Transmitter predistortion circuit and method therefor

• US 20050163252A1
• Filed: 12/14/2004
• Published: 07/28/2005
• Est. Priority Date: 01/27/2004
• Status: Active Grant

• Current Assignee: Crestcom Incorporated
• Original Assignee: Crestcom Incorporated
• Inventors: Brombaugh, Eric M., McCallister, Ronald Duane
• Granted Patent: US 7,469,491 B2
• Time in Patent Office: Days
• US Class Current: 375/296
• CPC Class Codes: H03F 1/3241 using predistortion circuits H03F1/3211, H03F1/3217 take precedence
• CPC Class Codes: H03F 1/3247 using feedback acting on predistortion circuits H03F1/3264 takes precedence
• CPC Class Codes: H03F 1/3252 using multiple parallel paths between input and output H03F1/3258, H03F1/3282, H03F1/3294 take precedence
• CPC Class Codes: H03F 1/3258 based on polynomial terms
• CPC Class Codes: H03F 1/3294 Acting on the real and imaginary components of the input signal
• CPC Class Codes: H03F 1/34 Negative-feedback-circuit arrangements with or without positive feedback H03F1/02 - H03F1/30, H03F1/38 - H03F1/50, H03F3/50 take precedence ; for rejection of common mode signals H03F3/45479
• CPC Class Codes: H03F 2200/336 A I/Q, i.e. phase quadrature, modulator or demodulator being used in an amplifying circuit
• CPC Class Codes: H03F 3/24 of transmitter output stages
• CPC Class Codes: H03H 17/0294 Variable filters; Programmable filters
• 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
• CPC Class Codes: H04B 2001/0433 with linearisation using feedback
• CPC Class Codes: H04L 27/2614 Peak power aspects
• CPC Class Codes: H04L 5/06 the signals being represented by different frequencies combined with time-division multiplexing H04L5/26

Abstract

A digital communications transmitter (100) includes a digital linear-and-nonlinear predistortion section (200, 1800, 2800) to compensate for linear and nonlinear distortion introduced by transmitter-analog components (120). A direct-digital-downconversion section (300) generates a complex digital return-data stream (254) from the analog components (120) without introducing quadrature imbalance. A relatively low resolution exhibited by the return-data stream (254) is effectively increased through arithmetic processing. Distortion introduced by an analog-to-digital converter (304) may be compensated using a variety of adaptive techniques. Linear distortion is compensated using adaptive techniques with an equalizer (246) positioned in the forward-data stream (112). Nonlinear distortion is then compensated using adaptive techniques with a plurality of equalizers (226) that filter a plurality of orthogonal, higher-ordered-basis functions (214) generated from the forward-data stream (112). The filtered-basis functions are combined together and subtracted from the forward-data stream (112).

Claims

1. A method of compensating for distortion introduced by analog-transmitter components of a digital communications transmitter, said method comprising:

obtaining a forward-data stream configured to convey a plurality of frequency-multiplexed communication channels;

processing said forward-data stream through said analog-transmitter components;

obtaining a return-data stream responsive to said forward-data stream after said forward-data stream has been influenced by said analog-transmitter components;

identifying a communication-signal strength for each of said communication channels in response to said forward-data stream;

identifying an error-signal strength for each of said communication channels in response to said return-data stream; and

counteracting distortion introduced by said analog-transmitter components in response to relative communication-signal strengths and error-signal strengths for said communication channels.

2. A method as claimed in claim 1 wherein said return-data-stream-obtaining activity comprises:

converting a return-analog signal into a return-raw-digitized-data stream through an analog-to-digital converter (A/D); and

generating said return-data stream by symmetrizing quantization error introduced into said return-raw-digitized-data stream by said A/D.

3. A method as claimed in claim 1 wherein said return-data-stream-obtaining activity comprises:

converting a return-analog signal into a return-raw-digitized-data stream through an analog-to-digital converter (A/D); and

generating said return-data stream by compensating said return-raw-digitized-data stream for quantization error introduced by said A/D.

4. A method as claimed in claim 3 wherein said generating activity is responsive to said forward-data stream.

5. A method as claimed in claim 3 wherein said A/D includes a quantizer, and said generating activity comprises:

evaluating said quantizer;

programming a quantizer-simulator in response to said evaluating activity;

operating said quantizer-simulator in response to said forward-data stream to generate a quantize-simulated-data stream; and

determining a difference between said forward-data stream and said quantize-simulated-data stream.

6. A method as claimed in claim 5 additionally comprising up-converting said forward-data stream to generate an IF-forward-data stream so that said quantizer-simulator quantizes said IF-forward-data stream.

7. A method as claimed in claim 6 wherein:

said difference between said forward-data stream and said quantize-simulated-data stream provides a quantization-error-data stream; and

said return-data-stream obtaining activity further comprises combining said quantization-error-data stream with said return-raw-digitized-data stream to form said return-data stream.

8. A method as claimed in claim 1 wherein said communication-signal-strength-identifying activity comprises separating said forward-data stream into a plurality of discrete communication signals corresponding to said plurality of frequency-multiplexed communication channels.

9. A method as claimed in claim 8 wherein said counteracting activity comprises:

scaling each of said discrete communication signals;

combining said discrete communication signals after said scaling activity to form a merged communication signal; and

adapting coefficients of an equalizer positioned in series with said analog-transmitter components in response to said merged communication signal.

10. A method as claimed in claim 9 wherein said scaling activity comprises forming scale factors in accordance with an algorithm that emphasizes the influence of weaker ones of said discrete communication signals in said adapting activity and deemphasizes the influence of stronger ones of said discrete communication signals in said adapting activity.

11. A method as claimed in claim 1 wherein said error-signal-strength-identifying activity comprises:

forming a first merged-error signal by subtracting one of said forward-data and return-data streams from the other of said forward-data and return-data streams; and

separating said first merged-error signal into a plurality of discrete error signals corresponding to said plurality of frequency-multiplexed communication channels.

12. A method as claimed in claim 11 wherein said counteracting activity comprises:

scaling each of said discrete error signals;

combining said discrete error signals after said scaling activity to form a second merged error signal; and

adapting coefficients of an equalizer positioned in series with said analog transmitter components in response to said second merged error signal.

13. A method as claimed in claim 12 wherein:

said equalizer is a complex equalizer having a direct path and a crossover path;

said scaling activity comprises a first multiplying activity which scales said discrete error signals for adapting coefficients in said direct path of said equalizer and a second multiplying activity which scales said discrete error signals for adapting coefficients in said crossover path of said equalizer; and

said combining activity forms a second merged-direct-path-error signal in response to said first multiplying activity and a second merged-crossover-path-error signal in response to said second multiplying activity.

14. A method as claimed in claim 12 wherein:

said communication-signal-strength-identifying activity comprises separating said forward-data stream into a plurality of discrete communication signals corresponding to said plurality of frequency-multiplexed communication channels; and

said scaling activity comprises forming scale factors in accordance with an algorithm that emphasizes influence in said adapting activity of said communication channels having weaker communication-signal strength and deemphasizes influence in said communication channels having stronger communication-signal strength.

15. A method as claimed in claim 1 wherein:

said processing activity processes said forward-data stream through an equalizer prior to processing said forward-data stream through said analog-transmitter components; and

said counteracting activity adjusts coefficients of said equalizer.

16. A method as claimed in claim 1 wherein said counteracting activity is configured so that distortion introduced by said analog-transmitter components is counteracted in response to relative error-vector magnitudes (EVM'"'"'s) of said communication channels.

17. A method as claimed in claim 1 wherein said counteracting activity is configured so that distortion introduced by said analog-transmitter components is counteracted more in response to communication channels having higher error-vector magnitudes (EVM'"'"'s) than in response to communication channels having lower EVM'"'"'s.

18. A method as claimed in claim 1 wherein:

said forward-data stream includes an in-phase component and a quadrature component;

said processing activity imparts different amounts of delay to said in-phase and quadrature components; and

said method additionally comprises dynamically compensating for said different amounts of delay imparted to said in-phase and quadrature components.

19. A method of compensating for distortion introduced by analog-transmitter components of a digital communications transmitter, said method comprising:

processing a forward-data stream to produce a processed-forward-data stream;

converting said processed-forward-data stream into a forward-analog signal using a digital-to-analog converter (D/A);

propagating said forward-analog signal through said analog-transmitter components to generate a return-analog signal;

converting said return-analog signal into a return-raw-digitized-data stream using an analog-to-digital converter (A/D);

compensating, in response to said forward-data stream, for errors introduced into said return-raw-digitized-data stream by said A/D to produce a return-data stream; and

altering said processed forward-data stream in response to said return-data stream.

20. A method as claimed in claim 19 wherein said D/A exhibits higher resolution than said A/D.

21. A method as claimed in claim 19 wherein said compensating activity compensates for quantization error introduced by said A/D.

22. A method as claimed in claim 21 wherein said A/D includes a quantizer, and said compensating activity comprises:

evaluating said quantizer;

programming a quantizer-simulator in response to said evaluating activity;

operating said quantizer-simulator in response to said forward-data stream to generate a quantize-simulated-data stream; and

determining a difference between said forward-data stream and said quantize-simulated-data stream.

23. A method as claimed in claim 22 additionally comprising up-converting said forward-data stream to generate an IF-forward-data stream so that said quantizer-simulator quantizes said IF-forward-data stream.

24. A method as claimed in claim 23 wherein:

said difference between said forward-data stream and said quantize-simulated-data stream provides a quantization-error-data stream; and

said compensating activity further comprises combining said quantization-error-data stream with said return-raw-digitized-data stream to form said return-data stream.

25. A method as claimed in claim 21 wherein said compensating activity additionally symmetrizes said quantization error introduced into said return-raw-digitized-data stream by said A/D.

26. A method as claimed in claim 19 wherein:

said processing activity applies said forward-data stream to an equalizer; and

said altering activity adjusts coefficients of said equalizer.

27. A method as claimed in claim 19 wherein:

said forward-data stream is configured to convey a plurality of frequency-multiplexed communication channels; and

said altering activity is configured so that distortion introduced by said analog-transmitter components is reduced, and said distortion is reduced in a way that achieves greater reduction in error-vector magnitude (EVM) for ones of said communication channels having higher EVM'"'"'s than is achieved for communication channels having lower EVM'"'"'s.

28. A method as claimed in claim 19 wherein said compensating activity symmetrizes quantization error introduced by said A/D.

29. A method as claimed in claim 19 wherein said compensating activity compensates for nonlinear distortion introduced by said A/D.

30. A method as claimed in claim 19 wherein said compensating activity compensates for linear distortion introduced by said A/D.

31. A predistortion circuit for compensating distortion introduced by analog-transmitter components of a digital communications transmitter, said predistortion circuit comprising:

an equalizer configured to receive a forward-data stream that conveys a plurality of frequency-multiplexed communication channels and to generate a processed-forward-data stream which propagates toward said analog-transmitter components;

an analog-to-digital converter (A/D) adapted to receive a return-analog signal from said analog-transmitter components;

a first signal-strength-measuring circuit adapted to receive said forward-data stream and configured to identify a communication-signal strength for each of said communication channels;

a second signal-strength-measuring circuit coupled to said A/D and configured to identify an error-signal strength for each of said communication channels; and

a control circuit coupled to said first and second signal-strength-measuring circuits and to said equalizer, said control circuit being configured to adapt coefficients of said equalizer in response to relative communication-signal strengths and error-signal strengths for said communication channels to counteract distortion introduced by said analog-transmitter components.

32. A predistortion circuit as claimed in claim 31 wherein said first signal-strength-measuring circuit is configured to separate said forward-data stream into a plurality of discrete communication signals corresponding to said plurality of frequency-multiplexed communication channels.

33. A predistortion circuit as claimed in claim 31 wherein said control circuit comprises a plurality of multipliers adapted to receive said plurality of discrete communication signals and to produce a plurality of scaled discrete communication signals, and said control circuit is configured to combine said scaled discrete communication signals to form a merged communication signal, said merged communication signal being used to adapt said coefficients of said equalizer.

34. A predistortion circuit as claimed in claim 31 wherein:

said A/D is configured to originate a return-data stream;

said second signal-strength-measuring circuit is responsive to a first merged-error signal formed by subtracting one of said forward-data and return-data streams from the other of said forward-data and return-data streams; and

said second signal-strength-measuring circuit is configured to separate said first merged-error signal into a plurality of discrete error signals corresponding to said plurality of frequency-multiplexed communication channels.

35. A predistortion circuit as claimed in claim 34 wherein said control circuit comprises a plurality of multipliers adapted to receive said plurality of discrete error signals and to produce a plurality of scaled discrete error signals, and said control circuit is configured to combine said scaled discrete error signals to form a second merged-error signal, said second merged-error signal being used to adapt said coefficients of said equalizer.

36. A predistortion circuit for compensating distortion introduced by analog-transmitter components of a digital communications transmitter, said predistortion circuit comprising:

an equalizer configured to receive a forward-data stream and to generate a processed-forward-data stream;

a digital-to-analog converter (D/A) coupled to said equalizer and configured to convert said processed-forward-data stream into a forward-analog signal which propagates through said analog-transmitter components;

an analog-to-digital converter (A/D) adapted to receive a return-analog signal from said analog-transmitter components, said return-analog signal being responsive to said forward-analog signal, and said A/D being configured to produce a return-raw-digitized-data stream;

an A/D compensation section adapted to receive said forward-data stream, said A/D compensation section being configured to compensate for errors introduced into said return-raw-digitized-data stream by said A/D and to produce a return-data stream; and

a control circuit coupled to said equalizer, said control circuit being configured to alter said processed-forward-data stream in response to said return-data stream.

37. A predistortion circuit as claimed in claim 36 wherein said D/A exhibits higher resolution than said A/D.

38. A predistortion circuit claimed in claim 36 wherein said A/D compensation section is configured to compensate for quantization error introduced by said A/D.

39. A predistortion circuit as claimed in claim 38 wherein said A/D includes a quantizer characterized by switching thresholds, and said A/D compensation section comprises:

a quantizer-simulator configured to simulate said quantizer when programmed with said switching thresholds which characterize said quantizer, said quantizer-simulator being adapted to receive said forward-data stream and to generate a quantize-simulated-data stream; and

a differencing circuit configured to determine a difference between said forward-data stream and said quantize-simulated-data stream.

40. A predistortion circuit as claimed in claim 39 wherein said A/D compensation section comprises a digital up-conversion section coupled to said quantizer-simulator and configured to generate an IF-forward-data stream so that said quantizer-simulator quantizes said IF-forward-data stream.

41. A predistortion circuit as claimed in claim 40 wherein:

said differencing circuit generates a quantization-error-data stream; and

said predistortion circuit additionally comprises a combining circuit which combines said quantization-error-data stream with said return-raw-digitized-data stream to form said return-data stream.

42. A predistortion circuit as claimed in claim 38 wherein said A/D compensation section is configured to symmetrize said quantization error introduced into said return-raw-digitized-data stream by said A/D.

43. A predistortion circuit as claimed in claim 36 wherein said control circuit is configured to alter said processed-forward-data stream by influencing adaptation of coefficients within said equalizer.

44. A predistortion circuit as claimed in claim 36 wherein:

said forward-data stream is configured to convey a plurality of frequency-multiplexed communication channels; and

said control circuit is configured so that distortion introduced by said analog-transmitter components is reduced, and said distortion is reduced in a way that achieves greater reduction in error-vector magnitude (EVM) for ones of said communication channels having higher EVM'"'"'s than is achieved for communication channels having lower EVM'"'"'s.