How does LTE manage Handover Preparation, Execution, and Completion phases?
LTE (Long-Term Evolution) networks manage handovers, which involve transferring a User Equipment (UE) from one cell to another, in a carefully orchestrated process that includes three main phases: Handover Preparation, Handover Execution, and Handover Completion. These phases ensure a smooth transition for the UE while maintaining the quality of service. Here's a technical breakdown of how LTE manages each of these phases:
1. Handover Preparation Phase:
a. Measurement and Evaluation:- Before initiating a handover, the network continuously monitors the UE's signal quality, including the Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) measurements. These measurements help assess the suitability of potential target cells.
b. Handover Decision:- The network uses the measurement reports provided by the UE to make an informed handover decision. When certain thresholds are met or exceeded (e.g., a drop in signal strength or quality), the network decides that a handover is necessary.
c. Target Cell Selection:- Based on the measurement reports and other criteria, the network selects a suitable target cell or eNB (Evolved Node B) for the handover. Factors considered include signal quality, available resources, network load, and UE capabilities.
d. Resource Allocation:- The network allocates the necessary radio resources for the handover, both in the source and target cells. This includes allocating time slots, frequency bands, and assigning temporary identifiers to the UE for the target cell.
e. Data Forwarding and Buffering:- To minimize service interruption, the network may buffer and forward data packets to the target cell before the handover is complete. This ensures that no data is lost during the transition.
2. Handover Execution Phase:
a. Handover Request:- The network sends a handover request message to the UE, instructing it to prepare for the handover. This message contains information about the target cell, the allocated resources, and timing information.
b. UE Synchronization:- The UE synchronizes its radio link with the target cell. This involves adjusting its timing and synchronization parameters to match those of the target cell. Precise synchronization is critical for a seamless handover.
c. Security Association Update:- The UE and the network update their security associations to ensure that the communication remains secure during and after the handover.
d. Quality of Service (QoS) Negotiation:- If necessary, QoS parameters are negotiated and set up between the UE and the target cell to ensure that the UE receives the required QoS after the handover.
e. Handover Confirmation:- Once the handover is executed successfully, the UE confirms the handover completion to the network. This confirmation may include measurement reports from the UE in its new location.
3. Handover Completion Phase:
a. Data Path Activation:- The network activates the data path in the target cell, ensuring that data packets are now routed through the new cell. This includes the activation of Radio Bearers (RBs) for the UE.
b. Data Packet Forwarding:- Any buffered data packets that were forwarded to the target cell are now transmitted to the UE.
c. Handover Optimization:- The network may perform optimizations, such as load balancing or power control, to further improve the UE's performance in its new location.
d. Release of Resources:- Resources in the source cell are released, and the UE is now fully connected to the target cell. The source cell may also update its neighbor cell list to reflect the UE's movement.
e. Handover Completion Confirmation:- Both the UE and the network confirm the successful completion of the handover. Measurement reports may continue to be exchanged to fine-tune the UE's performance in the new cell.
In summary, LTE networks manage handovers through a series of well-defined phases that include preparation, execution, and completion. These phases involve careful measurement, decision-making, synchronization, resource allocation, data buffering, and confirmation steps to ensure a seamless transition for the UE while maintaining service quality. The ultimate goal is to provide uninterrupted connectivity and service continuity for mobile users as they move within the network.