Signal framing in a coherent optical receiver

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

a processor of the receiver calculating a cross-correlation between a multi-bit digital signal corresponding to the received optical signal and a known pattern corresponding to the SYNC burst;

a framer processing samples of the cross-correlation to identify a candidate sub-block within which the SYNC burst is most likely located, wherein the candidate sub-block comprises a respective plurality of samples of the cross-correlation spanning more samples of the cross-correlation than the SYNC burst and less than a SYNC burst period; and

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

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

selecting one of a set of two or more 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.

3. The method as claimed in claim 2 , wherein the step of selecting one of a set of two or more known SYNC burst symbol sequences comprises a step of periodically selecting a different one of the set of two or more known SYNC burst symbol sequences.

4. The method as claimed in claim 1 , further comprising processing the multi-bit digital signal to compensate dispersion of the optical signal.

5. The method as claimed in claim 4 , wherein processing the multi-bit digital signal to compensate dispersion of the optical signal is performed before calculation of the cross-correlation.

6. The method as claimed in claim 4 , wherein processing the multi-bit digital signal to compensate dispersion of the optical signal is performed simultaneously with calculation of the cross-correlation.

7. The method of claim 1 , wherein the multi-bit digital signal comprises at least two parallel multi-bit sample streams, and wherein calculating the cross-correlation comprises steps of:

calculating a respective cross-correlation signal for each sample stream; and

combining the respective cross-correlation signals.

8. The method of claim 7 , wherein each cross-correlation signal is calculated using a respective pattern corresponding to a unique one of a set of SYNC bursts.

9. The method of claim 7 , wherein at least two cross-correlation signals are calculated using the respective pattern corresponding to a common SYNC burst.

10. The method of claim 7 , wherein combining the respective cross-correlation signals comprises computing a sum of squares of the respective cross-correlation signals.

11. 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:

accumulating a respective tail probability for one or more sub-blocks;

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.

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

13. The method as claimed in claim 12 , wherein the predetermined condition is an overflow condition of a counter for counting samples which are greater than the predetermined threshold.

14. 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 corresponding 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 within the candidate sub-block.

15. The method as claimed in claim 14 , wherein the predetermined condition is an overflow condition of a counter for counting samples which are greater than the predetermined threshold.

16. A receiver of an optical communications system, the receiver comprising:

a processor configured to calculate a cross-correlation between a multi-bit digital signal corresponding to a received optical signal and a known pattern corresponding to a periodic SYNC burst within the received optical signal;

a framer configured to process samples of the cross-correlation to identify a candidate sub-block within which the SYNC burst is most likely located, and to further process samples of the cross-correlation within the candidate sub-block to identify the most likely location of the SYNC burst within the candidate sub-block;

the candidate sub-block comprising a respective plurality of samples of the cross-correlation spanning more samples of the cross-correlation than the SYNC burst and less than a SYNC burst period.

17. A non-transitory computer readable storage medium storing software instructions for execution by a receiver of an optical communications system, the software instructions being configured to control the receiver to implement a method estimating a location of a periodic SYNC burst within an optical signal received by the receiver through the optical communications system, the method comprising steps of:

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

processing samples of the cross-correlation to identify a candidate sub-block within which the SYNC burst is most likely located, wherein the candidate sub-block comprises a respective plurality of samples of the cross-correlation spanning more samples of the cross-correlation than the SYNC burst and less than a SYNC burst period; and

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

18. The receiver as claimed in claim 16 , wherein the processor comprises a logic block configured to apply the known pattern to the multi-bit digital signal to calculate the cross-correlation.

19. The receiver as claimed in claim 16 , wherein the processor comprises:

a coefficient calculator configured to calculate a set of dispersion coefficients as a sum of a linear dispersive function and a Fast Fourier Transform of the known pattern; and

a frequency-domain dispersion compensator configured to apply the calculated set of dispersion coefficients to the multi-bit digital signal so as to simultaneously compensate dispersion of the received optical signal and calculate the cross-correlation.

20. The receiver as claimed in claim 16 , wherein the multi-bit digital signal comprises at least two parallel multi-bit sample streams, and wherein the processor is configured to process the parallel multi-bit sample streams to generate at least two parallel cross-correlation signals.

21. The receiver as claimed in claim 20 , wherein the processor further comprises a sum-of-squares network for computing a sum of the square of each one of the parallel cross-correlation signals to generate a combined cross-correlation signal, and wherein the framer is configured to process samples of the combined cross-correlation signal to identify the candidate sub-block and to identify the most likely location of the SYNC burst within the candidate sub-block.