Apparatus and method for blind LOFO estimation in coherent optical receiver

A digital signal processing apparatus implemented at least partially in hardware for estimating a local oscillator frequency offset (LOFO) of a coherent optical receiver, the digital signal processing apparatus comprising:

• a digital frequency shifter for digitally shifting a spectrum of a signal received by the coherent optical receiver according to one of a plurality of frequency shifts within a pre-determined first range for a possible LOFO;
• an equalizer module for equalizing the spectrum shifted according to each of the plurality of frequency shifts based on a hybrid shifted filter response of a receiver driver, the hybrid shifted filter response is based on a combination of a linear shifted response and a circular shifted filter response of the receiver driver;
• and
• a LOFO estimation module for estimating the LOFO,
• wherein the LOFO estimation module is configured to calculate a difference of power between two sides of each equalized spectrum, determine a minimum power difference among the plurality of frequency shifts within the first range, and output the frequency shift having the minimum power difference as an estimated LOFO, the power for each side of the spectrum being calculated within a first area centered around a frequency corresponding to half of a baud rate of the signal received by the coherent optical receiver.

A method for estimating a local oscillator frequency offset (LOFO) of a coherent optical receiver, the method comprising:

• receiving a signal by the coherent optical receiver;
• digitally shifting a spectrum of the received signal according to one of a plurality of frequency shifts within a first pre-determined range for a possible LOFO;
• equalizing the spectrum shifted according to each of the plurality of frequency shifts based on a shifted filter response of a receiver driver;
• calculating a difference of power between two sides of each equalized spectrum, the power for each side of the spectrum being calculated within a first area centered around a frequency corresponding to half of a baud rate of the received signal;
• determining a minimum power difference among the plurality of frequency shifts within the first range;
• output the frequency shift in the first range having the minimum power difference as an estimated LOFO;
• and
• determining if the estimated LOFO is within a pre-determined second range likely to be affected by channel filtering, the pre-determined second range being a subset of the first range, wherein
• if the estimated LOFO is within the second range, the method further comprises:
• calculating a difference of slopes between two sides of the equalized spectrum, the slope for each side of the spectrum being calculated within a second area located closer to a center of the spectrum than the first area;
• determining a minimum slope difference among the plurality of frequency shifts within a third range, the third range being from the estimated LOFO to an outer bound of the first range;
• and
• outputting the frequency shift in the third range having the minimum slope difference as a refined LOFO.