Techniques for adaptively adjusting decision levels of a PAM-N decision feedback equalizer

• US 8,576,903 B2
• Filed: 10/18/2011
• Issued: 11/05/2013
• Est. Priority Date: 10/18/2011
• Status: Active Grant

• Current Assignee: Cadence Design Systems Incorporated (Cadence Group)
• Original Assignee: Transwitch Corporation
• Inventors: Raphaeli, Dan, Slezak, Yaron
• Primary Examiner(s): Flores, Leon
• Application Number:
  US13/275,811
• Publication Number:
  US 20130094561A1
• Time in Patent Office: 749 Days
• Field of Search: 375/233, 375/229, 375/232, 375/346, 333/18, 333/28.R, 708/300, 455/296, 327/551
• US Class Current: 375/233
• CPC Class Codes: H04L 25/03057 with a recursive structure H04L25/03031 takes precedence
• CPC Class Codes: H04L 27/06 Demodulator circuits; Receiver circuits

Abstract

A PAM-N decision feedback equalizer (DFE) comprises a coefficient computation unit; a feedback unit that mitigates, using computed feedback coefficients, effects of interference from data symbols; an error-and-decision unit for at least computing a least error value respective to one of a plurality of decision levels, wherein the least error value indicates a difference of a pseudo equalized input PAM-N data symbol from an optimal position of the one of the plurality of decision levels, wherein the one of the plurality of decision levels corresponds to a modulation level used to modulate data in the input PAM-N data symbol; and a calibration unit for adaptively setting the plurality of decision levels based, in part, on the least error value, thereby enabling for compensating for gain changes resulted by a cable on which the input PAM-N data symbol is received and further compensating for embedded offsets of the error-and-decision unit.

Claims

1. A multi-level pulse amplitude modulation (PAM-N) decision feedback equalizer (DFE), comprising:

a coefficient computation unit for setting feedback coefficients of the DFE; and

a feedback unit configured to mitigate, using the feedback coefficients, effects of interference from data symbols that are adjacent in time to an input PAM-N data symbol;

an error-and-decision unit comprises a number of N of adders, wherein each of the N adders subtracts the pseudo equalized input PAM-N data symbol from one of the plurality of decision levels generated by the calibration unit to produce a number of N error values, a number of N−

1 comparators coupled to the N adders, wherein each of the N−

1 comparators generates a decision used to determine the least error value out of all the computed error values, a decoder for recovering the data based on the error values and for generating a selection signal, and a multiplexer coupled to the N adders and the decoder, the multiplexer outputs the least error out of the computed error values based on the selection signal, wherein the error-and-decision unit is configured to compute a least error value respective to one of a plurality of decision levels and to recover the data modulated in the input PAM-4 data symbol, wherein the least error value indicates a difference of a pseudo equalized input PAM-N data symbol from an optimal position of the one of the plurality of decision levels, wherein the one of the plurality of decision levels corresponds to a modulation level used to modulate data in the input PAM-N data symbol; and

a calibration unit for adaptively setting the plurality of decision levels based, in part, on the least error value, thereby enabling for compensating for gain changes resulted by a cable on which the input PAM-N data symbol is received and further compensating for embedded offsets of the error-and-decision unit.

2. The DFE of claim 1, further comprises:

an adder for cancelling intersymbol interference (ISI) of the input PAM-N data symbol by subtracting from the input PAM-N data symbol a sum of signals outputted by the feedback unit to generate the pseudo equalized input PAM-N data symbol.

3. The DFE of claim 1, further comprises:

a data sampler for sampling the recovered data output by the error-and-decision unit and feeding the sampled data to the coefficient computation unit and the feedback unit.

4. The DFE of claim 1, wherein N is the number of modulation levels of the PAM-N.

5. The DFE of claim 1, wherein the calibration unit is further configured to independently adjust each of the plurality of the decision levels.

6. The DFE of claim 5, wherein the adjustment of each of the plurality of the decision levels is determined by:

S_k(j)=S_k(j−

1)+μ

(sign(E_k(j))), wherein ‘

k’

is the one of the plurality of decision levels, μ

is an adaptation speed factor, ‘

E_k’

is the least error; and

‘

j’

is an index of the input PAM-N data symbol.

7. The DFE of claim 1, wherein the PAM-N DFE is a PAM-4 DFE, and the input PAM-N data symbol is a PAM-4 modulated signal.

8. A method for adaptively adjusting a plurality of decision levels of a N-pulse amplitude modulation (PAM-N) decision feedback equalizer (DFE), comprising:

setting feedback coefficients of the DFE;

mitigating, using the feedback coefficients, effects of interference from data symbols that are adjacent in time to an input PAM-N data symbol;

computing a least error respective of one of the plurality of decision levels, wherein the least error indicates a difference of a pseudo equalized input PAM-N data symbol from an optimal position of the one of the plurality of decision levels, wherein one of the plurality of decision levels corresponds to a modulation level used to modulate data in the input PAM-N data symbol, wherein computing the least error further comprises subtracting the pseudo equalized input PAM-N data symbol from each of the plurality of decision levels generated by the calibration unit to produce N error values, comparing two error values corresponding to two adjacent decision levels, generating N−

1 decisions based on the comparison results, wherein the generated N−

1 decisions are used to determine the least error out of the N computed error values, recovering the data based on the N−

1 decisions, and selecting the least error out of the computed N error values; and

adaptively setting the plurality of decision levels based, in part, on the least error, thereby compensating for gain changes resulted by a cable on which the input PAM-N data symbol is received.

9. The method of claim 8, further comprising:

cancelling intersymbol interference (ISI) of the input PAM-N data symbol by subtracting from the input PAM-N data symbol a sum of signals outputted by the feedback unit to produce the pseudo equalized input PAM-N data symbol.

10. The method of claim 9, further comprising:

recovering the data modulated in the input PAM-4 data symbol.

11. The method of claim 10, further comprising

sampling the recovered data;

and

using the sampled data for at least computing the feedback coefficients of the DFE.

12. The method of claim 8, wherein N is the number of modulation levels of the PAM-N.

13. The method of claim 8, further comprising:

independently adjusting each of the plurality of the decision levels.

14. The method of claim 13, wherein the adjustment of each of the plurality of the decision levels is determined by:

S_k(j)=S_k(j−

1)+μ

(sign(E_k(j))), wherein ‘

k’

is the one of the plurality of decision levels, μ

is an adaptation speed factor, ‘

E_k’

is the least error; and

‘

j’

is an index of the input PAM-N data symbol.

15. The method of claim 14, wherein the PAM-N DFE is a PAM-4 DFE, and the input PAM-N data symbol is a PAM-4 modulated signal.

16. The method of claim 8, wherein adaptively setting the plurality of decision levels further allows for compensating offsets that biased one or more of the plurality of decision levels.

17. A non-transitory computer readable medium having stored thereon instructions for causing one or more processing units to execute the method according to claim 8.

18. A multi-level pulse amplitude modulation (PAM-N) decision feedback equalizer (DFE), comprising:

a coefficient computation unit for setting feedback coefficients of the DFE;

a feedback unit that mitigates, using the feedback coefficients, effects of interference from data symbols that are adjacent in time to an input PAM-N data symbol;

an error-and-decision unit for at least computing a least error value respective to one of a plurality of decision levels, wherein the least error value indicates a difference of a pseudo equalized input PAM-N data symbol from an optimal position of the one of the plurality of decision levels, wherein the one of the plurality of decision levels corresponds to a modulation level used to modulate data in the input PAM-N data symbol; and

a calibration unit for adaptively setting the plurality of decision levels based, in part, on the least error value, thereby compensating for gain changes resulting from a cable on which the input PAM-N data symbol is received and further compensating for embedded offsets of the error-and-decision unit, wherein the calibration unit is further configured to independently adjust each of the plurality of the decision levels, the adjustment of each of the plurality of the decision levels is determined by S_k(j)=S_k(j−

1)+μ

(sign(E_k(j))), wherein ‘

k’

is the one of the plurality of decision levels, μ

is an adaptation speed factor, ‘

E_k’

is the least error; and

‘

j’

is an index of the input PAM-N data symbol.