A Joint Code-Doppler Acquisition Algorithm for DSSS-MSK Based on FFT

Direct sequence spread spectrum (DSSS) minimum frequency shift keying (MSK) modulation is very suitable for satellite communications with short frame, but how to quickly achieve DSSS-MSK signal acquisition under high dynamic large Doppler shift and low signal-to-noise ratio(SNR) situation has always been a problem. This paper proposes a fast DSSS-MSK signal acquisition algorithm based on fast Fourier transform (FFT). The algorithm uses the MSK baseband waveform of the pseudo-code sequence as the matching template. The correlation operation is performed in the frequency domain through FFT. The correlation peak is generated by cyclic shift search in the frequency domain. Finally, the position of the correlation peak in the two-dimensional plane is used to estimate frequency offset and code offset information. The simulation results show that the acquisition performance of the proposed algorithm is 10dB better than the traditional algorithm in terms of SNR, and it is 2-3dB better than the method of transforming DSSS-MSK signal to DSSS-BPSK signal. Moreover, the acquisition algorithm proposed in this paper can quickly search large Doppler frequency shifts by cyclic shift in the frequency domain. Finally, the proposed acquisition algorithm was implemented based on Xilinx's Artix-7 series FPGA and AD9361 hardware platform.