Handover
The procedure of transferring an active connection from one cell or base station to another while maintaining service continuity as the UE moves.
Handover is the network keeping your call alive while you move out of one cell's reach and into another's. The UE is constantly measuring its serving cell and neighbours and reporting back; when a neighbour gets sufficiently better (the classic trigger is the A3 event — neighbour exceeds serving by an offset, sustained for a time-to-trigger window), the source cell decides to hand you over and prepares the target before cutting you across.
LTE handover is hard handover — break-before-make, you're connected to exactly one cell at a time — but it's fast enough to be imperceptible when tuned well. The interface matters: an X2 handover goes directly eNB-to-eNB and is quick; an S1 handover routes through the MME and is used when no X2 exists between the cells. The real engineering is in the tuning. Set the thresholds too eager and you get ping-pong (bouncing back and forth between two cells, wasting signalling); too lazy and you get dropped calls at the boundary. That balancing act is exactly what the SON function MRO automates.
Frequently asked questions
- What causes a dropped call during handover?
- Usually a handover that happens too late or fails to complete. If thresholds are set conservatively, the UE can fall out of the serving cell before the target is ready, and the link drops. Other culprits are a missing neighbour relation (the network does not know about the cell you are moving into), interference at the cell edge corrupting the measurement reports, or the target cell being too congested to admit you. Tuning handover parameters and neighbour lists — often via SON functions like MRO and ANR — is how these get fixed.
Related terms
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