Baseband Carrier Recovery and Phase Tracking as a Doppler Compensation Technique for a zero-IF SDR

Compensating for the Doppler effect is a significant problem in low earth orbit (LEO) satellite communications. The effect of Doppler shift is that the baseband carrier signal is shifted in the frequency spectrum. This paper presents the design and analysis of a two-stage baseband carrier recovery algorithm for software-defined radio (SDR). The first-stage frequency-locked loop calculates a coarse estimate of the baseband carrier frequency. After a predetermined settling time, the coarse estimation stage switches to the second stage, in which fine tracking is performed by digital phase-locked loop. Two methods for the first-stage coarse carrier frequency estimation are compared. The first uses a feed-forward method, while the second method uses a feedback mechanism. The algorithms are implemented as part of an SDR modem running on a Freescale DSP56311 digital signal processor. Care is also taken in ensuring efficient implementation, e.g. by avoiding operations such as division, and by avoiding computationally intensive calculations.