JP4884959B2 - Optical digital transmission system and method - Google Patents

Description

  The present invention is used in an optical digital transmission system that transmits a digital data signal by modulating an optical carrier. In particular, the present invention relates to a technique for dealing with fluctuations in characteristics of optical transmission lines.

  In recent years, an optical multilevel transmission system has begun to be adopted to increase the speed of an optical digital transmission system. However, in the optical multilevel transmission system, conditions required for a transmission path become severe. In addition, the transmission / reception circuit becomes complicated.

  In optical digital transmission with a simple circuit configuration, direct detection is used. When direct detection is used, square detection is performed, so that the positive / negative (phase) of the amplitude cannot be determined, but the circuit becomes simple. In this case, the phase component can be extracted by using homodyne detection which is one of coherent detection.

  On the other hand, in wireless digital transmission and electrical digital transmission, pulse amplitude modulation (PAM), phase shift keying (PSK), quadrature amplitude amplitude modulation (Quadrature Amplitude Modulation): QAM) etc. are used (for example, refer nonpatent literature 1).

  In optical digital transmission, ternary duobinary modulation and quadruple phase shift keying (QPSK) are established techniques (see Non-Patent Document 2, for example). . Furthermore, a multi-value scheme is being studied from 8-phase PSK, 8QAM, 16QAM, and the like (for example, see Non-Patent Documents 3 to 5).

  Even in optical digital transmission, it is possible to adopt a quadrature amplitude intensity modulation (QAM) system by adopting wireless technology, but it has not yet matured until it uses the broadband property of optical communication (for example, see Non-Patent Document 6).

Proakis (translated by Yoshikazu Sakaiba) “Digital Communication” Science and Technology Publishing, 1999 A. Sano et al., ECOC 2006, paper Mo 3.2.1, 2006 E. Agrell and M.M. Karlsson, Optics Express, Vol. No. 14 4, p. 1700, 2006 C. Kim and G.K. Li, Optics Express Vol. 12 No. 15, p. 3415, 2004 N. Kikuchi et al., ECOC 2006, paper Tu3.2.1, 2006 M.M. Nakazawa et al., Electron Lett. 26 No. 12 C. Rasmussen et al., JLT 22, 203-207, 2004

  Although it is widely used in wireless transmission to vary transmission and reception conditions so that signal errors do not occur according to the transmission characteristics of the transmission line, in optical transmission there has been no significant change in the state of the optical transmission line. Until now, it has not been examined.

  However, as the optical network is scaled up, various states are being assumed in the optical transmission line, so the state of the optical transmission line is not always kept good. Communication becomes unstable when the state of the transmission path is bad.

  However, in a transmission / reception circuit for optical digital transmission constituted by an optical device, a change in circuit configuration cannot be easily realized by switching a switch or the like, and varying transmission / reception conditions such as a modulation method, Compared with a wireless digital transmission or electrical digital transmission / reception circuit composed only of electronic components.

  The present invention has been made under such a background, and an optical digital transmission system capable of changing various conditions of transmission / reception without changing the transmission / reception circuit according to the transmission characteristics of the optical transmission line, and It aims to provide a method.

  The present invention provides an optical device comprising: a transmitter that multi-value encodes digital data and modulates and transmits an optical carrier; and a receiver that identifies a received signal using a plurality of identification values corresponding to the multi-value encoding. A digital transmission system characterized in that the present invention is characterized in that the multi-level code of the multi-level code and / or the modulation scheme are both in accordance with transmission characteristics of an optical transmission path between the transmitter and the receiver. It is in the place with the means to change.

  According to this, in order to change the multi-value number of the multi-value code and / or the modulation system by changing only the data supply to the modulator without performing signal processing or the like, the transmitter / receiver circuit such as the modulator or demodulator is used. The transmission capacity can be changed without any modification and without performing electronic processing for signal processing.

  Thereby, when the transmission characteristic of the optical transmission line is deteriorated, the signal can be transmitted without error between transmission and reception by reducing the transmission capacity.

  For example, the variable means can be centrally provided for one or a plurality of the transmitters and the receivers. Alternatively, the variable means can be provided in the transmitter or the receiver.

  The present invention can also be viewed from the viewpoint of a transmitter. That is, the present invention is a transmitter that multi-values digital data and modulates and transmits an optical carrier to a receiver that identifies a received signal using a plurality of identification values corresponding to multi-level coding. A feature of the present invention is that it comprises means for varying the multi-level number of the multi-level code and / or the modulation scheme in accordance with the transmission characteristics of the optical transmission path between the transmitter and the receiver. There is.

  The present invention can also be viewed from the viewpoint of a receiver. That is, the present invention receives a transmission signal transmitted from a transmitter that multi-value encodes digital data and modulates and transmits an optical carrier, and receives a received signal using a plurality of identification values corresponding to the multi-value encoding. The present invention is characterized in that the multi-level number of the multi-level code in the transmitter according to the transmission characteristics of the optical transmission path between the transmitter and the own receiver. Alternatively, a means for changing the modulation system or both is provided.

  The present invention also includes a transmitter that multi-value encodes digital data and modulates and transmits an optical carrier, and a receiver that identifies a received signal using a plurality of identification values corresponding to the multi-value encoding. An optical digital transmission method applied to an optical digital transmission system, characterized in that the present invention is characterized in that one or a plurality of the transmitter and the receiver are managed in an integrated manner, the transmitter, The receiver is configured to change the multi-level number of the multi-level code and / or the modulation scheme according to transmission characteristics of an optical transmission path between the transmitter and the receiver.

  According to the present invention, since the transmission capacity can be easily changed without changing a transmission / reception circuit such as a modulator or a demodulator, good communication is always performed regardless of changes in the transmission characteristics of the optical transmission line. It can be secured.

  An embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a block diagram of the transmission capacity selection function of this embodiment. FIG. 2 is a flowchart showing the operation of the transmission capacity selection function of this embodiment. FIG. 3 is an overall configuration diagram of the optical digital transmission system of this embodiment provided with a transmission capacity selection device. FIG. 4 is an overall configuration diagram of the optical digital transmission system of this embodiment having a transmitter having a transmission capacity selection unit. FIG. 5 is an overall configuration diagram of the optical digital transmission system of this embodiment having a receiver having a transmission capacity selection unit.

  In this embodiment, as shown in FIG. 3, a transmitter 11 that multi-values digital data and modulates and transmits an optical carrier and a received signal using a plurality of identification values corresponding to the multi-value coding are used. An optical digital transmission system including a receiver 12 for identification, which is characterized by the fact that the multi-level code has a plurality of values according to the transmission characteristics of the optical transmission path 60 between the transmitter 11 and the receiver 12. There is a transmission capacity selection device 10 that can change the number of values and / or the modulation method.

  The transmission capacity selection device 10 has a transmission capacity selection function shown in FIG. 1, and the transmission characteristic observation unit 1 performs transmission characteristic observation by receiving transmission characteristic information from the receiver 12. The transmission capacity selection unit 2 selects a transmission capacity according to the observation result of the transmission characteristic observation unit 1, and notifies the selection result to the precoder of the transmitter 11 and the decoder of the receiver 12.

  In FIG. 3, one transmitter 11 and one receiver 12 are illustrated, but a plurality of transmitters 11 and receivers 12 can be managed by a single transmission capacity selection device 10.

  Alternatively, as illustrated in FIG. 4, the transmitter 20 may include a transmission capacity selection unit 21 having the transmission capacity selection function illustrated in FIG. 1. Alternatively, as shown in FIG. 5, the receiver 50 may include the transmission capacity selection unit 51 having the transmission capacity selection function shown in FIG. 1.

  In any case, as shown in FIG. 2, if the transmission characteristic observation unit 1 receives the transmission characteristic information from the receiver 12, 30 or the receiver unit 52 and the transmission result is good according to the observation result (S1), (S2) The transmission capacity is kept normal (S3). If the transmission characteristics are not good (S2), the transmission capacity is reduced (S4).

  Hereinafter, specific transmission capacity selection methods will be described as first to third embodiments.

(First embodiment)
FIG. 6 shows a schematic configuration of multilevel optical digital transmission. The modulator 71 modulates the carrier light with a plurality of input / output data strings input to the precoder 70, and then transmits the modulated light through the optical transmission line 60. The demodulator 72 demodulates the input / output data strings, and the decoder 73 restores the data. To do.

  FIG. 7 is a schematic configuration example of the present invention. By using only a part of data strings from among a plurality of input / output data strings, the transmission characteristics of the optical transmission line 60 are reduced by sacrificing the transmission capacity. Good transmission characteristics are ensured even in bad cases.

  Prior to the start of data transmission, the transmission capacity selection unit 2 determines how much transmission capacity can be communicated according to the observation result of the transmission characteristic observation unit 1. This determination method is a method widely used in telecommunications modems and wireless communications, and will not be described in detail.

  8 to 13 show schematic configuration examples of four-value amplitude modulation (PAM) transmission / reception circuits. As shown in FIG. 8, two synchronized binary data strings D1 and D2 are passed through a DA (digital to analog) converter that generates a four-valued data string of 0, 1, 2, and 3, Amplitude modulation may be performed by the amplitude modulator 80 with four amplitudes. As an example of the DA converter, a 4-value data string of 0, 1, 2, and 3 can be obtained by adding the amplitude of D2 by the adder 82 to the amplitude of D1 doubled by the multiplier 81. .

  FIG. 9 shows an example in which a logic circuit (XOR) 83 is inserted into D1 and D2, and precoding is performed. The receiving side determines the amplitude of these four values and performs AD (analog-to-digital) conversion to restore the digital signal. Examples thereof are shown in FIGS. In the level discriminating circuit 84, D1 can be determined to be “0” if it is smaller than the determination voltage th2, and “1” otherwise. That is, D1 does not depend on the determination voltages th1 and th3.

  In the case of FIG. 8, D2 can be determined to be “0” when it is smaller than the determination voltage th1, or larger than the determination voltage th2 and smaller than the determination voltage th3, and “1” in other cases. Further, in the case of FIG. 9, it can be judged as “0” if it is smaller than the judgment voltage th1 or larger than the judgment voltage th3, and “1” in other cases, and in the case of FIG. The dependency of the determination voltage th2 can be eliminated. Such a coding is well known as a Gray code. FIG. 10 shows the corresponding relationship between the data string and the amplitude, and FIG. 11 shows the signal point arrangement.

  In the case of FIG. 8, the data becomes "00" or "11" by putting the same data in D1 and D2. Of the four states, only the two states having the largest amplitude change can be used, and the transmission state is strongly deteriorated such as noise.

  In the case of FIG. 9, by always fixing the data of D2 to “0”, it is possible to use only the two states having the largest amplitude change among the four states, and it is strong against deterioration of the transmission state such as noise.

  In both cases, the modulation scheme is changed to a large-capacity four-value ASK when there is little noise in the optical transmission line, and to a two-value ASK that is resistant to noise when there is a lot of noise, without special measures on the receiving circuit. Even when the state of the optical transmission line is bad, it is possible to secure good transmission characteristics as compared with the four-value ASK.

(Second embodiment)
This embodiment is an example in which four amplitude values including positive and negative values are used in a four-value PAM. For example, as shown in FIG. 14, it is generated by subtracting 1.5 (by applying a bias) each of the four values 0, 1, 2, and 3 obtained in the first embodiment by a subtractor 85. it can.

  Further, since the negative amplitude is also a phase inversion, it can be generated by the phase modulator 86 and the intensity modulator 87 as shown in FIG. Here, the order of the phase modulator 86 and the intensity modulator 87 may be reversed.

  Since it is necessary to determine the phase on the receiving side, homodyne reception using local light 88 having the same frequency as the received light is performed as shown in FIG. As shown in FIG. 17, local light emission may not be used as the DPSK method. In the case of the DPSK method, the original transmission data is restored by decoding the data after identification or by using a precoder at the time of transmission (for example, see Non-Patent Document 7).

  The amplitude level in this embodiment is shown in FIG. 18, and the signal point arrangement is shown in FIG. When the state of the optical transmission line is bad, the code positions of -3 and +3 that are the most distant from each other among the codes may be used. In order to use this code position, in the case of FIG. 14, the same data may be input to D1 and D2 as in the first embodiment, and in the case of FIG. 15, D2 of the amplitude modulation unit. In this case, a code that always has the maximum transmittance is input, and the data only needs to be placed on D1.

  Further, if it is coded with the Gray code used in the first embodiment in FIG. 14, it is sufficient to place data only on D1 as in the first embodiment. In these cases, the modulation method is equivalent to binary PSK, and even when the transmission path is in a poor state, it is possible to secure good transmission characteristics as compared with the 4-value PAM.

(Third embodiment)
The third embodiment is an example using quaternary phase modulation (QPSK) or quaternary quadrature amplitude modulation (QAM).

  When QPSK or quaternary QAM is used, binary PSK (BPSK) is obtained if only two states on one diagonal are used. In the case of 4QAM, a diagonal line with a long intersymbol distance is used. This can be realized by putting data on only one side or putting the same data on both sides according to the signal point arrangement.

  FIG. 20 shows an example of a QPSK modulator, and FIGS. 21 to 23 show examples of signal point arrangement. 21 and 22 show examples of QPSK signal point arrangement, and FIG. 23 shows an example of QAM signal point arrangement. The QPSK modulator of FIG. 20 is an example using a Mach-Zehnder interferometer. Even when the state of the transmission line is bad, it is possible to ensure good transmission characteristics as compared with the four-value QAM and QPSK.

(Other embodiments)
In the embodiment described above, the transmission signal may be RZ-pulsed. In the above-described embodiment, only four values are shown, but a similar configuration can be considered for multi-value transmission. For example, 8PSK can be sequentially changed to QPSK, BPSK, and the modulation method in accordance with the state of the transmission path.

  According to the present invention, since it is possible to cope with a change in transmission characteristics of an optical transmission line in an optical digital transmission system, it is possible to contribute to the construction of an optical network under various environments.

The block block diagram of the transmission capacity selection function of this embodiment. The flowchart which shows operation | movement of the transmission capacity selection function of this embodiment. 1 is an overall configuration diagram of an optical digital transmission system according to an embodiment including a transmission capacity selection device. The whole block diagram of the optical digital transmission system of this embodiment which has a transmitter provided with the transmission capacity selection part. The whole block diagram of the optical digital transmission system of this embodiment which has a receiver provided with the transmission capacity selection part. The figure which shows the model structure of multi-value optical digital transmission. The figure which shows the schematic structural example of this invention. The block diagram of the DA converter of 1st embodiment (the 1). The block diagram of the DA converter of 1st embodiment (the 2). The figure which shows the corresponding relationship of a data sequence and an amplitude. The figure which shows signal point arrangement | positioning. 1 shows a configuration example (1) of an AD converter according to a first embodiment. Configuration example of the AD converter according to the first embodiment (part 2). The structural example (the 1) of the DA converter of 2nd embodiment. The structural example (the 2) of the DA converter of 2nd embodiment. The example of a structure of the AD converter of 2nd embodiment (the 1). The structural example (the 2) of AD converter of 2nd embodiment. The figure which shows the correlation of a data sequence and an amplitude. The figure which shows signal point arrangement | positioning. The figure which shows the example of the QPSK modulator of 3rd embodiment. The figure which shows the example of the signal point arrangement | positioning of QPSK (the 1) FIG. 5 is a diagram illustrating an example of signal point arrangement of QPSK (part 2); FIG. 6 is a diagram illustrating an example of signal point arrangement of QAM (part 3);

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Transmission characteristic observation part 2 Transmission capacity selection part 10 Transmission capacity selection apparatus 11, 20, 40 Transmitter 12, 30, 50 Receiver 21, 51 Transmission capacity selection part 22 Transmitter part 52 Receiver part 60 Optical transmission path 70 Precoder 71 Modulator 72 Demodulator 73 Decoder 80 Amplitude Modulator 81 Multiplier 82 Adder 83 Logic Circuit 84 Level Discriminator 85 Subtractor 86 Phase Modulator 87 Intensity Modulator 88 Local Light

Claims (6)

A transmitter for modulating and transmitting an optical carrier with multi-value encoded digital data by modulation means for performing quaternary amplitude modulation including positive and negative, quaternary phase modulation, or quaternary quadrature amplitude modulation;
An optical digital transmission system comprising: a receiver for identifying a received signal using a plurality of identification values corresponding to the multi-level encoding;
When the transmission characteristic of the optical transmission line between the transmitter and the receiver is deteriorated , the signal point of the code used in the modulation unit is changed to the modulation unit that performs the four-value amplitude modulation. In the modulation signal point arrangement diagram, in the case of the modulation means for performing the quaternary phase modulation or the quaternary quadrature amplitude modulation only in the two states where the inter-symbol distance is farthest in the modulation signal point arrangement diagram, An optical digital transmission system comprising means for changing to a modulation system that has only two states on one diagonal line .   2. The optical digital transmission system according to claim 1, wherein the changing means is provided in a transmission capacity selection device for notifying one or a plurality of the transmitters and the receivers of the selected transmission capacity.   The optical digital transmission system according to claim 1, wherein the changing unit is provided in the transmitter or the receiver. For a receiver that identifies a received signal using a plurality of identification values corresponding to multi-level encoding , optical carrier is subjected to quaternary amplitude modulation including positive and negative, quaternary phase modulation, or quaternary quadrature amplitude modulation. In a transmitter for modulating and transmitting with multi-value encoded digital data by a modulating means to perform,
When the transmission characteristic of the optical transmission path between the own transmitter and the receiver is deteriorated , the signal point of the code used in the modulation means is changed to the modulation means for performing the quaternary amplitude modulation. In the modulation signal point arrangement diagram, in the case of the modulation means for performing the quaternary phase modulation or the quaternary quadrature amplitude modulation only in the two states where the inter-symbol distance is farthest in the modulation signal point arrangement diagram, A transmitter characterized by comprising means for changing to a modulation system that has only two states on one diagonal line . The optical carrier is transmitted from a transmitter that modulates and transmits digital data that has been encoded with multilevel data by a modulation means that performs quaternary amplitude modulation including positive or negative, quaternary phase modulation, or quaternary quadrature amplitude modulation. In a receiver that receives the transmitted signal and identifies the received signal using a plurality of identification values corresponding to the multi-level encoding,
When the transmission characteristic of the optical transmission path between the transmitter and the receiver is deteriorated , the signal point of the code used in the modulation means is changed to the modulation means for performing the quaternary amplitude modulation. In the modulation signal point arrangement diagram, in the case of the modulation means for performing the quaternary phase modulation or the quaternary quadrature amplitude modulation only in the two states where the inter-symbol distance is farthest in the modulation signal point arrangement diagram, A receiver characterized by comprising means for changing to a modulation scheme that has only two states on one diagonal line . A transmitter for modulating and transmitting an optical carrier with multi-value encoded digital data by modulation means for performing quaternary amplitude modulation including positive and negative, quaternary phase modulation, or quaternary quadrature amplitude modulation;
In an optical digital transmission method applied to an optical digital transmission system comprising a receiver for identifying a received signal using a plurality of identification values corresponding to the multi-level encoding,
When the transmission characteristic of the optical transmission line between the transmitter and the receiver is deteriorated, the transmitter or the apparatus that manages the transmitter and the receiver or the receiver or the receiver, or the receiver, In the case of the modulation means for performing the quaternary amplitude modulation, the signal point of the code used in the modulation means is set to only the two states where the inter-symbol distance is farthest in the modulation signal point arrangement diagram. In the case of a modulation means that performs phase modulation or quaternary quadrature amplitude modulation, the optical digital transmission is changed to a modulation system that has only two states on one diagonal line in the modulation signal point arrangement diagram. Method.

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