Large-dynamic-range lookup table for a transmitter predistorter and system and method employing the same

• US 8,054,912 B2
• Filed: 06/20/2008
• Issued: 11/08/2011
• 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
• Primary Examiner(s): Payne; David C.
• Assistant Examiner(s): File; Erin
• Application Number:
  US12/142,890
• Publication Number:
  US 20090051426A1
• Time in Patent Office: 1,236 Days
• Field of Search: 375/297
• US Class Current: 375/297
• 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 addressing of said entries is based on amplitude of said input signal.

3. 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.

4. 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.

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

6. 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, wherein said compander is configured to employ a metric based on a constant signal signal-to-noise ratio (SNR) to compand said input signal; 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.

7. 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, wherein said nonlinear element is an amplifier contained in a WCDMA transmitter; 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.

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 entries are based on amplitude of said input signal.

10. 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.

11. 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.

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

13. 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, wherein said producing comprises employing a metric based on a constant signal signal-to-noise ratio (SNR) for said companding.

14. 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, wherein said nonlinear element is an amplifier contained in a WCDMA transmitter; 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.

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. 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, wherein said nonlinear element is an amplifier in a WCDMA transmitter; 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.

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. 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, wherein said nonlinear element is an amplifier in a WCDMA transmitter.

27. 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, wherein said nonlinear element is an amplifier; 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.

28. 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, wherein said nonlinear element is an amplifier; 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.

29. 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, wherein said nonlinear element is an amplifier.