Mixed-mode digital predistortion

1. A digital predistortion circuit to generate a predistorted signal for providing to a power amplifier, the digital predistortion circuit comprising:

an interpolator circuit configured to generate an interpolated complex baseband signal based at least in part on a complex baseband signal received from a digital upconverter, wherein the interpolated baseband signal has a sampling frequency higher than a sampling frequency of the complex baseband signal;

a complex-to-real circuit configured to generate a real mode signal based at least in part on the interpolated complex baseband signal; and

a real mode digital predistortion circuit configured to generate a predistorted signal based at least in part on the real mode signal.

2. The digital predistortion circuit of claim 1 , further comprising a digital upconverter to generate the complex baseband signal.

3. The digital predistortion circuit of claim 1 , further comprising an upcoverter configured to generate a translated complex signal based at least in part on the complex baseband signal, wherein the translated complex signal is at a carrier frequency higher than a baseband frequency of the complex baseband signal, and wherein the real mode signal is also based at least in part on the translated complex signal.

4. The digital predistortion circuit of claim 1 , wherein the complex-to-real circuit is configured to generate the real mode signal at a carrier frequency higher than a baseband frequency of the complex baseband signal.

5. The digital predistortion circuit of claim 1 , wherein the real mode digital predistortion circuit is also configured to perform operations comprising:

determining a real component based at least in part on the complex baseband signal;

determining a first value for the real component at a first time; and

selecting from a first nonlinear look up table (LUT) a first LUT value based at least in part on the first value for the real component, wherein the first LUT value is a based at least in part on a sum of terms omitting a first order term.

6. The digital predistortion circuit of claim 5 , wherein the real mode digital predistortion circuit is also configured to perform operations comprising:

selecting from a second LUT a second LUT value based at least in part on a second value for the real component at a second time before the first time; and

determining a sum of the first LUT value and the second LUT value, wherein the predistorted signal is based at least in part on the sum.

7. The digital predistortion circuit of claim 1 , wherein the real mode digital predistortion circuit is also configured to perform operations comprising:

determining a real component based at least in part on the complex baseband signal;

determining a first value for the real component at a first time;

determining I th an order power of the first value;

applying an I th order tap coefficient to the I th order power of the first value to generate an I th order tap value;

determining a J th order power of the first value; and

applying a J th order tap coefficient to the J th order power of the first value to generate a J th order tap value, wherein a nonlinear component of the predistorted signal is based at least in part a sum of the I th order tap value and the J th order tap value.

8. The digital predistortion circuit of claim 1 , further comprising a tilt reference filter circuit programmed to perform operations comprising applying a highpass frequency gain characteristic to a real component of the complex baseband signal to generate a tilt reference real component, wherein the real mode digital predistortion circuit is also programmed to perform operations comprising determining a tilt reference predistorted signal based at least in part on the tilt reference real component.

9. A method of generating a predistorted signal for providing to a power amplifier, comprising:

generating an interpolated complex baseband signal based at least in part on a complex baseband signal, wherein the interpolated baseband signal has a sampling frequency higher than a sampling frequency of the complex baseband signal;

generating a real mode signal based at least in part on the interpolated complex baseband signal; and

generating a predistorted signal based at least in part on the real mode signal.

10. The method of claim 9 , further comprising generating the complex baseband signal.

11. The method of claim 9 , further comprising generating a translated complex signal based at least in part on the complex baseband signal, wherein the translated complex signal is at a carrier frequency higher than a baseband frequency of the complex baseband signal, and wherein the real mode signal is also based at least in part on the translated complex signal.

12. The method of claim 9 , further comprising generating the real mode signal at a carrier frequency higher than a baseband frequency of the complex baseband signal.

13. The method of claim 9 , further comprising:

determining a real component based at least in part on the complex baseband signal;

determining a first value for the real component at a first time; and

selecting from a first nonlinear look up table (LUT) a first LUT value based at least in part on the first value for the real component, wherein the first LUT value is a based at least in part on a sum of terms omitting a first order term.

14. The method of claim 13 , further comprising:

selecting from a second a second LUT value based at least in part on a second value for the real component at a second time before the first time; and

determining a sum of the first LUT value and the second LUT value, wherein the predistorted signal is based at least in part on the sum.

15. The method of claim 9 , further comprising:

determining a real component based at least in part on the complex baseband signal;

determining a first value for the real component at a first time;

determining an I th order power of the first value;

applying an I th order tap coefficient to the J th order power of the first value to generate an I th order tap value;

determining a J th order power of the first value; and

applying a J th order tap coefficient to the J th order power of the first value to generate a J th order tap value, wherein a nonlinear component of the predistorted signal is based at least in part a sum of the I th order tap value and the J th order tap value.

16. The method of claim 9 , further comprising:

applying a highpass frequency gain characteristic to a real component of the complex baseband signal to generate a tilt reference real component; and

determining a tilt reference predistorted signal based at least in part on the tilt reference real component.

17. A system for generating a predistorted signal for providing to a power amplifier, comprising:

means for generating an interpolated complex baseband signal based at least in part on a complex baseband signal, wherein the interpolated baseband signal has a sampling frequency higher than a sampling frequency of the complex baseband signal;

means for generating a real mode signal based at least in part on the interpolated complex baseband signal; and

means for generating a predistorted signal based at least in part on the real mode signal.

18. The system of claim 17 , further comprising means for generating the complex baseband signal.

19. The system of claim 17 , further comprising means for generating a translated complex signal based at least in part on the complex baseband signal, wherein the translated complex signal is at a carrier frequency higher than a baseband frequency of the complex baseband signal, and wherein the real mode signal is also based at least in part on the translated complex signal.

20. The system of claim 17 , further comprising means for generating the real mode signal at a carrier frequency higher than a baseband frequency of the complex baseband signal.