3GPP standardized conditional handover (CHO) in Release 17 as a mechanism to enhance mobility robustness in non-terrestrial networks (NTNs). In addition to the traditional terrestrial NR triggering events (e.g., Event A3), two new triggers, location-based and time-based, were introduced. These enhancements address the limitations of relying solely on signal strength, which can be insufficient for distinguishing between satellites at similar altitudes due to comparable path-loss characteristics. In this paper, a time-to-exit (TTE) criterion, integrated with a location-based trigger, is proposed to select candidate cells for which the network estimates how long the D1 condition will remain valid after handover. A candidate cell is admitted if (i) it has the longest D1-based TTE and (ii) its signal quality exceeds a configured threshold; otherwise, it is discarded, with ties broken based on signal strength. This design arms only targets that are both strong enough and stable long enough to finish the handover, preventing late or unstable executions. Simulation results show that the proposed TTE-aware CHO significantly reduces handover failures and ping-pong events, and increases post-handover time-of-stay, compared with a baseline location-based CHO.

The space-air-ground integrated network (SAGIN) represents a major breakthrough in the advancement of Beyond 5G/6G, seamlessly combining terrestrial network (TN) and nonterrestrial networks (NTN) into a unified multi-layered system by creating a more cohesive and efficient communication infrastructure. However, the high mobility of NTN platforms creates significant challenges in managing user transitions across these interconnected layers. This article offers a concise overview of the unique demands of mobility management (MM) in SAGIN and the architectural developments required to meet the needs of various service categories, including enhanced mobile broadband (eMBB), massive machine type Communications (mMTC), and ultra reliable low latency Communications (URLLC). Additionally, the article details specific strategies for vertical handovers in SAGIN, as described in the literature, with a focus on the roles of software defined network (SDN), network function virtualization (NFV), and network slicing (NS).