Mixed-mode digital predistortion

1. A system comprising:

a linear digital predistortion (DPD) circuit programmed to perform operations comprising:

receiving a complex baseband signal; and

generating a pre-distorted signal linear component based at least in part on the complex baseband signal;

a nonlinear DPD circuit programmed to perform, operations comprising:

receiving the complex baseband signal;

determining a real-mode pre-distorted nonlinear component based at least in part on the complex baseband signal; and

generating a pre-distorted signal nonlinear component based at least in part on the real-mode pre-distorted nonlinear component; and

a mixer circuit programmed to generate a pre-distorted signal based at least in part on the pre-distorted signal linear component and the pre-distorted signal nonlinear component.

2. The system of claim 1 , further comprising a complex-to-real circuit programmed to determine the real-mode pre-distorted nonlinear component.

3. The system of claim 1 , further comprising:

an interpolator circuit to up-sample the complex baseband signal to generate an up-sampled complex baseband signal; and

an up-converter circuit to translate the up-sampled complex baseband signal to a carrier frequency to generate an up-converted complex signal, wherein the mixer circuit is also to generate the pre-distorted signal based at least in part on the up-converted complex signal.

4. The system of claim 1 , wherein the nonlinear DPD circuit is also programmed 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.

5. The system of claim 4 , wherein the nonlinear DPD circuit is also programmed 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 pre-distorted signal nonlinear component is based at least in part on the stum.

6. The system of claim 1 , wherein the nonlinear DPD circuit is also programmed 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 an I th 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 the pre-distorted signal nonlinear component is based at least in part a sum of the I th order tap value and the J th order tap value.

7. The system of claim 1 , wherein the nonlinear DPD circuit is further programmed to perform operations comprising

converting the real-mode pre-distorted nonlinear component to a complex pre-distorted nonlinear component.

8. The system of claim 7 , wherein the nonlinear DPD circuit is further programmed to perform operations comprising translating the complex pre-distorted nonlinear component to baseband.

9. The system of claim 1 , wherein the linear DPD circuit is further programmed to perform operations comprising applying a scalar gain stage to the complex baseband signal.

10. The system of claim 1 , further comprising a complex time align circuit programmed to align the pre-distorted linear component and the pre-distorted nonlinear component.

11. The system 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 nonlinear DPD circuit is also programmed to perform operations comprising determining a tilt reference real pre-distorted signal nonlinear component based at least in part on the tilt reference real component.

12. The system of claim 1 , wherein the nonlinear DPD circuit is further programmed to perform operations comprising generating a real pre-distorted signal nonlinear component at a carrier frequency, further comprising:

a complex decimation filter circuit programmed to perform operations comprising applying a passband to the real pre-distorted signal nonlinear component to generate a quadrature pre-distorted signal nonlinear component, wherein the passband has a bandwidth matching a bandwidth of the complex baseband signal centered at the carrier frequency; and

a demodulation circuit programmed to perform operations comprising demodulating the quadrature pre-distorted signal nonlinear component to baseband.

13. A method comprising:

receiving a complex baseband signal at a linear DPD circuit;

generating, with the linear DPD circuit, a pre-distorted signal linear component based at least in part on the complex baseband signal;

receiving the complex baseband signal at a nonlinear DPD circuit;

determining a real-mode pre-distorted nonlinear component based at least in part on the complex baseband signal;

generating, with the nonlinear DPD circuit, a pre-distorted signal nonlinear component based at least in part on the real-mode pre-distorted nonlinear component; and

generating a pre-distorted signal based at least in part on the pre-distorted signal linear component and the pre-distorted signal nonlinear component.

14. The method of claim 13 , further comprising:

up-sampling the complex baseband signal to generate an up-sampled complex baseband signal; and

translating the up-sampled complex baseband signal to a carrier frequency to generate an up-converted complex signal, wherein generating the pre-distorted signal is also based at least in part on the up-converted complex signal.

15. The method of claim 13 , 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.

16. The method of claim 15 , further 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 pre-distorted signal nonlinear component is based at least in part on the sum.

17. The method of claim 13 , 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 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 the pre-distorted signal nonlinear component is based at least in part a. sum of the I th order tap value and the J th order tap value.

18. The method of claim 13 , further comprising:

determining a real-mode pre-distorted nonlinear component; and

converting the real-mode pre-distorted nonlinear component to a complex pre-distorted nonlinear component.

19. The method of claim 13 , further comprising applying a scalar gain stage to the complex baseband signal.

20. The method of claim 13 , further comprising a complex time align circuit programmed to align the pre-distorted linear component and the pre-distorted nonlinear component.

21. The method of claim 13 , further comprising:

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

determining a tilt reference real pre-distorted signal nonlinear component based at least in part on the tilt reference real component; and

applying a tilt equalizer to the tilt reference real pre-distorted signal to generate the real pre-distorted signal nonlinear component.

22. The method of claim 13 , further comprising:

generating a real pre-distorted signal nonlinear component at a carrier frequency;

applying a passband to the real pre-distorted signal nonlinear component to generate a quadrature pre-distorted signal nonlinear component, wherein the passband has a bandwidth matching a bandwidth of the complex baseband signal centered at the carrier frequency; and

demodulating the quadrature pre-distorted signal nonlinear component to baseband.

23. A digital predistortion (DPD) apparatus, comprising:

means receiving a complex baseband signal;

means for generating a pre-distorted signal linear component based at least in part on the complex baseband signal;

means for receiving the complex baseband signal at a nonlinear DPD circuit; means for determining a real-mode pre-distorted nonlinear component based at least in part on the complex baseband signal;

means for generating a pre-distorted signal nonlinear component based at least in part on the real-mode pre-distorted nonlinear component; and

means for generating a pre-distorted signal based at least in part on the pie-distorted signal linear component and the pre-distorted signal nonlinear component.