.
Modify the RS power to cover the cell radius as expected.
4.5.2.4 Failing to Receive the RRC Connection Request Message
Can you explain why the eNodeB fails to receive the RRC Connection Request message that is sent from the UE?
If the RSRP is relatively low in the downlink, you can infer that it may be caused by a coverage problem.
If the RSRP is not too low (-105 dBm or more) in the downlink, you can infer that it may be caused by an RACH problem.
This problem may usually be caused by these potential factors:
Insufficient power ramping level
Too low output power (UE)
eNodeB fault (too high VWSR)
Improper cell radius configuration
4.5.2.5 Failing to Receive the RRC Connection Setup Message
After receiving the RRC Connection Request message from the UE, the eNodeB sends the RRC Connection Setup message, but the UE fails to receive the RRC Connection Setup message. This problem may usually be caused by these potential factors:
Poor coverage
Inappropriate cell selection and reselection parameters
To solve this problem, ZTE recommends the following solutions:
If this problem is caused by poor coverage, ZTE recommends you enhance the coverage if conditions permit. For example, you can add certain sites or optimize the antenna and feeder system. If conditions do not permit, ZTE recommends you improve the RS power and adjust the corresponding power allocation parameters.
If this problem is caused by inappropriate cell selection and reselection parameters, ZTE recommends you adjust the corresponding cell selection and reselection parameters to speed up the cell selection and reselection procedure.
4.5.2.6 Delivering the RRC Connection Reject Message
After receiving RRC Connection Request message, the eNodeB delivers the RRC Connection Reject message to the UE. When finding the RRC
Connection Reject message, you need to check the specific cause value:
Congestion: In this case, you need to check the network usage.
Unspecified: In this case, you need to check the log information.
When receiving the RRC Connection Setup message, the UE fails to deliver the RRC Connection Setup Complete message. If the signals in the downlink are normal, you can infer that this problem may be caused by a handset fault.
When the UE delivers the RRC Connection Setup Complete message, the eNodeB fails to receive the RRC Connection Setup Complete message. There is a very small probability that this problem will occur because the transmit power of the UE will be increased through the initial uplink power control.
Temporarily, no good solution is readily available to this problem.
4.5.3 Authentication and Encryption Failures
When an authentication failure occurs, you need to analyze potential factors, depending on the cause value (MAC Failure or Synch Failure) carried in the Authentication Failure message that is sent from the UE to the MME.
4.5.3.1 MAC Failure
During the authentication procedure, the UE checks the AUTN parameter carried in the Authentication Request message that is sent from the MME.
When finding incorrect MAC information, the UE delivers the Authentication Failure message that carries the cause value (MAC Failure) to the MME, as shown in Figure 4-71.
Figure 4-71 Authentication Failure Message (Cause Value: MAC Failure)
UE MME
Stop T3418
AUTHENTICATION REQUEST
Start T3418 Stop T3460
AUTHENTICATION FAILURE (cause = "MAC failure")
Start T3460
IDENTITY REQUEST
Stop T3470 IDENTITY RESPONSE (IMSI)
Start T3470
AUTHENTICATION REQUEST
Stop T3460 AUTHENTICATION RESPONSE
Start T3460
This problem may usually be caused by these potential factors:
Illegal subscriber
Ki or OPc configuration inconsistency between the USIM and the HLR
4.5.3.2 Synch Failure
During the authentication procedure, the UE checks the SQN parameter carried in the Authentication Request message that is sent from the MME. When finding incorrect SQN information, the UE delivers the Authentication Failure message that carries the cause value (Synch Failure) to the MME, as shown in Figure 4-72.
Figure 4-72 Authentication Failure Message (Cause Value: Synch Failure)
AUTHENTICATION FAILURE (cause = "synch failure")
Start T3460
This problem may usually be caused by these potential factors:
Illegal subscriber
Equipment fault
4.5.4 E-RAB Connection Establishment Failures
Based on the drive test data, the initial E-RAB connection establishment success rate is measured from the time when the UE sends out the PDN Connectivity Request message to the time when the UE returns the Activate Default EPS Bearer Context Accept message.
An E-RAB connection establishment failure may usually be caused by these potential factors:
Weak signals
UE/MME rejects
Parameter misconfiguration
Corner effect
Equipment faults
4.5.4.1 Weak Signals
A weak signal or blind spot can become a very critical factor for the UE to get access to the E-UTRAN, especially when the UE is put in mobility or the radio environment is sharply changed. When the UE is put in mobility, especially the RSRP is smaller than -110 dBm or the SINR is smaller than -3 dB (meaning that the UE is stationed in a high path loss or low SNR coverage area), the Samsung UE fails to demodulate signals and thereby experiences a radio access failure.
When the UE is motionlessly stationed in an isolated area (meaning that the RSRP is smaller than -120 dBm), the UE can successfully get access to the LTE network.
This problem may usually be caused by these potential factors:
Poor coverage
A poor coverage problem may occur in the uplink or downlink:
If a poor coverage problem occurs in the uplink, the eNodeB cannot receive or demodulate the response message received from the UE. In this case, you can check the RSSI to see if it is caused by radio interference in the uplink.
If a poor coverage problem occurs in the downlink, the demodulation function of the UE does not work very well. In this case, you need to optimize the RF.
To achieve optimum coverage, ZTE recommends the following solutions:
If a poor coverage problem occurs in the uplink, you need to check whether radio interference is present in the uplink.
If a poor coverage problem occurs in the downlink, you need to eliminate the malfunctioning demodulation factors:
Adding a new eNodeB
Optimizing the RF
Adjusting the antenna and feeder system
Optimize the RS power
The UE is not stationed in an optimum cell.
In the case of a quick signal change, the location update of the stationed cell cannot be implemented until the E-RAB connection is already established. As a result, the
In this case, you need to increase the intra-frequency cell reselection threshold and speed. This can force the UE to quickly station in an optimum cell.
4.5.4.2 UE/MME Rejects
The UE rejects may usually be caused by these potential factors:
The reject is resulted from the activated EPS bearer context.
The reject is resulted from the security mode of the NAS layer.
When the MME delivers the Attach Reject message, the cause value may include:
Network failure
EPS services not allowed in this PLMN
ESM failure
No EPS bearer context activated
For more information about the UE/MME rejects during the radio access procedure, please refer to the corresponding message description guide.
To solve this problem, ZTE recommends the following solutions:
If a UE reject problem occurs because the UE is mal-functioning, you need to upgrade the HW/SW version or replace the UE.
If an MME reject problem occurs, you need to check the STS signaling trace data on the eNodeB side to see if it is caused by a poor coverage or S1 link failure problem. If not, you need to hand this problem to the core network technical support team.
4.5.4.3 Parameter Misconfiguration
When a radio access failure occurs, we need to first compare the parameters of a well-functioning cell to those of a mal-functioning cell to see if they are
consistently configured. If not, check whether such a failure is caused by parameter misconfiguration. In normal cases, it is recommended to enable the intra-frequency measurement and cell reselection. To solve this problem, ZTE recommends you configure scenario-specific parameters as required.
4.5.4.4 Corner Effect
The corner effect is present when the original cell experiences a fast signal decrease but the target cell experiences a fast signal increase. For example, the signal of the original cell may be sharply decreased by 10 dB within one second.
On the contrary, the signal of the target cell may be sharply increased by 10 dB within one second.
This corner effect may be accompanied by a call drop problem or a call origination, especially in a densely-populated urban area. However, we now have very limited ways to tackle this issue.
To solve this problem, it is recommended to optimize the RF as below:
Adjusting the antenna or RS power to make the target cell bypass the corner effect, meaning that the cell reselection should be completed prior to the corner effect
Adjusting the antenna of the serving cell to avoid a fast signal change caused by the corner effect, and thus decreasing the call failure rate
4.6 Call Drop Analysis
The call drop rate reflects the system sustainability for different services; it’s the most important performance indicator that users can directly experience.
Generalized call drop rate definition should include both the EPC and eNodeB call drop, in this section; radio related call drops are emphasized. From high level point of view, most call drops are caused by the following three types of radio problems, that is, coverage, handover, and interference. They are described below, respectively.