A1 Event
Understanding the A1 Event
In LTE and 5G NR, the RRC layer configures measurement reports to drive mobility decisions. The A1 Event is the simplest: it evaluates only the serving cell against an absolute threshold. When the serving cell RSRP (or RSRQ, or SINR in NR) exceeds the configured threshold after applying hysteresis and surviving the Time-to-Trigger window, the UE sends a measurement report to the network.
The primary network response to an A1 report is to remove inter-frequency measurement configurations. By stopping measurement gaps (during which the UE tunes away from its serving frequency to scan neighbors), the UE reclaims those time slots for data reception, improving both throughput and battery life. This is particularly valuable for UEs in strong coverage near the cell center.
Ms − Hys > Thresh
Leave condition:
Ms + Hys < Thresh
Where:
Ms = serving cell measurement (RSRP, RSRQ, or SINR)
Hys = hysteresis (0-15 dB in 0.5 dB steps)
Thresh = configured threshold (e.g., -80 dBm RSRP)
UE must satisfy entry condition for the full TTT duration (0 ms to 5120 ms) before sending the report.
3GPP A-Series Measurement Events
| Event | Condition | Primary Action | Use Case |
|---|---|---|---|
| A1 | Serving > threshold | Stop inter-freq measurements | Battery saving, strong coverage |
| A2 | Serving < threshold | Start inter-freq measurements | Coverage degradation detection |
| A3 | Neighbor > serving + offset | Intra-freq handover | Mobility (strongest cell) |
| A4 | Neighbor > threshold | Inter-freq handover | Load balancing |
| A5 | Serving < thresh1 AND neighbor > thresh2 | Inter-freq handover | Coverage-triggered mobility |
Frequently Asked Questions
What is the opposite of an A1 Event?
The A2 Event. A2 triggers when the serving cell drops below a threshold (e.g., RSRP below -110 dBm). Upon receiving A2, the network commands the UE to begin inter-frequency measurements via measurement gaps. A1 and A2 work as a pair: A1 stops scanning when signal is strong, A2 restarts it when signal degrades.
Does the A1 Event trigger a handover?
No. A1 prevents unnecessary handover preparation. Because A1 confirms the serving cell is above threshold, the network deactivates measurement gaps, stopping the UE from scanning neighbor frequencies. This is the opposite of handover initiation, which requires A3, A4, or A5 events.
What are hysteresis and Time-to-Trigger?
Hysteresis (typically 1-3 dB) adds a guard band to the threshold to prevent rapid toggling between A1 and A2 states when RSRP hovers near the boundary. Time-to-Trigger (TTT) requires the condition to remain true for a configured duration (0 ms to 5120 ms) before the UE sends the report. Together they prevent ping-pong reporting that wastes battery and floods the network with unnecessary signaling.