S1 handover in LTE is a process of transferring a UE from one eNodeB (source eNodeB) to another eNodeB (target eNodeB) using the S1 interface. The S1 interface is a communication link between the eNodeB and the MME. It is used to exchange control and signaling information between the two entities.

S1 handover is typically used when the UE is moving between two eNodeBs that are served by different MMEs. This is known as an inter-MME handover. S1 handover can also be used for intra-MME handovers, but this is less common.

There are two main types of S1 handover:

• Measurement-triggered handover: This type of handover is initiated by the source eNodeB when it detects that the UE’s signal quality on the target eNodeB is better.
• Event-triggered handover: This type of handover is initiated by the source eNodeB when a specific event occurs, such as a change in the UE’s speed or direction of travel.

Handover Request:

• The MME (Mobility Management Entity) initiates the handover by sending a Handover Request message to the target eNodeB.
• IEs involved:
• Handover Type: Specifies the type of handover, such as Inter-eNodeB or Intra-eNodeB handover.
• UE Context: Contains the context information of the UE, including UE ID, security parameters, and EPS bearers.

Handover Request Acknowledgment:

• The target eNodeB acknowledges the Handover Request by sending a Handover Request Acknowledgment message to the MME.
• IEs involved:
• Cause: Indicates the reason for the acknowledgment, such as acceptance or rejection.
• Target Cell ID: Identifies the target cell where the UE will be handed over.

Handover Command:

• The target eNodeB sends a Handover Command message to the UE, instructing it to perform the handover to the target cell.
• IEs involved:
• Target Cell ID: Identifies the target cell.
• RRC Container: Carries the Radio Resource Control (RRC) information needed by the UE to perform the handover.

eNB Status Transfer:

• The source eNodeB sends an eNB Status Transfer message to the MME, containing the operational status and capabilities of the source eNodeB.
• This message allows the source eNodeB to inform the MME about the target eNodeB’s status, such as whether it is ready for handover or experiencing any issues.

MME Status Transfer:

• The MME responds to the eNB Status Transfer by sending an MME Status Transfer message to the target eNodeB.
• This message includes the operational status and capabilities of the MME, providing important information to the target eNodeB.

Handover Command Acknowledgment:

• The UE acknowledges the Handover Command by sending a Handover Command Acknowledgment message to the target eNodeB.

Handover Notify:

• The target eNodeB sends a Handover Notify message to the MME, notifying it about the ongoing handover process.
• IEs involved:
• Cause: Indicates the reason for the notification.
• UE Context: Contains the UE context information.

Modify Bearer Request

• The Modify Bearer Request message, in the context of S1 handover, enables the MME to request modifications to the bearer contexts for a specific UE. It allows the MME to update the QoS parameters or make other necessary changes to ensure a smooth transition of the UE to the target eNodeB during handover.

Modify Bearer Response

• The Modify Bearer Response message, in the context of S1 handover, provides the response from the target eNodeB to the MME regarding the requested modifications to the bearer contexts for a specific UE. It informs the MME about the successful modifications as well as any failures encountered during the modification process.

UE Context Release Command:

• The source eNodeB sends a UE Context Release Command message to the UE, instructing it to release the UE context.
• IEs involved:
• UE Context Release Command: Indicates the release command.

UE Context Release Complete:

• The UE acknowledges the UE Context Release Command by sending a UE Context Release Complete message to the source eNodeB.
• IEs involved:
• UE Context Release Complete: Indicates the completion of the UE context release.