Handover in LTE: Call Flow, Logs, and Debugging?

 X2 Handover occurs when a UE moves between two enodeB that have a direct X2 interface. The source eNodeB sends a handover request to the target eNodeB via X2 signaling. This is a faster and more efficient handover compared to S1 handover (which involves MME).X2 handover is used within the same MME pool. The X2 Handover is an intra-LTE handover.

 

Key Steps in X2 Handover Call Flow

1️. Handover Preparation
2️. Handover Execution
3️. Handover Completion

X2 Handover Call Flow with Entities

Step 1: Handover Preparation

1️. UE → Source eNodeB → Sends Measurement Report (e.g., signal strength, neighbouring Signal).
2️. Source eNodeB → Target eNodeB (via X2 interface) → Sends Handover Request with UE context, QoS, and security details.
3️. Target eNodeB → Source eNodeB → Replies with Handover Request Acknowledge after resource allocation.

Step 2: Handover Execution

4️. Source eNodeB → UE → Sends RRC Connection Reconfiguration with new parameters.
5️. UE → Target eNodeB → Synchronizes with the target cell and sends RRC Reconfiguration Complete.
6️. Source eNodeB → Target eNodeB → Forwards downlink data to prevent data loss.

Step 3: Handover Completion

7️. Target eNodeB → MME → Sends Path Switch Request to update routing.
8️. MME → S-GW → Updates the user plane path to forward traffic to the target eNodeB.
9️. S-GW → MME → Target eNodeB → Path switch is confirmed.
10. Source eNodeB → Target eNodeB → Sends Release Resource Message, freeing resources.