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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This paper proposes a novel automatic frequency control (AFC) method using the Discrete Wavelet Transform (DWT) for low earth orbit satellite, mobile and personal communications systems. The DWT is a useful analysis tool for non-stationary signals which uses short windows at high frequencies and long windows at low frequencies. The DWT AFC has its own simple structure and an intelligent characteristic of "observation and control according to frequency error", which makes it possible to achieve both fast initial acquisition and accurate tracking. Furthermore, it can make the irreducible frequency error arbitrarily small.

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