Minimum variance carrier recovery

1. In a coherent receiver of an optical communications system, a method of data symbol recovery comprising:

a phase noise estimator computing a respective probabilistic phase error for each of a plurality of data symbol estimates;

a phase rotator computing a phase rotation based on the probabilistic phase error estimates, using a filter function;

the phase rotator applying the phase rotation to at least one data symbol estimate to generate a corresponding rotated symbol estimate; and

a decision circuit processing each rotated symbol estimate to generate corresponding decision values of each data symbol;

wherein computing the respective probabilistic phase error comprises calculating an expected value of phase noise based on a conditional probability density function, the probability density function being conditioned on at least one symbol estimate.

2. The method of claim 1 , wherein the conditional probability density function is a probability density function of the phase error, or an operator thereof.

3. The method of claim 1 , wherein computing the respective probabilistic phase error comprises, for each data symbol estimate:

calculating respective polar coordinate symbol phase and magnitude values of the symbol estimate;

calculating a phase error of the symbol estimate, based on the calculated symbol phase; and

calculating the respective probabilistic phase error based on the calculated phase error and the symbol magnitude.

4. The method of claim 3 , wherein calculating the phase error comprises:

adjusting the detected symbol phase to compensate frequency offset between a transmit laser and a local oscillator of the coherent receiver, to generate a corrected symbol phase; and

calculating the phase error between the corrected symbol phase and a nearest symbol of an applicable encoding scheme.

5. The method of claim 1 , wherein computing a respective phase rotation comprises:

filtering minimum variance probabilistic phase errors calculated for a time series of successive symbol estimates to obtain a minimum variance phase error; and

calculating a minimum variance phase rotation using the minimum variance phase error.

6. The method of claim 5 , wherein filtering the probabilistic phase errors comprises any one or more of:

applying a Wiener filter to the probabilistic phase errors; and

computing a running average over the probabilistic phase errors.

7. The method of claim 5 , wherein computing the minimum variance phase rotation comprises accumulating, for each successive data symbol estimate, a corresponding incremental phase rotation corresponding to μ 1 Ψ+μ 2 Δφ(n+1), where μ 1 and μ 2 are a scaling factors defining a phase adjustment step size for each successive symbol estimate; Ψ is a phase slope indicative of unbounded phase error due to frequency offset; and Δφ(n+1) is the minimum variance phase error.

8. The method of claim 1 , further comprising an operation of emphasis of a subset of calculated probabilistic phase error values.

9. The method of claim 5 , wherein filtering the probabilistic phase errors comprises the operation of emphasis of a subset of the probabilistic phase error values being filtered.

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

a phase noise estimator configured to compute a respective probabilistic phase error for each of a plurality of data symbol estimates;

a phase rotator configured to compute a phase rotation based on the probabilistic phase error estimates, using a filter function, and for applying the phase rotation to at least one data symbol estimate to generate a corresponding rotated symbol estimate; and

a decision circuit configured to process each rotated symbol estimate to generate corresponding decision values of each data symbol;

wherein computing the respective probabilistic phase error comprises calculating an expected value of phase noise based on a conditional probability density function, the probability density function being conditioned on at least one symbol estimate.

11. The coherent receiver of claim 10 , wherein the conditional probability density function is a probability density function of the phase error, or an operator thereof.

12. The coherent receiver of claim 10 , wherein the phase noise estimator comprises:

a converter configured to calculate respective polar coordinate symbol phase and magnitude values of each symbol estimate;

a phase detector configured to calculate a phase error of each symbol estimate, based on the symbol phase; and

a Minimum Mean Square Error processor configured to calculate the respective probabilistic phase error based on the calculated phase error and the symbol magnitude.

13. The coherent receiver of claim 12 , wherein the Minimum Mean Square Error processor comprises a look-up-table.

14. The coherent receiver of claim 12 , further comprising a frequency correction block configured to adjust the symbol phase to compensate frequency offset between a transmit laser and a local oscillator of the coherent receiver, to generate a corrected symbol phase, wherein the phase detector is configured to calculate the phase error between the corrected symbol phase and a nearest symbol of an applicable encoding scheme.

15. The coherent receiver of claim 12 further comprising a filter block configured to filter probabilistic phase errors calculated for a time series of successive symbol estimates to obtain a minimum variance phase error.

16. The coherent receiver of claim 15 , wherein the phase rotator is configured to calculate a minimum variance phase rotation using the minimum variance phase error.

17. The coherent receiver of claim 16 , wherein the phase rotator is configured to compute the minimum variance phase rotation by accumulating, for each successive data symbol estimate, a corresponding incremental phase rotation corresponding to μ 1 Ψ+μ 2 Δφ(n+1), where μ 1 and μ 2 are a scaling factors defining a phase adjustment step size for each successive symbol estimate; Ψ is a phase slope indicative of unbounded phase error due to frequency offset; and Δφ(n+1) is the minimum variance phase error.

18. The coherent receiver of claim 15 , wherein the filter block comprises any one or more of:

a Wiener filter; and

an averaging circuit configured to compute a running average over the probabilistic phase errors.