JP2011223258A - Optical transmission method, optical transmission system, optical transmitter, and optical receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve anti-noise characteristic and improving receiver sensitivity in an optical transmission system.SOLUTION: An optical transmitter 1 performs trellis encoding for transmission data with a trellis encoder 11, and applies, to the transmission data, an optical modulation in combining a QPSK (Quadrature Phase Shift Keying) modulation with a polarization modulation using a QPSK modulator 12 and a polarization modulator 13, and transmits the transmission data. An optical transmitter 2 performs polarization multiplexing at an optical hybrid circuit 21 and decodes the received data with a trellis decoder 25. The data is encoded with the trellis encoder and transmitted in the optical modulation with a multilevel modulation and the polarization modulation combined, so that the receiver sensitivity is improved by trellis code without increasing the multilevel number of the multilevel modulation.

Description

  The present invention relates to an optical transmission method, an optical transmission system, an optical transmitter, and an optical receiver that transmit a digital data signal using an optical fiber.

  In recent years, the capacity of digital optical transmission systems has been increased. However, as the capacity of the digital optical transmission system increases, the amount of light allocated to one symbol decreases, making it difficult to ensure desired communication quality. For this reason, there is a demand for an optical transmission system that has high reception sensitivity and can ensure desired communication quality. In addition, if the configuration of the transmitter / receiver is complicated in order to increase the reception sensitivity and ensure high signal quality, the cost merit due to the increase in capacity is offset.

In wireless or wired digital transmission, it is known that trellis-coded transmission is effective as one method for improving reception sensitivity. In normal trellis encoding, an n-bit data signal is trellis-encoded into (n + 1) bits and subjected to multilevel modulation of M (= 2 ⁿ +1) values and transmitted. Trellis coded transmission has been put to practical use in high-speed modems, satellite broadcasting, and the like, and is sometimes used together with OFDM (Orthogonal Frequency Division Multiplex) transmission (for example, Non-Patent Document 1, Non-Patent Document). 2). Also in optical transmission, attention has been paid to improvement in reception sensitivity by trellis coding.

  In some cases, polarization modulation is used for transmission of an optical signal. Polarization separation in the case of polarization modulation includes a method of performing coherent reception and performing separation by digital signal processing, as described in Non-Patent Document 3, in addition to a method using a deflection beam splitter.

E. Biglieri et al. “Introduction to Trellis-Coded Modulation with Applications” Macmillian Publishing Company, 1991. Schlegel, “Trellis Coding” IEEE Press, NY, 1997. Han Sun, Kuang-Tsan Wu, and Kim Roberts, “Real-time measurements of a 40 Gb / s coherent system,” Opt. Express 16, 873-879 (2008)

  As a modulation method for optical transmission, quaternary QPSK (Quadrature Phase Shift Keying) modulation is generally used. In QPSK modulation, quaternary (2-bit) codes are mapped on an IQ plane composed of an I-axis and a Q-axis that are in quadrature. In QPSK modulation, quaternary codes are mapped in this way, but in order to further increase the transmission rate by multi-leveling, 8-level PSK modulation and 16-level QAM (Quadrature Amplitude Modulation) are used. Some are designed to be used.

  However, in the case of optical transmission, if the multi-value is increased, the influence on the stability and noise characteristics of the laser light increases. Further, since it is difficult to make the code point arrangement uniform due to the non-linear response of the optical modulator, there is a problem that the noise resistance characteristic deteriorates more than the theoretical characteristic.

  In view of the above problems, an object of the present invention is to provide an optical transmission method, an optical transmission system, an optical transmitter, and an optical receiver that can improve noise resistance and improve reception sensitivity.

  [1] In order to solve the above-mentioned problem, an optical transmission method according to the present invention is a method of optically modulating a trellis code in an optical transmission method in which transmission data is trellis-encoded and optically modulated and transmitted. The optical modulation is performed by combining value modulation and polarization modulation.

[2] The optical transmission system according to the present invention is the optical transmission system described above, wherein the optical modulation is a combination of four-phase phase modulation and two-polarization modulation, and the trellis code is 2 It is a trellis code of state or 8 states.

  [3] An optical transmission system according to the present invention includes: an optical transmitter that trellis-encodes transmission data; and optically modulates and transmits the encoded transmission data by a combination of multilevel modulation and polarization modulation; and the optical transmission And an optical receiver for decoding the received optical signal by optical polarization separation and trellis decoding.

  [4] An optical transmitter according to the present invention modulates trellis coding means for trellis coding transmission data, and a code trellis-coded by the coding means by a combination of multilevel modulation and polarization modulation. And a light modulation means.

  [5] The optical receiver according to the present invention includes receiving means for receiving an optical signal, and decoding means for decoding the optical signal received by the receiving means by optical polarization separation and trellis decoding. Features.

  According to the present invention, data to be transmitted is encoded with a trellis code, and optical modulation is performed by combining multi-level modulation and polarization modulation, so that the shortest inter-code distance can be increased, and noise resistance is improved. Characteristics can be improved. And the reception sensitivity in optical communication can be improved by improving the noise resistance.

It is a block diagram which shows the structure of the optical transmission system in embodiment of this invention. It is IQ top view which shows the mapping of the code | symbol in the embodiment. It is a block diagram of the specific example of the QPSK modulator and polarization modulator in the embodiment. It is a block diagram of the other specific example of the QPSK modulator and polarization modulator in the same embodiment. It is a trellis diagram of a two-state trellis code. It is IQ top view which shows the mapping of the code | symbol of 8PSK modulation. It is a trellis diagram in the case of an 8-state trellis code.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing the configuration of the optical transmission system of this embodiment. This optical transmission system includes an optical transmitter 1, an optical receiver 2, and an optical fiber 3.

  As shown in FIG. 1, the optical transmitter 1 includes a light source 10, a trellis encoder 11, a QPSK modulator (four-phase modulator) 12, a polarization modulator 13, and drivers 14 and 15. I have. The light source 10 emits laser light serving as carrier light. Laser light emitted from the light source 10 is supplied to the QPSK modulator 12.

The trellis encoder 11 encodes a signal indicating n-bit transmission data into (n + 1) bits using a trellis code. For example, in the case of (n = 2), the 2-bit data signal is encoded into 3 bits by the trellis encoder 11. This trellis encoding can be performed by a known technique. For example, transmission data can be encoded by the techniques described in Non-Patent Documents 1 and 2.
The trellis encoder 11 outputs the trellis-encoded 3-bit code (I, Q, D signal) to the QPSK modulator 12 and the polarization modulator 13 via the drivers 14 and 15.

The QPSK modulator 12 and the polarization modulator 13 perform optical modulation by combining QPSK modulation and polarization modulation on the laser light emitted from the light source 10 according to the code output from the trellis encoder 11. Do.
2A shows the arrangement of the IQ plane when mapping even-numbered codes in the QPSK modulator 12, and FIG. 2B shows the arrangement of the IQ plane when mapping odd-numbered codes. Yes. The trellis encoder 11 outputs 3-bit codes “0” to “7”. Here, the code “0” corresponds to a 3-bit code “000”, and the code “7” corresponds to a 3-bit code “111”. Among these codes, the QPSK modulator 12 maps even codes “0”, “2”, “4”, “6” on the IQ plane as shown in FIG. The odd numbers “1”, “3”, “5”, “7” are mapped on the IQ plane as shown in FIG.

  The QPSK modulator 12 modulates (maps) the input laser light according to the input I and Q signals, and outputs the modulated laser light to the polarization modulator 13. The polarization modulator 13 outputs laser light modulated by an even number code as X polarization in accordance with the input D signal, and outputs laser light modulated by an odd number code as Y polarization. Thus, the two-polarized QPSK-modulated optical signal is optically amplified as necessary, and then transmitted to the optical receiver 2 via the optical fiber 3.

Here, since a sign of (M = 8) value is used, a quaternary QPSK modulator 12 is used as a modulator. The relationship between the multi-level number of the trellis code, the multi-level number of the modulator (QPSK modulator 12) and the polarization number of the polarization modulator (polarization modulator 13) is as follows. When the multi-value number in V is V and the polarization number in the polarization modulator is W (where M, V, and W are integers), the relationship is (M = V × W).
Further, when encoding of (M = 16) value is used, for example, an optical transmitter may be configured by combining an 8-phase modulator and a polarization deflector.

FIG. 3 is a specific example of the QPSK modulator 12 and the polarization modulator 13.
In FIG. 3, the I component and the I and Q signals of the Q component output from the trellis encoder 11 (FIG. 1) are sent to the optical phase modulator 55 and the optical phase modulator 56 via the driver 51 and the driver 52. Supplied respectively. The polarization information D, which is a D signal, is supplied to the polarization modulator 57 via the driver 53. The optical phase modulator 55 performs optical modulation with a phase shift π on the input laser light in accordance with the I signal of the I component. The optical phase modulator 56 performs (π / 2) phase-shift optical modulation on the laser light output from the optical phase modulator 55 according to the Q signal of the Q component. As described above, the optical phase modulator 55 and the optical phase modulator 56 perform optical modulation with optical signals having phases different from each other by 90 degrees. Then, the polarization modulator 57 switches the polarization of the laser light output from the optical phase modulator 56 to either the X polarization or the Y polarization according to the input polarization information D. As a result, a two-polarized QPSK-modulated optical signal is output.

  FIG. 4 shows another specific example of the QPSK modulator 12 and the polarization modulator 13. In FIG. 4, the I component signal is supplied to the optical phase modulator 65 via the driver 61. The Q component signal is supplied to the optical phase modulator 66 via the driver 62. The polarization information D, which is a D signal, is supplied to the polarization modulator 67 via the driver 63.

  The input optical signal is branched into two optical paths. The optical phase modulator 65 performs optical phase modulation with a phase shift of π on the branched laser light in accordance with the I signal of the I component. Further, the optical phase modulator 66 performs optical phase modulation of phase shift π with respect to the laser light on the other branched optical path in accordance with the Q signal of the Q component. The phase shifter 68 performs phase shift (π / 2) optical modulation on the laser light optically modulated by the optical phase modulator 66.

  As described above, the laser beam branched into the two optical paths has a phase shift of π / 2 after the optical signal of one optical path is modulated by the I component and the optical signal of the other optical path is modulated by the Q component. Phase shifted. Then, each modulated laser beam is synthesized. Then, the polarization modulator 67 switches the synthesized laser light to either the X polarization or the Y polarization according to the polarization information D. As a result, a two-polarized QPSK-modulated optical signal is output.

  Next, the optical receiver 2 will be described. As shown in FIG. 1, the optical receiver 2 includes an optical hybrid circuit 21, a local light source 22, a photoelectric conversion / A / D converter 23, a compensation circuit 24, and a trellis decoder 25. The

  In FIG. 1, the laser light transmitted from the optical transmitter 1 through the optical fiber 3 is input to the optical hybrid circuit 21. The optical hybrid circuit 21 extracts the phase component of the received light by mixing the received laser light and the local light input from the local light source 22. The optical hybrid circuit 21 separates X-polarized laser light and Y-polarized laser light. The X-polarized and Y-polarized laser beams separated by the optical hybrid circuit 21 are output to the photoelectric conversion and A / D converter 23.

  The photoelectric conversion and A / D converter 23 converts the output of the optical hybrid circuit 21 from an optical signal to an electrical signal, and digitizes the analog electrical signal. The output of the photoelectric conversion / A / D converter 23 is output to the trellis decoder 25 via the compensation circuit 24. The compensation circuit 24 performs processing such as polarization compensation, waveform distortion compensation, and phase compensation by phase estimation on the input digital signal. The trellis decoder 25 considers the trellis transition state and decodes data by trellis decoding for the digital signal input from the compensation circuit 24 by soft decision processing and maximum likelihood decoding using, for example, the Viterbi algorithm. Output.

  The polarization separation for separating the X-polarized laser beam and the Y-polarized laser beam can be performed by a known technique. For example, in addition to a method using a deflecting beam splitter, a method of performing coherent reception and separating by digital signal processing may be used. (Non-Patent Document 3).

  In normal trellis coding, trellis-encoded 8-level code is transmitted after 8-level PSK modulation. On the other hand, in the present embodiment, as described above, by using QPSK modulation and two-polarization modulation in combination, the intersymbol distance is larger than that of normal trellis encoding (8-level PSK modulation). Thus, noise resistance can be improved. This will be described below.

  In trellis coding, state transitions are shown by trellis diagrams. FIG. 5 shows a trellis diagram of a two-state quaternary trellis code. The two-state trellis code can take two states, state S1 and state S2. In FIG. 5, four lines are output from each of the states S1 and S2 corresponding to the four-value codes. In the state S1, the codes “0”, “4”, There are four lines corresponding to “2” and “6”, and the state transitions according to this trellis diagram. In the state S2, four lines corresponding to the code “1”, the code “5”, the code “3”, and the code “7” come out from the top, and the state transitions according to the trellis diagram.

  For example, in the two-state trellis diagram shown in FIG. 5, the transition from the state S1 to the state S1 includes the following.

State transition (a): The code “2” is output from the state S1 to move to the state S2, and then the code “1” is output from the state S2 to return to the state S1.
State transition (b): The code “2” is output from the state S1 to move to the state S2, and then the code “5” is output to return to the state S1.
State transition (c): The code “4” is output from the state S1 and remains in the state S1.

  In addition, after the code “2” is output from the state S1 and moved to the state S2, the code “3” or the code “7” is output to remain in the state S2, and then return to the state S1. However, since the intersymbol distance is clearly longer than in the case of the state transition (a) or the state transition (b), the calculation is omitted here.

In 8PSK modulation, codes “0” to “7” are mapped on the IQ plane, as shown in FIG. In FIG. 6, in the normal 8PSK, eight code points are arranged at equal intervals on the circumference of unit length 1, and each code point is sequentially associated with code “0” to code “7”. In this case, the inter-code distance from the code “0” to each code is as follows. Here, the distance between the symbol a and the symbol b is written as d ² (a, b).

d ² (0,1) = 4 sin ² (π / 8) = 0.586
d ² (0,2) = 2
d ² (0,3) = 4 sin ² (3π / 8) = 3.414
d ² (0,4) = 4
d ² (0,5) = 4 sin ² (3π / 8) = 3.414
d ² (0,6) = 2
d ² (0,7) = 4 sin ² (π / 8) = 0.586

  Therefore, when the normal 8PSK modulation is used and each distance is calculated, in the case of the state transition (a), the code “2” is output and the code “1” is output. The distance is as follows.

d ² (0,2) + d ² (0,1) = 2 + 0.586 = 2.586

  In the case of state transition (b), since the code “2” is output and the code “5” is output, the distance between the codes is as follows.

d ² (0,2) + d ² (0,5) = 2 + 3.414 = 5.414

  In the case of state transition (c), since the code “4” is output, the distance between the codes is as follows.

d ² (0,4) = 4

  From the above results, when the 8PSK modulation is used, the distance between the codes becomes the shortest because the code “2” is output from the state S1 in (a), and the state S2 is moved. It can be seen that this is the case where the code “1” is output from the state and the state returns to the state S1. The inter-code distance at this time is 2.586.

Since the minimum inter-code distance of QPSK when the trellis code is not used is (d ² (0,2) = 2), as described above, the encoding when 8PSK modulation is performed by the two-state trellis encoding The gain is as follows.

    10 log (2.586 / 2) = 1.1 dB

In contrast, in the present embodiment, as described above, even-numbered codes are mapped as shown in FIG. 2A, and odd-numbered codes are mapped as shown in FIG. 2B. The In the two-polarization trellis of the present embodiment, the intersymbol distance in the case of making a transition across the polarization (X polarization to Y polarization or Y polarization to X polarization) depends on the degree of polarization separation. . When there is a separation of 10 dB, transition between polarizations causes a difference in intensity of 10 times in the received laser light, so that the determination of transition is not wrong. Furthermore, the degree of polarization separation by digital signal processing, optical processing (polarization beam splitter) or the like in the optical receiver 2 is generally 20 dB or more.
Therefore, in the calculation of the intersymbol distance, it can be regarded as a distance sufficiently larger than the shortest intersymbol distance in the same polarization. In the following calculation, the intersymbol distance between polarized waves is assumed to be infinite.
In this case, the distance between even codes is as follows.

d ² (0,1 ′) >> 1
d ² (0,2) = 2
d ² (0,3 ′) >> 1
d ² (0,4) = 4
d ² (0,5 ′) >> 1
d ² (0,6) = 2
d ² (0,7 ′) >> 1

  From the above consideration, when each distance in this embodiment is calculated, in the case of the state transition (a), the code “2” is output, the code “1” is output, and the code “2” is output. "Is an X polarization, and" 1 "is a Y polarization, and the degree of polarization separation is sufficient, so it can be determined that the distance between codes is sufficiently large.

  In the case of the state transition (b), the code “2” is output, the code “5” is output, the code “2” is the X polarization, the code “5” is the Y polarization, Since the degree of wave separation is sufficient, it can be determined that the intersymbol distance is sufficiently large.

  In the case of the state transition (c), the code “4” is output, and the distance between the codes is as follows.

d ² (0,4) = 4

  Therefore, in the present embodiment, the inter-code distance “2” when the code “4” is output from the state S1 of the state transition (c) and remains in the state S1 is the minimum inter-code distance. Therefore, the coding gain for QPSK of the present invention is as follows.

    10 log (4/2) = 3 dB

  As described above, when a transition is made from state S1 to state S1 with a two-state trellis code, the minimum inter-code distance is 2.586 in 8-level PSK modulation. On the other hand, in the present embodiment using 4PSK modulation and two-polarization modulation, the minimum intersymbol distance is 4. As described above, in this embodiment, by using 4PSK modulation and two-polarization modulation, the minimum intersymbol distance can be increased and noise resistance can be improved as compared with the case of using normal 8PSK modulation. be able to.

When polarization separation is not performed at all (0 dB), the intersymbol distance between the polarizations is zero, that is, d ² (0,1 ′) = 0, and the path distance due to the state transition (a) Since d ² (0,2) + d ² (0,1 ′) = 2 is the shortest, a gain due to trellis coding cannot be obtained.

  In the above description, the two-state trellis code is used. However, even in the case of other trellis codes, in the present embodiment, the minimum inter-code distance is increased and noise resistance can be improved. For example, the distance between codes when an 8-state trellis code is used is as follows.

  FIG. 7 shows a trellis diagram for an 8-state trellis code. In the 8-state trellis code, eight states from state S1 to state S8 can be taken. Similar to the above, the intersymbol distance in the case of transition from the state S1 to the state S1 will be considered. In the case of an 8-state trellis code, from the trellis diagram shown in FIG.

State transition (d) Outputs the code “4” from the state S1 to move to the state S2, outputs the code “1” to move to the state S5, outputs the code “2”, and returns to the state S1. .
State transition (e) Outputs the code “2” from S1 and moves to the state S3, outputs the code “4” and returns to the state S1 (broken line).
State transition (f) The code “6” is output from the state S1 to move to the state S4, the code “7” is output to move to the state S7, the code 6 is output, and the process returns to the state S1.

  In addition, after the code “2” is output from the state S1 and moved to the state S2, the code “3” or the code “7” is output to remain in the state S2, and then return to the state S1. However, since the inter-code distance in this case is clearly longer than that in the state transition (d) or the state transition (e), the calculation is omitted.

  In the normal 8-level PSK modulation, in the case of the state transition (d), the code “4”, the code “1”, and the code “2” are output, and the distance between the codes is as follows: Become.

d ² (0,4) + d ² (0,1) + d ² (0,2) = 4 + 0.586 + 2 = 6.586

  In the case of the state transition (e), the code “2” and the code “4” are output, and the distance between the codes is as follows.

d ² (0,2) + d ² (0,4) = 2 + 4 = 6

  In the case of the state transition (f), the code “6”, the code “7”, and the code “6” are output, and the distance between the codes is as follows.

d ² (0,6) + d ² (0,7) + d ² (0,6) = 2 + 0.586 + 2 = 4.586

  Therefore, in the normal 8-level PSK modulation, the minimum intersymbol distance is 4.586, and the coding gain is as follows.

    10 log (4.586 / 2) = 3.6 dB

  On the other hand, in the case of this embodiment, when the states S1, S3, S5, and S7 are X polarized waves, and the states S2, S4, S6, and S8 are Y polarized waves, odd states The intersymbol distance when transitioning from 1 to an even number of states, for example, the intersymbol distance when transitioning from the state S1 to the state S2 is very large, and therefore there is no need to consider in calculating the shortest intersymbol distance.

  In the present embodiment, in the case of the state transition (d), since the transition from the state S1 to the state S2 is made, it can be determined that the inter-code distance is sufficiently large.

  In the case of the state transition (e), it is a transition to the state S1, the state S3, and the state S1, and since the code “2” and the code “4” are output, the distance between the codes is as follows: become.

d ² (0,2) + d ² (0,4) = 2 + 4 = 6

  In the case of state transition (f), since the transition is made from state S1 to state S4, it can be determined that the intersymbol distance is sufficiently large.

  Therefore, in this embodiment, the state transition (e) is the minimum inter-code distance, and the minimum inter-code distance is 6 at this time, and the coding gain for QPSK is as follows.

    10 log (6/2)) = 4.7 dB

  As described above, even in the case of the 8-state trellis code, in this embodiment, the intersymbol distance is increased and the noise resistance is improved as compared with the trellis coding by the normal 8-level PSK modulation. Although the two-state trellis code and the eight-state trellis code have been described above, the same applies to other numbers of states.

Also, in the case of polarization multiplexing transmission, the transmission power per wavelength is doubled, so the transmission speed and noise characteristics equivalent to those of polarization multiplexing are increased by doubling the modulation rate and further doubling the transmission power. Can be obtained. Furthermore, by applying the present invention, it is possible to obtain noise resistance characteristics due to coding gain while realizing a transmission rate equivalent to polarization multiplexing transmission.
Thus, in this embodiment, the coding gain can be improved by combining multi-level modulation and polarization modulation. That is, the noise resistance can be improved, and the coding gain can be improved as compared with ordinary trellis coded modulation.

  The present invention is not limited to the above-described embodiments, and various modifications and applications can be made without departing from the gist of the present invention.

For example, in the above-described embodiment, in the two-state trellis code, the even code is the X polarization, and the odd code is the Y polarization, but the codes “0” to “3” are assigned to the X polarization, The codes “4” to “7” may be assigned to the Y polarization. Even if a code is assigned in this way, when a transition from X polarization to Y polarization is included, the inter-code distance becomes large, so that the coding gain can be improved.
Further, in the 8-state trellis code, the states S0 to S3 may be X polarized waves, and the states S4 to S7 may be Y polarized waves.

  In the above-described embodiment, the transmission apparatus has a configuration in which transmission data is 2 bits. However, encoding may be performed for each transmission data having a different number of bits. In that case, the trellis encoder may output (n + 1) bits with respect to an n-bit input, and may be configured to combine n-value multilevel modulation and polarization modulation.

  DESCRIPTION OF SYMBOLS 1 ... Optical transmitter, 2 ... Optical receiver, 3 ... Optical fiber, 10 ... Light source, 11 ... Trellis encoder, 12 ... QPSK modulator, 13, 57, 67 ... Polarization modulator, 21 ... Optical hybrid circuit , 22 ... Local light source, 23 ... Photoelectric conversion and A / D converter, 24 ... Compensation circuit, 25 ... Trellis decoder, 55, 56, 65, 66 ... Optical phase modulator, 68 ... Phase shifter

Claims (5)

In an optical transmission method in which transmission data is trellis encoded and optically modulated and transmitted,
When optically modulating the trellis code, optical modulation is performed by combining multilevel modulation and polarization modulation. 2. The optical transmission method according to claim 1, wherein the optical modulation is a combination of four-phase modulation and two-polarization modulation, and the trellis code is a two-state or eight-state trellis code. . An optical transmitter that trellis-encodes transmission data and optically modulates and transmits the encoded transmission data by a combination of multi-level modulation and polarization modulation;
An optical transmission system comprising: an optical receiver that receives an optical signal from the optical transmitter and decodes the received optical signal by optical polarization separation and trellis decoding. Trellis encoding means for trellis encoding transmission data;
An optical transmitter comprising: an optical modulation unit configured to modulate a code trellis-encoded by the encoding unit by a combination of multilevel modulation and polarization modulation. Receiving means for receiving an optical signal;
An optical receiver comprising: decoding means for decoding the optical signal received by the receiving means by optical polarization separation and trellis decoding.

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