Geometry-Aware Time-Window Conditional Handover for Non-Terrestrial Networks

Low Earth orbit (LEO) satellites are a key enabler for non-terrestrial networks (NTNs) in 6G, but the rapid motion of Earth-moving cells (EMCs) induces frequent handovers (HOs) and unstable connectivity. Conventional signal strength-based HO control (offset/TTT) is insufficient in NTN because large footprints cause small RSRP gradients, leading to both unnecessary HOs (UHOs) and delayed HOs. This paper proposes a time-window based conditional HO (TWCHO) that derives per-UE geometric timing bounds from the relative UE–beam mobility. The time window is explicitly calculated from the hexagonal cell boundary and the residence-time upper bound, and then mapped to the 3GPP technical specifications (TS) 38.331 CondEventT1 parameters. This realizes a dynamic, geometry-aware trigger window that suppresses HOs in the cell interior and executes them only in favorable regions. System-level simulations for 3GPP NTN study case 9 show that, compared with conventional HO mechanism, TWCHO significantly reduces UHOs and HO ping-pong (HOPP) while maintaining higher downlink signal-to-interference-plus-noise ratio (DL SINR) and lower radio link failure (RLF).