Overview
This study introduces a LEO satellite-assisted flexible faster-than-Nyquist (FFTN)-OTFS (LEO-FFTN-OTFS) scheme designed for high-mobility LEO satellite-to-ground communication scenarios. The proposed system addresses the dynamic link quality fluctuations and significant Doppler shifts inherent in LEO satellite communications. It aims to enhance spectral efficiency beyond the Nyquist limit while adapting to varying channel conditions.
Research Context
LEO satellite communications are characterized by drastic fluctuations in link quality, with a wide dynamic range from the horizon to the zenith during a satellite pass. Additionally, high relative velocities between the satellite and ground induce severe Doppler shifts. Orthogonal time frequency space (OTFS) modulation effectively manages doubly-selective fading, a common issue in such environments. However, the spectral efficiency of OTFS modulation is fundamentally constrained by the Nyquist limit. Conventional faster-than-Nyquist (FTN) signaling, with fixed parameters, can suffer from severe inter-symbol interference (ISI) at low elevation angles or be spectrally inefficient at the zenith. The research seeks to overcome these limitations by introducing adaptability to the FTN process.
Approach
The LEO-FFTN-OTFS scheme incorporates a flexible approach to FTN signaling. A key component of this approach is a low-complexity Look-Up Table (LUT) mechanism. This LUT is designed to adaptively optimize the time-domain compression factor based on the instantaneous signal-to-noise ratio (SNR). This adaptive optimization aims to maintain performance across the dynamic range of link qualities experienced during a satellite pass without suffering from the drawbacks of static FTN configurations.
At the receiver, a linear minimum mean-square error (LMMSE) detector is formulated. The purpose of this detector is to suppress colored noise and structured interference. It is designed to achieve this suppression with minimal computational overhead. The research also establishes a theoretical framework that incorporates 3GPP Tapped Delay Line (TDL) channel models. Within this framework, analytical expressions are derived for three specific performance metrics: effective throughput, energy efficiency, and bit error rate (BER) bounds.
Findings
Simulation results indicate that the proposed adaptive LEO-FFTN-OTFS scheme eliminates the irreducible error floor. This error floor is typically inherent in aggressive static FTN configurations, particularly at low signal-to-noise ratios (SNRs). Furthermore, the adaptive scheme maximizes the effective throughput across the entire elevation trajectory of a LEO satellite. The simulation outcomes demonstrate that the proposed system achieves a superior trade-off between spectral efficiency and transmission reliability under varying link conditions.