Optical receiving device and optical receiving method

Abstract

To reduce the power consumption required to calculate an initial value used when compensating for an optical frequency offset in a multilevel modulation scheme.SOLUTION: An optical receiving device includes: a receiving unit that receives an optical signal including a plurality of data symbols and pilot symbols periodically inserted between the plurality of data symbols, and modulated based on a multilevel modulation scheme; a first angle calculating unit that calculates a first phase angle used to calculate an initial value when compensating for an optical frequency offset based on the pilot symbols; a second angle calculating unit that calculates a second phase angle of the data symbols based on data symbols adjacent to the pilot symbols among the plurality of data symbols; an estimating unit that estimates the amount of the optical frequency offset based on a differential phase angle between the first phase angle and the second phase angle and amplitude of the data symbols; and a compensating unit that compensates for the optical frequency offset based on the amount of the optical frequency offset.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to an optical receiving device and an optical receiving method. [Background technology]

[0002] Receivers that use multilevel modulation methods in mobile communications and the like are known (see, for example, Patent Document 1). Known multilevel modulation methods include QPSK (Quadrature Phase Shift Keying) and 16QAM (Quadrature Amplitude Modulation) (see, for example, Patent Documents 2 and 3). In addition, in digital mobile radio systems, compensation for frequency offset using a digital signal processing method is also known (see, for example, Patent Document 4). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 10-098500 [Patent Document 2] US Patent Application Publication No. 2022 / 0294538 [Patent Document 3] International Publication No. 2014 / 187742 [Patent Document 4] Japanese Patent Application Publication No. 9-093302 Summary of the Invention [Problem to be solved by the invention]

[0004] The above-mentioned frequency offset compensation is not limited to digital mobile radio. For example, frequency offset compensation is also performed as compensation for optical frequency offset between an optical transmitter and an optical receiver that use a multilevel modulation method. In this case, an initial value of the optical frequency offset is calculated when the optical receiver is started, and the optical frequency offset is compensated based on the calculated initial value.

[0005] The initial value of the optical frequency offset may be calculated based on data symbols included in the transmission signal from the optical transmitter. In this case, if the number of data symbols is small, the initial value cannot be calculated accurately, and an error may occur in the initial value.

[0006] On the other hand, if a large number of data symbols are used to calculate the initial value, the amount of calculation required to calculate the initial value increases, making it difficult to calculate the initial value quickly. To calculate the initial value quickly, it is conceivable to use, for example, parallelized arithmetic circuits. However, in this case, the circuit size increases because arithmetic circuits corresponding to the number of parallel circuits are required. The increase in circuit size causes an increase in power consumption when calculating the initial value of the optical frequency offset.

[0007] An object of the present invention is to provide an optical receiving device and an optical receiving method that reduce the power consumption required to calculate an initial value used when compensating for an optical frequency offset in a multilevel modulation system. [Means for solving the problem]

[0008] In one embodiment, an optical receiving device includes: a receiving unit that receives an optical signal including a plurality of data symbols and pilot symbols periodically inserted between the plurality of data symbols, and modulated based on a multi-level modulation scheme; a first angle calculating unit that calculates, based on the pilot symbols, a first phase angle to be used for calculating an initial value when compensating for an optical frequency offset; a second angle calculating unit that calculates, based on a data symbol adjacent to the pilot symbol among the plurality of data symbols, a second phase angle of the data symbol; an estimating unit that estimates the amount of the optical frequency offset based on a differential phase angle between the first phase angle and the second phase angle and the amplitude of the data symbol; and a compensating unit that compensates for the optical frequency offset based on the amount of the optical frequency offset. [Effects of the Invention]

[0009] It is possible to reduce the power consumption required to calculate the initial value used when compensating for the optical frequency offset in the multilevel modulation method. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is an example of an optical transmission system. [Figure 2] 1 is an example of an optical signal. [Figure 3] FIG. 2 is a block diagram illustrating an example of a hardware configuration of an optical transmitting device. [Figure 4] FIG. 2 is a block diagram illustrating an example of a hardware configuration of an optical receiving device. [Figure 5] FIG. 2 is a block diagram showing an example of the functional configuration of an RxDSP. [Figure 6] FIG. 2 is a block diagram illustrating an example of the functional configuration of an FOC. [Figure 7] 10 is an example of a functional configuration of a first calculation unit. [Figure 8] 10 is a diagram illustrating an example of a phase angle difference before and after quadrupling the phase angle of an n-th pilot symbol modulated by QPSK. FIG. [Figure 9] (a) is an example of a constellation of data symbols modulated with 16QAM. (b) is an example of a first constellation. (c) is an example of a second constellation. [Figure 10] (a) is another example of a constellation of data symbols modulated with 16QAM, (b) is an example of a third constellation, and (c) is an example of a fourth constellation. [Figure 11] 10A and 10B are diagrams illustrating the phase angle difference and optical frequency offset before and after quadrupling processing of the phase angle of the (n+1)th data symbol modulated by 16QAM. [Figure 12] 10A and 10B are diagrams illustrating an example of a phase angle of a third constellation before and after quadrupling processing. [Figure 13] 10A and 10B are diagrams illustrating an example of a phase angle of a fourth constellation before and after quadrupling processing. [Figure 14]1A is an example of the functional configuration of a first estimation unit, and FIG. 1B is an example of the functional configuration of a correction unit. [Figure 15] 10 is a flowchart showing an example of the operation of a first calculation unit. [Figure 16] 10 is another example of the functional configuration of the reference calculation unit. [Figure 17] 10 is a diagram illustrating an example of a phase angle difference before and after multiplication of the phase angle of the n-th pilot symbol modulated by QPSK by −45 degrees. FIG. [Figure 18] FIG. 10 is a diagram illustrating another example of the phase angle difference before and after quadrupling the phase angle of the n-th pilot symbol modulated by QPSK. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0012] (First embodiment) As shown in Fig. 1, the optical transmission system ST includes an optical transmitter 10 and an optical receiver 20. The optical transmitter 10 and the optical receiver 20 are connected via an optical transmission path 30. The optical transmission path 30 includes optical fibers, optical repeaters, etc. Examples of optical repeaters include a ROADM (Reconfigurable Optical Add / Drop Multiplexer) and an ILA (Optical In-Line Amplifier Equipment). The optical transmitter 10 receives an electrical client signal in a digital format from a client network.

[0013] The client signal is, for example, an Ethernet (registered trademark) signal. The client signal may be a main signal, or may be a control signal that includes only parameters for adjusting transmission characteristics, etc. The optical transmitting device 10 converts the client signal into an optical signal 40 and transmits it to the optical transmission path 30. As a result, the optical signal 40 propagates through the optical transmission path 30. The optical receiving device 20 receives the optical signal 40 from the optical transmission path 30. Upon receiving the optical signal 40, the optical receiving device 20 converts the optical signal 40 into a client signal and transmits it to a client network.

[0014] 2, an optical signal 40 transmitted from the optical transmitter 10 includes, as transmission data, a plurality of data symbols 41 and pilot symbols 42 periodically inserted between the plurality of data symbols 41. The pilot symbols 42 are inserted between the data symbols 41 at a predetermined symbol interval "K." For example, the pilot symbols 42 are inserted at symbol interval "K" such as 32 symbol intervals or 64 symbol intervals.

[0015] For example, the data symbol 41 is modulated based on the 16QAM modulation method. The pilot symbol 42 is modulated based on the QPSK modulation method. The data symbol 41 and the pilot symbol 42 are modulated based on different multi-level modulation methods. Therefore, when using pilot symbols to calculate the initial value of the optical frequency offset, the amount of calculation required to calculate the initial value can be reduced compared to when pilot symbols are not used. This makes it possible to reduce the power consumption of the optical receiving device 20.

[0016] As will be described in detail later, the optical receiving device 20 calculates an initial value of the optical frequency offset and compensates the received signal based on the n-th pilot symbol 42 included in the optical signal 40, and the (n-1)-th data symbol 41A and the (n+1)-th data symbol 41B adjacent to the n-th pilot symbol 42, where n is a natural number.

[0017] The hardware configuration of the optical transmitter 10 will be described with reference to FIG.

[0018] As shown in FIG. 3, the optical transmitting device 10 includes a TxDSP (Tx Digital Signal Processor) 110, a DAC (Digital to Analogue Converter) 120, and a CDM (Coherent Driver Modulator) 130. The TxDSP 110 is a DSP mounted on the optical transmitting device 10. The CDM 130 includes a driver amplifier (represented as DRV in FIG. 3) 131 and an optical modulator (represented as MOD in FIG. 3) 132. The CDM 130 is an integrated circuit that houses the driver amplifier 131 and the optical modulator 132 in a single package. The optical transmitting device 10 also includes an ITLA (Integrable Tunable Laser Assembly) 140 and a transmission control unit 150. Although not shown, the ITLA 140 includes a transmission light source that outputs transmission light (specifically, laser light).

[0019] The TxDSP 110 performs various digital signal processing. For example, the TxDSP 110 accommodates a client signal in a transmission frame and generates a binary data bit sequence corresponding to the transmission frame. For example, the transmission frame is an OTU (Optical channel Transport Unit) frame. The TxDSP 110 also performs symbol mapping processing based on the modulation method set by the transmission control unit 150. The symbol mapping processing is processing for converting a binary data bit sequence corresponding to the transmission frame into a plurality of data symbol sequences.

[0020] The TxDSP 110 periodically inserts pilot symbols between data symbols. In addition, the TxDSP 110 compensates in advance for various losses that occur within the optical transmitting device 10 for the transmission signal composed of data symbols and pilot symbols. For example, the TxDSP 110 performs skew compensation and bandwidth characteristic compensation. The TxDSP 110 outputs the compensated transmission signal composed of data symbols and pilot symbols to the DAC 120 based on the setting value set by the transmission control unit 150.

[0021] The DAC 120 converts the transmission signal from digital format to analog format and outputs it to a driver amplifier 131 of the CDM 130. The driver amplifier 131 amplifies the signal amplitude of the transmission signal output from the DAC 120.

[0022] The optical modulator 132 modulates the transmission light (specifically, laser light) input from the ITLA 140 based on the signal amplitude amplified by the driver amplifier 131, to generate an optical signal 40 having an arbitrary optical waveform. The optical modulator 132 converts the electrical transmission signal into the optical signal 40 and outputs it to the optical transmission line 30. In this way, the CDM 130 converts the electrical transmission signal into the optical signal 40 and outputs it to the optical transmission line 30.

[0023] The transmission control unit 150 includes a processor and memory, and controls the operations of the TxDSP 110 and the ITLA 140. For example, the processor is a CPU (Central Processing Unit), and the memory is a volatile memory such as RAM (Random Access Memory) and a non-volatile memory such as ROM (Read Only Memory). The transmission control unit 150 may be an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit).

[0024] The transmission control unit 150 performs various settings on the TxDSP 110 in accordance with instructions from an operation terminal (not shown) and adjusts the frequency of the ITLA 140. The operation terminal may be a PC (Personal Computer) or a smart terminal (e.g., a tablet terminal). For example, when a signal type including a symbol rate, a multi-level modulation method, etc. is input from the operation terminal to the transmission control unit 150, the transmission control unit 150 sets the symbol rate, the multi-level modulation method, etc. in the TxDSP 110.

[0025] The hardware configuration of the optical receiving device 20 will be described with reference to FIG.

[0026] As shown in FIG. 4, the optical receiving device 20 includes an RxDSP 210, an ADC (Analogue to Digital Converter) 220, and an ICR (Integrated Coherent Receiver) 230. The RxDSP 210 is a DSP mounted on the optical receiving device 20. The ICR 230 includes a 90° optical hybrid circuit (simply indicated as 90° in FIG. 4) 231, a BPD (Balanced Photo Diode) 232, and a TIA (Transimpedance Amplifier) ​​233. The ICR 230 is an integrated circuit that houses the 90° optical hybrid circuit 231, the BPD 232, and the TIA 233 in a single package. The ICR 230 or the 90° optical hybrid circuit 231 is an example of a receiving unit. The optical receiving device 20 also includes an ITLA 240 and a receiving control unit 250. Although not shown, the ITLA 240 includes a local light source that outputs local light (specifically, laser light).

[0027] The 90° optical hybrid circuit 231 receives an optical signal 40 transmitted from the optical transmitter 10 and propagated through the optical transmission line 30. The 90° optical hybrid circuit 231 receives the optical signal 40 using the local oscillator light output from the ITLA 240 and outputs it to the BPD 232. The BPD 232 converts the optical signal 40 into a current signal and outputs it to the TIA 233. The TIA 233 converts the current signal output from the BPD 232 into a voltage signal, and amplifies the voltage signal to an amplitude suitable for the ADC 220, and outputs the amplified voltage signal to the ADC 220 as a received data string.

[0028] The ICR 230 converts the input optical signal 40 into an electrical analog received data sequence using the 90° optical hybrid circuit 231, the BPD 232, and the TIA 233. The ADC 220 converts the received data sequence from analog to digital format and outputs it to the RxDSP 210. The RxDSP 210 receives the received data sequence output from the ADC 220 based on a setting value set in the reception control unit 250.

[0029] The RxDSP 210 performs various digital signal processing. For example, the RxDSP 210 performs symbol demapping processing on received symbols (described later) based on the setting value of the multi-level modulation method set in the reception control unit 250. Specifically, the RxDSP 210 converts data symbols included in the received symbol sequence into a binary data bit sequence and reproduces the transfer frame. Thereafter, the RxDSP 210 extracts the client signal from the transfer frame and transmits the extracted client signal to the client network. Details of the digital signal processing performed by the RxDSP 210 will be described later.

[0030] The reception control unit 250 performs various settings on the RxDSP 210 in accordance with instructions from the operation terminal and adjusts the frequency of the ITLA 240. For example, when a signal type including a symbol rate, a multi-level modulation method, etc. is input from the operation terminal to the reception control unit 250, the reception control unit 250 sets the symbol rate, the multi-level modulation method, etc. in the RxDSP 210. Note that the hardware configuration of the reception control unit 250 is basically the same as the hardware configuration of the transmission control unit 150, and therefore a detailed description thereof will be omitted.

[0031] The functional configuration of the RxDSP 210 will be described with reference to FIG.

[0032] The RxDSP 210 includes a CDC (Chromatic Dispersion Compensation) 211, an AEQ (Adaptive Equalizer) 212, an FOC (Frequency Offset Compensation) 213, and a CPR (Carrier Phase Recovery) 214. Although not shown, the RxDSP 210 also includes a demodulation unit and the like that performs symbol demapping processing, error correction processing, and the like after the CPR 214 and outputs a client signal.

[0033] The CDC 211 performs fixed compensation for losses that occur in the optical transmitting device 10, the optical receiving device 20, and the optical transmission line 30 for the received data sequence output from the ADC 220. Specifically, the CDC 211 performs chromatic dispersion compensation, skew compensation, and bandwidth characteristic compensation. The AEQ 212 adaptively compensates for waveform distortion of the optical signal 40 that occurs due to polarization mode dispersion and polarization dependent loss that occur in the optical transmission line 30 for the received data sequence output from the CDC 211. At the same time, the AEQ 212 adjusts the sampling timing of the received data sequence output from the ADC 220, and outputs the symbol-by-symbol data sequence to the FOC 213 as a received symbol sequence.

[0034] The FOC 213 estimates the optical frequency offset representing the amount of optical frequency offset for the received symbol sequence output from the AEQ 212, and compensates the received symbol sequence with the estimated optical frequency offset. The optical frequency offset is the difference between the frequency of the transmitted light output by the ITLA 140 in the optical transmitter 10 and the frequency of the local oscillator light output by the ITLA 240 in the optical receiver 20. The frequency of the local oscillator light is approximately the same as the frequency of the transmitted light, but does not necessarily match. Therefore, the FOC 213 compensates for the difference between the frequency of the transmitted light and the frequency of the local oscillator light, absorbing the difference between the frequency of the transmitted light and the frequency of the local oscillator light. The CPR 214 compensates for fluctuations in the phase shift due to phase noise generated in the ITLA 240 and other components.

[0035] The FOC 213 will be described with reference to FIG.

[0036] FOC 213 has a first calculation unit 301, an extraction unit 302, a second calculation unit 303, a calculated value selection unit 304, an addition unit 305, a holding unit 306, an integrator unit 307, and a multiplier unit 309. Multiplier unit 309 is an example of a compensator that compensates for an optical frequency offset in a received symbol sequence. As will be described in detail later, integrator unit 307 outputs optical frequency offset compensation value 308 as an output value to multiplier unit 309.

[0037] The first calculation unit 301 extracts a pilot symbol and two data symbols adjacent to the pilot symbol before and after it from the received symbol sequence output from the AEQ 212. Specifically, the first calculation unit 301 receives position information of the pilot symbol 42 in the received symbol sequence from a synchronization unit (not shown) provided between the CDC 211 and the AEQ 212 or after the AEQ 212. The first calculation unit 301 extracts the pilot symbol and the data symbols before and after it from the position information of the pilot symbol. For example, if the pilot symbol is the nth symbol, the first calculation unit 301 extracts the (n-1)th data symbol and the (n+1)th data symbol.

[0038] The first calculation unit 301 extracts a pilot symbol and two data symbols adjacent to the pilot symbol, and then calculates an initial value of the optical frequency offset based on the pilot symbol and the two data symbols. After calculating the initial value of the optical frequency offset, the first calculation unit 301 outputs the initial value to the calculated value selection unit 304. Note that detailed processing when the first calculation unit 301 calculates the initial value of the optical frequency offset will be described later.

[0039] The extraction unit 302 extracts pilot symbols from the received symbol sequence output from the AEQ 212 and outputs the extracted pilot symbols to the second calculation unit 303. The extraction unit 302 can extract the pilot symbols by using the position information of the pilot symbols described above.

[0040] The second calculation unit 303 calculates a non-initial value excluding the initial value of the optical frequency offset based on the pilot symbol output from the extraction unit 302. The non-initial value corresponds to the amount of optical frequency offset that has fluctuated from the amount of optical frequency offset currently being compensated. After calculating the non-initial value, the second calculation unit 303 outputs the calculated non-initial value to the calculated value selection unit 304.

[0041] The calculated value selection unit 304 selects either the initial value or the non-initial value, and outputs the selected initial value or non-initial value to the addition unit 305. The calculated value selection unit 304 selects either the initial value or the non-initial value based on control by the reception control unit 250. For example, if the reception control unit 250 determines that optical frequency offset compensation has never been performed by the FOC 213, the calculated value selection unit 304 selects the initial value. On the other hand, if the reception control unit 250 determines that optical frequency offset compensation has been performed by the FOC 213 at least once, the calculated value selection unit 304 selects the non-initial value.

[0042] Adder 305 adds either the initial value or the non-initial value selected by calculation value selection unit 304 to the held value held by holding unit 306 provided downstream of adder 305, and outputs the addition result to holding unit 306. When the initial value is selected as the value to be output by calculation value selection unit 304, holding unit 306 holds a value of 0 (zero). Therefore, when the initial value is selected as the value to be output by calculation value selection unit 304, the addition result by adder 305 matches the initial value. Holding unit 306 holds the addition result as a new held value and outputs the addition result to integration unit 307. The held value is updated each time the initial value and non-initial value are output. When the initial value is selected as the value to be output by calculation value selection unit 304, adder 305 may be controlled to output the initial value output from calculation value selection unit 304 without adding the value to the held value held by holding unit 306.

[0043] Integrator 307 adds the held value output from holder 306 to the output value of integrator 307 for each symbol, integrates, and outputs the result as optical frequency offset compensation value 308. In other words, the output value of integrator 307 corresponds to the sum of the previous optical frequency offset compensation value 308 and the held value.

[0044] The multiplier 309 compensates for the optical frequency offset by multiplying the received symbol sequence output from the AEQ 212 by the optical frequency offset compensation value 308 output from the integrator 307, and outputs the result to the CPR 214. In this way, the FOC 213 compensates for the received symbol sequence using the optical frequency offset value calculated based on the initial value and non-initial value of the optical frequency offset.

[0045] The first calculation unit 301 will be described in detail with reference to FIGS.

[0046] The first calculation unit 301 has a first delay unit 410, a second delay unit 420, a first processing unit 430, a reference calculation unit 440, a second processing unit 450, an averaging unit 460, and a division unit 470. The reference calculation unit 440 is an example of a first angle calculation unit. The first processing unit 430 and the second processing unit 450 are examples of a second angle calculation unit. The division unit 470 is an example of a calculation unit that calculates an initial value of the optical frequency offset by dividing the amount of optical frequency offset estimated by the first processing unit 430 and the second processing unit 450 by four.

[0047] The first delay unit 410 delays the input object by one symbol and outputs it. The second delay unit 420 delays the input object by one symbol and outputs it. For example, suppose that the nth pilot symbol and two adjacent data symbols, the (n-1)th and (n+1)th symbols before and after the pilot symbol, are input to the first calculation unit 301. In this case, the (n+1)th data symbol adjacent after the nth pilot symbol is input to the first processing unit 430. The nth pilot symbol is input to the reference calculation unit 440. The (n-1)th data symbol adjacent before the nth pilot symbol is input to the second processing unit 450.

[0048] The first processing unit 430 has a phase angle calculation unit 431, a quadrupling unit 432, an amplitude calculation unit 433, an adder 434, and a first estimator 435. The reference calculation unit 440 has a phase angle calculation unit 441 and a quadrupling unit 442. The second processing unit 450 has a phase angle calculation unit 451, a quadrupling unit 452, an amplitude calculation unit 453, an adder 454, and a second estimator 455. The first estimator 435 and the second estimator 455 are examples of estimators that estimate the amount of optical frequency offset.

[0049] The reference calculation unit 440 will now be described. The phase angle calculation unit 441 calculates the phase angle of the n-th pilot symbol. For example, as shown in the upper part of FIG. 8, the phase angle difference between the phase angle of the n-th pilot symbol 42 at the time of transmission and the phase angle of the n-th pilot symbol 42R at the time of reception is expressed as θ n The n-th pilot symbol 42R is set to a phase angle θ of the n-th pilot symbol 42 at the time of transmission. nThe pilot symbol 42R is received at a position rotated by . The phase angle calculation unit 441 calculates the phase angle of the pilot symbol 42R and transmits it to the quadruple unit 442.

[0050] The pilot symbols 42 are modulated based on the QPSK modulation method. For example, in the first quadrant, the phase angle S n is uniquely identified as 45 degrees. Similarly to the first quadrant, the second, third, and fourth quadrants are uniquely identified as 135 degrees, 225 degrees, and 315 degrees, respectively.

[0051] The quadruple unit 442 performs a quadruple operation on the output of the phase angle calculation unit 441. This will be described with reference to FIG. 8. As shown in the upper part of FIG. 8, the pilot symbol 42R at the time of reception is a signal rotated by θ from the pilot symbol 42 at the time of transmission. As shown in the lower part of FIG. 8, when the quadruple operation is performed on the phase angles of the pilot symbols 42 at the time of transmission, the phase angles of all four pilot symbols 42 converge to 180 degrees, regardless of where the pilot symbols 42 at the time of transmission are mapped in the first to fourth quadrants. Similarly, when the quadruple operation is performed on the phase angles of the pilot symbols 42R at the time of reception, the phase angles of all pilot symbols 42R converge to a phase angle rotated by 4θ from 180 degrees, regardless of where the pilot symbols 42 at the time of transmission are mapped in the first to fourth quadrants. As a result, the quadruple unit 442 outputs the phase angle of 180 degrees + 4θ, which is rotated by 4θ from 180 degrees, as the first phase angle to the adders 434 and 454.

[0052] The first processing unit 430 will now be described. The phase angle calculation unit 431 calculates the phase angle of the (n+1)th received data symbol and outputs it to the quadrupling unit 432. The quadrupling unit 432 receives the (n+1)th received data symbol from the phase angle calculation unit 431, performs a quadrupling operation, and outputs the result to the addition unit 434. For example, it is assumed that the data symbol 41 is modulated based on the 16QAM modulation method. Specifically, as shown in Figures 9(a) and 10(a), the data symbol 41 is mapped to 16 symbol points on a constellation. As shown in Figures 9(a) and 10(a), the 16 symbol points of 16QAM can be classified into three types (amplitude Ra = radius R, radius 3R, radius √5R) based on the magnitude of the amplitude.

[0053] An explanation will be given for each classification.

[0054] The following describes the symbol points (symbol points of the first constellation) arranged on a circumference of a circle with amplitude Ra = radius R. As shown in FIG. 9(b), the symbol points of the first constellation can be represented by symbol points mapped to four data symbols 41 on a circumference of a circle with amplitude Ra = radius R. The radius R is the unit radius. The symbol points of the first constellation have the same phase angle as the symbol points of pilot symbols obtained by modulating the four data symbols 41 based on the QPSK modulation method.

[0055] The following describes the symbol points (symbol points of the second constellation) arranged on the circumference of a circle with amplitude Ra = radius 3R. The symbol points of the second constellation can be represented by symbol points where four data symbols 41 are mapped on the circumference of a circle with amplitude Ra = radius 3R, as shown in Fig. 9(c). Specifically, the symbol points of the second constellation have the same phase angle as the symbol points of pilot symbols where four data symbols 41 are modulated based on the QPSK modulation scheme, as in the case of the first constellation.

[0056] The following describes symbol points arranged on a circle with amplitude Ra = √5R. The symbol points arranged at amplitude Ra = √5R can be classified into Figure 10(b) and Figure 10(c) depending on the convergence point after four times the phase angle.

[0057] The symbol points in Figure 10(b) (symbol points of the third constellation) arranged on the circumference of a circle with amplitude Ra = √5R have amplitude Ra = radius 5 1 / 2 The symbol points of the third constellation are represented by the symbol points onto which four data symbols 41 on the circumference of R (√5R) are mapped. The symbol points of the third constellation have a different phase angle from the symbol points of the pilot symbols, which are obtained by modulating the four data symbols 41 at the time of transmission based on the QPSK modulation method.

[0058] The symbol points in Figure 10(c) (symbol points of the fourth constellation) arranged on the circumference of a circle with amplitude Ra = √5R have amplitude Ra = radius 5 1 / 2 It can be expressed by the symbol points where four data symbols 41 are mapped on the circumference of R. Like the symbol points of the third constellation, the symbols of the fourth constellation have a different phase angle from the symbol points of the pilot symbols where the four data symbols 41 at the time of transmission are modulated based on the QPSK modulation method.

[0059] The symbol points of the first and second constellations have the same phase angle as the symbol points of the pilot symbols modulated based on the QPSK modulation method. Therefore, when the calculation of the phase angle is performed four times, the phase angles of all four data symbols 41 converge to 180 degrees. For example, as shown in the upper part of FIG. 11, the phase angle S n+1 is 45 degrees, the phase angle of the (n+1)th data symbol 41B after the phase angle is multiplied by four converges to 180 degrees, as shown in the lower part of FIG. 11, similar to the pilot symbol 42 shown in the lower part of FIG. 8.

[0060] The symbols of the third and fourth constellations have different phase angles from the symbol points of the pilot symbols modulated based on the QPSK modulation scheme. Therefore, when the calculation of the phase angle is performed four times, the phase angles of the four data symbols 41 shown in Figures 10(b) and 10(c) do not converge to 180 degrees.

[0061] In this way, the symbol points of the first constellation shown in Figure 9(b) and the second constellation shown in Figure 9(c) can be treated in the same way as pilot symbols 42 modulated based on the QPSK modulation scheme. However, the symbol points of the third constellation shown in Figure 10(b) and the fourth constellation shown in Figure 10(c) cannot be treated in the same way as pilot symbols 42 modulated based on the QPSK modulation scheme.

[0062] The following description will be given for the case of the (n+1)th received data symbol 41R. For example, the phase angle of the (n+1)th received data symbol 41R is the phase angle θ n+1 In this case, the phase angle of the (n+1)th data symbol 41R is the phase angle of the (n+1)th data symbol 41B + θ n+1 The phase angle of the (n+1)th data symbol 41R can be expressed as the sum of the phase angle of the (n+1)th data symbol 41B and the phase angle difference θ n +optical frequency offset Δα.

[0063] A case will be described where the transmitted data symbol 41B corresponding to the (n+1)th received data symbol 41R is a symbol point of the first constellation shown in FIG. 9(b) or the second constellation shown in FIG. 9(c).

[0064] When the phase angle of the symbol points included in the first constellation and the second constellation is calculated four times, the phase angle of the (n+1)th data symbol 41R is 180 degrees + 4θ as shown in the lower part of FIG. n+1Therefore, the phase angle is 180 degrees + 4θ, which is four times the phase angle of the (n+1)th data symbol 41R. n+1 is 180 degrees + 4θ n It can also be expressed as +4Δα.

[0065] A case will be described where the transmission data symbol 41B corresponding to the (n+1)th data symbol 41R is the symbol point of the third constellation shown in the upper part of FIG.

[0066] When the phase angle is quadrupled for the symbol points of the third constellation shown in the upper part of Fig. 12, the arrangement of data symbol 41B shown in the lower part of Fig. 12 is obtained. Therefore, the phase angle after quadrupling is 74 degrees. This is different from the phase angle obtained by quadrupling the phase angles of the symbol points of the first constellation and the second constellation shown in the lower part of Fig. 11. Therefore, the phase angle obtained by quadrupling the phase angle of the (n+1)th data symbol 41R is 74 degrees + 4θ n+1 In addition, the phase angle obtained by multiplying the phase angle of the (n+1)th data symbol 41R by four is 74 degrees + 4θ n If a phase angle of 106 degrees is added to the phase angle obtained by multiplying the phase angle of the (n+1)th data symbol 41R by four, the result is 180 degrees + 4θ. n It becomes +4Δα.

[0067] A case will be described where the transmission data symbol 41B corresponding to the (n+1)th data symbol 41R is at the symbol point of the fourth constellation shown in the upper part of FIG.

[0068] When a quadruple operation is performed on the symbol points of the fourth constellation shown in the upper part of Fig. 13, the arrangement of data symbol 41B shown in the lower part of Fig. 13 is obtained. Therefore, the phase angle after quadruple operation is 286 degrees. This is different from the phase angle obtained by quadrupling the phase angles of the symbol points of the first constellation and the second constellation shown in the lower part of Fig. 11. Therefore, the phase angle obtained by quadrupling the phase angle of the (n+1)th data symbol 41R is 286 degrees + 4θ n+1 In addition, the phase angle obtained by multiplying the phase angle of the (n+1)th data symbol 41R by four is 286 degrees + 4θ n It can also be expressed as +4Δα. If a phase angle of 106 degrees is subtracted from the phase angle obtained by multiplying the phase angle of the (n+1)th data symbol 41R by four, the result is 180 degrees + 4θ. n It becomes +4Δα.

[0069] As will be described in detail later, the first estimator 435 can calculate the optical frequency offset in the same manner as in the case of the first and second constellations by subtracting 106 degrees from the phase angle of the output of the quadrupler 432 in the case of the third constellation and adding 106 degrees in the case of the fourth constellation. In the case of the first and second constellations, the first estimator 435 avoids such an angle correction of 106 degrees. The first estimator 435 determines whether the transmission data symbol 41B corresponding to the (n+1)th received data symbol 41R corresponds to the first constellation, the second constellation, the third constellation, or the fourth constellation, and adjusts the angle of the phase angle calculated from the received data symbol.

[0070] Returning to the description of the adder 434, the adder 434 subtracts the first phase angle output from the quadruple unit 442 from the second phase angle output from the quadruple unit 432, and inputs the result to the first estimation unit 435. Below, we will explain the cases where the transmission data symbol 41B corresponding to the (n+1)th received data symbol 41R input to the adder 434 is a symbol point of the first constellation, second constellation, third constellation, or fourth constellation.

[0071] A case will be described in which the transmission data symbol 41B is a symbol point of the first constellation or the second constellation. The quadrupling unit 442 uses 180 degrees + 4θ as the first phase angle. n The second phase angle output from the quadrupler 432 is 180 degrees + 4θ n Since the phase angle is +4Δα, when the difference from the first phase angle is calculated, the adder 434 outputs the value of 4Δα.

[0072] A case will be described where the transmission data symbol 41B is a symbol point of the third constellation. The quadrupling unit 442 multiplies the first phase angle by 180 degrees + 4θ. n The second phase angle output from the quadrupler 432 is 72 degrees + 4θ n Since the phase angle is +4Δα, when the difference from the first phase angle is calculated, adder 434 outputs a value corresponding to -106 degrees +4Δα. Therefore, by adding a phase angle of 106 degrees to the output result of adder 434, it is possible to calculate the same value of 4Δα as when transmission data symbol 41B is a symbol point of the first or second constellation.

[0073] A case will be described where the transmission data symbol 41B is a symbol of the fourth constellation. The quadrupling unit 442 multiplies the first phase angle by 180 degrees + 4θ. n The second phase angle output from the quadrupler 432 is 286 degrees + 4θ n+4Δα, so when the difference from the first phase angle is calculated, adder 434 outputs a value corresponding to 106 degrees + 4Δα. Therefore, by subtracting the phase angle of 106 degrees from the output result of adder 434, it is possible to calculate a value corresponding to 4Δα, the same as when transmission data symbol 41B is a symbol point of the first or second constellation.

[0074] From the above, if it is possible to estimate whether the (n+1)th transmitted data symbol 41B corresponds to a symbol point of the first, second, third, or fourth constellation, then by correcting the output value of the adder 434, it is possible to calculate an optical frequency offset 4Δα multiplied by four, which is the same as when the transmitted data symbol 41B is a symbol point of the first or second constellation.

[0075] On the other hand, the amplitude calculation unit 433 calculates the amplitude Ra of the (n+1)th data symbol 41R (specifically, the absolute value of the amplitude Ra) and outputs the calculated amplitude Ra to the first estimation unit 435. The first estimation unit 435 estimates an optical frequency offset multiplied by four, based on the value output from the addition unit 434 and the amplitude Ra output from the amplitude calculation unit 433. Thereafter, the first estimation unit 435 outputs the estimated optical frequency offset to the averaging unit 460.

[0076] The second processing unit 450 will now be described. The second processing unit 450 performs essentially the same processing as the first processing unit 430 on the (n-1)th data symbol 41A (see FIG. 2 ). The difference is that while the adder 434 of the first processing unit 430 inverts the sign of the first phase angle, the adder 454 of the second processing unit 450 does not invert the sign of the first phase angle. The adder 454 inverts the sign of the second phase angle output from the quadrupling unit 452 and adds the second phase angle and the first phase angle. Consequently, the adder 454 calculates the difference between the second phase angle and the first phase angle and inputs the calculated value to the second estimator 455. Similarly to the first estimator, the second estimator 455 calculates the optical frequency offset multiplied by four and outputs the result to the averaging unit 460.

[0077] The averaging unit 460 calculates the average value of the optical frequency offset output from the first estimating unit 435 and the optical frequency offset output from the second estimating unit 455, and outputs the average value to the dividing unit 470. The dividing unit 470 calculates the initial value of the optical frequency offset by dividing the average value output from the averaging unit 460 by four. After calculating the initial value, the dividing unit 470 outputs the initial value to the calculated value selecting unit 304.

[0078] 14(a), the first estimation unit 435 will be described. Note that the second estimation unit 455 is basically the same as the first estimation unit 435, and therefore a detailed description thereof will be omitted. The first estimation unit 435 has a determination unit 501, a correction unit 502, and an output value selection unit 503.

[0079] Based on the magnitude of the amplitude Ra output from the amplitude calculation section 433, the determination section 501 determines the area on the constellation that includes the (n+1)th data symbol 41R.

[0080] For example, amplitude Ra<(R+5 1 / 2 If (n+1) / 2 is satisfied, the decision unit 501 decides that the (n+1)th data symbol 41R is included in the first region. The first region is defined as a first circle with a radius of R and a circle with a radius of 5 1 / 2 It is an area inside the virtual first circle that is located halfway between the (n+1)th circle and the second circle of radius R. In this case, the transmitted data symbol 41B corresponding to the (n+1)th data symbol 41R is determined to be on radius R, and is therefore included in the first constellation described above.

[0081] (5 1 / 2 If (R+3R) / 2<amplitude Ra, the decision unit 501 decides that the (n+1)th data symbol 41R is included in the third region. 1 / 2 The area outside the virtual second circle that is located halfway between the second circle of radius R and the third circle of radius 3R. In this case, the transmitted data symbol 41B corresponding to the (n+1)th data symbol 41R is determined to be on the radius 3R, and is therefore included in the second constellation described above.

[0082] (R+5 1 / 2 R) / 2<amplitude Ra<(5 1 / 2 If the (n+1)th data symbol 41R satisfies the condition (R+3R) / 2, the decision unit 501 decides that the (n+1)th data symbol 41R is included in the second region. The second region is the region outside the virtual first circle and inside the virtual second circle. In this case, the transmission data symbol 41B corresponding to the (n+1)th data symbol 41R is included in the second region of the virtual first circle. 1 / 2 Since it is determined to be on R, it is included in the third or fourth constellation described above.

[0083] The correction unit 502 uses the value of the (n+1)th data symbol 41R output from the addition unit 434 to determine whether the transmission data symbol 41B corresponding to the (n+1)th data symbol 41R is included in the third constellation or the fourth constellation. Then, the correction unit 502 calculates a value based on the determination result. The method of determining whether the transmission data symbol 41B is included in the third constellation or the fourth constellation will be described later.

[0084] Based on the determination result output from the determination unit 501, the output value selection unit 503 outputs either the output value of the addition unit 434 or the output value of the correction unit 502. For example, when the determination result output from the determination unit 501 indicates that the amplitude Ra of the transmission data symbol 41B corresponds to the radius R or the radius 3R, the output value selection unit 503 selects the output value of the addition unit 434. On the other hand, when the determination result output from the determination unit 501 indicates that the amplitude Ra of the transmission data symbol 41B corresponds to the radius 5 1 / 2 If the output indicates that the value corresponds to R, the output value selection unit 503 selects the output value of the correction unit 502 .

[0085] 14(b), the correction unit 502 will be described. The correction unit 502 has a first adder 601, a second adder 602, an angle selector 603, a first absolute value calculator 604, a second absolute value calculator 605, and a minimum comparator 606.

[0086] The first adder 601 inverts the sign of 106 degrees and adds −106 degrees to the phase angle of the input target. That is, the first adder 601 subtracts 106 degrees from the phase angle of the input target. The correction unit 502 receives input of either −106 degrees + 4Δα or 106 degrees + 4Δα. For example, when −106 degrees + 4Δα is input, the first adder 601 subtracts 106 degrees from −106 degrees + 4Δα and outputs the subtraction result, −212 degrees + 4Δα, to the angle selection unit 603 and the first absolute value calculation unit 604. When 106 degrees + 4Δα is input, the first adder 601 subtracts 106 degrees from 106 degrees + 4Δα and outputs the subtraction result, 4Δα, to the angle selection unit 603 and the first absolute value calculation unit 604.

[0087] The second adder 602 adds 106 degrees to the phase angle of the input target without inverting the sign of 106 degrees. That is, the second adder 602 adds 106 degrees to the phase angle of the input target. As described above, either -106 degrees + 4Δα or 106 degrees + 4Δα is input to the correction unit 502. For example, when -106 degrees + 4Δα is input, the second adder 602 adds 106 degrees to -106 degrees + 4Δα and outputs the addition result, 4Δα, to the angle selection unit 603 and the second absolute value calculation unit 605. When 106 degrees + 4Δα is input, the second adder 602 adds 106 degrees to 106 degrees + 4Δα and outputs the addition result, 212 degrees + 4Δα, to the angle selection unit 603 and the second absolute value calculation unit 605.

[0088] A first absolute value calculation unit 604 calculates the absolute value of the output value of the first adder 601 and outputs it to a minimum comparison unit 606. A second absolute value calculation unit 605 calculates the absolute value of the output value of the second adder 602 and outputs it to a minimum comparison unit 606. The minimum comparison unit 606 compares the absolute value of the output value of the first adder 601 with the absolute value of the output value of the second adder 602 and notifies an angle selection unit 603 of the output value of the first adder 601 or the output value of the second adder 602, whichever has the smaller absolute value. The angle selection unit 603 selects the output value of the first adder 601 or the output value of the second adder 602 based on the notification from the minimum comparison unit 606. As a result, the angle selection unit 603 outputs the output value of the first adder 601 or the output value of the second adder 602, whichever is closer to the first phase angle.

[0089] The operation of the first calculation unit 301 will be described with reference to FIG.

[0090] The first calculation unit 301 first detects a pilot symbol and adjacent data symbols before and after the pilot symbol (step S1). Specifically, the first calculation unit 301 detects a pilot symbol and adjacent data symbols before and after the pilot symbol based on position information of the pilot symbol 42 identified by a synchronization unit (not shown) provided between the CDC 211 and the AEQ 212 or after the AEQ 212.

[0091] First calculation unit 301 uses the detected pilot symbols to calculate a first phase angle in reference calculation unit 440 (step S2). Specifically, phase angle calculation unit 441 and quadrupling unit 442 work together to calculate 180 degrees + 4θ as the first phase angle by quadrupling the phase angle of the pilot symbols. After reference calculation unit 440 calculates the first phase angle, first processing unit 430 and second processing unit 450 each calculate an amplitude and a phase angle using the detected data symbols (step S3).

[0092] Amplitude calculation units 433 and 453 calculate the amplitudes of adjacent data symbols before and after the pilot symbol. Phase angle calculation unit 431 and quadrupling unit 432 work together to calculate a second phase angle obtained by quadrupling the phase angle of the adjacent data symbol after the pilot symbol. Similarly, second processing unit 450, phase angle calculation unit 451 and quadrupling unit 452 work together to calculate a second phase angle obtained by quadrupling the phase angle of the adjacent data symbol before the pilot symbol.

[0093] Using the amplitude of the data symbol calculated by the amplitude calculation units 433 and 453 and the second phase angle calculated by the first processing unit 430 and the second processing unit 450, the first estimation unit 435 and the second estimation unit 455 each estimate the optical frequency offset amount multiplied by four (step S4).

[0094] The averaging unit 460 calculates the average value of the optical frequency offset from the amounts of the optical frequency offset estimated by the first estimating unit 435 and the second estimating unit 455 (step S5). The dividing unit 470 divides the average value calculated by the averaging unit 460 by four (step S6). Specifically, the dividing unit 470 divides the average value by four to calculate an initial value of the optical frequency offset. Once the dividing unit 470 calculates the initial value, the first processing unit 430 and the second processing unit 450 end their processing.

[0095] In this way, the optical receiving device 20 according to this embodiment calculates the initial value of the optical frequency offset based on the pilot symbol and the data symbols adjacent to the pilot symbol, and compensates for the optical frequency offset based on the calculated initial value. For example, if the initial value of the optical frequency offset is calculated based on all data symbols regardless of the pilot symbol, the amount of calculation required to calculate the initial value increases, making it difficult to calculate the initial value quickly.

[0096] For example, if the calculation circuits for calculating the initial values ​​are parallelized in order to calculate the initial values ​​quickly, the circuit size increases because the number of calculation circuits required corresponds to the number of parallel circuits. The increase in circuit size causes an increase in power consumption when calculating the initial value of the optical frequency offset. However, the optical receiving device 20 according to this embodiment calculates the initial value of the optical frequency offset based on the pilot symbol and the data symbols adjacent before and after the pilot symbol, thereby reducing the circuit size and power consumption.

[0097] (Second embodiment) Next, a second embodiment of the present invention will be described with reference to Fig. 16 to Fig. 18. As shown in Fig. 16, a reference calculation unit 440 according to the second embodiment differs from the reference calculation unit 440 according to the first embodiment in that it further includes a conjugation unit 443, a multiplication unit 444, and an addition unit 445.

[0098] For example, when 45 degrees in QPSK is input to conjugation section 443, conjugation section 443 inverts the sign and outputs −45 degrees to multiplication section 444. As a result, when an n-th pilot symbol is input, multiplication section 444 multiplies the n-th pilot symbol by −45 degrees and outputs the result to phase angle calculation section 441. As a result, as shown in FIG. 17, the n-th pilot symbols 42R at the time of reception are mapped to positions rotated by −45 degrees.

[0099] The phase angle calculation unit 441 calculates the phase angle difference θ from the n-th pilot symbol 42 at the time of transmission based on the relationship with the n-th pilot symbol 42 at the time of transmission. n The quadrupling unit 442 calculates the phase angle difference θ n As a result, as shown in FIG. 18, no matter where the n-th pilot symbol 42 is mapped in the first to fourth quadrants at the time of transmission, the phase angle of the pilot symbol 42R is always within the range of 0 degrees to 4θ n The phase angle converges to a phase angle rotated by

[0100] The adder 445 adds 180 degrees to the phase angle output from the quadruple unit 442. As a result, the reference calculation unit 440 calculates the first phase angle, as in the first embodiment (see the lower part of FIG. 8). In this way, even if the optical receiving device 20 includes the reference calculation unit 440 according to the second embodiment, the circuit size and power consumption are reduced, as in the first embodiment.

[0101] Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to the specific embodiments, and various modifications and variations are possible within the scope of the gist of the present invention as described in the claims.

[0102] In addition, the following supplementary notes are provided in relation to the above description. (Supplementary Note 1) An optical receiving device comprising: a receiving unit that receives an optical signal including a plurality of data symbols and pilot symbols periodically inserted between the plurality of data symbols, modulated based on a multi-level modulation scheme; a first angle calculating unit that calculates a first phase angle used for calculating an initial value when compensating for an optical frequency offset, based on the pilot symbols; a second angle calculating unit that calculates a second phase angle of the data symbol, based on a data symbol adjacent to the pilot symbol among the plurality of data symbols; an estimating unit that estimates an amount of the optical frequency offset, based on a differential phase angle between the first phase angle and the second phase angle and on the amplitude of the data symbol; and a compensating unit that compensates for the optical frequency offset, based on the amount of the optical frequency offset. (Supplementary Note 2) The optical receiving device according to Supplementary Note 1, further comprising a calculation unit that calculates the initial value by dividing the amount of the optical frequency offset by four, and the compensation unit compensates for the optical frequency offset based on the initial value. (Supplementary Note 3) The optical receiving device described in Supplementary Note 1 or 2, characterized in that the estimation unit determines an area containing the amplitude of the data symbol based on the magnitude of the amplitude, and determines whether to perform an angle correction of 106 degrees on the differential phase angle based on the determination result. (Supplementary Note 4) The estimation unit determines an area in which the amplitude of the data symbol is included based on the magnitude of the amplitude, and determines whether the data symbol is within a range of a unit radius and 5 of the unit radius. 1 / 2 Outside half of the sum of the unit radius and 1 / 2 3. The optical receiving device according to claim 1, wherein when the angle of the differential phase angle is within a region inside half the sum of 3 times the unit radius and 3 times the unit radius, an angle correction of 106 degrees is performed on the differential phase angle. (Supplementary Note 5) The estimation unit determines an area in which the amplitude of the data symbol is included based on the magnitude of the amplitude, and the data symbol is located within a range of a unit radius and 5 of the unit radius. 1 / 2 If it is included in the area inside half of the sum of the unit radius, 1 / 23. The optical receiving device according to claim 1, wherein angle correction for the differential phase angle is avoided when the differential phase angle is included in an area between an area outside half of the sum of 1 / 2 times the unit radius and 3 / 4 times the unit radius. (Supplementary Note 6) The optical receiving device described in Supplementary Note 3, characterized in that the estimation unit performs a first angle correction of subtracting 106 degrees from the differential phase angle and a second angle correction of adding 106 degrees to the differential phase angle, and selects the differential phase angle after performing the first angle correction or the differential phase angle after performing the second angle correction, whichever has a smaller absolute value after correction. (Supplementary Note 7) The optical receiving device according to Supplementary Note 2, wherein the second angle calculation unit calculates a first specific phase angle of the first data symbol based on a first data symbol that is adjacent before the pilot symbol among the plurality of data symbols, and calculates a second specific phase angle of the second data symbol based on a second data symbol that is adjacent after the pilot symbol among the plurality of data symbols; the estimation unit estimates an amount of a first optical frequency offset based on a first differential phase angle between the first phase angle and the first specific phase angle and a first amplitude of the first data symbol; and estimates an amount of a second optical frequency offset based on a second differential phase angle between the first phase angle and the second specific phase angle and a second amplitude of the second data symbol; and the calculation unit calculates the initial value by dividing an average value of the amount of the first optical frequency offset and the amount of the second optical frequency offset by four. (Supplementary Note 8) The optical receiving device according to Supplementary Note 2, characterized in that the second angle calculation unit calculates a first specific phase angle of the first data symbol based on a first data symbol adjacent to and preceding the pilot symbol among the plurality of data symbols, the estimation unit estimates an amount of a first optical frequency offset based on a first differential phase angle between the first phase angle and the first specific phase angle and a first amplitude of the first data symbol, and the calculation unit calculates the initial value by dividing an average value of the amount of the first optical frequency offset by four. (Supplementary Note 9) The optical receiving device according to Supplementary Note 2, wherein the second angle calculation unit calculates a second specific phase angle of the second data symbol based on a second data symbol adjacent to and subsequent to the pilot symbol among the plurality of data symbols, the estimation unit estimates an amount of second optical frequency offset based on a second differential phase angle between the first phase angle and the second specific phase angle and a second amplitude of the second data symbol, and the calculation unit calculates the initial value by dividing an average value of the amount of second optical frequency offset by four. (Supplementary Note 10) The optical receiving device according to any one of Supplementary Notes 1 to 4, characterized in that the optical signal comprises a signal including the plurality of data symbols and modulated by 16QAM belonging to the multi-level modulation scheme, and a signal including the pilot symbol and modulated by QPSK belonging to the multi-level modulation scheme. (Supplementary Note 11) An optical receiving method comprising: receiving an optical signal including a plurality of data symbols and pilot symbols periodically inserted between the plurality of data symbols, the optical signal being modulated based on a multi-level modulation scheme; calculating, based on the pilot symbols, a first phase angle to be used for calculating an initial value when compensating for an optical frequency offset; calculating, based on the data symbols adjacent to the pilot symbols among the plurality of data symbols, a second phase angle of the data symbols; estimating an amount of the optical frequency offset based on a differential phase angle between the first phase angle and the second phase angle and an amplitude of the data symbols; and compensating for the optical frequency offset based on the amount of the optical frequency offset. [Explanation of symbols]

[0103] 10 Optical transmitter 20 Optical receiving device 40 Optical Signal 41, 41A, 41B, 41R data symbols 42,42R Pilot symbol 213 FOC 250 Reception control section 301 First Calculation Unit 303 Second Calculation Unit 309 Multiplication section 430 First Processing Section 440 Standard calculation section 450 Second Processing Section 460 Averaging section 470 Division part

Claims

1. a receiver for receiving an optical signal modulated based on a multi-level modulation scheme, the optical signal including a plurality of data symbols and pilot symbols periodically inserted between the plurality of data symbols; a first angle calculation unit that calculates a first phase angle to be used for calculating an initial value when compensating for an optical frequency offset based on the pilot symbol; a second angle calculation unit that calculates a second phase angle of the data symbol based on a data symbol adjacent to the pilot symbol among the plurality of data symbols; an estimation unit that estimates the amount of the optical frequency offset based on a differential phase angle between the first phase angle and the second phase angle and an amplitude of the data symbol; a compensating unit that compensates for the optical frequency offset based on the amount of the optical frequency offset; An optical receiving device having:

2. a calculation unit that calculates the initial value by dividing the amount of the optical frequency offset by four, the compensating unit compensates for the optical frequency offset based on the initial value.

2. The optical receiving device according to claim 1.

3. the estimation unit determines an area including the amplitude of the data symbol based on the magnitude of the amplitude, and determines whether to perform angle correction of 106 degrees on the differential phase angle based on the determination result.

3. The optical receiving device according to claim 1 or 2.

4. The estimation unit determines an area in which the amplitude of the data symbol is included based on the magnitude of the amplitude, and determines whether the data symbol is within a range of a unit radius and 5 of the unit radius. １／２ Outside half of the sum of the unit radius and １／２ When the angle is within a region inside half the sum of the radius x and three times the unit radius, an angle correction of 106 degrees is performed on the differential phase angle.

3. The optical receiving device according to claim 1 or 2.

5. the estimation unit performs a first angle correction of subtracting 106 degrees from the differential phase angle and a second angle correction of adding 106 degrees to the differential phase angle, and selects the differential phase angle after the first angle correction or the differential phase angle after the second angle correction, whichever has a smaller absolute value after correction.

4. The optical receiving device according to claim 3.

6. the second angle calculation unit calculates a first specific phase angle of the first data symbol based on a first data symbol adjacent to and before the pilot symbol among the plurality of data symbols, and calculates a second specific phase angle of the second data symbol based on a second data symbol adjacent to and after the pilot symbol among the plurality of data symbols; the estimation unit estimates an amount of a first optical frequency offset based on a first differential phase angle between the first phase angle and the first specific phase angle and a first amplitude of the first data symbol, and estimates an amount of a second optical frequency offset based on a second differential phase angle between the first phase angle and the second specific phase angle and a second amplitude of the second data symbol; the calculation unit calculates the initial value by dividing an average value of the amount of the first optical frequency offset and the amount of the second optical frequency offset by four.

3. The optical receiving device according to claim 2.

7. the second angle calculation unit calculates a first specific phase angle of the first data symbol based on a first data symbol adjacent to and preceding the pilot symbol among the plurality of data symbols; the estimation unit estimates an amount of a first optical frequency offset based on a first differential phase angle between the first phase angle and the first specific phase angle and a first amplitude of the first data symbol; the calculation unit calculates the initial value by dividing an average value of the amount of the first optical frequency offset by four.

3. The optical receiving device according to claim 2.

8. the second angle calculation unit calculates a second specific phase angle of the second data symbol based on a second data symbol adjacent to and subsequent to the pilot symbol among the plurality of data symbols; the estimation unit estimates an amount of a second optical frequency offset based on a second differential phase angle between the first phase angle and the second specific phase angle and a second amplitude of the second data symbol; the calculation unit calculates the initial value by dividing an average value of the amount of the second optical frequency offset by four.

3. The optical receiving device according to claim 2.

9. the optical signal includes a signal including the plurality of data symbols and modulated by 16QAM (Quadrature Amplitude Modulation) belonging to the multi-level modulation method, and a signal including the pilot symbol and modulated by QPSK (Quadrature Phase Shift Keying) belonging to the multi-level modulation method.

3. The optical receiving device according to claim 1 or 2.

10. receiving an optical signal modulated based on a multi-level modulation scheme, the optical signal including a plurality of data symbols and pilot symbols periodically inserted between the plurality of data symbols; calculating a first phase angle to be used for calculating an initial value when compensating for an optical frequency offset based on the pilot symbol; calculating a second phase angle of the data symbol based on a data symbol adjacent to the pilot symbol among the plurality of data symbols; estimating the amount of the optical frequency offset based on a differential phase angle between the first phase angle and the second phase angle and an amplitude of the data symbol; Compensating for the optical frequency offset based on the amount of the optical frequency offset. Optical receiving method.

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