Abstract: The IoT NTN (Internet of Things Non-Terrestrial Network) satellite communication system, based on a transparent forwarding architecture, utilizes Orthogonal Frequency Division Multiple Access (OFDMA) technology. While effective, its performance is significantly sensitive to frequency offset. Currently, engineering practice lacks robust analytical tools to evaluate these effects. To provide a practical frequency offset analysis method for transparent IoT NTN systems, this paper proposes a frequency offset analysis model and offers design recommendations for related performance indicators. Focusing on the limited frequency offset tolerance of the return link, the systems operational limits were determined through rigorous simulation analysis and experimental testing. The study identifies ephemeris errors and clock frequency errors as the primary factors degrading system performance. Results indicate that the return link frequency offset should be maintained below 200Hz. To ensure compliance with this threshold, the following specifications are proposed: the ephemeris error should be kept within 20m, the ground segment clock frequency accuracy should not exceed 3ppb, and the space segment clock frequency accuracy should not exceed 10ppb. Beyond its application to transparent IoT NTN architecture, the research methodology presented here can be extended to the design of frequency offset indicators for broader communication systems, providing a valuable reference for engineering implementation.

Key words: IoT NTN, satellite communication system, transparent forwarding, frequency error, ephemeris, clock