In LTE, handover failures mainly occur due to which reason?

Concept: In cellular networks, a handover (handoff) transfers an active connection from one serving cell to a neighboring cell as the User Equipment (UE) moves, without dropping the session. The handover decision is triggered by the UE's periodic radio measurements of RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality) for the serving and neighboring cells, evaluated against network-configured measurement-event thresholds, hysteresis margins, and time-to-trigger timers (3GPP TS 36.331 measurement reporting, e.g. Event A3).

Application: When these RSRP/RSRQ thresholds, offsets, or timers are configured incorrectly, the handover trigger point becomes wrong: too high or too tight a threshold delays the handover decision (a too-late handover, risking radio link failure on the weakening serving cell), while too low or too loose a threshold triggers handover into a cell whose signal is not yet strong enough (a too-early handover, risking failure right after the switch) or repeated back-and-forth handovers between two cells. 3GPP's own Mobility Robustness Optimization study (TR 36.902) identifies incorrect handover-parameter configuration as a standard, well-documented cause of exactly these failure patterns — too-early, too-late, and wrong-cell handovers — among the scenarios (alongside factors such as UE mobility and coverage gaps) that TR 36.902 discusses.

Cross-check: Among the reasons offered here, the other three name real technical functions but are not the standard, commonly cited configuration-level cause of a handover failure in this sense:

• Loss of TDD-frame synchronization is a prerequisite for the TD-LTE radio link to operate at all (3GPP TS 36.133); its failure is documented as a general radio-link problem, not specifically as the too-early/too-late/ping-pong handover-trigger pattern — and this requirement does not even exist for FDD-LTE.

• LTE does derive fresh security keys for a target cell during handover itself (via the NH/NCC key-chaining mechanism in 3GPP TS 33.401), but this derivation is computed automatically by the UE and network from already-established security context, not externally provisioned or typically "missing." The initial encryption/ciphering keys are instead established once, at attach, via the EPS AKA procedure — a problem there is documented as an authentication/attach failure, distinct from the threshold-driven handover-trigger fault this question is about.

• "AP association" is IEEE 802.11 (Wi-Fi) terminology for a client joining an access point; LTE base stations are eNodeBs and use RRC connection setup/attach instead, so this option describes a different radio technology's connection procedure, not an LTE mechanism at all.

Incorrect RSRP/RSRQ threshold configuration is the option most directly and commonly documented, in 3GPP's own mobility-robustness literature, as a configuration-level cause of LTE handover failure — which is why it is identified as the reason handover failures are described as mainly occurring, among the reasons offered here.