JP6450210B2 - Optical transmitter and optical transmission method - Google Patents

Description

  The present invention relates to an optical transmitter and an optical transmission method.

  In a backbone network of an optical communication system, a DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) method (for example, Non-Patent Document 1) is a modulation method that is standardly adopted in 100 Gbps class transmission due to recent expansion of communication traffic. In the next-generation 400 Gbps class transmission, a DP-16QAM (Quadrature Amplitude Modulation) method (for example, see Non-Patent Document 2), which is a modulation method with higher frequency utilization efficiency than DP-QPSK, is being studied. In DP-16QAM, the polarization of each of the X polarization and the Y polarization is modulated independently by 16QAM. On the reception side, after input to a coherent receiver, analog / digital conversion (ADC; Analog to Digital) After being digitized by a converter, polarization separation is performed by digital signal processing, and symbol determination is performed as an independent signal.

  As a method for further increasing the transmission capacity, in the 1 Tbps class transmission, a further increase in the number of modulation signals of the DP-64QAM method or the like having a higher frequency utilization efficiency than DP-16QAM is being studied. However, the multi-level modulation signal reduces the minimum Euclidean distance between symbols, reduces noise resistance, and limits the transmission distance.

  In recent years, in response to the reduction in minimum Euclidean distance due to improved frequency utilization efficiency, the design dimension of signal point arrangement has been expanded in the polarization, time, and wavelength directions, which have been treated as independent dimensions so far, and the frequency utilization efficiency and minimum Euclidean distance have been expanded. By reducing the above relationship to the outline of the sphere filling problem in the N-dimensional space, a modulation method based on N-dimensional signal point arrangement that expands the minimum Euclidean distance has been proposed (see Non-Patent Document 3, for example).

  However, in order to realize the modulation method of the non-patent document 3, the number of amplitude levels is increased with respect to the conventional DP-QPSK and DP-16QAM, so that digital / analog conversion (DAC) is performed. However, there is a problem that the requirement for the bit resolution becomes high.

  In the method of Non-Patent Document 3, since there is no symmetry in signal point arrangement, a polarization separation algorithm based on the symmetry of signal point arrangement in digital signal processing on the receiving side (for example, see Non-Patent Document 4). And a receiving algorithm such as a carrier phase estimation algorithm (see, for example, Non-Patent Document 5) cannot be used.

OIF, "100G Ultra Long Haul DWDM Framework Document". T. J. Xia, S. Gringeri, and M. Tomizawa, "High-capacity optical transport networks," IEEE Commun. Mag., Vol. 50, no. 11, pp. 170-178, Nov. 2012. Koike-Akino, Toshiaki, et al. "Eight-dimensional modulation for coherent optical communications." Proc. ECOCTu 3 (2013). Yang, Jian, Jean-Jacques Werner, and Guy A. Dumont. "The multimodulus blind equalization and its generalized algorithms." IEEE Journal on Selected Areas in Communications 20.5 (2002): 997-1015. Viterbi, Andrew. "Nonlinear estimation of PSK-modulated carrier phase with application to burst digital transmission." Information Theory, IEEE Transactions on 29.4 (1983): 543-551.

  As described above, in the conventional optical transmitter, there is a problem that the minimum Euclidean distance between symbols is reduced due to the multi-level modulation signal, the resistance to noise is reduced, and the transmission distance is limited. is there.

  The present invention has been made in view of such circumstances, and an object thereof is to provide an optical transmitter and an optical transmission method that have high noise resistance and enable long-distance transmission.

The present invention is an optical transmitter used in an optical transmission device, when N (N is an integer of 4 or more) of the initial coordinate in dimensional space is determined, to determine the origin in response to the initial coordinate, N-dimensional The closest lattice point coordinate around the initial coordinate is calculated using the closest packed lattice in the space, the initial coordinate is added to the symbol candidate point, and the distance from the origin among the lattice points not adopted as the symbol candidate point Calculates a neighboring grid point around the smallest grid point, adds the grid point for which the neighboring grid point has been calculated to the symbol candidate point, and the symbol candidate until the number of the symbol candidate points reaches a specified number Among the lattice points that are not adopted as points, calculate the adjacent lattice points around the lattice point with the smallest distance from the origin, add the calculated lattice points to the symbol candidate points, and add the symbol candidate points Small norm from the origin of coordinates The symbol points are selected in order from the symbol candidate points, and when the norm values are the same, the symbol points having the smallest maximum amplitude in each dimension of the coordinates of the symbol candidate points are selected, and the input bit information And a symbol mapping circuit that converts the signal into a modulation signal by arranging the signal points, and an optical modulation circuit that modulates light by the modulation signal.

  In the present invention, the number of dimensions of the N dimension is 4, the number of amplitude levels is 4 at regular intervals, the respective levels are relatively −3, −1, 1, 3, and the signal point arrangement The number of is 64.

  In the present invention, the number of N-dimensional dimensions is 4, the number of amplitude levels is 6 at regular intervals, and the respective levels are relatively −5, −3, −1, 1, 3, 5 The number of signal point arrangements is 256.

  In the present invention, the number of dimensions of the N dimension is 8, the number of amplitude levels is 4 at regular intervals, and the respective levels are relatively −3, −1, 1, 3, and the signal point arrangement The number is 4096.

  In the present invention, the symbol mapping circuit includes a port that outputs two modulation signals having a length of N / 2 timeslot per symbol, and the optical modulation circuit includes one modulation signal of the two systems. The optical modulator is inputted and modulated.

  In the present invention, the symbol mapping circuit includes a port that outputs four modulation signals having a length of N / 4 timeslot per symbol, and the optical modulation circuit orthogonally converts the four modulation signals. It is characterized in that it is inputted to two optical modulators that modulate the polarization and modulated, and the modulated polarizations are combined.

  In the present invention, the symbol mapping circuit includes a port that outputs 4M systems of modulation signals having a length of N / M (M is an integer of 2 or more) timeslots, and the optical modulation circuit includes the 4M systems. A modulation signal is input, and both polarizations of wavelengths λ1 to λM are input to 2M optical modulators for modulation, and each modulated polarization is synthesized to synthesize all wavelengths. .

The present invention is an optical transmission method performed by an optical transmitter used in an optical transmission apparatus, and when an initial coordinate in an N (N is an integer of 4 or more) dimensional space is determined, an origin is determined according to the initial coordinate. And calculating the nearest grid point coordinates around the initial coordinates using the closest packed grid in the N-dimensional space, adding the initial coordinates to the symbol candidate points, and among the grid points not adopted as the symbol candidate points A neighboring grid point around a grid point having a minimum distance from the origin is calculated, the grid point for which the neighboring grid point is calculated is added to the symbol candidate point , and the norm of the norm from the origin of the coordinate of the symbol candidate point is calculated. A symbol mapping step of selecting symbol points in order from the smallest symbol candidate point, and selecting a symbol point having a small maximum amplitude in each dimension of the coordinate of the symbol candidate point when the norm value is the same; It is characterized by that.

  According to the present invention, it is possible to obtain a longer distance transmission than the conventional method by increasing the minimum Euclidean distance between symbols by increasing the number of modulation signals and increasing noise resistance.

It is a flowchart which shows the processing operation which searches the signal point arrangement | positioning in N-dimensional space by 1st Embodiment of this invention. It is explanatory drawing which shows the concept of the determination of an initial coordinate. It is explanatory drawing which shows the concept of adjacent lattice point calculation. It is explanatory drawing which shows the minimum Euclidean distance of the searched modulation format. It is a figure which shows the result by the searched modulation format. It is a block diagram which shows the structure of the optical transmitter by 2nd Embodiment of this invention. It is explanatory drawing which shows the searched signal point arrangement | positioning. It is explanatory drawing which shows the searched signal point arrangement | positioning. It is explanatory drawing which shows the searched signal point arrangement | positioning. It is a block diagram which shows the structure of the optical transmitter by 6th Embodiment of this invention. It is a block diagram which shows the structure of the optical transmitter by 7th Embodiment of this invention. It is a block diagram which shows the structure of the optical transmitter by 8th Embodiment of this invention.

<First Embodiment>
Hereinafter, an optical transmitter according to a first embodiment of the present invention will be described with reference to the drawings. First, a method for searching for a signal point arrangement in an N-dimensional space will be described. FIG. 1 is a flowchart showing a processing operation for searching for a signal point arrangement in an N-dimensional space. Here, the signal point arrangement in the N-dimensional space is defined as a modulation format.

  First, the optical transmitter sets the dimension N to be searched (N is an integer of 4 or more), the initial coordinates of the N-dimensional space where the search is started, and the frequency utilization efficiency of the modulation format to be searched (step S1). When the initial coordinate is an N-dimensional vector A0, the value of A0 can be arbitrarily selected, and the signal point arrangement changes depending on the set value. Here, for the sake of space, a conceptual explanation in a state of projection onto a two-dimensional plane will be given using FIG. When the initial coordinate A0 is set as shown in the left diagram of FIG. 2, the signal point is not arranged on a concentric circle from the origin. This state is defined as no symmetry. Similarly, when A0 is set as shown in the right diagram of FIG. 2, the pixels are arranged on a concentric circle from the origin, and this state is defined as having symmetry.

Next, the optical transmitter calculates the proximity point of the closest packed lattice (see Reference 1) in the N-dimensional space with respect to the center coordinate using the N-dimensional initial coordinate as the center coordinate using the method of Reference 1. All the coordinate values of the grid points around the center coordinates are held (step S2).
Reference 1: JHCONWAY, NJASLOANE "SPHERE PACKINGS, LATTICES AND GROUPS" Springer, 1999. pp.12-20, pp.94-135 Vol. 290.

  The number of amplitude levels changes depending on the construction method of the adjacent lattice points. This will be specifically described with reference to FIG. For the sake of space, FIG. 3 is a diagram in which two components in the N-dimensional space are projected onto a two-dimensional plane, and there are points that can be taken around the center coordinates. Although it is limited by the values of other dimensions that are not, the proximity lattice points have a degree of freedom with respect to the rotation direction around the center coordinates. When the configuration example 2 is adopted with respect to the configuration example 1, the number of amplitude levels can be reduced.

  Next, the optical transmitter has calculated all the adjacent lattice points around the initial coordinates, and therefore adds the initial coordinates to the symbol candidate points (step S3). Subsequently, the optical transmitter calculates the norm of the lattice point coordinates calculated in the process so far, and for all the lattice point coordinates having the minimum value, the adjacent lattice points are obtained in the same manner as in step S2. And the coordinates of the calculated grid points are held (step S4).

  Next, the optical transmitter adds the coordinates of the lattice points for which the proximity lattice points have been calculated in step S4 to the symbol candidate points (step S5). Subsequently, the optical transmitter proceeds to the next step if the number of symbol candidate points calculated in the process so far is equal to or greater than the number of symbol points corresponding to the frequency utilization efficiency set in step S1, and the number of symbol candidate points is the specified number. If not, the process returns to step S4.

  Next, the optical transmitter calculates a distance (norm) from the origin of the symbol candidate point coordinates in order to maximize the minimum Euclidean distance with respect to the transmission power, and sequentially converts the symbol points of the modulation format from the symbol candidate point with the smallest norm. The symbol point is selected by adopting (Step S7). When the norm values are the same, it is possible to reduce the number of amplitude levels while maintaining the minimum Euclidean distance by employing symbol candidate points having a small maximum amplitude in each dimension of the coordinates of the symbol candidate points. . Then, the process is repeated until the number of symbols adopted reaches the number of symbols corresponding to the frequency utilization efficiency set in step S1. If the number of amplitude levels needs to be further reduced, it is possible to adopt from symbol candidate points with a small maximum amplitude level, and to adopt candidate points with the same maximum amplitude from candidate points with a small norm. is there.

  FIG. 4 is a diagram showing a comparison result of the minimum Euclidean distance between the four-dimensional and eight-dimensional modulation formats and the conventional two-dimensional modulation calculated by the method of searching for the N-dimensional signal point arrangement in the present embodiment. The minimum Euclidean distance is compared under the condition that the transmission power is constant, and it can be confirmed that the minimum Euclidean distance is increased in both the 4th and 8th dimensions as compared with the conventional method.

  FIG. 5 is a diagram showing a result of the searched modulation format. The vertical axis represents frequency utilization efficiency, and the horizontal axis represents the required OSNR penalty from QPSK obtained from the minimum Euclidean distance, and the prior art is two-dimensional modulation of polarization multiplexing. From FIG. 5, improvement can be confirmed for all frequency utilization efficiencies.

Second Embodiment
Next, an optical transmitter according to a second embodiment of the present invention will be described. FIG. 6 is a block diagram showing the configuration of the optical transmitter according to the embodiment. The optical transmitter in FIG. 6 converts transmission data, which is binary information, into a multi-level modulation signal by the symbol mapping circuit 1 using the modulation format calculated in the first embodiment. Then, the modulation signal is converted into an optical signal by the optical modulator 3. At this time, the signal point arrangement in the N dimension is mapped in whole or in part to the in-phase carrier, the quadrature carrier, time, polarization, wavelength, and space (multimode, multicore).

<Third Embodiment>
Next, an optical transmitter according to a third embodiment of the present invention will be described. FIG. 7 is a diagram showing a signal point arrangement with a frequency utilization efficiency of 6 bps / Hz in a four-dimensional space searched by the above-described search method. In the signal point arrangement shown in FIG. 7, the number of dimensions is 4, the number of amplitude levels is 4 at regular intervals, and the levels are relatively -3, -1, 1, 3 respectively. Dimension 1 and Dimension 2, Dimension 3 and Dimension 4 do not take 3 in absolute value of amplitude level at the same time, and the sum of Dimension 1 and Dimension 2, Dimension 3 and Dimension 4 divided by 2 is an odd number. When group 1 (circle in FIG. 7) and even number is group 2 (× in FIG. 7), the possible signal point arrangement is √ (2) from the origin of dimension 1 and 2 At some point, dimensions 3 and 4 can take all the coordinates of the same group as dimensions 1 and 2, and when the distance from the origin of dimensions 1 and 2 is √ (10), dimensions 3 and 4 are dimensions 1 and 2 The coordinates of the same group as dimensions 1 and 2 can be taken, and the number of signal point arrangements is 64.

<Fourth embodiment>
Next, an optical transmitter according to a fourth embodiment of the present invention will be described. FIG. 8 is a diagram showing a signal point arrangement with a frequency utilization efficiency of 8 bps / Hz in the four-dimensional space searched by the above-described search method. In the signal point arrangement shown in FIG. 8, the number of dimensions is 4, the number of amplitude levels is 6 at regular intervals, and the respective levels are -5, -3, -1, 1, 3, 5 relatively. Take. Dimension 1 and Dimension 2, Dimension 3 and Dimension 4 do not take 5 in absolute value of the amplitude level at the same time, and the sum of Dimension 1 and Dimension 2, Dimension 3 and Dimension 4 divided by 2 is an odd number. When group 1 (◯ in FIG. 8) and group 2 (× in FIG. 8) are even numbers, when the distance from the origin of dimensions 1 and 2 is √ (2), dimensions 3 and 4 are All coordinates in the same group as dimensions 1 and 2 can be taken.

  When the distance from the origin of dimensions 1 and 2 is √ (10), dimensions 3 and 4 are the same group as dimensions 1 and 2 and the distance from the origin is √ (2), √ (10) or 3√ ( 2) or √ (26) can be taken. When the distance from the origin of dimensions 1 and 2 is 3√ (2), dimensions 3 and 4 are the same group as dimensions 1 and 2 and the distance from the origin is √ (2) or √ (10) or 3√. The coordinates of (2) can be taken. In addition, when the distance from the origin of dimensions 1 and 2 is √ (26), dimensions 3 and 4 are the same group as dimensions 1 and 2 and coordinates where the distance from the origin is √ (2) or √ (10). Can be taken. Also, when the distance from the origin of dimensions 1 and 2 is 6, dimensions 3 and 4 can take the same group as dimensions 1 and 2 and the coordinates where the distance from the origin is √ (2), and the signal point arrangement Is 256.

<Fifth Embodiment>
Next, an optical transmitter according to a fifth embodiment of the invention will be described. FIG. 9 is a diagram showing a signal point arrangement with a frequency utilization efficiency of 6 bps / Hz in an 8-dimensional space searched by the above-described search method. In the signal point arrangement shown in FIG. 9, the number of dimensions is 8, the number of amplitude levels is 4 at regular intervals, and the levels are relatively −3, −1, 1, 3 respectively. The sum of dimension 1 and dimension 2, dimension 3 and dimension 4, dimension 5 and dimension 6, dimension 7 and dimension 8 divided by 2 is an odd number, and the amplitude level of dimension 1, dimension 3, dimension 5, and dimension 7 is 1 Or -3 is group 1 (-in FIG. 9). An odd number, a dimension 1, a dimension 3, a dimension 5, and a dimension 7 having an amplitude level of −1 or 3 is defined as a group 2 (× in FIG. 9). A group 3 (◯ in FIG. 9) is an even number having dimension 1, dimension 3, dimension 5, and dimension 7 having an amplitude level of 1 or -3. A group 4 (Δ in FIG. 9) is an even-numbered, dimension 1, dimension 3, dimension 5, or dimension 7 whose amplitude level is −1 or 3.

  In this case, all of dimensions 1, 2, 3, 4, 4, 5, 6, and 7, 8 take the coordinates included in group 1. Alternatively, all of dimensions 1, 2, 3, 4, 5, 5, 6, and 7 and 8 take coordinates included in group 2. Alternatively, two of the dimensions 1, 2, 3, 4, 5, 5, 6, and 7 and 8 take the coordinates included in the group 1, and the remaining two take the coordinates included in the group 2. Alternatively, all of dimensions 1, 2, 3, 4, 4, 5, 6, and 7 and 8 are coordinates included in group 3. Alternatively, all of dimensions 1, 2, 3, 4, 5, 5, 6, and 7 and 8 take coordinates included in group 4. Alternatively, two of the dimensions 1, 2, 3, 4, 5, 5, 6, and 7 and 8 take the coordinates included in the group 3, and the remaining two take the coordinates included in the group 4. The number is 4096.

<Sixth Embodiment>
Next, an optical transmitter according to a sixth embodiment of the present invention will be described. FIG. 10 is a block diagram showing the configuration of the symbol mapping circuit 11 and the optical modulation circuit 21 in the optical transmitter according to the embodiment. The symbol mapping circuit 11 calculates an N-dimensional vector as a symbol corresponding to an input bit, using a modulation format searched by a method of searching for signal point arrangements in N dimensions. Then, the N-dimensional vector is output as a modulation signal from the 2-port output terminal over N / 2 timeslots.

  The optical modulation circuit 21 is composed of one optical modulator 211, and modulates the in-phase carrier and the quadrature carrier using the two modulation signals (modulation signal 1 and modulation signal 2) output from the symbol mapping circuit 11. , Output an optical signal.

<Seventh embodiment>
Next, an optical transmitter according to a seventh embodiment of the present invention will be described. FIG. 11 is a block diagram showing the configuration of the symbol mapping circuit 12 and the optical modulation circuit 22 in the optical transmitter according to the embodiment. The symbol mapping circuit 12 calculates an N-dimensional vector as a symbol corresponding to the input bit using the modulation format searched by the method of searching for the signal point arrangement in the N dimension. Then, the N-dimensional vector is output as a modulation signal from the 4-port output terminal over N / 4 timeslots.

  The optical modulation circuit 22 includes an optical modulator (X polarization) 221 that modulates X polarization and an optical modulator (Y modulation) 222 that modulates Y polarization, and is output from the symbol mapping circuit 12. Light in which the in-phase carrier and the quadrature-phase carrier of X polarization and Y polarization are modulated by four modulation signals (modulation signals 1 to 4) and the X polarization and Y polarization are synthesized by the multiplexer 223. Output a signal.

<Eighth Embodiment>
Next, an optical transmitter according to an eighth embodiment of the invention will be described. FIG. 11 is a block diagram showing the configuration of the symbol mapping circuit 13 and the optical modulation circuit 23 in the optical transmitter according to the embodiment. The symbol mapping circuit 13 calculates an N-dimensional vector as a symbol corresponding to the input bit using the modulation format searched by the method of searching for the signal point arrangement in the N dimension. If the number of wavelengths to be used is M (M is an integer greater than or equal to 2 and less than or equal to N / 4), an N-dimensional vector is output from the output terminal of the 4M port as a modulation signal over N / 4M timeslots.

  The optical modulation circuit 23 is composed of 2M optical modulators 231-1 and 23M-1 and 2 that modulate the X polarization and the Y polarization with respect to all wavelengths, and is output from the symbol mapping circuit 13. Based on the modulated signals of the 4M system, the in-phase carrier and the quadrature-phase carrier of X polarization and Y polarization are modulated at all wavelengths. Then, after combining the X polarized wave and the Y polarized wave by the multiplexers 232-1 to 23M, an optical signal in which all wavelengths are multiplexed by the multiplexer 233 is output.

  In this way, in the method of searching for the signal point arrangement in the N dimension, using the closest packed grid in the N dimension space, the searched signal point arrangement can limit the number of amplitude levels, and the signal point arrangement is symmetric. It was made to have sex.

  In the optical transmitter, the symbol mapping circuit includes a circuit that converts the input bit information into a modulation signal by the N-dimensional signal point arrangement searched by the N-dimensional signal point arrangement search method described above. The optical modulation circuit includes a modulator that modulates light by the modulation signal.

  In the optical transmitter, the symbol mapping circuit includes a port that outputs two modulation signals having a length of N / 2 timeslot per symbol, and the optical modulation circuit outputs two modulation signals to one symbol. Input to the optical modulator for modulation.

  Further, in the optical transmitter, the symbol mapping circuit includes a port for outputting four modulation signals having a length of N / 4 timeslot per symbol, and the optical modulation circuit converts the four modulation signals into orthogonal polarization. The waves are input to two optical modulators that modulate the wave and modulated, and the modulated polarized waves are combined.

  In the optical transmitter, the symbol mapping circuit includes a port that outputs 4M systems of modulation signals having a length corresponding to the number of time slots N / M (M is an integer of 2 or more), and the optical modulation circuit includes 4M systems. A modulation signal is input, and both polarizations of wavelengths λ1 to λM are input to 2M optical modulators for modulation, and the modulated polarizations are combined to combine all wavelengths.

  As described above, the signal points in the N dimension designed by the search method of the signal point arrangement in the N dimension, using the closest packed grid, having the symmetry of the signal point arrangement, and capable of limiting the number of amplitude levels. Based on the arrangement, it is possible to realize an optical transmitter that modulates optical signals in a plurality of dimensions.

  According to this configuration, in the N-dimensional signal point space, a method for searching for an N-dimensional signal point arrangement in which the minimum Euclidean distance is expanded, the signal point arrangement is symmetric, and the number of amplitude levels can be limited. Can be provided. Further, by using this N-dimensional signal point arrangement, the minimum Euclidean distance is increased and noise resistance is increased, so that transmission over a longer distance than the conventional method is possible, and the requirements of the DAC can be satisfied. It is possible to provide an optical transmitter in which the complexity of the signal processing algorithm is eliminated.

  You may make it implement | achieve the optical transmitter in embodiment mentioned above with a computer. In that case, a program for realizing this function may be recorded on a computer-readable recording medium, and the program recorded on this recording medium may be read into a computer system and executed. Here, the “computer system” includes an OS and hardware such as peripheral devices. The “computer-readable recording medium” refers to a storage device such as a flexible medium, a magneto-optical disk, a portable medium such as a ROM and a CD-ROM, and a hard disk incorporated in a computer system. Furthermore, the “computer-readable recording medium” dynamically holds a program for a short time like a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line. In this case, a volatile memory inside a computer system serving as a server or a client in that case may be included and a program held for a certain period of time. Further, the program may be a program for realizing a part of the above-described functions, and may be a program capable of realizing the functions described above in combination with a program already recorded in a computer system. It may be realized using hardware such as PLD (Programmable Logic Device) or FPGA (Field Programmable Gate Array).

  As mentioned above, although embodiment of this invention has been described with reference to drawings, the said embodiment is only the illustration of this invention, and it is clear that this invention is not limited to the said embodiment. is there. Therefore, additions, omissions, substitutions, and other modifications of the components may be made without departing from the technical idea and scope of the present invention.

  It can be applied to uses where noise tolerance is high and long distance transmission is indispensable.

  DESCRIPTION OF SYMBOLS 1, 11, 12, 13 ... Symbol mapping circuit, 2, 211, 221, 222, 231, 23M ... Optical modulator, 21, 22, 23 ... Optical modulation circuit, 223, 232, 233 ..Multiplexer

Claims (8)

An optical transmitter used in an optical transmission device,
When N (N is an integer of 4 or more) of the initial coordinate in dimensional space is determined, to determine the origin in accordance with the initial coordinates,
Calculate the nearest grid point coordinates around the initial coordinates using the closest packed grid in the N-dimensional space,
Adding the initial coordinates to the symbol candidate points;
Calculating neighboring grid points around a grid point having a minimum distance from the origin among grid points not adopted as the symbol candidate points;
Adding the lattice points for which the adjacent lattice points have been calculated to the symbol candidate points;
The neighboring grid points are calculated by calculating the neighboring grid points around the grid point with the smallest distance from the origin among the grid points that are not adopted as the symbol candidate points until the number of the symbol candidate points reaches a specified number. Added to the symbol candidate points,
Symbol points are selected in order from the symbol candidate point with the smallest norm from the origin of the coordinate of the symbol candidate point, and when the norm value is the same, the maximum amplitude in each dimension of the coordinate of the symbol candidate point is A symbol mapping circuit that selects a small symbol point and converts the input bit information into a modulation signal by arranging signal points;
An optical transmitter comprising: an optical modulation circuit that modulates light by the modulation signal. The N-dimensional dimension number is 4,
The number of amplitude levels is 4 at regular intervals.
Each level is relatively -3, -1, 1, 3,
2. The optical transmitter according to claim 1, wherein the number of signal point arrangements is 64. The N-dimensional dimension number is 4,
The number of amplitude levels is 6 at regular intervals.
Each level is -5, -3, -1, 1, 3, 5,
2. The optical transmitter according to claim 1, wherein the number of signal point arrangements is 256. The N-dimensional dimension number is 8,
The number of amplitude levels is 4 at regular intervals.
Each level is relatively -3, -1, 1, 3,
2. The optical transmitter according to claim 1, wherein the number of signal point arrangements is 4096. The symbol mapping circuit includes:
It is equipped with a port for outputting two systems of modulation signals with a length of N / 2 timeslot per symbol,
The light modulation circuit is
The optical transmitter according to any one of claims 1 to 4, wherein the modulation signals of the two systems are input to one optical modulator to perform modulation. The symbol mapping circuit includes:
It is equipped with a port that outputs four modulation signals with a length of N / 4 timeslots per symbol,
The light modulation circuit is
5. The system according to claim 1, wherein the four modulation signals are input to two optical modulators that modulate orthogonally polarized waves, are modulated, and each of the modulated polarized waves is synthesized. The optical transmitter according to claim 1. The symbol mapping circuit includes:
It has a port that outputs 4M systems of modulation signals having a length corresponding to the number of time slots N / M (M is an integer of 2 or more),
The light modulation circuit is
Input the modulation signal of the 4M system, input both polarizations of wavelengths λ1 to λM to 2M optical modulators, perform modulation, synthesize each of the modulated polarizations, and synthesize all wavelengths. The optical transmitter according to any one of claims 1 to 4, wherein: An optical transmission method performed by an optical transmitter used in an optical transmission device,
When the initial coordinates in the N (N is an integer of 4 or more) dimensional space are determined, the origin is determined according to the initial coordinates,
Calculate the nearest grid point coordinates around the initial coordinates using the closest packed grid in the N-dimensional space,
Adding the initial coordinates to the symbol candidate points;
Calculating neighboring grid points around a grid point having a minimum distance from the origin among grid points not adopted as the symbol candidate points;
Adding the lattice points for which the adjacent lattice points have been calculated to the symbol candidate points;
The neighboring grid points are calculated by calculating the neighboring grid points around the grid point having the smallest distance from the origin among the grid points not adopted as the symbol candidate points until the number of the symbol candidate points reaches a specified number. Added to the symbol candidate points,
Symbol points are selected in order from the symbol candidate point with the smallest norm from the origin of the coordinate of the symbol candidate point, and when the norm value is the same, the maximum amplitude in each dimension of the coordinate of the symbol candidate point is An optical transmission method comprising a symbol mapping step of selecting a small symbol point.

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