DTFT-based frequency lock loop for GNSS applications

Precise frequency estimation is one of the most stringent constraints for Global Navigation Satellite System (GNSS) receivers, that generally uses a Frequency Locked Loop (FLL) to provide a reliable evaluation of the Doppler frequency of the received signal. The proposed FLL is a two step algorithm based on an initial coarse estimation followed by an iterative procedure providing frequency refinements. The coarse estimation stage is implemented by finding the sampled frequency that maximizes the envelop of the FFT of a windowed portion of the input signal. The iterative procedure is based on the phase difference of two Discrete Time Fourier Transforms (DTFTs) evaluated over two overlapping data segments at the previous frequency estimate. The FLL has been integrated with denoising blocks, and median filters have been added to reduce the effect of outliers in the frequency estimation: simulations have proved the effectiveness of the system and the obtained results are very encouraging and show the feasibility of an FLL based on DTFT.