JP2009038553A - Digital transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To transmit a sub-signal such as a monitoring signal without deteriorating transmission characteristics of a main signal and without reducing transmission efficiency. <P>SOLUTION: The main signal is transmitted while switching a plurality of modulation modes with almost equal transmission characteristics, and the modulation modes are switched according to the sub-signal. For example, QAM modulation is used for modulation, a code whose amplification ratio between an i axis component and a q axis component of the QAM modulation is 1:√3 is used, amplitude of the i axis component and the q axis component are replaced by the sub-signal to transmit the sub-signal by amplitude components of the i axis component and the q axis component. Or, QAM modulation using the code whose amplification ratio between the i axis component and the q axis component is 1:√3, and QPSK modulation in which amplitude of the i axis component and the q axis component are equal to each other and having amplitude of √2 when it is standardized by the amplitude of the code are used for modulation, the two modulation modes are switched by the sub-signal to transmit the sub-signal by the amplitude components of the i axis component and the q axis component. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention is used for a multi-value transmission system using a QAM code or a QPSK code. In particular, the present invention relates to a method for transmitting a sub signal used for monitoring or the like together with a main signal. In the following background art and embodiments, an example of an optical digital transmission system will be described. However, the scope of the present invention is not limited to optical communication.

  In order to increase the speed and capacity of optical digital transmission systems in recent years, optical multilevel transmission systems have begun to be adopted. On the other hand, techniques similar to those in the past have been used for transmission of sub-signals such as monitoring signals, and studies on methods for transmitting sub-signals without sacrificing the bandwidth and transmission characteristics of the main signal have not progressed.

  Conventionally, a main signal is subjected to slight amplitude modulation or phase modulation at a low frequency, and a monitor signal is put on the low frequency component. However, since modulation is mixed, transmission characteristics are deteriorated. Further, although the monitoring sub-signal can be transmitted at a different wavelength, a new band is used for sub-signal transmission, which is inefficient.

JP 2000-2011137 A JP-A-6-97885 Ohmsha, "Optical amplifier and its application", supervised by Hideki Ishio, 1992 Science and Technology Publishing, "Digital Communication", Proakis, 1999

  For example, when transmission of the sub signal is performed by amplitude modulation, it is equivalent to the intensity fluctuation of the main signal corresponding to the modulation intensity of the sub signal. For example, when modulation of 1 dB is performed, the intensity of 1 dB The margin will be used for sub-signal transmission.

  The present invention has been made to solve such a problem, and transmits a sub-signal such as a monitoring signal without degrading the transmission characteristics of the main signal and without reducing the transmission efficiency. It is an object of the present invention to provide a digital transmission system that can perform the above-described process.

  The present invention uses a plurality of modulation forms having substantially the same transmission characteristics for main signal transmission, and transmits a sub-signal such as a monitoring signal by switching the sub-signal transmission without deterioration of the main signal transmission characteristics. Features.

  That is, the present invention includes a transmission apparatus that multi-values digital data, modulates and transmits a carrier signal, and a reception apparatus that identifies a received signal using a plurality of identification values corresponding to the multi-level code. Digital transmission system.

  Here, a feature of the present invention is that it includes means for transmitting a main signal while switching a plurality of modulation forms having substantially the same transmission characteristics, and this means for transmitting switches the modulation form in accordance with a sub-signal. is there.

  For example, the modulation uses quaternary quadrature amplitude modulation (QAM), a code whose amplitude ratio between the i-axis component and the q-axis component is 1: √3, and the i-axis component and the q-axis by the sub signal. By substituting the amplitude with the component, the sub-signal is transmitted with the amplitude component of the i-axis component and the q-axis component.

  Alternatively, for the modulation, QAM modulation using a code in which the amplitude ratio of the i-axis component and the q-axis component is 1: √3, and the amplitude of the i-axis component and the q-axis component are equal, and the amplitude of the code is specified. In this case, the QPSK modulation having an amplitude of √2 is used, and the two modulation modes are switched by the sub signal to transmit the sub signal with the amplitude component of the i-axis component and the q-axis component.

  The present invention can also be viewed from the viewpoint of a sub-signal transmission method. That is, the present invention is a sub-signal transmission method provided in a digital transmission system, wherein digital data is multi-level encoded, a carrier signal is modulated and transmitted, and the transmission apparatus includes a plurality of modulations having substantially the same transmission characteristics. The main signal is transmitted while switching the form, and at the time of switching, the modulation form is switched according to the sub signal, and the receiving apparatus demodulates the sub signal by extracting the switching pattern of the modulation form. And

  The present invention can also be viewed from the viewpoint of a transmission apparatus. That is, the present invention is a transmission apparatus that is provided in a digital transmission system and multi-level encodes digital data, modulates and transmits a carrier signal, and performs four-phase phase modulation on the carrier signal to generate an i-axis component and a q-axis. And means for generating a code point arrangement signal with substantially equal component amplitudes, and means for changing the amplitude ratio between the i-axis component and the q-axis component of the signal in accordance with the sub-signal. .

  Alternatively, the transmitting apparatus of the present invention branches the carrier signal into two branches, one of which is an i-axis component and the other is a q-axis component, so that the phase of the q-axis component is shifted by 90 degrees and then multiplexed. And means for changing the amplitude ratio between the amplitude of the i-axis component and the amplitude of the q-axis component in accordance with the sub-signal.

  The present invention can also be viewed from the viewpoint of a receiving device. That is, the present invention is a receiving device that is provided in a digital transmission system and identifies a received signal using a plurality of identification values corresponding to a multilevel code, and the transmission signal of the transmitting device of the present invention is a received signal, Means is provided for demodulating the sub-signal based on the output levels of the demodulated i-axis component signal and q-axis component signal.

  Further, in order to improve the identification sensitivity, a means for differentially identifying the output of the i-axis component and the output of the q-axis component may be provided.

  According to the present invention, it is possible to transmit a sub signal such as a monitoring signal without degrading the transmission characteristics of the main signal and without degrading the transmission efficiency.

(First embodiment)
A transmission apparatus according to a first embodiment of the present invention will be described with reference to FIGS. 1, 2 and 3 show signal point arrangement examples in the phase space of a four-value QAM code (hereinafter referred to as QAM code). FIG. 3 is equivalent to a QPSK code, and it has been found that the distance between codes can be maximized at a certain transmission power.

  1 and FIG. 2 are the same as FIG. 3 in the case of the same transmission power as FIG. 3 by setting the amplitude ratio of the i-axis component and the q-axis component to 1: √3 when the amplitude of FIG. This is the distance between codes. Therefore, it is known that these transmission characteristics are almost equal. 1 and 2, when the amplitude ratio of the i-axis component and the q-axis component is shifted from 1: √3 while the transmission power is kept constant, the intersymbol distance becomes narrow and the characteristics deteriorate (for example, Non-patent document 2).

  It is clear that even if the amplitudes of the i-axis component and the q-axis component are switched in the QAM code as shown in FIG. 1 or FIG. In the conventional QAM code, only the signal point arrangement of either FIG. 1 or FIG. 2 is used. A method for modulating the amplitude of a signal at a sufficiently low frequency compared to the main signal and transmitting the sub-signal is described in, for example, Patent Document 1, but the transmission output corresponding to the fluctuation of the amplitude must be changed. However, a margin for transmission is required. That is, transmission characteristics are deteriorated.

  On the other hand, in the present invention, two kinds of signal point arrangement of the QAM code are switched by the sub signal. Since the transmission characteristics of the two signal point arrangements are equivalent, the transmission characteristics of the main signal are not deteriorated. The amplitude of these two signal point arrangements is A and √3A, where the amplitude of the component of the axis with a small amplitude is A (A is a positive real number), and the ratio is about 4.8 dB. That is, 4.8 dB characteristic deterioration can be prevented as compared with the case where sub-signal transmission having the same amplitude ratio is performed by the conventional method.

  FIG. 4 shows a schematic configuration of the transmission apparatus. As shown in FIG. 4, the transmission code of the present invention has a four-phase phase modulator 2 and an amplitude modulator 3 arranged in series, and performs four-phase phase modulation and amplitude modulation in series, or FIG. As shown, amplitude modulation is performed on the i-axis component and the q-axis component by the amplitude modulators 6 and 7, respectively, and the phase of the i-axis component or the q-axis component is shifted by 90 degrees using the 90-degree phase shifter 8. These can be generated by vector synthesis.

  In order to generate the code arrangement shown in FIG. 1 with the configuration shown in FIG. 4, first, the signal from the laser 1 is modulated by the four-phase modulator 2 to generate a signal having the code point arrangement shown in FIG. Amplitude modulation is performed by the amplitude modulator 3 so that the amplitude is 1 when the point is on the i-axis and the amplitude is √3 when the point is on the q-axis.

  The amplitude is set in the amplitude modulator 3, and the switching is performed by a signal from the encoder 4-1. That is, an analog signal corresponding to the sub signal corresponding to the corresponding amplitude is output from the encoder 4-1, and the amplitude modulator 3 is driven by the analog signal.

  In order to generate the code arrangement of FIG. 2 with the configuration of FIG. 4, the operation of the amplitude modulator 3 is as follows: amplitude √3A when the code point is on the i axis, amplitude A when the code point is on the q axis. It may be changed so as to become. In the present invention, the code arrangement of FIGS. 1 and 2 is switched by the sub-signal, and this switching is performed by the encoder 4-1.

  In addition, in order to generate the code arrangement of FIG. 1 by the transmission apparatus having the configuration of FIG. 5, the light from the laser 1 is branched into two by the branching unit 5, and one is an i-axis component and the other is a q-axis component. The optical phase of the q-axis component is shifted by 90 degrees phase shifter 8 and then multiplexed by multiplexing unit 9. The encoder 4-2 drives the amplitude modulator 6 or 7 so that the i-axis component signal or the q-axis component signal light is transmitted according to the code point of the main signal. The same applies when the code point arrangements of FIGS. 2 and 3 are generated. The switching is performed by switching the operation of the encoder 4-2 according to the sub signal.

  FIG. 6 shows a schematic configuration example of a QAM receiving circuit for demodulating a sub-signal according to the present invention. FIG. 7 shows a schematic configuration example of a conventional QAM receiving circuit. In order to demodulate the sub-signal of the present invention, it is as follows.

  The received signal suppresses power fluctuation due to noise or the like by the automatic level adjustment unit 10. The carrier reproducing unit 11 reproduces a carrier signal from the received signal and generates local light. The phase of the local light is adjusted by the phase adjustment unit 12, and the signals of the i-axis component and the q-axis component are demodulated by the interferometers 13-1 and 13-2. Interferometers 13-1 and 13-2 have 45 degree phase shifters 14-1 and 14-2. Each signal is converted from an optical signal to an electric signal by the light receiving units 15-1 and 15-2, and then synchronized with the clock component by the synchronizing units 16-1 and 16-2, and the multilevel discriminating unit 17-1. And 17-2 to perform multi-level identification. The sub-signal can be demodulated by the output level of the multi-level discriminating unit 17-1 using the level discriminating unit 19.

  Further, by using balanced light receiving units such as the light receiving units 15-1 and 15-2, the outputs of the i-axis component and the q-axis component can be identified differentially for identifying the sub-signals. The sensitivity of signal identification can be improved. The multi-level discriminators 17-1 and 17-2 can identify at least four levels of ± √3A and ± A, but may be constituted by an AD converter or the like.

  The main signal is decoded by reading the position on the phase plane by the Euclidean distance metric comparison unit 18, but the main signal is transmitted in any of the code point arrangements of FIGS. 1 to 3 by demodulating the sub signal. Since the main signal can be determined and demodulated based on the signal point arrangement, the transmission characteristics of the main signal do not deteriorate.

  In the present invention, since the average power at the time of transmission is constant, it is possible to perform automatic level adjustment on the received signal and correct the level fluctuation due to noise during transmission, but this is shown in FIG. As described above, when the sub signal is transmitted by the amplitude component in the conventional method, the sub signal needs to be separated before the automatic level adjustment unit 10 performs the automatic level adjustment. Since signal determination is performed using this, it is necessary to keep the transmission capacity of the sub signal extremely low in order to distinguish it from level fluctuations due to noise or the like. Although it is possible to detect amplitude fluctuations after demodulating the main signal without performing automatic level adjustment, the deterioration of the transmission characteristics of the main signal becomes large.

(Second embodiment)
The signal point arrangement in the phase space of the four-phase phase modulation code (hereinafter referred to as QPSK code) is the same as that in FIG. 3, and can be regarded as a special case of the QAM code. In order to achieve the same transmission power as the QAM code of the first embodiment, the amplitude is √2A. When such an amplitude ratio is taken and QAM and QPSK are set to the same transmission power, both have the same distance in the phase space, and therefore the transmission characteristics are almost equal (see, for example, Non-Patent Document 2).

  Further, since the QPSK code is a special case of the QAM code, it can be demodulated by the same receiving apparatus shown in FIG. 6 as the QAM code. That is, even when transmission is performed while switching between the QAM code and the QPSK code according to the sub-signal, the receiving apparatus does not need to be changed and the change in the transmission characteristic of the main signal is small.

  Here, focusing on the amplitude component of the i-axis component or the q-axis component as in the first embodiment, binary (one of FIG. 1 or FIG. 2 and FIG. 3 (QPSK)) or ternary (FIG. 1 and FIG. 2). The amplitude modulation shown in FIG. 3 (QPSK) can be performed. In the case of ternary amplitude modulation, the level identifying unit 19 of the receiving apparatus uses a circuit that can identify three levels.

  Also in this case, the sensitivity of the i-axis component and the q-axis component can be improved by receiving them differentially. When ternary values are used, codes that are resistant to transmission distortion, such as duobinary codes, can be used.

  The multi-level discriminators 17-1 and 17-2 are at least four levels of ± √2A and ± √3A or ± √2A and ± A in the case of binary and ± √3A in the case of ternary, ± Although 6 levels of √2A and ± A can be identified, an AD converter or the like may be used.

  When binary is used, the amplitude of the two signal point arrangements is √2A and √3A, and the ratio is approximately 1.8 dB. That is, the characteristic degradation of 1.8 dB can be prevented as compared with the conventional method. When three values are used, the amplitudes of the three signal point arrangements are A, √2A, and √3A, and the maximum ratio is approximately 4.8 dB as in the first embodiment. That is, the characteristic degradation of 4.8 dB can be prevented as compared with the conventional method.

  Also in the second embodiment, by changing the intensity of amplitude modulation, the transmission device having the configuration of FIG. 4 or FIG. 5 can be used as in the first embodiment.

  In the embodiment described above, only the case where the QAM code and the QPSK code are used is shown. However, the modulation code is not limited to this, and a plurality of modulation schemes having substantially the same transmission characteristics can be used.

  According to the present invention, it is possible to transmit a sub-signal such as a monitoring signal without degrading the transmission characteristics of the main signal and without reducing the transmission efficiency, thereby increasing the transmission efficiency of the entire digital transmission system. Available to:

The figure which shows the example of signal point arrangement | positioning in the phase space of 4 value QAM code (i axis A, q axis √3A). The figure which shows the example of a signal point arrangement | positioning in the phase space of 4 value QAM code (i axis √3A, q axis A). The figure which shows the example of signal point arrangement | positioning in the phase space of 4 value QAM code (i axis and q axis √2A). The schematic block diagram of the transmission apparatus of a present Example (the 1). The schematic block diagram (the 2) of the transmitter of a present Example. The schematic block diagram of the receiver of a present Example. The schematic block diagram of the conventional conventional apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser 2 4 phase phase modulator 3, 6, 7 Amplitude modulator 4-1, 4-2 Encoder 5 Branch part 8 90 degree phase shifter 9 Combiner part 10 Automatic level adjuster 11 Carrier reproducing part 12 Phase adjuster 13-1, 13-2 Interferometers 14-1, 14-2 45 degree phase shifters 15-1, 15-2, 20 Light receiving units 16-1, 16-2, 21 Synchronizing units 17-1, 17-2 Multi-level identification unit 18 Euclidean distance metric comparison unit 19, 22 Level identification unit

Claims (10)

A transmitter that multi-value encodes digital data and modulates and transmits a carrier signal;
In a digital transmission system comprising a receiving device that identifies a received signal using a plurality of identification values corresponding to the multilevel code,
Comprising means for transmitting a main signal while switching a plurality of modulation forms having substantially the same transmission characteristics;
The digital transmission system is characterized in that the means for transmitting switches the modulation form according to the sub-signal.   For the modulation, quaternary quadrature amplitude modulation (QAM) is used, and a code having an amplitude ratio of 1: √3 between the i-axis component and the q-axis component is used. The digital transmission system according to claim 1, wherein the sub-signal is transmitted with the amplitude component of the i-axis component and the q-axis component by exchanging the amplitudes of.   For the modulation, QAM modulation using a code in which the amplitude ratio of the i-axis component and the q-axis component is 1: √3, and the amplitude of the i-axis component and the q-axis component are equal and normalized by the amplitude of the code 2. The digital signal according to claim 1, wherein QPSK modulation having an amplitude of {square root over (2)} is used and the sub-signal is transmitted with the amplitude component of the i-axis component and the q-axis component by switching between these two modulation modes by the sub-signal. Transmission system. In a sub-signal transmission method provided in a digital transmission system, which multi-value encodes digital data and modulates and transmits a carrier signal,
The transmission device transmits the main signal while switching a plurality of modulation forms having substantially the same transmission characteristics, and at the time of this switching, the modulation form is switched according to the sub-signal,
A sub-signal transmission method, wherein the receiving device demodulates the sub-signal by extracting the modulation pattern switching pattern.   For the modulation, four-value quadrature amplitude modulation (QAM) is used, and a code having an amplitude ratio of 1: √3 between the i-axis component and the q-axis component is used. The sub-signal transmission method according to claim 4, wherein the sub-signal is transmitted with the amplitude components of the i-axis component and the q-axis component by exchanging the amplitude.   For the modulation, QAM modulation using a code in which the amplitude ratio of the i-axis component and the q-axis component is 1: √3, and the amplitude of the i-axis component and the q-axis component are equal and normalized by the amplitude of the code 5. The sub-signal according to claim 4, wherein the sub-signal is transmitted with the amplitude component of the i-axis component and the q-axis component by using QPSK modulation having an amplitude of √2 and switching between these two modulation modes by the sub-signal. Signal transmission method. In a transmission apparatus that is provided in a digital transmission system and that multi-values digital data and modulates and transmits a carrier signal,
Means for four-phase phase-modulating the carrier signal to generate a signal with a code point arrangement in which the amplitudes of the i-axis component and the q-axis component are substantially equal;
Means for changing an amplitude ratio between an i-axis component and a q-axis component in the signal in accordance with a sub-signal. In a transmission apparatus that is provided in a digital transmission system and that multi-value encodes digital data and modulates and transmits a carrier signal,
Means for branching the carrier signal into two parts, one being the i-axis component and the other being the q-axis component, so as to multiplex after shifting the phase of the q-axis component by 90 degrees;
A transmission device comprising: means for changing an amplitude ratio between the amplitude of the branched i-axis component and the amplitude of the q-axis component according to a sub-signal. In a receiving apparatus that is provided in a digital transmission system and identifies a received signal using a plurality of identification values corresponding to a multilevel code,
The transmission signal of the transmission device according to claim 7 or 8 as a reception signal,
A receiving apparatus comprising: means for demodulating a sub-signal based on the output levels of the demodulated i-axis component signal and q-axis component signal.   10. The receiving apparatus according to claim 9, further comprising means for differentially discriminating between the output of the i-axis component and the output of the q-axis component.

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