LINEARIZATION OF A TRANSMIT AMPLIFIER

• US 20070190952A1
• Filed: 02/15/2007
• Published: 08/16/2007
• Est. Priority Date: 02/15/2006
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Staszewski, Robert B., Waheed, Khurram, Rezeq, Sameh S., Eliezer, Oren E.
• Granted Patent: US 8,195,103 B2
• Time in Patent Office: Days
• US Class Current: 455/114.3
• CPC Class Codes: H03F 1/3241 using predistortion circuits H03F1/3211, H03F1/3217 take precedence
• CPC Class Codes: H04B 1/0475 with means for limiting noise, interference or distortion H04B1/0483 takes precedence
• CPC Class Codes: H04B 2001/0425 with linearisation using predistortion

Abstract

A novel apparatus and method of linearization of a digitally controlled pre-power amplifier (DPA) and RF power amplifier (PA). The mechanism is operative to perform predistortion calibration to compensate for nonlinearities in the DPA and PA circuits. A predistortion look up table (LUT) stores measured distortion compensation data that is applied to the TX data before being input to the digital to frequency converter (DFC), DPA and PA. The mechanism of the invention takes advantage of the on-chip receiver, which is normally inactive during the TX burst in a half-duplex operation, to demodulate the RF PA output and use the digital I/Q RX outputs to perform calibration of the TX pre-distortion tables. Controlled RF coupling is used to provide a sample of the RF output signal that to the receiver chain. The contents of the predistortion LUT are typically updated during the PA power up or down ramp. While the digitally-controlled PA (DPA) code is increasing (or decreasing), the amplitude and phase of the recovered I/Q samples are used to determine the instantaneous value of the AM/AM and AM/PM pre-distortion from which an update to the predistortion tables may be computed.

Claims

1. A method of linearization of a transmit amplifier, said method comprising the steps of:

scanning a plurality of power amplifier codes;

generating a transmit amplifier output signal in accordance with each code scanned; and

determining one or more predistortion values in accordance with said transmit amplifier output signal.

2. The method according to claim 1, wherein said step of scanning comprises ramping said plurality of codes either up or down.

3. The method according to claim 1, wherein said step of scanning comprises using codes corresponding to the amplitude modulation waveform used during a transmitted payload of data.

4. The method according to claim 1, wherein said step of determining comprises the step of inputting said transmit amplifier output signal to an on-chip receiver chain operative to generate I and Q samples therefrom.

5. The method according to claim 1, wherein said predistortion values comprises AM/AM and AM/PM predistortion values.

6. The method according to claim 1, wherein said transmit amplifier comprises a combination of digitally controlled pre-power amplifier and radio frequency (RF) power amplifier.

7. The method according to claim 1, wherein said step of determining comprises incorporating information from one or more on-chip sensors in the calculation of said predistortion values.

8. A method of compensating for predistortion of a transmit amplifier, said method comprising the steps of:

scanning a plurality of transmit amplifier codes;

generating a transmit amplifier output signal in accordance with each code scanned;

providing controlled leakage of said transmit amplifier output signal to a receiver chain operative to demodulate said transmit amplifier output signal to yield I and Q samples therefrom;

calculating amplitude and phase distortion values in accordance with said I and Q samples; and

calibrating a predistortion table with said amplitude and phase distortion values.

9. The method according to claim 8, wherein said receiver chain is co-located on-chip with a transmitter operative to generate said transmit amplifier output signal.

10. The method according to claim 8, wherein said step of scanning comprises the step of applying a ramp up or ramp down signal to a transmitter circuit.

11. The method according to claim 8, wherein said step of calibrating comprises the step of storing said amplitude and phase distortion values in a predistortion look up table (LUT).

12. The method according to claim 8, wherein said step of generating a transmit amplifier output signal is performed during a power ramp up or ramp down of a GGE burst.

13. The method according to claim 8, wherein said step of generating a transmit amplifier output signal is performed during the payload of a burst having a known waveform of varying amplitude.

14. The method according to claim 8, wherein said transmit amplifier comprises a combination of digitally controlled pre-power amplifier and radio frequency (RF) power amplifier.

15. A method of predistortion calibration of a transmit amplifier, said method comprising the steps of:

stepping through a dynamic range of codes of said transmit amplifier;

inputting each code to a transmit chain comprising a predistortion look up table (LUT) of transmit amplifier predistortion values to generate a radio frequency (RF) output signal in response thereto;

demodulating said RF output signal to recover I and Q samples therefrom;

calculating an instantaneous correction to the predistortion values stored in said predistortion LUT in accordance with said recovered I and Q values; and

updating the contents of said predistortion LUT in accordance with said calculated instantaneous correction.

16. The method according to claim 15, wherein said step of stepping is performed during the ramp up portion of a GSM/GPRS/EDGE (GGE) burst transmission.

17. The method according to claim 15, wherein said step of stepping is performed during the ramp down portion of a GSM/GPRS/EDGE (GGE) burst transmission.

18. The method according to claim 15, wherein said step of demodulating is performed utilizing a receiver chain co-located on-chip with said transmit chain.

19. The method according to claim 15, wherein said predistortion values comprise AM/AM and AM/PM predistortion values.

20. The method according to claim 15, wherein said step of updating comprises the steps of:

reading the predistortion values currently stored in said predistortion LUT;

applying said instantaneous correction to the predistortion values read from said predistortion LUT to yield corrected predistortion values thereby; and

saving said corrected predistortion values back into said predistortion LUT.

21. The method according to claim 20, further comprising the step of interpolating the updated contents of said predistortion LUT for any missing correction values.

22. The method according to claim 15, wherein said step of updating comprises the steps of:

reading the predistortion values currently stored in said predistortion LUT;

applying said instantaneous correction to the predistortion values read from said predistortion LUT to yield filtered corrected predistortion values; and

saving said corrected predistortion values back into said predistortion LUT.

23. The method according to claim 22, wherein said filtered corrected predistortion values are generated by applying a moving average filter to a plurality of instantaneous correction values.

24. The method according to claim 22, wherein said filtered corrected predistortion values are generated by applying an infinite impulse response (IIR) filter to a plurality of instantaneous correction values.

25. The method according to claim 22, wherein said filtered corrected predistortion values are generated by applying an auto-regressive moving average (ARMA) filter to a plurality of instantaneous correction values.

26. The method according to claim 22, wherein said filtered corrected predistortion values are generated by applying a nonlinear threshold function permitting said step of correcting to ignore inputs exceeding a predetermined range of expected values caused by strong interference induced into the receiver.

27. The method according to claim 15, wherein said transmit amplifier comprises a combination of digitally controlled pre-power amplifier and radio frequency (RF) power amplifier.

28. An apparatus for calibrating predistortion of a digitally controlled pre-power amplifier (DPA) and power amplifier (PA), comprising:

means for stepping through a range of codes of said DPA;

a transmit chain comprising a predistortion look up table (LUT) adapted to store a plurality of amplifier predistortion values, said transmit chain operative to generate a radio frequency (RF) output signal in response to each code;

means for coupling said RF output signal to a receiver chain;

said receive chain operative to demodulate said RF output signal to generate recovered I and Q samples therefrom;

means for calculating corrections to the predistortion values stored in said predistortion LUT in accordance with said recovered I and Q values; and

update means for updating the contents of said predistortion LUT in accordance with said calculated corrections.

29. The apparatus according to claim 28, wherein said means for calculating comprises means for incorporating information from one or more on-chip sensors in the calculation of said predistortion values.

30. The apparatus according to claim 29, wherein said on-chip sensor comprises a temperature sensor operative to generate temperature information.

31. The apparatus according to claim 28, wherein said means for stepping comprises means for generating said range of codes during the ramp up portion of a GSM/GPRS/EDGE (GGE) burst transmission.

32. The apparatus according to claim 28, wherein said means for stepping comprises means for generating said range of codes during the ramp down portion of a GSM/GPRS/EDGE (GGE) burst transmission.

33. The apparatus according to claim 28, wherein said receive chain is co-located on-chip with said transmit chain.

34. The apparatus according to claim 28, wherein said predistortion values comprise AM/AM and AM/PM predistortion values.

35. The apparatus according to claim 28, wherein said update means comprises means for applying a filter function to said corrections.

36. The apparatus according to claim 35, wherein said filter function comprises a moving average function.

37. The apparatus according to claim 35, wherein said filter function comprises an infinite impulse response (IIR) filter function.

38. The apparatus according to claim 35, wherein said filter function comprises an auto-regressive moving average (ARMA) filter function.

39. The apparatus according to claim 35, wherein said filter function comprises a threshold function that would determine whether the computed update exceeds a predefined magnitude.

40. The apparatus according to claim 35, wherein said range of codes are stepped through, corresponding correction values calculated and said predistortion LUT updated during a time wherein said transmit chain is inactive.

41. A method of hybrid linearization compensation for use in a transceiver, said method comprising the steps of:

first performing predistortion compensation during the ramp portion of a first end of a transmission burst to generate relative power level information feedback to update a predistortion look up table (LUT) thereby;

second performing power control level (PCL) compensation during the ramp portion of a second end of said transmission burst to generate absolute power level information feedback and to update a power control LUT thereby; and

tracking changes in said power control LUT over an operational range of said transceiver in accordance with said relative power level information feedback and said absolute power level information feedback.

42. The method according to claim 41, wherein said of performing predistortion compensation comprises the steps of:

scanning a plurality of power amplifier codes;

generating a transmit amplifier output signal in accordance with each code scanned; and

determining one or more predistortion values in accordance with said transmit amplifier output signal and updating a predistortion look up table (LUT) in accordance therewith.

43. The method according to claim 41, wherein said power control LUT defines a transformation between required average power at the output of said transceiver to an average transmit code.