Handover in LTE (Long-Term Evolution) refers to the process of transferring an ongoing communication session from one base station (eNodeB) to another without interrupting the session. Handover is necessary in LTE to maintain a seamless connection as a mobile device moves through different areas or cells within the network.

There are three types of Handovers in LTE:

• Intra-Frequency Handover

• Intra-Frequency Handover is a specific type of handover that occurs within the same frequency band or carrier in an LTE network.
• Intra-Frequency Handover is a specific type of handover that occurs within the same frequency band or carrier in an LTE network.

• Inter-Frequency Handover

• Inter-Frequency Handover is a type of handover that occurs between different frequency bands or carriers in an LTE network. It involves transferring an ongoing communication session from a cell operating on one frequency band to a cell operating on a different frequency band.
• Event A4 and A5 which is used for LTE Inter-Frequency handover

• Inter-RAT Handover

• Inter-RAT (Radio Access Technology) Handover, also known as Inter-Technology Handover, occurs when a mobile device needs to switch between different radio access technologies, such as LTE (Long-Term Evolution), 3G (UMTS), 2G (GSM), or even non-cellular technologies like Wi-Fi. It involves transitioning from one network using a specific technology to another network using a different technology.
• Event B1 and B2 which is used for I-RAT handover.

LTE measurement Events  

Below is the table explaining the LTE measurements events.

These events are essential for determining when to initiate handover procedures in LTE networks, ensuring smooth and seamless transitions between cells or different radio access technologies. The specific utilization of these events may vary based on the network configuration and requirements.

Event Type	Description	Handover Type
A1	Serving cell signal becomes better than threshold	Inter-Frequency HO & used to stop the measurement
A2	Serving cell signal becomes worse than threshold	Inter-Technology HO & used to start the measurement
A3	Neighbour cell signal becomes offset better than serving	Intra-Frequency and Inter-Frequency HO
A4	Neighbour cell signal becomes better than threshold	Inter-Frequency HO
A5	Serving cell becomes worse than threshold1 and neighbour becomes better than threshold2	Inter-Frequency HO
A6	Neighbour cell becomes offset better than serving (Carrier Aggregation)	Intra-Frequency (CA)
B1	Inter-RAT neighbour cell becomes better than threshold	Inter-Technology (Inter-RAT)
B2	Serving cell becomes worse than threshold1 and inter-RAT neighbour becomes better than threshold2	Inter-Technology (Inter-RAT)

Event A1: This event occurs when the signal strength or quality of the serving cell becomes better than a predefined threshold. It is used in intra-frequency handovers and indicates that the serving cell’s conditions have improved, potentially eliminating the need for a handover. In inter-frequency handovers, this event is used to stop measuring neighbouring cells.

Event A2: When the signal strength or quality of the serving cell falls below a predefined threshold, event A2 is triggered. It is used in inter-technology handovers and suggests that the serving cell’s conditions have deteriorated, indicating a potential need for a handover. In inter-frequency handovers, this event is used to start measuring neighbouring cells.

Event A3: This event occurs when a neighbouring cell’s signal becomes offset better than the serving cell. It is used in both intra-frequency and inter-frequency handovers. When event A3 is triggered, it signifies that the neighbouring cell’s conditions have significantly improved, making it a potential target for handover.

Event A4: In inter-frequency handovers, event A4 is triggered when a neighbouring cell’s signal strength or quality becomes better than a predefined threshold. This event suggests that the neighbouring cell’s conditions are superior, indicating a potential need for a handover.

Event A5: When the serving cell’s signal becomes worse than threshold1, and a neighbouring cell’s signal becomes better than threshold2, event A5 is triggered in inter-frequency handovers. This event indicates that the serving cell’s conditions have degraded while a neighbouring cell offers better signal conditions, suggesting a potential handover.

Event A6: Introduced in Release 10 for Carrier Aggregation (CA), event A6 is triggered when a neighbors cell becomes offset better than the serving cell. This event is specific to intra-frequency handovers with carrier aggregation, where multiple component carriers are used.

Event B1: This event is used in inter-technology handovers (inter-RAT). It occurs when an inter-RAT neighbour cell’s signal becomes better than a predefined threshold. It suggests that the inter-RAT neighbour cell’s conditions are favourable for a potential handover.

Event B2: In inter-technology handovers, event B2 is triggered when the serving cell’s signal becomes worse than threshold1, and an inter-RAT neighbors cell’s signal becomes better than threshold2. This event indicates that the serving cell’s conditions have deteriorated, while the inter-RAT neighbors cell provides better signal conditions, indicating a potential handover.

Categorization of Handover (EPC Entities are Involved or not) in two types

• X2 Handover

X2 handover is a type of handover that occurs between two neighbouring eNodeBs (base stations) connected via the X2 interface.

• S1 Handover

S1 handover is a type of handover that involves the transfer of the mobile device’s connection from one eNodeB to another via the Evolved Packet Core (EPC). It typically occurs when a mobile device moves between LTE networks of different operators or when transitioning from LTE to other technologies, such as 3G (UMTS) or 2G (GSM).

The handover procedure in LTE (Long-Term Evolution) networks involves the seamless transfer of a mobile device’s connection from one base station (eNodeB) to another while maintaining an ongoing communication session. It ensures uninterrupted service and optimal network performance as the mobile device moves within the network coverage area. Here is a high-level overview of the handover procedure:

Measurement and Evaluation:

• The mobile device continuously measures the signal quality and strength of the serving cell and neighbouring cells.
• It reports these measurements to the serving eNodeB, which evaluates the radio conditions and determines if a handover is required.

Handover Decision:

• Based on the measurement reports and predefined criteria, the serving eNodeB makes a decision whether to initiate a handover.
• If the conditions in the target cell are deemed better than the serving cell, the handover decision is made to transfer the connection.

Handover Preparation:

• The serving eNodeB initiates the handover preparation phase.
• It selects the most suitable target cell for the handover and provides the necessary handover parameters to the target eNodeB.
• The target eNodeB is prepared to receive the handover and configure resources for the incoming mobile device.

Handover Execution:

• The handover execution phase involves signalling messages exchanged between the serving eNodeB, target eNodeB, and the mobile device.
• The mobile device is instructed to switch its connection from the serving cell to the target cell at an optimal timing determined by the network.
• The serving eNodeB and target eNodeB coordinate to ensure a smooth transition of the communication session.

Handover Completion:

• Once the handover is successfully executed, the mobile device establishes a connection with the target cell and resumes its communication session.
• The target eNodeB becomes the serving eNodeB for the mobile device.
• The data path is redirected, and ongoing data packets are forwarded from the serving eNodeB to the target eNodeB.
• The handover procedure is considered complete when the mobile device continues its communication without interruption in the target cell.

X2 Handover Call flow

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.

S1 Handover Call flow:

The S1 handover call flow in LTE (Long-Term Evolution) involves the transfer of a user equipment (UE) from the source eNodeB to the target eNodeB through the S1 interface.

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.

There are 3 main things to focus for measurement related rrc reconfiguration message in handover:

1. Measurement object id

Measurement id is the combination of Measurement object id and Report config id

Measurement Object ID is an identifier used to reference a specific measurement object within the Radio Resource Control (RRC) protocol. Measurement objects are used to define the cells or frequencies on which measurements are performed by the mobile device. Here is an explanation of the Information Elements (IEs) associated with the

Measurement Object ID in LTE:

Measurement Object ID (IE Type: Measurement Object ID):

• Description: It is a unique identifier assigned to a measurement object within the RRC protocol.
  • Usage: The Measurement Object ID is used to reference and identify the specific measurement object within the network.

Purpose (IE Type: Purpose):

• Description: Specifies the purpose or objective of the measurement object.
• Usage: It helps the network understand the intention behind configuring the measurement object.

Carrier Frequency (IE Type: Carrier Frequency):

• Description: Specifies the frequency or frequency range associated with the measurement object.
• Usage: It identifies the carrier frequency on which the measurements are performed.

Allowed Measurement Bandwidth (IE Type: Allowed Measurement Bandwidth):

• Description: Indicates the bandwidth allocated for the measurement object.
• Usage: It defines the bandwidth within which the measurements are valid.

Presence Antenna Port 1 (IE Type: Presence Antenna Port 1):

• Description: Indicates whether the measurement object is associated with antenna port 1.
• Usage: It helps in determining the antenna port configuration for the measurement object.

System Frame Number (IE Type: System Frame Number):

• Description: Indicates the system frame number associated with the measurement object.
• Usage: It provides a reference for synchronization and timing purposes.

Measurement Bandwidth (IE Type: Measurement Bandwidth):

• Description: Specifies the measurement bandwidth for the measurement object.
• Usage: It defines the bandwidth over which the measurements are performed.

Offset Frequency (IE Type: Offset Frequency):

• Description: Indicates the frequency offset applied to the measurement object.
• Usage: It helps in aligning the measurement object with the desired frequency.

1. Report config id

Report Config ID is an identifier used to reference a specific reporting configuration within the Radio Resource Control (RRC) protocol. Reporting configurations define how the mobile device reports measurement results to the network.

Report Config ID (IE Type: Report Config ID):

• Description: It is a unique identifier assigned to a reporting configuration within the RRC protocol.
• Usage: The Report Config ID is used to reference and identify the specific reporting configuration within the network.

Trigger Type (IE Type: Trigger Type):

• Description: Specifies the type of trigger that initiates a measurement report.
• Usage: It defines the event or condition that triggers the reporting of measurement results.

Report Interval (IE Type: Report Interval):

• Description: Indicates the interval at which measurement reports are sent to the network.
• Usage: It determines the periodicity of the measurement reports.

Report Amount (IE Type: Report Amount):

• Description: Specifies the number of measurement results included in each measurement report.
• Usage: It determines the quantity of measurement results reported in each report.

Report Quantity (IE Type: Report Quantity):

• Description: Indicates the type of measurement quantity included in the measurement reports.
• Usage: It defines the specific measurement parameter (e.g., RSRP, RSRQ, SINR) to be reported.

Event Trigger Quantity (IE Type: Event Trigger Quantity):

• Description: Specifies the measurement quantity used as a trigger for reporting.
• Usage: It defines the specific measurement parameter that triggers the measurement report.

Report Quantity Cell (IE Type: Report Quantity Cell):

• Description: Indicates the measurement quantity specific to a particular cell.
• Usage: It defines the specific measurement parameter (e.g., RSRP, RSRQ, SINR) for a specific cell in the reporting configuration.

1. Measurement id

Measurement ID is an identifier used to reference a specific measurement configuration within the Radio Resource Control (RRC) protocol. Measurement configurations define how the mobile device performs measurements on neighboring cells.

Measurement ID (IE Type: Measurement ID):

• Description: It is a unique identifier assigned to a measurement configuration within the RRC protocol.
• Usage: The Measurement ID is used to reference and identify the specific measurement configuration within the network.

Measurement Purpose (IE Type: Measurement Purpose):

• Description: Specifies the purpose or objective of the measurement configuration.
• Usage: It helps the network understand the intention behind configuring the measurement.

Report Config ID (IE Type: Report Config ID):

• Description: Indicates the identifier of the reporting configuration associated with the measurement.
• Usage: It links the measurement configuration with the appropriate reporting configuration for reporting measurement results.

Quantity Configuration (IE Type: Quantity Configuration):

• Description: Specifies the measurement quantity or parameters to be measured.
• Usage: It defines the specific measurement parameter (e.g., RSRP, RSRQ, SINR) to be measured.

Measurement Timing Configuration (IE Type: Measurement Timing Configuration):

• Description: Defines the timing parameters for performing measurements.
• Usage: It determines the measurement interval and duration, and the timing relationship between different measurement configurations.

Measurement Control Purpose (IE Type: Measurement Control Purpose):

• Description: Specifies the purpose or reason for controlling the measurements.
• Usage: It helps in understanding the objective behind controlling the measurement procedures.

Measurement Object List (IE Type: Measurement Object List):

• Description: Contains a list of measurement objects associated with the measurement configuration.
• Usage: It specifies the cells or frequencies on which the measurements are performed.

Measurement Gap Configuration (IE Type: Measurement Gap Configuration):

• Description: Specifies the configuration of measurement gaps for performing measurements.
• Usage: It defines the time intervals during which the mobile device is allowed to perform measurements without interrupting ongoing data transmission.

Measurement GAP

• Measurement GAP is introduced to measure the different frequencies of neighbor cell and inter RAT from the current cell in that mentioned GAP, so when neighboring inter frequency Cell has better signal than serving Cell then UE will measure the RSRP in that GAP and informs to current cell in measurement report. During this gap ue will not listen or transmit from/to serving cell.
• Ultimately the network makes the decision, but the gap provides the UE sufficient time to change frequency, make a measurement, and switch back to the active channel

• There is two profile gp0 and gp1 ..
  Gp0 for fast moving traffic as it’s periodicity is 40 ms
  Gp1 for slow moving as it’s periodicity is 80 ms(like small cell application)