When analyzing drive test data, the initial E-RAB (Evolved Radio Access Bearer) connection establishment success rate is assessed from the time the User Equipment (UE) sends out the PDN Connectivity Request message to time when it returns the Activate Default EPS Bearer Context Accept message.
Common Causes of E-RAB Connection Establishment Failures:
- Weak Signals: Poor signal strength can lead to connection failures, as the UE may struggle to establish a reliable link with the network.
- UE/MME Rejections: The UE or the Mobility Management Entity (MME) might reject the connection request due to various reasons, such as resource limitations or policy restrictions.
- Parameter Misconfiguration: Incorrectly configured parameters in the network can prevent successful E-RAB establishment. This could involve settings on the UE, eNodeB, or core network elements.
- Corner Effect: Signal degradation or interference caused by physical obstructions, like buildings at a street corner, can disrupt the connection process.
- Equipment Faults: Hardware or software issues in the network equipment, such as faulty eNodeBs or UEs, can also cause E-RAB establishment failures.
Weak Signals
Weak signals can significantly impact a UE’s ability to access the LTE network, especially in situations where the radio environment changes rapidly or the UE is in motion. When the received signal strength (RSRP) is below -110 dBm or the signal-to-noise ratio (SINR) is less than -3 dB, the UE is likely in an area with high path loss or low signal quality, making it difficult to demodulate signals and resulting in radio access failures. If the UE is stationary in an isolated area with an RSRP below -120 dBm, it may still access the network, but weak signal conditions remain a concern.
These issues are often caused by poor coverage in either the uplink or downlink. If poor coverage affects the uplink, the eNodeB may struggle to receive or demodulate responses from the UE, possibly due to radio interference. If the downlink is affected, the UE’s demodulation performance may suffer, necessitating RF optimization.
To improve coverage, potential solutions include checking for uplink interference, addressing malfunctioning factors in the downlink, adding new eNodeBs, optimizing the RF, and adjusting the antenna and feeder systems. Additionally, ensuring the UE is connected to the optimal cell is crucial, particularly when signals change quickly. Increasing the intra-frequency cell reselection threshold and speed can help the UE station itself in the best possible cell, reducing the likelihood of E-RAB connection establishment failures in weak-signal conditions.
UE/MME Rejects.
UE rejects during the radio access procedure can occur due to several reasons, often related to the EPS bearer context or the security mode of the NAS layer. When the MME sends an Attach Reject message, the cause might include network failure, the PLMN not allowing EPS services, ESM failure, or the absence of an activated EPS bearer context.
To resolve UE reject issues:
- If the problem is with the UE: Check if the UE is malfunctioning. If it is, upgrading the hardware or software version, or replacing the UE may solve the issue.
- If the problem is with the MME: Examine the STS signaling trace data on the eNodeB to determine if the reject is due to poor coverage or S1 link failure. If these are not the causes, the issue should be escalated to the core network technical support team for further investigation.
Parameter Misconfiguration.
When a radio access failure occurs, the first step is to compare the parameters of the malfunctioning cell with those of a well-functioning cell. If inconsistencies are found, the failure might be due to parameter misconfiguration. It’s important to ensure that critical features like intra-frequency measurement and cell reselection are enabled. ZTE recommends configuring parameters according to specific scenarios to resolve this issue.
Corner Effect.
The corner effect happens when the signal from the original cell rapidly decreases while the signal from the target cell quickly increases. For example, the signal strength might drop by 10 dB in one second in the original cell, while simultaneously increasing by 10 dB in the target cell. This can lead to call drops or issues during call initiation, especially in densely populated urban areas.
To mitigate this, RF optimization is recommended:
- Adjust the antenna or RS power so that cell reselection occurs before the corner effect can take place.
- Modify the antenna of the serving cell to prevent rapid signal changes caused by the corner effect, reducing the call failure rate.