Robust connectivity in time-frequency dual fading channels for the 6G satellite IoT: chirp and FRFT

With the development of Sixth-Generation (6G) mobile communication technologies, Low Earth Orbit (LEO) satellite communication systems have become extremely important in mobile communications owing to their large coverage, high efficiency, and low cost. However, the high dynamic LEO satellite channels cause serious time-frequency dual selective fading, significantly impairing the performance of conventional single time or frequency domain synchronization algorithms and limiting their applicability. To address these challenges, this paper proposes a synchronization algorithm based on Linear Frequency Modulation (LFM) signals and the Fractional Fourier Transform (FRFT). Exploiting the inherent robustness of LFM signals against frequency deviations and multipath effects, coupled with their energy concentration property in the optimal fractional Fourier domain, the proposed algorithm enables efficient synchronization with enhanced resilience to time-frequency variations. Furthermore, LFM preamble sequences are optimally designed for diverse channel conditions. This work presents a theoretical analysis of the time-frequency nonstationary characteristics of LEO satellite channels and discusses the performance limitations of traditional synchronization algorithms. The proposed integrated FRFT-LFM synchronization framework and sequence optimization scheme are rigorously evaluated via comprehensive simulations. The results demonstrate substantial improvements in synchronization accuracy and computational efficiency compared with conventional methods, particularly under time-frequency dual selective fading LEO satellite channels. The algorithm provides a robust and reliable solution for time-frequency synchronization in LEO satellite communication systems, thereby enhancing overall system performance and reliability.