Performance Evaluation of L1/L2-Triggered Mobility in Non-Terrestrial Networks

Abstract

Low Earth orbit (LEO)-based non-terrestrial networks (NTN) are emerging as a key component of 6G systems, enabling seamless connectivity over wide geographical areas. However, in Earth-moving cell (EMC)-based NTN scenarios, rapid beam movement induces frequent cell boundary crossings, resulting in excessive handover operations and significant signaling overhead. This makes efficient mobility management a critical challenge. Recently, L1/L2-triggered mobility (LTM) and conditional LTM (C-LTM), specified in 3GPP Releases 18 and 19, respectively, have been proposed as advanced handover mechanisms to enhance mobility robustness. Despite their potential, their effectiveness in highly dynamic NTN environments has not yet been systematically evaluated. This paper presents a comprehensive system-level evaluation of 3GPP handover mechanisms in NTN, including baseline handover (BHO), conditional handover (CHO), LTM, and C-LTM. A dedicated simulator is developed to capture the unique characteristics of LEO satellite networks and realistic handover procedures under EMC conditions. The results show that schemes based on LTM and C-LTM significantly improve mobility robustness by reducing radio link failures and interruption time. However, these gains come at the cost of increased handover frequency and potentially increased signaling overhead associated with frequent mobility events and measurement reporting in EMC-based NTN environments. This reveals a fundamental tradeoff between mobility robustness and signaling efficiency. The findings provide quantitative insights into the performance of emerging 3GPP mobility solutions in NTN and offer practical guidelines for designing efficient handover strategies in highly dynamic 6G NTN environments.