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Explain the concept of the S1 Handover procedure in LTE for inter-eNodeB handovers.

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The S1 handover procedure in LTE (Long-Term Evolution) is crucial for maintaining seamless communication during inter-eNodeB handovers. It facilitates the transfer of UE's session and context from the source eNodeB (Serving eNodeB) to the target eNodeB, ensuring continuous connectivity as the UE moves between different coverage areas. Here's a technical, detailed explanation of the S1 handover procedure for inter-eNodeB handovers:

1. Introduction:

  • Objective: Transfer the UE's ongoing communication session, radio context, and associated information from the source eNodeB to the target eNodeB to ensure uninterrupted connectivity during handovers.
  • Components: S1-MME (S1 Application Protocol for the Mobility Management Entity) and S1-U (S1 User Plane) interfaces play a vital role in S1 handovers.

2. Handover Trigger:

  • Measurement Reporting: The UE continuously measures the signal quality and strength of neighboring cells.
  • Event Trigger: Measurement reports trigger an event when the neighboring cell's quality exceeds a defined threshold, indicating a need for a handover.

3. Preparation at Source eNodeB (S-eNB):

a. Context Transfer:

  • UE Context: The source eNodeB gathers UE context, including radio and session context, from the EPC (Evolved Packet Core) via the S1 interface.
  • Security Context: Ensures secure transfer of context information.

b. Forwarding Information to Target eNodeB (T-eNB):

  • The S-eNB forwards the UE's context and forwarding information (e.g., target cell details) to the T-eNB over the X2 interface.
  • T-eNB prepares to receive the UE's context and complete the handover.

4. Handover Execution:

a. Handover Request:

  • Handover Preparation: The UE, S-eNB, and T-eNB prepare for the handover.
  • Handover Request: The S-eNB sends a handover request to the T-eNB over the X2 interface, indicating the upcoming handover.

b. Data Forwarding:

  • The S-eNB forwards the uplink and downlink data packets from the UE to the T-eNB via the S1-U interface to minimize data loss during handover.

c. Path Switch and UE Context Transfer:

  • Path Switching: The UE switches its connection from the S-eNB to the T-eNB as per the handover preparation.
  • UE Context Transfer: The T-eNB receives the UE's context from the S-eNB over the S1 interface and updates its context accordingly.

5. Handover Completion:

a. Handover Command:

  • The T-eNB sends a handover command to the UE, instructing it to complete the handover process.
  • The UE adjusts its radio parameters and connections as per the command.

b. Handover Acknowledgment:

  • The UE acknowledges the handover to the T-eNB.
  • The T-eNB confirms the successful handover, and the UE resumes communication over the new cell.

6. Post-Handover:

  • Communication Continuation: The UE continues its communication session using the services of the new serving cell under the T-eNB's coverage.

In summary, the S1 handover procedure in LTE ensures a smooth transition of the UE's ongoing session and context from the source eNodeB to the target eNodeB, enabling seamless communication as the UE moves between different cells and coverage areas. The handover process involves the transfer of context, forwarding of data, path switching, and ensuring the UE's connection continuity in the target cell.