Fast fourier transform based phase locked loop for navigational receivers
View Patent ↗Fast Fourier Transform (FFT) based Phase Lock Loops (PLLs) are provided for use in navigational signal receivers. In an embodiment, a navigation receiver correlates a received navigational signal with a locally generated signal into correlation samples, e.g., one-millisecond correlation samples. The navigation receiver includes a FFT based PLL that corrects phase shifts in the correlation samples due to the Doppler frequency by considering both the Doppler frequency and its rate of change, which are obtained from a FFT computation with interpolation. The phase corrected correlation samples are then integrated over a length of a navigation data bit, e.g., 20 milliseconds, to determine the sign of the data bit of the received signal. In another embodiment, a soft decision feedback technique involving integration extending over the present data bit and several prior data bits is used to determine the sign of a present data bit of the received signal.
1. A method for tracking satellite signals at a satellite navigational receiver, wherein the receiver correlates a received satellite signal with a local signal into correlation samples, comprising:
computing a Doppler frequency and a rate of change of the Doppler frequency;
computing a phase for each correlation sample based on the computed Doppler frequency and the computed rate of change of the Doppler frequency;
compensating for a phase shift of each sample with the computed phase for the respective sample;
coherently integrating the phase compensated samples over the length of one or more data bit for an in-phase (I) component to obtain an I value;
coherently integrating the phase compensated samples over the length of one or more data bit for a quadrature (Q) component to obtain a Q value;
determining a sign of each data bit of the received signal based on a relative value of the I value with respect to the Q value; and
determining a quality of tracking value based on the relative value of the I value with respect to the Q value.
2. The method of claim 1 , wherein the computed phase for each sample comprises a sum of contributions from a prior phase, the Doppler frequency, and the rate of change of the Doppler frequency.
3. The method of claim 1 , wherein the rate of change of the Doppler frequency is computed by a difference between the computed Doppler frequency and a prior Doppler frequency divided by a time duration.
4. The method of claim 1 , wherein the length of one data bit is about 20 milliseconds.
5. The method of claim 1 , further comprising determining the sign of the data bit based on a sign of the I value if the quality of tracking value is greater than a threshold value.
6. The method of claim 1 , further comprising using a soft decision feedback weighting factor based on the quality of tracking value to determine the sign of the data bit.
7. The method of claim 1 , wherein computing the Doppler frequency comprises:
computing a Fast Fourier Transfom (FFT) of several of the samples; and
locating a peak power frequency component of the FFT using a quadratic or higher order polynomial interpolation.
8. The method of claim 7 , wherein the FFT is computed using a sliding integration window, wherein a present sliding integration window comprises a plurality of known old samples from a prior sliding integration window and at least one new sample, and has a sliding ratio of old samples to new samples.
9. The method of claim 8 , wherein a size of the sliding integration window is a function of a satellite signal strength.
10. The method of claim 8 , wherein the sliding ratio of old samples to new samples of the sliding integration window is variable.
11. The method of claim 8 , wherein the sliding ratio of old samples to new samples of the sliding integration window depends upon Position, Velocity and Time (PVT) and frequency update requirements.
12. The method of claim 11 , vherein the sliding ratio of old samples to new samples of the sliding integration window is increased if higher PVT update rate is required.
13. A navigational satellite receiver, comprising:
a radio frequency front-end for receiving satellite signals;
a baseband section for processing the received signals into correlation samples; and
a processor coupled to the baseband section, wherein the processor computes a Doppler frequency and a rate of change of the Doppler frequency, computes a phase for each correlation sample based on the computed Doppler frequency and the computed rate of change of the Doppler frequency, compensates for a phase shift of each sample with the computed phase for the respective sample, coherently integrates the phase compensated samples over the length of one or more data bit for an in-phase (I) component to obtain an I value, coherently integrates the phase compensated samples over the length of one or more data bit for a quadrature (Q) component to obtain a Q value, determines a sign of each data bit of a received signal based on a relative value of the I value with respect to the value, and determines a quality of tracking value based on the relative value of the I value with respect to the Q value.
14. The navigational receiver of claim 13 , wherein the computed phase for each sample comprises a sum of contributions from a prior phase, the computed Doppler frequency, and the computed rate of change of the Doppler frequency.
15. The navigational receiver of claim 13 , wherein processor computes the rate of change of the Doppler frequency by a difference between the computed Doppler frequency and a prior Doppler frequency divided by a time duration.
16. The navigational receiver of claim 13 , wherein the length of one data bit is about 20 milliseconds.
17. The navigational receiver of claim 13 , wherein the processor determines the sign of the data bit based on a sign of the I value if the quality of tracking value is greater than a threshold value.
18. The navigational receiver of claim 13 , wherein the processor uses a soft decision feedback weighting factor based on the quality of tracking value to determine the sign of the data bit.
19. The navigational receiver of claim 13 , wherein the processor computes the Doppler frequency by computing a Fast Fourier Transform (FFT) of several of the samples, and locating a peak power frequency component of the FFT using a quadratic or higher order polynomial interpolation.
20. The navigational receiver of claim 19 , wherein processor uses a sliding integration window to compute the FFT, wherein a present sliding integration window comprises a plurality of known old samples from a prior sliding integration window and at least one new sample, and has a sliding ratio of old samples to new samples.
21. The navigational receiver of claim 20 , wherein a size of the sliding integration window is a function of a satellite signal strength.
22. The navigational receiver of claim 20 , wherein the sliding ratio of old samples to new samples of the sliding integration window is variable.
23. The navigational receiver of claim 20 , wherein the sliding ratio of old samples to new samples of the sliding integration window depends upon Position, Velocity and Time (PVT) and frequency update requirements.
24. The navigational receiver of claim 23 , wherein the sliding ratio of old samples to new samples of the sliding integration window is increased if higher PVT update rate is required.