High-speed adaptive decision feedback equalizer

• US 8,301,036 B2
• Filed: 11/15/2009
• Issued: 10/30/2012
• Est. Priority Date: 11/15/2009
• Status: Active Grant

• Current Assignee: Credo Technology Group Limited (Credo Technology Group Holding Ltd.)
• Original Assignee: Credo Semiconductor (Hong Kong) Limited
• Inventors: He, Runsheng
• Primary Examiner(s): Phan, Hanh
• Application Number:
  US12/618,735
• Publication Number:
  US 20110116806A1
• Time in Patent Office: 1,080 Days
• Field of Search: 398/202, 398/208, 398/209, 398/158, 398/159, 398/154, 398/155, 398/149, 398/135, 398/136, 398/137, 398/138, 398/214, 398/139, 398/161, 398/164, 398/25, 398/26, 398/27, 398/29, 398/102, 375/233, 375/232, 375/234, 375/376, 375/317, 375/316, 375/371, 375/346, 375/348, 375/350
• US Class Current: 398/202
• CPC Class Codes: H04L 25/03019 adaptive, i.e. capable of adjustment during data reception
• CPC Class Codes: H04L 25/03057 with a recursive structure H04L25/03031 takes precedence

Abstract

A decision-feedback equalizer (DFE) can be operated at higher frequencies when parallelization and pre-computation techniques are employed. Disclosed herein is a DFE design that operates at frequencies above 10 GHz, making it feasible to employ decision feedback equalization in optical transceiver modules. An adaptation technique is also disclosed to maximize communications reliability. The adaptation module can be treated as a straightforward extension of the pre-computation unit. At least some method embodiments include, in each time interval: sampling a signal that is partially compensated by a feedback signal; comparing the sampled signal to a set of thresholds to determine multiple speculative decisions; selecting and outputting one of the speculative decisions based on preceding decisions; and updating a counter if the sampled signal falls within a window proximate to a given threshold. Once a predetermined interval has elapsed, the value accumulated by the counter is used to adjust the given threshold.

Claims

1. A high speed equalization method that comprises, at each of multiple consecutive time intervals:

sampling a signal that is partially compensated for trailing intersymbol interference dependent upon an error signal produced by a feedback filter operating on preceding decisions, wherein said sampling produces a sampled signal;

comparing the sampled signal to a plurality of threshold values to determine a set of speculative decisions;

selecting and outputting one of the speculative decisions based on a plurality of preceding decisions; and

updating a counter if the sampled signal falls within a range of values about a given one of the threshold values.

2. The method of claim 1, further comprising:

accumulating a counter value over a fixed interval; and

adjusting the given one of the threshold values based on the counter value.

3. The method of claim 2, further comprising:

performing said updating, accumulating, adjusting, and determining for each of said threshold values.

4. The method of claim 2, wherein said updating, accumulating, and adjusting are performed concurrently for each of said threshold values.

5. The method of claim 2, wherein said sampling and comparing and selecting are performed by a plurality of pre-computation units operating in parallel.

6. The method of claim 5, wherein each pre-computation unit performs said updating, accumulating, and adjusting operations independent of the other pre-computation units.

7. The method of claim 1, wherein the counter is incremented when the sampled signal falls in a range of values above the given threshold, and wherein the counter is decremented when the sampled signal falls in a range of values below the given threshold, and wherein the given threshold value is adjusted upward if the accumulated counter value exceeds a count threshold value.

8. A high speed equalizer that comprises:

an analog filter that enhances a signal-to-noise ratio of a receive signal to provide a filtered receive signal;

a summer that combines the filtered receive signal with a feedback signal to provide a combined signal;

at least one pre-computation unit that samples the combined signal and compares the combined signal to multiple threshold values to generate a set of speculative decisions;

a multiplexer that, based on previous decisions, selects from the set of speculative decisions to provide a selected decision for output from the equalizer;

a feedback filter that operates on the selected decision to provide said feedback signal; and

at least one adaptation unit that estimates a number of times a sampled signal is in a first range of values above a given one of the threshold values versus a number of times the sampled signal is in a second range of values below the given one of the threshold values.

9. The equalizer of claim 8, wherein the at least one adaptation unit uses the estimate of the number of times the sampled signal is in the first range of values versus the number of times the sampled signal is in the second range of values to adjust the given one of the threshold values.

10. The equalizer of claim 8, wherein said at least one pre-computation unit comprises a plurality of pre-computation units that take turns sampling the combined signal.

11. The equalizer of claim 10, wherein each of said pre-computation units has a corresponding adaptation unit that adjusts each of the multiple threshold values.

12. The equalizer of claim 8, wherein the equalizer outputs the selected decision at a rate in excess of 10 GHz.

13. A fiber optic interface module that comprises a receiver having:

a sensor that converts incident optical signals into analog electrical signals;

an analog filter that enhances the signal-to-noise ratio of the analog electrical signals;

a summer that combines a feedback signal with an output signal from the analog filter;

a plurality of pre-computation units that take turns sampling a combined signal produced by the summer, wherein each pre-computation unit compares a sample of the combined signal with a set of threshold values to generate a set of speculative decisions, wherein each pre-computation unit further selects one of the speculative decisions for output based on previous decisions from other ones of the pre-computation units;

a device interface that provides the outputs of the pre-computation units as a received data stream; and

at least one adaptation unit that periodically adjusts the threshold values.

14. The module of claim 13, further comprising a transmitter that receives a transmit data stream from said device interface and converts said transmit data stream to an optical transmit signal.

15. The module of claim 14, wherein the received data stream carries at least 10 Gbit/s.

16. The module of claim 13, further comprising a feedback filter that generates said feedback signal based on previous decisions.

17. The module of claim 13, wherein the at least one adaptation unit counts a number of times a sampled signal is within a first range of values above a given threshold value versus a number of times the sampled signal is within a second range of values below the given threshold value, and adjusts the given threshold value to reduce the count.

18. The module of claim 17, wherein the at least one adaptation unit adjusts each of the threshold values.

19. The module of claim 17, wherein each pre-computation unit has a corresponding adaptation unit.

20. The module of claim 17, further comprising an adaptation unit for each threshold value.