JP5653561B2 - Optical modulation / demodulation method and optical transceiver - Google Patents

Description

  The present invention relates to an optical modulation / demodulation method and an optical transceiver, and more particularly, to an optical modulation / demodulation method and an optical transceiver using a digital coherent method.

  For large-capacity optical transmission, overcoming the limitations of optical signal versus noise power and high-density wavelength multiplexing are challenges.

  As a technique for overcoming the limitations of optical signal-to-noise power, binary phase-shift keying (BPSK) or quaternary PSK (Quaternary) is used instead of conventional on-off keying (OOK). Phase-Shift Keying (QPSK) is used.

  For high-density wavelength multiplexing, a system is used in which the number of transmission bits per symbol is doubled by polarization multiplexing in which independent signals are assigned to two orthogonal polarization components. Alternatively, as another method, there is known a method of increasing the signal multiplicity and increasing the number of transmission bits per symbol, such as QPSK or 16-value quadrature amplitude modulation (16 QAM). QPSK and 16QAM are transmitted by assigning signals to the same phase axis (In-Phase axis: I axis) and quadrature phase axis (Quadrature-Phase axis: Q axis) in the optical transmitter.

  Further, a digital coherent system that receives these optical modulation signals by combining digital signal processing with the synchronous detection system has attracted attention (for example, see Non-Patent Document 1). In this method, chromatic dispersion generated in a transmission line and polarization mode dispersion (Polarization-polarization-) are obtained by linear photoelectric conversion by synchronous detection and fixed, semi-fixed, and adaptive linear equalization by digital signal processing. It is possible to achieve excellent equalization characteristics and excellent noise tolerance against linear waveform distortion caused by Mode Dispersion (PMD).

  As a method for performing carrier phase offset compensation at the receiving unit in the digital coherent method, an M-th power method (for example, see Non-Patent Document 2) or a provisional determination type algorithm (for example, see Patent Document 1) has been used. In these methods, a slip of an angle (2π / M) × N may occur when the carrier phase is restored under conditions where noise and waveform distortion are large, and a large-scale continuous error may occur (M: PSK phase). A natural number indicating a number, N: a natural number). Conventionally, differential encoding / decoding has been generally used as a method for preventing phase slip.

  Also, a method for avoiding phase slip without using differential encoding / decoding has been studied.

  In Patent Document 2, in estimating the phase of a carrier wave of a coherent optical receiver, the period count (cycle count) value is increased or decreased on the condition that the transition of the estimated carrier phase between symbols exceeds a positive or negative threshold value. It is disclosed that carrier phase estimation is accurately performed based on the period count value.

  In Patent Document 3, a reference signal (pilot signal) is inserted between data signals on the transmission side in coherent optical communication. On the receiving side, the carrier phase is estimated from the reference signal. For symbols having a large phase variation in the data signal, it is disclosed that the carrier phase estimated based on the reference signal is linearly interpolated to guarantee the estimation accuracy of the carrier phase.

  Patent Document 4 discloses that in coherent optical communication, a reference signal (SYNC burst) is inserted between data signals on the transmission side, and a continuous error that may occur due to phase slip is relieved by error correction. In addition, phase slip can be generated by checking whether the decoding results of the same symbol decoded in each direction are inconsistent after overlapping and collating the two-way data decoding in the time series order and the time series reverse order. Is also detected and corrected by error correction.

US 2011/0217043 A1 US 2006/0245766 A1 US 2011/0274442 A1 US 7522841 B2

Optical Internetworking Forum, “100G Ultra Long Haul DWDM Framework Document” http://www.oiforum.com/public/documents/OIF-FD-100G-DWDM-01.0.pdf, June 2009. Andrew J. Viterbi and Audrey M. Viterbi “Nonlinear Establishment of PSK—Modulated Carrier Phase with Application to Burst Digital Transmission”, Trans Inf. Theory, vol. IT-29, no. 4, pp. 543-551, 1983.

  However, according to the prior art using differential encoding / decoding, one symbol error in the transmission channel propagates to about two symbol errors, which degrades the code error rate. There was a problem of being.

  The method described in Patent Document 2 has a problem that the detection accuracy of the phase slip is insufficient, and a large-scale continuous error due to the missed phase slip is unavoidable.

  In the methods described in Patent Document 3 and Patent Document 4, it is necessary to insert a reference signal, and there is a problem in that redundancy is increased and transmission speed is increased.

  In the method described in Patent Document 4, there is a problem that a restriction is added to the coding rule of the error correction code to reduce the resistance against random errors in order to increase the resistance to consecutive errors in error correction.

  The present invention has been made in order to solve the above-described problem, an optical modulation / demodulation method capable of preventing transmission quality deterioration due to phase slip, improving transmission quality, and improving noise immunity, and The purpose is to obtain an optical transceiver.

  According to the present invention, an optical transmission unit generates an optical signal having a signal point arrangement that is asymmetrical in each of the X polarization and the Y polarization, and multiplexes them. A step of transmitting the multiplexed optical signal; a step of receiving the multiplexed optical signal by an optical receiver; and a local oscillation light that oscillates at a predetermined wavelength by the optical receiver. A step of mixing the received optical signal and the local oscillation light by the optical receiving unit to perform optical / electrical conversion, performing analog / digital conversion, and outputting a digital signal; and A step of simultaneously estimating a carrier frequency and a carrier phase of the received optical signal based on the digital signal by the receiving unit using the X polarization and the Y polarization; and an estimated carrier frequency And a step of outputting a frequency / phase compensated digital signal for each phase slip state that can be taken for each symbol based on the carrier phase, and a likelihood for each phase slip state based on the frequency / phase compensated digital signal Calculating the frequency / phase compensated digital signal based on a preset threshold value for each phase slip state, and outputting a decoding result for each phase slip state, and the phase And a step of selecting and outputting a decoding result corresponding to a phase slip state having the maximum likelihood among the decoding results for each slip state. .

  According to the present invention, an optical transmission unit generates an optical signal having a signal point arrangement that is asymmetrical in each of the X polarization and the Y polarization, and multiplexes them. A step of transmitting the multiplexed optical signal; a step of receiving the multiplexed optical signal by an optical receiver; and a local oscillation light that oscillates at a predetermined wavelength by the optical receiver. A step of mixing the received optical signal and the local oscillation light by the optical receiving unit to perform optical / electrical conversion, performing analog / digital conversion, and outputting a digital signal; and A step of simultaneously estimating a carrier frequency and a carrier phase of the received optical signal based on the digital signal by the receiving unit using the X polarization and the Y polarization; and an estimated carrier frequency And a step of outputting a frequency / phase compensated digital signal for each phase slip state that can be taken for each symbol based on the carrier phase, and a likelihood for each phase slip state based on the frequency / phase compensated digital signal Calculating the frequency / phase compensated digital signal based on a preset threshold value for each phase slip state, and outputting a decoding result for each phase slip state, and the phase And a step of selecting and outputting a decoding result corresponding to a phase slip state having the maximum likelihood among the decoding results for each slip state. Therefore, transmission quality deterioration due to phase slip can be prevented, and transmission quality can be improved and noise tolerance can be improved.

It is a figure which shows the optical transmission / reception structure concerning Embodiment 1 of this invention. It is a figure which shows polarization | polarized-light phase space signal point arrangement | positioning of a polarization multiplexing BPSK system. It is a figure which shows polarization | polarized-light phase space signal point arrangement | positioning concerning Embodiment 1 of this invention. It is a figure which shows the input / output table of the symbol mapper which implement | achieves the signal point arrangement | positioning concerning Embodiment 1 of this invention. It is a figure which shows signal point arrangement | positioning for every phase slip state concerning Embodiment 1 of this invention. It is a figure which shows the detailed functional block structural example of the decoding part concerning Embodiment 1 of this invention. It is a figure which shows the detailed functional block structural example of the phase slip state estimation part concerning Embodiment 1 of this invention. It is a figure which shows the structural example of the likelihood production | generation part in the phase slip state estimation part concerning Embodiment 1 of this invention. It is a figure which shows the simulation conditions concerning Embodiment 1 of this invention. It is a figure which shows the simulation result concerning Embodiment 1 of this invention.

  Embodiments of an optical transceiver and an optical modulation / demodulation method according to the present invention will be described below in detail with reference to the drawings. The embodiment described below is an embodiment for embodying the present invention, and is not intended to limit the present invention to its category.

Embodiment 1 FIG.
FIG. 1 is a diagram illustrating a configuration example of an optical transceiver according to Embodiment 1 of the present invention. As illustrated in FIG. 1, the optical transceiver according to the first embodiment includes an optical transmission unit 100, a transmission channel 200, and an optical reception unit 300. The optical transmission unit 100 includes a light source 101, a symbol mapper 102, and an optical modulation unit 103. The optical receiver 300 includes a light source 301, an optical demodulator 302, a waveform equalizer 303, a frequency / phase estimator 304 (X / Y combination), a decoder 305 (X / Y simultaneously), a phase slip The state estimation unit 306 and the selection unit 307 are configured. The optical transmitter 100 transmits an optical signal to a counterpart optical transceiver (not shown) via the transmission channel 200. The optical receiving unit 300 receives an optical signal transmitted from a partner optical transceiver (not shown) via the transmission channel 200. The counterpart optical transmitter / receiver is not shown in FIG. 1, but has the same configuration as the optical transmitter / receiver shown in FIG.

  Hereinafter, the optical modulation / demodulation method and the optical transceiver according to the first embodiment will be described.

  First, the optical transmission unit 100 will be described. The light source 101 generates a carrier light signal that oscillates at a predetermined wavelength set in advance, and outputs the carrier wave signal to the light modulator 103.

  The symbol mapper 102 performs signal point arrangement based on transmission data (two series of codes A and B, see FIG. 4) input from the outside (not shown), and outputs a 4-lane binary electrical signal.

  FIG. 2 is a signal point arrangement in a polarization phase space of a polarization-multiplexed BPSK (DP-BPSK: Dual-Polarization BPSK) system. The horizontal axis represents the optical phase of X polarization, which is one polarization component when orthogonal polarization multiplexing is performed, and the vertical axis is the optical phase of the Y polarization, which is the other polarization component when orthogonal polarization multiplexing is performed. Indicates. Ordinary BPSK is represented by binary values of phase 0 and phase π. In FIG. 2, these are indicated by phase π / 4 and phase −3π / 4 obtained by rotating them by π / 4. Since each of the X polarization and the Y polarization takes a binary phase, four signal points (symbols) 0, 1, 2, and 3 can be taken in FIG. The arrangement of these signal points is symmetrical (point symmetry, line symmetry) between the X polarization and the Y polarization.

  FIG. 3 is a signal point arrangement in the polarization phase space according to the first embodiment of the present invention. As shown in FIG. 3, the signal point arrangement is an asymmetric arrangement between the X polarization and the Y polarization, and the signal point arrangement differs between the X polarization and the Y polarization. Specifically, as shown in FIG. 3, in DP-BPSK, only when the phase of X polarization is −3π / 4 (signal points 1 and 3), from the signal point arrangement of FIG. Shift the phase by + π / 2. When the phase of the X polarization is π / 4 (signal points 0 and 2), the phase of the Y polarization is the same as the signal point arrangement in FIG. As a result, the arrangement of the signal points 0, 1, 2, and 3 is neither point-symmetric nor line-symmetric. As described above, in the first embodiment, the signal point arrangement of both X / Y polarization is binary phase shift keying, and one side phase of one binary phase shift keying of X / Y polarization is performed. Only in this case, the other signal point of the X / Y polarization is shifted by π / 2. Thus, in the first embodiment, the arrangement of the signal points is asymmetric between the X polarization and the Y polarization.

The input / output relationship of the signal point arrangement according to Embodiment 1 of the present invention is shown in FIG. In the optical transmitter 100, the symbol mapper 102 receives two series of codes A and B as transmission data from the outside (not shown), and is a 4-lane binary (+/−) electrical signal XI, XQ. , YI, YQ are generated and output to the light modulation unit 103. The Y polarization is apparently QPSK, but the QPSK side is not gray-coded, as can be seen from FIG. 3, between 0-1 and 0-3 rather than between symbols 0-2 and 1-3. Since the polarization phase space distance is longer between 2-1 and 2-1 and 2-3, this is because the hamming distance on the far side is increased. However, the scope of the present invention is not limited to this encoding rule. This processing can be realized by table processing in which A and B are input and XI, XQ, YI, and YQ are output. Alternatively, it can be obtained by the following logical operation.
XI = NOT {B}
XQ = NOT {B}
YI = EXOR {B}
YQ = NOT {A}

  As shown in FIG. 4, in the symbol 0, the code A is “0”, the code B is “0”, the X phase is π / 4, and the Y phase is π / 4. At this time, the symbol mapper 102 generates a four-lane binary signal with XI “+”, XQ “+”, YI “+”, and YQ “+”. The same applies to the symbols 1, 2, and 3, and the codes A and B shown in FIG. 4 are inputted, respectively, and based on them, a binary signal of 4 lanes shown in FIG. 4 is generated.

  The optical modulation unit 103 modulates the carrier optical signal input from the light source 101 with the 4-lane binary electrical signals (XI, XQ, YI, YQ) input from the symbol mapper 102, multiplexes them, and FIG. An optical signal having the signal point arrangement shown in FIG. A signal (binary electrical signal) input from the symbol mapper 102 is generally amplified by a driver (not shown) to drive the light modulator 103, but the driver is omitted in the configuration of FIG. Yes. The optical modulation unit 103 divides the carrier wave input from the light source 101 into two systems, respectively performs quadrature phase (I / Q) modulation with the binary electric signal from the symbol mapper 102, and performs orthogonal polarization multiplexing. Further, the transmission unit 100 may be provided with a digital / analog conversion unit (not shown) to perform digital signal processing such as predistortion of waveform distortion at the transmission end.

  The transmission channel 200 transmits an optical signal input from the optical modulation unit 103 in the optical transmission unit 100 and outputs the optical signal to the optical reception unit 300 of the counterpart optical transceiver. The transmission channel 200 includes devices and components generally used for transmitting optical signals, such as a wavelength multiplexing / demultiplexing device, an optical amplifying device, and a transmission line optical fiber.

  Next, the optical receiver 300 will be described. In the optical receiving unit 300, the light source 301 generates a local oscillation optical signal that oscillates at a predetermined wavelength set in advance, and outputs the local oscillation optical signal to the optical demodulation unit 302. In general, the local oscillation optical signal is oscillated at substantially the same wavelength as the carrier optical signal generated by the light source 101.

  The optical demodulator 302 decomposes the optical signal transmitted via the transmission channel 200 and the local oscillation optical signal input from the light source 301 into orthogonal polarization components and orthogonal phase components. Next, the optical demodulator 302 performs coherent detection that mixes the decomposed optical signal and the local oscillation optical signal, performs photoelectric conversion, and converts the signal into a four-lane electrical signal. Further, these electric signals are converted into digital signals by analog / digital conversion, and quantized and sampled. The 4-lane digital signal thus obtained is output to the waveform equalization unit 303. In the analog-digital conversion, quantization is usually performed with a resolution of 6 bits or more, and sampling is performed at a sampling rate that is twice or more the baud rate.

  The waveform equalization unit 303 receives a 4-lane digital signal from the optical demodulation unit 302 and generates waveform distortion caused by chromatic dispersion, polarization rotation, polarization mode dispersion, fiber nonlinear optical effect, or the like generated in the transmission channel 200. And a 4-lane digital signal separated by orthogonal polarization is output to the frequency / phase estimator 304.

  The frequency / phase estimation unit 304 receives 4-lane digital signals from the waveform equalization unit 303 and combines them between orthogonal polarizations (between X / Y polarizations) to estimate and compensate for the carrier frequency / phase. . That is, the frequency / phase estimation unit 304 compensates for the center frequency difference between the carrier optical signal and the local oscillation optical signal, which is present in the 4-lane digital signal input from the waveform equalization unit 303, and the M multiplication method. Also, carrier frequency estimation and carrier phase estimation are performed based on the provisional determination method and the like, and the 4-lane digital signal after frequency / phase compensation is output to the decoding unit 305 and the phase slip state estimation unit 306. Here, the carrier frequency estimation and the carrier phase estimation are not performed independently between the orthogonal polarizations, but are performed simultaneously by combining between the orthogonal polarizations (between the X / Y polarizations).

When carrier frequency estimation and carrier phase estimation are performed simultaneously with X / Y polarization, the phase slip state includes (1) a state in which the phase is not slipped in both X / Y polarization, and (2) X / Y polarization. There are a total of four states: a state where both slip +90 degrees, a state where (3) both X / Y polarized waves slip 180 degrees, and a state where both (4) X / Y polarized waves slip -90 degrees. If the carrier frequency and phase are estimated independently for X / Y polarization, slips will occur independently for each polarization. Therefore, the slip does not fit in these four ways, but the square of 4 (4 ² = 16), and phase slip detection described later becomes difficult.

  FIG. 5 shows signal point arrangements after frequency / phase compensation for four types of phase slip states obtained by combining with X / Y polarization and simultaneously performing carrier frequency / phase estimation. . In FIG. 5, the phase slip state State-0 is when there is no phase slip, the phase slip state State-1 is when the phase slip +90 degrees, the phase slip state State-2 is when the phase slip +180 degrees, and the phase slip state State- 3 is the case of phase slip -90 degrees. It can be seen that the signal points do not overlap between the phase slip states, and the signal point arrangement is unique for each phase slip state. In addition, since these signal point arrangements are signal point arrangements that cannot be taken when there is no phase slip, it is possible to detect and compensate for a phase slip state. Symbol Z ′ (Z ′ = 0 ′, 1 ′, 2 ′, 3 ′) in State-1 is obtained by slipping +90 degrees on symbol Z (Z = 0, 1, 2, 3) in State-0. . A symbol Z ″ (Z ″ = 0 ″, 1 ″, 2 ″, 3 ″) in State-2 is obtained by slipping the symbol Z by 180 degrees. The symbol Z ″ ″ (Z ″ ″ = 0 ″ ″, 1 ″ ″, 2 ″ ″, 3 ″ ″) in State-3 is obtained by slipping the symbol Z by −90 degrees.

  Decoding section 305 performs decoding for all possible phase slip states that can be taken for each symbol. That is, the decoding unit 305 decodes the 4-lane digital signal input from the frequency / phase estimation unit 304 based on the threshold value indicated by the dotted line in the signal point arrangement of each State in FIG. Is output to the selection unit 307.

  FIG. 6 illustrates a configuration example of the decoding unit 305. The decoding unit 305 includes four two-dimensional phase identifiers (2-D Phase Slicers) 500, 501, 502, and 503. The 4-lane digital signals input from the frequency / phase estimation unit 304 are equally input to the two-dimensional phase identifiers 500, 501, 502, and 503. The two-dimensional phase discriminator 500 performs decoding for each 4-lane digital signal corresponding to State-0, and outputs the decoding result to the selection unit 307 (see FIG. 1). The two-dimensional phase identifier 501 performs decoding for each signal of the 4-lane digital signal corresponding to State-1, and outputs the decoding result to the selection unit 307. The two-dimensional phase discriminator 502 performs decoding for each signal of the 4-lane digital signal corresponding to State-2, and outputs the decoding result to the selection unit 307. The two-dimensional phase discriminator 503 performs decoding for each 4-lane digital signal corresponding to State-3, and outputs the decoding result to the selection unit 307. Symbols Z ′, Z ″, and Z ″ ″ are decoded as symbols Z, respectively. In the decoding, not only a hard decision but also a soft value may be passed to the selection unit 307 as a soft decision to give reliability information according to the distance from the signal point center in the polarization phase space.

  The phase slip state estimation unit 306 calculates likelihoods for all possible phase slip states for each signal (symbol), compares them, and estimates which phase slip state has the maximum likelihood. That is, the phase slip state estimation unit 306 estimates the maximum likelihood phase slip state based on the 4-lane digital signal input from the frequency / phase estimation unit 304, and outputs it to the selection unit 307.

  FIG. 7 illustrates a configuration example of the phase slip state estimation unit 306. The phase slip state estimation unit 306 includes four likelihood generation units (metric generators) 600, 601, 602, and 603 and one maximum likelihood state estimation unit (maximum state state estimator) 610. The 4-lane digital signals input from the frequency / phase estimation unit 304 (see FIG. 1) are equally output to the likelihood generation units 600, 601, 602, and 603. The likelihood generation unit 600 calculates the likelihood that the phase slip state is State-0, and outputs the likelihood information to the maximum likelihood state estimation unit 610. The likelihood generation unit 601 calculates the likelihood that the phase slip state is State-1, and outputs the likelihood information to the maximum likelihood state estimation unit 610. The likelihood generation unit 602 calculates the likelihood that the phase slip state is State-2, and outputs the likelihood information to the maximum likelihood state estimation unit 610. The likelihood generation unit 603 calculates the likelihood that the phase slip state is State-3, and outputs the likelihood information to the maximum likelihood state estimation unit 610. The maximum likelihood state estimation unit 610 is based on the likelihood information of the phase slip states State-0, State-1, State-2, and State-3 input from the likelihood generation units 600, 601, 602, and 603. The most likely phase slip state is determined from the slip states State-0, State-1, State-2, and State-3, and is output to the selection unit 307 (not shown in FIG. 7).

  FIG. 8 illustrates a configuration example of the likelihood generation units 600, 601, 602, and 603 in the phase slip state estimation unit 306. Since the likelihood generation units 600, 601, 602, and 603 each have the same functional block, the case of the likelihood generation unit 600 that calculates the likelihood of State-0 will be described here.

  The likelihood generation unit 600 includes four polarization phase distance calculation units 700, 701, 702, and 703, one selection unit 710, n delay units 720A, 720B,. , 721X and one adder (sum) 722. The multipliers 721A, 721B,. The delay units 720A, 720B,..., 720X are connected in series as shown in FIG. The multipliers 721A, 721B,..., 721X are connected to delay units 720A, 720B,. The 4-lane digital signals input from the frequency / phase estimation unit 304 are equally output to the polarization phase distance calculation units 700, 701, 702, and 703.

The polarization phase space distance calculation unit 700 applies the polarization phase from the central signal point 0 to the input 4-lane digital signal based on the State-0 signal point arrangement rule where there is no phase slip. Find the spatial distance. For example, the sum of squares of the phase distance D_ph_x (0) in the X polarization direction and the phase distance D_ph_y (0) in the Y polarization direction ((D_ph_x (0)) ² + (D_ph_y (0)) ² ) is calculated. The result is output to the selection unit 710. The polarization phase space distance calculation unit 701 obtains the polarization phase space distance from the signal point 1 based on the signal point arrangement rule of State-0, which is in a state without phase slip, for the 4-lane digital signal. For example, the sum of squares ((D_ph_x (1)) ² + (D_ph_y (1)) ² ) of the phase distance D_ph_x (1) in the X polarization direction and the phase distance D_ph_y (1) in the Y polarization direction is calculated. The result is output to the selection unit 710. The polarization phase space distance calculation unit 702 obtains the polarization phase space distance from the signal point 2 based on the signal point arrangement rule of State-0 that is in a state where there is no phase slip for the 4-lane digital signal. For example, the sum of squares ((D_ph_x (2)) ² + (D_ph_y (2)) ² ) of the phase distance D_ph_x (2) in the X polarization direction and the phase distance D_ph_y (2) in the Y polarization direction is calculated. The result is output to the selection unit 710. The polarization phase space distance calculation unit 703 obtains the polarization phase space distance from the signal point 3 based on the signal point arrangement rule of State-0, which is in a state without phase slip, for the 4-lane digital signal. For example, the sum of squares ((D_ph_x (3)) ² + (D_ph_y (3)) ² ) of the phase distance D_ph_x (3) in the X polarization direction and the phase distance D_ph_y (3) in the Y polarization direction is calculated. The result is output to the selection unit 710.

  The selection unit 710 selects the minimum polarization phase space distance from the four polarization phase space distances input from the polarization phase space distance calculation units 700, 701, 702, and 703, and outputs the selected polarization phase space distance to the delay unit 720A. To do.

  The delay unit 720A holds the digital signal input from the selection unit 710 for one symbol time, and outputs the digital signal to the delay unit 720B and the multiplication unit 721A.

  Multiplier 721A multiplies the digital signal input from delay unit 720A by the coefficient C [0], and outputs the result to adder 722.

  The delay unit 720B holds the digital signal input from the delay unit 720A for one symbol time, and outputs the digital signal to the delay unit 720C (not shown) and the multiplication unit 721B connected in series to the delay unit 720B.

  Multiplier 721B outputs the result of multiplying the digital signal input from delay unit 720B by the coefficient C [1] to adder 722.

  Thereafter, the same processing is repeated, and the n-th delay unit 720X holds the digital signal input from the (n−1) -th delay unit 720W (not shown) for one symbol time, and outputs the digital signal to the multiplication unit 721X. .

  The multiplier 721X outputs a result obtained by multiplying the digital signal input from the delay unit 720X by the coefficient C [n−1] to the adder 722.

  The adder 722 calculates the sum of the n multiplication results input from the multipliers 721A, 721B,..., 721X, and outputs the sum to the maximum likelihood state estimator 610 (see FIG. 7). Here, it should be noted that the smaller the sum value, the higher the likelihood.

  The delay units 720A, 720B,..., 720X, the multipliers 721A, 721B,..., 721X, and the adder 722 constitute a finite impulse response filter. The instantaneous likelihood at the time is averaged. Therefore, delay units 720A, 720B,..., 720X, multiplication units 721A, 721B,..., 721X, and addition unit 722 constitute an averaging unit that averages the signal output from selection unit 710. doing. The filter constituting the averaging unit is not limited to a finite impulse response filter, and a forgetting infinite impulse response filter may be used. In that case, maximum likelihood sequence estimation and posterior probability maximization can be performed more complicatedly.

  The likelihood generation units 601, 602, and 603 also perform the same processing as the processing in the likelihood generation unit 600. Here, the description is omitted.

  In this way, four total values are input to the maximum likelihood state estimation unit 610 from the likelihood generation units 600, 601, 602, and 603. The maximum likelihood state estimation unit 610 includes a total sum of phase slip states State-0, State-1, State-2, and State-3 input from the likelihood generation units 600, 601, 602, and 603. The phase slip state having the smallest sum value is obtained as the “maximum likelihood phase slip state” and is output to the selection unit 307 (see FIG. 1).

  The selection unit 307 compares the four decoding results input from the decoding unit 305 based on the phase slip state estimated by the phase slip state estimation unit 306, and corresponds to the phase slip state from those decoding results. The decoding result to be selected is selected as the maximum likelihood decoding result, and is output to an external device (not shown) connected to the optical transceiver.

  A simulation of the code error rate characteristics when this embodiment is used was performed. Simulation conditions are shown in FIG. The bit rate was 128 Gb / s, and the 11-stage pseudo-random binary sequence was used as the code sequence. The number of symbols was 16384, and the calculation was repeated with the noise pattern changed 100 times. The transmission line was an additive white Gaussian noise (AWGN) channel, and the noise bandwidth when defining the optical signal power to noise power ratio was 0.1 nm. The phase noise between the carrier optical signal and the local oscillation optical signal was 0, and the frequency difference between the carrier optical signal and the local oscillation optical signal was 0. Polarization demultiplexing is ideally performed. For the carrier phase estimation, the fourth power method was used, the phase errors detected in each polarization were averaged, and equal phase compensation was performed for each polarization. The averaging of the carrier phase estimation was a 17-symbol moving average, and an unwrapping process was performed so that the phase change from the carrier phase one symbol before was within ± π / 4. The averaging of the phase slip state estimation was performed by a 17 symbol moving average. After obtaining the code error rate, it was converted into a Q value through a complementary error function and evaluated.

  FIG. 10 shows the simulation result. For comparison, a DP-DE (Differential Encoded) QPSK signal in which differential encoding / decoding is applied to the DP-QPSK method and a case in which there is no phase slip in the DP-QPSK signal (No Slip) were also calculated. In FIG. 10, the horizontal axis is OSNR, and the vertical axis is Q value (Q-factor). In the present embodiment, as shown in FIG. 10, in the region where the Q value is 9 dB or more, a Q value equivalent to that without slip is obtained, and good characteristics are exhibited. In the region where the Q value is 7 dB or less, an error occurs in the phase slip estimation according to the present embodiment. Therefore, the Q value of the present embodiment is lower than the Q value when there is no phase slip, but the DP-DEQPSK signal However, the Q value still shows an excellent Q value of 0.5 dB or more.

  As described above, in the present embodiment, signal point arrangement that is asymmetrical between orthogonally polarized waves as shown in FIG. 3 is performed in optical transmitter 100, and carrier frequency / phase estimation is performed between orthogonally polarized waves in receiver 300. The phase slip can be detected by carrying out simultaneously. Thereby, the phase slip state can be estimated, and the most likely slip state can be estimated. In addition, phase slip compensation can be performed by selecting a decoding result based on a decoding rule corresponding to the estimated slip state. Thus, according to the present embodiment, when a phase slip occurs, the received signal point has a unique arrangement according to the phase slip state, and in the case of no phase slip, the arrangement of signal points that cannot be taken It becomes. Thereby, the phase slip state can be detected and the phase slip compensation can be performed. Therefore, the phase slip state can be estimated on a signal basis without using differential encoding / decoding and without increasing the redundancy. Also, decoding according to the estimated slip state can be performed. As a result, transmission quality deterioration due to phase slip can be avoided, noise tolerance can be improved, and transmission quality can be improved.

Industrial applicability

  As described above, the optical modulation / demodulation method according to the present invention is useful for an optical transmission system using a digital coherent method, and is particularly suitable for an optical transmission system with a low code error rate.

  DESCRIPTION OF SYMBOLS 100 Optical transmission part, 101 Light source, 102 Symbol mapper, 103 Optical modulation part, 200 Transmission channel, 300 Optical receiving part, 301 Light source, 302 Optical demodulation part, 303 Waveform equalization part, 304 Frequency / phase estimation part, 305 Decoding part 306, phase slip state estimation unit, 307 selection unit, 500, 501, 502, 503 two-dimensional phase identifier, 600, 601, 602, 603 likelihood generation unit, 610 maximum likelihood state estimation unit, 700, 701, 702 703 Polarization phase space distance calculation unit, 710 selection unit, 720A, 720B, 720C, 720X delay unit, 721A, 721B, 721X multiplication unit, 722 addition unit.

Claims (8)

A step of generating an optical signal having a signal point arrangement that is asymmetrical in each of the orthogonal polarizations of the X polarization and the Y polarization by the optical transmission unit, and multiplexing them;
Transmitting the multiplexed optical signal by the optical transmitter; and
Receiving the multiplexed optical signal by an optical receiver;
Generating local oscillation light that oscillates at a predetermined wavelength by the optical receiver;
The optical receiving unit mixes the received optical signal and the local oscillation light to perform optical / electrical conversion, performs analog / digital conversion, and outputs a digital signal;
Estimating the carrier frequency and the carrier phase of the received optical signal simultaneously with the X-polarized wave and the Y-polarized wave by the optical receiver based on the digital signal;
Outputting a digital signal after frequency / phase compensation for each phase slip state that can be taken for each symbol based on the estimated carrier frequency and carrier phase;
Based on the frequency / phase compensated digital signal, calculating a likelihood for each phase slip state;
Decoding the frequency / phase compensated digital signal based on a preset threshold value for each phase slip state, and outputting a decoding result for each phase slip state;
An optical modulation / demodulation method comprising: selecting a decoding result corresponding to a phase slip state having a maximum likelihood from the decoding results for each phase slip state; . The signal point arrangement of both the X polarization and the Y polarization is binary phase shift keying,
The other signal point of the X polarization and the Y polarization is shifted by π / 2 only in the case of one side phase of the binary phase shift keying of one of the X polarization and the Y polarization. The optical modulation / demodulation method according to claim 1. An optical transceiver including an optical transmitter and an optical receiver,
The optical transmitter is
A light source that outputs an optical signal;
A symbol mapper that outputs an electrical signal for orthogonal polarization multiplexing based on code data input from the outside;
Light having a signal point arrangement in which an optical signal output from the light source is modulated with an electric signal output from the symbol mapper and orthogonally polarized, and is asymmetrical in each of the orthogonally polarized waves of X polarization and Y polarization An optical modulation unit for generating a signal,
The optical receiver is
A local oscillation light source that outputs a local oscillation light signal;
An optical demodulator that receives an optical signal, mixes the received optical signal and the local oscillation optical signal to perform optical / electrical conversion, performs analog / digital conversion, and outputs a digital signal;
A waveform equalizer for compensating for waveform distortion of the digital signal;
A carrier frequency / phase estimator for simultaneously estimating and compensating the carrier frequency and the carrier phase of the received optical signal with the X polarization and the Y polarization based on the digital signal by the optical receiver;
Decoding the frequency / phase compensated digital signal based on a preset threshold for each phase slip state, and outputting a decoding result for each phase slip state;
A phase slip state estimator that calculates a likelihood for each phase slip state based on the frequency / phase compensated digital signal and compares the likelihoods to estimate a maximum likelihood phase slip state; ,
A selection unit that selects and outputs a decoding result corresponding to the phase slip state of the maximum likelihood from decoding results for each phase slip state based on the phase slip state estimated by the phase slip state estimation unit; An optical transceiver characterized by comprising. The symbol mapper is
Two series of binary electrical signal series are input,
The four-sequence binary electric signal to be output is such that the signal point arrangement of both of the X / Y polarizations is binary phase shift keying, and one of the X / Y polarizations is the binary phase shift keying. 4. The optical transceiver according to claim 3, wherein the other signal point of the X / Y polarized wave is shifted by π / 2 only in the case of one side phase. The decoding unit
When there is no phase slip in the X polarization and the Y polarization,
When the phase slip is both +90 degrees in the X polarization and the Y polarization,
When the phase slip is both +180 degrees in the X polarization and the Y polarization, and
Decoding is performed for each phase slip state when the phase slip is -90 degrees in both the X polarization and the Y polarization, and a total of four decoding results are obtained, one for each phase slip state. The optical transceiver according to claim 3 or 4, The phase slip state estimator is
When there is no phase slip in the X polarization and the Y polarization,
When the phase slip is both +90 degrees in the X polarization and the Y polarization,
When the phase slip is both +180 degrees in the X polarization and the Y polarization, and
A likelihood generation unit that calculates the likelihood that each of the X-polarized wave and the Y-polarized wave is in a phase slip state when both phase slips are −90 degrees;
The optical transceiver according to any one of claims 3 to 5, wherein the phase slip state of the maximum likelihood is estimated based on the likelihood. The likelihood generator is
A polarization phase space distance calculation unit for calculating a polarization phase space distance from the symbol center in the polarization phase space;
A selector for selecting a minimum value of the polarization phase space distance;
The optical transceiver according to claim 6, further comprising: an averaging unit that averages a signal output from the selection unit. The optical transceiver according to claim 7, wherein the averaging unit includes a finite impulse response filter or a forgetting type infinite impulse response filter.

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