Optical transmission device, control circuit, and program storage medium

Abstract

A transmission-side optical transmission device (1) comprises: a transmission sorting unit (13) that sorts a plurality of signals in a case where transmission signals are grouped into signals corresponding to each of a plurality of digital subcarriers; and an LDPC encoding unit (14) that uses a low-density parity-check code to encode the plurality of signals sorted by the transmission sorting unit (13). In a case where the plurality of digital subcarriers are arranged in a transmission line, the transmission sorting unit (13) assigns a signal corresponding to a digital subcarrier at a relatively large distance from a center frequency to an encoding position in which the column weight of a check matrix of the low-density parity-check code is relatively large.

Description

Optical transmission device, control circuit, and program storage medium

[0001] The present disclosure relates to an optical transmission device, a control circuit, and a program storage medium.

[0002] In backbone optical networks, high-capacity transmissions exceeding 100 Gbps are being carried out over a single optical fiber. One of the technologies supporting high-capacity transmissions is related to the broadening of optical signal bandwidth. For example, in Non-Patent Document 1, OpenROADM defines a wideband optical signal interface ranging from 31.57 Gbaud QPSK (100 Gbps) to 131 Gbaud PCS-16QAM (800 Gbps). With such wideband optical signals, EEPN (Equalization-Enhanced Phase Noise) is a major issue. Note that QPSK is an abbreviation for "Quadrature Phase Shift Keying," PCS is an abbreviation for "Probabilistic Constellation Shaping," and QAM is an abbreviation for "Quadrature Amplitude Modulation."

[0003] In coherent transmission systems, instantaneous frequency fluctuations caused by phase noise in the transmitting and receiving light sources are amplified by chromatic dispersion compensation using digital signal processing. In recent years, the application of digital subcarriers has been considered as a solution to this problem. Digital subcarriers are a technology that electrically generates wavelength-multiplexed signals using digital signal processing, transmits the wavelength-multiplexed signals on a single optical carrier, and then receives them. For example, by transmitting four digital subcarrier signals for a wide optical signal bandwidth of 131 GHz, narrowing the bandwidth per subcarrier to 32.75 GHz, the effects of EEPN can be reduced, thereby enabling the transmission distance of optical signals to be extended.

[0004] In digital subcarrier multiplexed signals, a technique for adjusting the amount of information for each subcarrier to increase communication capacity is often used in the field of wireless communications, where an Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme is used, which aggregates orthogonal frequency subcarriers. For example, a technique is used in which a high-order modulation scheme is mapped to subcarriers in which errors are unlikely to occur, and a low-order modulation scheme is mapped to subcarriers in which errors are likely to occur, thereby equalizing the error rate. A technique is known in which an error-correcting code with high correction performance is applied to subcarriers (bits) in which errors are likely to occur. Patent Document 1 discloses a technique in which, for an output signal of an irregular Low Density Parity Check (LDPC) code having high error correction performance, coded bit sequences are sorted according to the column weights of the LDPC code's check matrix, and the sorted signals are mapped in descending order of the number of transmission path errors.

[0005] Patent No. 3875693

[0006] OpenROADM, retrieved March 4, 2024, Internet <URL: http: / / openroadm.org / >

[0007] Prior art has proposed a method of sorting encoded code bits and switching mapping depending on the susceptibility of errors in the transmission path for each subcarrier and time series. This method requires sequential monitoring of the transmission path and switching mapping according to the monitoring results. In today's optical communications, the transmission capacity per carrier (wavelength) exceeds 800 Gbps, and the code length of the applied LDPC code is long. Considering this, the need for a sorting circuit raises concerns about an increase in circuit size. The same process, i.e., reverse sorting, must be applied on the receiving side. However, since one bit of information is represented by several bits on the receiving side using soft-decision processing, the impact on circuit size is significant.

[0008] Furthermore, in recent optical communication systems, a technology called PCS is applied before error correction coding or after error correction decoding, which controls the probability of occurrence of each symbol in multi-level modulation, such as 16QAM or 32QAM. This technology increases the probability of occurrence of the center symbol by limiting the amount of information in the basic QAM signal, for example, by limiting the number of bits per channel from 2 bits in 16QAM to 1.5 bits. The means for implementing this technology are typically placed before error correction coding or after error correction decoding. Conventional technology only mentions processing of encoded bit strings, but this does not allow this technology to be utilized.

[0009] The present disclosure has been made in view of the above, and aims to provide an optical transmission device that can improve error tolerance in an optical fiber communication system that uses irregular LDPC codes and digital subcarriers with a simple configuration.

[0010] In order to solve the above-mentioned problems and achieve the object, an optical transmission device according to the present disclosure includes a transmission reordering unit that reorders a plurality of signals when a transmission signal is grouped into signals corresponding to each of a plurality of digital subcarriers, and a low-density parity check encoding unit that encodes the plurality of signals reordered by the transmission reordering unit using a low-density parity check code. When the plurality of digital subcarriers are arranged on a transmission path, the transmission reordering unit assigns signals corresponding to digital subcarriers that are relatively far from a center frequency to encoding positions with relatively large column weights in a check matrix of the low-density parity check code.

[0011] The optical transmission device according to the present disclosure has an effect of being able to improve the error tolerance in an optical fiber communication system that uses irregular LDPC codes and digital subcarriers with a simple configuration.

[0012] 1. A block diagram showing the configuration of a transmitting-side optical transmission device according to a first embodiment. 2. A block diagram showing the configuration of a receiving-side optical transmission device according to the first embodiment. 3. A diagram showing an example of a check matrix of an LDPC code. 4. A diagram showing an example of a Tanner graph. 5. A diagram showing an example of a transmitted optical signal. 6. A diagram showing an example of degradation of a transmitted optical signal. 7. A diagram showing an example of a column weight distribution with respect to the position number of a variable node. 8. A diagram for explaining the operation of a transmission rearrangement unit included in a transmitting-side optical transmission device according to the first embodiment. 9. A diagram showing that a subcarrier signal with greater characteristic degradation is assigned to a subcarrier with a larger column weight. 10. A diagram for explaining the operation of a transmission de-arrangement unit included in a transmitting-side optical transmission device according to the first embodiment. FIG. 1 is a diagram for explaining the operation of a transmission reordering unit included in an optical transmission device. FIG. 2 is a diagram illustrating a processor in a case where at least some functions of N PCS coding units, a hard-decision error correction coding unit, a transmission reordering unit, an LDPC coding unit, a transmission deordering unit, N interleavers, a transmission signal processing unit, and an optical transmitter included in a transmission-side optical transmission device according to embodiment 1 are realized by the processor. FIG. 3 is a diagram illustrating a processing circuit in a case where at least some functions of N PCS coding units, a hard-decision error correction coding unit, a transmission reordering unit, an LDPC coding unit, a transmission deordering unit, N interleavers, a transmission signal processing unit, and an optical transmitter included in a transmission-side optical transmission device according to embodiment 1 are realized by the processing circuit. FIG. 4 is a diagram illustrating a control circuit for controlling the operation performed by the transmission-side optical transmission device according to embodiment 1. FIG. 5 is a diagram illustrating a program storage medium storing a program for controlling the transmission-side optical transmission device 1 according to embodiment 1.

[0013] An optical transmission device, a control circuit, and a program storage medium according to an embodiment will be described in detail below with reference to the accompanying drawings.

[0014] First Embodiment. FIG. 1 is a block diagram showing the configuration of a transmitting-side optical transmission device 1 according to a first embodiment. FIG. 2 is a block diagram showing the configuration of a receiving-side optical transmission device 2 according to the first embodiment. Hereinafter, the transmitting-side optical transmission device 1 may be simply referred to as the "optical transmission device 1," and the receiving-side optical transmission device 2 may be simply referred to as the "optical transmission device 2." The optical transmission device 1 converts a transmission signal into an electrical signal for each digital subcarrier, and then converts the electrical signal into an optical signal. The optical signal obtained by the optical transmission device 1 is input to the optical transmission device 2 shown in FIG. 2 via an optical fiber transmission line. The optical fiber transmission line is not shown. The optical transmission device 2 shown in FIG. 2 receives an optical signal, converts the optical signal into digital data, and then performs signal processing to extract the transmitted data. In FIG. 2, the transmitted data is a received signal.

[0015] 1, the optical transmission device 1 on the transmitting side includes N PCS encoders, a hard-decision error correction encoder 12, a transmission reordering unit 13, an LDPC encoder 14, a transmission deordering unit 15, N interleavers, a transmission signal processor 17, and an optical transmitter 18. As shown in Fig. 2, the optical transmission device 2 on the receiving side includes an optical receiver 21, a reception signal processor 22, N deinterleavers, a reception reordering unit 24, an LDPC soft-decision decoder 25, a reception deordering unit 26, a hard-decision error correction decoder 27, and N PCS decoders. The functions of the components of the optical transmission device 1 and the optical transmission device 2 will be described when describing the operation of each of the optical transmission device 1 and the optical transmission device 2.

[0016] The operation of a transmitting optical transmission device 1 according to the first embodiment will be described with reference to FIG. 1 . In the optical transmission device 1, a transmission signal undergoes probability shaping processing by N PCS encoders, where N is a natural number equal to or greater than 3. The N PCS encoders are PCS encoder 11A, PCS encoder 11B, ..., PCS encoder 11N. Each of the N PCS encoders modulates the input signal and performs distribution matching, converting the modulated symbol generation probability into a distribution suitable for the communication path. The modulation is mapping. There are several methods for performing distribution matching, including a computational implementation method and a method using a look-up table (LUT), and any method may be employed. Because each of the N PCS encoders may control the amount of information for each subcarrier using a different modulation scheme, FIG. 1 shows each subcarrier as a separate block to distinguish it from other PCS encoders.

[0017] The signals output from each of the N PCS encoders are coded by a hard-decision error correction encoder 12 using a hard-decision error correction code (HD-FEC: Hard Decision-Forward Error Correction), then processed by a transmission rearrangement unit 13, and then coded by an LDPC encoder 14. Various codes such as a BCH (Bose Chaudhuri Hocquenghem) code or an RS (Reed-Solomon) code can be used as the hard-decision error correction code, and any code may be employed.

[0018] The signals that have been subjected to hard-decision error correction coding processing by the hard-decision error correction coding unit 12 are rearranged by the transmission rearrangement unit 13 into a form suitable for the subsequent LDPC coding unit 14. The purpose and means of this rearrangement will be described later.

[0019] The signal output from the transmission rearrangement unit 13 is subjected to encoding processing by the LDPC encoding unit 14. The LDPC encoding is performed based on a generator matrix G. The generator matrix G is obtained as a matrix that satisfies H×G=0 when multiplied by a check matrix H used in the correction processing of the LDPC code. The check matrix is ​​a parity check matrix. The LDPC encoding processing is performed by multiplying the data to be transmitted by the generator matrix G. The LDPC code in the first embodiment is an irregular LDPC code.

[0020] An irregular LDPC code will be explained with reference to FIG. 3. FIG. 3 is a diagram showing an example of a check matrix H of an LDPC code. The check matrix H is not only related to the generator matrix G described above, but also has a structure of H×c for a code c. Ｔ = 0. An LDPC code in which the column weights of all columns of the check matrix H are the same is a regular LDPC code, whereas an LDPC code in which the column weights are not all the same is an irregular LDPC code. The column weight means the number of 1s in a column. Regular LDPC codes are characterized by a low error floor region, while irregular LDPC codes are characterized by good performance in the waterfall region. Therefore, for error correcting codes that require high correction performance, such as those used in optical communication networks, a concatenated code of an irregular LDPC code and a hard-decision error correcting code that eliminates the error floor is often used.

[0021] In the check matrix H of an LDPC code, the column direction corresponds to the code bits, and the row direction corresponds to the parity bits. A decoding method is used for the decoding process, utilizing the connection relationship of a graph called a Tanner graph, as shown in FIG. 4, to update the reliability of a received signal. FIG. 4 is a diagram showing an example of a Tanner graph. The upper part of the Tanner graph corresponds to the columns of the check matrix, and is called a bit node, which corresponds to the coded bits. The lower part of the Tanner graph corresponds to the rows of the check matrix, and is called a check node. Connections are provided between the bit nodes and the check nodes, corresponding to 1 in the check matrix, and reliability update calculations are performed on these connections. The degree of a variable node corresponds to a column weight, and generally, the heavier this weight, the more it can be influenced by other nodes, thereby enabling higher error correction performance.

[0022] The signals output from the LDPC encoder 14 are rearranged by the transmission de-arrangement unit 15 so as to be distributed for each subcarrier.

[0023] The signal output from the transmission de-arrangement unit 15 is interleaved for each subcarrier by an interleaver unit, and then sent to the transmission signal processing unit 17. N interleaver units are shown in Fig. 1. The N interleaver units are interleaver unit 16A, interleaver unit 16B, ..., interleaver unit 16N.

[0024] The transmission signal processing unit 17 has a multiplexer (MUX) function for subcarrier signals, a frame function for inserting pilots and the like, a mapping function for mapping to a multilevel signal, a pre-equalization function for compensating in advance for the influence of the transmission path, and a digital to analog converter (DAC) function for converting digital data into an analog electrical signal, etc. The electrical signal output from the transmission signal processing unit 17 is converted into an optical signal by the optical transmitter 18.

[0025] Fig. 5 is a diagram showing an example of a transmitted optical signal. Fig. 5 shows an example of an optical signal composed of eight digital subcarriers. In the example of Fig. 5, there are eight digital subcarrier data #1 to #8 ranging from the low-frequency side to the high-frequency side, and the data of these eight digital subcarriers is transmitted on a single transmission carrier. The transmitted optical signal is generally subjected to equalization processing so that the performance of each subcarrier is uniform, and as shown in Fig. 5, the signal is one in which the power of each subcarrier is uniform.

[0026] The optical signal emitted from the optical transmitter 18 reaches the receiving optical transmission device 2 via a transmission path made up of optical fiber. In the transmission path, the optical signal passes through relay nodes made up of optical fiber, at least one stage of wavelength selective switch (WSS), multilayer arrayed waveguide grating (AWG), optical filter, etc., and the signal is attenuated by narrowing of the filter in a high-frequency region away from the center frequency of the optical signal, causing degradation of characteristics.

[0027] Unlike wireless communication, the transmission path in optical fiber communication is relatively static, so the further out the subcarrier, the more its characteristics tend to deteriorate, making it clear which subcarriers will experience fixed deterioration. The phrase "the further out" above refers to the further away from the center frequency. Figure 6 shows an example of deterioration in a transmitted optical signal. In the example of Figure 6, subcarriers #1 and #8 have the most degraded characteristics, followed by subcarriers #2 and #7, and subcarriers #3 to #6 are hardly affected by band narrowing.

[0028] In the first embodiment, an LDPC code having a column weight distribution as shown in FIG. 7 is applied. FIG. 7 is a diagram illustrating an example of a column weight distribution with respect to the position number of a variable node. In the example of FIG. 7, the region indicated by the solid line in the first half is the variable node corresponding to the information bits, and the distribution region indicated by the dashed line is the variable node corresponding to the parity bits. As shown in FIG. 7, if there is a characteristic in which the column weights decrease from the first half to the second half of the variable bits, the transmission reordering unit 13 reorders the signals in order of the subcarriers with the greatest characteristic degradation, as shown in FIG. 8. FIG. 8 is a diagram illustrating the operation of the transmission reordering unit 13 included in the optical transmission device 1 on the transmitting side according to the first embodiment. In FIG. 8, "SC" refers to a subcarrier. Note that, because PCS coding controls the symbol generation probability of multilevel modulation, the hard-decision error correction code and LDPC code after PCS coding must be configured as systematic codes. The post-HD-FEC signal of each subcarrier shown in the example of FIG. 8 is a signal generated by distributing the parity bits generated by hard-decision error correction coding to each subcarrier in the PCS-encoded signal of each subcarrier.

[0029] By performing the above-described reordering by the transmission reordering unit 13, the LDPC encoder 14 performs encoding processing, assigning subcarrier signals with greater performance degradation to those with larger column weights, as shown in FIG. 9 . FIG. 9 illustrates the assignment of subcarrier signals with greater performance degradation to those with larger column weights. "SC" in FIG. 9 refers to a subcarrier. Parity bits are generated by encoding. If the generated parity bits are listed as P1 to P8 in descending order of weight, performing transmission reordering as shown in the example of FIG. 10 makes it possible to assign data consisting of LDPC-coded signals and parity bits in a manner consistent with the interleaver units present for each subsequent subcarrier. FIG. 10 is a diagram illustrating the operation of the transmission reordering unit 15 included in the transmitting-side optical transmission device 1 according to the first embodiment. "SC" in FIG. 10 refers to a subcarrier. Eight interleavers 16A-16H corresponding to eight digital subcarriers are shown in FIG. 10 .

[0030] Next, the operation of the optical transmission device 2 on the receiving side according to the first embodiment will be described with reference to FIG. 2 . The optical transmission device 2 on the receiving side performs a decoding process corresponding to the inverse process of the above-described transmission process or the above-described transmission process. An optical signal input to the optical receiver 21 is converted into an electrical signal by the optical receiver 21, and the electrical signal undergoes received signal processing by the received signal processing unit 22. The received signal processing includes an analog-to-digital converter (ADC) process that converts an analog electrical signal into digital data, an equalization process that compensates for distortion occurring on the transmission path, a process that realizes a de-framing function that detects frames by finding pilot signals inserted on the transmitting side, a demapping process that extracts soft decision values ​​of digital data from received QAM symbols, and a process that realizes a subcarrier DeMUX (Demultiplexer) function.

[0031] The signals demodulated by the received signal processing are deinterleaved for each subcarrier by a deinterleaver unit. N deinterleaver units are shown in Fig. 2. The N deinterleaver units are deinterleaver unit 23A, deinterleaver unit 23B, ..., deinterleaver unit 23N.

[0032] The output signals from each of the N deinterleavers are rearranged by the reception rearrangement unit 24. This rearrangement is performed in the reverse form of the rearrangement performed by the transmission inverse rearrangement unit 15 shown in FIG. 1 so as to correspond to the parity check matrix of the subsequent LDPC soft decision decoding unit 25.

[0033] The signals rearranged by the reception rearrangement unit 24 are subjected to error correction decoding processing by the LDPC soft decision decoding unit 25. As described above, in the decoding processing, iterative decoding is performed to update the reliability of the received signal by utilizing the connection relationships of a graph called a Tanner graph as shown in Fig. 4. There are several known methods for iterative decoding, such as the Sum-Product decoding method, a simplified Min-Sum decoding method, and an Offset Min-Sum decoding method, but the type of decoding method is not limited.

[0034] The signals corrected by the LDPC soft-decision decoder 25 are then rearranged by a receiver de-arrangement unit 26. This rearrangement is the inverse of the process performed by the transmitter de-arrangement unit 13 in FIG. 1. The signals output from the receiver de-arrangement unit 26 are subjected to hard-decision error correction by a hard-decision error correction decoder 27, and the processed signals are decoded by a PCS decoder to extract the received signals. FIG. 2 shows N PCS decoders. The N PCS decoders are PCS decoder 28A, PCS decoder 28B, ..., PCS decoder 28N.

[0035] As described above, by assigning code positions with high error correction performance to digital subcarrier signals that are likely to experience degradation in characteristics due to bandwidth narrowing, it is possible to improve the error tolerance of optical communication systems.

[0036] In the first embodiment, a code in which the column weight increases from the variable node with the smaller position number as shown in Figure 7 has been described as an example. When starting the decoding process from the beginning of the code, a code design in which the column weight increases from the variable node with the smaller position number is generally used so that the improved reliability is propagated to subsequent stages by strongly correcting initial errors. However, this is not always the case. Even in such cases, it is possible to propagate relatively high reliability to subsequent stages by assigning digital subcarrier signals that are more likely to have performance degradation to code positions with large column weights.

[0037] In the first embodiment, a case has been described in which the same data weight is assigned to all signals of subcarriers with poor characteristics. The check matrix H in an LDPC code is an important parameter that affects error correction performance and the implementation circuit. Therefore, signals belonging to the same subcarrier and signals of subcarriers that are likely to have the same performance degradation may not be assigned to code positions with the same column weight. In this case, the transmission rearrangement unit 13 rearranges the signals, starting from the signals of subcarriers that are most likely to cause performance degradation, so that signals of subcarriers that are relatively likely to cause performance degradation are assigned to code positions with relatively large column weights.

[0038] In addition, in the first embodiment, the order of allocation of subcarrier signals that may cause the same deterioration in characteristics, subcarrier #1 and subcarrier #8, and subcarrier #2 and subcarrier #7, can be considered in several ways, such as from the smallest subcarrier number, from the largest subcarrier number, or alternately, and any of these methods may be adopted. Also, a method may be adopted in which a signal that is mapped to a least significant bit (LSB), which is a multi-level signal among subcarrier signals and in which errors are more likely to occur, is allocated to a code position with a larger column weight.

[0039] The optical transmission device 1 on the transmitting side according to the first embodiment includes a transmission reordering unit 13 that reorders the plurality of signals when a transmission signal is grouped into signals corresponding to each of a plurality of digital subcarriers, and an LDPC encoding unit 14 that encodes the plurality of signals reordered by the transmission reordering unit 13 using a low-density parity-check code. When the plurality of digital subcarriers are arranged on a transmission path, the transmission reordering unit 13 assigns signals corresponding to digital subcarriers that are relatively far from a center frequency to encoding positions with relatively large column weights in a check matrix of the low-density parity-check code. For example, when the plurality of digital subcarriers are arranged on a transmission path, the transmission reordering unit 13 assigns the signal corresponding to the digital subcarrier that is farthest from the center frequency to the encoding position with the largest column weight in the check matrix of the low-density parity-check code, assigns the signal corresponding to the digital subcarrier that is second farthest from the center frequency to the encoding position with the second largest column weight in the check matrix, and assigns the signal corresponding to the digital subcarrier that is third farthest from the center frequency to the encoding position with the third largest column weight in the check matrix. As a result, the optical transmission device 1 can increase the error tolerance in an optical fiber communication system that uses irregular LDPC codes and digital subcarriers with a simple configuration.

[0040] The optical transmission device 1 further includes a transmission de-arrangement unit 15 that rearranges the multiple signals output from the LDPC encoding unit 14 so that each of the multiple signals output from the LDPC encoding unit 14 is assigned to a corresponding one of the multiple digital subcarriers. The transmission de-arrangement unit 15 may have an interleaver function. If the transmission de-arrangement unit 15 has an interleaver function, the optical transmission device 1 does not have N interleaver units.

[0041] The optical transmission device 2 on the receiving side according to the first embodiment includes a receiving reordering unit 24 that reorders the received signals when the received signals are grouped into signals corresponding to each of a plurality of digital subcarriers, and an LDPC soft-decision decoding unit 25 that decodes the plurality of signals reordered by the receiving reordering unit 24 using a low-density parity-check code. The receiving reordering unit 24 assigns signals corresponding to digital subcarriers that are relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to coding positions with relatively large column weights in a check matrix of the low-density parity-check code. Specifically, the receiving reordering unit 24 assigns signals corresponding to digital subcarriers that are farthest from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to coding positions with the largest column weights in a check matrix of the low-density parity-check code, assigns signals corresponding to digital subcarriers that are second farthest from the center frequency to coding positions with the second largest column weights in the check matrix, and assigns signals corresponding to digital subcarriers that are third farthest from the center frequency to coding positions with the third largest column weights in the check matrix. As a result, the optical transmission device 2 can increase the error tolerance in an optical fiber communication system that uses irregular LDPC codes and digital subcarriers with a simple configuration.

[0042] The optical transmission device 2 on the receiving side further includes a receiving de-sorting unit 26 that sorts the multiple signals output from the LDPC soft-decision decoding unit 25 so that each of the multiple signals output from the LDPC soft-decision decoding unit 25 is assigned to a corresponding digital subcarrier among the multiple digital subcarriers.

[0043] Embodiment 2. In the first embodiment, a form was described in which rearrangement was performed to match the check matrix H of the LDPC code. In the first embodiment, characteristic degradation occurs in the signals of the subcarriers arranged on the highest and lowest frequency sides, such as subcarrier #1 and subcarrier #8. Therefore, for example, in the example shown in Fig. 8, the wiring length of subcarrier #8 is the longest, and there is a possibility that wiring delay will occur in circuit implementation. In the second embodiment, a form will be described in which rearrangement to match the check matrix H of the LDPC code is not necessary.

[0044] Fig. 11 is a block diagram showing a configuration of a transmitting-side optical transmission device 1A according to embodiment 2. Fig. 12 is a block diagram showing a configuration of a receiving-side optical transmission device 2A according to embodiment 2. Hereinafter, the transmitting-side optical transmission device 1A may be simply referred to as "optical transmission device 1A," and the receiving-side optical transmission device 2A may be simply referred to as "optical transmission device 2A."

[0045] The optical transmission device 1A shown in Fig. 11 does not have the transmission reordering unit 13 that is arranged between the hard-decision error correction coding unit 12 and the LDPC coding unit 14 in the optical transmission device 1 shown in Fig. 1. The optical transmission device 1A has an LDPC coding unit 14A instead of the LDPC coding unit 14 that the optical transmission device 1 has, and has a transmission reordering unit 15A instead of the transmission unreordering unit 15 that the optical transmission device 1 has.

[0046] The optical transmission device 2A shown in Fig. 12 corresponds to the optical transmission device 1A shown in Fig. 11, and does not have the reception inverse rearrangement unit 26 arranged between the LDPC soft-decision decoder 25 and the hard-decision error correction decoder 27 in the optical transmission device 2 shown in Fig. 2. The optical transmission device 2A has an LDPC soft-decision decoder 25A instead of the LDPC soft-decision decoder 25 of the optical transmission device 2. The basic operation of the second embodiment is the same as the basic operation of the first embodiment, and only the differences from the first embodiment will be described below.

[0047] As described above, since the operation is performed in combination with PCS, the hard-decision error correcting code and LDPC code used are systematic codes that retain transmitted bits after information is encoded. Assuming that the encoding process is a process of adding parity bits following input information bits, the LDPC encoder 14A adds parity bits for eight digital subcarriers using hard-decision error correcting codes after the PCS-coded signal of eight digital subcarriers, and then adds parity bits for eight digital subcarriers using LDPC encoding after that, as shown in FIG. 13 . FIG. 13 is a diagram for explaining the operation of the LDPC encoder 14A included in the transmitting-side optical transmission device 1A according to the second embodiment. In FIG. 13 , "SC" refers to subcarriers. In FIG. 13 , "FEC" refers to error correcting codes.

[0048] Assume that the parity bits generated by the hard-decision error correcting code and the parity bits generated by the LDPC code are divided into eight parts, and the parity bits generated by the hard-decision error correcting code and the parity bits generated by the LDPC code after division into eight parts are assigned to each of a plurality of subcarriers from subcarrier #1 to subcarrier #8. In this case, the LDPC encoder 14A in the second embodiment uses a parity check matrix having the column weight distribution shown in Fig. 13. By performing column permutation on the parity check matrix with large column weights from the beginning of the code shown in Fig. 7, it is possible to generate a parity check matrix that is adapted to the input / output interface of the LDPC encoder 14A and has the column weight distribution shown in Fig. 13.

[0049] FIG. 14 is a diagram illustrating the operation of the transmission reordering unit 15A included in the optical transmission device 1A on the transmitting side according to the second embodiment. In FIG. 14, "SC" refers to subcarrier. In FIG. 14, "FEC" refers to error correction code. The transmission reordering unit 15A aggregates signals assigned to the same subcarrier from the signals processed by the LDPC encoder 14A. The aggregated signals are input to the subsequent interleaver unit arranged for each subcarrier. Signals assigned to the same subcarrier are assigned the same index. The optical transmission device 2A shown in FIG. 12 performs reception reordering processing to undo this reordering and performs decoding processing corresponding to the parity check matrix. The configuration of the second embodiment makes it possible to omit the transmission reordering unit 13 arranged before the LDPC encoder 14 and the reception deordering unit 26 arranged after the LDPC soft-decision decoder 25 in the first embodiment, thereby improving wiring efficiency.

[0050] The optical transmission device 1A on the transmitting side according to the second embodiment includes an LDPC encoder 14A that encodes a plurality of signals obtained by grouping a transmission signal into signals corresponding to each of a plurality of digital subcarriers using a low-density parity-check code, and a transmission reordering unit 15A that rearranges the plurality of signals output from the LDPC encoder 14A. The LDPC encoder 14A performs encoding using a parity check matrix that assigns signals corresponding to digital subcarriers that are relatively far from a center frequency to encoding positions with relatively large column weights when the plurality of digital subcarriers are arranged on a transmission path. The transmission reordering unit 15A rearranges the plurality of signals output from the LDPC encoder 14A so that each of the plurality of signals is assigned to a corresponding digital subcarrier among the plurality of digital subcarriers. The transmission reordering unit 15A may have an interleaver function. If the transmission reordering unit 15A has an interleaver function, the optical transmission device 1A does not have N interleavers.

[0051] The optical transmission device 2A on the receiving side according to the second embodiment includes a reception reordering unit 24 that reorders the received signals when the received signals are grouped into signals corresponding to each of the plurality of digital subcarriers, and an LDPC soft-decision decoding unit 25A that decodes the plurality of signals reordered by the reception reordering unit 24 using a low-density parity-check code. The LDPC soft-decision decoding unit 25A assigns signals corresponding to digital subcarriers that are relatively far from the center frequency when the plurality of digital subcarriers are arranged on a transmission path to coding positions in the low-density parity-check code having relatively large column weights in a check matrix, and performs decoding using the check matrix. The LDPC soft-decision decoding unit 25A reorders the plurality of decoded signals so that each of the plurality of decoded signals is assigned to a corresponding one of the plurality of digital subcarriers, and outputs the reordered signals to a hard-decision error correction decoding unit 27.

[0052] In the second embodiment, the configuration and operation are described on the assumption that the coding process is a process of adding parity bits following input information bits. In the hard-decision error correction coding process and the LDPC coding process, if a signal is output in subcarrier units so as to be compatible with the subsequent interleaver unit, the rearrangement described in the second embodiment is not necessary.

[0053] Furthermore, in the first and second embodiments, an example in which the number of subcarriers is eight has been described, but the number of subcarriers is not limited to eight.

[0054] 15 is a diagram illustrating a processor 91 in a case where at least some of the functions of the N PCS encoders, the hard-decision error correction encoder 12, the transmission reordering unit 13, the LDPC encoder 14, the transmission deordering unit 15, the N interleavers, the transmission signal processing unit 17, and the optical transmitter 18 included in the transmitting-side optical transmission device 1 according to the first embodiment are realized by the processor 91. That is, at least some of the functions of the N PCS encoders, the hard-decision error correction encoder 12, the transmission reordering unit 13, the LDPC encoder 14, the transmission deordering unit 15, the N interleavers, the transmission signal processing unit 17, and the optical transmitter 18 may be realized by the processor 91 executing a program stored in a memory 92. The processor 91 is a CPU (Central Processing Unit), a processing system, an arithmetic system, a microprocessor, or a DSP (Digital Signal Processor). The memory 92 is also shown in FIG. 15 .

[0055] When at least some of the functions of the N PCS encoders, the hard-decision error correction encoder 12, the transmission reordering unit 13, the LDPC encoder 14, the transmission deordering unit 15, the N interleavers, the transmission signal processing unit 17, and the optical transmitter 18 are implemented by the processor 91, the functions are implemented by the processor 91 together with software, firmware, or a combination of software and firmware. The software or firmware is written as a program and stored in the memory 92. The processor 91 reads and executes the program stored in the memory 92, thereby implementing at least some of the functions of the N PCS encoders, the hard-decision error correction encoder 12, the transmission reordering unit 13, the LDPC encoder 14, the transmission deordering unit 15, the N interleavers, the transmission signal processing unit 17, and the optical transmitter 18.

[0056] When at least some of the functions of the N PCS encoders, the hard-decision error correction encoder 12, the transmission reordering unit 13, the LDPC encoder 14, the transmission deordering unit 15, the N interleavers, the transmission signal processing unit 17, and the optical transmitter 18 are realized by the processor 91, the optical transmission device 1 has a memory 92 for storing a program that results in the execution of at least some of the steps executed by the N PCS encoders, the hard-decision error correction encoder 12, the transmission reordering unit 13, the LDPC encoder 14, the transmission deordering unit 15, the N interleavers, the transmission signal processing unit 17, and the optical transmitter 18. It can also be said that the program stored in the memory 92 causes a computer to execute at least some of the procedures or methods executed by the N PCS encoders, the hard-decision error correction encoder 12, the transmission reordering unit 13, the LDPC encoder 14, the transmission deordering unit 15, the N interleavers, the transmission signal processing unit 17, and the optical transmitter 18.

[0057] The memory 92 may be, for example, a non-volatile or volatile semiconductor memory such as a RAM (Random Access Memory), a ROM (Read Only Memory), a flash memory, an EPROM (Erasable Programmable Read Only Memory), an EEPROM (registered trademark) (Electrically Erasable Programmable Read-Only Memory), a magnetic disk, a flexible disk, an optical disk, a compact disk, a mini disk, or a DVD (Digital Versatile Disk).

[0058] 16 is a diagram illustrating a processing circuit 93 in a case where at least some of the functions of the N PCS encoders, the hard-decision error correction encoder 12, the transmission reordering unit 13, the LDPC encoder 14, the transmission deordering unit 15, the N interleavers, the transmission signal processing unit 17, and the optical transmitter 18 included in the transmitting-side optical transmission device 1 according to embodiment 1 are realized by the processing circuit 93. In other words, at least some of the functions of the N PCS encoders, the hard-decision error correction encoder 12, the transmission reordering unit 13, the LDPC encoder 14, the transmission deordering unit 15, the N interleavers, the transmission signal processing unit 17, and the optical transmitter 18 may be realized by the processing circuit 93.

[0059] The processing circuitry 93 is dedicated hardware, and may be, for example, a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a combination thereof.

[0060] Some of the functions of the N PCS encoding units, the hard-decision error correction encoding unit 12, the transmission rearrangement unit 13, the LDPC encoding unit 14, the transmission de-arrangement unit 15, the N interleaver units, the transmission signal processing unit 17, and the optical transmitter 18 may be realized by dedicated hardware that is separate from the hardware that realizes the remaining functions.

[0061] With regard to the multiple functions of the N PCS encoders, the hard-decision error correction encoder 12, the transmission reordering unit 13, the LDPC encoder 14, the transmission deordering unit 15, the N interleavers, the transmission signal processing unit 17, and the optical transmitter 18, some of the multiple functions may be realized by software or firmware, and the remaining multiple functions may be realized by dedicated hardware. In this way, the multiple functions of the N PCS encoders, the hard-decision error correction encoder 12, the transmission reordering unit 13, the LDPC encoder 14, the transmission deordering unit 15, the N interleavers, the transmission signal processing unit 17, and the optical transmitter 18 can be realized by hardware, software, firmware, or a combination thereof.

[0062] At least some of the functions of the optical receiver 21, the received signal processing unit 22, the N deinterleavers, the received signal reordering unit 24, the LDPC soft-decision decoding unit 25, the received signal deordering unit 26, the hard-decision error correction decoding unit 27, and the N PCS decoding units included in the receiving-side optical transmission device 2 according to the first embodiment may be implemented by a processor that executes a program stored in a memory. The memory is the same as the memory 92. The processor is the same as the processor 91. At least some of the functions of the optical receiver 21, the received signal processing unit 22, the N deinterleavers, the received signal reordering unit 24, the LDPC soft-decision decoding unit 25, the received signal deordering unit 26, the hard-decision error correction decoding unit 27, and the N PCS decoding units may be implemented by a processing circuit. The processing circuit is the same as the processing circuit 93.

[0063] At least some of the functions of the N PCS encoders, the hard-decision error correction encoder 12, the LDPC encoder 14A, the transmission reordering unit 15A, the N interleavers, the transmission signal processing unit 17, and the optical transmitter 18 included in the transmitting-side optical transmission device 1A according to the second embodiment may be implemented by a processor that executes a program stored in a memory. The memory is the same as the memory 92. The processor is the same as the processor 91. At least some of the functions of the N PCS encoders, the hard-decision error correction encoder 12, the LDPC encoder 14A, the transmission reordering unit 15A, the N interleavers, the transmission signal processing unit 17, and the optical transmitter 18 may be implemented by a processing circuit. The processing circuit is the same as the processing circuit 93.

[0064] At least some of the functions of the optical receiver 21, the received signal processing unit 22, the N deinterleavers, the reception reordering unit 24, the LDPC soft-decision decoding unit 25A, the hard-decision error correction decoding unit 27, and the N PCS decoding units included in the receiving-side optical transmission device 2A according to the second embodiment may be implemented by a processor that executes a program stored in a memory. The memory is the same as the memory 92. The processor is the same as the processor 91. At least some of the functions of the optical receiver 21, the received signal processing unit 22, the N deinterleavers, the reception reordering unit 24, the LDPC soft-decision decoding unit 25A, the hard-decision error correction decoding unit 27, and the N PCS decoding units may be implemented by a processing circuit. The processing circuit is the same as the processing circuit 93.

[0065] 17 is a diagram showing a control circuit 94 that controls the operation performed by the transmitting-side optical transmission device 1 according to the first embodiment. The control circuit 94 is a control circuit that causes the optical transmission device 1 to perform an operation of rearranging a plurality of signals when a transmission signal is grouped into signals corresponding to each of a plurality of digital subcarriers, and an operation of encoding the rearranged plurality of signals using a low-density parity-check code. The operation of rearranging the plurality of signals includes an operation of assigning a signal corresponding to a digital subcarrier that is relatively far from the center frequency when the plurality of digital subcarriers are arranged on a transmission path to an encoding position having a relatively large column weight in the parity check matrix of the low-density parity-check code.

[0066] The operations performed by the optical transmission device 2 on the receiving side according to the first embodiment may be controlled by a control circuit. The control circuit causes the optical transmission device 2 to perform an operation of rearranging the received signals when the received signals are grouped into signals corresponding to each of a plurality of digital subcarriers, and an operation of decoding the rearranged signals using a low-density parity-check code. The operation of rearranging the signals includes an operation of assigning signals corresponding to digital subcarriers that are relatively far from the center frequency when the plurality of digital subcarriers are arranged on a transmission path to coding positions with relatively large column weights in the check matrix of the low-density parity-check code. The control circuit is a circuit similar to the control circuit 94.

[0067] The operations performed by the transmitting optical transmission device 1A according to the second embodiment may be controlled by a control circuit. The control circuit causes the optical transmission device 1A to perform an operation of encoding a plurality of signals when a transmission signal is grouped into signals corresponding to each of a plurality of digital subcarriers using a low-density parity-check code, and an operation of rearranging the encoded plurality of signals. The operation of encoding the plurality of signals includes an operation of performing encoding using a parity check matrix that assigns signals corresponding to digital subcarriers that are relatively far from a center frequency to encoding positions with relatively large column weights when the plurality of digital subcarriers are arranged on a transmission path. The operation of rearranging the plurality of signals includes an operation of rearranging the encoded plurality of signals so that each of the encoded plurality of signals is assigned to a corresponding digital subcarrier among the plurality of digital subcarriers. The control circuit is a circuit similar to the control circuit 94.

[0068] The operations performed by the receiving-side optical transmission device 2A according to the second embodiment may be controlled by a control circuit. The control circuit causes the optical transmission device 2A to perform an operation of rearranging the received signals when the received signals are grouped into signals corresponding to each of a plurality of digital subcarriers, and an operation of decoding the rearranged plurality of signals using a low-density parity-check code. The operation of decoding the plurality of signals includes an operation of assigning signals corresponding to digital subcarriers that are relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to coding positions in the low-density parity-check code where the column weight of the check matrix is ​​relatively large, performing decoding using the check matrix, and rearranging and outputting the plurality of decoded signals so that each of the plurality of decoded signals is assigned to a corresponding digital subcarrier among the plurality of digital subcarriers. The control circuit is a circuit similar to the control circuit 94.

[0069] 18 is a diagram showing a program storage medium 95 storing a program for controlling the optical transmission device 1 on the transmitting side according to the first embodiment. The program causes the optical transmission device 1 to execute an operation of rearranging a plurality of signals when a transmission signal is grouped into signals corresponding to each of a plurality of digital subcarriers, and an operation of encoding the rearranged plurality of signals using a low-density parity-check code. The operation of rearranging the plurality of signals includes an operation of assigning a signal corresponding to a digital subcarrier that is relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to an encoding position having a relatively large column weight in a parity check matrix of the low-density parity-check code.

[0070] The optical transmission device 2 on the receiving side according to the first embodiment may be controlled by a program stored in a program storage medium. The program causes the optical transmission device 2 to perform an operation of rearranging received signals, when the received signals are grouped into signals corresponding to each of a plurality of digital subcarriers, and an operation of decoding the rearranged plurality of signals using a low-density parity-check code. The operation of rearranging the plurality of signals includes an operation of assigning signals corresponding to digital subcarriers that are relatively far from a center frequency, when the plurality of digital subcarriers are arranged on a transmission path, to coding positions with relatively large column weights in the parity check matrix of the low-density parity-check code. The program storage medium storing the program is a storage medium similar to the program storage medium 95.

[0071] The transmitting-side optical transmission device 1A according to the second embodiment may be controlled by a program stored in a program storage medium. The program causes the optical transmission device 1A to perform an operation of encoding a plurality of signals when a transmission signal is grouped into signals corresponding to each of a plurality of digital subcarriers using a low-density parity-check code, and an operation of rearranging the encoded plurality of signals. The operation of encoding the plurality of signals includes an operation of performing encoding using a parity check matrix that assigns signals corresponding to digital subcarriers that are relatively far from a center frequency to encoding positions with relatively large column weights when the plurality of digital subcarriers are arranged on a transmission path. The operation of rearranging the plurality of signals includes an operation of rearranging the encoded plurality of signals so that each of the encoded plurality of signals is assigned to a corresponding digital subcarrier among the plurality of digital subcarriers. The program storage medium storing the program is a storage medium similar to the program storage medium 95.

[0072] The receiving-side optical transmission device 2A according to the second embodiment may be controlled by a program stored in a program storage medium. The program causes the optical transmission device 2A to perform an operation of rearranging received signals when the received signals are grouped into signals corresponding to each of a plurality of digital subcarriers, and an operation of decoding the rearranged plurality of signals using a low-density parity-check code. The operation of decoding the plurality of signals includes an operation of assigning signals corresponding to digital subcarriers that are relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to coding positions in a check matrix of the low-density parity-check code that have relatively large column weights, performing decoding using the check matrix, and rearranging and outputting the plurality of decoded signals so that each of the plurality of signals after decoding is assigned to a corresponding digital subcarrier among the plurality of digital subcarriers. The program storage medium storing the program is a storage medium similar to the program storage medium 95.

[0073] The configurations shown in the above embodiments are merely examples, and may be combined with other known technologies, or different embodiments may be combined with each other, or part of the configuration may be omitted or modified within the scope of the gist of the invention.

[0074] 1, 1A Optical transmission device on the transmitting side, 2, 2A Optical transmission device on the receiving side, 11A, 11B, 11N PCS encoding unit, 12 Hard decision error correction encoding unit, 13, 15A Transmission rearrangement unit, 14, 14A LDPC encoding unit, 15 Transmission de-arrangement unit, 16A-16H, 16N Interleaver unit, 17 Transmission signal processing unit, 18 Optical transmitter, 21 Optical receiver, 22 Received signal processing unit, 23A, 23B, 23N Deinterleaver unit, 24 Received rearrangement unit, 25, 25A LDPC soft decision decoding unit, 26 Received de-arrangement unit, 27 Hard decision error correction decoding unit, 28A, 28B, 28N PCS decoding unit, 91 Processor, 92 Memory, 93 Processing circuit, 94 Control circuit, 95 Program storage medium.

Claims

1. An optical transmission device comprising: a transmission rearrangement unit that rearranges a plurality of signals when transmission signals are grouped into signals corresponding to each of a plurality of digital subcarriers; and a low-density parity check encoding unit that encodes the plurality of signals rearranged by the transmission rearrangement unit using a low-density parity check code, wherein the transmission rearrangement unit assigns signals corresponding to digital subcarriers that are relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to encoding positions with relatively large column weights in the check matrix of the low-density parity check code.

2. The optical transmission device according to claim 1, further comprising a transmission de-sorting unit that sorts the multiple signals output from the low-density parity check coding unit so that each of the multiple signals output from the low-density parity check coding unit is assigned to a corresponding digital subcarrier among the multiple digital subcarriers.

3. The optical transmission device according to claim 2, wherein the transmission de-arrangement unit has an interleaver function.

4. An optical transmission device comprising: a receiving reordering unit that reorders a plurality of signals when received signals are grouped into signals corresponding to each of a plurality of digital subcarriers; and a low-density parity check soft-decision decoding unit that decodes the plurality of signals reordered by the receiving reordering unit using a low-density parity check code, wherein the receiving reordering unit assigns signals corresponding to digital subcarriers that are relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to coding positions with relatively large column weights in a check matrix of the low-density parity check code.

5. The optical transmission device according to claim 4, further comprising a receiving inverse rearrangement unit that rearranges the multiple signals output from the low-density parity check soft-decision decoding unit so that each of the multiple signals output from the low-density parity check soft-decision decoding unit is assigned to a corresponding digital subcarrier among the multiple digital subcarriers.

6. An optical transmission device comprising: a low-density parity check coding unit that encodes a plurality of signals when a transmission signal is grouped into signals corresponding to each of a plurality of digital subcarriers using a low-density parity check code; and a transmission reordering unit that rearranges the plurality of signals output from the low-density parity check coding unit, wherein the low-density parity check coding unit performs encoding using a check matrix that assigns signals corresponding to digital subcarriers that are relatively far from a center frequency to coding positions with relatively large column weights when the plurality of digital subcarriers are arranged on a transmission path, and the transmission reordering unit rearranges the plurality of signals output from the low-density parity check coding unit so that each of the plurality of signals output from the low-density parity check coding unit is assigned to a corresponding digital subcarrier among the plurality of digital subcarriers.

7. The optical transmission device according to claim 6, wherein the transmission rearrangement unit has an interleaver function.

8. An optical transmission device comprising: a receiving reordering unit that reorders a plurality of signals when received signals are grouped into signals corresponding to each of a plurality of digital subcarriers; and a low-density parity check code soft-decision decoding unit that decodes the plurality of signals reordered by the receiving reordering unit using a low-density parity check code, wherein the low-density parity check code soft-decision decoding unit assigns signals corresponding to digital subcarriers that are relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to coding positions in a check matrix of the low-density parity check code that have relatively large column weights, and performs decoding using the check matrix, and reorders and outputs the plurality of signals after decoding so that each of the plurality of signals after decoding is assigned to a corresponding digital subcarrier among the plurality of digital subcarriers.

9. A control circuit that causes an optical transmission device to perform an operation of rearranging a plurality of signals when a transmission signal is grouped into signals corresponding to each of a plurality of digital subcarriers, and an operation of encoding the rearranged plurality of signals using a low-density parity check code, wherein the operation of rearranging the plurality of signals includes an operation of assigning a signal corresponding to a digital subcarrier that is relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to an encoding position with a relatively large column weight in a check matrix of the low-density parity check code.

10. A control circuit that causes an optical transmission device to perform an operation of rearranging a plurality of signals when received signals are grouped into signals corresponding to each of a plurality of digital subcarriers, and an operation of decoding the rearranged plurality of signals using a low-density parity check code, wherein the operation of rearranging the plurality of signals includes an operation of assigning a signal corresponding to a digital subcarrier that is relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to an encoding position with a relatively large column weight in a check matrix of the low-density parity check code.

11. A control circuit that causes an optical transmission device to perform an operation of encoding a plurality of signals when a transmission signal is grouped into signals corresponding to each of a plurality of digital subcarriers using a low-density parity check code, and an operation of rearranging the encoded plurality of signals, wherein the operation of encoding the plurality of signals includes an operation of performing encoding using a parity check matrix that assigns signals corresponding to digital subcarriers that are relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to encoding positions with relatively large column weights, and the operation of rearranging the plurality of signals includes an operation of rearranging the encoded plurality of signals so that each of the encoded plurality of signals is assigned to a corresponding digital subcarrier among the plurality of digital subcarriers.

12. A control circuit that causes an optical transmission device to perform an operation of rearranging a plurality of signals when received signals are grouped into signals corresponding to each of a plurality of digital subcarriers, and an operation of decoding the rearranged plurality of signals using a low-density parity check code, wherein the operation of decoding the plurality of signals includes an operation of assigning signals corresponding to digital subcarriers that are relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to coding positions in a check matrix of the low-density parity check code that have relatively large column weights, and decoding the signals using the check matrix, and rearranging and outputting the plurality of signals after decoding so that each of the plurality of signals after decoding is assigned to a corresponding digital subcarrier among the plurality of digital subcarriers.

13. A program storage medium storing a program for controlling an optical transmission device, the program causing the optical transmission device to perform an operation of rearranging a plurality of signals when a transmission signal is grouped into signals corresponding to each of a plurality of digital subcarriers, and an operation of encoding the rearranged plurality of signals using a low-density parity check code, characterized in that the operation of rearranging the plurality of signals includes an operation of assigning a signal corresponding to a digital subcarrier that is relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to an encoding position having a relatively large column weight in a check matrix of the low-density parity check code.

14. A program storage medium storing a program for controlling an optical transmission device, the program causing the optical transmission device to perform an operation of rearranging a plurality of signals when received signals are grouped into signals corresponding to each of a plurality of digital subcarriers, and an operation of decoding the rearranged plurality of signals using a low-density parity check code, characterized in that the operation of rearranging the plurality of signals includes an operation of assigning a signal corresponding to a digital subcarrier that is relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to an encoding position with a relatively large column weight in a check matrix of the low-density parity check code.

15. A program storage medium storing a program for controlling an optical transmission device, the program causing the optical transmission device to execute an operation of encoding a plurality of signals when a transmission signal is grouped into signals corresponding to each of a plurality of digital subcarriers using a low-density parity check code, and an operation of rearranging the encoded plurality of signals, wherein the operation of encoding the plurality of signals includes an operation of performing encoding using a parity check matrix that assigns signals corresponding to digital subcarriers that are relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to encoding positions with relatively large column weights, and the operation of rearranging the plurality of signals includes an operation of rearranging the encoded plurality of signals so that each of the encoded plurality of signals is assigned to a corresponding digital subcarrier among the plurality of digital subcarriers.

16. A program storage medium storing a program for controlling an optical transmission device, the program causing the optical transmission device to perform an operation of rearranging a plurality of signals when received signals are grouped into signals corresponding to each of a plurality of digital subcarriers, and an operation of decoding the rearranged plurality of signals using a low-density parity check code, wherein the operation of decoding the plurality of signals includes an operation of assigning a signal corresponding to a digital subcarrier that is relatively far from a center frequency when the plurality of digital subcarriers are arranged on a transmission path to an encoding position in a check matrix of the low-density parity check code having a relatively large column weight, and decoding the signal using the check matrix, and rearranging and outputting the plurality of signals after decoding so that each of the plurality of signals after decoding is assigned to a corresponding digital subcarrier among the plurality of digital subcarriers.

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