Cycle slip location and correction
View Patent ↗Methods and techniques are disclosed for correcting the effect of cycle slips in a coherent communications system. A signal comprising SYNC bursts having a predetermined periodicity and a plurality of known symbols at predetermined locations between successive SYNC bursts is received. The received signal is partitioned into data blocks. Each data block encompasses at least data symbols and a set of check symbols corresponding to the plurality of known symbols at predetermined locations between a respective pair of successive SYNC bursts in the signal. Each data block is processed to detect a cycle slip. When a cycle slip is detected, the set of check symbols of the data block are examined to identify a first slipped check symbol, and a phase correction applied to data symbols of the data block lying between the first slipped check symbol and an end of the data block.
1. A method of correcting an effect of cycle slips in an optical communications system, the method comprising steps of:
receiving an optical signal through the communications system, the signal comprising SYNC bursts having a predetermined periodicity and a plurality of known symbols at predetermined locations between successive SYNC bursts;
partitioning the received optical signal into data blocks, wherein each data block encompasses at least data symbols and a set of check symbols corresponding to the plurality of known symbols at predetermined locations between a respective pair of successive SYNC bursts in the optical signal;
processing each data block to detect a cycle slip, and, when a cycle slip is detected:
examining the set of check symbols of the data block to identify a first slipped check symbol; and
applying a phase correction to data symbols of the data block lying between the first slipped check symbol and an end of the data block.
2. The method of claim 1 wherein examining the set of check symbols of the data block to identify the first slipped check symbol comprises steps of:
for each check symbol:
applying a common phase rotation to the check symbols of the data block starting at the check symbol and extending to the end of the data block; and
computing an error rate across at least the phase rotated check symbols, and associating the calculated error rate with the check symbol; and
identifying the first slipped check symbol as the check symbol associated with the lowest calculated error rate.
3. The method of claim 1 wherein the first slipped check symbol is a first check symbol in a continuous sequence of errored check symbols extending from the first slipped check symbol to the last check symbol of the data block.
4. The method of claim 3 wherein examining the set of check symbols of the data block to identify the first slipped check symbol comprises steps of:
examining each check symbol in turn, starting at the last check symbol and progressing towards a leading check symbol of the data block, until a first correct check symbol is found; and
identified a check symbol immediately following the first correct check symbol as the first slipped check symbol.
5. A coherent receiver for receiving an optical signal through an optical communications system, the coherent receiver comprising:
a detector for receiving the optical signal, the optical signal comprising SYNC bursts having a predetermined periodicity and a plurality of known symbols at predetermined locations between successive SYNC bursts;
a framer for partitioning the received optical signal into data blocks, wherein each data block encompasses at least data symbols and a set of check symbols corresponding to the plurality of known symbols at predetermined locations between a respective pair of successive SYNC bursts in the optical signal;
a decoder processing each data block to detect a cycle slip; and,
a post processor responsive to the decoder, for processing a data block affected by a cycle slip, the post processor implementing steps of:
examining the set of check symbols of the affected data block to identify a first slipped check symbol; and
applying a phase correction to data symbols of the affected data block lying between the first slipped check symbol and an end of the affected data block.
6. The coherent receiver of claim 5 wherein the first slipped check symbol is a first check symbol in a continuous sequence of errored check symbols extending from the first slipped check symbol to the last check symbol of the affected data block.
7. The coherent receiver of claim 6 wherein examining the set of check symbols of the data block to identify the first slipped check symbol comprises:
examining each check symbol in turn, starting at the last check symbol and progressing towards a leading check symbol of the data block, until a first correct check symbol is found; and
identified a check symbol immediately following the first correct check symbol as the first slipped check symbol.
8. The coherent receiver of claim 5 wherein examining the set of check symbols of the data block to identify the first slipped check symbol comprises:
for each check symbol:
applying a common phase rotation to the check symbols of the data block starting at the check symbol and extending to the end of the data block; and
computing an error rate across at least the phase rotated check symbols, and associating the calculated error rate with the check symbol; and
identifying the first slipped check symbol as the check symbol associated with the lowest calculated error rate.