A5 Event
Understanding the A5 Event
The A5 Event is the most conservative inter-frequency handover trigger. While A4 triggers based solely on neighbor cell quality (useful for load balancing), A5 adds a second guard: the serving cell must also be degrading. This dual-condition logic makes A5 the preferred event for coverage-driven inter-frequency handover, where the goal is to catch a failing connection before RLF occurs.
A typical deployment uses A5 to govern the transition between high-speed and coverage layers. A user on a 3.5 GHz C-Band carrier stays there as long as the signal is viable. Only when the C-Band signal drops below Threshold 1 (e.g., -105 dBm) and a 700 MHz carrier exceeds Threshold 2 (e.g., -110 dBm) will the handover execute. This keeps users on the faster layer as long as physically possible.
Condition 1: Ms + Hys < Thresh1
Condition 2: Mn + Ocn − Hys > Thresh2
Leave condition (either true):
Ms − Hys > Thresh1 OR
Mn + Ocn + Hys < Thresh2
Where:
Ms = serving cell measurement
Mn = neighbor cell measurement
Thresh1 = serving cell minimum (e.g., -105 dBm)
Thresh2 = neighbor cell minimum (e.g., -110 dBm)
Both conditions must hold for the full TTT duration before report is sent.
A5 Threshold Configuration by Scenario
| Scenario | Serving Layer | Thresh1 | Target Layer | Thresh2 |
|---|---|---|---|---|
| C-Band to low-band | 3.5 GHz | -105 dBm | 700 MHz | -115 dBm |
| mmWave to Sub-6 | 28 GHz | -95 dBm | 3.5 GHz | -105 dBm |
| NR to LTE fallback | NR n77 | -110 dBm | LTE B7 | -100 dBm |
Frequently Asked Questions
What if only one A5 condition is met?
The event does not trigger. Both conditions must be true simultaneously after hysteresis and TTT. If the serving drops below Threshold 1 but no neighbor exceeds Threshold 2, the UE stays on the degrading connection. This prevents handover to a cell that cannot provide adequate service.
Why use A5 instead of A3 for inter-frequency?
A3 compares RSRP directly, which fails across bands: 700 MHz always appears stronger than 3.5 GHz at the same distance. A5 uses independent absolute thresholds, keeping users on the high-speed layer until it genuinely fails (Threshold 1), then falling back only if the target is viable (Threshold 2).
How is A5 used in 5G mmWave fallback?
In networks with mmWave (28 GHz) and Sub-6 GHz layers, A5 governs the fallback. Threshold 1 is mmWave minimum viable signal (e.g., -95 dBm). Threshold 2 is Sub-6 minimum (e.g., -105 dBm). When the UE loses mmWave line-of-sight around a corner, A5 triggers the fallback to Sub-6 GHz.