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The 5G handover procedure utilizing the Xn interface facilitates the transition of the primary serving cell between two 5G Base Stations(Source and Target gNB). This process is enabled when the 5G Base Stations (Source and Target gNB) are interconnected through the Xn interface, allowing for seamless communication continuity during the handover. The handover procedure can be triggered e.g. by an RRC: MeasurementReport (e.g. Event A3) from the UE.
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The handover process within 5G network is divided into three phases: Handover Preparation, Handover Execution, and Handover Completion. The Handover Preparation and Handover Execution phases are confined to the Radio Access Network’s domain, focusing on the readiness and actual switching of the user’s connection between 5G base stations (Source and Target gNB). The Completion phase also involves the Core Network, ensuring that the handover is finalized and stable.
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Handover preparation:
- The UE initiates the process by submitting a measurement report.
- The source gNB evaluates this report and makes a handover decision.
- A Handover Request is sent from the source gNB to the target gNB via the XnAP protocol.
- The target gNB applies its Admission Control checks after receiving the Handover Request message
- The target gNB then proceeds to complete the Handover Preparation phase by returning the Handover Request Acknowledge message , which includes the Handover Command message. This message encapsulates the
RRCReconfiguration message which is to be sent to the UE.
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Handover Execution:
- The source gNB transmits the RRC Reconfiguration message to the UE.
- The source gNB forwards the SN Status Transfer message to the target gNB. This message is used to provide the target gNB with information regarding the Sequence Number status at the PDCP layer for both Uplink and Downlink direction.
- The downlink data is forwarded and s buffered at the target gNB until the UE completes the Random Access procedure and forwards the RRCReconfigurationComplete message.
- The UE transitions from the source NR cell to the target NR cell, executes the Random Access Procedure, and transmits an RRC Reconfiguration Complete message to the target gNB.
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During the Handover Execution phase, Downlink Data Forwarding is a crucial procedure that ensures uninterrupted data delivery to the User Equipment (UE) as it transitions from the source cell to the target cell. This process involves the source gNB forwarding any pending downlink data to the target gNB, which then buffers it until the UE has successfully completed the handover. This strategy prevents any data loss and maintains a continuous data flow to the UE during the handover phase. Here I try to depict how this process is conducted during Handover Execution phase.
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