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This paper proposes a novel frequency-shift keying (FSK) demodulation method using the short-time discrete Fourier transform (ST-DFT) analysis for low-earth-orbit (LEO) satellite communication systems. The ST-DFT-based FSK demodulation method is simple and robust to a large and time-variant frequency offset because it expands the received signal in a time-frequency plane and demodulates it only by searching the instantaneous spectral peaks with no complicated carrier-recovery circuit. Two kinds of demodulation strategies are proposed: a bit-by-bit demodulation algorithm and an efficient demodulation-algorithm frequency-sequence estimation (FSE) based on the Viterbi algorithm. In addition, in order to carry out an accurate ST-DFT window synchronization, a simple DFT-based ST-DFT windowsynchronization method is proposed.

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