Digital Predistortion for nonlinear RF power amplifiers

1. A method, comprising:

generating a digital transmit signal;

receiving a digital receive signal that is based on an amplified analog version of the digital transmit signal, wherein the amplified analog version is produced by a power amplifier having one or more nonlinear characteristics;

storing signal samples, the signal samples including (i) transmit samples based on the digital transmit signal and (i) receive samples based on the digital receive signal;

arranging sample pairs in sample pair groups, each sample pair comprising a transmit sample of the transmit samples and a receive sample of the receive samples, wherein the receive samples of the sample pairs are aligned with the corresponding transmit samples of the sample pairs;

determining representative sample pairs for at least a portion of the sample pair groups;

determining gain coefficients based on at least a portion of the signal samples, the gain coefficients being associated with respective first operational ranges of the power amplifier, wherein determining the gain coefficients comprises using the representative sample pairs;

determining predistortion values, the predistortion values including

(i) first values based on the gain coefficients, the first values being associated with the first operational ranges, and

(ii) one or more second values based on an extrapolation of at least two of the gain coefficients, the one or more second values being associated with respective one or more second operational ranges of the power amplifier; and

using the predistortion values to predistort digital signals to compensate for the one or more nonlinear characteristics of the power amplifier.

2. The method of claim 1 , wherein determining the predistortion values comprises performing the extrapolation with respect to a threshold parameter, the first operational ranges meeting or surpassing the threshold parameter, and the one or more second operational ranges not surpassing the threshold parameter.

3. The method of claim 2 , wherein arranging the sample pairs in the sample pair groups comprises:

selecting samples of the transmit samples based on the threshold parameter; and

ordering the selected samples, wherein at least a portion of the sample pair groups comprise different ordered portions of the selected samples.

4. The method of claim 2 , wherein determining the representative sample pairs comprises determining a representative receive sample of a sample pair group based on an average of digital receive samples included in the sample pair group.

5. The method of claim 1 , further comprising:

determining phase coefficients based on at least a portion of the signal samples, wherein the predistortion values include values based on additive inverses of the phase coefficients, respectively, and wherein the first values are based on multiplicative inverses of the gain coefficients.

6. The method of claim 1 , further comprising:

storing the predistortion values in a data structure that is indexed by input amplitude values.

7. The method of claim 1 , wherein using the predistortion values comprises:

accessing a sample value of a digital signal;

retrieving a predistortion value from the data structure based on the accessed sample value; and

applying the retrieved predistortion value to the accessed sample value.

8. A system, comprising:

processor electronics configured to produce a digital transmit signal;

a first converter to convert the digital transmit signal to an analog transmit signal;

a power amplifier to amplify the analog transmit signal, wherein the power amplifier has one or more nonlinear characteristics;

receiver circuitry to receive an analog receive signal; and

a second converter to convert the analog receive signal to a digital receive signal,

wherein the processor electronics are configured to perform operations comprising

causing the receiver circuitry to receive the amplified analog transmit signal as the analog receive signal,

storing signal samples, the signal samples including (i) transmit samples based on the digital transmit signal and (ii) receive samples based on the digital receive signal;

arranging sample pairs in sample pair groups, each sample pair comprising a transmit sample of the transmit samples and a receive sample of the receive samples, wherein the receive samples of the sample pairs are aligned with the corresponding transmit samples of the sample pairs;

determining representative sample pairs for at least a portion of the sample pair groups;

determining gain coefficients based on at least a portion of the signal samples, the gain coefficients being associated with respective first operational ranges of the power amplifier, wherein determining the gain coefficients comprises using the representative sample pairs;

determining predistortion values, the predistortion values including

(i) first values based on the gain coefficients, the first values being associated with the first operational ranges, and

(ii) one or more second values based on an extrapolation of at least two of the gain coefficients, the one or more second values being associated with respective one or more second operational ranges of the power amplifier; and

using the predistortion values to predistort digital signals to compensate for the one or more nonlinear characteristics of the power amplifier.

9. The system of claim 8 , wherein determining the predistortion values comprises performing the extrapolation with respect to a threshold parameter, the first operational ranges meeting or surpassing the threshold parameter, and the one or more second operational ranges not surpassing the threshold parameter.

10. The system of claim 9 , wherein arranging the sample pairs in the sample pair groups comprises:

selecting samples of the transmit samples based on the threshold parameter; and

ordering the selected samples, wherein at least a portion of the sample pair groups comprise different ordered portions of the selected samples.

11. The system of claim 9 , wherein determining the representative sample pairs comprises determining a representative receive sample of a sample pair group based on an average of digital receive samples included in the sample pair group.

12. The system of claim 8 , wherein the operations further comprise:

determining phase coefficients based on at least a portion of the signal samples, wherein the predistortion values include values based on additive inverses of the phase coefficients, respectively, and wherein the first values are based on multiplicative inverses of the gain coefficients.

13. The system of claim 8 , wherein the operations further comprise:

storing the predistortion values in a data structure that is indexed by input amplitude values.

14. The system of claim 8 , wherein using the predistortion values comprises:

accessing a sample value of a digital signal;

retrieving a predistortion value from the data structure based on the accessed sample value; and

applying the retrieved predistortion value to the accessed sample value.

15. An apparatus, comprising:

circuitry configured to (i) produce signals including a digital transmit signal and (ii) receive signals including a digital receive signal that is based on an amplified analog version of the digital transmit signal, the amplified analog version being produced by a power amplifier having one or more nonlinear characteristics; and

a memory to store signal samples, the signal samples including (i) transmit samples based on the digital transmit signal and (ii) receive samples based on the digital receive signal,

wherein the circuitry is configured to perform operations comprising

determining gain coefficients based on at least a portion of the signal samples, the gain coefficients being associated with respective first operational ranges of the power amplifier;

determining predistortion values with respect to a threshold parameter, the predistortion values comprising first values and one or more second values, wherein determining the predistortion values comprises:

(i) determining the first values based on the gain coefficients, the first values being associated with the first operational ranges, the first operational ranges meeting or surpassing the threshold parameter, and

(ii) determining the one or more second values based on an extrapolation of at least two of the gain coefficients, the one or more second values being associated with respective one or more second operational ranges of the power amplifier, the one or more second operational ranges not surpassing the threshold parameter; and

using the predistortion values to predistort digital signals to compensate for the one or more nonlinear characteristics of the power amplifier.

16. The apparatus of claim 15 , wherein the threshold parameter is selected based on a noise floor.

17. The apparatus of claim 15 , wherein the operations further comprise:

arranging sample pairs in sample pair groups, each sample pair comprising a transmit sample of the transmit samples and a receive sample of the receive samples, wherein the receive samples of the sample pairs are aligned with the corresponding transmit samples of the sample pairs; and

determining representative sample pairs for at least a portion of the sample pair groups, wherein determining the gain coefficients comprises using the representative sample pairs.

18. The apparatus of claim 17 , wherein arranging the sample pairs in the sample pair groups comprises:

selecting samples of the transmit samples based on the threshold parameter; and

ordering the selected samples, wherein at least a portion of the sample pair groups comprise different ordered portions of the selected samples.

19. The apparatus of claim 17 , wherein determining the representative sample pairs comprises determining a representative receive sample of a sample pair group based on an average of digital receive samples included in the sample pair group.

20. The apparatus of claim 15 , wherein the operations further comprise:

determining phase coefficients based on at least a portion of the signal samples, wherein the predistortion values include values based on additive inverses of the phase coefficients, respectively, and wherein the first values are based on multiplicative inverses of the gain coefficients.

21. The apparatus of claim 15 , wherein the operations further comprise:

storing the predistortion values in a data structure that is indexed by input amplitude values.

22. The apparatus of claim 15 , wherein using the predistortion values comprises:

accessing a sample value of a digital signal;

retrieving a predistortion value from the data structure based on the accessed sample value; and

applying the retrieved predistortion value to the accessed sample value.

23. The apparatus of claim 15 , wherein the threshold parameter is selected based on a voltage associated with a divergence between the power amplifier and an ideal power amplifier that is free of nonlinear characteristics.