Method and system for decoding multilevel signals

• US 20020196510A1
• Filed: 03/28/2002
• Published: 12/26/2002
• Est. Priority Date: 04/04/2001
• Status: Active Grant

• Current Assignee: Intersil Americas LLC (Renesas Electronics Corporation)
• Original Assignee: Quellan, Inc. (Renesas Electronics Corporation)
• Inventors: Kim, Andrew Joo, Hietala, Vincent Mark
• Granted Patent: US 7,215,721 B2
• Time in Patent Office: Days
• US Class Current: 398/202
• CPC Class Codes: H04L 1/004 by using forward error control H04L1/0618 takes precedence; coding, decoding or code conversion, for error detection or correction H03M13/00
• CPC Class Codes: H04L 25/03343 Arrangements at the transmitter end
• CPC Class Codes: H04L 27/02 Amplitude-modulated carrier systems, e.g. using on-off keying; Single sideband or vestigial sideband modulation H04L27/32 takes precedence
• CPC Class Codes: H04L 27/367 using predistortion

Abstract

A multilevel optical receiver can comprise a plurality of comparators that generally correspond with the number of levels in a multilevel data stream. Each comparator can be individually controlled and fed a decision threshold in order to decode a multilevel signal. The multilevel optical receiver can generate a statistical characterization of the received symbols in the form of a marginal cumulative distribution function (CDF) or probability density function (pdf). This characterization can be used to produce a set of ε-support estimates from which conditional pdfs are derived for each of the transmission symbols. These conditional pdfs may then be used to determine decision thresholds for decoding the received signal. The conditional pdfs may further be used to continuously estimate the fidelity or error rate of the received signal without the transmission of a testing sequence. The supports may further be used to automatically control the gain on the receiver.

Claims

1. An optical receiver for receiving and decoding a multilevel optical communication signal, comprising:

a plurality of first comparators for estimating a transmitted level based on the received multilevel signal;

a decoder coupled to the first comparators;

a second comparator connected in parallel with the first comparators, for indicating when the received signal exceeds a voltage;

a filter coupled to the second comparator, for calculating a cumulative distribution function for the received multilevel signal;

an analog-to-digital converter coupled to the filter for sampling a cumulative distribution function; and

a microcontroller for processing the cumulative distribution function to determine threshold voltage values, for feeding the threshold voltage values to the first comparators for decoding the multilevel signal.

2. The optical receiver of claim 1, wherein the multilevel signal comprises at least two levels.

3. The optical receiver of claim 1, further comprising a latch coupled between the second comparator and the filter, the latch passing a portion of the thresholded signal to the filter, the portion of the signal being synchronized with a clock and the plurality of first comparators.

4. The optical receiver of claim 1, wherein the first comparators and the second comparator are manufactured on a single integrated circuit chip.

5. The optical receiver of claim 1, wherein the microcontroller further calculates a probability density function based on the cumulative distribution function.

6. The optical receiver of claim 1, wherein the microcontroller further estimates a conditional probability density function associated with each symbol of the multilevel signal based on an associated ε

-support region and determines a probability of error for the channel based upon the conditional probability density functions and decision thresholds.

7. The optical receiver of claim 6, further comprising programmable analog signal processing modules that process the multilevel signal prior to the comparators, where the microcontroller selects an operating point of the programmable analog signal processing modules based upon the determination of the fidelity measure of the multilevel signal.

8. The optical receiver of claim 7, wherein the analog signal processing modules comprise one of an equalizer and signal conditioning filter.

9. The optical receiver of claim 7, wherein the analog signal processing modules comprise a clock recovery circuit.

10. The optical receiver of claim 1, wherein the filter averages the output of the second comparator over time.

11. The optical receiver of claim 1, wherein the microcontroller iteratively sets a reference voltage and samples the voltage from the filter, wherein the microcontroller sweeps the reference voltage over a range of voltage levels in order to generate the cumulative distribution function.

13. The optical receiver of claim 12, wherein the multilevel signal comprises at least two levels.

14. The optical receiver of claim 12, further comprising a latch coupled between the second pair of comparators and the filter, the latch passing a portion of the thresholded signal to the filter, the portion of the signal being synchronized with a clock and the plurality of first comparators.

15. The optical receiver of claim 12, wherein the microcontroller further estimates a conditional probability density function associated with each symbol of the multilevel signal based on an associated ε

-support region and determines a probability of error for the channel based upon the conditional probability density functions and decision thresholds.

16. The optical receiver of claim 15, further comprising programmable analog signal processing modules that process the multilevel signal prior to the comparators, where the microcontroller selects an operating point of the programmable analog signal processing modules based upon the determination of the fidelity measure of the multilevel signal.

17. The optical receiver of claim 16, wherein the analog signal processing modules comprise one of an equalizer and signal conditioning filter.

18. The optical receiver of claim 16. wherein the analog signal processing modules comprise a clock recovery circuit.

19. The optical receiver of claim 12, wherein the filter averages the output of the logic circuit over time.

20. The optical receiver of claim 12, wherein the microcontroller iteratively sets a reference voltage and samples the voltage from the filter, wherein the microcontroller sweeps the reference voltage over a range of voltage levels in order to generate the probability density function.

70. The optical receiver of claim 12, wherein the first comparators and the second comparators are manufactured on a single integrated circuit chip.

12. An optical receiver for receiving and decoding a multilevel optical communication signal, comprising:

a plurality of first comparators for estimating a transmitted level based on the received multilevel signal;

a decoder coupled to the first comparators;

a pair of second comparators connected in parallel with the first comparators, to indicate when the received signal falls within controlled voltage ranges;

a logic circuit coupled to the second comparators, to indicate when the received signal is in the intersection of the two controlled voltage ranges;

a filter coupled to the logic circuit, for calculating a probability density function for the received multilevel signal;

an analog-to-digital converter coupled to the filter for sampling a probability density function; and

a microcontroller for processing the probability density function to determine threshold voltage values, for feeding the threshold voltage values to the first comparators for decoding the multilevel signal.

21. A desymbolizer for receiving and decoding a multilevel optical communication signal, comprising:

an analog-to-digital converter for receiving and decoding the multilevel signal;

a signal integrity unit coupled to the analog-to-digital converter for calculating and controlling one or more threshold voltages supplied to the analog-to-digital converter, for calculating at least one of a cumulative distribution function and a probability density function derived from the multilevel signal and associated with the one or more threshold voltages.

22. The desymbolizer of claim 21, wherein the analog-to-digital converter comprises a holding circuit for sampling the multilevel signal in response to control signals generated by the signal integrity unit.

23. The desymbolizer of claim 21, wherein the analog-to-digital converter comprises:

a plurality of comparators for receiving the multilevel signal; and

a decoder coupled to the outputs of the first comparators.

24. The desymbolizer of claim 21, wherein the analog-to-digital converter is a first analog-digital converter, and the signal integrity unit further comprises:

a digital-to-analog controller coupled to inputs of respective comparators located in the first analog-to-digital converter;

a microcontroller coupled to the digital-to-analog controller for controlling a threshold voltage of each comparator located in the first analog-to-digital controller;

a second analog-to-digital converter coupled to the first analog-to-digital controller and microcontroller of the signal integrity unit, for converting the signals received from the first digital-to-analog controller and for feeding the converted signals to the microcontroller for statistical analysis.

25. The desymbolizer of claim 24, wherein the output of the first analog-to-digital converter is coupled to a low-pass filter and the output of the low-pass filter is coupled to the second analog-to-digital converter.

26. The desymbolizer of claim 22, wherein the holding circuit comprises one of a track-and-hold circuit and a sample-and-hold circuit.

27. The desymbolizer of claim 22, wherein the holding circuit samples the multilevel signal at random intervals.

28. The desymbolizer of claim 22, wherein the holding circuit samples the multilevel signal at a periodic sampling rate wherein the period is an integer multiple of the symbol period.

29. The desymbolizer of claim 22, wherein the holding circuit samples the multilevel signal at a periodic sample rate that is not harmonically related to a data rate of the multilevel signal.

30. The desymbolizer of claim 21, wherein the signal integrity unit further calculates a conditional probability density function for each symbol of the multilevel signal based ε

-support estimates.

31. The desymbolizer of claim 30, wherein the signal integrity unit further determines a probability of error for a channel based upon the conditional probability density function and threshold levels.

32. The desymbolizer of claim 21, further comprising programmable analog signal processing modules that equalizes and optimally filters the multilevel signal prior to the analog-to-digital converter, where the signal integrity unit selects an operating point of the programmable analog signal processing modules based upon the determination of the fidelity of the multilevel signal.

33. The desymbolizer of claim 32, wherein the analog signal processing modules comprise at least one of an equalizer and signal conditioning filter.

34. The desymbolizer of claim 32, where the analog signal processing modules comprise at least one clock recovery circuit.

36. The method of claim 35, further comprising the step of calculating both a cumulative distribution function and probability density function, the probability density function being derived from the calculated cumulative distribution function.

37. The method of claim 35, further comprising the step of determining one or more decision thresholds positioned at local minima in a calculated probability density function.

38. The method of claim 35, further comprising the steps of:

estimating a conditional probability density function for each symbol of the multilevel signal; and

determining a probability of error for the multilevel signal based upon the conditional probability density function of each symbol and the decision levels.

39. The method of claim 38, further comprising the step of selecting an operating point of programmable analog signal processing modules based upon the determination of the probability of error for the multilevel signal.

40. The method of claim 35, further comprising the step of assigning threshold voltage levels to respective comparators of a plurality of comparators.

41. The method of claim 35, wherein the step of receiving a multilevel signal further comprises receiving a multilevel signal where the level corresponds to one of an amplitude, phase, or frequency that has been modulated according to the transmitted symbol.

35. A method for receiving and converting a multilevel signal into a plurality of data streams, comprising the steps of:

receiving a multilevel signal;

calculating one of a cumulative distribution function and a probability density function based on the received multilevel signal;

determining one or more decision thresholds based upon one of the cumulative distribution function and probability density function;

associating one or more threshold voltage levels based on the decision thresholds;

comparing the multilevel signal with the threshold voltage levels; and

decoding the multilevel symbol into one or more bits based on the comparison of the multilevel signal with the threshold voltage levels.

42. A method for determining threshold voltages of comparators in a multilevel signal receiver system, comprising the steps of:

estimating a probability density function;

determining one or more statistical centers positioned on local minima that are present in the probability density function; and

associating one or more threshold voltage levels based on the one or more statistical centers positioned according to the local minima.

43. The method of claim 42, further comprising the step of estimating a cumulative distribution function based on a received multilevel signal.

44. The method of claim 43, wherein the step of estimating a cumulative distribution function further comprises sampling voltages with a periodic sample rate.

45. The method of claim 42, wherein the step of estimating the probability density function comprises directly computing the probability density function from the received multilevel signal by collecting sampling voltage levels of the multilevel signal at random time intervals.

47. The method of claim 46, where in the determining one or more decision thresholds further comprises normalizing a cumulative distribution function.

48. The method of claim 46, where in the determining step further comprises combining ε

-supports.

49. The method of claim 46, further comprising calculating a cumulative distribution function based on the received multilevel signal.

46. A method for receiving and converting a multilevel signal into a plurality of data streams, comprising the steps of:

receiving a multilevel signal;

determining one or more decision thresholds by calculating an initial set of ε

-support estimates;

associating one or more threshold voltage levels based on the decision thresholds;

comparing the multilevel signal with the threshold voltage levels; and

decoding the multilevel symbol into one or more bits based on the comparison of the multilevel signal with the threshold voltage levels.

50. A desymbolizer for receiving and decoding a multilevel optical communication signal, comprising:

an analog-to-digital converter for measuring a voltage of the multilevel signal; and

a digital signal processor for calculating a probability density function derived from the multilevel signal and for identifying two or more signals that form the multilevel signal based upon the probability density function.

51. The desymbolizer of claim 50, wherein the digital signal processor calculates a cumulative distribution function instead of a probability density function.

52. The desymbolizer of claim 50, wherein the digital signal processor executes software to identify the two or more signals that form the multilevel signal.

53. The desymbolizer of claim 50, wherein the digital signal processor determines the threshold voltages for the received multilevel signal.

54. A method for determining a set of decoding thresholds from a marginal probability density function comprising the steps of:

thresholding the probability density function to obtain an initial set of intervals conveying regions of significant probability for received signal;

calculating ε

-support regions by merging intervals until the number of regions is substantially equal to the number of candidate symbol transmission levels;

associating each ε

-support region with a mode of a conditional probability density function corresponding to a transmission level; and

computing a decision threshold based on the conditional probability density functions.

55. The method of claim 54, wherein the step of calculating ε

-support regions comprises iteratively merging intervals wherein a pair of closest intervals is merged and a measure of closeness conveys a notion of distance between sets.

56. The method of claim 55, wherein closeness is defined by the minimum distance between any two points in the set, the distance between two sets A and B being defined as

57. The method of claim 55, wherein closeness is defined by a minimum weighted-distance between any two points in the set, the weighted-distance between two sets A and B being defined as

58. The method of claim 54, wherein the step of calculating ε

-support regions further comprises merging any two regions A and B by taking a convex hull of their union.

59. The method of claim 54, further comprising the step of associating each ε

-support with a Gaussian conditional probability density function.

60. The method of claim 54, wherein the step of computing a decision threshold comprises determining a midpoint between ε

-support regions.

61. The method of claim 54, wherein the step of computing a decision threshold comprises determining a weighted midpoint between ε

-support regions.

62. The method of claim 54, further comprising the step of calculating an error rate for each transmitted symbol by using the calculated conditional probability density functions and the determined thresholds.

63. The method of claim 54, further comprising the step of estimating an aggregate error rate for a communication link based upon each error rate calculated for each of the symbols.

64. The method of claim 54, further comprising the step of determining a gain control to be applied to the received signal based upon calculated ε

-supports.

65. The method of claim 63, further comprising the step of determining an automatic gain voltage V_agc, the automatic gain voltage V_agc comprising a reciprocal of a size of a convex hull of all of the ε

-support regions calculated wherein;

66. A system for receiving and converting a multilevel signal into a plurality of data streams comprising:

means for determining one or more decision thresholds by calculating an initial set of ε

-support estimates;

means for associating one or more threshold voltage levels based on the decision thresholds;

means for comparing the multilevel signal with the threshold voltage levels; and

means for decoding the multilevel symbol into one or more bits based on the comparison of the multilevel signal with the threshold voltage levels.

67. The system of claim 66, wherein the means for determining one or more decision thresholds further comprises means for normalizing a cumulative distribution function.

68. The system of claim 66, wherein the means for determining one or more decision thresholds further comprises means for combining ε

-supports.

69. The system of claim 66, further comprising means for calculating a cumulative distribution function based on the received multilevel signal.