AFDM-Based Preamble Sequence Transmission for 6G Mobile Satellite Communication Systems

The vision for the 6th generation wireless systems (6G) aims to achieve a space-air-ground integrated seamless global coverage network. Low Earth Orbit (LEO) satellite communication is a cost-effective solution that offers extensive coverage over terrestrial communications. Nevertheless, the relative motion between LEO satellites and user terminals results in a larger Carrier Frequency Offset (CFO) compared to terrestrial communications. Meanwhile, the maximum round-trip delay within the satellite communication beam is considerably greater. To address these challenges, this paper creatively proposes the utilization of Affine Frequency Division Multiplexing (AFDM) waveform for the transmission of preambles for the first time. Specifically, this paper firstly focuses on the Discrete Fourier Transform (DFT) based AFDM waveform for Random Access (RA) preamble transmission in LEO satellite communications. It is demonstrated that AFDM modulation is able to eliminate the CFO of the Zadoff-Chu sequence. Then, to enhance the accuracy of Timing Advance (TA) detection, a detection algorithm is proposed to distinguish the first-path delay. Subsequently, three distinct preamble formats are designed for different scenarios proposed in the 6G use-case requirements. Finally, the simulation results confirm that the AFDM preambles outperform the preambles based on Orthogonal Frequency Division Multiplexing (OFDM) in mitigating the impact of CFO in LEO satellite communications, without the requirement of pre-compensation.