Signal processing circuit, optical signal receiving device, signal processing method, and program

Abstract

This enables compensation for lane mismatches during operation without reducing bandwidth utilization efficiency for signals received through optical sampling. [Solution] The signal processing circuit includes a MIMO filter that performs polarization separation on a received signal obtained by receiving an optical signal, which is a polarization multiplexed signal, in parallel using multiple local light pulses, and a coefficient update unit that updates the coefficients of the MIMO filter based on the periodicity based on the number of parallel processes in the optical sampling reception.

Description

[Technical Field]

[0001] The present invention relates to a signal processing circuit, an optical signal receiving device, a signal processing method, and a program. [Background technology]

[0002] Optical sampling reception is a known technique for receiving optical signals (see, for example, Patent Document 1). Optical sampling reception is a technique that parallelizes a received optical signal into a predetermined number of parallel optical signals and captures the entire signal using multiple local light pulses. By sampling the optical signal with multiple light pulses, it becomes possible to reduce the speed and bandwidth of the receiving device per lane receiving the optical sampling signal, which is expected to lead to further increases in the speed of optical signals and a reduction in the cost of optical receiving devices. In optical sampling reception, the received optical signal is parallelized into, for example, four optical signals. In other words, the received optical signal is branched into four lanes. The four branched optical signals are coherently received in each lane using light pulses that are repeated at a predetermined period.

[0003] Generally, the optical pulses output from a light source are delayed by a predetermined delay time according to the lane using an optical delay device, and then input to the coherent receiver for each lane. For example, the optical pulses are delayed in the optical delay device so that they are shifted by 1 / 4 of the repetition period of the optical pulses. However, due to factors such as the equipment environment, a difference in the delay time of the optical pulses used for coherent reception may occur between lanes. If a difference in the delay time of the optical pulses occurs, the characteristics of the received signal will deteriorate.

[0004] Techniques for adjusting timing mismatches in optical sampling reception include those described in Non-Patent Documents 1 and 2. Non-Patent Document 1 adjusts and calibrates timing mismatches using a reference signal. Non-Patent Document 2 compensates for timing mismatches using spectral linewidth. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2011-097253 [Non-patent literature]

[0006] [Non-Patent Document 1] JK Fischer, et al., “High-Speed ​​Digital Coherent Receiver Based on Parallel Optical Sampling”, Journal of Lightwave Technology, Vol. 29, No. 4, 378-385 (2011) [Non-Patent Document 2] P. Johannisson, et al., “A Blind Phase Stabilization Algorithm for Parallel Coherent Receivers”, Journal of Lightwave Technology, Vol. 29, No. 24, 3737-3743(2011) [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, Non-Patent Document 1 has the problem that timing discrepancies cannot be adjusted during system operation. Also, Non-Patent Document 2 has the problem that a guard band is required and bandwidth utilization efficiency is poor.

[0008] One exemplary object of this disclosure is to provide a signal processing circuit, an optical signal receiving device, a signal processing method, and a program that can compensate for lane mismatches during operation with respect to optically sampled signals without reducing bandwidth utilization efficiency. [Means for solving the problem]

[0009] The signal processing circuit according to the first aspect of the present disclosure includes a multi input multi output (MIMO) filter that performs polarization separation on a received signal obtained by optically sampling and receiving in parallel an optical signal that is a polarization multiplexed signal using a plurality of local lights using optical pulses, and a coefficient update unit that updates the coefficients of the MIMO filter based on periodicity based on the number of parallel elements in the optical sampling reception.

[0010] The optical signal receiving apparatus according to the second aspect of the present disclosure includes the above signal processing circuit, an optical receiver that optically samples and receives in parallel the optical signal using a plurality of local lights using the optical pulses, and a synthesizer that synthesizes a plurality of signals obtained by the optical receiver optically sampling and receiving in parallel the optical signal, and inputs the synthesized plurality of signals as the received signal to the signal processing circuit.

[0011] The signal processing method according to the third aspect of the present disclosure includes performing polarization separation on a received signal obtained by optically sampling and receiving in parallel an optical signal that is a polarization multiplexed signal using a plurality of local lights using optical pulses using a multi input multi output (MIMO) filter, and updating the coefficients of the MIMO filter based on periodicity based on the number of parallel elements in the optical sampling reception.

[0012] The program according to the fourth aspect of the present disclosure causes a processor to execute a process of performing polarization separation on a received signal obtained by optically sampling and receiving in parallel an optical signal that is a polarization multiplexed signal using a plurality of local lights using optical pulses using a multi input multi output (MIMO) filter, and updating the coefficients of the MIMO filter based on periodicity based on the number of parallel elements in the optical sampling reception.

Advantages of the Invention

[0013] The signal processing circuit, optical signal receiving device, signal processing method, and program relating to this disclosure can compensate for lane mismatches during operation without reducing bandwidth utilization efficiency. [Brief explanation of the drawing]

[0014] [Figure 1] This block diagram shows an example of a schematic configuration of an optical signal receiving device relating to this disclosure. [Figure 2] This block diagram shows an example configuration of a communication system including an optical signal receiving device related to this disclosure. [Figure 3] This is a block diagram showing an example configuration of an optical signal receiver. [Figure 4] This is a waveform diagram showing the sampling of an optical signal. [Figure 5] This block diagram shows an example configuration of a MIMO signal processing circuit. [Figure 6] This diagram schematically shows the MIMO filtering process when m=2. [Figure 7] This diagram schematically shows the MIMO filtering process when m=4. [Figure 8] This is a flowchart showing the operating procedure of an optical signal receiver. [Figure 9] This is a constellation of the output signals of an optical signal receiver when the number of parallel connections is 4. [Figure 10] This is a constellation of the output signals of an optical signal receiver when the number of parallel connections is 4. [Figure 11] This is a constellation of the output signals of an optical signal receiver when the number of parallel connections is 8. [Figure 12] This is a constellation of the output signals of an optical signal receiver when the number of parallel connections is 8. [Figure 13] This is a constellation of the output signals of an optical signal receiver when there is a gain difference between lanes. [Figure 14] This is a constellation of the output signals of an optical signal receiver when there is a gain difference between lanes. [Figure 15] This block diagram shows an example configuration of a MIMO signal processing circuit. [Figure 16] This diagram schematically illustrates MIMO filtering. [Figure 17] This is a constellation of the output signals of an optical signal receiver when the number of parallel connections is 4. [Figure 18] This is a block diagram showing an example of a signal processing circuit configuration. [Modes for carrying out the invention]

[0015] Prior to describing embodiments of this disclosure, an overview of this disclosure will be provided. Figure 1 is a block diagram showing an example of a schematic configuration of an optical signal receiving device according to this disclosure. The optical signal receiving device 10 includes an optical receiver 20, a combiner 30, and a signal processing circuit 40. The signal processing circuit 40 includes a MIMO filter 41 and a coefficient update unit 42.

[0016] The optical receiver 20 receives optical signals, which are polarization-multiplexed signals, in parallel using optical sampling with multiple local light pulses. The combiner 30 combines the multiple signals obtained by the optical receiver 20 receiving the optical signals in parallel using optical sampling. The combiner 30 outputs the combined signal to the signal processing circuit 40.

[0017] In the signal processing circuit 40, the MIMO filter 41 is a filter that performs polarization separation on the signal input from the combiner 30, i.e., the received signal. The MIMO filter 41 has multiple types of coefficients and performs MIMO processing based on the periodicity derived from the number of parallel processes in optical sampling reception. The coefficient update unit 42 updates the coefficients of the MIMO filter 41 based on the periodicity derived from the number of parallel processes in optical sampling reception.

[0018] In this disclosure, the coefficient update unit 42 of the signal processing circuit 40 updates the coefficients of the MIMO filter 41 for each type of coefficient using coefficients of multiple types of MIMO filters 41 based on periodicity based on the number of parallel processes in optical sampling reception. In this disclosure, the signal processing circuit 40 performs MIMO processing based on periodicity based on the number of parallel processes in optical sampling reception using coefficients of multiple types of MIMO filters 41 in the MIMO filter 41 used for polarization separation. In this way, the MIMO filter 41 can compensate for timing mismatches between lanes of signals sampled and received using optical pulses in parallel simultaneously with polarization separation.

[0019] The signal processing circuit 40 according to this disclosure can compensate for timing mismatch without using special signals such as reference signals. Therefore, the signal processing circuit 40 can compensate for timing mismatch during system operation. Furthermore, in this disclosure, it is not necessary to provide a guard band for timing mismatch. Accordingly, the signal processing circuit 40 can compensate for timing mismatch without reducing bandwidth efficiency.

[0020] The embodiments of this disclosure will be described in detail below. Note that the following descriptions and drawings have been omitted and simplified as appropriate for clarity of explanation. Furthermore, in each drawing, the same elements and similar elements are denoted by the same reference numerals, and redundant explanations have been omitted where necessary.

[0021] A first embodiment will be described. Figure 2 is a block diagram showing an example configuration of a communication system including an optical signal receiving device according to the present disclosure. The optical fiber communication system 100 includes an optical signal transmitter 110, a transmission line 130, and an optical signal receiver 150. In the first embodiment, it is assumed that the optical fiber communication system 100 employs a polarization multiplexing multilevel modulation scheme and is an optical fiber communication system that performs coherent reception.

[0022] The optical signal transmitter 110 generates a polarization-multiplexed optical signal and outputs the generated polarization-multiplexed optical signal to the transmission path 130. The transmission path 130 transmits the polarization-multiplexed optical signal output from the optical signal transmitter 110 to the optical signal receiver 150. The transmission path 130 includes an optical fiber for guiding the optical signal. The transmission path 130 also includes an optical amplifier to compensate for propagation loss in the optical fiber. The optical signal receiver 150 receives the polarization-multiplexed optical signal transmitted via the transmission path 130.

[0023] Figure 3 is a block diagram showing an example configuration of the optical signal receiver 150. The optical signal receiver 150 includes a local oscillator (LO) 151, optical delayers 152-1 to 152-3, coherent receivers 153-0 to 153-3, and a digital signal processor (DSP) 154. The optical signal receiver 150 corresponds to the optical signal receiving device 10 shown in Figure 1.

[0024] In the optical signal receiver 150, an optical splitter, such as an optical coupler (not shown), splits the optical signal transmitted via the transmission path 130, i.e., the polarization-multiplexed optical signal, according to the number of parallel samples of the optical signal. In the example in Figure 3, the number of parallel samples is 4. In the example in Figure 3, the optical signal is split into 4 lanes, and the 4 split optical signals are input to the coherent receivers 153-0 to 153-3 for each lane.

[0025] LO151 outputs an optical pulse LO at a predetermined repetition period. LO151 includes a pulse light source. LO151 includes, for example, a modulator-integrated semiconductor laser. Optical delay unit 152-1 delays the optical pulse LO output from LO151 and outputs the delayed optical pulse LO1. Optical delay unit 152-2 delays the optical pulse LO1 output from optical delay unit 152-1 and outputs the delayed optical pulse LO2. Optical delay unit 152-3 delays the optical pulse LO2 output from optical delay unit 152-2 and outputs the delayed optical pulse LO3.

[0026] The delay amount of optical delay unit 152-1 is expressed as τ + δτ1. The delay amount of optical delay unit 152-2 is expressed as τ + δτ2. The delay amount of optical delay unit 152-3 is expressed as τ + δτ3. When performing optical sampling of 2 samples / 1 symbol to an optical signal with a baud rate of 32 G Aud, the delay time τ is set to τ = 1 / (32 G × 2) [seconds]. δτ1, δτ2, and δτ3 represent fluctuating components that change due to factors such as the equipment environment.

[0027] The coherent receivers 153-0 to 153-3 perform coherent detection using optical pulses on optical signals that are branched in parallel. In the first embodiment, the coherent receivers 153-0 to 153-3 use optical pulses LO, LO1, LO2, and LO3 as local optics and sample the optical signals in parallel. For example, when performing optical sampling of 2 samples / 1 symbol in four parallel circuits on an optical signal with a baud rate of 32 GBaud, the coherent receivers 153-0 to 153-3 each sample the input optical signal at a period corresponding to 16 GHz.

[0028] Figure 4 is a waveform diagram showing the sampling of the optical signal. The repetition period of the optical pulse output from LO151 is T. LO Let's assume that the delay time τ given to the optical pulse in the optical delay devices 152-1 to 152-3 is T LO It is set to / 4. The optical pulse LO1 is set to T relative to the optical pulse LO. LO This is an optical pulse delayed by 4. Optical pulse LO2 is 2×T relative to optical pulse LO. LO This is an optical pulse delayed by 4. Optical pulse LO3 is 3 × T relative to optical pulse LO. LO This is a light pulse delayed by 4.

[0029] Coherent receiver 153-0 performs coherent detection of the optical signal using the optical pulse LO output from LO151. Coherent receiver 153-1 performs coherent detection of the optical signal using the optical pulse LO1 output from optical delay unit 152-1. Coherent receiver 153-2 performs coherent detection of the optical signal using the optical pulse LO2 output from optical delay unit 152-2. Coherent receiver 153-3 performs coherent detection of the optical signal using the optical pulse LO3 output from optical delay unit 152-3. In Figure 4, the white circles shown on the optical signal waveform indicate the sampling points of the optical signal.

[0030] Coherent receivers 153-0 to 153-3 each include a 90° hybrid and a photoelectric converter. Each coherent receiver 153-0 to 153-3 outputs four sequences of received signals (electrical signals) corresponding to the I and Q components of coherently detected X-polarization and Y-polarization. The received signals output in parallel from coherent receivers 153-0 to 153-3 are converted from analog signals to digital signals using an analog-to-digital converter (ADC) (not shown). The received signals converted to digital signals are input to DSP 154. LO 151, optical delayers 152-1 to 152-3, and coherent receivers 153-0 to 153-3 correspond to the optical receiver 20 shown in Figure 1.

[0031] The DSP154 performs digital signal processing on the received signal. The DSP154 includes upsampling units 155-0 to 155-3, delay units 156-1 to 156-3, a combiner 157, an equalizer 158, a multi-input multi-output (MIMO) signal processing circuit 159, and a carrier phase recovery (CPR) compensator 160. The DSP154 may be configured, for example, as a device having one or more processors and one or more memories. At least some of the functions of each part in the DSP154 may be realized by the processor executing processing according to instructions read from memory. At least some of the functions of each part in the DSP154 may be realized by dedicated hardware circuits.

[0032] The upsampling units 155-0 to 155-3 each upsample the received signal output from the coherent receivers 153-0 to 153-3. For example, the upsampling units 155-0 to 155-3 upsample a 16GHz received signal to a 64GHz signal. More specifically, the upsampling unit 155-0 inserts "0" into the signal output from the coherent receiver 153-0 as a signal corresponding to the timing of the optical pulses LO1, LO2, and LO3. The upsampling unit 155-1 inserts "0" into the signal output from the coherent receiver 153-1 as a signal corresponding to the timing of the optical pulses LO, LO2, and LO3. The upsampling unit 155-2 inserts "0" into the signal output from the coherent receiver 153-2 as a signal corresponding to the timing of the optical pulses LO, LO1, and LO3. The upsampling unit 155-3 inserts "0" into the signal output from the coherent receiver 153-3 as a signal corresponding to the timing of the optical pulses LO, LO1, and LO2.

[0033] Delay units 156-1 to 156-3 each delay the upsampled signals output from the upsampling units 155-1 to 155-3, adjusting the timing of each signal. The combiner 157 combines the upsampled received signals output from the upsampling unit 155-0 and the delay units 156-1 to 156-3. In other words, the combiner 157 combines four received signals sampled in parallel. The combiner 157 outputs, for example, complex number signals representing the I and Q components of the X polarization and complex number signals representing the I and Q components of the Y polarization to the equalizer 158. The combiner 157 corresponds to the combiner 30 shown in Figure 1.

[0034] In the first embodiment, the optical signals are optically sampled and received in parallel so that the received signals become signals with 2 samples / 1 symbol, in order to perform wavelength dispersion compensation, polarization mode dispersion compensation, and polarization separation on the combined received signals. For example, if the number of parallel receivers is 4 and the optical signal is a 32GBaud signal, the optical signals are sampled at a period corresponding to 16GHz in each of the coherent receivers 153-0 to 153-3, as described above. If the number of parallel receivers is 8, the optical signals are sampled at a period corresponding to 8GHz in each of the 8 coherent receivers.

[0035] The equalizer 158 performs equalization processing to compensate for static distortion in the synthesized received signal. The equalizer 158 compensates for static distortion using, for example, a fixed filter with statically set coefficients. The MIMO signal processing circuit 159 includes an adaptive MIMO filter whose coefficients are adaptively controlled. The MIMO signal processing circuit 159 performs, for example, polarization separation, polarization mode dispersion compensation, and frequency response compensation. The CPR compensator 160 performs frequency offset compensation and carrier phase recovery. The signal output from the CPR compensator 160 is then processed, such as symbol recovery and decoding.

[0036] Here, for example, if δτ3 is not 0 in the optical delay unit 152-3, the timing of the optical pulse LO3 will deviate from the timing shown in Figure 4. In that case, the timing of the optical signal sampling in the coherent receiver 153-3 will deviate from the timing of the white circles shown in Figure 4, and the sampling interval of the optical signal will no longer be equal. When the timing of the optical pulse LO3 is shifted, the coherent receiver 153-3 will acquire a signal different from the signal acquired at the original sampling point. This shift in the sampling interval is also called a timing mismatch. In the first embodiment, the MIMO signal processing circuit 159 also performs timing mismatch compensation between lanes.

[0037] Figure 5 is a block diagram showing an example configuration of the MIMO signal processing circuit 159. The MIMO signal processing circuit 159 includes a MIMO filter 171 and a coefficient update unit 172. The MIMO signal processing circuit 159 corresponds to the signal processing circuit 40 shown in Figure 1. The MIMO filter 171 corresponds to the MIMO filter 41 shown in Figure 1. The coefficient update unit 172 corresponds to the coefficient update unit 42 shown in Figure 1.

[0038] The MIMO filter 171 performs polarization separation on the received signal. The MIMO filter 171 includes a Finite Impulse Response (FIR) filter with a 2x2 butterfly structure to separate the two polarization components. The tap length of each FIR filter is assumed to be "l". The MIMO filter 171 has m types of coefficients h0~h m The MIMO filter 171 has a set of coefficients. The MIMO filter 171 applies m types of coefficients to the received signal based on the periodicity determined by the number of parallel optical sampling receivers. The value of m is determined according to the number of parallel optical sampling receivers. For example, if the number of parallel receivers is 4, m=2, and two types of coefficients, h0 and h1, are used in rotation. If the number of parallel receivers is 8, m=4, and four types of coefficients, h0, h1, h2, and h3, are used in rotation. If the number of parallel receivers is 16, m=8, and eight types of coefficients are used in rotation. If the number of parallel receivers is 32, m=16, and sixteen types of coefficients are used in rotation.

[0039] Let the input signal of the MIMO filter 171 be u[i] = {X in [i], Y in [i]}. Here, i indicates the index of the input signal. Assume that the input signal u[i] is a 2-fold oversampled signal, that is, a signal with 2 samples / 1 symbol. Also, let the output signal of the MIMO filter 171 be v[j] = {X out [j], Y out [j]}. Here, j indicates the index of the output signal. Assume that the output signal v[j] is a 1-fold oversampled signal, that is, a signal with 1 sample / 1 symbol. The output signal v[j] is expressed by the following formula, where j|m is the remainder of j with respect to m. TIFF2026067111000002.tif1146

[0040] The coefficient update unit 172 periodically updates the m types of coefficients based on the periodicity based on the parallel number of optical sampling reception. The coefficient update unit 172 updates the coefficient h m|j so that the difference between the output signal v[j] of the MIMO filter 171 and the identification signal corresponding to v[j] becomes small. For example, the coefficient update unit 172 calculates the error ∂E between the output signal v[j] and the identification signal for each of the m types of coefficients. The coefficient update unit 172 uses the following formula with Δ as the step width in coefficient update to update the coefficient h j|m so that the error becomes small. h j|m →h j|m -Δ∂E

[0041] Figure 6 is a diagram schematically showing the MIMO filter processing in the case of m = 2. In the example of Figure 6, the tap length of the FIR filter is l = 5. The lane numbers shown in Figure 6 indicate the numbers of the lanes before the synthesis of the input signal u[i]. The MIMO filter 171 switches the coefficient h[[ID=#]] j|m applied to the input signal according to the index j of the output signal, and applies the coefficient h j|m to the input signals u[i] to u[i - 4].

[0042] For example, the coefficients of the MIMO filter 171 are h0 = {h 0XX ,h 0XY ,h 0YX ,h 0YY It is set to}. In that case, the coefficient of each FIR filter is h XX =h 0XX ,h XY =h 0XY ,h YX =h 0YX ,h YY =h 0YY It is set to this. The coefficient update unit 172 calculates the error ∂E at j=0 and updates the coefficient h0 used at j=2 by h0→h0-Δ∂E. On the other hand, the coefficient of the MIMO filter 171 is set to h1={h 1XX ,h 1XY ,h 1YX ,h 1YY It is set to}. The coefficient update unit 172 calculates the error ∂E at j=1 and updates the coefficient h1 by h1→h1-Δ∂E. In Figure 6, the coefficient update unit 172 may update the coefficient h0 at all j where the index j is j=0, 2, ... and j|m=0. Alternatively, the coefficient h0 does not necessarily need to be updated at all j where j|m=0, and the coefficient update unit 172 may update the coefficient h0 at an appropriate frequency in accordance with the required circuit scale or the time scale at which the timing mismatch between lanes changes. Similarly, the coefficient update unit 172 may update the coefficient h1 at all j where the index j is j|m=1, or it may update the coefficient h1 at an appropriate frequency.

[0043] Here, the input signal u[i] is a signal synthesized from pulse reception data received by optical sampling in parallel across four lanes. In the first embodiment, the order of lanes for the duration of the tap length l of u[i] is the same whether h0 is used as the filter coefficient or h1 is used as the filter coefficient. In Figure 6, the order of lanes when h0 is used is 0→3→2→1→0, which is the same as the order of lanes for v[j] when h0 is used. Similarly, the order of lanes when h1 is used is 2→1→0→3→2, which is the same as the order of lanes for v[j] when h1 is used. In the first embodiment, the coefficient update unit 172 updates the coefficients h0 and h1 individually. In this way, even if there is a timing mismatch between lanes, the MIMO filter 171 can perform MIMO processing according to the order of the lanes, and the timing mismatch can be compensated. Furthermore, even if there is a gain difference (signal amplitude difference) between lanes due to variations in the signal amplification ratio of each device, the gain mismatch between lanes can be compensated.

[0044] If we assume that the same coefficient h is used without switching the coefficient, the order of the lanes for the duration of the tap length l of the input signal u[i] will differ between v[0] and v[1]. In Figure 6, two different lane orders coexist: 0→3→2→1→0 and 2→1→0→3→2. Therefore, if we update the coefficient h to minimize the error at each time step, it is unlikely that the timing mismatch between lanes will be properly compensated. Furthermore, it is unlikely that the gain mismatch between lanes will be properly compensated.

[0045] Figure 7 schematically shows the MIMO filtering process when m=4. In the example in Figure 7, the tap length of the FIR filter is l=5. The coefficient of the MIMO filter 171 is set to h0 for j=0 and j=4. The coefficient of the MIMO filter 171 is set to h1 for j=1. The coefficient of the MIMO filter 171 is set to h2 for j=2. The coefficient of the MIMO filter 171 is set to h3 for j=3. In this case as well, as in the case of m=2, the order of lanes for the period of tap length l of u[i] is the same for each of the cases where h0, h1, h2, and h3 are used. Therefore, the coefficient update unit 172 sets the coefficient h 0、 By updating h1, h2, and h3 individually, the MIMO filter 171 can compensate for timing mismatches between lanes. Furthermore, it can compensate for gain mismatches between lanes, even if there are gain differences between them.

[0046] Next, the operation procedure of the optical signal receiver 150 will be explained. Figure 8 is a flowchart showing the operation procedure of the optical signal receiver 150. In the optical signal receiver 150, a splitter such as an optical coupler splits the received optical signal into parallel optical pulses (step S1). Coherent receivers 153-0 to 153-3 each use optical pulses with delay times corresponding to their respective lanes to optically sample and receive the optical signals split in step S1 in parallel (step S2).

[0047] The upsampling units 155-0 to 155-3 upsample the received signals from each lane (step S3). The combiner 157 combines the upsampled received signals (step S4). The MIMO signal processing circuit 159 performs MIMO filtering on the combined received signals using the MIMO filter 171 (step S5). The coefficient update unit 172 updates the coefficients of the MIMO filter 171 based on the periodicity derived from the number of parallel processes in optical sampling reception (step S6). Steps S5 and S6 correspond to the signal processing methods performed in the MIMO signal processing circuit 159.

[0048] The inventors verified the effects of the first embodiment through simulation. A 32 Gbaud polarization-multiplexed Quadrature Phase Shift Keying (QPSK) signal was used in the simulation. The chromatic dispersion in the transmission line was set to CD = 2000 ps / nm. The Optical signal-to-noise ratio (OSNR) was set to 25 dB. The tap length of the MIMO filter 171 was set to 51 taps. Error Vector Magnitude (EVM), which represents the difference between the modulated / demodulated symbol position and the identification symbol position, was used to evaluate the reception quality.

[0049] Figures 9 and 10 show the constellation of the output signals of the optical signal receiver 150 when the number of parallel connections is 4, as obtained by simulation. In the simulation, the delay time errors of the optical delayers 152-1 to 152-3 were set to δτ1 = -4 / 32 × 1 symbol time, δτ2 = 2 / 32 × 1 symbol time, and δτ3 = 6 / 32 × 1 symbol time, respectively.

[0050] Figure 9 shows the output signal constellation when coefficient updates in a standard MIMO filter are used. As shown in Figure 9, when coefficient updates in a standard MIMO filter are used, that is, when only one type of coefficient is updated regardless of the index j of the output signal, the spread of the four signal points is large. When coefficient updates in a standard MIMO filter are used, the EVM was 15.6%.

[0051] Figure 10 shows the output signal constellation when the coefficients are updated based on the periodicity derived from the number of parallel optical sampling receivers. As shown in Figure 10, when the two coefficients h0 and h1 are updated periodically, the spread of the four signal points is smaller compared to the case in Figure 9. When the coefficients are updated based on the periodicity derived from the number of parallels, the EVM was 9.7%. Therefore, it was confirmed that when the coefficient update of the MIMO filter 171 in the first embodiment is used, the reception characteristics can be improved compared to when the coefficient update of a normal MIMO filter is used.

[0052] Figures 11 and 12 show the constellation of the output signals of the optical signal receiver 150 when the number of parallel connections is 8, as obtained by simulation. In the simulation, the delay time errors of the seven optical delayers were set as follows: δτ1 = -2 / 32 × 1 symbol time, δτ2 = 4 / 32 × 1 symbol time, δτ3 = 0, δτ4 = 1 / 32 × 1 symbol time, δτ5 = -4 / 32 × 1 symbol time, δτ6 = 3 / 32 × 1 symbol time, and δτ7 = 4 / 32 × 1 symbol time, respectively.

[0053] Figure 11 shows the output signal constellation when coefficient updating is used in a standard MIMO filter. As shown in Figure 11, when coefficient updating is used in a standard MIMO filter, the spread of the four signal points is large, similar to the case in Figure 9. When coefficient updating is used in a standard MIMO filter, the EVM was 13.1%.

[0054] Figure 12 shows the output signal constellation when the coefficients are updated based on the periodicity derived from the number of parallel optical sampling receivers. As shown in Figure 12, when the four coefficients are updated periodically, the spread of the four signal points is smaller compared to the case in Figure 11. When the coefficients are updated based on the periodicity derived from the number of parallel optical sampling receivers, the EVM was 9.9%. Therefore, it was confirmed that the first embodiment can improve the reception characteristics when the number of parallel optical sampling receivers is 8, just as it does when the number of parallel optical sampling receivers is 4.

[0055] Figures 13 and 14 show the constellation of the output signals of optical signal receiver 150 when there is a gain difference between lanes, as obtained by simulation. In the simulation, the output signal of coherent receiver 153-0 was amplified by 1.4 times, and the output signal of coherent receiver 153-1 was amplified by 0.8 times. Also, the output signal of coherent receiver 153-2 was amplified by 0.9 times, and the output signal of coherent receiver 153-3 was amplified by 1 time. In the simulation, the delay time errors of optical delayers 152-1 to 152-3 were set to 0.

[0056] Figure 13 shows the output signal constellation when coefficient updating is used in a standard MIMO filter. As shown in Figure 9, when coefficient updating is used in a standard MIMO filter, the spread of the four signal points is large due to the gain difference between lanes, resulting in poor reception characteristics. When coefficient updating is used in a standard MIMO filter, the EVM was 35.8%.

[0057] Figure 14 shows the output signal constellation when the coefficients are updated based on the periodicity derived from the number of parallel optical sampling receivers. As shown in Figure 14, when the two coefficients h0 and h1 are updated periodically, the spread of the four signal points is smaller compared to the case in Figure 13. When the coefficients are updated based on the periodicity derived from the number of parallels, the EVM was 9.8%. Therefore, the first embodiment was confirmed to be able to compensate for the gain difference between lanes in the MIMO filter 171 even when there is a gain difference between lanes.

[0058] In the first embodiment, the MIMO signal processing circuit 159 can compensate for timing mismatch in optical sampling reception in addition to polarization separation. In the first embodiment, a reference signal is not required, and the MIMO signal processing circuit 159 can compensate for timing mismatch that may dynamically change during normal operation of the system. Furthermore, in the first embodiment, a guard band is not required, and the MIMO signal processing circuit 159 can compensate for timing mismatch without reducing bandwidth utilization efficiency. Moreover, in the first embodiment, the MIMO signal processing circuit 159 can compensate for gain differences between lanes, thereby improving reception characteristics.

[0059] Next, a second embodiment will be described. In the second embodiment, the MIMO signal processing circuit 159 has m MIMO filters. In the second embodiment, the received signal output from the equalizer 158 is split into m signals, and the m split received signals are input to m MIMO filters, respectively. In the first embodiment, m types of coefficients were switched in one MIMO filter 171, and the m types of coefficients were updated based on the periodicity based on the number of parallel optical sampling receptions. In contrast, in the second embodiment, the received signal is split into m MIMO filters, and filtering and coefficient updating are performed in the m MIMO filters.

[0060] Figure 15 is a block diagram showing an example configuration of a MIMO signal processing circuit. In the example shown in Figure 15, the number of parallel optical sampling receivers is assumed to be 4. The MIMO signal processing circuit 159a shown in Figure 15 has MIMO filters 175 and 176 and a delay circuit 177. The received signal output from the equalizer 158 is split into two, and one of the received signals is input to the MIMO filter 175. The other of the split received signals is input to the delay circuit 177. The delay circuit 177 delays the input received signal by 2 symbols. The delayed received signal is input to the MIMO filter 176.

[0061] MIMO filters 175 and 176 are each configured as 2x2 MIMO filters. The coefficient of MIMO filter 175 is h0, and the coefficient of MIMO filter 176 is h1. The coefficient update unit 172 updates the coefficient h0 based on the output signal and identification signal of MIMO filter 175. The coefficient update unit 172 also updates the coefficient h1 based on the output signal and identification signal of MIMO filter 176.

[0062] Figure 16 schematically shows the MIMO filtering process in the MIMO signal processing circuit 159. In the example in Figure 16, the tap length of the FIR filter of each MIMO filter is l=5. The MIMO signal processing circuit 159a switches the MIMO filter used for filtering between MIMO filter 175 and MIMO filter 176 depending on the index j of the output signal. MIMO filter 175 applies coefficient h0 to the input signal u[i]~u[i-4] for j|m=0. MIMO filter 175 applies coefficient h1 to the input signal u[i]~u[i-4] for j|m=1.

[0063] As shown in Figure 16, the MIMO filter 175 applies coefficient h0 to the input signal at j=0 and j=2, respectively. The coefficient update unit 172 calculates the error ∂E between the output of the MIMO filter 175 and the identification signal at j=0. The coefficient update unit 172 updates the coefficient h0 used at j=2 by h0 → h0 - Δ∂E. On the other hand, the MIMO filter 176 applies coefficient h1 to the input signal at j=1. The coefficient update unit 172 calculates the error ∂E between the output of the MIMO filter 176 and the identification signal at j=1. The coefficient update unit 172 updates the coefficient h1 by h1 → h1 - Δ∂E.

[0064] The inventors verified the effects of the second embodiment through simulation. The main simulation conditions were the same as those in the simulation described in the first embodiment. Figure 17 shows the constellation of the output signals of the optical signal receiver obtained by simulation when the number of parallel connections is 4. In the simulation, the delay time errors of the optical delayers 152-1 to 152-3 were set to δτ1 = -4 / 32 × 1 symbol time, δτ2 = 2 / 32 × 1 symbol time, and δτ3 = 6 / 32 × 1 symbol time, respectively.

[0065] In the simulation, a MIMO filter with coefficient h0 and a MIMO filter with coefficient h1 were used, switching periodically based on the periodicity corresponding to the number of parallel optical sampling receivers. Referring to Figure 17, it can be seen that the spread of the four signal points is smaller compared to the case in Figure 9 where a MIMO filter with one type of coefficient is used. When the MIMO filter with coefficient h0 and the MIMO filter with coefficient h1 were used in a switching manner, the EVM was 9.7%. Therefore, it was confirmed that in the second embodiment as well, the reception characteristics can be improved compared to when the coefficient update of a normal MIMO filter is used.

[0066] In the second embodiment, the MIMO signal processing circuit 159a has m MIMO filters. In this case, unlike in the first embodiment, the MIMO signal processing circuit 159a does not need to rapidly switch between m types of coefficients in a single MIMO filter. The filtering operation on index j in the second embodiment is the same as the filtering operation on index j in the first embodiment. In the second embodiment, the same effects as those obtained in the first embodiment can be obtained.

[0067] In the first and second embodiments, the MIMO signal processing circuit 159 can be configured as any digital signal processing circuit. Figure 18 is a block diagram showing an example configuration of a signal processing circuit that can be used in the MIMO signal processing circuit 159. The signal processing circuit 400 includes one or more processors 410 and one or more memories 420. The processor 410 reads a program stored in the memory 420 and performs processing such as switching m types of coefficients in the MIMO filter and updating the coefficients of the MIMO filter.

[0068] The above program, when loaded into a processor, includes a set of instructions (or software code) for causing the processor to perform one or more of the functions described in the embodiments. The program may be stored on a non-temporary computer-readable medium or a physical storage medium. Examples, but not limited to, include random-access memory (RAM), read-only memory (ROM), flash memory, solid-state drive (SSD) or other memory technologies, CD (compact disc)-ROM, digital versatile disc (DVD), Blu-ray® disc or other optical disc storage, magnetic cassette, magnetic tape, magnetic disk storage or other magnetic storage devices. The program may be transmitted over a temporary computer-readable medium or a communication medium. Examples, but not limited to, include temporary computer-readable medium or a communication medium that includes electrically, optically, acoustically, or otherwise propagating signals.

[0069] Although the present disclosure has been described above with reference to embodiments, the present disclosure is not limited to the embodiments described above. Various modifications to the structure and details of the present disclosure can be made as can be understood by those skilled in the art within the scope of the present disclosure. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0070] Each drawing is merely illustrative to illustrate one or more embodiments. Each drawing may be associated with one or more other embodiments rather than with only one specific embodiment. As those skilled in the art will understand, various features or steps described with reference to any one drawing can be combined with features or steps shown in one or more other drawings, for example, to create embodiments not explicitly shown or described. Not all features or steps shown in any one drawing to illustrate an exemplary embodiment are necessarily required, and some features or steps may be omitted. The order of steps shown in any of the drawings may be changed as appropriate.

[0071] Some or all of the above embodiments may also be described as follows, but are not limited to the following:

[0072] [Note 1] A multi-input multi-output (MIMO) filter performs polarization separation on a received signal obtained by parallel optical sampling and reception of an optical signal, which is a polarization-multiplexed signal, using multiple local light pulses, and A signal processing circuit having a coefficient update unit that updates the coefficients of the MIMO filter based on the periodicity derived from the number of parallelisms in the optical sampling reception.

[0073] [Note 2] The MIMO filter uses m coefficients h0 to h, where m is an integer determined according to the number of parallelisms. m This is applied to the received signal based on the periodicity derived from the number of parallel connections, The coefficient update unit is a signal processing circuit as described in Appendix 1, which individually updates m types of coefficients based on the periodicity determined by the number of parallel connections.

[0074] [Note 3] The MIMO filter takes the index of the output signal as j, and sets the coefficient h as the remainder of m with respect to j, where j|m is the remainder of m with respect to j. j|mA signal processing circuit as described in Appendix 2, which switches and applies to the received signal.

[0075] [Note 4] The coefficient update unit adjusts the coefficient h so that the error between the output signal of the MIMO filter and the identification signal is reduced. j|m The signal processing circuit described in Appendix 3 updates the signal.

[0076] [Note 5] The signal processing circuit described in Appendix 2 includes m MIMO filters, each of which receives a branched signal.

[0077] [Note 6] The signal processing circuit described in any one of the appendices 1 to 5, wherein the received signal input to the MIMO filter is a signal obtained by combining multiple signals obtained by optically sampling and receiving the optical signal in parallel using the optical pulses.

[0078] [Note 7] A signal processing circuit described in any one of the appendices 1 to 6, An optical receiver that receives the optical signal in parallel using multiple local light sources with the aforementioned light pulses, An optical signal receiving device comprising: an optical receiver; a synthesizer that synthesizes a plurality of signals obtained by optical sampling reception of the optical signal in parallel; and inputs the synthesized plurality of signals as the received signal to the signal processing circuit.

[0079] [Note 8] The aforementioned optical receiver is, A pulse light source that outputs the light pulse at a predetermined repetition period, The optical signal receiving device according to Appendix 7, further comprising an optical delay unit that delays the optical pulse output from the pulse light source by a delay time determined according to the repetition period and the number of parallel connections, thereby generating a plurality of local lights.

[0080] [Note 9] Using a multi-input multi-output (MIMO) filter, polarization separation is performed on the received signal obtained by parallel optical sampling and reception of an optical signal, which is a polarization-multiplexed signal, using multiple local light pulses. A signal processing method for updating the coefficients of the MIMO filter based on the periodicity derived from the number of parallelisms in the optical sampling reception.

[0081] [Note 10] Using a multi-input multi-output (MIMO) filter, polarization separation is performed on the received signal obtained by parallel optical sampling and reception of an optical signal, which is a polarization-multiplexed signal, using multiple local light pulses. A program that causes a processor to perform a process of updating the coefficients of the MIMO filter based on the periodicity derived from the number of parallelisms in the optical sampling reception.

[0082] Some or all of the elements (e.g., configuration and function) described in Appendices 2 to 6 that are dependent on Appendice 1 may also be dependent on Appendices 9 and 10 in the same manner as those described in Appendices 2 to 6. Some or all of the elements described in any appendice may be applicable to various hardware, software, recording means, systems, and methods for recording software. [Explanation of symbols]

[0083] 10: Optical signal receiving device 20: Optical receiver 30: Synthesizer 40: Signal Processing Circuits 41: MIMO filter 42: Coefficient update section 100: Fiber Optic Communication System 110: Optical signal transmitter 130: Transmission line 150: Optical signal receiver 151:LO 152-1~152-3: Optical delay device 153-0~153-3: Coherent Receiver 154:DSP 155-0~155-3: Upsampling section 156-1~156-3: Delay devices 157:Synthesizer 158: Equalizer 159: MIMO signal processing circuit 160:CPR compensator 171, 175, 176: MIMO filters 172: Coefficient update section 177: Delay Circuit

Claims

1. A multi-input multi-output (MIMO) filter performs polarization separation on a received signal obtained by optical sampling and receiving an optical signal, which is a polarization-multiplexed signal, in parallel using multiple local light pulses, and A signal processing circuit having a coefficient update unit that updates the coefficients of the MIMO filter based on the periodicity derived from the number of parallelisms in the optical sampling reception.

2. The MIMO filter has m types of coefficients h, where m is an integer determined according to the number of parallelisms. ０ ~h ｍ This is applied to the received signal based on the periodicity derived from the number of parallel connections, The signal processing circuit according to claim 1, wherein the coefficient update unit individually updates m types of coefficients based on the periodicity determined by the number of parallel connections.

3. The MIMO filter takes the index of the output signal as j, and the coefficient h is determined by taking j|m as the remainder of m with respect to j, according to the index j of the output signal. ｊ｜ｍ The signal processing circuit according to claim 2, which switches and applies to the received signal.

4. The coefficient update unit adjusts the coefficient h so that the error between the output signal of the MIMO filter and the identification signal is reduced. ｊ｜ｍ A signal processing circuit according to claim 3, which updates the signal.

5. The signal processing circuit according to claim 2, wherein the MIMO filter includes m MIMO filters, each of which the received signal is branched and input.

6. The signal processing circuit according to any one of claims 1 to 5, wherein the received signal input to the MIMO filter is a signal obtained by synthesizing a plurality of signals obtained by optically sampling and receiving the optical signal in parallel using the optical pulses.

7. A signal processing circuit according to any one of claims 1 to 5, An optical receiver that receives the optical signal in parallel using multiple local light sources with the aforementioned light pulses, An optical signal receiving device comprising: an optical receiver; a synthesizer that synthesizes a plurality of signals obtained by optical sampling reception of the optical signal in parallel; and inputs the synthesized plurality of signals as the received signal to the signal processing circuit.

8. The aforementioned optical receiver is, A pulse light source that outputs the light pulse at a predetermined repetition period, The optical signal receiving device according to claim 7, further comprising an optical delay unit that delays the optical pulse output from the pulse light source by a delay time determined according to the repetition period and the number of parallel connections, thereby generating a plurality of local lights.

9. By using a multi-input multi-output (MIMO) filter, polarization separation is performed on the received signal obtained by parallel optical sampling and reception of an optical signal, which is a polarization-multiplexed signal, using multiple local light sources generated by optical pulses. A signal processing method for updating the coefficients of the MIMO filter based on the periodicity derived from the number of parallelisms in the optical sampling reception.

10. By using a multi-input multi-output (MIMO) filter, polarization separation is performed on the received signal obtained by parallel optical sampling and reception of an optical signal, which is a polarization-multiplexed signal, using multiple local light sources generated by optical pulses. A program that causes a processor to perform a process of updating the coefficients of the MIMO filter based on the periodicity derived from the number of parallelisms in the optical sampling reception.

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