A3 Event
An RRC measurement event triggered when a neighbor cell becomes better than the serving cell by an offset, used to initiate intra-frequency handover.
A3 is the bread-and-butter handover trigger you'll see constantly in measurement logs. The condition is relative, not absolute: the neighbour has to beat the serving cell by a configured offset. In full form it's Mn + Ofn + Ocn − Hys > Mp + Ofp + Ocp + Off — the neighbour measurement plus its frequency and cell offsets, minus hysteresis, has to exceed the serving cell's equivalent plus the A3 offset. The hysteresis and offset are there to stop it firing on noise.
Because it's a comparison between cells rather than a fixed threshold, A3 is the natural fit for intra-frequency handover, where serving and neighbour are on the same carrier and directly comparable. (Inter-frequency or inter-RAT mobility more often uses A4/A5, which bring absolute thresholds into play.) When you tune handover boundaries — including what MRO adjusts automatically — the A3 offset and Time to Trigger are the two knobs you're usually turning. Note the entry condition has to hold continuously for the whole TTT before the UE actually sends the measurement report.
Frequently asked questions
- What is the difference between the A3 offset and hysteresis?
- They both add margin so a handover doesn't trigger on a marginal, noisy difference, and in practice they stack. The cleanest way to think about it: the A3 offset sets how much better the neighbour must be, and hysteresis is symmetric padding around that condition to damp oscillation near the threshold. Tuning either one (along with Time to Trigger) shifts the effective handover boundary; most teams keep hysteresis modest and do the real shaping with the offset.
Related terms
Want to truly understand A3 Event? Learn it in context — free for 7 days.
A3 Event is taught inside our 5G Optimization & Troubleshooting course with diagrams, labs and a TelcoMentor AI coach. Start a free 7-day Pro trial — no credit card.
- No credit card
- Full Pro access
- 21 verifiable certs
- TELCOMA since 2009
Get weekly 5G/LTE engineering deep-dives
One technical breakdown every Tuesday — plus first access to new tools and lessons. No spam, no marketing, just engineering. Unsubscribe in one click.