Large-Dynamic-Range Lookup Table for a Transmitter Predistorter and System and Method Employing the Same

• US 20090051426A1
• Filed: 06/20/2008
• Published: 02/26/2009
• Est. Priority Date: 08/21/2007
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Ba, Seydou N., Waheed, Khurram
• Granted Patent: US 8,054,912 B2
• Time in Patent Office: Days
• US Class Current: 330/149
• CPC Class Codes: H03F 1/3247 using feedback acting on predistortion circuits H03F1/3264 takes precedence
• CPC Class Codes: H03F 1/3294 Acting on the real and imaginary components of the input signal
• CPC Class Codes: H03F 2201/3233 Adaptive predistortion using lookup table, e.g. memory, RAM, ROM, LUT, to generate the predistortion
• CPC Class Codes: H03F 3/19 with semiconductor devices only

Abstract

A predistorters for use with a nonlinear element and methods of predistorting for a nonlinear element for use in a 3G, e.g., WCDMA transmitter. In one embodiment, the predistorter includes: (1) a lookup table having non-uniformly spaced entries therein, (2) a compander configured to compand an input signal based on a nonlinearity of the nonlinear element to address the entries and (3) an interpolation offset calculation circuit associated with the lookup table and configured to produce an output based on a value of the input signal and a linear interpolation involving at least two entries from the lookup table.

Claims

1. A predistorter for use with a nonlinear element, comprising:

a lookup table having non-uniformly spaced entries therein;

a compander configured to compand an input signal based on a nonlinearity of said nonlinear element to address said entries; and

an interpolation offset calculation circuit associated with said lookup table and configured to produce an output based on a value of said input signal and a linear interpolation involving at least two entries from said lookup table.

2. The predistorter as recited in claim 1 wherein said compander is configured to employ a metric based on a constant signal signal-to-noise ratio (SNR) to compand said input signal.

3. The predistorter as recited in claim 1 wherein addressing of said entries is based on amplitude of said input signal.

4. The predistorter as recited in claim 1 wherein values of said entries contain at least one selected from the group consisting of:

an output amplitude,

a phase,

a real gain, and

a complex gain for at least one modulation signal.

5. The predistorter as recited in claim 1 wherein said predistorter causes said nonlinear element to produce an output signal having a substantially constant signal-to-noise ratio over an amplitude range of interest.

6. The predistorter as recited in claim 1 wherein said lookup table is an inverse-gain-mapped lookup table.

7. The predistorter as recited in claim 1 wherein said nonlinear element is an amplifier contained in a WCDMA transmitter.

8. A method of predistorting for a nonlinear element, comprising:

companding an input signal based on a nonlinearity of said nonlinear element to address non-uniformly spaced entries contained in a lookup table; and

producing an output from said lookup table based on a linear interpolation involving a value of said input signal and at least two of said entries.

9. The method as recited in claim 8 wherein said producing comprises employing a metric based on a constant signal signal-to-noise ratio (SNR) for said companding.

10. The method as recited in claim 8 wherein said entries are based on amplitude of said input signal.

11. The method as recited in claim 8 wherein values of said entries contain at least one selected from the group consisting of:

an output amplitude,

a phase,

a real gain, and

a complex gain for at least one modulation signal.

12. The method as recited in claim 8 further comprising causing said nonlinear element to produce an output signal having a substantially constant signal-to-noise ratio over an amplitude range of interest.

13. The method as recited in claim 8 wherein said lookup table is an inverse-gain-mapped lookup table.

14. The method as recited in claim 8 wherein said nonlinear element is an amplifier contained in a WCDMA transmitter.

15. A predistorter for use with a nonlinear element, comprising:

a lookup table containing base-2-spaced entries;

an address calculation block associated with said lookup table and configured to calculate addresses for said entries based on a mapping between an amplitude of an input signal and bits of said addresses; and

an interpolation circuit associated with said lookup table and configured to produce a selected one of an interpolation factor and an interpolated output based on a bin width and an offset derived from said input signal.

16. The predistorter as recited in claim 15 wherein said bin width is a power of two.

17. The predistorter as recited in claim 15 wherein intervals in said lookup table is at least ten and said address calculation block employs a 13 most significant bits of said amplitude to calculate said addresses.

18. The predistorter as recited in claim 15 wherein said lookup table is an inverse-gain-mapped lookup table.

19. The predistorter as recited in claim 15 wherein intervals in said lookup table contain different numbers of entries.

20. The predistorter as recited in claim 15 wherein said nonlinear element is an amplifier in a WCDMA transmitter.

21. A method of predistorting for a nonlinear element, comprising:

calculating addresses for base-2-spaced entries in a lookup table based on a mapping between an amplitude of an input signal and bits of said addresses; and

producing a selected one of an interpolation factor and an interpolated output based on a bin width and an offset derived from said input signal.

22. The method as recited in claim 21 wherein said bin width is a power of two.

23. The method as recited in claim 21 wherein at least ten of said intervals are in said lookup table and said producing comprises employing a 13 most significant bits of said amplitude to calculate said addresses.

24. The method as recited in claim 21 wherein said lookup table is an inverse-gain-mapped lookup table.

25. The method as recited in claim 21 wherein intervals in said lookup table contain different numbers of entries.

26. The method as recited in claim 21 wherein said nonlinear element is an amplifier in a WCDMA transmitter.