X2 handover in LTE is a process of transferring a UE from one eNodeB (source eNodeB) to another eNodeB (target eNodeB) using the X2 interface. The X2 interface is a direct communication link between two eNodeBs. It is used to exchange information about UEs and network conditions, and to coordinate handover and other procedures.

X2 handover is typically used when the UE is moving between two eNodeBs that are served by the same MME. This is known as an intra-MME handover. X2 handover can also be used for inter-MME handovers, but this is less common.

There are two main types of X2 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.

The X2 handover process is as follows:

1. The source eNodeB sends a measurement report to the target eNodeB. This report contains information about the UE’s signal quality on the source eNodeB and the target eNodeB.
2. The target eNodeB evaluates the measurement report and decides whether to accept the handover.
3. If the target eNodeB accepts the handover, it sends a handover request to the source eNodeB.
4. The source eNodeB acknowledges the handover request and sends a handover command to the UE.
5. The UE switches to the target eNodeB and starts communicating with it.

Measurement and Handover Triggering:

• The source eNodeB measures the radio conditions of the neighboring cells, including the target cell.
• Measurement results are reported to the Mobility Management Entity (MME) via the source eNodeB.
• The MME evaluates the measurement reports and determines that a handover is required.

Handover Request:

• The MME sends a Handover Request message to the target eNodeB via the X2 interface.
• IEs involved:
• UE Context (UE-related information for the target eNodeB to establish a connection).
• E-RABs to Be Setup List (list of bearers to be established in the target eNodeB).

Handover Preparation:

• The target eNodeB receives the Handover Request and starts preparing for the handover.
• It allocates radio resources for the target cell and configures the necessary parameters.
• IEs involved:
• UE Context (received from the MME).
• E-RABs to Be Setup List (received from the MME).

Handover Decision and Resource Allocation:

• The target eNodeB evaluates the handover request and determines if it can accept the handover.
• It checks the available resources and capacity in the target cell.
• If the target eNodeB can accept the handover, it sends a Handover Request Acknowledgment message to the source eNodeB.
• IEs involved:
• UE Context (received from the MME).

Handover Command:

• The source eNodeB receives the Handover Request Acknowledgment and prepares for the handover command.
• It sends a Handover Command message to the mobile device, instructing it to perform the handover to the target cell.
• IEs involved:
• Target Cell ID (identifies the target cell).
• Measurement Configuration (configuration parameters for measurement in the target cell).

Handover Execution:

• The mobile device receives the Handover Command and starts the handover execution.
• It performs necessary measurements on the target cell.
• Once the mobile device confirms the successful measurements, it sends a Handover Complete message to the target eNodeB.
• IEs involved:
• Measurement Results (measurement reports of the target cell).

SN Status Transfer

• It is exchanged between the source eNodeB and the target eNodeB during the X2 handover procedure to transfer the status of the Sequence Numbers (SNs) of the User Plane Data Forwarding.
• IEs involved:
• UE ID (User Equipment ID): It identifies the specific user equipment (mobile device) involved in the handover.
• E-RAB ID (E-RAB Identity): It identifies the specific E-RAB (Bearer) associated with the ongoing data transmission.
• UL SN (UpLink Sequence Number): It represents the sequence number of the last uplink data packet transmitted by the mobile device before the handover.
• DL SN (DownLink Sequence Number): It represents the sequence number of the last downlink data packet transmitted by the source eNodeB before the handover.

Handover Completion:

• The target eNodeB receives the Handover Complete message and verifies the successful handover.
• It establishes the radio link with the mobile device and updates the UE context accordingly.
• The target eNodeB sends a Handover Request Acknowledgment to the MME to acknowledge the successful handover.
• IEs involved:
• Measurement Results (received from the mobile device).

Data Forwarding:

• The target eNodeB starts forwarding the ongoing data packets to the mobile device, ensuring continuity of the communication session.
• The handover procedure is complete, and the mobile device continues its communication in the target cell.

Path Switch Request:

• The target eNodeB sends a Path Switch Request message to the source eNodeB, requesting a path switch for the data forwarding.
• IEs involved:
• Target Cell ID (identifies the target cell).

Modify Bearer Request:

Modify Bearer Request message is used in the handover procedure to request modifications to the bearer configuration of a user equipment (UE).

Modify Bearer Response:

Modify Bearer Response message is used in the handover procedure to indicate the outcome of the requested modifications to bearer configurations during a handover. After receiving the Modify Bearer Request message from the MME (Mobility Management Entity), the target eNodeB processes the request and generates a Modify Bearer Response to communicate the result back to the MME.

Path Switch Request Acknowledgment:

• The source eNodeB receives the Path Switch Request and acknowledges it by sending a Path Switch Request Acknowledgment message to the target eNodeB.
• IEs involved:
• Target Cell ID (received from the target eNodeB).

Path Switch Request Acknowledgment with UE Context Release:

• If the source eNodeB has no further use for the UE context, it may include the UE Context Release IE in the Path Switch Request Acknowledgment message to release the UE context.
• IEs involved:
• UE Context Release (optional).

Data Forwarding:

• The target eNodeB starts forwarding the data packets received from the source eNodeB to the mobile device.
• It may use the path established through the X2 interface or directly over the S1 interface.
• IEs involved:
• Data packets.

Source eNodeB Resource Release:

• Once the handover is successfully completed, the source eNodeB releases the radio resources and stops forwarding the data packets.
• It sends a Resource Release message to release the resources.
• IEs involved:
• Resource Release information.

Handover Command with Bearer Status Transfer:

• In some cases, when the target eNodeB needs to know the status of the bearers established in the source eNodeB, the Handover Command message may include the Bearer Status Transfer IE.
• IEs involved:
• Bearer Status Transfer (optional).

UE Context Release Command:

• If the source eNodeB decides to release the UE context after the handover, it sends a UE Context Release Command message to the mobile device, instructing it to release the context.
• IEs involved:
• UE Context Release Command.

UE Context Release Complete:

• The mobile device acknowledges the UE Context Release Command by sending a UE Context Release Complete message to the source eNodeB.
• IEs involved:
• UE Context Release Complete.