JPH04239825A - Echo canceller - Google Patents

Abstract

PURPOSE:To revise a tap coefficient accurately with respect to the echo canceller for canceling an echo component. CONSTITUTION:The echo canceller is provided with a pseudo echo generating section 2 to cancel an echo component bypassed via a hybrid circuit 1 with a pseudo echo component, a line equalizer 3 equalizing a reception signal with a residual echo component superimposed thereon, a discrimination feedback equalizer 4 equalizing inter-code interference being a remained component of equalization and a filter 5 receiving an error signal for tap coefficient revision of the discrimination feedback equalizer 4 to the pseudo echo generating section 2 as a signal for tap coefficient revision and the characteristic of the filter 5 is selected to be opposite to the characteristic of the line equalizer 3.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an echo canceller that cancels echo components that go around to a receiving side via a hybrid circuit that performs two-wire and four-wire conversion. In order to connect the transmitter and receiver on the 4-wire side to the transmission path on the 2-wire side, a hybrid circuit that performs 2-wire to 4-wire conversion is connected. A part of the transmitted data from the transmitting section goes around to the receiving section via this hybrid circuit and is superimposed on the received signal as an echo component, causing an error in data identification. Therefore, an echo canceller is provided to cancel this echo component, and it is desired to cancel the echo component correctly. [0002] FIG. 2 is a block diagram of the main parts of a digital subscriber line transmission device, in which transmission data is transmitted through a coder section (COD
) 16, it is converted into a transmission code such as an AMI code or a 2B1Q code, and is added to the pseudo-echo forming unit (EC) 12 of the echo canceller 20 and the line driver 17 (DRV), and is sent from the line driver 17 to the hybrid circuit ( HYB) 11 to the two-wire subscriber line 19 and transmitted to the receiving side. The received signal via the two-wire subscriber line 19 is sent from the hybrid circuit 11 to an AD converter (A
The received signal, which is applied to DC) 18 and converted into a digital signal, is applied to the line equalizer 13 via the adder 15 of the echo canceller 20, and is then applied to the line equalizer 13 to eliminate the distortion caused by the loss characteristics of the two-wire subscriber line 19. However, the line equalizer 13 with the opposite characteristic
is equalized by Then, the intersymbol interference remaining after equalization is equalized by the decision feedback equalizer 14, and the decision output becomes received data. [0003] Also, in the pseudo echo forming section 12 of the echo canceller 20 for canceling the echo component that the transmitted data goes around to the receiving section via the hybrid circuit 11, the tap coefficient is updated based on the transmitted data and the error signal. A pseudo echo component is formed, and the adder 15 subtracts it from the digital signal output from the AD converter 18, thereby canceling out the echo component. FIG. 3 is a block diagram of main parts of a conventional echo canceller, in which 21 is a tap coefficient updating section, 22 is an adding section, 23 is a coefficient unit (K), and 24 is a delay circuit (T). Further, aej is a transmission signal at time j, Xej is an echo component that goes around via the hybrid circuit 11, Rej is a pseudo echo component, and εej is an error signal obtained by subtracting the pseudo echo component Rej from the echo component Xej by the adder 15. . The transmission signal aej is sequentially delayed by a delay time T corresponding to one sampling period by the delay circuit 24 and inputted to the tap coefficient update unit 21 and the coefficient multiplier 23, respectively.
The output signals of
The cancellation of ej is performed. Further, the tap coefficients K0j, K1j, . . . be exposed. FIG. 4 is a block diagram of the main parts of a decision feedback equalizer, in which 31 is a tap coefficient update section, 32 is an adder section, and 33 is a tap coefficient update section.
is a coefficient unit, 34 is a delay circuit (T), 35 is a judgment circuit, 3
6 and 37 are adders. Further, Xrj is a received signal at time j, Rrj is a pseudo inter-symbol interference signal, Frj is an equalized signal obtained by subtracting pseudo inter-symbol interference signal Rrj from the received signal Xrj in an adder 36, and arj is an equalized signal of the judgment circuit 35. The output judgment signal εrj is the equalization signal Fr from the judgment signal arj.
This is an error signal obtained by subtracting j in an adder 37. In the determination circuit 35, the equalization signal Frj
The level of arj is determined at the determination timing to generate binary received data, and the determination signal arj is sequentially delayed by a delay circuit 34 and input to the tap coefficient updating unit 31 and the coefficient multiplier 33, respectively. The output signals of 33 are added by an adder 32 to generate a pseudo intersymbol interference signal Rrj
Then, in the adder 36, the intersymbol interference component included in the received signal Xrj is canceled and becomes an equalized signal Frj,
The equalized signal Frj is applied to a determination circuit 35 and an adder 37. The adder 37 subtracts the equalization signal Frj from the determination signal arj and adds an error signal εrj to the tap coefficient updating unit 31. The adder 37 subtracts the equalization signal Frj from the determination signal arj and adds the error signal εrj to the tap coefficient updating unit 31. tap coefficient C1j,C
2j, . . . Cmj are controlled. The waveform of the received isolated pulse is, for example, as shown in FIG. 5, where C0 is the value of the judgment timing,
Also, since C1, C2, C3, C4, C5,... are intersymbol interference components, the tap coefficients C1j, C2j, C3j, C4j, C are set so that they have the same value as these components.
5j, . I will do it. An example of the tap coefficient updating algorithm in the pseudo echo forming unit 12 and the decision feedback equalizer 14 is as follows.
For example, it is as shown in the following equation. Kk,j+1 =Kk,j +αe ・ae,j−
k-1 ・εe,j-1 (k=0~n)...(
1) Ck,j+1 =Ck,j +αr・ar
,j-k-1 ・εr,j-1 (k=1~m
)...(2) [0009] In the configuration shown in FIG. This makes it difficult to update the tap coefficients of the echo forming section 12 accurately. Therefore, in the configuration shown in FIG. 6 in which the line equalizer 13 in FIG. 2 is omitted, the decision signal a in the decision feedback equalizer 14 is
It is conceivable to use the error signal εrj, which is the difference between rj and the equalized signal Frj, to update the tap coefficients of the pseudo echo forming section 12 of the echo canceller 20. With such a configuration, it is possible to accurately obtain the residual echo component and update the tap coefficients of the pseudo echo forming section 12. However, two-wire subscriber lines 19 of various lengths
is connected, and the received signal via the hybrid circuit 11 is subjected to distortion corresponding to the transmission loss of the two-wire subscriber line 19, so the judgment at the receiving section on the four-wire side is Equalization using only the feedback equalizer 14 becomes difficult. Therefore, in the configuration shown in FIG. 6, a configuration shown in FIG. 7 can be considered in which the line equalizer 13 is connected between the adder 15 and the decision feedback type equalizer 14. In this FIG. 7, the same reference numerals as in FIG. 2 or FIG. 6 indicate the same parts. [0011] In the configuration shown in FIG. 7, the line equalizer 13 is used to
The loss distortion caused by the transmission path on the line side is equalized, the intersymbol interference is equalized by the decision feedback equalizer 14, and the error signal εrj is used to update the tap coefficient of the pseudo echo forming unit 12, and the echo component is However, since the residual echo component is affected by the line equalizer 13, the decision feedback equalizer 14 cannot obtain an accurate residual echo component for the echo canceller. Become. Therefore, there is a drawback that the tap coefficients of the pseudo echo forming section 12 cannot be updated accurately. An object of the present invention is to accurately update tap coefficients for canceling echo components. [Means for Solving the Problems] In the echo canceller of the present invention, transmission data sent from a transmitter on the 4-wire side to a transmission line on the 2-wire side is sent to the receiver on the 4-wire side via a hybrid circuit 1. In an echo canceller equipped with a pseudo-echo forming section 2 that forms a pseudo-echo component for canceling the echo component that wraps around the echo component, the received signal containing the residual echo component is equalized by the line equalizer 3, and the The error signal for updating tap coefficients in the decision feedback equalizer 4 that equalizes intersymbol interference is passed through a filter 5 with opposite characteristics to the line equalizer 3 to update the tap coefficients of the pseudo echo forming unit 2. The configuration is such that it is added as a signal. [Operation] The received signal via the hybrid circuit 1 includes 4
An echo component that is a part of the transmission data from the line-side transmitter is superimposed, but the echo component is canceled by the pseudo echo component from the pseudo echo forming section 2. In this case, residual echo components that are not completely canceled out are equalized together with the received signal by a line equalizer 3, and input to a decision feedback equalizer 4, where intersymbol interference is equalized. The error signal in the decision feedback equalizer 4 is used to update the tap coefficients of the pseudo echo forming section 2. Since the residual echo component is inputted, a different error signal is generated than when the residual echo component is directly inputted to the decision feedback equalizer 4. However, since it is applied to the pseudo echo forming section 2 via a filter 5 with characteristics opposite to that of the line equalizer 3, it is used to update the tap coefficients of the decision feedback equalizer 4 when the line equalizer 3 is not provided. This is a signal equivalent to the error signal. Therefore, the tap coefficients of the pseudo echo forming section can be updated accurately. [Embodiment] FIG. 1 is a block diagram of the main parts of an embodiment of the present invention, in which 1 is a hybrid circuit that performs 2-wire and 4-wire conversion, 2 is a pseudo echo forming section, 3 is a line equalizer, and 4 is a decision feedback equalizer, 5 is a filter, 6 is a coder section (COD), 7 is a line driver (DRV), 8 is an AD converter (ADC), 9
is an adder. The transmitting section on the 4-line side includes a coder section 6 and a line driver 7, and the receiving section on the 4-line side includes an AD converter 8. The hybrid circuit 1 is connected to the transmission line of the hybrid circuit 1. The transmission data is converted into a transmission line code by the coder section 6, and is applied to the line driver 7 and the pseudo echo forming section 2 of the echo canceller.
from the hybrid circuit 1 to the transmission line on the two-wire side. At that time, the echo component that has passed through the hybrid circuit 1 is superimposed on the received signal that is applied from the transmission line on the 2-wire side to the receiver on the 4-wire side via the hybrid circuit 1, and converted into a digital signal by the AD converter 8. is converted to This received digital signal is added to the adder 9 of the echo canceller, where the echo component is canceled by the pseudo echo component from the pseudo echo forming section 2, and the output signal of the adder 9 is converted into a two-line signal by the line equalizer 3. Distortion due to the loss characteristics of the side transmission path is equalized, and the intersymbol interference remaining after equalization is equalized by the decision feedback equalizer 4. The pseudo echo forming section 2 includes, for example, as shown in FIG. 3, a tap coefficient updating section 21, an adding section 22, a plurality of coefficient units 33, and a plurality of delay circuits 34. 21, an error signal εrj for updating the tap coefficients of the decision feedback equalizer 4 is added via the filter 5 instead of the error signal εej. Further, the decision feedback equalizer 4 has, for example, the configuration shown in FIG. It will be done. This filter 5 has a characteristic opposite to that of the line equalizer 3, and for example, the characteristic E(Z) of the line equalizer 3 is
E(Z)=1-a・Z-1

...(3) Then, the characteristic F(Z) of the filter 5 is F(Z)=1/(1-a・Z-1)

...(4). With this filter 5, the error signal εrj of the decision feedback equalizer 4 becomes a signal for updating tap coefficients equivalent to that in the case where the line equalizer 3 is not provided. It becomes possible to accurately update the tap coefficients according to the residual echo component. Furthermore, when the characteristic E(Z) of the line equalizer 3 is changed, it is only necessary to change the characteristic of the filter 5 accordingly, and it is possible to form a pseudo echo component corresponding to the residual echo component. . Note that the line equalizer 3 is not limited to the characteristics expressed by the above-mentioned equation (3), but can be configured to have various characteristics corresponding to the characteristics of the transmission line on the two-wire side. As explained above, the present invention uses the line equalizer 3 to equalize the loss characteristics of the transmission line on the two-line side, and uses the decision feedback equalizer 4 to eliminate intersymbol interference. At the same time, the error signal εrj in the decision feedback equalizer 4 is
This is added to update the tap coefficients of the pseudo echo forming unit 2 via the filter 5 with the opposite characteristics to the line equalizer 3. Even if the residual echo component is affected by the equalization characteristics of the line equalizer 3, the filter 5, it can be added to the pseudo echo forming section 2 as an error signal equivalent to the case where the line equalizer 3 is not provided, so there is an advantage that the tap coefficients corresponding to the residual echo component can be updated.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the invention.

FIG. 2 is a block diagram of main parts of a digital subscriber line transmission device.

FIG. 3 is a block diagram of main parts of a conventional echo canceller.

FIG. 4 is a block diagram of main parts of a decision feedback equalizer.

FIG. 5 is an explanatory diagram of a received isolated pulse.

FIG. 6 is a block diagram of a main part of a conventional example using an error signal of a decision feedback equalizer.

FIG. 7 is a block diagram of a main part of a conventional example using an error signal of a decision feedback equalizer.

[Explanation of symbols]

1 Hybrid circuit 2 Pseudo echo forming part 3 Line equalizer 4 Decision feedback equalizer 5 Filter 6 Coder section (COD) 7 Line driver (DRV) 8 AD converter (ADC) 9 Adder

Claims (1)

[Claims] Claim 1: Transmission data sent from the transmitter on the 4-wire side to the transmission path on the 2-wire side generates a pseudo echo component for canceling the echo component that goes around to the receiver on the 4-wire side via the hybrid circuit (1). In an echo canceller equipped with a pseudo echo forming unit (2), a received signal containing a residual echo component is equalized by a line equalizer (3), and a decision feedback is performed to equalize the intersymbol interference remaining after equalization. The error signal for updating tap coefficients in the line equalizer (4) is sent to the pseudo echo forming unit (
2) An echo canceller characterized in that it is added as a signal for updating tap coefficients.