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

1. A system for time aligning widely frequency spaced signals, the system comprising:

a digital predistortion (DPD) processor having a DPD input and a DPD output;

a power amplifier coupled to the DPD output 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) unit 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 unit and configured to delay the feedback signal based on a fraction of the sampling rate;

a delay estimator coupled to the adaptive fractional delay filter and the DPD output; 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 adaptive fractional delay filter comprises a delay parameter.

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

12. The system of claim 10 wherein the delay parameter is calculated by a counter multiplied by a step size parameter.

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

14. The system of claim 10 wherein the delay parameter ranges from 0 to 1.

15. A system for time aligning widely frequency spaced signals, the 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) unit 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 unit;

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 based on a fraction of the sampling rate of the ADC unit; and

a DPD coefficient estimator coupled to the delay estimator.

16. The system of claim 15 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.

17. The system of claim 15 wherein the delay is a function of a delay parameter equal to a counter multiplied by a step size parameter.

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