A2 Event
Understanding the A2 Event
As a UE moves away from its serving cell, RSRP gradually decreases. The A2 Event acts as the early-warning system: once the serving cell degrades below the configured threshold, the network activates inter-frequency measurements. This is the first step in a potential inter-frequency handover chain, where A2 enables measurement and a subsequent A4 or A5 event triggers the actual handover.
During measurement gaps (typically 6 ms gaps every 40 or 80 ms per 3GPP gap patterns 0 and 1), the UE retuning its receiver to scan neighbor frequencies. This temporarily reduces serving cell throughput by approximately 15-7.5%, creating a direct trade-off between measurement agility and user experience. Properly tuning the A2 threshold is critical: too high triggers premature scanning that wastes throughput, too low risks Radio Link Failure before the UE finds a target cell.
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., -110 dBm RSRP)
Example: Thresh=-110 dBm, Hys=2 dB. Entry when RSRP < -108 dBm. Leave when RSRP > -112 dBm.
A2 Threshold Tuning by Frequency Band
| Band | Frequency | Typical A2 Threshold | Rationale |
|---|---|---|---|
| n71 | 600 MHz | -120 dBm | Strong penetration; hold until edge |
| n5 | 850 MHz | -115 dBm | Good coverage; moderate threshold |
| n77 | 3.5 GHz | -105 dBm | Fragile; escape early |
| n258 | 26 GHz (mmWave) | -95 dBm | Very fragile; aggressive escape |
Frequently Asked Questions
Does the A2 Event execute a handover?
No. A2 only initiates the search phase. It prompts the network to configure inter-frequency measurement gaps. The actual handover requires a subsequent event: typically A3 for intra-frequency, A4 for inter-frequency load balancing, or A5 for coverage-triggered inter-frequency mobility.
What happens if no suitable neighbor is found?
The connection will eventually suffer Radio Link Failure (RLF). The UE detects RLF when N310 consecutive out-of-sync indications are received and timer T310 expires without recovery. The UE enters RRC_IDLE, drops the connection, and attempts cell reselection from scratch.
Do different frequencies use different A2 thresholds?
Yes. Engineers configure per-frequency A2 thresholds in MeasConfig. A fragile 3.5 GHz carrier might use -105 dBm so the UE escapes early. A robust 700 MHz carrier might use -120 dBm, keeping the UE on the strong low-band signal until absolute coverage edge. This per-carrier tuning is critical for multi-layer optimization.