Hello everyone!
Today, I'm going to give you a brief introduction to the LTE handover process.
What is Handover?
When the UE is in the connected state, changing the serving cell without interrupting services is referred to as a handover.
LTE Handover Type
Intra-RAT (Intra-RAT Handover)
1. Handover between cells within an eNodeB.
2. Inter-eNodeB Inter-Cell Handover Through the X2 Interface (MME Not Changed).
3. Inter-eNodeB inter-cell handover over the S1 interface (MME/S-GW may change).
Note
If no X2 link is configured, inter-eNodeB handovers are performed over the S1 interface.
If both the X2 and S1 interfaces are configured, the X2 interface is preferentially used for inter-eNodeB handovers.
Intra-RAT (Inter-System Handover)
1. WCDMA Handover.
2. GSM/EDEG Handover.
3. CDMA2000 Handover.
LTE intra-frequency and inter-frequency handovers
intra-frequency handover means that UEs are handed over between cells on the same frequency. inter-frequency handover means that UEs are handed over between cells on different frequencies.
Intra-frequency and inter-frequency handovers are triggered by handover events.
LTE Handover Event
Event A1: The signal quality of the serving cell is higher than the corresponding threshold. The eNodeB stops inter-frequency or inter-RAT measurement when the cell information that meets the triggering conditions is reported.
Event A2: The signal quality of the serving cell is lower than the corresponding threshold. The eNodeB starts inter-frequency or inter-RAT measurement when the cell information that meets the triggering conditions is reported.
Event A3: The signal quality of the neighboring cell of the PCell is higher than the corresponding threshold of the PCell. The eNodeB initiates an intra-frequency or inter-frequency handover request when the cell information that meets the triggering conditions is reported.
Event A4: The quality of an inter-frequency neighboring cell is higher than the corresponding threshold. The eNodeB initiates an inter-frequency handover request when the cell information that meets the triggering conditions is reported (based on frequency priority).
Event A5: The signal quality of the serving cell is lower than threshold 1 and the signal quality of the inter-frequency neighboring cell is higher than threshold 2. The eNodeB initiates an inter-frequency handover request when the cell information that meets the triggering conditions is reported (Coverage-based).
Event A6: The signal quality of an intra-frequency neighboring cell of an SCell is higher than that of an SCell. The eNodeB changes the SCell without changing the PCell when the cell information that meets the triggering conditions is reported.
Event B1: The signal quality of an inter-RAT neighboring cell is higher than the corresponding threshold. The eNodeB initiates an inter-RAT handover request when the cell information that meets the triggering conditions is reported. (Event B1 is usually combined with event A2. Event B1 is initiated only when the serving cell is lower than the A2 threshold).
Event B2: The signal quality of the serving cell becomes lower than threshold 1 and the signal quality of the inter-RAT neighboring cell becomes higher than threshold 2. The eNodeB initiates an inter-RAT handover request when the cell information that meets the triggering conditions is reported. (Event B2 is configured independently. Event B2 = Event A2 + Event B1).
LTE Handover Process
The handover process can be divided into four phases: handover measurement, handover preparation, handover execution, and handover completion.
Figure 1 LTE Handover Procedure
Handover measurement
The UE measures the neighboring cells that meet the handover conditions and reports the measurement results to the eNodeB.
Interaction between the UE and the eNodeB. The eNodeB instructs the UE to measure the signal strength of the cell where the UE is located and its neighboring cells. and reports the signal strength to the source eNodeB based on the reporting threshold. Then, the source eNodeB needs to determine the following three items:
1. Determine whether the handover can be performed.
2. Filter the target cell and check whether the target cell is a blocklisted cell, inter-PLMN, handover barred area, and access barred area.
3. The eNodeB determines the priority of the target cell based on the reported cell signals. If the measurement results are the same, intra-frequency or inter-frequency eNodeBs are considered, and intra-frequency eNodeBs are preferred.
Handover preparation
The eNodeB selects a proper cell based on the cell information reported by the UE and sends a handover request to the cell to obtain related resources.
If the source eNodeB decides to perform a handover, the source eNodeB sends a handover request to the target eNodeB with the highest priority based on the priority list generated in the previous step. The signaling contains the UE information. Resources such as the C-RNTI and preamble are available, and the target eNodeB is ready. Respond to the handover request, indicating that the handover is ready. If resources such as the C-RNTI and preamble are unavailable, the eNodeB returns a handover failure message to the eNodeB. If the source eNodeB receives a handover failure message, the source eNodeB sends a handover request to the next target cell in the priority list until all the attempts are complete. If the eNodeB still does not receive the ACK message for the handover request, the eNodeB stops sending the handover request and waits for the next round of measurement reports. Then, the eNodeB initiates a next round of handover attempts in the target cell list.
Handover execution
When the handover resources meet the conditions, the eNodeB initiates a handover and hands over the bearer to another cell.
The source eNB notifies, on the radio interface by using RRC connection reconfiguration signaling, the UE to perform handover, disconnects the RRC connection from the UE during data forwarding, the source eNB buffers data to the S-GW, forwards the data to the target eNB, and finally The UE successfully accesses the target eNB and can perform data exchange. After receiving the handover command, the UE starts the T304 timer, and then initiates random access to the target eNB. After the random access succeeds, the UE sends RRC connection reconfiguration complete signaling to the target eNB, indicating that the handover succeeds, and stops the T304 timer. If the random access is not complete before T304 expires, the handover fails and the UE starts cell search and RRC connection reestablishment.
Handover completion
After receiving the RRC connection reconfiguration complete message, the target eNB sends a path change request to the MME, to notify the core network that the serving cell of the UE has changed. The MME controls the S-GW to change the transmission path of downlink data and responds to the target eNB after the path change is complete. After responding, the target MME sends a UE context release message to the source eNB, instructing the source eNB to completely release the radio and control plane resources related to the UE. The handover procedure is complete.
Figure 2 LTE Handover Procedure
These are the details I would like to share about LTE handovers. Welcome to read and leave a message.
