Optical receiving apparatus, optical transmitting apparatus, and frequency offset estimation method

1. An optical receiving apparatus for receiving a coherent optical signal that includes a training signal, the optical receiving apparatus comprising:

a timing detection unit configured to detect a reception training signal corresponding to a training code sequence for frequency offset estimation within a reception signal acquired by converting the received coherent optical signal into an electric signal;

a frequency offset estimation unit configured to calculate a plurality of delay differential components on basis of the reception training signal detected by the timing detection unit and at least two delay signals of the reception training signal, each delay signal having a different number of delay symbols, and calculates an averaged frequency offset amount using the calculated plurality of delay differential components; and

a frequency offset compensation processing unit configured to perform frequency offset compensation on the reception signal using the frequency offset amount calculated by the frequency offset estimation unit,

wherein the training code sequence for frequency offset estimation is a code sequence constituted by multi-value phase modulation symbols, in which a deviation angle of a vector average of a one-symbol delay differential component of the training signal generated on the basis of the code sequence has a prescribed angle, and a modulation phase difference between adjacent symbols has a fixed, repeated pattern,

wherein each of the timing detection unit, the frequency offset estimation unit and the frequency offset compensation processing unit is implemented by:

i) computer executable instructions executed by at least one processor,

ii) at least one circuitry or

iii) a combination of computer executable instructions executed by at least one processor and at least one circuitry.

2. The optical receiving apparatus according to claim 1 , wherein a sequence length of the training code sequence for frequency offset estimation is a multiple of a number of elements included in the fixed, repeated pattern of the modulation phase difference between the adjacent symbols.

3. The optical receiving apparatus according to claim 1 , wherein, when the frequency offset estimation unit calculates a one-symbol delay differential component and a two-symbol delay differential component of the reception training signal, the frequency offset estimation unit calculates a first vector of a differential component between a vector average of the one-symbol delay differential component of the reception training signal and the vector average of the one-symbol delay differential component of the training signal, calculates a second vector in which a phase component of a predetermined angle, determined in advance on the basis of the prescribed angle and a total angle of the elements of the modulation phase difference included in the repeated pattern, has been removed from a vector of a differential component between respective vector averages of the one-symbol delay differential component and the two-symbol delay differential component of the reception training signal, and calculates the frequency offset amount by averaging the calculated first and second vectors.

4. The optical receiving apparatus according to claim 3 , wherein, when the frequency offset estimation unit calculates a p (where p is an integer of at least 2) symbol delay differential component and a (p+1)-symbol delay differential component of the reception training signal, the frequency offset estimation unit calculates a third vector in which a phase component of a predetermined angle, determined in advance on the basis of the prescribed angle, has been removed from a vector of a differential component between respective vector averages of the p-symbol delay differential component and the (p+1) symbol delay differential component of the reception training signal, calculates the frequency offset amount by averaging the first vector, the second vector, and the third vector when p includes 2, and calculates the frequency offset amount by averaging the first vector and the third vector when p does not include 2.

5. The optical receiving apparatus according to claim 4 , wherein, when the number of elements included in the fixed, repeated pattern of the modulation phase difference between adjacent symbols is set as M (where M represents number of elements in the repeated pattern of the modulation phase difference), p takes a value of M−1, or M, or M+1, or all of these values.

6. The optical receiving apparatus according to claim 1 , wherein the timing detection unit, when detecting the reception training signal corresponding to the training code sequence for frequency offset estimation within the reception signal, detects, as the reception training signal, an interval of the reception signal including all or a part of the training code sequence for frequency offset estimation, the interval having a length that corresponds to sequence length of the training code sequence for frequency offset estimation.

7. A frequency offset estimation method comprising:

receiving, by a processor, a coherent optical signal that includes a training signal generated using a code sequence constituted by multi-value phase modulation symbols, in which a deviation angle of a vector average of a one-symbol delay differential component of a signal generated on the basis of the code sequence has a prescribed angle, and a modulation phase difference between adjacent symbols has a fixed, repeated pattern;

detecting, by a processor, a reception training signal corresponding to a training code sequence for frequency offset estimation within a reception signal acquired by converting the received coherent optical signal into an electric signal; and

calculating, by a processor, a plurality of delay differential components on the basis of the detected reception training signal and at least two delay signals of the reception training signal, each delay signal having a different number of delay symbols, and calculating an averaged frequency offset amount, which is used to compensate for a frequency offset in the reception signal, using the calculated plurality of delay differential components.