LTE handovers are controlled by measurement events that tell the UE when to trigger a handover to another cell. The events are defined in 3GPP specifications and are used for intra‑LTE and inter‑RAT (Radio Access Technology) handovers.
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Event A1 – Serving cell becomes better than threshold
Description: The signal quality/strength of the serving cell rises above a configured threshold, indicating good coverage.
Example: A mobile moving into the center of a cell measures RSRP > −108 dBm (threshold). The UE reports the event, and the network may decide to keep the UE in the same cell or optimize resources.
Trigger: First event when coverage improves significantly. -
Event A2 – Serving cell becomes worse than threshold
Description: The serving cell’s signal drops below a threshold, signalling deteriorating coverage and the need to search for a better cell.
Example: RSRP falls below −110 dBm. The UE reports A2, initiating a search for neighbor cells for potential handover.
Trigger: Usually triggered after A1 if conditions degrade; it starts the handover preparation phase. -
Event A3 – Neighbor cell becomes offset better than serving cell
Description: A neighbor cell’s signal is better than the serving cell by a configured offset, indicating a better candidate for handover.
Example: Neighbor RSRP is 4 dB stronger than the serving cell (after applying an offset). The UE reports A3, prompting the network to consider handover to that neighbor.
Trigger: Follows A2 when a suitable neighbor is found; it decides which cell to hand over to. -
Event A4 – Neighbor cell becomes better than threshold
Description: A neighbor cell’s signal exceeds a specific threshold, making it a strong candidate even without comparing to the serving cell.
Example: Neighbor RSRP > −106 dBm. The UE reports A4, informing the network of a strong neighbor for potential load balancing or handover.
Trigger: Can be triggered independently of A2/A3, often used for load balancing. -
Event A5 – Serving worse than threshold1 & neighbor better than threshold2
Description: Two conditions must be met: the serving cell falls below threshold1 and a neighbor exceeds threshold2, triggering a handover.
Example: Serving RSRP < −110 dBm (Th1) and neighbor RSRP > −106 dBm (Th2). The UE reports A5, forcing a handover to the neighbor.
Trigger: Typically the final event leading to an actual handover execution, combining A2 and A4 conditions.
Sequence & triggering logic
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Typical intra‑LTE sequence: A1 → A2 → A3/A4 → A5 (or directly A2 → A3).
- A1 is optional (quality improvement).
- A2 initiates handover search.
- A3 decides the target cell (offset comparison).
- A5 finalizes the handover if dual thresholds are used.
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Intra‑frequency vs inter‑frequency:
- Intra‑frequency handovers usually rely on A3/A5.
- Inter‑frequency uses A4 to detect other frequencies and may involve A5 for strict conditions.
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