Research on the Digital Carrier Tracking Technique for High Dynamic Spread Spectrum Receiver

High dynamic condition will cause Doppler shift acutely, then digital carrier loop may fail to track signal and wrong demodulation may occur in spread spectrum receiver. This paper researches on the digital carrier tracking technique and introduces a structure of frequency-locked loop (FLL)-assisted phase-locked loop (PLL) with separately designed FLL and PLL. The frequency and phase decision are very easy to realize by programmable digital device. Due to the frequency pulling of FLL, the pass-band of the filter in PLL can be made very narrow to suppress the noise, and the PLL can lock the phase of carrier with high accuracy. In this paper, the implementation of the structure adopts the algorithms of cross-product auto frequency tracking (CPAFC) and four phase frequency discrimination (atan2), and the improved digital Costas loop to achieve carrier synchronization. The performance of the improved digital Costas loop is verified by simulation and this method works well in the environment when input signal changes in a large dynamic range.