JP5940966B2 - Optical transmission system, optical transmitter, optical receiver, optical transmission method, optical reception method - Google Patents

Description

  The present invention relates to an optical transmission system, an optical transmitter, an optical receiver, an optical transmission method, and an optical reception method. More specifically, the present invention relates to a signal to be transmitted that is represented by symbols whose occurrence probability follows a uniform distribution. By converting and multiplexing signals represented by symbols that follow a Gaussian distribution, the generation probability of optical signals with large amplitude is reduced while the signal power is kept approximately constant, and nonlinearity generated in the optical fiber transmission line The present invention relates to an optical signal transmission technique for reducing the influence of an optical effect.

  As the demand for data communication increases, optical transmission networks using optical signal modulation technology and optical signal multiplexing technology that enable transmission of large-capacity traffic are becoming widespread. On the other hand, along with an increase in data transmission speed, optical signal waveform degradation due to nonlinear optical effects that occur in optical fiber transmission lines is one of the main factors that limit transmission speed and transmission distance (non- Patent Document 1). Under such circumstances, an optical signal transmission system that is not easily affected by the nonlinear optical effect is desired.

  The nonlinear optical effect generated in the optical fiber transmission line depends on the power of the optical signal, that is, the intensity of the optical signal. Since the intensity of the optical signal is expressed by the square of the amplitude, the optical signal waveform deterioration due to the nonlinear optical effect becomes remarkable for the optical signal having a large amplitude.

  FIG. 14 shows an example of the relationship between the incident power of the optical signal to the optical fiber transmission line and the received power (reception sensitivity) of the optical signal as an example of optical signal waveform deterioration due to the nonlinear optical effect. As illustrated in the figure, when the incident power is increased, the reception sensitivity is rapidly deteriorated. In the example shown in the figure, the reception power rapidly decreases when the incident power is around 7 [dBm]. This means that when the power of the optical signal incident on the optical fiber transmission mechanism exceeds a certain threshold value, the optical signal waveform is greatly deteriorated.

S. Yamamoto, et al., “Suppression and Compensation of Linear / Nonlinear Crosstalk for Spectrum-Overlapped Signal in Carrier-Phase-Locked WDM,” Proceeding of ECOC2010, P4.10, 2010

  In general, in optical fiber communication, data information is superimposed on the amplitude and phase of an optical signal. Different data information corresponds to different amplitude / phase, and the amplitude / phase information corresponding to each data information is called a symbol. Usually, the occurrence probability of each symbol shows a uniform distribution, and the occurrence frequency of each symbol is constant regardless of the amplitude. On the other hand, optical signal waveform deterioration due to the nonlinear optical effect is a phenomenon that occurs remarkably with respect to an optical signal having a large signal power (intensity). Therefore, in order to suppress the optical signal waveform deterioration due to the nonlinear optical effect, it is desired to reduce the probability of generating a symbol with a large amplitude and increase the probability of generating a symbol with a small amplitude.

  The present invention has been made in view of the above circumstances, and an optical transmission system, an optical transmitter, an optical receiver, an optical transmission method, which can suppress optical signal waveform deterioration due to a nonlinear optical effect in an optical transmission line, An object of the present invention is to provide an optical receiving method.

According to the invention, this object is achieved by the means indicated in the claims.
That is, an aspect of the optical transmitter according to the present invention includes an optical transmitter and an optical receiver connected to each other via an optical transmission path, and the optical reception from the optical transmitter via the optical transmission path. An optical transmission system for transmitting an optical signal to a transmitter, wherein the optical transmitter transmits an input data string represented by symbols according to a uniform distribution of occurrence probabilities and a transmission represented by symbols according to a Gaussian distribution of occurrence probabilities A coding unit that codes the data sequence; and a multiplexing unit that multiplexes the transmission data sequence obtained by the coding unit into the optical signal, wherein the optical receiver has an occurrence probability of Gaussian from the optical signal. A separation unit that separates a reception data sequence represented by symbols according to a distribution, and the reception data sequence obtained by the separation unit are decoded into an output data sequence represented by symbols that have a uniform occurrence probability Recovery It has a section, a configuration of an optical transmission system comprising a.

In one aspect of the optical transmission system, for example, the input data string and the output data string are (A ₁ , A ₂ ,... A _N ), and the transmission data string and the received data string are (B ₁ , B ₂ ,... B _N ) and the N × N Hadamard matrix is H _n , the encoding unit encodes the input data sequence into the transmission data sequence according to the following equation (1A), and the decoding unit Decodes the received data string into the output data string according to the following equation (2A).

  An aspect of the optical transmitter according to the present invention is an optical transmitter for sending an optical signal to an optical transmission line, and an input data sequence represented by symbols having a uniform probability of occurrence is represented by a Gaussian distribution. And a multiplexing unit that multiplexes the transmission data sequence obtained by the encoding unit into the optical signal.

In one aspect of the optical transmitter, for example, the input data sequence is (A ₁ , A ₂ ,... A _N ), the transmission data sequence is (B ₁ , B ₂ ,... B _N ), and N When the × N Hadamard matrix is set to H _n , the encoding unit encodes the input data string into the transmission data string according to the previous equation (1A).

  An aspect of the optical receiver according to the present invention is an optical receiver that receives an optical signal from an optical transmission line, and separates a received data sequence represented by a symbol whose occurrence probability follows a Gaussian distribution from the optical signal. And a decoding unit that decodes the received data sequence obtained by the demultiplexing unit into an output data sequence represented by symbols according to a distribution with a uniform occurrence probability. .

In one aspect of the optical receiver, for example, the received data string is (B ₁ , B ₂ ,... B _N ), the output data string is (A ₁ , A ₂ ,... A _N ), and N When the × N Hadamard matrix is H _n , the decoding unit decodes the received data sequence into the output data sequence according to the previous equation (2A).

  One aspect of an optical transmission method according to the present invention is an optical transmission method for transmitting an optical signal to an optical transmission line, wherein an input data sequence represented by symbols having a uniform distribution probability is represented by a Gaussian distribution. And a multiplexing step of multiplexing the transmission data sequence obtained by the encoding step into the optical signal.

  One aspect of the optical receiving method according to the present invention is an optical receiving method for receiving an optical signal from an optical transmission line, and separating the received data string represented by symbols whose occurrence probability follows a Gaussian distribution from the optical signal. And a decoding step of decoding the received data sequence obtained by the separation step into an output data sequence represented by symbols having a uniform occurrence probability.

The above-described present invention can be restated as follows.
One aspect of an optical transmitter according to the present invention is an optical transmitter that generates an optical signal and sends it to an optical fiber transmission line, and encodes data sequences A1, A2,... AN having a plurality of different data information. By performing processing, the probability of occurrence of each symbol in each data string is converted from a uniform distribution to a Gaussian distribution, and the probability of occurrence of a symbol having a large signal power is reduced. Have. Here, it is assumed that the data strings A1, A2,..., AN have amplitude / phase information of the optical signal, and the values are represented by arbitrary complex numbers.

  One aspect of an optical transmitter according to the present invention is an optical transmitter that generates an optical signal and sends it to an optical fiber transmission line, and N × for data strings A1, A2,... AN having a plurality of different data information By performing the encoding process shown in the above equation (1A) using the N Hadamard matrix, the generation probability of each symbol of each data string generates data strings B1, B2,. It has the structure of the characteristic optical transmitter. Here, it is assumed that the data sequences A1, A2,..., AN, and the data sequences B1, B2,..., BN have amplitude / phase information of the optical signal, and the values are represented by arbitrary complex numbers.

  One aspect of an optical transceiver according to the present invention is an optical transceiver including an optical transmitter that generates an optical signal and transmits the optical signal to an optical fiber transmission line, and an optical receiver that detects the optical signal and restores the transmitted signal. By applying the encoding process shown in the previous equation (1A) to the data strings A1, A2,... AN having a plurality of different data information, the probability of occurrence of each symbol in each data string follows a Gaussian distribution. The data string B1, B2,... BN is generated, and the data string A1 is obtained by performing the decoding process shown in the above equation (2A) on the data string B1, B2,. , A2,..., AN is restored. Here, it is assumed that the data sequences A1, A2,..., AN, and the data sequences B1, B2,..., BN have amplitude / phase information of the optical signal, and the values are represented by arbitrary complex numbers. Further, the threshold determination for data is performed on the data strings A1, A2,..., AN.

  In one aspect of the optical transmitter and optical transmitter / receiver, for example, the optical transmitter includes an encoder, an optical modulator, a light source, and a multiplexer, and each data sequence A1, A2,. A digital electric signal is input to the encoder and subjected to the encoding process shown in the previous equation (1A), thereby generating data sequences B1, B2,..., BN in which the probability distribution of each symbol follows a Gaussian distribution, Data information included in each data string is input as a digital electric signal to different optical modulators, and is superimposed on light having different wavelengths λ1, λ2, ..., λN as amplitude / phase information and generated in this way. The optical signal is wavelength-multiplexed using a multiplexer and sent to an optical fiber transmission line. When the data sequence is represented by a complex number, an optical vector modulator (IQ modulator) is used as the optical modulator, and the real number component of the data sequence is used as the in-phase component and the imaginary number component is used as the quadrature phase component. Further, the threshold determination for data is performed on the data strings A1, A2,..., AN.

  In one aspect of the optical transceiver, for example, the optical receiver is composed of a demultiplexer, a digital coherent receiver, a delay adjuster, and a decoder, and data strings B1, B2,. Wavelength-multiplexed optical signals are separated by a demultiplexer, and each optical signal is incident on a digital coherent receiver to be converted into a digital electrical signal while retaining the amplitude / phase information of the optical signal. In addition, linear optical signal waveform deterioration caused by chromatic dispersion and polarization mode dispersion caused in the optical fiber transmission line is compensated, and between each data string generated by the wavelength dispersion of the optical fiber transmission line by the delay adjuster. Data strings B1, B2,..., BN are obtained as digital electric signals by compensating the delay difference with a delay adjuster such as a variable delay line, and further, the arithmetic processing shown in the above equation (2A) is performed by the decoder. Especially Thus, the data strings A1, A2,..., AN are restored. Further, the threshold determination for data is performed on the data strings A1, A2,..., AN.

  In one aspect of the optical transceiver, for example, the optical transmitter is composed of an encoder, a time division multiplexer, an optical modulator, and a light source, and each data sequence A1, A2,..., AN is a digital electric signal. Is input to the encoder, and the encoding process shown in the previous equation (1A) is performed to generate the data sequences B1, B2,..., BN in which the probability distribution of each symbol follows a Gaussian distribution, and time division A data string C in which the data strings B1, B2,..., BN are time-multiplexed is generated by the multiplexer, and the data information of the data string C is input to the optical modulator as a digital electrical signal, and is converted into light of an arbitrary wavelength λ. The optical signal superimposed on the amplitude / phase information and transmitted in this way is sent to the optical fiber transmission line. When the data sequence is represented by a complex number, an optical vector modulator (IQ modulator) is used as the optical modulator, and the real number component of the data sequence is used as the in-phase component and the imaginary number component is used as the quadrature phase component. Further, the threshold determination for data is performed on the data strings A1, A2,..., AN.

  In one aspect of the optical transceiver, for example, the optical receiver is composed of a digital coherent receiver, a time division multiplexer, and a decoder, and an optical signal on which the data sequence C is superimposed is a digital coherent receiver. Incident light converts the optical signal amplitude / phase information into a digital electrical signal and reduces linear optical signal waveform degradation due to chromatic dispersion and polarization mode dispersion generated in the optical fiber transmission line. By compensating and further time-separating the data sequence C by a time division multiplexer, the data sequences B1, B2,... BN are obtained, and further the arithmetic processing shown in the previous equation (2A) is performed by the decoder. , Data strings A1, A2,..., AN are restored. Further, the threshold determination for data is performed on the data strings A1, A2,..., AN.

In one aspect of the optical transceiver, for example, the optical transmitter is composed of an encoder, an optical modulator, a light source, a polarization coupler, and a multiplexer, and each data string A1, A2,. Is input to the encoder as a digital electrical signal, and the data string B1, B2,..., BN in which the probability distribution of each symbol follows a Gaussian distribution is generated by performing the encoding process shown in the previous equation (1A). The data information of each data string is input as a digital electrical signal to different optical modulators, superimposed as amplitude / phase information on light having different arbitrary wavelengths λ1, λ2,. A polarization multiplexed signal is generated by polarization multiplexing optical signals of the same wavelength using a wave coupler, and the optical signal thus generated is wavelength multiplexed using a multiplexer, and an optical fiber transmission line It is characterized by being sent to. At this time, light of the same wavelength λ1 is incident on the optical modulator that superimposes B1 and the optical modulator that superimposes B2. Similarly, an optical modulator that superimposes Bn and an optical modulator that superimposes Bn + 1, such that light of the same wavelength λ2 is incident on the optical modulator that superimposes B3 and the optical modulator that superimposes B4 It is assumed that light having the same wavelength is incident on. Here, n is an odd number.
When the data sequence is represented by a complex number, an optical vector modulator (IQ modulator) is used as the optical modulator, and the real number component of the data sequence is used as the in-phase component and the imaginary number component is used as the quadrature phase component. Further, the threshold determination for data is performed on the data strings A1, A2,..., AN.

  In one aspect of the optical transceiver, for example, the optical receiver includes a duplexer, a digital coherent receiver, a delay adjuster, and a decoder, and the data strings B1, B2, and B2 according to claim 4, ..., polarization-multiplexed and wavelength-multiplexed optical signals with superimposed BN are wavelength-separated by a demultiplexer, and each optical signal is incident on a digital coherent receiver, so that the amplitude / phase information of the optical signal is obtained. The digital signal is converted into a digital electric signal in the held state, and linear optical signal waveform deterioration due to chromatic dispersion and polarization mode dispersion generated in the optical fiber transmission line is compensated, polarization separation is performed, and a delay adjuster is used. Data strings B1, B2, ..., BN are obtained as digital electrical signals by compensating for the delay difference between the data strings generated by chromatic dispersion in the optical fiber transmission line by a delay adjuster such as a variable delay line. Decryption By performing the arithmetic processing shown in Equation (2A) in the data string A1, A2, ..., characterized by restoring the AN. Further, the threshold determination for data is performed on the data strings A1, A2,..., AN.

In one aspect of the optical transceiver, for example, the optical transmitter is composed of an encoder, a time division multiplexer, an optical modulator, a light source, a polarization coupler, and a multiplexer, and each data string A1, A2,..., AN are input to the encoder as digital electrical signals and subjected to the encoding process shown in the previous equation (1A), whereby the occurrence probability distribution of each symbol is a data string B1, B2,. , BN is generated, and data sequences B1, B2,..., BN are time-multiplexed by the time division multiplexer, and data sequences C1, C2,..., CN / M are generated. As an electrical signal, it is input to the optical modulator and superimposed as amplitude / phase information on light having arbitrary different wavelengths λ1, λ2, ..., λN / 2M, and optical signals of the same wavelength are also connected using a polarization coupler. Polarization multiplexed signal is generated by polarization multiplexing, and the optical signal thus generated is generated. The wavelength-multiplexed with a multiplexer, characterized in that it sent to the optical fiber transmission line. At this time, light of the same wavelength λ1 is incident on the optical modulator that superimposes C1 and the optical modulator that superimposes C2. Similarly, an optical modulator that superimposes Cn and an optical modulator that superimposes Cn + 1, such that light having the same wavelength λ2 is incident on the optical modulator that superimposes C3 and the optical modulator that superimposes C4. It is assumed that light having the same wavelength is incident on. Here, n is an odd number. Also, M is the number of time division multiplexing.
When the data sequence is represented by a complex number, an optical vector modulator (IQ modulator) is used as the optical modulator, and the real number component of the data sequence is used as the in-phase component and the imaginary number component is used as the quadrature phase component. Further, the threshold determination for data is performed on the data strings A1, A2,..., AN.

  In one aspect of the optical transceiver, the optical receiver is composed of a demultiplexer, a digital coherent receiver, a delay adjuster, a time division multiplexer, and a decoder, and a data sequence C1, C2,. Wavelength-separated polarization multiplexed and wavelength multiplexed optical signals with / M superimposed, and each optical signal is incident on a digital coherent receiver to hold the amplitude / phase information of the optical signal In this state, the signal is converted into a digital electrical signal, and linear optical signal waveform deterioration caused by chromatic dispersion and polarization mode dispersion caused in the optical fiber transmission line is compensated, polarization separation is performed, and a delay adjuster is used. By compensating for the delay difference between each data sequence generated by the chromatic dispersion of the fiber transmission line by a delay adjuster such as a variable delay line, the data sequences C1, C2, ..., CN / M are obtained as digital electrical signals, Time division multiplexing To obtain data strings B1, B2, ..., BN by time-separating the data strings C1, C2, ..., CN / M, and the operation shown in the previous formula (2A) by the decoder for these data strings By performing the processing, the data strings A1, A2,..., AN are restored. Further, the threshold determination for data is performed on the data strings A1, A2,..., AN.

  According to the present invention, it is possible to reduce the probability of occurrence of a symbol that increases the amplitude of an optical signal to be transmitted and increase the probability of occurrence of a symbol that decreases the amplitude of the optical signal. Therefore, it is possible to generate an optical signal that is not easily affected by the nonlinear optical effect, and it is possible to suppress the waveform deterioration of the optical signal due to the nonlinear optical effect in the optical transmission path.

It is explanatory drawing for demonstrating the principle of this invention, and is a figure for demonstrating the influence which distribution of the generation probability of a symbol has on distribution of signal power. It is explanatory drawing for demonstrating the principle of this invention, and is a figure which shows an example of the relationship between the signal power in the case of using 64QAM as a modulation system, and the frequency | count of generation. It is explanatory drawing for demonstrating the principle of this invention, and is a figure which shows an example of the relationship between the signal power and the frequency | count of generation in the case of using 16QAM as a modulation system. It is a block diagram which shows an example of a structure of the optical transmitter with which the optical transmission system by 1st Embodiment of this invention is provided. It is a block diagram which shows an example of a structure of the optical receiver with which the optical transmission system by 1st Embodiment of this invention is provided. It is a block diagram which shows an example of a structure of the optical transmitter with which the optical transmission system by 2nd Embodiment of this invention is provided. It is a block diagram which shows an example of a structure of the optical receiver with which the optical transmission system by 2nd Embodiment of this invention is provided. It is explanatory drawing for demonstrating the time multiplexing process and the time separation process by the time division multiplexing part in the optical transmission system by 2nd Embodiment of this invention. It is a characteristic view for demonstrating the characteristic of the optical signal in case the 16QAM is used as a modulation system in the optical transmission system by 2nd Embodiment of this invention. It is a block diagram which shows an example of a structure of the optical transmitter with which the optical transmission system by 3rd Embodiment of this invention is provided. It is a block diagram which shows an example of a structure of the optical receiver with which the optical transmission system by 3rd Embodiment of this invention is provided. It is a block diagram which shows an example of a structure of the optical transmitter with which the optical transmission system by 4th Embodiment of this invention is provided. It is a block diagram which shows an example of a structure of the optical receiver with which the optical transmission system by 4th Embodiment of this invention is provided. It is a figure which shows an example of the relationship between the incident power to the optical fiber transmission line of the optical signal in a prior art, and the receiving sensitivity of an optical signal.

Next, embodiments of the present invention will be described with reference to the drawings.
Note that the same reference numeral represents the same element throughout all the embodiments and all the drawings.
(Description of the principle of the present invention)
Prior to the description of the embodiments of the present invention, the principle of the present invention will be described.
In general, the present invention encodes data information to be superimposed on an optical signal, that is, amplitude / phase information of the optical signal using, for example, a Hadamard matrix, and uniformly distributes the probability of occurrence of each symbol based on the central limit theorem. This is a technique for reducing the probability of occurrence of a symbol with a large amplitude and increasing the probability of occurrence of a symbol with a small amplitude by approximately converting to a Gaussian distribution. Details of the Hadamard matrix are described, for example, on page 422 of the document “Digital Communication”. However, the matrix is not limited to a Hadamard matrix, and any matrix can be used as long as the data string is averaged by a plurality of different methods and the data string corresponds one-to-one before and after the conversion.

  FIG. 1 is an explanatory diagram for explaining the principle of the present invention, and for explaining the influence of the distribution of symbol occurrence probability on the distribution of signal power. FIG. 4A shows the relationship between the signal amplitude and the probability of occurrence when the occurrence probability of each symbol follows a uniform distribution and the occurrence probability of each symbol follows a Gaussian distribution in a situation where the average signal power is equal. An example is shown. As shown by a solid line in FIG. 6A, when the probability of occurrence of each symbol follows a Gaussian distribution, the probability of occurrence of a symbol with a large signal amplitude is smaller than when the probability of occurrence of a symbol follows a uniform distribution indicated by a dotted line. It shows a tendency that the probability of occurrence of symbols increases.

  FIG. 1B shows the signal power generation probability distribution in the same situation where the average signal power is the same. As shown by a solid line in FIG. 5B, when the symbol occurrence probability follows a Gaussian distribution, the symbol generation probability with a small signal power increases as compared to when the symbol occurrence probability follows a uniform distribution indicated by a dotted line. . For example, the probability that a symbol having a power smaller than the average of the signal power is generated is 53% in the case of uniform distribution and 64% in the case of Gaussian distribution. This means that the optical signal waveform deterioration due to the nonlinear optical effect is less likely to occur in the case of the Gaussian distribution than in the case of the uniform distribution.

The method of the present invention for converting the distribution of occurrence probability of each symbol from a uniform distribution to a Gaussian distribution will be described. First, consider data strings A ₁ , A 2,..., A _N (N is an integer of 2 or more) having a plurality of different data information. Here, A _1, A2, ..., each data string of A _N represents the amplitude / phase information of the optical signal, and those represented by arbitrary complex. Each data string follows a uniform distribution independent of each other. In general, since the occurrence probability of each symbol corresponding to the data information indicates a uniform distribution, the data sequence A _1, A2, ..., occurrence probability of each symbol in A _N follows the independent uniform distribution. Here, using the N × N Hadamard matrix Hn shown in the following equation (1) for these data strings A ₁ , A 2,..., A _N , an operation shown in the following equation (2), that is, encoding is executed. . This arithmetic operation means that the data strings A ₁ , A 2,..., A _N are multiplexed (averaged) by N different methods. Here, in this embodiment, N is referred to as a multiplexing number.
Note that the N value of the N × N Hadamard matrix is such that the frequency distribution of the light amplitude that is equal to or less than the threshold value of the optical signal waveform deterioration is larger than a desired value, for example, a conventional optical transceiver. Any value may be used as long as the frequency distribution is approximately Gaussian.

In formula (2), B ₁ , B ₂ ,..., B _N represent encoded data strings. For example, when N = 4, the encoded data strings B ₁ , B _{2 ,.} , Represented by equation (3).

FIG. 2 shows an example of the relationship between the signal power and the number of occurrences when 64QAM is used as the modulation method. Here, FIG. 2A shows the signal power and the number of occurrences of optical signals corresponding to the data strings A ₁ , A 2,..., A _N before encoding (when the occurrence probability of symbols follows a uniform distribution). And the optical signal corresponding to the data sequence B ₁ , B ₂ ,..., B _N after encoding the data sequence with the multiplexing number N being 4 (when the probability of symbol occurrence follows a Gaussian distribution) The relationship between the signal power distribution and the number of occurrences (solid line) is shown. FIG. 2B shows the relationship between the signal power and the number of occurrences before encoding (when the occurrence probability of symbols follows a uniform distribution) (dotted line), and the number of multiplexing N is 8, and the data string is encoded. The relationship (solid line) between the distribution of signal power and the number of occurrences is shown (when the occurrence probability of symbols follows a Gaussian distribution). Further, FIG. 2C shows the relationship between the signal power and the number of occurrences before the encoding (when the occurrence probability of symbols follows a uniform distribution) (dotted line), and the number of multiplexing N is 16, and the data string is encoded. The relationship (solid line) between the distribution of signal power and the number of occurrences is shown (when the occurrence probability of symbols follows a Gaussian distribution). Here, in each of FIGS. 2A to 2C, the average of the signal power before encoding and the average of the signal power after encoding are both equal to 0.21.

  As can be understood from FIGS. 2A to 2C, by performing the encoding, the generation of symbols is concentrated at low power. In addition, as the multiplexing number N increases, the generation of symbols tends to concentrate on the lower power side. In this example, since there is no significant difference in the signal power distribution between the multiplexing number N = 8 and the multiplexing number N = 16, it can be said that the multiplexing number N is about 8 to 16. The probability that a signal of less than average power is generated is 49% when the probability of symbol occurrence follows a uniform distribution (before encoding), and the probability of occurrence of a symbol follows a Gaussian distribution with a multiplexing number N = 16 (encoding). The latter is 62%.

FIG. 3 shows an example of the relationship between the signal power and the number of occurrences when 16QAM is used as the modulation method. Here, FIG. 3A shows the signal power and the number of occurrences of the optical signals corresponding to the data strings A ₁ , A 2,..., A _N before encoding (when the occurrence probability of symbols follows a uniform distribution). And the optical signal corresponding to the data sequence B ₁ , B ₂ ,..., B _N after encoding the data sequence with the multiplexing number N being 4 (when the probability of symbol occurrence follows a Gaussian distribution) The relationship between the signal power distribution and the number of occurrences (solid line) is shown. FIG. 3B shows the relationship between the signal power before the encoding (when the probability of occurrence of symbols follows a uniform distribution) and the number of occurrences (dotted line), and the number of multiplexing N is 8, and the data string is encoded. The relationship (solid line) between the distribution of signal power and the number of occurrences is shown (when the occurrence probability of symbols follows a Gaussian distribution). Further, FIG. 3C shows the relationship between the signal power before the encoding (when the probability of occurrence of symbols follows a uniform distribution) and the number of occurrences (dotted line), and the number of multiplexing N is 16, and the data string is encoded. The relationship (solid line) between the distribution of signal power and the number of occurrences is shown (when the occurrence probability of symbols follows a Gaussian distribution). Here, in each of FIGS. 3A to 3C, the average of the signal power before encoding and the average of the signal power after encoding are both equal to 0.27.

  As can be understood from FIGS. 3A to 3C, as in the case where 64QAM is used as the modulation method, the generation of symbols is concentrated at low power by performing encoding. In addition, as the multiplexing number N increases, the generation of symbols tends to concentrate on the lower power side. In this example, there is no significant difference in signal power distribution between the multiplexing number N = 4, the multiplexing number N = 8, and the multiplexing number N = 16, so the multiplexing number N is about 4 to 16. That's enough. The probability of occurrence of a signal of less than average power is 25% when the symbol occurrence probability follows a uniform distribution (before encoding), and the symbol occurrence probability follows a Gaussian distribution with a multiplexing number N = 16 (encoding). The latter is 62%.

By transmitting the optical signals corresponding to the encoded data strings B ₁ , B ₂ ,..., B _{N in} which the generation of the above-described symbols is concentrated at low power from the transmission side to the reception side via the optical transmission path. It is possible to suppress optical signal waveform deterioration due to the nonlinear optical effect on the optical transmission line.

On the other hand, on the receiving side, using the N × N Hadamard matrix shown in the same formula and the coding (1) on the transmission side of the above, the data sequence B ₁ after _encoding, B 2, _..., with respect to B _N Then, the calculation shown in the following formula (4) is executed. Thus, at the receiving side, the data sequence _B 1 after _encoding, B 2, ..., data sequence _{B N} is pre-encoding _A 1, A2, ..., is decoded into _{A N.} Threshold values are determined for each of the decoded data strings A ₁ , A 2,..., A _N to identify symbols.

  The present invention suppresses waveform degradation of an optical signal due to a nonlinear optical effect centered on a third-order nonlinear optical effect that is influenced by the cube of the amplitude of light. In particular, the present invention encodes the amplitude / phase information of an optical signal using, for example, a Hadamard matrix, thereby converting the probability of occurrence of each symbol from a uniform distribution to an approximately Gaussian distribution based on the central limit theorem. . As a result, the probability of occurrence of a symbol having a small amplitude is increased under the situation where the average signal power is equal. According to the present invention, there is an effect that the frequency of waveform deterioration can be reduced particularly in a situation where the waveform deterioration of the optical signal cannot be ignored in the form of a step function with respect to the value of the optical amplitude serving as a certain threshold value.

  2 and 3, the average value of the optical amplitude is used as the value of the amplitude of the light whose waveform deterioration cannot be ignored. However, the optical transmission according to the present invention is used as the value of the amplitude of the light whose waveform deterioration cannot be ignored. The present invention is effective in the case of an arbitrary light amplitude value in which the frequency exceeding the light amplitude of the conventional optical transceiver is larger than the frequency exceeding the light amplitude of the optical receiver and the optical receiver. is there.

(First embodiment)
Next, a first embodiment of the present invention will be described with reference to FIG. 4 and FIG.
In the present embodiment, the encoded data strings B ₁ , B ₂ ,..., B _N generated based on the principle of the present invention described above are wavelength-multiplexed and transmitted.

[Description of configuration]
The optical transmission system according to the present embodiment includes the optical transmitter 100 in FIG. 4 and the optical receiver 200 in FIG. 5 that are connected to each other via an optical transmission path 900. The optical transmission path 900 is connected to the optical transmitter 100 from the optical transmitter 100. An optical signal is transmitted to the optical receiver 200 via the optical receiver 200. In this embodiment, an optical fiber transmission line is assumed as the optical transmission line 900. However, the present invention is not limited to this example, and any medium can be used as long as an optical signal can be transmitted.

FIG. 4 shows an example of the configuration of the optical transmitter 100 included in the optical transmission system according to the present embodiment. The optical transmitter 100 generates an optical signal S and sends it to the optical transmission line 900, and includes an encoding unit 110 and a multiplexing unit 120. Here, the encoding unit 110 uses the input data sequences A ₁ , A ₂ ,..., A _N represented by symbols according to a uniform distribution of occurrence probabilities as transmission data sequences represented by symbols according to a Gaussian distribution of occurrence probabilities. B ₁ , B ₂ ,..., B _N are encoded (converted). The multiplexing unit 120 multiplexes the transmission data sequence B ₁ , B ₂ ,..., B _N obtained by the encoding unit 110 into the optical signal S. Here, the input data strings A ₁ , A ₂ ,..., _AN are data strings having a plurality of different data, and represent the amplitude and phase information of the optical signal, and are represented by arbitrary complex numbers. Each data string follows a uniform distribution independent of each other.
In FIG. 4, solid arrows represent optical signals, and dotted arrows represent electrical signals. The same applies to the other drawings.

In the present embodiment, the encoding unit 110 converts the input data sequences A ₁ , A ₂ ,..., A _N into transmission data sequences B ₁ , B ₂ ,. .., _BN includes an arithmetic processing unit 111 that performs a matrix operation for encoding into _N. Processing unit 111, by performing the calculation of Expression (2) described above, the input data sequence _{A 1,} A 2, ..., transmits the _{A N} data sequence _{B 1,} B 2, ..., coded _{B N} To do.

The multiplexing unit 120 includes N light sources 121 _{1 to} 121 _N , N light modulation units 122 _{1 to} 122 _N, and a multiplexing unit 123. Light source ₁₂₁ 1 to 121 _N, respectively, is intended to generate continuous light of wavelength lambda ₁ to [lambda] _N a (CW light). Optical modulating sections ₁₂₂ 1 to 122 _N, the transmission data sequence continuous light source ₁₂₁ 1 to 121 _N is caused _{B 1,} B 2, ..., by optical modulation based on _{B N,} the wavelength lambda ₁ to [lambda] _N modulated lights corresponding to N are generated. Multiplexing section 123 is to be a light modulation unit ₁₂₂ 1 to 122 _N N pieces of modulated light corresponding to the generated wavelength lambda ₁ to [lambda] _N by combining generates the optical signal S. The optical signal S generated by the multiplexing unit 123 is sent to the optical transmission line 900.

FIG. 5 shows an example of the configuration of the optical receiver 200 included in the optical transmission system according to the present embodiment. The optical receiver 200 receives an optical signal S from the optical transmission line 900 and restores a data string from the optical signal S, and includes a separation unit 210 and a decoding unit 220. Among these, the separation unit 210 separates the received data sequence represented by symbols whose occurrence probability follows a Gaussian distribution from the optical signal S. Here, in the present embodiment, the reception data sequence obtained by separation by the separation unit 210 corresponds to the transmission data sequences B ₁ , B ₂ ,..., B _N in the optical transmitter 100 described above. Hereinafter, similarly to the above-described transmission data sequence, the reception data sequence obtained by the separation unit 210 is referred to as B ₁ , B ₂ ,..., B _N.

The separation unit 210 includes a demultiplexing unit 211, N digital coherent reception units 212 _{1 to} 212 _N , and delay adjustment units 213 _{1 to} 213 _N. Of these, the demultiplexing unit 211 demultiplexes (wavelength-separates) the optical signal S incident from the optical transmission line 900 into N optical signals for each wavelength. The digital coherent receiving units 212 _{1 to} 212 _N convert the detection results of the N optical signals demultiplexed by the demultiplexing unit 211 and local light (not shown) into digital signals and output the digital signals. As the digital coherent receivers 212 _{1 to} 212 _N , a general digital coherent receiver conventionally used in optical digital coherent communication can be used. The delay adjusting units 213 _{1 to} 213 _N perform processing for compensating for a delay caused by chromatic dispersion or the like in the optical transmission path 900 for the digital signals output from the digital coherent receiving units 212 _{1 to} 212 _N. It is. From the delay adjustment units 213 _{1 to} 213 _N , received data strings B ₁ , B ₂ ,..., B _N are output as output signals of the separation unit 210.

The decoding unit 220 decodes the received data sequence B ₁ , B ₂ ,..., B _N obtained by the separation unit 210 into an output data sequence represented by symbols having a uniform probability of occurrence. . Here, in the present embodiment, the output data sequence decoded by the decoding unit 220 corresponds to the input data sequences A ₁ , A ₂ ,..., A _N in the optical transmitter 100 described above. Hereinafter, similarly to the above-described input data sequence, output data sequences obtained by decoding by the decoding unit 220 are referred to as A ₁ , A ₂ ,..., A _N.

In this embodiment, the decoding unit 220 converts the received data sequences B ₁ , B ₂ ,..., B _N into output data sequences A ₁ , A ₂ ,. ..., and a processing unit 221 for performing the matrix operation to be coded into a _N. Processing unit 221, by performing the calculation of Equation (4) described above, the received data sequence _{B 1,} B 2, ..., output _{B N} data sequences _{A 1,} A 2, ..., decoded _{A N} To do.

Next, operations of the optical transmitter 100 and the optical receiver 200 constituting the optical transmission system according to the present embodiment will be described.
In the present embodiment, the optical transmitter 100 performs wavelength multiplexing by superimposing a plurality of data strings B ₁ , B ₂ ,..., B _N obtained by encoding in the encoding unit 110 on light of different wavelengths. And transmit.

In the optical transmitter 100 of FIG. 4, input data sequences A ₁ , A ₂ ,..., A _N are input as digital electric signals to the encoding unit 110 and subjected to the arithmetic processing represented by the above-described equation (2). As a result, transmission data strings B ₁ , B ₂ ,..., B _N are generated. Transmission data sequence _{B 1,} B 2, ..., each data row _{B 1} constituting the _{B N,} the data sequence _{B 2,} ..., data information data sequence _{B N} has, respectively, different light modulation unit as a digital electric signal 122 _{1 to} 122 _N and is superimposed as amplitude / phase information on light of different wavelengths λ ₁ , λ ₂ ,..., Λ _N. The optical signal S generated in this way is wavelength-multiplexed using the multiplexing unit 123 and then transmitted to the optical transmission line 900. When each data sequence is a complex number, for example, an optical vector modulator (IQ modulator) is used as the optical modulators 122 _{1 to} 122 _N , the real number component of the data sequence is set to the in-phase component, and the imaginary number component is set to the quadrature phase component. Apply modulation.

On the other hand, in the optical receiver 200 of FIG. 5, the wavelength-multiplexed optical signal S is demultiplexed (wavelength separated) by the demultiplexing unit 211, and each demultiplexed optical signal is sent to the digital coherent receiving units 212 _{1 to} 212 _N. Incident. The digital coherent receiving units 121 _{1 to} 212 _N convert the incident optical signal into a digital electric signal while holding the amplitude / phase information. Also, the digital coherent receiving units 121 _{1 to} 212 _N perform processing for compensating for linear optical signal waveform deterioration caused by chromatic dispersion, polarization mode dispersion, or the like that occurs in the optical transmission line 900, and receive data strings B ₁ , B ₂ ,..., B _N are obtained.

Here, the received data sequence B _1, B 2, _..., in between the data string of B _N, the wavelength dispersion and the optical transmission path 900, known delay due to path differences between the data in the optical transmitter 100 Since a difference arises, this is compensated by the delay adjustment units 213 _{1 to} 213 _N. As the delay adjusting units 213 _{1 to} 213 _N , a variable delay line or the like can be used. The reception data strings B ₁ , B ₂ ,..., B _N thus subjected to the delay adjustment are input to the decoding unit 220 as digital electric signals.

The decoding unit 220 performs output processing from the received data strings B ₁ , B ₂ ,..., B _N to the output data strings A ₁ , A _{2 ,.} Thus, the input data strings A ₁ , A ₂ ,..., A _N on the transmission side are restored. Output data array A ₁ obtained by the decoding unit _220, A 2, _..., threshold determination for each data A _N is performed, the symbol is identified.

  According to the first embodiment described above, the probability of occurrence of each symbol is not uniformly distributed, the probability of occurrence of a symbol with a large amplitude can be reduced, and the probability of occurrence of a symbol with a small amplitude can be increased. Thereby, optical signal waveform deterioration by a nonlinear optical effect can be suppressed.

(Second Embodiment)
Next, a second embodiment of the present invention will be described.
In the present embodiment, the encoded data strings B ₁ , B ₂ ,..., B _N generated based on the principle of the present invention described above are time-division multiplexed to be superimposed on a single wavelength of light and transmitted. To do.

  FIG. 6 is a block diagram illustrating an exemplary configuration of the optical transmitter 300 included in the optical transmission system according to the present embodiment. FIG. 7 illustrates an exemplary configuration of the optical receiver 400 included in the optical transmission system according to the present embodiment. FIG.

As shown in FIG. 6, the optical transmitter 300 according to the present embodiment includes an encoding unit 110 and a multiplexing unit 320. Among these, the multiplexing unit 320 includes a time division multiplexing unit 321, a light source 322, and an optical modulation unit 323. Here, the time division multiplexing unit 321 time-multiplexes the transmission data string B ₁ , B ₂ ,..., B _N to the data string C of the digital electric signal. The optical modulation unit 323 optically modulates the continuous light having the wavelength λ generated by the light source 322 based on the transmission data strings B ₁ , B ₂ ,..., B _N multiplexed by time division. Is generated. The optical signal S is sent to the optical transmission line 900.

On the other hand, as illustrated in FIG. 7, the optical receiver 400 includes a separation unit 410 and a decoding unit 220. In the present embodiment, the separation unit 410 includes a digital coherent reception unit 411 and a time division multiplexing unit 412. Here, the digital coherent receiving unit 411 converts the detection result of the optical signal S incident from the optical transmission line 900 and local light (not shown) into a data sequence C of a digital electric signal and outputs the data. As the digital coherent receiving unit 411, a general digital coherent receiver conventionally used in optical digital coherent communication can be used. The time division multiplexing unit 412 time-separates the digital signal C into received data strings B ₁ , B ₂ ,..., B _N , and performs processing reverse to the time division multiplexing unit 321 in the optical transmitter 300 described above. To implement. Received data strings B ₁ , B ₂ ,..., B _N are input to decoding section 220.

It is assumed that input data strings A ₁ , A ₂ ,..., A _N having a plurality of different data information represent amplitude / phase information of the optical signal and are represented by arbitrary complex numbers. The input data sequence A _1, A 2, _..., each data string of A _N shall comply with the mutually independent uniform distribution.

Next, operations of the optical transmitter 300 and the optical receiver 400 constituting the optical transmission system according to the present embodiment will be described.
In the optical transmitter 300 of FIG. 6, each data sequence of input data sequences A ₁ , A ₂ ,..., A _N is input to the encoding unit 110 as a digital electrical signal. The encoding unit 110 generates transmission data strings B ₁ , B ₂ ,..., B _N by performing the arithmetic processing shown in the above-described equation (2). Data information of each data sequence of the transmission data sequence B ₁ , B ₂ ,..., B _N is input to the time division multiplexing unit 321 as a digital electric signal, and the time division multiplexing unit 321 performs a new data sequence by time multiplexing. C is generated. The data string C is input to the light modulation unit 323 as a digital electric signal. The optical modulation unit 323 generates the optical signal S by superimposing the data string C on the local light of the wavelength λ in the form of amplitude / phase information. The optical signal S generated in this way is sent to the optical transmission line 900. When the data sequence is a complex number, for example, an optical vector modulator (IQ modulator) is used as the optical modulation unit 323, and the real number component of the data sequence is set to the in-phase component and the imaginary number component is modulated to the quadrature phase component. .

On the other hand, in the optical receiver 400 of FIG. 7, the optical signal S enters the digital coherent receiving unit 411 from the optical transmission line 900. The digital coherent receiving unit 411 converts the optical signal S into a digital electrical signal while maintaining the amplitude / phase information, and linear optical signal waveform degradation caused by chromatic dispersion or polarization mode dispersion generated in the optical transmission line 900 To obtain the data string C. The data string C is input to the time division multiplexing unit 412 as a digital electric signal. Time division multiplexing unit 412 receives and separates at a data string C data sequence _{B 1,} B 2, ..., and outputs the _{B N.} The received data strings B ₁ , B ₂ ,..., B _N are input to the decoding unit 220 as digital electric signals.

The decoding unit 220 performs output processing from the received data strings B ₁ , B ₂ ,..., B _N to the output data strings A ₁ , A _{2 ,.} Thus, the input data strings A ₁ , A ₂ ,..., A _N on the transmission side are restored. Output data array A ₁ generated by the decoding unit _220, A 2, _..., threshold determination for each data A _N is performed, the symbol is identified.

FIG. 8 is an explanatory diagram for explaining time multiplexing by the time division multiplexing unit 321 and time separation by the time division multiplexing unit 421 in the optical transmission system according to the present embodiment. Here, the value in the time slot t of the data string Bm is Bm (t). As shown in FIG. 6 (a), the received data sequence _{B 1, B 2, ...,} the data sequence _{B 1} constituting the _{B N,} the data sequence _{B 2,} ... a data sequence _{B N} in series in accordance with a predetermined rule data By generating the column C, the received data columns B ₁ , B ₂ ,..., B _N are time-multiplexed with the data column C. Also, as shown in FIG. 5B, by parallelizing the data strings constituting the data string C according to the rule opposite to the time multiplexing, the data string C is received data strings B ₁ , B ₂ , ..., in a time separated into _{B N.}

FIG. 9 is a characteristic diagram for explaining the characteristics of an optical signal when 16QAM is used as a modulation method in the optical transmission system according to the present embodiment. FIG (a) and FIG. (B), the data sequence _{A 1,} A 2, ..., in the case of using the 64QAM as _{A N,} the amplitude occurrence distribution of the in-phase component data string _{B 1} and a data row C Each is shown. Here, the multiplexing number N is four. Further, FIG. (C) and FIG (d) is shown in the same conditions, the data sequence B ₁ and a data row C of the occurrence distribution of the signal power, respectively. As illustrated in FIGS. 6A and 6B, the amplitude distribution of the data string C, which is a time-multiplexed data string, is the distribution of the data string B ₁ that is the data before time-multiplexing. Similar to the amplitude distribution, a Gaussian distribution is obtained. Further, as illustrated in FIG. (C) and FIG (d), the signal power of the data string C show almost the same occurrence distribution and signal power data string B _1. Here, the data sequence _{A 1,} A 2, ..., because of the use of 64QAM as _{A N,} the data sequence _{A 1,} A 2, ..., generation distribution of the signal power of _{A N} is the dotted line in FIGS. 2 (a) As shown in the characteristics.

  According to the present embodiment, delay adjustment is unnecessary in the optical receiver 400 as compared to the first embodiment described above. Further, it is not necessary to prepare the number of optical modulation units corresponding to the multiplexing number N in the optical transmitter 300. Therefore, the configuration of the optical transmitter 300 and the optical receiver 400 can be simplified.

(Third embodiment)
Next, a third embodiment of the present invention will be described.
In the present embodiment, encoded data strings B ₁ , B ₂ ,..., B _N generated based on the principle of the present invention described above are superimposed on light of different wavelengths and transmitted as polarization multiplexed signals.

FIG. 10 is a block diagram illustrating an example of the configuration of the optical transmitter 500 included in the optical transmission system according to the present embodiment. The optical transmitter 500 includes a coding unit 110 and a multiplexing unit 520. Among these, the multiplexing unit 520 includes light sources 521 _{1 to} 521 _{N / 2} , light modulation units 522 _{1 to} 522 _N , polarization coupling units 523 _{1 to} 523 _{N / 2} , and a combining unit 524. The light sources 521 _{1 to} 521 _{N / 2} generate light having wavelengths λ _{1 to} λN _{/ 2} . Optical modulating sections ₅₂₂ 1 _{~522 N,} the transmission data sequence _{B 1,} B 2, ..., and the light modulating light wavelength λ ₁ ~λ _{N / 2} based on _{B N} modulated light (x-polarization, y-polarized Wave). Polarization coupling unit ₅₂₃ 1 _{~523 N / 2} is intended to generate a light signal _D 1 _{~D N / 2} by the modulated light from the optical modulation unit ₅₂₂ 1 _{~522 N} and polarization coupling. The optical signals D _{1 to} D _{N / 2} are input to the multiplexing unit 524. The multiplexing unit 524 combines the optical signals D _{1 to} D _{N / 2} with the optical signal S. The optical signal S is sent to the optical transmission line 900.

FIG. 11 shows an example of the configuration of the optical receiver 600 included in the optical transmission system according to the present embodiment. The optical receiver 600 receives the optical signal S from the optical transmission line 900 and restores the data string, and includes a separation unit 610 and a decoding unit 220. Among these, the separation unit 610 separates the received data strings B ₁ , B ₂ ,..., B _N represented by symbols whose occurrence probabilities follow a Gaussian distribution from the optical signal S.

The separating unit 610 includes a demultiplexing unit 611, N digital coherent receiving units 612 _{1 to} 612 _{N / 2} , and delay adjusting units 613 _{1 to} 613 _N. Among these, the demultiplexing unit 611 demultiplexes (wavelength-separates) the optical signal S incident from the optical transmission line 900 into N / 2 optical signals for each wavelength. The digital coherent receiving units 612 _{1 to} 612 _{N / 2} convert the detection results of N / 2 optical signals demultiplexed by the demultiplexing unit 611 and local light (not shown) into digital electric signals and output the digital electric signals. Is. The delay adjustment units 613 _{1 to} 613 _N perform processing for compensating for delay caused by chromatic dispersion in the optical transmission line 900 for the digital signals output from the digital coherent reception units 612 _{1 to} 612 _{N / 2.} To do. The received data strings B ₁ , B ₂ ,..., B _N from the delay adjustment units 613 _{1 to} 613 _N are output to the decoding unit 220 as output signals of the separation unit 610.

It is assumed that input data strings A ₁ , A ₂ ,..., A _N having a plurality of different data information represent amplitude / phase information of the optical signal and are represented by arbitrary complex numbers. The input data sequence A _1, A 2, _..., each data string of A _N shall comply with the mutually independent uniform distribution.

Next, operations of the optical transmitter 500 and the optical receiver 600 included in the optical transmission system according to the present embodiment will be described.
In FIG. 10, each data string of input data strings A ₁ , A ₂ ,..., A _N is input to the encoding unit 110 as a digital electric signal. The encoding unit 110 generates data strings B ₁ , B ₂ ,..., B _N by performing the arithmetic processing shown in the above-described equation (2). Data information included in each data string is input as a digital electric signal to different optical modulators 522 _{1 to} 522 _N , and is superimposed on light of different wavelengths λ ₁ , λ ₂ ,..., Λ _{N / 2} as amplitude / phase information. Is done. At this time, the optical modulator 522 ₂ for superimposing an optical modulator 522 ₁ and the data sequence _{B 2} which superimposes a data sequence _{B 1} is the same wavelength lambda ₁ of the light is incident. Similarly, as such the optical modulator 522 ₄ for superimposing an optical modulator 522 ₃ and data string B ₄ for superimposing a data sequence B ₃ are the same wavelength lambda ₂ of light is incident, superimposes the data sequence B _n The light of the same wavelength is incident on the optical modulator that superimposes the data modulator _{Bn + 1} and the data modulator _Bn . Here, n is an odd number.

The polarization of the optical signals generated by the optical modulators 522 _{1 to} 522 _N in this manner is polarization-multiplexed by using the polarization couplers 523 _{1 to} 523 _{N / 2} to polarize and multiplex the optical signals. generating a Shigeru Hata signal _D 1 _{~D N / 2.} Further, the multiplexing unit 524 generates the optical signal S by wavelength multiplexing the polarization multiplexed signals D _{1 to} D _{N / 2} and sends it to the optical transmission line 900. When the data sequence is a complex number, for example, an optical vector modulator (IQ modulator) is used as the optical modulators 522 _{1 to} 522 _N , the real number component of the data sequence is the in-phase component, and the imaginary number component is the quadrature phase component. As a modulation.

In the optical receiver 600 of FIG. 11, the demultiplexing unit 611 wavelength-separates the wavelength-multiplexed optical signal S, and each of the wavelength-separated optical signals enters the digital coherent receiving units 612 _{1 to} 612 _{N / 2.} . The digital coherent receivers 612 _{1 to} 612 _{N / 2} convert optical signals into digital electrical signals while maintaining amplitude / phase information, and are linear due to chromatic dispersion or polarization mode dispersion generated in the optical transmission line 900. The optical signal waveform degradation is compensated and polarization separation is performed. In this way, received data strings B ₁ , B ₂ ,..., B _N are obtained. Since there is a known delay difference due to the chromatic dispersion of the optical transmission line 900 and the path difference between the data in the transmitter between the data strings, this is compensated by the delay adjusters 613 _{1 to} 613 _N. To do. As the delay adjusters 613 _{1 to} 613 _N , a variable delay line or the like can be used. The data strings B ₁ , B ₂ ,..., B _N thus subjected to the delay adjustment are input to the decoding unit 220 as digital electric signals. The decoding unit 220 performs output processing from the received data strings B ₁ , B ₂ ,..., B _N to the output data strings A ₁ , A _{2 ,.} Thus, the input data strings A ₁ , A ₂ ,..., A _N on the transmission side are restored. Output data array A ₁ generated by the decoding unit _220, A 2, _..., threshold determination for each data A _N is performed, the symbol is identified.

(Fourth embodiment)
Next, a fourth embodiment of the present invention will be described.
This embodiment is a combination of the first embodiment, the second embodiment, and the third embodiment, that is, a combination of wavelength multiplexing, time division multiplexing, and polarization multiplexing.

  FIG. 12 is a block diagram illustrating an exemplary configuration of the optical transmitter 700 included in the optical transmission system according to the present embodiment. FIG. 13 illustrates an exemplary configuration of the optical receiver 800 included in the optical transmission system according to the present embodiment. FIG.

An optical transmitter 700 in FIG. 12 includes an encoding unit 110 and a multiplexing unit 720. Of these, multiplexing section 720, time division multiplexing unit ₇₂₁ 1 _{~721 N / M,} the light source ₇₂₂ 1 _{~722 N / 2M,} the light modulation unit ₇₂₃ 1 _{~723 N / M,} the polarization coupling unit ₇₂₄ 1 _{~724 N / 2M} and a combining unit 725. Here, time division multiplexing unit ₇₂₁ 1 _{~721 N / M} is an element corresponding to the division multiplexing unit 321 when the second embodiment, the transmission data sequence _{B 1,} B 2, ..., time-multiplexed _{B N} The transmission data strings C ₁ , C ₂ ,..., C _{N / M} are generated. Optical modulating sections ₇₂₃ 1 _{~723 N / M} is an element corresponding to the light modulation unit ₅₂₂ 1 _{~522 N} of the third embodiment, when the transmission is multiplexed data stream _{C 1,} C 2, ..., to _{C N} Based on this, the modulated light (x polarization, y polarization) is generated by optically modulating the continuous light of the wavelengths λ _1, λ ₂ ,..., Λ _{N / 2M} generated by the light sources 722 _{1 to} 722 _{N / 2M.} Is. Polarization coupling unit ₇₂₄ 1 _{~724 N / 2M} is the element corresponding to ₅₂₃ 1 _{~523 N / 2} of the third embodiment, the light modulation unit ₇₂₃ 1-723 modulated light from _{N / M} (x polarization , Y-polarization) to generate optical signals D _{1 to} D _{N / 2M} by polarization coupling. The optical signals D _{1 to} D _{N / 2M} are input to the multiplexing unit 725. The multiplexing unit 725 combines the optical signals D _{1 to} D _{N / 2M} with the optical signal S. The optical signal S is sent to the optical transmission line 900.

The optical receiver 800 in FIG. 13 includes a separation unit 810 and a decoding unit 220. Among them, the separation unit 810 includes a demultiplexing unit 811, a digital coherent receiving unit 812 _{1 to} 812 _{N / 2M} , a delay adjusting unit 813 _{1 to} 813 _{N / M} , and a time division multiplexing unit 814 _{1 to} 814 _{N / M.} The Among these, the demultiplexing unit 811 is an element corresponding to the demultiplexing unit 611 of the third embodiment, and the optical signal S incident from the optical transmission line 900 is divided into N / 2 optical signals D ₁ to D 2 for each wavelength. _{Demultiplexes} (wavelength separation) into _{DN / 2M} . Digital coherent receiver ₈₁₂ 1 _{~812 N / 2M} is the element corresponding to the digital coherent receiver ₆₁₂ 1 _{~612 N / 2} of the third embodiment, the demultiplexing unit 811 demultiplexed N / 2M pieces of The detection result of the optical signal and local light (not shown) is converted into a digital signal and output. Delay adjustment units 813 _{1 to} 813 _{N / 2M} are elements corresponding to the delay adjustment units 613 _{1 to} 613 _N of the third embodiment, and are digital signals output from the digital coherent reception units 813 _{1 to} 813 _{N / 2M.} On the other hand, processing for compensating for a delay caused by chromatic dispersion or the like in the optical transmission line 900 is performed. The data strings C ₁ , C ₂ ,..., C _{N / M} from the delay adjustment units 813 _{1 to} 813 _{N / 2M} are input to the time division multiplexing units 814 _{1 to} 814 _{N / M.} Time division multiplexing unit ₈₁₄ 1 _{~814 N / M,} the data sequence _{C 1, C 2, ...,} C N / M the received data sequence _{B 1,} B 2, ..., is to time separate _{B N.}

It is assumed that input data strings A ₁ , A ₂ ,..., A _N having a plurality of different data information represent amplitude / phase information of the optical signal and are represented by arbitrary complex numbers. The input data sequence A _1, A 2, _..., each data string of A _N shall comply with the mutually independent uniform distribution.

Next, operations of the optical transmitter 700 and the optical receiver 800 constituting the optical transmission system according to the present embodiment will be described.
In FIG. 12, each data sequence of input data sequences A ₁ , A ₂ ,..., A _N is input to the encoding unit 110 as a digital electrical signal. The encoding unit 110 generates transmission data strings B ₁ , B ₂ ,..., B _N by performing the arithmetic processing shown in the above-described equation (2). These transmission data strings are input to the time division multiplexing units 721 _{1 to} 721 _{N / M.} Time division multiplexing unit ₇₂₁ 1 _{~721 N / M,} the transmission data sequence _{B 1,} B 2, ..., the data string by time multiplexing the _{B N C 1, C 2,} ..., to produce a _{C N / M.} Here, M is the number of time division multiplexing. Data information of each data string is input to different optical modulators 723 _{1 to} 723 _{N / M} as digital electric signals, and has different wavelengths λ ₁ , λ ₂ ,..., Λ _{N / 2M} as amplitude / phase information. Superimposed on the light. At this time, the optical modulator 723 ₂ for superimposing an optical modulator 723 ₁ and the data sequence _{C 2} of superimposing data sequence _{C 1} is the same wavelength lambda ₁ of the light is incident. Similarly, as such the optical modulator 723 ₄ for superimposing an optical modulator 723 ₃ and data string C ₄ to superimpose a data string C ₃ are the same wavelength lambda ₂ of light is incident, superimposes the data sequence C _n The light of the same wavelength is incident on the optical modulator that superimposes the data modulator Cn _{+ 1} and the data modulator to be performed. Here, n is an odd number.

In this way, the optical signals generated by the optical modulation units 723 _{1 to} 723 _{N / M} are polarization-multiplexed with a pair of optical signals having the same wavelength using the polarization coupling units 724 _{1 to} 724 _{N / 2M} , Polarization multiplexed signals D _{1 to} D _{N / 2M} are generated. Further, the multiplexing unit 725 generates an optical signal S by wavelength multiplexing the polarization multiplexed signals D _{1 to} D _{N / 2M} and sends it to the optical transmission line 900. When the data sequence is a complex number, for example, an optical vector modulator (IQ modulator) is used as the optical modulators 724 _{1 to} 724 _{N / 2M} , the real number component of the data sequence is the in-phase component, and the imaginary number component is orthogonal. Modulation is performed as a phase component.

In the optical receiver 800 of FIG. 13, the demultiplexing unit 811 wavelength-separates the wavelength-multiplexed optical signals, and the respective optical signals D ₁ , D ₂ ,..., _{DN / 2M} are digitally coherent receiving units 812 ₁ to Incident at 612 _{N / 2M} . In the digital coherent receiving units 812 _{1 to} 812 _{N / 2M} , the optical signal is converted into a digital electric signal while maintaining the amplitude / phase information, and is caused by chromatic dispersion or polarization mode dispersion generated in the optical transmission line 900. Linear optical signal waveform degradation is compensated and polarization separation is performed. In this way, received data strings C ₁ , C ₂ ,..., C _{N / M} are obtained.

Here, since there is a known delay difference due to the chromatic dispersion of the optical transmission line 900 and the path difference between the data in the transmitter between the data strings, the delay adjusters 813 _{1 to} 813 _{N /} This is compensated by _2M . As the delay adjusters 813 _{1 to} 813 _{N / M} , a variable delay line or the like can be used. The data strings C ₁ , C ₂ ,..., C _{N / M} subjected to the delay adjustment in this way are input to the time division multiplexing units 814 _{1 to} 814 _{N / M} as digital electric signals. Time division multiplexing unit ₈₁₄ 1 _{~814 N / M,} the data sequence _{C 1,} C 2, ..., received by time division _{C N} data sequence _{B 1,} B 2, ..., and generates a _{B N.} Received data strings B ₁ , B ₂ ,..., B _N are input to decoding section 220. The decoding unit 220 performs output processing from the received data strings B ₁ , B ₂ ,..., B _N to the output data strings A ₁ , A _{2 ,.} Thus, the input data strings A ₁ , A ₂ ,..., A _N on the transmission side are restored. Threshold values are determined for each data of the data strings A ₁ , A ₂ ,..., A _N to identify symbols.

In each of the embodiments described above, the present invention is expressed as an optical transmitter and an optical receiver. However, the present invention can also be expressed as an optical transmission method and an optical reception method. In this case, the optical transmission method according to the present invention is an optical transmission method for sending an optical signal to an optical transmission line, and an input data sequence represented by symbols having a uniform probability of occurrence is represented by a Gaussian distribution. It can be expressed as an optical transmission method including a code step for encoding a transmission data sequence represented by a symbol according to the above and a multiplexing step for multiplexing the transmission data sequence obtained by the code step to the optical signal. .
The optical reception method according to the present invention is an optical reception method for receiving an optical signal from an optical transmission line, and a separation step of separating a received data sequence represented by symbols having an occurrence probability following a Gaussian distribution from the optical signal. And a decoding step of decoding the received data sequence obtained by the separation step into an output data sequence represented by symbols having a uniform probability of occurrence. .

Although the embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment, and can be arbitrarily modified or modified without departing from the gist of the present invention.
For example, the optical transmitters and optical receivers according to the above-described embodiments may be code-decoded using either phase or amplitude information, or may be processed by analog signal processing other than digital signal processing. .

100 ... optical transmitter, 110 ... encoding unit, 120 ... multiplexing _unit, 121 1 to 121 N _... light _source, 122 1 to 122 N _... optical modulation section, 123 ... multiplexing unit, 200 ... optical receiver, 210 ... separating portion , 211 ... demultiplexing unit, 212 _{1 to} 212 _N ... digital coherent receiving unit, 213 ₁ ... 213 _N ... delay adjustment unit, 220 decoding unit, 300 ... optical transmitter, 310 ... encoding unit, 320 ... multiplexing unit, 321 ... Time division multiplexing unit, 322... Light source, 323 .. optical modulation unit, 400... Optical receiver, 410 .. separation unit, 411... Digital coherent reception unit, 412. 521 _{1 to} 521 _{N / 2} ... Light source, 522 _{1 to} 522 _N ... Optical modulation unit, 523 _{1 to} 523 _{N / 2} ... Polarization coupling unit, 524 ... multiplexing unit, 600 ... optical receiver, 610. 611 ... branching unit, 612 _{1 to} 612 _{N / 2} ... digital coherent receiving unit, 613 _{1 to} 613 _N ... delay adjustment unit, 700 ... optical transmitter, 720 ... multiplexing unit, 721 _{1 to} 721 _{N / M} ... time division multiplexing , 722 _{1 to} 722 _{N / 2M} ... light source, 723 _{1 to} 723 _{N / 2M} ... optical modulator, 724 _{1 to} 724 _{N / 2M} ... polarization coupling part, 725 ... multiplexing part, 800 ... optical receiver, 810 ... separation unit, 811 ... demultiplexing unit, 812 _{1 to} 812 _{N / 2M} ... digital coherent reception unit, 813 _{1 to} 813 _{N / M} ... delay adjustment unit, 814 _{1 to} 814 _{N / M} ... time division multiplexing unit, 900 ... Optical transmission line (optical fiber).

Claims (6)

An optical transmission system comprising an optical transmitter and an optical receiver connected to each other via an optical transmission path, and transmitting an optical signal from the optical transmitter to the optical receiver via the optical transmission path. ,
The optical transmitter is
An input data sequence (A1, A2,... AN) represented by symbols according to a uniform distribution of occurrence probabilities is changed to a transmission data sequence (B1, B2,... BN) represented by symbols according to a Gaussian distribution of occurrence probabilities . A coding unit that codes using the N × N Hadamard matrix Hn shown in the following equation (1) ;
A multiplexing unit for multiplexing said obtained by the encoding unit transmission data sequence (B1, B2, ... BN) to the optical signal,
With
The optical receiver is:
A separation unit that separates a received data sequence (B1, B2,... BN) represented by symbols having an occurrence probability according to a Gaussian distribution from the optical signal;
The received data sequence (B1, B2,... BN) obtained by the separation unit is converted into an output data sequence (A1, A2,... AN) represented by symbols according to a distribution with a uniform occurrence probability by the following formula ( A decoding unit for decoding using the N × N Hadamard matrix Hn shown in 2) ;
Equipped with a,
An optical transmission system in which the N value of the N × N Hadamard matrix is set based on a frequency distribution of the amplitude of light that is equal to or less than a threshold for waveform degradation of the optical signal .

An optical transmitter for sending an optical signal to an optical transmission line,
An input data sequence (A1, A2,... AN) represented by symbols according to a uniform distribution of occurrence probabilities is changed to a transmission data sequence (B1, B2,... BN) represented by symbols according to a Gaussian distribution of occurrence probabilities . A coding unit that codes using the N × N Hadamard matrix Hn shown in the following equation (3) ;
A multiplexing unit for multiplexing said obtained by the encoding unit transmission data sequence (B1, B2, ... BN) to the optical signal,
Equipped with a,
An optical transmitter in which the N value of the N × N Hadamard matrix is set based on a frequency distribution of the amplitude of light that is equal to or less than a threshold for waveform degradation of the optical signal .

An optical receiver for receiving an optical signal from an optical transmission line,
A separation unit that separates a received data sequence (B1, B2,... BN) represented by symbols having an occurrence probability according to a Gaussian distribution from the optical signal;
The received data sequence (B1, B2,... BN) obtained by the separation unit is converted into an output data sequence (A1, A2,... AN) represented by symbols according to a distribution with a uniform occurrence probability by the following formula ( A decoding unit for decoding using the N × N Hadamard matrix Hn shown in 4) ,
Equipped with a,
An optical receiver in which the N value of the N × N Hadamard matrix is set based on a frequency distribution of the amplitude of light that is equal to or less than a threshold for waveform degradation of the optical signal .

An optical transmission method for sending an optical signal to an optical transmission line,
An input data sequence (A1, A2,... AN) represented by symbols according to a uniform distribution of occurrence probabilities is changed to a transmission data sequence (B1, B2,... BN) represented by symbols according to a Gaussian distribution of occurrence probabilities . A coding stage for encoding using the N × N Hadamard matrix Hn shown in the following equation (5), which is set based on the frequency distribution of the amplitude of the light whose N value is equal to or less than the threshold value of the waveform degradation of the optical signal ;
A multiplexing step of multiplexing said obtained by the code phase transmission data sequence (B1, B2, ... BN) to the optical signal,
Including an optical transmission method.

An optical receiving method for receiving an optical signal from an optical transmission line,
Separating from the optical signal a received data sequence represented by symbols whose occurrence probability follows a Gaussian distribution;
The obtained by the separation step the received data sequence (B1, B2, ... BN) , the output data string probability is represented by a symbol according to a uniform distribution (A1, A2, ... AN) to, N values A decoding step of decoding using an N × N Hadamard matrix Hn represented by the following equation (6), which is set based on a frequency distribution of the amplitude of light that is equal to or less than a threshold for waveform degradation of the optical signal ;
Including an optical receiving method.

An optical transmission system comprising an optical transmitter and an optical receiver connected to each other via an optical transmission path, and transmitting an optical signal from the optical transmitter to the optical receiver via the optical transmission path. ,
The optical transmitter is
  An input data sequence (A1, A2,... AN) represented by symbols according to a uniform distribution of occurrence probabilities is changed to a transmission data sequence (B1, B2,... BN) represented by symbols according to a Gaussian distribution of occurrence probabilities. An encoding unit that encodes using an N × N Hadamard matrix Hn shown in the following equation (7);
A time division multiplexing unit that superimposes and transmits an optical signal having a single wavelength by time division multiplexing the transmission data sequence (B1, B2,... BN) obtained by the encoding unit;
With
The optical receiver is:
A separation unit that obtains a received data sequence (B1, B2,... BN) represented by symbols whose occurrence probability follows a Gaussian distribution by time-separating the optical signal transmitted from the time-division multiplexing unit;
The received data sequence (B1, B2,... BN) obtained by the separation unit is converted into an output data sequence (A1, A2,... AN) represented by symbols according to a distribution with a uniform occurrence probability by the following formula ( A decoding unit for decoding using the N × N Hadamard matrix Hn shown in 8),
An optical transmission system comprising:

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