JPH0681080B2 - Equalizer - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to an equalizer for removing waveform distortion.

[Technical background of the invention and its problems]

In data transmission and the like, removing waveform distortion is a basic problem. Specific solutions to this problem include, for example, a transversal type automatic equalizer and an echo canceller.

According to these techniques, the waveform distortion can be removed, but there is a drawback that it takes too long to remove. In other words, these techniques treat the waveform distortion as an error amount and control the system so that this error amount becomes zero (minimum value). Therefore, it naturally took time to follow the dynamic changes.

When this is called a transversal type automatic equalizer, it means that the convergence time of the tap coefficient value becomes long, and that the echo canceller has a long echo path estimation time. Many proposals have been made for these drawbacks, but there were drawbacks that only the improvement proposal based on the above-mentioned basic concept made the hardware configuration large and the control too complicated. .

[Object of the Invention]

It is an object of the present invention to provide an equalization method that can eliminate the above-mentioned drawbacks and remove waveform distortion in a short time.

[Outline of Invention]

According to the present invention, in an equalizer having an equalizer for removing waveform distortion generated in a transmission line, a first converter for converting a received signal including a reference signal known on the receiving side into a frequency domain, and the first converter. Estimating means for estimating the characteristics of the transmission line in the frequency domain by performing a predetermined arithmetic processing with the received signal converted into the frequency domain by the converting means and the signal in the frequency domain of the reference signal known on the receiving side. And second conversion means for converting the estimated transmission path characteristics into the time domain, and determining the state of the equalization means using the estimated transmission path characteristics in the time domain. It is a feature.

EFFECTS OF THE INVENTION According to the present invention, the received signal is converted into the frequency domain, the characteristics of the transmission path in the frequency domain are estimated, and the internal state of the equalization processing is determined by this estimation, so that equalization is performed in a short time. Once the scheme is trained, we can move on to the transmission of valid data.

Example of Invention

Next, an embodiment of the present invention will be described with reference to FIG. In this embodiment, a transversal type equalizer is used as the equalizing means.

The entire system is composed of a transmitting side and a receiving side connected to this transmitting side via a transmission line (2). The transmitting side has a transmitting means, which generates a symbol sequence of cycle l,
A signal based on the symbol sequence is transmitted. The transmitted signal is usually waveform-shaped.

On the receiving side, the signal sent via the transmission line (2) is received and captured by the reception filter (3). This signal is supplied to the first conversion means (4) and the equalization means (8).
The first conversion means (4) converts at least one cycle of the received signal into the frequency domain. For example, it is composed of a FET.

The output of the first converting means (4) is supplied to the normalizing means (6). A signal obtained by converting the reference signal into the frequency domain is supplied to the other input terminal of the normalizing means (6). This reference signal is a signal corresponding to the symbol sequence to be sent from the transmitting side and is not affected by transmission path distortion. Further, this signal is known to the receiving side in advance.

Based on these inputs, the normalizing means (6) converts the reference signal into the frequency domain into the first converting means (4).
The output signal of is divided. The reference signal is a signal that is not affected by the transmission line distortion, and dividing this by the received signal containing the information of the transmission line distortion, as described below,
This is equivalent to estimating the characteristics of the transmission line in the frequency domain.

The output of the normalizing means (6) is supplied to the second converting means (7) and converted into the time domain. This converted output value is supplied to the equalizing means (8). The equalization state of the equalization means (8) is defined by the output of the second conversion means (7).

That is, the received signal is converted into the frequency domain, the reference signal known in the frequency domain is divided by the received signal to estimate the characteristics of the transmission path in the frequency domain, and this is converted into the time domain to obtain the real time Estimated information of the characteristics of the transmission path of is obtained. By determining the equalization processing state based on this information, an equalized waveform from which the waveform distortion has been removed can be obtained as the output from the equalization means (8).

Next, the operation of the above-described configuration that obtains such an output will be described. First, a qualitative explanation will be given.

Spectrum D of the symbol sequence transmitted from the transmitting means (1)
(Ω) (ω is the angular frequency), R is the waveform shaping spectrum
_{If 1} (ω), the transmitted symbol sequence is D (ω) · R
It becomes ₁ (ω). If the spectrum of the transmission line (2) and the spectrum of the reception filter (6) are C (ω) and R ₂ (ω), respectively, the reception filter
The output spectrum of (6) is D (ω) ・ R ₁ (ω) ・ R
It is given by ₂ (ω) · C (ω).

In general, R ₁ (ω) · R ₂ (ω) is generally selected as the roll-off filter characteristic R (ω) that does not cause intersymbol interference, and this embodiment also sets such a value. Then,
The spectrum of the receiving filter (3) is D (ω) ・ R (ω) ・
It becomes C (ω).

By the way, as described above, the output of the first conversion means (4) is obtained by converting the output of the reception filter (3) into the frequency domain. Therefore, the output of the first conversion means (4) is the above D.
(Ω) · R (ω) · C (ω).

On the other hand, the reference signal is set as follows.

That is, the transmitting means (1) transmits a signal of a symbol sequence for at least one period before transmitting effective data. The signal for one cycle is assumed to be predetermined,
It is assumed that the receiving side also knows. This signal is not distorted by the transmission line (2) and the like, and is a signal of the same symbol sequence as that transmitted from the transmission side. The signal D (ω) · R (ω) obtained by further converting this into the frequency domain is used as the reference signal.

Then, the calculation in the first normalizing means (6) is D (ω) ·
R (ω) / D (ω) ・ R (ω) ・ C (ω) becomes 1 / C
(Ω) is obtained. As described above, this output is the impulse response waveform of the inverse filter of the transmission line (2) in the frequency domain. It is clear that the impulse response waveform estimated and converted into the time domain corresponds to the tap coefficient of the transversal type automatic equalizer constituting the equalizing means (8).

In this way, the transmission side generates a symbol sequence with a cycle of l and transmits a signal based on the symbol sequence, receives at least one period of the signal, and estimates the transmission path characteristics in the frequency domain. Then, by determining the state of the equalizing means (8) from the transmission line characteristics in the converted time domain, the initial state of the equalizing means (8) is immediately set to the optimum one for the system. It will be.

That is, the initial training by the equalizing means (8) can be executed in an extremely short time. Further, in the reference signal or the signal for one cycle described above, if the cycle l is lengthened or the signal of the cycle l is repeatedly sent from the transmitting side, the accuracy can be further improved.

Next, a quantitative explanation will be given. First, the impulse response of the transmission line (2) Is sufficiently represented by l variables, Then, the received signal x _n in the steady state at the receiving side is (However, ai is a transmission signal), and x _n can be expressed by convolution of the transmission data sequence and the transmission path impulse sequence. Here, a transmission signal sequence (a ₀ , a ₁ ,
... With _{a l-1),} the received signal sequence _{x n} likewise periodically l (x _0, x _1, can be represented by ... _{x l).}

Therefore, age, On the other hand, when the Fourier transform of each discrete system is performed, each Fourier transform H (f), A (f), X (f) becomes Becomes Also, x _i is When Since it can be found by convolution of Can be expressed as Therefore, equation (7) becomes Becomes Where the transmitted signal sequence Has a period of l, Becomes From this, equation (8) is Becomes

Furthermore, the tap coefficient sequence of the transversal type automatic equalizer that constitutes the equalizing means (8) is And the fractional Fourier transform is R (f), Becomes

According to the configuration shown in FIG. 1, the received data string Will be equalized by Let Y (f) be the discrete Fourier transform of and let l = m, Becomes

When the equalizer is operated with one baud sample, the purpose of the equalization is to set A (f) = Y (f), so equation (12) is Becomes

Converting this to the time domain, (However, f ₀ = 1 / lT, T is the sample interval time). Therefore, the equalized impulse train can be obtained from equation (14).

The correspondence between the output of the second conversion means (7) and the coefficient of the equalization means (8) depends on the mode of correspondence between the output of the first conversion means (4) and the respective periodic signals of the reference signal. Dependent. However, the second
If the maximum absolute value in the output sequence of the transform means (7) is cyclically shifted to the position of the predetermined center tab to form the equalized tab coefficient, the first transform means ( The correspondence between the output of 4) and the periodic signal of the reference signal may be arbitrary.

In the above embodiment, the first and second conversion means
(4) and (7) are not limited to the Fourier transform and may be any one that can transform the time domain into the frequency domain or the inverse of the lie, and can be realized by the chirp Z transform or the chirp inverse Z transform. Good.

Although the embodiment of the present invention has been described above, the equalizing means (8) is initially equalized at high speed by the configuration of this embodiment.
However, after that, a known method may be adaptively equalized by using, for example, the steepest descent method.

[Other Embodiments of the Invention]

Next, a telephone using an echo canceller as an equalizing means will be described with reference to FIG. In the figure, the same numbers are used for the same elements as those in the above-described embodiment.

The spectrum transmitted from the transmission means (1) is D (ω) · R
₁ (ω), which is supplied to the 4-wire / 2-wire conversion circuit (10). This circuit is a normal hybrid circuit. The transmission signal is transmitted to the terminal (11) through the 4-wire / 2-wire conversion circuit (10) and transmitted to the partner station. In contrast, the terminal (11)
From the partner station (not shown) is input to the reception filter (3) via the 4-wire / 2-wire conversion circuit (10) and sent to the terminal (9) via the subtractor (13). .

At this time, transmitting means via the 4-wire / 2-wire conversion circuit (10)
If the transmission signal from (1) leaks into the reception filter (3) as an echo, it will be transmitted to the terminal (9) via the subtractor (13). Transmission means (1) to reception filter
Letting C (ω) be the frequency spectrum of the transmission relation up to (3), the echo component of the output of the reception filter (3) is D
(Ω) · R ₁ (ω) · R ₂ (ω) · C (ω). However,
R ₂ (ω) is the spectrum of the reception filter (3).
Further, since R ₁ (ω) and R ₂ (ω) are usually selected as roll-off characteristics R (ω) without intersymbol interference, they are set in this way also in this embodiment. Then, the output of the first conversion means (4) becomes D (ω) · R (ω) · C (ω).

Now, next, as the reference signal used in the normalizing means (6),
In this embodiment, the signal without distortion of the same symbol sequence from the transmitting means (1) and converted into the frequency domain is used, but the reciprocal thereof is used unlike the above embodiment. That is, 1 / D (ω) · R (ω) is adopted as a reference signal, and this is supplied to the normalizing means (6) by combining the terminal (5).

The normalizing means (6) in this embodiment has a multiplication function and takes the product of two inputs. Therefore, D (ω) · R
The calculation (multiplication) of (ω) · C (ω) / D (ω) · R (ω) is performed to obtain C (ω). This C (ω) is the transmission means
It is the frequency spectrum of the transfer function from (1) to the receiving filter (3), and it can be seen that the characteristics of the transmission line from the transmitting means (1) to the receiving filter (3) in the frequency domain can be estimated by this standardization process. .

Next, this C (ω) is transformed into the time domain by the second transformation means (7) and supplied to the echo canceller (12). The output of the second conversion means (7) shows the impulse response waveform of the transfer function from the transmission means (1) to the 4-wire / 2-wire conversion circuit (10) and the conversion filter (3). )
It is used as the tap coefficient of.

In the echo canceller (12), a replica of the 4-wire / 2-wire conversion circuit (10) is generated, and a convolution operation with the symbol sequence from the transmission means (1) is performed, so that an echo replica dependent on the transmission symbol sequence is generated. Is generated. This echo replica is subtracted from the output of the reception filter (3) in the subtractor (13). Therefore, the echo component is removed from the signal supplied to the terminal (9).

In the above embodiments, the structure of the standardization means (6) and (6) is as follows.
Since an automatic equalizer of transversal type is used as the equalizing means (8), the reverse characteristic of the transmission line is used in the first embodiment, and a method of estimating the forward characteristic of the transmission line is used in the embodiments described later. It has been shown, and if the equalization means changes, it naturally changes accordingly.

For example, if a feedback type equalizer is used as the equalizer, the characteristics of the transmission line to be estimated are reversed in the above embodiments.

[Application Example of Invention]

Next, an example of a preferable system to which the present invention is applied will be described. With the recent development of computers, computer networks, that is, computers and terminals, and communication between computers have become popular. For example, a transmission line is set for a computer,
It is assumed that a plurality of terminals (or computers) are connected to this transmission line. It is necessary to carry out appropriate processing and calculation and send the obtained data to the (host) computer at the end. In this case, it is effective to apply the present invention when data is periodically or irregularly transmitted from each of a plurality of terminals.

That is, the amount of individual data transmitted from each terminal is not so large. Rather, it is often a small amount of data. In such a case, if it takes a long time to reach the equalization state between the terminal and the (host) computer, it is meaningless and the use efficiency of the transmission path is poor. That is, before sending valid data, it is not possible to know what the transmission is for sending hundreds of bits of control data from the terminal side to the (host) computer side to realize the equalization state. Further, since it is necessary to set the equalization state for each communication between the terminal and the (host) computer, the effect of the present invention is great.

The embodiments of the present invention have been described above.
The invention is not limited to these examples, and the scope of application is not limited to the above examples.

For example, the equalizing means may be anything as long as it removes distortion, and the standardizing means may be a multiplier or a divider, or a dedicated CPU may be used. Further, various changes can be made such as storing the calculation result in the memory and using the output of the first converting means as an address, and these are also included in the present invention.

[Brief description of drawings]

1 and 2 are diagrams showing an embodiment of the present invention. 1 ... Transmission means 2 ... Transmission path 3 ... Reception filter 4 ... First conversion means 6, 6 '... Normalization means 7 ... Second conversion means 8 ... Equalization means

Claims (5)

[Claims] 1. A waveform distortion generated in a transmission line is converted into an error amount,
In an equalizer including an equalizer that controls an internal state so as to minimize an error amount based on a time-series received signal and removes waveform distortion, a received signal including a reference signal known on the receiving side is received. Predetermined arithmetic processing by first transforming means for transforming into the frequency domain, the received signal transformed into the frequency domain by the first transforming means, and a signal in the frequency domain of the reference signal known by the receiving side. And estimating means for estimating the characteristics of the transmission line in the frequency domain, and second conversion means for converting the characteristics of the transmission line in the frequency domain estimated here into the time domain. An equalizer, characterized in that the internal state of the equalizing means is determined by using the characteristics of the transmission line in the time domain. 2. The estimating means for estimating the characteristic of the transmission line,
The equalizer according to claim 1, wherein the internal state of the equalizer is determined at the beginning of reception of one time-series signal. 3. The first exchanging means transforms the time-series received signals into a frequency domain by Fourier transform, and the second transforming means transforms the characteristic of the transmission line in the estimated frequency domain. The equalization device according to claim 1, wherein the equalization device is for converting into the time domain. 4. The equalizing means is configured by using a transversal type automatic equalizer, and the estimating means uses the received signal converted into the frequency domain by the first converting means on the receiving side. The present invention is intended to estimate a reverse characteristic of a transmission line in the frequency domain, including arithmetic processing for dividing a signal of a known received signal in the frequency domain, and the transversal type automatic equalization is performed based on the estimated reverse characteristic of the transmission line. The equalization device according to claim 1, wherein a tap coefficient of the converter is determined. 5. The equalization means is configured by using a convolution operation circuit and a subtractor, and the estimation means is known by the reception side and the reception signal converted in the frequency domain by the first conversion means. For estimating the forward characteristic of the transmission line in the frequency domain, the convolution operation of the equalizing means being performed based on the estimated forward characteristic of the transmission line. The equalization device according to claim 1, wherein a convolution coefficient in the circuit is determined.