In low Earth orbit (LEO) satellite communications, the link quality fluctuates drastically during a satellite pass, exhibiting a wide dynamic range from the horizon to the zenith. Moreover, the high relative velocity induces severe Doppler shifts. While orthogonal time frequency space (OTFS) modulation effectively resolves the doubly-selective fading, its spectral efficiency is fundamentally bounded by the Nyquist limit. To break this bottleneck while adapting to dynamic channel variations, this paper proposes a LEO satellite-assisted flexible faster-than-Nyquist (FFTN)-OTFS (LEO-FFTN-OTFS) scheme. Conventional fixed-parameter FTN signaling suffers from severe inter-symbol interference at low elevation angles or spectral inefficiency at the zenith. To overcome this, a low-complexity Look-Up Table (LUT) mechanism is designed to adaptively optimize the time-domain compression factor based on the instantaneous signal-to-noise ratio. At the receiver, a linear minimum mean-square error (LMMSE) detector is formulated to suppress the colored noise and structured interference with minimal computational overhead. Besides, a rigorous theoretical framework is established incorporating 3GPP Tapped Delay Line (TDL) channel models to derive analytical expressions for effective throughput, energy efficiency, and bit error rate (BER) bounds.Simulation results demonstrate that the proposed adaptive scheme eliminates the irreducible error floor inherent in aggressive static FTN configurations at low SNRs, and maximizes the effective throughput across the entire elevation trajectory, achieving a superior trade-off between spectral efficiency and transmission reliability.

翻译：在低地球轨道（LEO）卫星通信中，链路质量在卫星过境期间剧烈波动，呈现出从地平线到天顶的宽动态范围。此外，极高的相对速度会引发严重的多普勒频移。虽然正交时频空（OTFS）调制能有效对抗双选择性衰落，但其频谱效率在根本上受限于奈奎斯特极限。为突破这一瓶颈并适应动态信道变化，本文提出了一种低轨卫星辅助灵活超奈奎斯特（FFTN）-OTFS方案（LEO-FFTN-OTFS）。传统固定参数的FTN信令在低仰角时遭受严重码间干扰，或在天顶时频谱效率低下。为克服此问题，本文设计了一种低复杂度的查找表（LUT）机制，基于瞬时信噪比自适应优化时域压缩因子。接收端采用线性最小均方误差（LMMSE）检测器，以最小计算开销抑制色噪声与结构化干扰。此外，本文建立了严格的理论框架，结合3GPP抽头延迟线（TDL）信道模型，推导了有效吞吐量、能量效率和误码率（BER）界等解析表达式。仿真结果表明，所提出的自适应方案消除了低信噪比下激进静态FTN配置固有的不可消除错误平层，并在整个仰角轨迹上最大化有效吞吐量，实现了频谱效率与传输可靠性的优越折中。