Abstract
Constant envelope signals such as MSK, GMSK have been widely used in satellite system for their power and spectral efficiency. Shaped Offset Quadrature Phase Shift Keying (SOQPSK) belongs to excellent Continuous Phase Modulation (CPM) waveforms. With constant envelope and continuous phase, SOQPSK signal has both high power and spectral efficiency, which makes it quite suitable for satellite communication. But, satellite's high velocity expands the carrier frequency of the SOQPSK signal. To adapt it, this paper proposes a novel carrier synchronization technology based on a modified COSTAS loop, which is assisted by Fast Fourier Transformation (FFT). Firstly, FFT is used to get a rough estimation of the Doppler frequency and limit the frequency offset to a smaller range. Secondly, a modified COSTAS loop is used to track the residual frequency offset and the phase error. Simulation results show that the proposed algorithm can not only track large Doppler frequency, but also performs better than traditional COSTAS loop when it comes to track small Doppler frequency.
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