Method and system for aligning signals widely spaced in frequency for wideband digital predistortion in wireless communication systems
A system for time aligning widely frequency spaced signals includes a digital predistortion (DPD) processor and a power amplifier coupled to the DPD processor and operable to provide a transmit signal at a power amplifier output. The system also includes a feedback loop coupled to the power amplifier output. The feedback loop comprises an adaptive fractional delay filter, a delay estimator coupled to the adaptive fractional delay filter, and a DPD coefficient estimator coupled to the delay estimator.
1. A method of temporally aligning signals, the method comprising:
a) computing a value of a delay parameter of a delay device using a counter;
b) receiving a plurality of transmit signals;
c) receiving a plurality of feedback signals;
d) determining a function of a timing error using the plurality of transmit signals and the plurality of feedback signals;
e) determining that the function of the timing error is greater than or equal to a predetermined threshold;
f) incrementing the counter;
g) repeating a) through f) one or more times;
h) determining that the function of the timing error is less than the predetermined threshold; and
i) fixing the delay parameter of the delay device.
2. The method of claim 1 further comprising estimating predistortion coefficients using the delay parameter.
3. The method of claim 1 wherein computing the value of the delay parameter comprises multiplying the counter times a step size parameter.
4. The method of claim 3 wherein the step size parameter ranges from 0 to 1.
5. The method of claim 1 wherein determining the function of the timing error using the plurality of transmit signals and the plurality of feedback signals comprises filtering the plurality of transmit signals and the plurality of feedback signals and estimating the timing error.
6. The method of claim 1 wherein the function of the timing error comprises a magnitude squared function.
7. The method of claim 1 wherein the function of the timing error comprises a magnitude peak correlation function.
8. The method of claim 1 wherein the function of the timing error comprises an error vector magnitude (EVM) function.