Detection and removal of cycle slip from received signals

1. A method comprising:

(a) receiving, at an optical receiver, a signal comprising a plurality of phase-modulated symbols;

(b) generating, for each phase-modulated symbol, a phase-error estimate of a phase error of the phase-modulated symbol;

(c) detecting presence of a cycle slip based on the phase-error estimates; and

(d) adjusting a phase of each of one or more of the received phase-modulated symbols, downstream of a local oscillator of the receiver, to remove the detected cycle slip without adjusting the local oscillator; and

wherein, in step (c), the cycle slip is detected by comparing a current block of the phase-error estimates to one or more previous blocks of the phase-error estimates.

2. The method of claim 1 , wherein step (b) comprises generating each phase-error estimate by performing at least fine-phase estimation.

3. The method of claim 2 , wherein step (b) comprises performing both (i) pilot-aided coarse-phase estimation and (ii) the fine-phase estimation.

4. The method of claim 1 , wherein step (b) comprises:

(b1) performing, for each phase-modulated symbol, at least one of (i) coarse phase estimation to generate a coarse phase-error estimate and (ii) fine phase estimation to generate a fine phase-error estimate;

(b2) adjusting, for each of one or more of the phase-modulated symbols, the phase of the phase-modulated symbol based on at least one of (i) the coarse phase-error estimate and (ii) the fine phase-error estimate; and

(b3) generating, for each phase-modulated symbol, the phase-error estimate of the phase error of the phase-modulated symbol after the phase-modulated symbol is adjusted in step (b2).

5. The method of claim 1 , wherein step (c) comprises:

(c1) generating a current mean phase-error estimate of the current block of the phase-error estimates;

(c2) generating a previous mean phase-error estimate of the one or more previous blocks of the phase-error estimates; and

(c3) comparing the current mean phase-error estimate to the previous mean phase-error estimate to detect the cycle slip.

6. The method of claim 5 , wherein step (c3) comprises:

(c3i) comparing (i) a difference between the current mean phase-error estimate and the previous mean phase-error estimate to (ii) a first threshold;

(c3ii) comparing the current mean phase-error estimate to a second threshold; and

(c3iii) detecting the cycle slip based on the comparisons of (c3i) and (c3ii).

7. The method of claim 1 , wherein step (c) comprises:

(c1) comparing a first current block of the phase-error estimates to a first set of one or more previous blocks of the phase-error estimates to detect the cycle slip, wherein the first current block and each block in the first set has a first number of phase-modulated symbols; and

(c2) comparing a second current block of the phase-error estimates to a second set of one or more previous blocks of the phase-error estimates to detect the cycle slip, wherein the second block and each block in the second set has a second number of phase-modulated symbols, different from the first number of phase-modulated symbols.

8. The method of claim 1 , wherein step (c) comprises:

(c1) comparing a first current block of the phase-error estimates to a first set of one or more previous blocks of the phase-error estimates to detect the cycle slip, wherein the first current block and each block in the first set consist of phase-modulated symbols corresponding to data; and

(c2) comparing a second current block of the phase-error estimates to a second set of one or more previous blocks of the phase-error estimates to detect the cycle slip, wherein the second block and each block in the second set consist of phase-modulated symbols corresponding to pilot information.

9. The method of claim 1 , wherein:

the phase-error estimates in step (b) are wrapped phase-error estimates;

step (b) comprises unwrapping the wrapped phase-error estimates to generate unwrapped phase-error estimates; and

step (c) comprises detecting the presence of the cycle slip based on the unwrapped phase-error estimates.

10. A method comprising:

configuring a digital signal processor of an optical receiver to generate, for each phase-modulated symbol carried by an optical signal received by the optical receiver, a phase-error estimate of a phase error of the phase-modulated symbol;

configuring the digital signal processor to detect presence of a cycle slip by comparing a current block of the phase-error estimates to one or more previous blocks of the phase-error estimates; and

configuring the digital signal processor to adjust a phase of each of one or more of received phase-modulated symbols, downstream of a local oscillator of the optical receiver, to remove a detected cycle slip without causing an adjustment of the local oscillator.

11. An apparatus comprising:

a phase-error estimator configured to generate a phase-error estimate of a phase error of each phase-modulated symbol of a plurality of phase-modulated symbols of a received optical signal;

a cycle slip detector configured to detect presence of a cycle slip based on the phase-error estimates; and

a cycle slip remover configured to adjust, downstream of a local oscillator of a receiver, a phase of each of one or more of the received phase-modulated symbols to remove the detected cycle slip without adjusting the local oscillator; and

wherein the cycle slip detector is configured to detect the cycle slip by comparing a current block of the phase-error estimates to one or more previous blocks of the phase-error estimates.

12. The apparatus of claim 11 , wherein the phase-error estimator comprises at least a fine-phase estimator configured to generate each of the phase-error estimates.

13. The apparatus of claim 12 , wherein the phase-error estimator comprises both (i) a coarse-phase estimator and (ii) the fine-phase estimator.

14. The apparatus of claim 11 , the cycle slip detector is configured to:

generate a current mean phase-error estimate of the current block of the phase-error estimates;

generate a previous mean phase-error estimate of the one or more previous blocks of the phase-error estimates; and

comparing the current mean phase-error estimate to the previous mean phase-error estimate to detect the cycle slip.

15. The apparatus of claim 14 , wherein the cycle slip detector is configured to:

(a) compare (i) a difference between the current mean phase-error estimate and the previous mean phase-error estimate to (ii) a first threshold;

(b) compare the current mean phase-error estimate to a second threshold; and

detect the cycle slip based on the comparisons of (a) and (b).

16. The apparatus of claim 11 , wherein the cycle slip detector is configured to:

compare a first current block of the phase-error estimates to a first set of one or more previous blocks of the phase-error estimates to detect the cycle slip, wherein the first current block and each block in the first set has a first number of phase-modulated symbols; and

compare a second current block of the phase-error estimates to a second set of one or more previous blocks of the phase-error estimates to detect the cycle slip, wherein the second block and each block in the second set has a second number of phase-modulated symbols, different from the first number of phase-modulated symbols.

17. The apparatus of claim 11 , wherein the cycle slip detector is configured to:

compare a first current block of the phase-error estimates to a first set of one or more previous blocks of the phase-error estimates to detect the cycle slip, wherein the first current block and each block in the first set consist of phase-modulated symbols corresponding to data; and

compare a second current block of the phase-error estimates to a second set of one or more previous blocks of the phase-error estimates to detect the cycle slip, wherein the second block and each block in the second set consist of phase-modulated symbols corresponding to pilot information.

18. The apparatus of claim 11 , wherein:

the phase-error estimates generated by the phase-error estimator are wrapped phase-error estimates;

the apparatus further comprises an unwrapping block configured to unwrap the wrapped phase-error estimates to generate unwrapped phase-error estimates; and

the cycle slip detector is configured to detect the presence of the cycle slip based on the unwrapped phase-error estimates.

19. The apparatus of claim 11 , wherein:

the apparatus comprises at least one of:

(1) a coarse phase estimator and compensator configured to (i) generate, for each phase-modulated symbol, a coarse phase-error estimate and (ii) adjust, for each of one or more of the phase-modulated symbols, the phase of the phase-modulated symbol based on the coarse phase-error estimate; and

(2) a fine phase estimator and compensator configured to (i) generate, for each phase-modulated symbol, a fine phase-error estimate and (ii) adjust, for each of one or more of the phase-modulated symbols, the phase of the phase-modulated symbol based on the fine phase-error estimate; and

the phase-error estimator is configured to generate, for each phase-modulated symbol, the phase-error estimate of the phase error of the phase-modulated symbol after the phase-modulated symbol is adjusted by at least one of (a) the coarse phase estimator and compensator and (b) the fine phase estimator and compensator.

20. The apparatus of claim 11 , further comprising a digital signal processor configured to implement the phase-error estimator, the cycle slip detector and the cycle slip remover.

21. The apparatus of claim 11 , further comprising a coherent optical receiver configured to recover the plurality of phase-modulated symbols from the received optical signal.