In our last post, we looked at how the serving cell alone can trigger events like A1 (strong signal, stay connected) and A2 (weak signal, prepare to handover). Now, let’s explore how LTE gets smarter by looking beyond the serving cell.
Introducing A-Series Events for Neighbor Comparisons
A3 Event – A Neighbor is Better
Triggered when a neighboring cell’s RSRP is offset stronger than the serving cell.
The offset helps prevent ping-pong handovers (e.g., only trigger if the neighbor is 3 dB better).
Ensures the UE hands over only when truly beneficial.
A5 Event – Dual Threshold Monitoring
Involves two absolute thresholds:
Serving cell gets worse (below Threshold 1)
Neighbor becomes good (above Threshold 2)
Offers more stability by checking both sides of the handover equation.
Practical Scenario: City Commute Example
You’re on a moving bus:
While leaving one cell’s coverage (serving RSRP drops), a nearby cell’s RSRP improves.
Serving falls below A5 Threshold 1
Neighbor rises above A5 Threshold 2
A5 event fires → UE triggers measurement report → Handover initiated
For A3:
If the neighbor consistently shows better performance than serving by set offset → handover is triggered even if the current signal is not weak yet.
Graph Idea:
Here is the graph showing the RSRP (Reference Signal Received Power) of the serving and neighbor cells over time, including the A3 and A5 event trigger zones:
A3 Trigger Zone (green): Neighbor cell becomes stronger than the serving cell by an offset (3 dB).
A5 Trigger Zone (red): Serving RSRP drops below Threshold 1 (−88 dBm) and Neighbor RSRP exceeds Threshold 2 (−82 dBm).
Optimization Tip:
Use A3 with carefully tuned offsets in urban areas to avoid unnecessary handovers, and A5 in high-speed environments (like highways or railways) where serving degradation and neighbor improvement can be clearly observed.
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