Real-Time Ephemeris Update Mechanism for Effective Doppler and Timing Pre-Compensation in 5G NR-NTN gNB

Abstract

Non-Terrestrial Networks (NTN) are an important feature of 5G NR that extends cellular connectivity through satellites. A major issue is the strong Doppler shift and variable propagation delay caused by fast-moving satellites, especially in Low Earth Orbit (LEO). 3GPP specifies that Operation and Maintenance (OAM) shall provide ephemeris (orbital trajectory information) data to the gNB. The gNB needs to pre-compensate the Doppler and Timing Advance (TA) before sending any residual corrections to the UE via SIB19. This paper develops a mathematical model that explains how outdated ephemeris data (i.e., inaccurate satellite position/velocity information) causes errors in Doppler shift and Timing Advance estimates at the gNB/UE. We show that short update intervals of 1 to 5 seconds are enough for LEO at 2 GHz. However, using higher carrier frequencies or longer update gaps leads to significant residuals. Simulations of LEO, MEO, and GEO orbits provide design guidelines for ephemeris update timing. The results highlight the importance of real-time ephemeris delivery and demonstrate the trade-offs between update frequency, signalling cost and UE performance.