Optical receiving apparatus and optical receiving method

Abstract

To provide an optical receiving apparatus and an optical receiving method that suppress a reduction in phase detection sensitivity of a sampling phase when PCS (Probabilistic Constellation Shaping) is applied to a multilevel modulation scheme.SOLUTION: An optical receiving apparatus includes: a receiving unit that performs digital coherent reception on an optical signal to which PCS is applied; a conversion unit that samples an electric field signal in an analog format representing an optical electric field component of the optical signal and converts the same into a digital signal in a digital format; a detection unit that detects a sampling phase of the digital signal and generates sampling phase information corresponding to the sampling phase; a compensation unit that compensates the sampling phase of the digital signal based on the sampling phase information; and an adjustment unit that adjusts a signal strength of the digital signal before generating the sampling phase information.SELECTED DRAWING: Figure 4

Description

[Technical Field]

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

[0002] An optical receiver having three functions of equalization amplification, retiming, and regeneration is known (see, for example, Patent Document 1). Also known are coherent optical transmitters including a light source and an optical modulator, and coherent optical receivers including an ADC (Analogue to Digital Converter) and a DSP (Digital Signal Processor) (see, for example, Patent Documents 2 and 3). In addition, an optical transmission system that performs optical transmission control is also known (see, for example, Patent Document 4). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-198467 [Patent Document 2] US Patent Application Publication No. 2018 / 0269985 [Patent Document 3] US Patent Application Publication No. 2018 / 0183631 [Patent Document 4] Japanese Patent Application Laid-Open No. 2003-037562 Summary of the Invention [Problem to be solved by the invention]

[0004] Coherent optical transmitters sometimes use a multilevel modulation method called 16QAM (Quadrature Amplitude Modulation) to modulate optical signals. 16QAM is a multilevel modulation method that can transmit 4 bits (i.e., 16 values) of information per symbol (signal point). When 16QAM is used, 16 types of symbols are arranged on a constellation according to the combination of the phase and amplitude of the optical signal.

[0005] The outer symbols located farther from the center of the constellation have a larger amplitude of the optical signal, and more power is required to transmit the optical signal. In other words, inner symbols located closer to the center of the constellation can transmit the optical signal with less power. In the case of 16QAM, 16 types of symbols are used with equal probability when transmitting an optical signal.

[0006] Coherent optical transmitters sometimes use a multi-level modulation method called PCS-16QAM, which applies a technology called PCS (Probabilistic Constellation Shaping) to 16QAM. When PCS-16QAM is used, inner symbols located near the center of the constellation are used probabilistically more frequently when transmitting optical signals. Conversely, outer symbols located farther from the center of the constellation are used probabilistically less frequently when transmitting optical signals.

[0007] In this way, when PCS-16QAM is used, the inner symbols are used in preference to the outer symbols. This reduces the power required to transmit the optical signal compared to 16QAM. In addition, the inner symbols, which have a smaller amplitude, are used in preference, improving the discrimination between symbols and improving noise resistance. Coherent optical receivers can receive optical signals modulated using this PCS-16QAM.

[0008] When a coherent optical receiver receives an optical signal, it converts the analog electric field signal representing the optical field component of the optical signal into a digital signal and detects the sampling phase of the digital signal. However, when PCS-16QAM is used in a coherent optical transmitter, the optical signal received by the coherent optical receiver is unlikely to contain outer symbols with large amplitudes. This reduces the signal strength of the optical signal, which depends on the amplitude of the optical signal. When the signal strength of the optical signal decreases, the signal strength of the digital signal also decreases, reducing the detection sensitivity of the sampling phase of the digital signal. In this case, the coherent optical receiver may not be able to accurately demodulate the digital signal.

[0009] Therefore, in one aspect, an object of the present invention is to provide an optical receiving device and an optical receiving method that suppress a decrease in the phase detection sensitivity of the sampling phase when PCS is applied to a multi-level modulation method. [Means for solving the problem]

[0010] In one embodiment, an optical receiving device includes a receiving unit that digitally and coherently receives an optical signal to which PCS is applied, a converting unit that samples an analog-format electric field signal representing an optical electric field component of the optical signal and converts it into a digital-format digital signal, a detecting unit that detects the sampling phase of the digital signal and generates sampling phase information corresponding to the sampling phase, a compensating unit that compensates for the sampling phase of the digital signal based on the sampling phase information, and an adjusting unit that adjusts the signal strength of the digital signal before generating the sampling phase information. [Effects of the Invention]

[0011] It is possible to suppress a decrease in the phase detection sensitivity of the sampling phase when PCS is applied to a multi-level modulation method. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is an example of an optical transmission system. [Figure 2] (a) is an example of a 16QAM constellation. (b) is an example of a PCS-16QAM constellation. [Figure 3] 1 is an example of an optical receiving device. [Figure 4] 3 is an example of a sampling phase synchronization unit according to the first embodiment. [Figure 5] FIG. 10 is a diagram illustrating an example of an effect. [Figure 6] FIG. 10 is a diagram illustrating another example of the effect. [Figure 7] 4 is a flowchart showing an example of the operation of the optical receiving device according to the first embodiment. [Figure 8] 10 is an example of a sampling phase synchronization unit according to the second embodiment. [Figure 9] 10 is an example of a control unit according to the second embodiment. [Figure 10] FIG. 2 is a diagram illustrating an example of a first DB (Data Base). [Figure 11] 10 is a flowchart showing an example of the operation of the optical receiving device according to the second embodiment. [Figure 12] 10 is another example of the control unit according to the second embodiment. [Figure 13] FIG. 10 is a diagram illustrating an example of a second DB. [Figure 14] 10 is a flowchart showing another example of the operation of the optical receiving device according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0014] (First embodiment) As shown in Fig. 1, the optical transmission system ST includes an optical transmitter 10T and an optical receiver 10R. The optical receiver 10R includes a digital coherent receiver. The optical transmitter 10T and the optical receiver 10R are connected by a transmission path 10Z such as an optical fiber. A repeater such as an optical amplifier may be provided in the transmission path 10Z.

[0015] When transmission data is input, the optical transmitting device 10T modulates the transmission data and transmits the modulated optical signal to the transmission path 10Z. For example, the optical transmitting device 10T transmits an optical signal modulated by PCS-16QAM based on the transmission data. The optical signal propagates through the transmission path 10Z. The optical receiving device 10R receives the optical signal transmitted from the optical transmitting device 10T via the transmission path 10Z, demodulates it, and outputs demodulated data corresponding to the optical signal.

[0016] Next, the difference between 16QAM and PCS-16QAM will be described with reference to FIGS.

[0017] First, as shown in Figure 2(a), when an optical signal is modulated using 16QAM, 16 types of symbols are arranged on a constellation according to the combination of the optical signal's phase Ph and amplitude Am. The constellation is a two-dimensional plane where the I (in-phase) axis and Q (quadrature) axis are orthogonal. Each symbol is mapped to four different bits. Each symbol has the same probability of being used when transmitting an optical signal. Note that in Figure 2(a), each symbol is shown as the same size. Having each symbol the same size indicates that each symbol has the same probability of being used.

[0018] The outer symbols located farther from the center O of the constellation have a larger amplitude of the optical signal, and more power is consumed when transmitting the optical signal. For example, outer symbol 71, located farthest from the center O of the constellation, has the largest amplitude of the optical signal. Therefore, symbol 71 consumes more power to transmit the optical signal than inner symbol 72, located closest to the center O of the constellation. Furthermore, symbol 71 consumes more power to transmit the optical signal than symbol 73, located next closest to symbol 72 from the center O of the constellation. Therefore, inner symbols located closer to the center O of the constellation can transmit optical signals with less power.

[0019] Next, as shown in Figure 2(b), when PCS-16QAM is used to modulate the optical signal, 16 types of symbols are arranged on the constellation, just as in the case of 16QAM. When PCS-16QAM is used, the probability that each symbol will be used when transmitting the optical signal differs. For this reason, in Figure 2(b), 16 types of symbols are shown with three different sizes. The three different sizes of the 16 types of symbols represent the fact that these symbols will be used with three different probabilities.

[0020] For example, outer symbol 81, which is positioned farthest from center O of the constellation, is shown with the smallest size. That is, when transmitting an optical signal, symbol 81 is used with the lowest probability. Conversely, inner symbol 82, which is positioned closest to center O of the constellation, is shown with the largest size. That is, when transmitting an optical signal, symbol 82 is used with the highest probability. Symbol 83, which has a size between the sizes of symbols 81 and 82, is used with a probability between the probability of symbol 81 being used and the probability of symbol 82 being used.

[0021] Furthermore, as described above, symbol 81 has the smallest size. Therefore, even if the size of symbol 81 increases slightly due to increased variability caused by noise, a sufficient Euclidean distance is maintained between symbol 81 and adjacent symbol 82. Therefore, when PCS-16QAM is used, erroneous determination of symbol 81 and symbol 82 is suppressed, and the discrimination between symbols 81 and 82 is improved. Similarly, even if the size of symbol 81 increases slightly due to increased variability caused by noise, a sufficient Euclidean distance is maintained between symbol 81 and adjacent symbol 83. Therefore, when PCS-16QAM is used, erroneous determination of symbol 81 and symbol 83 is suppressed, and the discrimination between symbols 81 and 83 is also improved. In this way, when PCS-16QAM is used, the noise resistance of symbol 81 is improved.

[0022] However, when PCS-16QAM is used, the average signal strength (hereinafter referred to as signal power), which is expressed as the sum of the squares of the I and Q components of a symbol, is lower than when 16QAM is used. Specifically, whether PCS-16QAM or 16QAM is used, the closer a symbol is positioned to the center O of the constellation, the smaller the amplitude becomes, and the lower the signal power, which depends on the amplitude of the optical signal. However, with PCS-16QAM, the probability that an inner symbol will be used is higher than with 16QAM. Also, with PCS-16QAM, the probability that an outer symbol will be used is lower than with 16QAM. Therefore, when the probability that a symbol will be used is taken into account, the signal power when PCS-16QAM is used is lower than the signal power when 16QAM is used.

[0023] As will be described in detail later, when signal power decreases in this manner, if the optical receiving device 10R detects the sampling phase using a predetermined phase detection method such as the Gardner method, the detection sensitivity may decrease. This is because a predetermined phase detection method such as the Gardner method requires signal power greater than the signal power when 16QAM is used, for example. When the detection sensitivity decreases in this manner, it becomes difficult for the optical receiving device 10R to accurately generate sampling phase information corresponding to the sampling phase. In other words, it becomes difficult for the optical receiving device 10R to accurately generate sampling phase information including a sampling phase error.

[0024] The optical receiving device 10R compensates for the sampling phase error of the digital signal corresponding to the optical signal based on the sampling phase information. Therefore, if the accuracy of the sampling phase information is low, the accuracy of the sampling phase compensation decreases. As a result, the optical receiving device 10R may not be able to demodulate the digital signal with high accuracy. Therefore, in the first embodiment, an optical receiving device 10R that increases the signal power before generating the sampling phase information will be described.

[0025] The optical receiving device 10R will be described in detail with reference to FIG.

[0026] When optical receiving device 10R receives an optical signal from transmission line 10Z, the optical signal is input to PBS (Polarization Beam Splitter) 11. PBS 11 splits the optical signal into an X-polarized component and a Y-polarized component. The X-polarized component is input to 90° optical hybrid circuit 14. The Y-polarized component is input to 90° optical hybrid circuit 15. Local light output from LO (Local Oscillator) 12 is split by PBS 13. The local light is input to each of 90° optical hybrid circuits 14 and 15.

[0027] The 90° optical hybrid circuit 14 detects the X polarization component using the local light, outputs the in-phase interference component (I component) to a BPD (Balanced Photo Diode) 21 serving as an optoelectric converter, and outputs the 90° phase-shifted interference component (Q component) to BPD 22. The 90° optical hybrid circuit 15 detects the Y polarization component using the local light, outputs the in-phase interference component (I component) to BPD 23, and outputs the 90° phase-shifted interference component (Q component) to BPD 24. In this way, the 90° optical hybrid circuits 14 and 15 separate the optical signal into a total of four channel optical signals: the X polarization I component, the X polarization Q component, the Y polarization I component, and the Y polarization Q component, and output them to the corresponding BPDs 21, 22, 23, and 24. The PBSs 11 and 13, the 90° optical hybrid circuits 14 and 15, and the BPDs 21, 22, 23, and 24 constitute an optical front-end module as a receiving section.

[0028] Each of the BPDs 21, 22, 23, and 24 converts the input optical signal into an analog electric field signal representing the optical electric field component of the optical signal. Each electric field signal is input to a corresponding ADC 31, 32, 33, or 34 in the ADC group 30. The ADCs 31, 32, 33, and 34 are examples of conversion units. Each of the ADCs 31, 32, 33, and 34 performs digital sampling at a sampling timing synchronized with a sampling frequency output from a variable frequency oscillator (not shown). For example, each of the ADCs 31, 32, 33, and 34 performs two samples per symbol (2x oversampling).

[0029] This converts the analog values ​​of each electric field signal into digital values ​​and expands them in parallel up to a clock speed that can be realized by an LSI (Large-Scale Integration) such as a CMOS (Complementary Metal-Oxide-Semiconductor). The digital signals of the X-polarized I component and the X-polarized Q component are added together in an adder circuit 35 and input as an X-channel digital signal to a receiver DSP (denoted as RxDSP in FIG. 3) 40. The digital signals of the Y-polarized I component and the Y-polarized Q component are added together in an adder circuit 36 ​​and input as a Y-channel digital signal to a receiver DSP 40.

[0030] The receiving-side DSP 40 includes a chromatic dispersion compensator 41, a sampling phase synchronizer 42, an adaptive equalizer 43, etc. Although not shown in Fig. 3, the receiving-side DSP 40 also includes, for example, an error correction decoder following the adaptive equalizer 43, which demodulates the digital signal and outputs the demodulated data. The chromatic dispersion compensator 41, sampling phase synchronizer 42, adaptive equalizer 43, etc. may be implemented by a single DSP or by separate DSPs.

[0031] The sampling phase synchronization unit 42 is controlled by a control unit 60 that controls the entire optical receiving device 10R. The control unit 60 is provided in the optical receiving device 10R. The control unit 60 can be realized by a hardware circuit. The hardware circuit may be a processor such as a CPU (Central Processing Unit), an FPGA (Field Programmable Gate Array), or an ASIC (Application Specific Integrated Circuit).

[0032] The chromatic dispersion compensator 41 estimates the waveform distortion of the X-polarized component of the optical signal due to chromatic dispersion based on the X-channel digital signal output from the adder circuit 35, and compensates for the waveform distortion due to chromatic dispersion. Similarly, the chromatic dispersion compensator 41 estimates the waveform distortion of the Y-polarized component of the optical signal due to chromatic dispersion based on the Y-channel digital signal output from the adder circuit 36, and compensates for the waveform distortion due to chromatic dispersion.

[0033] The sampling phase synchronization unit 42 compensates for the sampling phase of each of the X-channel and Y-channel digital signals, for which waveform distortion due to chromatic dispersion has been compensated, and outputs the compensated signals to the downstream adaptive equalization unit 43. The adaptive equalization unit 43 compensates for the characteristics of the transmission path 10Z, based on the tap coefficients, for each of the X-channel and Y-channel digital signals for which sampling phase compensation has been performed.

[0034] For example, the adaptive equalizer 43 compensates for waveform distortion caused by PMD (Polarization Mode Dispersion) and PDL (Polarization Dependent Loss). After compensating for the characteristics of the transmission path 10Z, the adaptive equalizer 43 outputs the compensated digital signals of the X and Y channels to an error correction decoder or the like provided downstream of the adaptive equalizer 43. The error correction decoder demodulates the digital signals of the X and Y channels and outputs demodulated data. As a result, the demodulated data is output from the optical receiving device 10R.

[0035] The sampling phase synchronization unit 42 will be described in detail with reference to FIGS.

[0036] The sampling phase synchronization unit 42 includes a sampling phase compensation unit 51 as a compensation unit, a level adjustment unit 52 as an adjustment unit, and a sampling phase detection unit 53 as a detection unit. The digital signals of the X channel and Y channel, whose waveform distortion due to chromatic dispersion has been compensated for by the chromatic dispersion compensation unit 41, are input to the level adjustment unit 52.

[0037] Before generating sampling phase information, the level adjustment unit 52 adjusts the signal levels of the digital signals of the X and Y channels based on the level adjustment value set by the control unit 60. The level adjustment value is a fixed magnification such as 1 or 1.5. When the signal level is adjusted, the signal power changes.

[0038] For example, if 1.5 times is selected as the level adjustment value, the signal power increases. If 1 time is selected as the level adjustment value, the increased signal power is nullified and the signal power returns to the original value before the increase. In this way, the level adjustment unit 52 adjusts the signal power of each digital signal of the X channel and Y channel and outputs the adjusted signal power to the sampling phase compensation unit 51.

[0039] The sampling phase compensation unit 51 branches the X-channel and Y-channel digital signals output from the level adjustment unit 52 and outputs them to the sampling phase detection unit 53. The sampling phase detection unit 53 detects the sampling phase of each X-channel and Y-channel digital signal and generates sampling phase information corresponding to the sampling phase. The sampling phase indicates the timing at which sampling is actually performed within one symbol interval, with the ideal sampling timing as the reference. The ideal sampling timing is the timing at which each symbol arrives at the optical receiving device 10R.

[0040] More specifically, the sampling phase detector 53 detects the sampling phase error of each of the X-channel and Y-channel digital signals using a predetermined phase detection method, such as Gardner filtering. The sampling phase error represents the magnitude and direction of the timing error between the ideal sampling timing and the actual sampling timing relative to the symbol phase of the digital signal in the ADC group 30. The direction of the timing error is the advance or delay of the sampling timing, and can be expressed, for example, by a positive or negative sign. To always maintain ideal sampling timing, the optical receiving device 10R detects and compensates for the sampling phase error. This improves the signal quality of the digital signal.

[0041] When the sampling phase detection unit 53 detects a sampling phase error, it generates sampling phase information including a compensation amount according to the sampling phase error and notifies the sampling phase compensation unit 51. For example, the sampling phase detection unit 53 removes noise from the sampling phase error using a loop filter, and generates and notifies sampling phase information including the sampling phase error after the noise removal as a compensation amount.

[0042] The sampling phase compensation unit 51 compensates for the sampling phase of each of the X-channel and Y-channel digital signals based on the compensation amount included in the sampling phase information, and outputs the compensated signals to the adaptive equalization unit 43. Note that the sampling phase detection unit 53 can be implemented by referring to, for example, Japanese Patent Application Laid-Open No. 2011-009956 and Japanese Patent Application Laid-Open No. 2012-253461.

[0043] The control unit 60 uniquely sets a level adjustment value corresponding to the operation mode (hereinafter simply referred to as the mode) of the optical transmission system ST in the level adjustment unit 52. The control unit 60 holds a plurality of modes and level adjustment values ​​corresponding to each of the plurality of modes. When one of the plurality of modes is selected, the control unit 60 identifies the level adjustment value corresponding to that mode and sets the identified level adjustment value in the level adjustment unit 52.

[0044] At the time of initial startup before the optical receiving device 10R begins operation, the control unit 60 is instructed to select one of a plurality of modes. The instruction to the control unit 60 can be given, for example, from an external terminal device connected to the optical receiving device 10R. The external terminal device includes, for example, a PC (Personal Computer) or a dedicated terminal.

[0045] Here, when 1.5 times is set as the level adjustment value, the signal power of each of the X-channel and Y-channel digital signals input to the sampling phase detector 53 is increased. By applying an adjustment that increases the signal power in this way, the amplitude of the digital signal is amplified and the variation increases compared to when no adjustment is applied, as shown in Fig. 5. This allows the sampling phase detector 53 to suppress a decrease in the phase detection sensitivity of the sampling phase.

[0046] The effect of this case will be described with reference to FIG.

[0047] First, when a magnification of 1.5 or a value close to that is selected as the level adjustment value, the detection sensitivity of the sampling phase by the sampling phase detection unit 53 improves. For example, when a magnification of 1.5 or a value close to that is selected as the level adjustment value, the detection sensitivity improves by several times compared to when 1 is selected as the level adjustment value. On the other hand, when a magnification exceeding a magnification close to 1.5 is selected as the level adjustment value, the detection sensitivity gradually decreases as the magnification increases. Therefore, when considering detection sensitivity alone, it is desirable to select a magnification of 1.5 or a value close to that.

[0048] On the other hand, if a magnification exceeding 1.5x is selected as the level adjustment value, the number of clips will increase. The number of clips is the total number of symbols in which clipping occurs. Clipping is a phenomenon in which the signal power of a digital signal partially adheres to the upper limit value of, for example, the receiving DSP 40 or the sampling phase detection unit 53. In digital signal processing, it is desirable for the number of clips to be as close to 0 (zero). For this reason, when considering both the detection sensitivity and the number of clips, it is desirable to select a magnification of 1.5x or less as the level adjustment value.

[0049] An example of the operation of the optical receiving device 10R according to the first embodiment will be described with reference to Fig. 7. The optical receiving method of this case is realized by the control unit 60 executing a program according to the flowchart shown in Fig. 7.

[0050] First, the control unit 60 sets a level adjustment value (step S1). For example, when the control unit 60 detects an instruction from an external terminal device connected to the optical receiving device 10R, the control unit 60 sets the level adjustment value. After the control unit 60 sets the level adjustment value, the level adjustment unit 52 adjusts the signal power of each digital signal of the X channel and the Y channel (step S2).

[0051] Once the level adjuster 52 has adjusted the signal power, the sampling phase detector 53 synchronizes the sampling phase (step S3). More specifically, the sampling phase detector 53 detects a sampling phase error and, in cooperation with the sampling phase compensator 51, executes an initial phase pull-in process to converge the sampling phase error. This shifts and adjusts the input position of the parallel-input digital signal, adjusting the sampled digital signal to its intended phase position. The sampling phase detector 53 continues synchronizing the sampling phase until synchronization is complete (step S4: NO).

[0052] When the synchronization of the sampling phase is completed (step S4: YES), the control unit 60 cancels the level adjustment value (step S5), and the optical receiving device 10R ends the initial startup process. More specifically, when the synchronization of the sampling phase is completed, the sampling phase detection unit 53 generates sampling phase information including a compensation amount according to the sampling phase error and notifies the sampling phase compensation unit 51. After notifying the sampling phase information, the sampling phase detection unit 53 instructs the control unit 60 to cancel the level adjustment value. This causes the control unit 60 to cancel the level adjustment value. For example, if the control unit 60 set the level adjustment value to 1.5 times, the control unit 60 sets the level adjustment value to 1 time to cancel the level adjustment value.

[0053] In the optical receiving device 10R according to the first embodiment, the level adjustment unit 52 is provided before the sampling phase compensation unit 51. If the level adjustment value is maintained at 1.5 times and the optical receiving device 10R starts operating, this may affect the demodulation of the digital signal. For this reason, in the first embodiment, once synchronization of the sampling phase is completed, the control unit 60 sets the level adjustment value to 1 time. This prevents the level adjustment value from affecting the demodulation of the digital signal.

[0054] (Second embodiment) Next, a second embodiment of the present invention will be described with reference to Fig. 8 to Fig. 14. Note that the same components as those of the optical receiving device 10R described in the first embodiment are basically denoted by the same reference numerals, and detailed description thereof will be omitted.

[0055] 8, the sampling phase synchronization unit 42 according to the second embodiment differs from the sampling phase synchronization unit 42 according to the first embodiment in the arrangement of the level adjustment unit 52. The level adjustment unit 52 according to the second embodiment is provided in a non-demodulation path P2 that is arranged between the sampling phase compensation unit 51 and the sampling phase detection unit 53, independent of the demodulation path P1 that demodulates the digital signal.

[0056] In this way, by providing the level adjustment unit 52 on the non-demodulation path P2, the optical receiving device 10R according to the second embodiment does not affect the demodulation of the digital signal. Therefore, in the second embodiment, the processing of step S5 described in the first embodiment is not necessary, and the processing load on the control unit 60 is reduced. Note that in this case, as in the first embodiment, one of a plurality of modes is instructed to the control unit 60. This allows the control unit 60 to uniquely set a level adjustment value corresponding to the mode in the level adjustment unit 52.

[0057] Next, a first modified example of the optical receiving device 10R according to the second embodiment will be described with reference to FIGS.

[0058] 9, the control unit 60 according to the first modification includes a first DB 61, a power detection unit 62, and a level determination unit 63. The first DB 61 defines the relationship between the magnitude of the signal power and the magnitude of the level adjustment value. The smaller the signal power, the larger the level adjustment value associated with it.

[0059] 10, the first DB 61 stores a plurality of criteria and level adjustment values ​​corresponding to each criterion. The criteria are used to determine the relationship between the signal power and three thresholds A, B, and C. Threshold A is the smallest value among thresholds A, B, and C. Threshold C is the largest value among thresholds A, B, and C. Threshold B is a value between thresholds A and C. Instead of the three thresholds A, B, and C, two thresholds A and B may be used. Furthermore, instead of the three thresholds A, B, and C, four thresholds A, B, C, and D may be used.

[0060] The four level adjustment values ​​K, L, M, and N are multiplication factors, such as 1x or 1.5x. The level adjustment value K is the largest multiplication factor among the level adjustment values ​​K, L, M, and N. The level adjustment value N is the smallest multiplication factor among the level adjustment values ​​K, L, M, and N. The level adjustment values ​​L and M are both multiplication factors between the level adjustment value K and the level adjustment value N. Note that the level adjustment value L is a multiplication factor greater than the level adjustment value M. Instead of the four level adjustment values ​​K, L, M, and N, three level adjustment values ​​K, L, and M may be used. Furthermore, instead of the four level adjustment values ​​K, L, M, and N, five level adjustment values ​​J, K, L, M, and N may be used.

[0061] The power detector 62 acquires the digital signals of the X channel and the Y channel from the non-demodulated path P2. Upon acquiring each digital signal, the power detector 62 calculates the signal power of each digital signal for each channel and detects the overall signal power of the digital signal by adding up the calculated signal powers. Upon detecting the signal power, the power detector 62 outputs the signal power to the level determiner 63.

[0062] When signal power is input, level determination unit 63 accesses first DB 61, acquires a level adjustment value corresponding to the signal power, and determines the level adjustment value to be set in level adjustment unit 52. After determining the level adjustment value, level determination unit 63 sets the determined level adjustment value in level adjustment unit 52. As a result, unlike the first embodiment, a level adjustment value corresponding to the signal power detected based on each digital signal of the X channel and Y channel is dynamically set in level adjustment unit 52. This eliminates the need for the external terminal device described in the first embodiment, and reduces the operational burden on the person operating the terminal device.

[0063] Next, an example of the operation of the first modified example of the optical receiving device 10R according to the second embodiment will be described with reference to FIG.

[0064] First, the control unit 60 detects the signal power (step S11). Specifically, the power detection unit 62 of the control unit 60 acquires the digital signals of the X channel and the Y channel, and detects the signal power based on the acquired digital signals. Once the signal power is detected, the control unit 60 determines a level adjustment value (step S12). As described above, the level determination unit 63 of the control unit 60 determines the level adjustment value based on the signal power. Once the level determination unit 63 determines the level adjustment value, it sets the determined level adjustment value in the level adjustment unit 52.

[0065] Once the control unit 60 has determined the level adjustment values, the level adjustment unit 52 adjusts the signal levels of the X-channel and Y-channel digital signals (step S13). As a result, the X-channel and Y-channel digital signals after their signal levels have been adjusted are input to the sampling phase detection unit 53. The sampling phase detection unit 53 generates sampling phase information based on the input digital signals and notifies the sampling phase compensation unit 51. Once the sampling phase detection unit 53 has notified the sampling phase information, the optical receiving device 10R ends its processing.

[0066] Next, a second modified example of the optical receiving device 10R according to the second embodiment will be described with reference to FIGS.

[0067] 12, the control unit 60 according to the second modification includes a second DB 64, a sensitivity acquisition unit 65, and a level determination unit 66. The second DB 64 defines the relationship between the number of times the sensitivity is determined and the magnitude of the level adjustment value. The greater the number of times the sensitivity is determined, the greater the corresponding level adjustment value.

[0068] Specifically, as shown in Fig. 13, the second DB 64 stores a plurality of determination counts and level adjustment values ​​corresponding to each determination count. The determination count is the number of times it has been determined whether the phase detection sensitivity of the sampling phase is equal to or greater than the threshold sensitivity. If the phase detection sensitivity is less than the threshold sensitivity, the level adjustment value is changed (specifically, increased), and it is determined again whether the phase detection sensitivity is equal to or greater than the threshold sensitivity. Although Fig. 13 shows up to three determination counts, the determination count may be up to two, four, or five.

[0069] The three level adjustment values ​​V, U, and T are multiplication factors such as 1x and 1.5x, for example. The level adjustment value V is the smallest multiplication factor among the level adjustment values ​​V, U, and T. The level adjustment value T is the largest multiplication factor among the level adjustment values ​​V, U, and T. The level adjustment value U is the multiplication factor between the level adjustment values ​​V and T. Instead of the three level adjustment values ​​V, U, and T, two level adjustment values ​​V and U may be used. Also, instead of the three level adjustment values ​​V, U, and T, four level adjustment values ​​V, U, T, and J may be used.

[0070] The sensitivity acquisition unit 65 acquires the phase detection sensitivity of the sampling phase from the sampling phase detection unit 53. When the sensitivity acquisition unit 65 acquires the phase detection sensitivity, it outputs the phase detection sensitivity to the level determination unit 66. When the phase detection sensitivity is input, the level determination unit 66 determines whether the phase detection sensitivity is equal to or greater than the threshold sensitivity and counts the number of times the determination is made. If the phase detection sensitivity is less than the threshold sensitivity, the level determination unit 66 changes the level adjustment value. For example, if a level adjustment value V is set in the level adjustment unit 52 as an initial value, the level determination unit 66 acquires a level adjustment value U from the second DB 64 and sets it in the level adjustment unit 52. This causes the level determination unit 66 to once again determine whether the phase detection sensitivity is equal to or greater than the threshold sensitivity.

[0071] Next, an example of the operation of the second modified example of the optical receiving device 10R according to the second embodiment will be described with reference to FIG.

[0072] First, the level adjustment unit 52 adjusts the signal levels of the digital signals of the X channel and the Y channel (step S21). For example, if a level adjustment value V is set in the level adjustment unit 52, the level adjustment unit 52 adjusts the signal levels of the digital signals of the X channel and the Y channel based on the level adjustment value V.

[0073] Next, the control unit 60 acquires the sensitivity (step S22). Specifically, the sensitivity acquisition unit 65 of the control unit 60 acquires the phase detection sensitivity of the sampling phase from the sampling phase detection unit 53, and outputs the acquired phase detection sensitivity to the level determination unit 66. Next, the level determination unit 66 determines whether the sensitivity is equal to or greater than a threshold value (step S23). That is, the level determination unit 66 determines whether the phase detection sensitivity is equal to or greater than the threshold sensitivity.

[0074] If the phase detection sensitivity is less than the threshold sensitivity (step S23: NO), the level determination unit 66 changes the level adjustment value (step S24). For example, the level determination unit 66 changes the level adjustment value V to the level adjustment value U. When the level determination unit 66 changes the level adjustment value, the level adjustment unit 52 executes the process of step S21. The processes of steps S21 to S24 are repeated until the phase detection sensitivity becomes equal to or greater than the threshold sensitivity. Then, when the phase detection sensitivity becomes equal to or greater than the threshold sensitivity (step S23: YES), the level determination unit 66 ends the process.

[0075] In this way, when the phase detection sensitivity becomes equal to or greater than the threshold sensitivity, the change in the level adjustment value stops. This allows the level determination unit 66 to stop continuously increasing the level adjustment value. Therefore, the optical receiving device 10R can avoid a significant increase in the number of clips (see FIG. 6).

[0076] 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.

[0077] For example, in the above-described embodiment, PCS-16QAM is used as an example of a multi-level modulation scheme to which PCS is applied, but the multi-level modulation scheme to which PCS is applied is not limited to this. For example, PCS-64QAM or the like may be used as a multi-level modulation scheme to which PCS is applied.

[0078] In the above-described embodiment, the Gardner method is used as an example of a phase detection method, but a diversity addition type phase detection method may be used instead of or in addition to the Gardner method. Furthermore, the level adjustment value is not limited to being set by the control unit 60, and the level adjustment value may be set in advance in the level adjustment unit 52.

[0079] In addition, the following supplementary notes are provided in relation to the above description. (Supplementary Note 1) An optical receiving device having a receiving unit that digitally coherently receives an optical signal to which PCS is applied, a converting unit that samples an analog-format electric field signal representing an optical electric field component of the optical signal and converts it into a digital signal, a detecting unit that detects the sampling phase of the digital signal and generates sampling phase information corresponding to the sampling phase, a compensating unit that compensates for the sampling phase of the digital signal based on the sampling phase information, and an adjusting unit that adjusts the signal strength of the digital signal before generating the sampling phase information. (Appendix 2) The optical receiving device described in Appendix 1 further comprises a control unit that sets an adjustment value that increases the signal strength in the adjustment unit and cancels the adjustment value after the sampling phase information is notified to the compensation unit from the detection unit, and the adjustment unit is provided in a stage before the compensation unit and adjusts the signal strength based on the adjustment value. (Appendix 3) The optical receiving device described in Appendix 1 further comprises a control unit that sets an adjustment value to the adjustment unit to increase the signal strength, the adjustment unit being provided in a non-demodulation path that is arranged between the compensation unit and the detection unit, independent of a demodulation path that demodulates the digital signal, and characterized in that the signal strength is adjusted based on the adjustment value. (Appendix 4) The optical receiving device described in Appendix 3, characterized in that the control unit acquires the digital signal from the non-demodulated path, detects the signal strength of the acquired digital signal, and sets the adjustment value in the adjustment unit according to the detected signal strength. (Appendix 5) The optical receiving device described in Appendix 3, characterized in that the control unit acquires the phase detection sensitivity of the sampling phase from the detection unit and sets the adjustment value corresponding to the acquired phase detection sensitivity in the adjustment unit. (Supplementary Note 6) The optical receiving device according to any one of Supplementary Notes 1 to 3, wherein the detector detects the sampling phase based on a phase detection method including the Gardner method. (Supplementary Note 7) An optical receiving method comprising: digitally coherently receiving an optical signal to which PCS is applied; sampling an analog electric field signal representing an optical electric field component of the optical signal; converting the analog electric field signal into a digital signal; detecting a sampling phase of the digital signal; generating sampling phase information corresponding to the sampling phase; compensating for the sampling phase of the digital signal based on the sampling phase information; and adjusting the signal strength of the digital signal before generating the sampling phase information. (Supplementary Note 8) The optical receiving method according to Supplementary Note 7, wherein the process of detecting the sampling phase detects the sampling phase based on a phase detection method including the Gardner method. [Explanation of symbols]

[0080] ST Optical Transmission System 10T optical transmitter 10R optical receiver 10Z transmission line 42 Sampling phase synchronization section 51 Sampling phase compensation section 52 Level adjustment section 53 Sampling phase detector 60 Control Unit

Claims

1. a receiving unit that digitally and coherently receives an optical signal to which PCS (Probabilistic Constellation Shaping) is applied; a converter that samples an analog electric field signal representing an optical electric field component of the optical signal and converts it into a digital signal; a detection unit that detects a sampling phase of the digital signal and generates sampling phase information corresponding to the sampling phase; a compensating unit that compensates for the sampling phase of the digital signal based on the sampling phase information; an adjustment unit that adjusts the signal intensity of the digital signal before generating the sampling phase information; An optical receiving device having:

2. a control unit that sets an adjustment value that increases the signal strength in the adjustment unit, and cancels the adjustment value after the sampling phase information is notified to the compensation unit from the detection unit, the adjustment unit is provided before the compensation unit and adjusts the signal strength based on the adjustment value.

2. The optical receiving device according to claim 1.

3. a control unit that sets an adjustment value that increases the signal strength in the adjustment unit; the adjustment unit is provided in a non-demodulation path that is disposed between the compensation unit and the detection unit, independent of a demodulation path that demodulates the digital signal, and adjusts the signal strength based on the adjustment value.

2. The optical receiving device according to claim 1.

4. the control unit acquires the digital signal from the non-demodulation path, detects the signal strength of the acquired digital signal, and sets the adjustment value corresponding to the detected signal strength in the adjustment unit.

4. The optical receiving device according to claim 3.

5. the control unit acquires the phase detection sensitivity of the sampling phase from the detection unit, and sets the adjustment value corresponding to the acquired phase detection sensitivity in the adjustment unit.

4. The optical receiving device according to claim 3.

6. A digital coherent optical signal to which PCS (Probabilistic Constellation Shaping) is applied is received, sampling an analog electric field signal representing an optical electric field component of the optical signal and converting it into a digital signal; detecting a sampling phase of the digital signal and generating sampling phase information corresponding to the sampling phase; Compensating the sampling phase of the digital signal based on the sampling phase information; adjusting the signal strength of the digital signal before generating the sampling phase information; Optical receiving method.

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