Estimating frequency offset using a feedback loop
According to particular embodiments, a signal communicated from a transmitter to a receiver is received. A frequency offset estimate of the signal is determined. The frequency offset estimate indicates a frequency difference between the transmitter and the receiver. The frequency offset estimate is provided as feedback. A next frequency offset is compensated for according to the feedback.
1. A method comprising:
receiving an optical signal communicated from a transmitter to a receiver;
separating the optical signal into an X-polarized component and a Y-polarized component;
determining a frequency offset estimate for each of the X-polarized component and the Y-polarized component of the signal, the frequency offset estimate indicating a frequency difference between the transmitter and the receiver;
averaging the frequency offset estimates for the X-polarized component with the Y-polarized component to determine an average frequency offset estimate;
providing the average frequency offset estimate as feedback; and
compensating for a next frequency offset according to the feedback using digital signal processing.
2. The method of claim 1 , wherein the determining of the frequency offset estimate of the X-polarized component and the Y-polarized component of the signal comprises:
identifying a plurality of data symbols of the X-polarized component and the Y-polarized component of the signal;
respectively removing the identified plurality of data symbols of the X-polarized component and the Y-polarized component from the X-polarized component and the Y-polarized component of the signal; and
determining the frequency offset estimate of the X-polarized component and the Y-polarized component from the respective remaining X-polarized component and the Y-polarized component of the signal.
3. The method of claim 1 , wherein the determining of the frequency offset estimate of the X-polarized component and the Y-polarized component of the signal comprises determining a plurality of data symbols of the X-polarized component and the Y-polarized component of the signal by:
determining an angle difference between consecutive symbols of a plurality of symbols of the X-polarized component and the Y-polarized component of the signal to yield a plurality of angle differences for each of the X-polarized component and the Y-polarized component; and
identifying the plurality of data symbols of the X-polarized component and the Y-polarized component from the respective plurality of angle differences.
4. The method of claim 1 , wherein the determining of the frequency offset estimate of the X-polarized component and the Y-polarized component of the signal comprises:
removing a plurality of data symbols from the X-polarized component and the Y-polarized component of the signal;
determining an average angle difference between consecutive samples of the remaining X-polarized component and the Y-polarized component of the signal for a plurality of consecutive samples to yield an average angle difference for the X-polarized component and an average angle difference for the Y-polarized component; and
determining the frequency offset estimate of the X-polarized component and the Y-polarized component from the respective average angle difference.
5. The method of claim 1 , wherein the determining of the frequency offset estimate of the X-polarized component and the Y-polarized component of the signal comprises:
removing a plurality of data symbols from the X-polarized component and the Y-polarized component of the signal;
determining an angle difference between consecutive samples of the remaining X-polarized component and the Y-polarized component of the signal for a plurality of consecutive samples to yield a plurality of angle differences for each of the X-polarized component and the Y-polarized component; and
summing the respective plurality of angle differences.
6. The method of claim 1 , wherein the receiving of the signal comprises:
receiving the signal from a frequency offset compensator configured to receive the signal from an equalizer, the equalizer configured to reduce inter-symbol interference.
7. The method of claim 1 :
wherein the receiving of the signal comprises receiving the signal from an equalizer configured to reduce inter-symbol interference, the equalizer processing n samples per symbol period, wherein n is an integer greater than zero; and
the compensating for the next frequency offset according to the feedback comprises:
compensating the next frequency offset in accordance with the n samples per symbol period; and
sending the signal with the compensated next frequency offset to the equalizer.
8. The method of claim 1 , the frequency difference is a frequency difference between a transmitter laser of the transmitter and a local oscillator of the receiver.
9. An apparatus comprising:
an interface configured to:
receive an optical signal communicated from a transmitter to a receiver; and
separate the optical signal into an X-polarized component and a Y-polarized component; and
one or more processors configured to:
determine a frequency offset estimate for each of the X-polarized component and the Y-polarized component of the signal, the frequency offset estimate indicating a frequency difference between the transmitter and the receiver;
average the frequency offset estimates for the X-polarized component with the Y-polarized component to determine an average frequency offset estimate;
provide the average frequency offset estimate as feedback; and
compensate for a next frequency offset according to the feedback using digital signal processing.
10. The apparatus of claim 9 , wherein the one or more processors is configured to determine the frequency offset estimate of the X-polarized component and the Y-polarized component of the signal by being configured to:
identify a plurality of data symbols of the X-polarized component and the Y-polarized component of the signal;
respectively remove the identified plurality of data symbols of the X-polarized component and the Y-polarized component from the X-polarized component and the Y-polarized component of the signal; and
determine the frequency offset estimate of the X-polarized component and the Y-polarized component from the respective remaining X-polarized component and the Y-polarized component of the signal.
11. The apparatus of claim 9 , wherein the one or more processors is configured to determine the frequency offset estimate of the X-polarized component and the Y-polarized component of the signal by determining a plurality of data symbols of the signal by being configured to:
determine an angle difference between consecutive symbols of a plurality of symbols of the X-polarized component and the Y-polarized component of the signal to yield a plurality of angle differences for each of the X-polarized component and the Y-polarized component; and
identify the plurality of data symbols of the X-polarized component and the Y-polarized component from the respective plurality of angle differences.
12. The apparatus of claim 9 , wherein the one or more processors is configured to determine the frequency offset estimate of the X-polarized component and the Y-polarized component of the signal by being configured to:
remove a plurality of data symbols from the X-polarized component and the Y-polarized component of the signal;
determine an average angle difference between consecutive samples of the remaining X-polarized component and the Y-polarized component of the signal for a plurality of consecutive samples to yield an average angle difference for the X-polarized component and an average angle difference for the Y-polarized component; and
determine the frequency offset estimate of the X-polarized component and the Y-polarized component from the respective average angle difference.
13. The apparatus of claim 9 , wherein the one or more processors is configured to determine the frequency offset estimate of the X-polarized component and the Y-polarized component of the signal by being configured to:
remove a plurality of data symbols from the X-polarized component and the Y-polarized component of the signal;
determine an angle difference between consecutive samples of the remaining X-polarized component and the Y-polarized component of the signal for a plurality of consecutive samples to yield a plurality of angle differences for each of the X-polarized component and the Y-polarized component; and
sum the respective plurality of angle differences.
14. The apparatus of claim 9 , wherein the interface is configured to receive the signal by being configured to:
receive the signal from a frequency offset compensator configured to receive the signal from an equalizer, the equalizer configured to reduce inter-symbol interference.
15. The apparatus of claim 9 :
wherein the interface is configured to receive the signal from an equalizer configured to reduce inter-symbol interference, the equalizer is configured to process n samples per symbol period, wherein n is an integer greater than zero; and
the one or more processors is configured to compensate for the next frequency offset according to the feedback by being configured to:
compensate the next frequency offset in accordance with the n samples per symbol period; and
send the signal with the compensated next frequency offset to the equalizer.
16. The apparatus of claim 9 , the frequency difference is a frequency difference between a transmitter laser of the transmitter and a local oscillator of the receiver.
17. One or more processors configured to execute instructions to:
receive an optical signal communicated from a transmitter to a receiver;
separate the optical signal into an X-polarized component and a Y-polarized component;
determine a frequency offset estimate for each of the X-polarized component and the Y-polarized component of the signal, the frequency offset estimate indicating a frequency difference between the transmitter and the receiver, the frequency difference being a frequency difference between a transmitter laser of the transmitter and a local oscillator of the receiver;
average the frequency offset estimates for the X-polarized component with the Y-polarized component to determine an average frequency offset estimate;
provide the average frequency offset estimate as feedback; and
compensate for a next frequency offset according to the feedback using digital signal processing.
18. The processors of claim 17 , wherein the one or more processors is configured to determine the frequency offset estimate of the X-polarized component and the Y-polarized component of the signal by being configured to:
identify a plurality of data symbols of the X-polarized component and the Y-polarized component of the signal;
respectively remove the identified plurality of data symbols of the X-polarized component and the Y-polarized component from the X-polarized component and the Y-polarized component of the signal; and
determine the frequency offset estimate of the X-polarized component and the Y-polarized component from the respective remaining X-polarized component and the Y-polarized component of the signal.
19. The processors of claim 17 , wherein the one or more processors is configured to determine the frequency offset estimate of the X-polarized component and the Y-polarized component of the signal by determining a plurality of data symbols of the X-polarized component and the Y-polarized component of the signal by being configured to:
determine an angle difference between consecutive symbols of a plurality of symbols of the X-polarized component and the Y-polarized component of the signal to yield a plurality of angle differences for each of the X-polarized component and the Y-polarized component; and
identify the plurality of data symbols of the X-polarized component and the Y-polarized component from the respective plurality of angle differences.
20. The processors of claim 17 , wherein the one or more processors is configured to determine the frequency offset estimate of the X-polarized component and the Y-polarized component of the signal by being configured to:
remove a plurality of data symbols from the X-polarized component and the Y-polarized component of the signal;
determine an average angle difference between consecutive samples of the remaining X-polarized component and the Y-polarized component of the signal for a plurality of consecutive samples to yield an average angle difference for the X-polarized component and an average angle difference for the Y-polarized component; and
determine the frequency offset estimate of the X-polarized component and the Y-polarized component from the respective average angle difference.