JPH0936783A - Equalizer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a f<1/2> equalizer capable of high speed processing with small size by which a f<1/2> attenuation characteristic caused in a digital subscriber line transmission network is compensated. SOLUTION: A signal given to an AGC circuit 12 via an input terminal 11 is amplified up to a predetermined voltage level. The AGC circuit 12 informs an amplification factor to a coefficient selection section 14. The coefficient selection section 14 selects an optimum coefficient for waveform equalization processing from a coefficient storage section 13 accordingly and sets the selected coefficient to a coefficient memory 18 of a step filter 15. Thus, the signal received by the step filter 15 is subject to waveform-equalization processing and the resulting signal is outputted from an output terminal 16.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an equalizer, and more particularly to an equalizer for compensating a transmission signal which is attenuated depending on a signal frequency.

[0002]

2. Description of the Related Art ISDN (Integrated Se)
A metallic line is used for a transmission line in the subscriber line transmission. When transmitting a digital signal using a metallic wire, the quality of the digital signal deteriorates due to various factors. One of them is the √f attenuation characteristic. This is because when a digital signal containing a high frequency component is transmitted, the digital signal is attenuated according to the frequency. Therefore, at each reception point of the digital signal, an equalizer is required to estimate the amount of attenuation due to the √f attenuation characteristic and equalize the waveform at the transmission point of the digital signal.

FIG. 2 is a block diagram showing the overall structure of a conventional equalizer. The equalizer includes an input terminal 21, an AGC circuit 22, an adaptive FIR filter 23, and a ternary value determining unit 2
4, the coefficient estimation unit 25, the difference unit 26, and the output terminal 27.
With. The input terminal 21 inputs a digital signal transmitted via a metallic wire (not shown), that is, a signal to be waveform-equalized. An AGC circuit 22 for amplifying a digital signal to a preset voltage level is installed in the subsequent stage of the input terminal 21. In the subsequent stage of the AGC circuit 22, an adaptive FIR filter 23 (details will be described later) for performing waveform equalization processing of a digital signal.
Is installed. The digital signal output from the adaptive FIR filter 23 is branched into two, one of which is input to the difference unit 26. The other is input to the ternary value determining unit 24. In the three-value determination unit 24, the voltage level is divided into three in a predetermined range in advance,
A representative value (“−1”, “0”,
"+1" etc.) is assigned. The three-value determination unit 24 samples the input digital signal and assigns a specific representative value corresponding to the voltage level at the sampling point. The digital signal output from the ternary value determining unit 24 is branched into two, and one of them is output from the output terminal 27. The other is the difference unit 2
Enter in 6. The difference unit 26 uses the adaptive FIR filter 2
The difference between the digital signal input from 3 and the digital signal input from the ternary value determining unit 24 is calculated. Difference part 2
A coefficient estimator 25 for setting the coefficient of the adaptive FIR filter 23 is installed in the subsequent stage of 6. The coefficient estimator 25 sets the coefficient in a coefficient memory (not shown, details will be described later) with reference to the difference calculation result.

Here, the FIR filter 23 will be described. The FIR filter 23 includes a coefficient memory and a delay circuit (not shown) inside. Transfer function H of FIR filter
₁ (z) is represented by the following formula (1). _{H 1 (z) = c 0} + c 1 · z -1 + c 2 · z -2 + ... + c n · z -n ... (1) _{where, c 0, c 1 ...,} c n is set to the coefficient memory Is a coefficient that is The FIR filter 23 performs waveform equalization processing on this transfer function based on H ₁ (z).

Next, the operation of the conventional equalizer shown in FIG. 2 will be described. First, the equalizer performs an initial setting called a training mode. The training mode is an operation of setting coefficients in the coefficient memory so that the equalizer can accurately perform waveform equalization processing on an unknown transmission line (metallic line). Hereinafter, an example of a conventional training mode will be described.

First, at the time of initial setting of the equalizer (for example, at the time of installing a new equalizer), through the metallic line,
An initial training signal is sent to the equalizer. This initial training signal is a signal having a predetermined amplitude, and is transmitted via the input terminal 21 to the AGC circuit 22.
Given to. The AGC circuit 22 amplifies the initial training signal to a preset voltage level. The amplified initial training signal is given to the adaptive FIR filter 23. The adaptive FIR filter 23 performs waveform equalization processing of the initial training signal based on the coefficient set by the coefficient estimation unit 25. The waveform-equalized initial training signal is branched into two, and one is given to the difference unit 26. The other is given to the ternary value judging unit 24. The ternary determination unit 24 samples the initial training signal and assigns a specific representative value corresponding to the voltage level at the sampling point. The ternary determination unit 24 branches the ternary-determined initial training signal into two, and outputs one of them to the output terminal 2
7 and the other to the difference unit 26. Difference unit 26
Calculates the difference between the initial training signal input from the adaptive FIR filter 23 and the initial training signal input from the ternary determination unit 24. The calculation result is given to the coefficient estimation unit 25. When the difference is not zero, the coefficient estimation unit 25 determines that the waveform equalization is incomplete, and resets the coefficient of the adaptive FIR filter 23. The equalizer repeatedly executes the above operation until the difference becomes zero.
When the difference is zero, the coefficient estimation unit 25 determines that the waveform equalization processing is complete, and the adaptive FIR filter 2
The coefficient memory of 3 is fixedly set without changing.

[0007]

The fact that this FIR filter has no pole is apparent from the above equation (1). Therefore, the output of the FIR filter does not diverge and is always stable. Conventionally, this FIR filter has been used for an equalizer largely because of this stability problem.

However, in the FIR filter,
In order to perform accurate waveform equalization processing, a high-order filter must be used. In the case of implementing such an FIR filter by hardware, the FIR filter must be provided with multipliers for the order of the above equation (1). Therefore, there is a problem that the size of the equalizer is necessarily increased.

On the other hand, in the case of realizing with software,
Since the number of calculations in the FIR filter is large, there is a problem that the calculation time is too long. In particular, this problem is a big problem in the ping-pong transmission system in which pulses are transmitted and received in a time division manner using a pair of cables.

Therefore, it is an object of the present invention to provide an equalizer that can be reduced in size when the equalizer is configured by hardware. Also,
An object of the present invention is to provide an equalizer capable of performing waveform equalization processing at high speed when the equalizer is configured by software.

[0011]

The invention according to claim 1 is
An equalizer for compensating a transmission signal attenuated depending on a frequency of the transmission signal, the equalizer having a pole in a transfer function.
A waveform equalization processing unit that includes an IR filter and executes the waveform equalization processing of the transmission signal, and a coefficient group that can perform the waveform equalization processing of the transmission signal and whose stability of the transfer function has been confirmed in advance. Based on the transmission distance estimated by the transmission distance estimation means for estimating the transmission distance over which the transmission signal has been transmitted, the coefficient group storage means for storing,
Coefficient selecting means for selecting a coefficient group from the coefficient group storing means,
Coefficient setting means for setting the coefficient group selected by the coefficient selecting means in the waveform equalization processing means.

[0012]

According to the first aspect of the invention, the coefficient group storage means stores the coefficient group capable of executing the waveform equalization processing of the transmission signal and the stability of the transfer function of which has been confirmed in advance. The transmission distance estimation means estimates the transmission distance of the transmission signal and notifies the coefficient selection means of this. The coefficient selecting means selects, from the coefficient group storing means, an optimum coefficient for waveform equalizing the transmission signal based on the transmission distance. The coefficient setting means sets the selected coefficient in the waveform equalization processing means. The waveform equalization means executes waveform equalization processing of the transmission signal based on the set coefficient.

[0013]

1 is a block diagram showing the structure of an equalizer according to an embodiment of the present invention. The equalizer has an input terminal 11
An AGC circuit 12, a coefficient storage unit 13, a coefficient selection unit 14, a step filter 15, and an output terminal 16.

The input terminal 11 inputs a digital signal transmitted via a metallic line (not shown), that is, a signal to be waveform-equalized. An AGC circuit 12 for amplifying a digital signal to a preset voltage level is installed in the subsequent stage of the input terminal 11. The AGC circuit 22 outputs the amplified digital signal to the step filter 15 installed in the subsequent stage. The AGC circuit 22 is
At the same time, the amplification factor ((predetermined level value) / (peak value of input signal)) of the digital signal is calculated and notified to the coefficient selection unit 14. The coefficient storage unit 13 is configured by a storage medium such as a ROM, and stores a coefficient group set in a coefficient memory 18 (described later) of the step filter 15. The coefficient selection unit 14 refers to the amplification factor notified from the AGC circuit 12, selects a coefficient to be set in the coefficient memory 18 (described later) of the step filter 15 from the coefficient storage unit 13, and selects these coefficients from the corresponding coefficient memory. 18 is stored. The step filter 15 is provided with a step filter 15 (details will be described later) for performing waveform equalization processing of a digital signal. The waveform equalized digital signal is output from the output terminal 16.

Here, regarding the step filter 15,
This will be described in detail. The step filter 15 is a second-order II
R (Infinite Impulse Response)
ce) filter. The IIR filter includes a coefficient memory 18 and a delay circuit 17 inside. Where 2
The next IIR filter is II having two delay circuits.
It is an R filter, and its transfer function H ₂ (z) is expressed by the following equation (2). _{H 2 (z) = (a} 0 + a 1 z -1 + a 2 z -2) / (1-b 1 z -1 + b 2 z -2) ... (2) _{where, a 0, a 1, a} 2 , B ₁ and b ₂ are coefficient memories 1
The coefficient is set to 8. As is clear from the above equation (3), since the IIR filter may have poles, II
The output of the R filter may diverge. Therefore, the coefficients a ₀ , a ₁ , and a for confirming the stability of the transfer function H ₂ (z) are confirmed.
Only ₂ , b ₁ and b ₂ are stored in the coefficient storage unit 13.
Therefore, the coefficient storage unit 13 stores a plurality of coefficient groups each including five coefficients. The coefficient selection unit 14 selects the optimum coefficient group for waveform equalization processing from these coefficient groups by a predetermined method (details will be described later) and sets it in the corresponding coefficient memory 18.

The equalizer as described above is a DSU (Digit).
Although it is installed in the al Service Unit, an initial setting called a training mode is performed in order to actually enable equalization processing of the attenuated signal. This is the same as that described in "Prior Art". This training mode is shown below in Figure 1.
This will be described in detail with reference to FIG.

A switching center (not shown) sends a training signal to this DSU. This training signal is attenuated due to the attenuation due to the transmission distance and the √f attenuation characteristic of the metallic line.

The training signal is input to the AGC circuit 12 via the input terminal 11. The AGC circuit 12 first performs peak hold processing for detecting the voltage peak value of the attenuated training signal, and amplifies the detected voltage peak value so that the detected voltage peak value becomes a predetermined voltage level. Furthermore, the AGC circuit 12 calculates the amplification factor ((predetermined voltage level value) / (voltage peak value of the training signal)) and notifies the coefficient selection circuit 14 of it. The coefficient selection unit 14 can obtain the transmission distance of the digital signal from the amplification factor. The coefficient selection unit 14 refers to this transmission distance and selects the optimum coefficient group for waveform equalization processing from the coefficient storage unit 13. The coefficient selection unit 14 stores the selected coefficient group in the corresponding coefficient memory 18. This allows the equalizer to perform accurate waveform equalization processing when receiving a digital signal. After that, digital communication can be freely performed.

Here, the equalizer according to this embodiment and the conventional equalizer will be compared. As a comparative example, the inventor of the present application performed a simulation in the case where a digital signal was transmitted on a digital subscriber line transmission line 8 km using a paper cable having a diameter of 0.65 mm. According to the result, when it is attempted to obtain the equalization performance which is almost equal to that of the equalizer using the second-order IIR filter of the present embodiment with the conventional equalizer, the FIR filter has the following formula (1): n = 16 or more is required. Therefore, the equalizer according to the present embodiment can easily improve the equalization performance. Moreover, when the equalizer according to the present embodiment is configured by hardware, the circuit scale can be reduced. On the other hand, when it is configured by software, the calculation time can be shortened.

Further, as described above, the IIR filter is inferior in stability as compared with the FIR filter. However, since the coefficient group stored in the coefficient storage unit 13 stores the one whose stability is previously determined, this problem is solved.

[0021]

According to the invention of claim 1, the waveform equalization processing means executes the waveform equalization processing of the transmission signal based on the transfer function having the poles. The coefficient group storage means is capable of executing the waveform equalization processing of the transmission signal, and stores the coefficient group for which the stability of the transfer function of the waveform equalization processing means is confirmed.
Therefore, the signal output by the waveform equalization processing means does not diverge. Therefore, the waveform equalization processing means can perform accurate waveform equalization processing by reducing the number of coefficient storage means and delay circuits. That is, since the number of calculations in the waveform equalization means is reduced, it is possible to perform high-speed waveform equalization processing. Moreover, since the number of constituent parts of the equalizer can be reduced, the equalizer can be downsized.

[Brief description of drawings]

FIG. 1 is a block diagram showing an overall configuration of an equalizer according to an exemplary embodiment of the present invention.

FIG. 2 is a block diagram showing an overall configuration of a conventional equalizer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Input terminal 12 ... AGC circuit 13 ... Coefficient storage part 14 ... Coefficient selection part 15 ... Step filter 16 ... Output terminal 17 ... Delay circuit 18 ... Coefficient memory

Claims (1)

[Claims] 1. An equalizer for compensating a transmission signal attenuated depending on the frequency of the transmission signal, including an IIR filter having a pole in a transfer function, and performing waveform equalization processing of the transmission signal. Waveform equalization processing means to be executed, coefficient group storage means capable of performing waveform equalization processing of the transmission signal, and storing coefficient groups whose stability of the transfer function has been confirmed in advance, and the transmission signal Transmission distance estimation means for estimating the transmission distance that has been transmitted, coefficient selection means for selecting a coefficient group from the coefficient group storage means based on the transmission distance estimated by the transmission distance estimation means, and the coefficient selection And a coefficient setting means for setting the coefficient group selected by the means in the waveform equalization processing means.