Optical transmitter, optical communication system, and optical communication method

The present invention relates to an optical transmitter and an optical communication method, and more particularly to an optical transmitter and an optical communication method for performing optical coding modulation using a digital signal.

In an optical communication system using an optical fiber, it is important to improve the reception sensitivity and the frequency utilization efficiency per optical fiber in order to perform long-distance and large-capacity communication. In optical communication, there is a trade-off between the expansion of the transmission distance and the improvement of the frequency utilization efficiency. Therefore, various modulations in which the reachable transmission distance and the frequency utilization efficiency differ depending on the required transmission distance and communication capacity. It has been proposed to switch the method and use it. Examples of modulation methods having different reachable transmission distances and frequency utilization efficiencies include BPSK (binary phase shift keying), QPSK (quadrature phase shift keying), 8QAM (quadrature amplitude modulation), and 16QAM.

Patent Document 1 describes an example of an optical transmitter that is used by switching the modulation method in this way. The related optical transmitter described in Patent Document 1 includes a client accommodating unit, a variable frame mapping unit, a variable coding unit, an optical modulation unit, and a communication method setting unit.

The client accommodating unit terminates the client signal transmitted from the client. The variable frame mapping unit accommodates the client signal terminated by the client accommodation unit in a predetermined transmission frame. At this time, the variable frame mapping unit performs mapping according to the communication method selected by the communication method setting unit.

The variable coding unit generates a modulation signal for carrying the transmission frame generated by the variable frame mapping unit. At this time, the variable coding unit generates a modulated signal according to the communication method selected by the communication method setting unit. The optical modulation unit generates and outputs a modulated optical signal from the modulation signal generated by the variable coding unit. At this time, the optical modulation unit generates a modulated optical signal according to the communication method selected by the communication method setting unit.

The communication method setting unit selects a communication method corresponding to the speed of the client signal from a plurality of communication methods provided by the optical transmitter. Then, the communication method setting unit notifies the variable frame mapping unit, the variable coding unit, and the optical modulation unit of the communication method information representing the selected communication method.

As described above, the related optical transmitter is configured to transmit the client signal by the communication method according to the speed of the client signal. Therefore, even when the client speed is slow, the amount of useless signal transmission is small and the transmission efficiency is improved.

Further, as a related technique, there is a technique described in Patent Documents 2 to 4.

<p><patcit num="1"><text>Japanese Unexamined Patent Publication No. 2011-250291 (paragraphs "0016" to "0025")</text></patcit><patcit num="2"><text>Japanese Patent Application Laid-Open No. 2003-087345</text></patcit><patcit num="3"><text>Japanese Unexamined Patent Publication No. 2009-105748</text></patcit><patcit num="4"><text>Special Table 2011-514736 Gazette</text></patcit></p>

<p><nplcit><text>Leonardo D. Coelho and Norbert Hanik, "Global Optimization of Fiber-Optic Communication Systems using Four-Dimensional Modulation Formats," in European Conference on Optical Communications (ECOC2011), Technical Digest, paper Mo.2.B.4.</text></nplcit><nplcit><text>J.Renaudier, A. Voicila, O.Bertran-Pardo, O.Rival, M. Karlsson, G. Charlet, and S.Bigo, "Comparison of Set-Partitioned Two-Polarization 16QAM Formats with PDM-QPSK and PDM-8QAM for Optical Transmission Systems with Error-Correction Coding ", in European Conference on Optical Communications (ECOC2012), Technical Digest, paper We.1.C.5.</text></nplcit></p>

<p>When switching between multiple modulation methods such as BPSK, QPSK, 8QAM, and 16QAM with a single optical transmitter, as in the related optical transmitters described above, multiple algorithms and bits that support multiple modulation methods. It will be necessary to implement a signal processing circuit with accuracy. Therefore, there is a problem that the power consumption of the optical transmitter and the optical receiver increases and the control becomes complicated.</p><p>As described above, if the modulation method used in the optical communication system can be switched according to the transmission conditions, there is a problem that power consumption increases and control becomes complicated.</p><p>An object of the present invention is to solve the above-mentioned problem that if the modulation method used in the optical communication system can be switched according to the transmission conditions, the power consumption increases and the control becomes complicated. To provide an optical transmitter and an optical communication method.</p>

<p>The optical transmitter of the present invention encodes an input digital signal using one of a plurality of coding methods, outputs a encoder, and optical a digital signal using an optical carrier. Depending on the transmission, the controller that identifies one coding scheme and the input coding signal are modulated by setting at least one of the coding scheme's generation polynomial, constraint length, and coding factor. It has a mapping device that outputs in association with a symbol, and an optical modulator that modulates an optical carrier using an input symbol-corresponding signal.</p><p>The optical communication method of the present invention encodes an input digital signal using one of a plurality of coding methods, outputs the signal, and responds to optical transmission of the digital signal using an optical carrier. By setting at least one of the generated polynomial, constraint length, and coding rate of the coding method, one coding method is specified, and the input coded signal is associated with the modulation symbol and output. Then, the optical carrier is modulated using the input symbol-corresponding signal.</p>

<p>According to the optical transmitter and the optical communication method of the present invention, even when the modulation method used in the optical communication system can be switched according to the transmission conditions, the power consumption can be reduced and the control can be facilitated. Can be planned.</p>

<figref num="1">It is a block diagram which shows the structure of the optical transmitter which concerns on 1st Embodiment of this invention.</figref><figref num="2">It is a block diagram which shows the structure of the optical transmitter which concerns on 2nd Embodiment of this invention.</figref><figref num="3A">It is a constellation diagram for demonstrating the case where the set division method is used for the 16QAM signal as a coding method in the optical transmitter which concerns on 2nd Embodiment of this invention, and is the constellation diagram by ordinary 16QAM modulation. ..</figref><figref num="3B">It is a constellation diagram for demonstrating the case where the set division method is used for the 16QAM signal as a coding method in the optical transmitter which concerns on 2nd Embodiment of this invention, and is the constellation diagram by SP8-16QAM modulation. ..</figref><figref num="4">It is a block diagram which shows the structure of the optical transmitter which concerns on 3rd Embodiment of this invention.</figref><figref num="5">It is a block diagram which shows the structure of the convolutional coder provided in the optical transmitter which concerns on 3rd Embodiment of this invention.</figref><figref num="6">It is a constellation diagram for demonstrating the operation of the optical transmitter which concerns on 3rd Embodiment of this invention.</figref><figref num="7">It is a block diagram which shows the structure of the optical communication system which concerns on 4th Embodiment of this invention.</figref><figref num="8">It is a block diagram which shows the structure of the optical transmitter which concerns on 5th Embodiment of this invention.</figref><figref num="9">It is a block diagram which shows the structure of the encoder provided in the optical transmitter which concerns on 5th Embodiment of this invention.</figref><figref num="10">It is a logical formula for explaining the operation of the 2nd encoder provided in the optical transmitter which concerns on 5th Embodiment of this invention.</figref><figref num="11">It is a constellation diagram of 16QAM for demonstrating the operation of the optical transmitter which concerns on 5th Embodiment of this invention.</figref><figref num="12">It is a constellation diagram of 12QAM for demonstrating the operation of the optical transmitter which concerns on 5th Embodiment of this invention.</figref><figref num="13A">It is a constellation diagram of 12QAM in X polarization and Y polarization for demonstrating the operation of the optical transmitter which concerns on 5th Embodiment of this invention.</figref><figref num="13B">FIG. 3 is another constellation diagram of 12QAM in X and Y polarizations for explaining the operation of the optical transmitter according to the fifth embodiment of the present invention.</figref><figref num="13C">FIG. 3 is yet another constellation diagram of 12QAM in X and Y polarization to illustrate the operation of the optical transmitter according to the fifth embodiment of the present invention.</figref><figref num="13D">FIG. 3 is yet another constellation diagram of 12QAM in X and Y polarization to illustrate the operation of the optical transmitter according to the fifth embodiment of the present invention.</figref>

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The direction of the arrow in the drawing is an example, and does not limit the direction of the signal between the blocks.

[First Embodiment] FIG. 1 is a block diagram showing a configuration of an optical transmitter 100 according to a first embodiment of the present invention. The optical transmitter 100 includes a encoder 110, a coding control unit 120, a mapping unit 130, and an optical modulation unit 140.

The encoder 110 encodes a digital signal transmitted by an optical carrier wave under predetermined transmission conditions by one of a plurality of coding methods. The coding control unit 120 selects a predetermined coding method corresponding to a predetermined transmission condition from a plurality of coding methods, and operates the encoder 110 with the predetermined coding method. The mapping unit 130 associates the output bit signal output by the encoder 110 with the modulation symbol. Then, the optical modulation unit 140 modulates the optical carrier wave based on the symbol signal output by the mapping unit 130.

Next, the operation of the optical transmitter 100 according to the present embodiment will be described.

M-bit digital signal input to the optical transmitter 100 a₁~ a_mIs input to the encoder 110. The encoder 110 encodes based on the coding method set by the coding control unit 120, and is a bit string b of n bits which is an output bit signal.₁~ b_nIs output. The mapping unit 130 is a bit string b₁~ b_nIs symbol-mapped, and S is a symbol signal.₁, S₂, ..., S_pThe p (dimensional) data strings of are output to the optical modulation unit 140.

The optical modulation unit 140 is S₁, S₂, ..., S_pOptical modulation is performed based on each data of the above, and an optical code-modulated transmission optical signal is output. Here, the optical modulator 140 includes a D / A converter (digital-to-analog converter), a modulator driver, an optical modulator, a light source, and the like (not shown).

As shown in FIG. 1, the encoder 110 can be selected from the k coding methods of the coding method k from the coding method 1 to set a predetermined coding method. Here, the encoder 110 may be configured to perform coding by any of a plurality of convolutional coding methods in which at least one of the generated polynomial, the constraint length, and the coding rate in the convolutional coding method is different. At this time, the coding control unit 120 selects a predetermined coding method by setting at least one of the generation polynomial, the constraint length, and the coding rate. That is, the coding control unit 120 selects a suitable coding method from the coding method 1 to the coding method k according to predetermined transmission conditions such as the transmission distance and the transmission capacity required for each communication. It is possible to set it.

At least one of the transmission capacity, the transmission distance, the error rate, and the optical signal-to-noise ratio can be used as the transmission conditions described above.

The symbol signal that drives the optical modulator 140, that is, S₁, S₂, ..., S_pAs the data string of, a signal having at least one dimension of the optical phase (I component and Q component), polarization (X polarization component and Y polarization component), wavelength, and time of the optical carrier is used. be able to. Further, by combining these plurality of dimensions, it is possible to perform higher-dimensional optical coding modulation.

The light modulator that constitutes the light modulator 140 is lithium niobate (LiNbO).₃) And any of the ferroelectric materials and semiconductor materials. By using such an optical modulator alone or in combination of two or more, it is possible to multiplex and transmit a digital signal by at least one of polarization multiplexing, wavelength multiplexing, and time division multiplexing. ..

Next, the optical communication method according to the present embodiment will be described.

In the optical communication method of the present embodiment, first, a digital signal transmitted by an optical carrier wave under predetermined transmission conditions is encoded by selecting a predetermined coding method corresponding to the predetermined transmission conditions. Then, the coded bit signal is associated with the modulation symbol to generate a symbol signal. Finally, an optical modulation signal obtained by modulating the optical carrier wave based on this symbol signal is generated.

As described above, in the optical transmitter 100 and the optical communication method of the present embodiment, a predetermined coding method corresponding to a predetermined transmission condition is selected and encoded. With such a configuration, it is possible to select a suitable modulation method corresponding to a predetermined transmission condition only by changing the coding method. Therefore, it is possible to minimize changes in digital signal processing. As a result, even when the modulation method used in the optical communication system can be switched according to the transmission conditions, it is possible to reduce the power consumption and facilitate the control.

[Second Embodiment] Next, a second embodiment of the present invention will be described. FIG. 2 is a block diagram showing a configuration of an optical transmitter 200 according to a second embodiment of the present invention.

The optical transmitter 200 includes a encoder 110, a coding control unit 120, a mapping unit 230, and an optical modulation unit 140. The mapping unit 230 included in the optical transmitter 200 according to the present embodiment has a configuration including a set division unit 231 and a symbol selection unit 232. Since other configurations are the same as those of the optical transmitter 100 according to the first embodiment, detailed description thereof will be omitted.

The set division unit 231 divides the modulation symbol into a plurality of subsets and selects one of the plurality of subsets based on the output bit signal. The symbol selection unit 232 selects one modulation symbol based on the output bit signal from the modulation symbols included in the selection subset selected by the set division unit 231, and associates the output bit signal with the selected modulation symbol.

Next, the operation of the optical transmitter 200 according to the present embodiment will be described.

The encoder 110 is an m-bit digital signal a input to the optical transmitter 200.₁~ a_mEnter. Then, after coding based on the coding method set by the coding control unit 120, the bit string b of n bits which is an output bit signal b.₁~ b_nIs output.

Bit string b₁~ b_nIs symbol-mapped in the mapping unit 230, and then, for example, the data strings of the X-polarized light phase components XI-ch and XQ-ch, and the Y-polarized light phase components YI-ch and YQ-ch. Is input to the optical modulation unit 140. The optical modulation unit 140 performs optical modulation based on each data of XI-ch, XQ-ch, YI-ch, and YQ-ch, and outputs an optical code-modulated transmission optical signal.

Here, the set division unit 231 divides the constellation of two-dimensional or higher-order QAM modulation into L partial constellations (states) based on the set division method. And bit string b₁~ b_nOf the above-mentioned L partial constellations (states), one of the above-mentioned partial constellations (states) is selected by using the coded α bit. Then, the symbol selection unit 232 is the bit string b.₁~ b_nSelect one suitable symbol from the partial constellations selected using the uncoded β bits, and select the data corresponding to XI-ch, XQ-ch, YI-ch, and YQ-ch. Output.

The above-mentioned set-partitioning (SP) method is a method of increasing the minimum code-to-code distance by thinning out the most adjacent points from the symbol points. For example, Non-Patent Document 1 discloses SP-16QAM modulation in which a symbol mapped to a four-dimensional signal space of phase information and polarization information is divided into symbols by set division. Since the distance between symbols is increased by the set division, the reception sensitivity can be improved.

Next, the operation of the encoder 110 will be specifically described.

FIGS. 3A and 3B are constellation diagrams for explaining a case where the above-mentioned set division method is used for a two-dimensional 16QAM signal as a coding method. Figure 3A is a normal 16QAM constellation diagram with the minimum inter-symbol distance d._minIs. FIG. 3B is a constellation diagram showing symbol mapping by SP8-16QAM coding modulation with every other symbol thinned out from the 16QAM constellation shown in FIG. 3A. In this case, the minimum intersymbol distance is 2^1/2d_minIt can be seen that the reception sensitivity is improved by this.

Here, in the SP8-16QAM coding method shown in FIG. 3B, the coding rate r is halved because the symbols are thinned out. Therefore, the transmission rate is lowered. Therefore, for applications where the transmission rate (capacity) is important, the coding control unit 120 can select the normal 16QAM coding method shown in FIG. 3A. On the other hand, for applications that require long-distance transmission, the coding control unit 120 can select the coding method using SP8-16QAM shown in FIG. 3B. In this way, the coding control unit 120 is a coding method corresponding to transmission conditions from normal 16QAM (SP16-16QAM) without thinning, SP8-16QAM described above, SP4-16QAM with further thinned symbols from SP8-16QAM, and the like. Can be selected.

For the sake of simplicity, the above explanation describes the set division in 2D 16QAM on a simple IQ plane. Not limited to this, it is also possible to use a four-dimensional PM-16QAM that is extended by adding two dimensions of X polarization and Y polarization by polarization multiplexing (PM).

In the case of 4D PM-16QAM, a symbol point can be created by combining 16 symbols for X polarization (XI, XQ) and 16 symbols for Y polarization (YI, YQ), so 256 (= 16 × 16). There are a number of symbol points. Therefore, PM-16QAM without normal thinning can be described as SP256-PM-16QAM, and SP128-PM-16QAM can be obtained by thinning every other PM-16QAM, and SP64-PM-16QAM and SP32-PM can be obtained by further thinning. -16QAM etc. can be obtained.

In this case, the coding control unit 120 can select a coding method such as SP32-PM-16QAM or SP128-PM-16QAM. That is, PM-16QAM, SP32-PM-16QAM, SP128-PM-16QAM, etc. can be supported as the coding method of the encoder 110 (coding method 1 to coding method k).

Then, the set dividing unit 231 is used to output a bit signal (bit string b).₁~ b_n), Select one subset by selecting the modulation symbol corresponding to one of the polarization states of the optical carrier.

Since the reception sensitivity and the coding rate can be set differently from the coding method 1 to the coding method k described above, a suitable coding method is used according to the required transmission distance and transmission capacity. It is possible to select the modulation method.

Further, a suitable modulation method can be selected only by changing the coding method of the encoder 110 from the coding method 1 to the coding method k based on the constellation by the usual 16QAM modulation. Therefore, it is possible to minimize changes in digital signal processing. As a result, even when the modulation method used in the optical communication system can be switched according to the transmission conditions, it is possible to reduce the power consumption and facilitate the control. Further, since the physical interface such as the optical modulator can be shared in each coding method, the number of parts can be reduced. This also makes it possible to reduce costs and facilitate control.

Next, the optical communication method according to the present embodiment will be described.

In the optical communication method of the present embodiment, first, a digital signal transmitted by an optical carrier wave under predetermined transmission conditions is encoded by selecting a predetermined coding method corresponding to the predetermined transmission conditions. Then, the coded bit signal is associated with the modulation symbol to generate a symbol signal. Finally, an optical modulation signal obtained by modulating the optical carrier wave based on this symbol signal is generated.

Here, when generating the symbol signal described above, the modulated symbol is divided into a plurality of subsets, and one subset of the plurality of subsets is selected based on the bit signal. Then, one modulation symbol can be selected from the modulation symbols included in the selected selection subset based on the bit signal, and the bit signal can be associated with the selected modulation symbol.

According to the optical communication method of the present embodiment, even when the modulation method used in the optical communication system can be switched according to the transmission conditions, it is possible to reduce the power consumption and facilitate the control. can.

[Third Embodiment] Next, a third embodiment of the present invention will be described. FIG. 4 is a block diagram showing the configuration of the optical transmitter 300 according to the third embodiment of the present invention.

In the optical transmitter 300 of the present embodiment, the encoder is a convolutional encoder 310. The convolutional coder 310 encodes with one of a plurality of convolutional coding schemes that differ in at least one of the generated polynomials, constraint lengths, and coding rates in the convolutional coding scheme. The same components as those of the optical transmitter 200 according to the second embodiment shown in FIG. 2 are designated by the same reference numerals, and the description thereof will be omitted.

The operation of the optical transmitter 300 according to the present embodiment will be described below.

The convolutional encoder 310 is a digital signal a of m bits input to the optical transmitter 300.₁~ a_mEnter. Then, after coding based on the coding method set by the coding control unit 120, the bit string b of n bits which is an output bit signal b.₁~ b_nIs output.

The mapping unit 230 is a bit string b₁~ b_nIs symbol-mapped, and S is a symbol signal.₁, S₂, ..., S_pThe p (dimensional) data strings of are output to the optical modulation unit 140. The optical modulation unit 140 is S₁, S₂, ..., S_pOptical modulation is performed based on each data of the above, and an optical code-modulated transmission optical signal is output.

Here, the convolutional coder 310 can be set by selecting one from the k coding methods of the coding method k from the coding method 1 having different generation polynomials, constraint lengths, coding rates, etc. The configuration was as follows. The coding control unit 120 selects a suitable coding method according to the required transmission distance and transmission capacity from the coding method 1 to the coding method k. Then, the convolutional coder 310 is set to operate in the selected coding method.

FIG. 5 shows the configuration of a convolutional coder having a constraint length of 4 and a code rate of 2/3 as an example of the convolutional coder 310. The convolutional coder 310 shown in FIG. 5 has the input a.₁And input a₂Is coded for, and the coded bit b₁~ b₃Is output. Also, input a₃~ a₇For b, remain uncoded₄~ b₈Is output as.

Fig. 6 shows the case where the 4D PM-16QAM modulation using optical phase (I, Q) and polarization (X, Y) is convolutionally coded by the convolutional coder 310 shown in Fig. 5. An example of the set division of is shown. In FIG. 6, for the sake of simplicity, only the constellation for X polarization is shown, but in reality, it is a four-dimensional constellation combined with the constellation for Y polarization. As shown in FIG. 6, eight states (partial constellations) of S1 to S8 exist by two set divisions. Then, since it is a four-dimensional (I, Q, X, Y) constellation in which the constellations of Y polarization are combined for each state of S1 to S8, 2 for each state.⁵Contains 32 symbol points.

Next, the operation of the symbol selection unit 232 when the convolutional coder 310 is used will be described.

The symbol selection unit 232 is first convolutionally coded by the convolutional coder 310 shown in FIG. 5. b₁~ b₃Select one of the eight states (S1 to S8) using the 3 bits of. In addition, it is an uncoded bit, 5 bits (b).₄~ b₈) Is used to select one of the 32 symbols contained in one of the eight selected states.

This makes it possible to expand the minimum squared distance between code sequences by convolutional coding and make it greater than or equal to the squared distance of the distance between signals in the state divided by the set division method. Therefore, by simply changing the settings of the convolutional coder 310, which has multiple convolutional coding methods with different constraint lengths and code rates, the required transmission distance and transmission capacity can be obtained based on PM-16QAM modulation. It is possible to select the appropriate modulation method. The convolutional coding method may be determined by setting a generated polynomial instead of the constraint length and the coding rate.

As described above, according to the optical transmitter 300 of the present embodiment, even when the modulation method used in the optical communication system is configured to be switchable according to the transmission conditions, the power consumption is reduced and the control is performed. Can be facilitated.

[Fourth Embodiment] Next, a fourth embodiment of the present invention will be described. FIG. 7 is a block diagram showing a configuration of the optical communication system 1000 according to the fourth embodiment of the present invention.

The optical communication system 1000 includes an optical transmitter 100 that sends an optical modulation signal to a communication path (optical transmission medium) 600, and an optical receiver 400 that receives an optical modulation signal propagated through the communication path 600.

The optical transmitter 100 includes a encoder 110, a coding control unit 120, a mapping unit 130, and an optical modulation unit 140. Since the configuration and operation of the optical transmitter 100 are the same as those of the optical transmitter according to the first embodiment, detailed description thereof will be omitted.

The optical receiver 400 includes a photoelectric conversion unit 410, a demapping unit 420, a decoder 430, and a decoding control unit 440.

The photoelectric conversion unit 410 receives the optical modulation signal, converts it into an electric signal, and outputs the received signal. The demapping unit 420 demaps the received signal and outputs the received bit signal. The decoder 430 inputs a received bit signal and decodes it by one of a plurality of decoding methods. Then, the decoding control unit 440 selects a predetermined decoding method from the plurality of decoding methods, and operates the decoder 430 with the predetermined decoding method.

Next, the operation of the optical communication system 1000 according to the present embodiment will be described. Since the operation of the optical transmitter 100 to output the optical code-modulated optical signal is the same as that in the first embodiment, the description thereof will be omitted.

The optical signal output from the optical modulation unit 140 included in the optical transmitter 100 passes through the communication path 600 and is received by the photoelectric conversion unit 410 included in the optical receiver 400. The photoelectric conversion unit 410 converts the received optical signal into an electric signal, and outputs the received signal as a digital signal in each lane of XI-ch, XQ-ch, YI-ch, and YQ-ch. Here, the photoelectric converter includes a 90 ° hybrid, a photodiode, a transimpedance amplifier, an A / D converter (analog-to-digital converter), and the like (not shown).

The demapping unit 420 performs symbol identification on the XI-ch, XQ-ch, YI-ch, and YQ-ch data strings, and n-bit bit strings c as received bit signals.₁~ c_nIs output. Bit string c₁~ c_nIs input to the decoder 430 having a plurality of decoding methods.

The decoder 430 selects one of a plurality of decoding methods according to the setting of the decoding control unit 440. The decoder 430 is a bit string d of m bits which is a decoded bit string.₁~ d_mIs output. Here, as the above-mentioned decoding method, a Viterbi decoding method, which is maximum likelihood decoding, or a sequential decoding method for a convolutional code having a longer constraint length can be used.

The optical communication system 1000 may be configured to further include an optical network control unit 450. The optical network control unit 450 determines a predetermined coding method and a predetermined decoding method corresponding to a predetermined transmission condition, and notifies the coding control unit 120 and the decoding control unit 440 in synchronization with each other.

Next, the operation of the optical network control unit 450 will be described in more detail.

The optical network control unit 450 selects a suitable coding method and decoding method based on communication quality information such as transmission distance and transmission capacity, which are transmission conditions required from the system operation side. Then, the selection result is notified to the coding control unit 120 and the decoding control unit 440 in synchronization. Specifically, the optical network control unit 450 instructs the coding control unit 120 to set the generated polynomial, the constraint length, the coding rate, and the like, and the decoding control unit 440 is instructed to set the constraint length, the code length, and the number of soft determination bits. Instruct settings such as. At this time, a suitable reception state can be maintained by synchronously changing the settings of the coding control unit 120 and the decoding control unit 440.

The optical network control unit 450 does not necessarily have to acquire the communication quality information used for the above-mentioned control from the system operation side. For example, it is also possible to select a suitable coding method and decoding method by using information such as an optical signal-to-noise ratio and an error rate.

Next, the optical communication method according to the present embodiment will be described.

In the optical communication method of the present embodiment, first, a digital signal transmitted by an optical carrier wave under predetermined transmission conditions is encoded by selecting a predetermined coding method corresponding to the predetermined transmission conditions. Then, the coded bit signal is associated with the modulation symbol to generate a symbol signal. An optical modulation signal obtained by modulating an optical carrier wave is generated based on this symbol signal.

Next, this optical modulation signal is received and a received signal converted into an electric signal is generated. Then, the received signal is demapped to generate a received bit signal. Finally, the received bit signal is decoded by selecting a predetermined decoding method from the plurality of decoding methods.

As described above, according to the optical communication system 1000 and the optical communication method of the present embodiment, low power consumption is achieved even when the modulation method used in the optical communication system can be switched according to the transmission conditions. It is possible to facilitate the conversion and control.

[Fifth Embodiment] Next, a fifth embodiment of the present invention will be described. FIG. 8 is a block diagram showing the configuration of the optical transmitter 500 according to the fifth embodiment of the present invention.

The optical transmitter 500 includes a encoder 510, a coding control unit 120, a mapping unit 530, and an optical modulation unit 140. The optical transmitter 500 according to the present embodiment is different from the encoder 110 and the mapping unit 130 included in the optical transmitter 100 according to the first embodiment in the configuration and operation of the encoder 510 and the mapping unit 530. Since other configurations and operations are the same as those of the optical transmitter 100 according to the first embodiment, detailed description thereof will be omitted.

In the following, the encoder 510 is a first encoder 511, a second encoder 512, and a third encoding corresponding to the coding method 1, the coding method 2, and the coding method 3, respectively. A case where the device 513 is provided will be described. At this time, the coding control unit 120 selects the optimum coding method from the coding methods 1 to 3 according to predetermined transmission conditions such as the transmission distance and the transmission capacity required for communication. Then, the operation method of the encoder 510, the mapping unit 530, and the optical modulation unit 140 is set.

Next, the operation of the encoder 510 and the mapping unit 530 will be described in detail.

FIG. 9 shows the configuration of the encoder 510. As described above, the encoder 510 includes three encoders, that is, a first encoder 511, a second encoder 512, and a third encoder 513. These encoders have a configuration in which the number of input bits is 5, 6, and 7 bits, respectively. Since the configuration and operation of the first encoder 511 and the third encoder 513 are described in Non-Patent Document 2, detailed description thereof will be omitted here.

The second encoder 512 calculates the exclusive OR of the 6-bit inputs b1 to b6 and outputs it as b7. Next, the four-dimensional encoder 512E included in the second encoder 512 converts the 7-bit input of b1 to b7 into an 8-bit output. FIG. 10 shows a specific logical expression of the four-dimensional encoder 512E.

The mapping unit 530 takes the 8-bit output of the encoder 510 as an input and allocates it to the symbol so that the coding gain can be obtained. It is assumed that the symbol selected here belongs to the four-dimensional symbol space. In the present embodiment, the optical phase (I component and Q component) and polarization (X polarization component and Y polarization component) of the optical carrier wave are used as the four-dimensional signal space, and the symbol-mapped symbol signal is optical-modulated. Output to unit 140.

Specifically, the symbols are assigned using the mapping symbols shown in FIG. The mapping unit 530 assigns a transmission symbol in X polarization using the bits B1 to B4 output by the encoder 510, and assigns a transmission symbol in Y polarization using the bits B5 to B8. At this time, as shown in FIG. 12, the mapping unit 530 assigns a symbol so that the symbol becomes 12QAM excluding the points at the four corners of 16QAM. As can be seen from the figure, the 12QAM signal points are divided into 8 points with large amplitude and 4 points with small amplitude.

Further, in the optical transmitter 500 of the present embodiment, in the encoder 510, the amplitude of the optical signal modulated by the optical modulator 140 is between two types of polarization (X polarization, Y polarization) of the optical carrier wave. In the configuration, the coding is performed so as to have a correlation. Then, the mapping unit 530 is configured to assign a symbol to the signal point having the maximum amplitude in at least one of the polarized waves.

The operation of the mapping unit 530 will be described in more detail by using the 12QAM constellation diagram in the X-polarized wave and the Y-polarized wave shown in FIGS. 13A to 13D. As shown in FIGS. 13A to 13C, there is the following correlation in amplitude between the X-polarized wave symbol and the Y-polarized wave symbol. That is, in the case shown in FIG. 13A, it has a symbol point having a large amplitude in both the X-polarized wave and the Y-polarized wave. Further, in the case shown in FIG. 13B, the X-polarized wave has a symbol point having a small amplitude, and the Y-polarized wave has a symbol point having a large amplitude. On the contrary, in the case shown in FIG. 13C, the X-polarized wave has a symbol point having a large amplitude, and the Y-polarized wave has a symbol point having a small amplitude. However, the case where only the symbol points having a small amplitude are included in both the X-polarized wave and the Y-polarized wave as shown in FIG. 13D is excluded. In this way, the mapping unit 530 assigns a symbol to the signal point having the maximum amplitude in at least one of the polarized waves, that is, the configuration excluding the symbol arrangement shown in FIG. 13D, thereby reducing the number of adjacent points. Can be reduced. This makes it possible to reduce the bit error rate.

The signal points shown in FIGS. 13A, 13B, and 13C are 64 (= 8 × 8) points, 32 (= 4 × 8) points, and 32 (= 8 × 4) points, respectively, for a total of 128 points. It becomes a four-dimensional symbol point of the point. However, since the four-dimensional set division is performed by generating the parity bit in the second encoder 512 and the symbol points are thinned out in half, the second encoder 512 and the mapping unit 530 are generated. The number of symbol points to be used is 64. This is equivalent to the conventional PM-8QAM.

Here, assuming that the transmission power per polarization of the transmission symbol is Es, Es is the average of the squares of the amplitudes for all symbols, so the distance between signal points is 0.63Es in 16QAM.^1/2Is. On the other hand, in the case of 12QAM, the distance between signal points is 0.71Es.^1/2Expand to. This is the effect of removing the symbols at the four corners of the 16QAM symbol point.

Furthermore, the distance between the 4D signal points of 12QAM is 2 due to the 4D set division performed in the 2nd encoder 512.^1/2Double 1.0Es^1/2The distance between signal points in the conventional PM-8QAM is 0.92Es.^1/2Will be bigger. Therefore, by setting it to 12QAM, performance equal to or better than PM-8QAM can be obtained.

As described above, it is possible to improve the reception sensitivity by correlating the amplitude between the two types of polarizations of the optical carrier wave by using the second encoder 512 according to the present embodiment. Needless to say, the configuration in which the amplitude is correlated between the polarizations can be applied not only to other QAM signals but also to any symbol arrangement. Further, instead of the configuration excluding the case shown in FIG. 13D described above, it can be applied to the configuration excluding the case where the symbol point having a large amplitude is obtained in both the X polarization and the Y polarization shown in FIG. 13A.

Next, the optical communication method according to the present embodiment will be described.

In the optical communication method of the present embodiment, first, a digital signal transmitted by an optical carrier wave under predetermined transmission conditions is encoded by selecting a predetermined coding method corresponding to the predetermined transmission conditions. Then, the coded bit signal is associated with the modulation symbol to generate a symbol signal. Finally, an optical modulation signal obtained by modulating the optical carrier wave based on this symbol signal is generated.

As described above, in the optical transmitter 500 and the optical communication method of the present embodiment, a predetermined coding method corresponding to a predetermined transmission condition is selected and encoded. With such a configuration, it is possible to select a suitable modulation method corresponding to a predetermined transmission condition by changing the coding method.

Further, even when the coding method is switched, the base symbol arrangement does not change, so that it is possible to minimize the change in digital signal processing. As a result, even when the modulation method used in the optical communication system can be switched according to the transmission conditions, it is possible to reduce the power consumption and facilitate the control. At the same time, the effect of reducing the bit error rate and extending the transmission distance can be obtained. Further, since the physical interface such as the optical modulator can be shared in each coding method, the number of parts can be reduced. This also makes it possible to reduce costs and facilitate control.

The present invention has been described above by using the above-described embodiment as a model example. However, the invention is not limited to the embodiments described above. That is, the present invention can apply various aspects that can be understood by those skilled in the art within the scope of the present invention.

100, 200, 300, 500 optical transmitter

110, 510 encoder

120 Coding control unit

130, 230, 530 Mapping part

140 Optical modulator

231 Set division

232 Symbol selection section

310 310 Convolutional coder

400 Optical receiver

410 Photoelectric conversion unit

420 Demapping section

430 Decoder

440 Decoding control unit

450 Optical network control unit

511 First encoder

512 Second encoder

512E 4D encoder

513 Third encoder

600 Channel

1000 Optical communication system