Iterative post-equalization for coherent optical receivers

Patent metadata

Field	Value
Publication number	US-9912414-B2
Application number	US-201514941472-A
Country	US
Kind code	B2
Filing date	Nov 13, 2015
Priority date	Nov 14, 2014
Publication date	Mar 6, 2018
Grant date	Mar 6, 2018

Abstract

A post-equalization technique for recovering data bits from a coherent modulation optical signal is implemented in the digital domain by iteratively performing a decision-directed least mean square channel equalization step, a digital post filter step and a maximum likelihood sequence estimation step so that the symbol decisions of the previous iteration are fed to the decision directed least mean square channel equalization step to successively improve the symbol decisions. In an experimental setup, the iterative technique demonstrated performance improvement mitigating the bandwidth limitation as compared to a corresponding non-iterative technique.

First claim

What is claimed is what is described and illustrated, including:

1. A method of recovering information bits from a modulated optical signal, the method comprising:
receiving the modulated optical signal over an optical transmission medium, wherein the modulated optical signal comprises information bits modulated using polarization division multiplexing;
generating a stream of digital samples by performing analog-to-digital conversion of the received modulated signal;
processing the stream of digital samples through a first decoding stage in which carrier frequency and phase recovery is performed to obtain a stream of recovered digital samples;
performing, after the first decoding stage, post-equalization on the stream of recovered digital samples, wherein the performing post-equalization includes iteratively performing:
a channel equalization on the stream of recovered digital samples;
a digital filtering operation on output of the channel equalization using a digital post filter, wherein a response of the digital post filter is adjusted adaptively, and
a symbol estimation operation on output of the digital post filter, wherein parameters for the symbol estimation operation are adjusted adaptively based on the response of the digital post filter;

until an iteration termination criterion is satisfied;
wherein an output of the symbol estimation operation is fed back into the channel equalization operation; and
recovering, after termination of iterations, the information bits from the output of the symbol estimation process.

2. The method of claim 1 , further comprising:
adaptively adjusting, using the estimates of the information bits, coefficients of the digital post filter.

3. The method of claim 1 , further comprising:
performing error correction to generate the output of the symbol estimation operation.

4. The method of claim 1 , wherein the iteration termination criterion comprises estimating improvement in a bit error ratio over a previous iteration.

5. The method of claim 1 , further including:
performing symbol alignment on the output of the symbol estimation process.

6. The method of claim 1 , wherein the performing the channel equalization on the stream of samples includes estimating characteristics of the optical transmission medium using a decision directed least mean square in which coefficients of a channel equalizer are adjusted based on the output of the symbol estimation operation.

7. The method of claim 1 , wherein the symbol estimation operation uses a maximum likelihood sequence estimation (MLSE) criterion.

8. An optical communication receiver apparatus for recovering information bits from a modulated optical signal, comprising:
an optical front end that receives the modulated optical signal over an optical transmission medium, the modulated optical signal comprises information bits modulated using polarization division multiplexing;
an analog to digital converter that generates a stream of digital samples by performing analog-to-digital conversion of the received modulated signal;
a memory that stores executable instructions; and
a digital signal processor that reads the executable instructions from the memory and implements a method, including:
processing the stream of digital samples through a first decoding stage in which carrier recovery and phase recovery is performed to obtain a stream of recovered digital samples;
performing, after the first decoding stage, post-equalization on the stream of recovered digital samples, wherein the performing post-equalization includes iteratively performing:
a channel equalization on the stream of recovered digital samples;
a digital filtering operation on output of the channel equalization using a digital post filter, wherein a response of the digital post filter is adjusted adaptively, and
a symbol estimation operation on output of the digital post filter, wherein parameters for the symbol estimation operation are adjusted adaptively based on the response of the digital post filter;

until an iteration termination criterion is satisfied;
wherein an output of the symbol estimation operation is fed back into the channel equalization operation; and
recovering, after termination of iterations, the information bits from the output of the symbol estimation process.

9. The optical communication receiver apparatus of claim 8 , wherein the method further includes:
adaptively adjusting, using the estimates of the information bits, coefficients of the digital post filter.

10. The optical communication receiver apparatus of claim 8 , wherein the method further includes:
performing error correction to generate the output of the symbol estimation operation.

11. The optical communication receiver apparatus of claim 8 , wherein the iteration termination criterion comprises estimating improvement in a bit error ratio over a previous iteration.

12. The optical communication receiver apparatus of claim 8 , wherein the method further includes:
performing symbol alignment on the output of the symbol estimation process.

13. The optical communication receiver apparatus of claim 8 , wherein the performing the channel equalization on the stream of samples includes estimating characteristics of the optical transmission medium using a decision directed least mean square in which coefficients of a channel equalizer are adjusted based on the output of the symbol estimation operation.

14. The optical communication receiver apparatus of claim 8 , wherein the symbol estimation operation uses a maximum likelihood sequence estimation (MLSE) criterion.

15. An optical communication system comprising:
an optical signal transmitter that produces an optical signal comprising information bits modulated using polarization division multiplexing; and
an optical signal receiver that recovers the information bits from the modulated optical signal by:
receiving the modulated signal over an optical transmission medium;
generating a stream of digital samples by performing analog-to-digital conversion of the received modulated signal;
processing the stream of digital samples through a first decoding stage in which carrier recovery and phase recovery is performed to obtain a stream of recovered digital samples;
performing, after the first decoding stage, post-equalization on the stream of recovered digital samples, wherein the performing post-equalization includes iteratively performing:
a channel equalization on the stream of recovered digital samples;
a digital filtering operation on output of the channel equalization using a digital post filter, wherein a response of the digital post filter is adjusted adaptively, and
a symbol estimation operation on output of the digital post filter, wherein parameters for the symbol estimation operation are adjusted adaptively based on the response of the digital post filter;

until an iteration termination criterion is satisfied;
wherein an output of the symbol estimation operation is fed back into the channel equalization operation; and
recovering, after termination of iterations, the information bits from the output of the symbol estimation process.

16. The optical communication system of claim 15 , wherein the receiver-implemented method further includes:
adaptively adjusting, using the estimates of the information bits, coefficients of the digital post filter.

17. The optical communication system of claim 15 , wherein the receiver-implemented method further includes:
performing error correction to generate the output of the symbol estimation operation.

Assignees

• Zte Corp

Inventors

• Jia Zhensheng
• Cai Yi

Classifications

• H04L1/0054

  Maximum-likelihood or sequential decoding, e.g. Viterbi, Fano, ZJ algorithms · CPC title

• H04L7/048

  using the properties of error detecting or error correcting codes, e.g. parity as synchronisation signal · CPC title

• H04L25/03267

  with decision feedback equalisers · CPC title

• H04B10/6165 Primary

  Estimation of the phase of the received optical signal, phase error estimation or phase error correction · CPC title

• H04L25/03019 Primary

  adaptive, i.e. capable of adjustment during data reception · CPC title