The handover procedure in 5G, also known as the Xn-based (NG-RAN to NG-RAN) or NG handover, is a critical mechanism that allows seamless and continuous communication for User Equipment (UE) as it moves between different cells or gNBs (gNodeBs) within the 5G network. The handover process is designed to maintain the quality of service, optimize resource usage, and ensure uninterrupted connectivity for UEs. Here’s a detailed explanation of the handover procedure in 5G:
- Triggering Event:
- The handover procedure in 5G can be triggered by various events, such as deterioration of the radio signal quality in the current serving cell, changes in network conditions, or the UE’s movement across cell boundaries.
- Measurement and Evaluation:
- The UE continuously measures the radio signal quality from neighboring cells. The measurements include parameters like signal strength, signal quality, and interference levels.
- Based on these measurements, the UE evaluates the quality of neighboring cells and determines whether a handover is necessary.
- Handover Decision:
- The decision to perform a handover is typically made by the gNB (gNodeB) that serves the UE. The gNB considers the measurement reports received from the UE and determines whether a handover to a neighboring gNB is beneficial in terms of improving signal quality or optimizing resource usage.
- Xn Interface Preparation:
- The gNBs involved in the handover exchange relevant information over the Xn interface, which connects different gNBs within the NG-RAN (Next-Generation Radio Access Network). This information includes details about the UE, the source gNB, and the target gNB.
- Path Switching Preparation:
- The target gNB prepares the resources and configuration necessary to accommodate the handover. This involves setting up radio bearers, allocating resources, and configuring the target cell to ensure a smooth transition for the UE.
- Handover Command:
- Once all preparations are complete, the source gNB sends a handover command to the UE, instructing it to initiate the handover process. The handover command includes information about the target gNB and the radio resources allocated for the handover.
- UE Handover Execution:
- Upon receiving the handover command, the UE starts the handover execution process. It adjusts its communication parameters and establishes connections with the target gNB while maintaining the existing connection with the source gNB.
- Data Forwarding:
- During the handover process, the source gNB continues to forward data to the UE until the handover is completed. This ensures a smooth transition without service interruption.
- Data Forwarding Handover Completed:
- Once the UE is successfully connected to the target gNB, the handover is considered completed. The target gNB takes over the communication with the UE, and the source gNB releases the allocated resources.
- Handover Confirmation:
- The target gNB sends a handover completion message to the source gNB, confirming the successful handover. The source gNB acknowledges the completion, and the handover process concludes.
- Radio Bearer Reconfiguration:
- After the handover, the gNBs may reconfigure the radio bearers to optimize resource usage and adapt to the new network conditions.
In summary, the handover procedure in 5G is a complex yet efficient process that ensures continuous and uninterrupted connectivity for UEs as they move between cells or gNBs within the NG-RAN. It involves measurement, evaluation, decision-making, interface preparation, path switching, command execution, data forwarding, and confirmation, all orchestrated to maintain high-quality communication for mobile users.