Signal framing in a coherent optical receiver

1. In a coherent optical receiver of an optical communications system, a method of estimating a location of a periodic SYNC burst within an optical signal received through the optical communications system, the method comprising steps of:

calculating a cross-correlation between a multi-bit digital signal corresponding to the optical signal received through the optical communications system and a known pattern corresponding to the SYNC burst;

logically dividing the cross-correlation into at least one sub-block, each sub-block comprising a plurality of contiguous samples of the cross-correlation spanning less than a SYNC burst period and having a repetition rate that is synchronous with the digital signal;

processing each sub-block to identify a candidate sub-block within which the SYNC burst is most likely located; and

processing samples of the cross-correlation within the candidate sub-block to identify the most likely location of the SYNC burst;

wherein the steps of calculating the cross-correlation, processing each sub-block and processing samples of the cross-correlation within the candidate sub-block to identify the most likely location of the SYNC burst are performed before detection of symbols modulated on the received optical signal.

2. The method as claimed in claim 1 , wherein the multi-bit digital signal has a non-uniform sample timing.

3. The method as claimed in claim 1 , wherein the known pattern corresponds with an expected sequence of samples generated by sampling the SYNC burst.

4. The method as claimed in claim 1 , wherein the step of calculating the cross-correlation comprises steps of:

selecting one of a set of known SYNC burst symbol sequences; and

applying a respective sample pattern corresponding to the selected SYNC burst symbol sequence to the multi-bit digital signal.

5. The method as claimed in claim 4 , wherein the step of selecting one of a set of known SYNC burst symbol sequences comprising a step of periodically selecting a different one of the set of known SYNC burst symbol sequences.

6. The method as claimed in claim 1 , wherein the step of logically dividing the cross-correlation into at least one sub-blocks comprises a step of defining the at least one sub-block having a predetermined size and repetition period.

7. The method as claimed in claim 6 , wherein the step of defining at least one sub-block comprises defining a number of sub-blocks and the size of each sub-block such that the sub-blocks can encompass one SYNC burst and any data symbols lying between that SYNC burst and a next successive SYNC burst of the optical signal.

8. The method as claimed in claim 6 , wherein the repetition period of the sub-block is greater than an expected repetition period of the SYNC burst.

9. The method as claimed in claim 6 , further comprising a step of adjusting a phase of the sub-block relative to the optical signal.

10. The method as claimed in claim 1 , wherein the step of identifying a candidate sub-block in which the SYNC burst is most likely located comprises steps of:

for each sub-block, accumulating a respective tail probability;

detecting a first tail probability to reach a predetermined condition; and

identifying the corresponding sub-block as the candidate sub-block in which the SYNC burst is most likely located.

11. The method as claimed in claim 10 , wherein the tail probability is represented by a count of samples which are greater than a predetermined threshold.

12. The method as claimed in claim 1 , wherein the step of processing samples of the cross-correlation within the candidate sub-block comprises steps of:

for each sample of the candidate sub-block, accumulating a respective count of instances in which the sample value is greater than a predetermined threshold;

detecting a first count to reach a predetermined condition; and

identifying the corresponding sample as a most likely location of the SYNC burst.

13. The method as claimed in claim 1 , wherein calculating the cross-correlation comprises compensating dispersion of the optical signal.

14. The method as claimed in claim 13 , wherein compensating dispersion of the optical signal is performed before calculation of the cross-correlation.

15. The method as claimed in claim 13 , wherein compensating dispersion of the optical signal is performed simultaneously with calculation of the cross-correlation.

16. The method as claimed in claim 1 , wherein one SYNC burst is composed of a set of contiguous symbols.

17. The method as claimed in claim 15 , wherein the step of calculating the cross-correlation comprises incorporating the known pattern into dispersion compensation coefficients used by the dispersion compensator to generate multi-bit dispersion compensated digital signals.