Frequency and phase compensation for modulation formats using multiple sub-carriers

Abstract

An optical receiver may receive input signals carried by respective sub-carriers. The optical receiver may determine, based on the input signals, a compensation value to be used to modify an input signal. The optical receiver may use the compensation value to adjust the input signal to form a modified input signal. The compensation value may be used to modify a frequency or a phase of the input signal. The optical receiver may determine, based on the modified input signal, a phase estimate value that represents an estimated phase associated with the input signal. The optical receiver may combine the compensation value and the phase estimate value to form a phase adjustment signal, may combine the input signal and the phase adjustment signal to form an output signal, and may output the output signal.

What is claimed is: 
     
       1. An optical receiver, comprising:
 a digital signal processor configured to:
 receive a plurality of input signals carried by a respective plurality of sub-carriers; 
 determine, based on the plurality of input signals, a frequency compensation value and a phase compensation value to be used to modify an input signal of the plurality of input signals; 
 modify the input signal, to form a modified input signal, using the frequency compensation value and the phase compensation value,
 the frequency compensation value being used to modify a frequency of the input signal, 
 the phase compensation value being used to modify a phase of the input signal; 
 
 determine, based on the modified input signal, a phase estimate value that represents an estimated phase for the input signal; 
 combine the frequency compensation value, the phase compensation value, and the phase estimate value to generate a phase adjustment signal; 
 combine the input signal and the phase adjustment signal to form an output signal; and 
 output the output signal. 
 
 
     
     
       2. The optical receiver of  claim 1 , where the digital signal processor, when determining the frequency compensation value, is further configured to:
 determine a plurality of frequency compensation error values corresponding to the plurality of sub-carriers; and 
 determine the frequency compensation value based on the plurality of frequency compensation error values. 
 
     
     
       3. The optical receiver of  claim 2 , where the digital signal processor, when determining the plurality of frequency compensation error values, is further configured to:
 receive a plurality of samples, associated with the input signal, carried by a particular sub-carrier of the plurality of sub-carriers; and 
 determine a frequency compensation error value, of the plurality of frequency compensation error values, based on the plurality of samples. 
 
     
     
       4. The optical receiver of  claim 1 , where the input signal has a first polarization and a second polarization; and
 where the digital signal processor, when determining the phase compensation value, is further configured to:
 determine a first phase estimate value for the input signal,
 the first phase estimate value being associated with the first polarization of the input signal; 
 
 determine a second phase estimate value for the input signal,
 the second phase estimate value being associated with the second polarization of the input signal,
 the second polarization being different than the first polarization; 
 
 
 calculate a difference between the first phase estimate value and the second phase estimate value; and 
 determine the phase compensation value based on the difference between the first phase estimate value and the second phase estimate value. 
 
 
     
     
       5. The optical receiver of  claim 4 , where the digital signal processor, when determining the first phase estimate value, is further configured to:
 apply a plurality of test phases to the input signal on the first polarization; 
 calculate a plurality of error values that each corresponds to one of the plurality of test phases; 
 select a test phase, of the plurality of test phases, that corresponds to a lowest error value, of the plurality of error values, as compared to other error values included in the plurality of error values; and 
 interpolate the first phase estimate value based on the selected test phase. 
 
     
     
       6. The optical receiver of  claim 1 , where the digital signal processor, when determining the phase compensation value, is further configured to:
 determine a plurality of first phase estimate values, corresponding to the plurality of sub-carriers, associated with a first polarization of the plurality of input signals; 
 determine a plurality of second phase estimate values, corresponding to the plurality of sub-carriers, associated with a second polarization of the plurality of input signals,
 the second polarization being different than the first polarization; 
 
 calculate a first difference between at least one first phase estimate value, of the plurality of first phase estimate values, and at least one other first phase estimate value of the plurality of first phase estimate values; 
 calculate a second difference between at least one second phase estimate value, of the plurality of second phase estimate values, and at least one other second phase estimate value of the plurality of second phase estimate values; and 
 determine the phase compensation value based on the first difference and the second difference. 
 
     
     
       7. The optical receiver of  claim 1 , where the digital signal processor, when determining the phase compensation value, is further configured to:
 determine a first phase difference between a first signal of a first polarization and a second signal of a second polarization,
 the first signal and the second signal being included in the modified input signal; 
 
 determine a second phase difference between a third signal, included in the modified input signal and carried via a first sub-carrier of the plurality of sub-carriers, and a fourth signal carried via a second sub-carrier of the plurality of sub-carriers; and 
 determine the phase compensation value based on the first phase difference and the second phase difference. 
 
     
     
       8. The optical receiver of  claim 1 , where the digital signal processor is further configured to:
 provide the output signal as a feedback signal; 
 determine, based on the feedback signal, another phase compensation value to be used to modify another input signal; and 
 modify the other input signal using the other phase compensation value. 
 
     
     
       9. A system, comprising:
 an optical receiver configured to:
 receive a plurality of input signals carried by a respective plurality of sub-carriers; 
 determine, based on the plurality of input signals, a frequency compensation value or a phase compensation value to be used to modify an input signal of the plurality of input signals; 
 modify the input signal, to form a modified input signal, using the frequency compensation value or the phase compensation value,
 the frequency compensation value being used to modify a frequency of the input signal, 
 the phase compensation value being used to modify a phase of the input signal; 
 
 determine, based on the modified input signal, a phase estimate value that represents an estimated phase for the input signal; 
 combine the phase estimate value and the frequency compensation value or the phase compensation value to generate a phase adjustment signal; 
 combine the input signal and the phase adjustment signal to form an output signal; and 
 output the output signal. 
 
 
     
     
       10. The system of  claim 9 , where the optical receiver, when determining the frequency compensation value, is further configured to:
 determine a plurality of frequency compensation error values corresponding to the plurality of sub-carriers; and 
 determine the frequency compensation value based on the plurality of frequency compensation error values. 
 
     
     
       11. The system of  claim 9 , where the optical receiver, when determining the frequency compensation value, is further configured to:
 determine a mode indicator value that indicates whether to combine a plurality of frequency compensation error values, corresponding to the plurality of sub-carriers, to determine the frequency compensation value; and 
 selectively perform a first action or a second action based on the mode indicator value,
 the first action including using a single frequency compensation error value, corresponding to a single sub-carrier of the plurality of sub-carriers, to determine the frequency compensation value when the mode indicator value is equal to a first value, and 
 the second action including using at least two frequency compensation error values, corresponding to at least two sub-carriers of the plurality of sub-carriers, to determine the frequency compensation value when the mode indicator value is equal to a second value,
 the second value being different than the first value. 
 
 
 
     
     
       12. The system of  claim 9 , where the optical receiver, when determining the phase compensation value, is further configured to:
 determine a first phase estimate value for the input signal,
 the first phase estimate value being associated with a first polarization of the input signal; 
 
 determine a second phase estimate value for the input signal,
 the second phase estimate value being associated with a second polarization of the input signal,
 the second polarization being different than the first polarization; 
 
 
 calculate a difference between the first phase estimate value and the second phase estimate value; and 
 determine the phase compensation value based on the difference between the first phase estimate value and the second phase estimate value. 
 
     
     
       13. The system of  claim 9 , where the optical receiver, when determining the phase compensation value, is further configured to:
 determine a plurality of first phase estimate values, corresponding to the plurality of sub-carriers, associated with a first polarization of the plurality of input signals; 
 determine a plurality of second phase estimate values, corresponding to the plurality of sub-carriers, associated with a second polarization of the plurality of input signals,
 the second polarization being different than the first polarization; 
 
 calculate a first difference between at least one first phase estimate value, of the plurality of first phase estimate values, and at least one other first phase estimate value of the plurality of first phase estimate values; 
 calculate a second difference between at least one second phase estimate value, of the plurality of second phase estimate values, and at least one other second phase estimate value of the plurality of second phase estimate values; and 
 determine the phase compensation value based on the first difference and the second difference. 
 
     
     
       14. The system of  claim 9 , where the optical receiver, when determining the phase compensation value, is further configured to:
 determine a mode indicator value that indicates whether to combine a plurality of phase estimate values, corresponding to the plurality of sub-carriers, to determine the phase compensation value; and 
 selectively perform a first action or a second action based on the mode indicator value,
 the first action including using a single phase estimate value, corresponding to a single sub-carrier of the plurality of sub-carriers, to determine the phase compensation value when the mode indicator value is equal to a first value, and 
 the second action including using at least two phase estimate values, corresponding to at least two sub-carriers of the plurality of sub-carriers, to determine the phase compensation value when the mode indicator value is equal to a second value,
 the second value being different than the first value. 
 
 
 
     
     
       15. A method, comprising:
 receiving, by an optical receiver, a plurality of input signals carried by a respective plurality of sub-carriers; 
 determining, by the optical receiver and based on the plurality of input signals, a compensation value to be used to modify an input signal of the plurality of input signals; 
 adjusting, by the optical receiver and using the compensation value, the input signal to form a modified input signal,
 the compensation value being used to modify a frequency or a phase of the input signal; 
 
 determining, by the optical receiver and based on the modified input signal, a phase estimate value that represents an estimated phase associated with the input signal; 
 combining, by the optical receiver, the compensation value and the phase estimate value to form a phase adjustment signal; 
 combining, by the optical receiver, the input signal and the phase adjustment signal to form an output signal; and 
 outputting, by the optical receiver, the output signal. 
 
     
     
       16. The method of  claim 15 , where determining the compensation value further comprises:
 determining a plurality of frequency compensation error values corresponding to the plurality of sub-carriers; and 
 determining the compensation value based on the plurality of frequency compensation error values. 
 
     
     
       17. The method of  claim 15 , where determining the compensation value further comprises:
 determining a first phase estimate value associated with the input signal,
 the first phase estimate value representing a first estimated phase associated with a first polarization of the input signal; 
 
 determining a second phase estimate value associated with the input signal,
 the second phase estimate value representing a second estimated phase associated with a second polarization of the input signal,
 the second polarization being different than the first polarization; 
 
 
 calculating a difference between the first phase estimate value and the second phase estimate value; and 
 determining the compensation value based on the difference between the first phase estimate value and the second phase estimate value. 
 
     
     
       18. The method of  claim 15 , where determining the compensation value further comprises:
 determining a plurality of first phase estimate values, corresponding to the plurality of input signals, that represent a first estimated phase associated with a first polarization; 
 determining a plurality of second phase estimate values, corresponding to the plurality of input signals, that represent a second estimated phase associated with a second polarization;
 the second polarization being different than the first polarization; 
 
 calculating a first difference between at least one first phase estimate value, of the plurality of first phase estimate values, and at least one other first phase estimate value of the plurality of first phase estimate values; 
 calculating a second difference between at least one second phase estimate value, of the plurality of second phase estimate values, and at least one other second phase estimate value of the plurality of second phase estimate values; and 
 determining the compensation value based on the first difference and the second difference. 
 
     
     
       19. The method of  claim 15 , where determining the compensation value further comprises:
 determining the compensation value based on at least one of:
 a plurality of frequency compensation error values associated with different sub-carriers of the plurality of sub-carriers, or 
 a plurality of phase estimate values associated with the different sub-carriers. 
 
 
     
     
       20. The method of  claim 15 , further comprising:
 determining a feedback signal based on the output signal; 
 determining, based on the feedback signal, another compensation value to be used to modify another input signal; and 
 modifying the other input signal using the other compensation value.