Method and system for aligning signals widely spaced in frequency for wideband digital predistortion in wireless communication systems

1. A system comprising:

a digital predistortion (DPD) processor;

a power amplifier coupled to the DPD processor and operable to provide a transmit signal at a power amplifier output; and

a feedback loop coupled to the power amplifier output, wherein the feedback loop comprises:

an analog-to-digital converter (ADC) coupled to the power amplifier output and outputting a feedback signal, wherein the ADC has a sampling rate;

an adaptive fractional delay filter coupled to the ADC unit and comprising a delay parameter, wherein the adaptive fractional delay filter is configured to delay the feedback signal by a delay that is variable based on a fraction of the sampling rate and on the delay parameter;

a delay estimator coupled to the adaptive fractional delay filter and configured to set the delay parameter; and

a DPD coefficient estimator coupled to the delay estimator.

2. The system of claim 1 , wherein the delay estimator further comprises a control algorithm.

3. The system of claim 1 , wherein the adaptive fractional delay filter comprises a third or higher order Lagrange Farrow filter.

4. The system of claim 1 , wherein the delay estimator is operable to calculate a function of a timing error.

5. The system of claim 4 , wherein the delay estimator is operable to calculate the function of the timing error by filtering the transmit signal and an output of the adaptive fractional delay filter.

6. The system of claim 4 , wherein the function of the timing error comprises a magnitude squared function.

7. The system of claim 4 , wherein the function of the timing error comprises a magnitude peak correlation function.

8. The system of claim 4 , wherein the function of the timing error comprises an error vector magnitude (EVM) function.

9. The system of claim 1 , wherein the adaptive fractional delay filter is a low pass filter.

10. The system of claim 1 , wherein the DPD coefficient estimator is operable to estimate predistortion coefficients using the delay parameter.

11. The system of claim 1 , wherein the delay parameter is calculated by a counter multiplied by a step size parameter.

12. The system of claim 11 , wherein the step size parameter ranges from 0 to 1.

13. The system of claim 1 , wherein the delay parameter ranges from 0 to 1.

14. A system comprising:

a digital predistortion (DPD) processor including an output operable to output a predistorted signal;

a power amplifier coupled to the DPD processor and operable to provide a transmit signal at a power amplifier output; and

a feedback loop coupled to the power amplifier output, wherein the feedback loop comprises:

an analog-to-digital converter (ADC) coupled to the power amplifier output and outputting a feedback signal, wherein the ADC unit has a sampling rate;

an adaptive fractional delay filter coupled to the ADC and comprising a delay parameter;

a delay estimator coupled to the adaptive fractional delay filter and the output of the DPD processor and configured to determine a delay between the predistorted signal and the feedback signal, wherein the delay is variable based on a fraction of the sampling rate of the ADC unit and on the delay parameter, wherein the delay estimator is further configured to set the delay parameter; and

a DPD coefficient estimator coupled to the delay estimator.

15. The system of claim 14 , wherein the transmit signal comprises an in-phase signal and a quadrature-phase signal and the feedback signal comprises an in-phase feedback signal and a quadrature-phase feedback signal.

16. The system of claim 14 , wherein the delay is a function of a delay parameter equal to a counter multiplied by a step size parameter.

17. The system of claim 16 , wherein the delay equals the delay parameter divided by the sampling rate.