What is Handover?

Detailed explanation

In LTE, handover is entirely network-controlled. The serving eNodeB configures the device to measure and report radio conditions of neighbour cells. When a measurement report indicates that a neighbour cell provides better coverage (event A3), the serving cell decides to initiate a handover by sending an RRC Reconfiguration message with mobility control information.

The A3 event is the most common handover trigger: it fires when the neighbour RSRP exceeds the serving RSRP by a configured offset for a specified time-to-trigger. The hysteresis and time-to-trigger parameters control the sensitivity: too aggressive causes ping-pong between cells, too conservative causes too-late handovers at cell edges.

Other measurement events include A1 (serving cell becomes good enough), A2 (serving cell becomes worse than threshold), A4 (neighbour becomes better than threshold) and A5 (serving becomes weak AND neighbour becomes strong). In 5G NR, additional events support beam-level measurements for multi-beam handover scenarios.

Handover failures are a major cause of dropped calls and data interruptions. The three main failure types are too-late handover (RLF before handover completes), too-early handover (connection fails immediately after handover) and handover to wrong cell (connection fails because the target cell cannot sustain the connection).