JP2009188788A - Transmission/reception system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a transmission/reception system which can obtain a signal at a stable level from a transmitter, and can prevent a deterioration of a receiving signal. <P>SOLUTION: This transmission/reception system includes: a transmitting side filter 12 for adding characteristics corresponding to a transmitting side tap coefficient to a transmitting signal which is input into a transmitter 1 and is modulated; a receiving side filter 24 for adding characteristics corresponding to a receiving side tap coefficient to the transmitting signal which is sent out from the transmitting side filter 12 and transmitted with an optical transmission line 3 received to a receiver 2; a transmission line characteristics computing part 4 for computing transmission line characteristics of the optical transmission line 3; and a tap coefficient computing part 5 for computing the transmitting side tap coefficient and the receiving side tap coefficient. The tap coefficient computing part 5 combines the transmission line characteristics with an additional characteristics set in response to the transmission line characteristics to compute a composite characteristics, computes the transmitting side tap coefficient based on reverse characteristics of the composite characteristics, and also computes the receiving side tap coefficient based on the additional characteristics. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a transmission / reception system that compensates for signal distortion caused by transmission path characteristics when a signal is transmitted through an optical transmission path.

A conventional compensation system includes a transmission line characteristic calculation circuit, an inverse characteristic calculation circuit, a transmitter including a filter circuit, and a receiver. When a signal is transmitted through an optical transmission line, the compensation system is caused by the transmission line characteristic. The signal distortion generated in this way is compensated (see, for example, Patent Document 1).
The transmission line characteristic calculation circuit calculates the transmission line characteristic of an optical transmission line composed of an optical fiber or the like. The reverse characteristic calculation circuit calculates a reverse characteristic of the transmission line characteristic calculated by the transmission line characteristic calculation circuit.

  Based on the inverse characteristic of the transmission path characteristic calculated by the inverse characteristic calculation circuit, the transmitter adds the inverse characteristic to the input transmission signal modulated by the filter circuit in the electrical domain. In addition, the transmitter converts the transmission signal to which the reverse characteristic is added from an electric signal to an optical signal by an optical modulator and outputs the converted signal to the optical transmission line.

Here, since the reverse characteristic of the transmission line characteristic is added to the transmission signal, the distortion of the signal caused by the transmission line characteristic in the optical transmission line is offset by the distortion of the transmission signal due to the reverse characteristic. Removed.
The receiver receives a transmission signal that has passed through the optical transmission line as a reception signal, converts the reception signal from an optical signal to an electrical signal, and receives transmitted data.

  As a filter circuit that adds a reverse characteristic of the transmission path characteristic to the transmission signal, a digital filter called an FIR (Finite Impulse Response) filter or a transversal filter is used. In place of these filters, a RAM that stores a lookup table value and outputs an integration result by memory access may be used.

JP 2006-522508 A

However, the prior art has the following problems.
That is, even when a reverse characteristic of the transmission path characteristic is added to the same transmission signal, the average level of the signal output from the filter circuit varies depending on the magnitude of the transmission path characteristic.
Here, when the transmission path characteristic is large, the distortion of the signal generated in the optical transmission path increases, and when the transmission path characteristic is low, the distortion of the signal generated in the optical transmission path decreases.

Therefore, the average level of the signal output from the filter circuit differs greatly between the case where the transmission path characteristics are very small and the case where the transmission path characteristics are large to some extent when the inverse characteristics of these transmission path characteristics are added to the transmission signal. .
For this reason, there is a problem that a signal having a stable level cannot be obtained from the filter circuit, and the level of the signal output from the transmitter is not stabilized. Further, since the level of the signal output from the transmitter is not stable, there is a problem that the received signal transmitted through the optical transmission path and received by the receiver may be deteriorated.

Note that the value of the imaginary part of the signal output from the filter circuit is particularly obtained by adding an inverse characteristic when the transmission line characteristic is very small to a transmission signal modulated by OOK (On-Off Keying). Is known to be very small.
In this case, in an optical modulator that performs orthogonal transformation based on the real part and the imaginary part of the signal output from the filter circuit, the transmission SNR (Signal to Noise Ratio) cannot be increased. .
Therefore, when the transmission path characteristic is very small, there is a problem that the received signal is further deteriorated.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a transmission / reception system capable of obtaining a stable level signal from a transmitter and preventing deterioration of a received signal. It is to provide.

  A transmission / reception system according to the present invention is a transmission / reception system in which a transmitter and a receiver are connected to each other via an optical transmission line, and the transmission / reception system is provided in the transmitter and receives a modulated transmission signal input to the transmitter. Transmission-side characteristic adding means for adding characteristics according to the transmission-side tap coefficient, and a receiver that is provided in the receiver and that is output from the transmission-side characteristic adding means and transmitted through the optical transmission line. Receiving side characteristic adding means for adding characteristics according to the tap coefficient, transmission path characteristic calculating means for calculating the transmission path characteristic of the optical transmission path, tap coefficient calculating means for calculating the transmitting side tap coefficient and the receiving side tap coefficient; The tap coefficient calculation means calculates the combined characteristic by combining the transmission line characteristic and the additional characteristic set according to the transmission line characteristic, and calculates the transmission side tap coefficient based on the inverse characteristic of the combined characteristic. Rutotomoni, and calculates the reception side tap coefficients based on the additional properties.

According to the transmission / reception system of the present invention, the tap coefficient calculation means calculates the combined characteristic by combining the transmission line characteristic and the additional characteristic set according to the transmission line characteristic, and transmits the transmission side based on the inverse characteristic of the combined characteristic. While calculating a tap coefficient, a receiving side tap coefficient is calculated based on an additional characteristic. The transmission-side characteristic adding unit adds a characteristic corresponding to the transmission-side tap coefficient to the transmission signal that is input to the transmitter and modulated. Further, the reception side characteristic adding means adds a characteristic corresponding to the reception side tap coefficient to the transmission signal received as a reception signal by the receiver.
Therefore, it is possible to obtain a stable level signal from the transmitter and to prevent deterioration of the received signal.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the same or corresponding parts will be described with the same reference numerals.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing a transmission / reception system according to Embodiment 1 of the present invention.
1, this transmission / reception system includes a transmitter 1, a receiver 2, an optical transmission line 3, a transmission line characteristic calculator 4 (transmission line characteristic calculation means), and a tap coefficient calculation unit 5 (tap coefficient calculation means). ).

The transmitter 1 includes a transmission data generation unit 11, a transmission side filter 12 (transmission side characteristic adding means), a D / A (Digital to Analog) converter 13, an E / O (Electronic to Optical) converter 14, and Is included. The receiver 2 includes an O / E converter 21, a waveform shaping filter 22, an A / D converter 23, a reception filter 24 (reception side characteristic adding means), an equalizer 25, and data determination. Part 26.
The optical transmission line 3 is composed of, for example, an optical fiber.

First, functions on the transmitter 1 side will be described.
The transmission data generation unit 11 performs signal mapping on a transmission information sequence that is a digital signal input to the transmitter 1. For the signal mapping, for example, a method such as OOK or QPSK (Quadrature Phase Shift Keying) can be considered. When it is assumed that the receiver 2 uses the delay detection method, the transmission data generation unit 11 also executes differential encoding.
The transmission data generation unit 11 outputs a signal obtained by signal mapping to the transmission filter 12 as a transmission signal.

  The transmission filter 12 has a complex transversal filter that is a digital filter. The complex transversal filter includes a plurality of delay units, a multiplier that multiplies the signal delayed by each delay unit by a tap coefficient, and an adder that adds an output from each multiplier. In addition, the complex transversal filter may perform signal processing in the time domain and execute signal processing in parallel when high speed processing is required.

The transmission-side filter 12 adds a characteristic corresponding to a transmission-side tap coefficient (described later) calculated by the tap coefficient calculation unit 5 to the transmission signal from the transmission data generation unit 11 using a complex transversal filter. To do. The transmission-side tap coefficient is calculated according to a combined characteristic (described later) in which the transmission line characteristic of the optical transmission line 3 and an additional characteristic set according to the transmission line characteristic are combined.
The transmission filter 12 outputs a signal obtained by adding a reverse characteristic of the composite characteristic to the transmission signal to the D / A converter 13 as a filtered transmission signal.

The D / A converter 13 converts the filtered transmission signal from the transmission filter 12 from a digital signal to an analog signal, and outputs the analog transmission signal to the E / O converter 14.
The E / O converter 14 converts the analog transmission signal from the D / A converter 13 from an electric signal to an optical signal and outputs the optical transmission signal to the optical transmission line 3 as an optical transmission signal. At this time, the analog transmission signal is orthogonally modulated based on the real part signal and the imaginary part signal of the complex signal. As the E / O converter 14, for example, a two-dimensional optical modulator disclosed in Patent Document 1 is used.
The optical transmission line 3 transmits the optical transmission signal from the E / O converter 14 and outputs it to the O / E converter 21 of the receiver 2.

Next, functions on the receiver 2 side will be described.
The O / E converter 21 receives an optical transmission signal transmitted through the optical transmission path 3 as an optical reception signal. The O / E converter 21 converts the optical reception signal from an optical signal to an electric signal, and outputs the signal to the waveform shaping filter 22 as an analog reception signal. At this time, the optical reception signal is subjected to quadrature detection and output as a complex signal having a real part signal and an imaginary part signal.

The waveform shaping filter 22 is configured by, for example, a bandpass filter or the like, and executes analog filter processing for limiting the reception band on the analog reception signal from the O / E converter 21. The waveform shaping filter 22 outputs a signal in which the reception band of the analog reception signal is limited to the A / D converter 23 as a shaped reception signal.
The A / D converter 23 converts the received signal after shaping from the waveform shaping filter 22 from an analog signal to a digital signal, and outputs it as a digital received signal to the receiving filter 24.

Similar to the transmission filter 12, the reception filter 24 has a complex transversal filter. The reception filter 24 adds the above-described additional characteristics to the digital reception signal from the A / D converter 23 based on the reception side tap coefficient (described later) calculated by the tap coefficient calculation unit 5.
The reception-side filter 24 outputs a signal obtained by adding an additional characteristic to the digital reception signal to the equalizer 25 as a filtered reception signal.

The equalizer 25 includes filters such as a complex transversal filter and a feedback filter, and a tap coefficient estimator that estimates tap coefficients of these filters. The equalizer 25 removes signal distortion caused by causes other than the transmission path characteristics and additional characteristics of the optical transmission path 3 described above from the filtered received signal from the reception-side filter 24. The equalizer 25 also removes signal distortion and the like caused by the fluctuation even when the transmission line characteristics of the optical transmission line 3 fluctuate with time. As a tap coefficient estimation method, for example, a LMS (Least Mean Square) algorithm or the like is used.
The equalizer 25 outputs a signal obtained by removing these signal distortions from the filtered received signal to the data determination unit 26 as a compensated received signal.

  The data determination unit 26 performs data determination on the compensated received signal from the equalizer 25 based on the mapping rule. The data determination unit 26 outputs the data determination result to the outside as an output signal.

Next, the transmission path characteristic calculator 4 and the tap coefficient calculator 5 will be described.
The transmission line characteristic calculator 4 calculates the transmission line characteristic of the optical transmission line 3 based on the optical transmission signal transmitted through the optical transmission line 3 and outputs the transmission line characteristic to the tap coefficient calculation unit 5. The transmission path characteristic calculator 4 calculates a transmission path impulse response together with the transmission path characteristics of the optical transmission path 3.
In addition to the optical transmission signal, the transmission path characteristic calculator 4 receives an analog reception signal from the O / E converter 21, a shaped reception signal from the waveform shaping filter 22, or a digital reception from the A / D converter 23. The transmission path characteristic of the optical transmission path 3 can also be calculated based on the signal.

The transmission path characteristics of the optical transmission path 3 are determined by four-wave mixing, nonlinear distortion of the optical fiber such as self-phase modulation or cross-phase modulation, wavelength dispersion, or polarization mode dispersion. In the following description, a case where the transmission path characteristics are determined by chromatic dispersion is shown as an example.
Further, the transmission path characteristic of the optical transmission path 3 is expressed as a complex number having a real part and an imaginary part.

  If the transmission line characteristic of the optical transmission line 3 calculated by the transmission line characteristic calculator 4 is the transmission line characteristic H, and the transmission line characteristic H expressed in the frequency domain is the transmission line frequency characteristic H (f), the transmission line frequency The characteristic H (f) is expressed by the following formula (1).

    H (f) = exp [jφ (DL)] (1)

  In equation (1), φ is a preset constant, D is a loss per unit length determined according to the material of the optical fiber, L is the fiber length, and the product of D and L is the wavelength. The amount of dispersion.

  The tap coefficient calculation unit 5 calculates a transmission side tap coefficient for the transmission side filter 12 and a reception side tap coefficient for the reception side filter 24 based on the transmission line characteristic H calculated by the transmission line characteristic calculator 4. The tap coefficient calculation unit 5 outputs the calculated transmission side tap coefficient and reception side tap coefficient to the transmission side filter 12 and the reception side filter 24, respectively.

First, the tap coefficient calculation unit 5 calculates the composite characteristic S by combining the transmission line characteristic H and the additional characteristic ΔH set according to the transmission line characteristic H.
Assuming that the additional characteristic ΔH is expressed in the frequency domain as the additional frequency characteristic ΔH (f), and the synthetic characteristic S expressed in the frequency domain is the synthetic frequency characteristic S (f), the additional frequency characteristic ΔH (f) and the synthesis The frequency characteristics S (f) are expressed by the following expressions (2) and (3), respectively.

ΔH (f) = exp [jφ (R)] (2)
S (f) = H (f) ΔH (f) (3)

  In the formula (2), R is set to a value of 5 ns / nm or more, for example. The additional frequency characteristic ΔH (f) is set so that the combined frequency characteristic S (f) is larger than a predetermined value even when the transmission line frequency characteristic H (f) is very small. The predetermined value is a value that does not cause the level of the signal output from the transmitter to become unstable or deteriorate the received signal when the inverse characteristic of the composite characteristic is added to the transmission signal. It is set according to the path 3.

Subsequently, the tap coefficient calculation unit 5 calculates an inverse characteristic C (f) of the combined frequency characteristic S (f).
As a method for calculating the inverse characteristic, for example, a method based on ZF (Zero Focusing), a method based on MMSE (Minimum Mean Square Error), or a transmission / reception signal obtained in advance is reversed. A method of calculating a matrix can be considered.
Here, for example, when a method based on ZF is used, the inverse characteristic C (f) of the combined frequency characteristic S (f) shown in Expression (3) is expressed by the following Expression (4).

Next, the tap coefficient calculation unit 5 calculates the transmission-side tap coefficient T _Tr based on the inverse characteristic C (f) of the combined frequency characteristic S (f).
Sender tap coefficient _{T Tr} is, IDTF (Inverse Discrete Fourier Transform: inverse discrete Fourier transform) is used to be expressed by the following equation (5) as a complex number in the time domain.

T _Tr (n) = IDFT [C (f)] (n: 0... N−1) (5)

In Expression (5), n represents the number of taps of the complex transversal filter of the transmission filter 12. The tap number n is set by the tap coefficient calculator 5 as a value (N in this case) longer than the transmission path impulse response.
In addition, when the transmission side tap coefficient _TTr is expressed in the frequency domain, the inverse characteristic C (f) is obtained.

Subsequently, the tap coefficient calculation unit 5 calculates the reception-side tap coefficient T _Re based on the additional frequency characteristic ΔH (f).
The reception side tap coefficient T _Re is calculated in the same manner as the transmission side tap coefficient T _Tr and is expressed by the following equation (6).

T _Re (m) = IDFT [ΔH (f)] (m: 0... M−1) (6)

In Expression (6), m represents the number of taps of the complex transversal filter of the reception-side filter 24. The tap number m is set by the tap coefficient calculation unit 5 as a value (here, M) larger than the tap number n of the complex transversal filter of the transmission filter 12.
Note that when representing the reception side tap coefficient T _Re in the frequency domain, the additional frequency characteristics [Delta] H (f).

The operation of the transmission / reception system configured as described above will be described below.
First, the digital signal input to the transmitter 1 is signal mapped and output as a transmission signal.
The transmission signal input to the transmission-side filter 12 is added with the inverse characteristic C (f) of the combined frequency characteristic S (f) according to the transmission-side tap coefficient T _Tr and is output as a filtered transmission signal that is a complex signal. The
Here, if the transmission signal is x (t) and the transmission signal x (t) and the filtered transmission signal are expressed in the frequency domain as X (f) and F (f), respectively, F (f) is Using the inverse characteristic C (f) of the synthesized frequency characteristic S (f), it is expressed by the following equation (7).

Subsequently, the filtered transmission signal is converted into an analog transmission signal by the D / A converter 13, converted into an optical transmission signal by the E / O converter 14, and transmitted through the optical transmission line 3.
Here, if the optical transmission signal transmitted by the optical transmission line 3 is expressed in the frequency domain as R (f), R (f) uses the transmission line frequency characteristic H (f) of the optical transmission line 3. Is represented by the following equation (8).

Next, the optical transmission signal transmitted through the optical transmission line 3 is converted into an analog reception signal by the O / E converter, the reception band is limited by the waveform shaping filter 22, and the digital signal is converted by the A / D converter 23. It is converted and output as a digital received signal.
The digital reception signal input to the reception-side filter 24 is added with an additional frequency characteristic ΔH (f) according to the reception-side tap coefficient T _Re and output as a filtered reception signal.

  Here, assuming that the filtered received signal expressed in the frequency domain is D (f), D (f) is expressed by the following equation (9) using the additional frequency characteristic ΔH (f).

    D (f) = R (f) ΔH (f) = X (f) (9)

  In Expression (9), since the transmission signal x (t) expressed in the frequency domain is X (f), the received signal after filtering is free from distortion caused by the transmission line characteristic H of the optical transmission line 3. It can be seen that the signal is changed.

Subsequently, the filtered received signal input to the equalizer 25 is free from distortion of the signal generated due to causes other than the transmission path characteristic H and the additional characteristic ΔH (f) of the optical transmission path 3, and the compensated received signal Is output as
Next, the compensated received signal input to the data determination unit 26 is subjected to data determination and output as an output signal.

According to the transmission / reception system according to Embodiment 1 of the present invention, the tap coefficient calculation unit 5 combines the transmission line frequency characteristic H (f) and the additional frequency characteristic ΔH (f) to obtain the combined frequency characteristic S (f). The transmission side tap coefficient _TTr is calculated based on the combined frequency characteristic S (f). Further, the tap coefficient calculation unit 5 calculates the reception-side tap coefficient T _Re based on the additional frequency characteristic ΔH (f). The transmission-side filter 12 adds an inverse characteristic C (f) of the combined frequency characteristic S (f) to the transmission signal according to the transmission-side tap coefficient _TTr . The reception-side filter 24 adds an additional frequency characteristic ΔH (f) to the digital reception signal in accordance with the reception-side tap coefficient T _Re .
Therefore, it is possible to obtain a stable level signal from the transmitter 1 and to prevent deterioration of the signal received by the receiver 2.

In the first embodiment, the case where the transmission path characteristics are determined by chromatic dispersion has been described. However, the present invention is not limited to this.
Even when transmission path characteristics are determined by four-wave mixing, nonlinear distortion of optical fiber such as self-phase modulation or cross-phase modulation, or polarization mode dispersion, or when dispersion and distortion are mixed In addition, signal distortion can be removed.

Embodiment 2. FIG.
FIG. 2 is a block configuration diagram showing a transmission / reception system according to Embodiment 2 of the present invention.
2, this transmission / reception system includes a transmission-side table value calculation unit 15 (table value calculation means) and a transmission-side RAM 16 instead of the transmission-side filter 12 shown in FIG. Further, in place of the reception side filter 24, a reception side table value calculation unit 26 (table value calculation means) and a reception side RAM 27 are provided.
Other configurations are the same as those of the first embodiment, and the description thereof is omitted.

The transmission side table value calculation unit 15 calculates a lookup table value for the transmission side RAM 16 based on the transmission side tap coefficient T _Tr calculated by the tap coefficient calculation unit 5 and stores the lookup table value in the transmission side RAM 16.
Specifically, the transmission-side table value calculation unit 15 uses a signal sequence of a transmission signal that is considered to be input to the transmission-side RAM 16 as an input value, and multiplies each input value by a transmission-side tap coefficient _TTr. The table value to be output is calculated.

The reception side table value calculation unit 26 calculates a lookup table value for the reception side RAM 27 based on the reception side tap coefficient T _Re calculated by the tap coefficient calculation unit 5 in the same manner as the transmission side table value calculation unit 15. And stored in the receiving RAM 27.
Note that a plurality of transmission side RAMs 16 and reception side RAMs 27 may be provided to cope with high-speed processing, and the processing may be executed in parallel.

According to the transmission / reception system according to the second embodiment of the present invention, instead of the transmission-side filter 12 and the reception-side filter 24 shown in the first embodiment, a transmission-side table value calculation unit 15 and a reception-side table value calculation unit 26 are used. And a transmission side RAM 16 and a reception side RAM 27.
This eliminates the need for multiplication processing required for the transmission side filter 12 and the reception side filter 24, thereby reducing the amount of calculation and reducing the circuit scale.

In the second embodiment, the transmission side table value calculation unit 15 and the reception side table value calculation unit 26 are provided in the transmitter 1A and the receiver 1B, respectively. However, the present invention is not limited to this, and the transmission-side table value calculation unit 15 and the reception-side table value calculation unit 26 may be provided outside the transmitter 1A and the receiver 1B.
Also in this case, the same effects as those of the second embodiment can be obtained.

It is a block block diagram which shows the transmission / reception system which concerns on Embodiment 1 of this invention. It is a block block diagram which shows the transmission / reception system which concerns on Embodiment 2 of this invention.

Explanation of symbols

  1, 1A transmitter, 2, 2B receiver, 3 optical transmission path, 4 transmission path characteristic calculator (transmission path characteristic calculation means), 5 tap coefficient calculation section (tap coefficient calculation means), 12 transmission side filter (transmission side) Characteristics adding means), 15 transmitting side table value calculating section (table value calculating means), 16 transmitting side RAM, 24 receiving side filter (receiving side characteristic adding means), 26 receiving side table value calculating section (table value calculating means), 27 Receiving side RAM.

Claims (3)

A transmitter / receiver system in which a transmitter and a receiver are connected to each other via an optical transmission line;
Transmission side characteristic adding means for adding a characteristic according to a transmission side tap coefficient to a transmission signal provided in the transmitter and modulated by being input to the transmitter;
A reception-side characteristic adding unit that is provided in the receiver and adds a characteristic according to a reception-side tap coefficient to the transmission signal output from the transmission-side characteristic addition unit and transmitted by the optical transmission line;
Transmission line characteristic calculating means for calculating the transmission line characteristic of the optical transmission line;
Tap coefficient calculation means for calculating the transmission side tap coefficient and the reception side tap coefficient, and
The tap coefficient calculation means calculates a combined characteristic by combining the transmission line characteristic and an additional characteristic set according to the transmission line characteristic, and calculates the transmission side tap coefficient based on an inverse characteristic of the combined characteristic And the reception side tap coefficient is calculated based on the additional characteristic.   The transmission / reception system according to claim 1, wherein the transmission side characteristic addition unit and the reception side characteristic addition unit are transversal filters. The transmitting side characteristic adding unit and the receiving side characteristic adding unit are:
Table value calculation means for calculating a lookup table value based on the transmission side tap coefficient or the reception side tap coefficient;
RAM storing the lookup table value;
The transmission / reception system according to claim 1, comprising:

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