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NR-Handover From E-UTRA Failure

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In 5G New Radio (NR) networks, the "NR-Handover from E-UTRA Failure" refers to a technical procedure that occurs when a handover attempt from a 4G LTE (E-UTRA) cell to a 5G NR cell is unsuccessful. Handovers are essential for maintaining seamless connectivity as a User Equipment (UE) moves between different cells or network technologies. A handover failure can occur due to various reasons, and the procedure is initiated to handle such failures effectively. Here's a detailed technical explanation of the NR-Handover from E-UTRA Failure procedure:

Background: Dual-Connectivity and Handovers:

  • In dual-connectivity scenarios, a UE is simultaneously connected to both LTE (E-UTRA) and NR cells, allowing for better performance and mobility.
  • Handovers are performed when the network decides to move the UE's connection from one cell to another, typically based on factors like signal quality, interference, and load balancing.

Trigger for Handover Failure:

  1. The handover from E-UTRA to NR is initiated either by the UE, the network, or a combination of both. The trigger for handover failure could be due to various reasons, including:
  • Radio Conditions: The signal quality or interference levels in the target NR cell are not sufficient for a successful handover.
  • Network Issues: Network congestion, resource unavailability, or other issues prevent the handover.
  • UE Issues: The UE may not be able to configure resources in the NR cell correctly, or it may not meet the prerequisites for the handover.

Handover Failure Indication:

  1. When the handover attempt fails, the UE or the LTE (E-UTRA) cell detects the failure.
  2. The LTE (E-UTRA) cell sends a failure indication message to the UE, informing it that the handover was unsuccessful.

UE Reaction to Handover Failure:

  1. Upon receiving the handover failure indication, the UE follows specific procedures:
  • It may remain connected to the LTE (E-UTRA) cell and continue to use LTE resources for communication.
  • The UE may report the failure to the network, providing details about the failure cause and conditions.

Network Reaction to Handover Failure:

  1. The network, upon receiving the failure report from the UE, may:
  • Analyze the reported conditions and reasons for the failure.
  • Determine whether to attempt a handover to another cell or technology.
  • Adjust network parameters or configuration to improve handover success rates.

Possible Handover Retry:

  1. In some cases, the network may decide to retry the handover to the NR cell.
  2. A new handover request is initiated with adjusted parameters, taking into account the lessons learned from the previous failure.

Alternative Handover Paths:

  1. In situations where a direct handover from E-UTRA to NR is not feasible, the network may opt for alternative handover paths. For example, the UE may hand over to an intermediate cell (such as an LTE cell) first and then transition to the NR cell.

Continuous Monitoring and Optimization:

  1. The network and UE continue to monitor radio conditions, and handover decisions may be revisited periodically as conditions change.
  2. Continuous optimization ensures that the UE remains connected to the most suitable cell or technology.

The NR-Handover from E-UTRA Failure procedure is essential for maintaining connectivity in situations where a handover attempt from LTE to NR is unsuccessful. It allows the network and UE to react appropriately to such failures, facilitating continuous communication while taking steps to improve handover success rates and overall network performance. The specific actions taken during a handover failure can vary depending on network configurations and mobility management strategies.