JPH0496522A - Echo canceller for full duplex modem - Google Patents

Abstract

PURPOSE:To realize an echo canceller whose echo cancellation characteristic including a convergence time is excellent by providing a delay means and a phase tracking means to a canceller for a remote echo signal so as to compensate a delay time of the remote echo signal and a fluctuation of the delay time. CONSTITUTION:A reception signal subject to elimination processing for a near echo signal by an adder 24 receives Hilbert transformation at a Hilbert transformation filter 25 and is converted into a reception signal (complex vector signal) comprising an in-phase component and an orthogonal component and inputted to an adder 26. A pseudo remote echo signal yf' generated by a remote echo canceller 40 is given to the adder 26 and the adder 26 makes subtractive synthesis of the pseudo remote echo signal yf' to the reception signal to eliminate the remote echo signal included in the reception signal. An in-phase component outputted from the adder 26 is given to a demodulator 27, in which it is demodulated and subject to code conversion by a code converter 28 and the code to be sent from an opposite station is reproduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a two-wire full-duplex modem echo canceller device.

[Prior Art] Conventionally, as this type of echo canceller device, the one shown in FIG. 2 by Nobunari is known (Japanese Patent Application Laid-Open No. 61242127).

In FIG. 2, this Co-A-Yancer device 3 is provided between a transmitter 1 and a receiver 2, and includes a fixed filter 4, an adaptive filter 5, an adder 6, and a gain adaptive filter. It is made up of circuit 7.

The transmission signal S whose code has been converted by the transmission section 1 becomes an input to a fixed filter 4 and an adaptive filter 5.

The fixed filter 4 has approximate near-end echo path characteristics, and the adaptive filter 5 estimates the difference between the fixed filter 4 and the actual echo path characteristics including the far-end echo path. The output from the fixed filter 4 and the output from the adaptive filter 5 are added by an adder 6. The sum signal of each filter output obtained by the addition is controlled so that the error signal I= is minimized through a gain adaptation circuit 7, and is sent as a pseudo echo signal to an adder 8 provided in the receiving section 2. is input.

Thus, the adder 8 adds the echo signal and the pseudo echo signal in the received signal to cancel the echo signal.

``Problems to be Solved by the Invention L7'' However, in this echo canceller device 3, 1.
Since the two adaptive filters estimate the difference between the actual near-end echo path characteristics and the fixed filter 4, and also estimate the far-end echo path characteristics, the number of taps in the adaptive filter 5 increases, requiring more filter calculations. This required a number of iterations, and convergence was slow.

Further, when the delay time of the far-end echo signal is large, filter calculations corresponding to the delay time are wasted.

Furthermore, considering the case where the far-end echo signal is large and the frequency offset, which is a time-varying element, is large, it is necessary to increase the number of filter taps, which increases the convergence time and prevents echo cancellation. There were also problems such as deterioration of characteristics.

The present invention has been made in consideration of the above points, and eliminates wasteful filter calculations of an adaptive filter corresponding to the delay time of a far-end echo signal, and also eliminates the need for a high-level far-end echo signal to have a large frequency offset. It is an object of the present invention to provide an echo canceller device for a full-duplex modem that reduces the deterioration of the echo cancellation characteristic when waiting for the transmission and has better echo cancellation characteristics than the conventional one.

[Means for Solving the Problem] In order to solve the problem, the present invention provides an echo canceller device for a full-duplex modem provided between a transmitter and a receiver of a two-wire full-duplex modem. , a near-end echo canceller for removing a near-end echo signal and a far-end echo canceler for removing a far-end echo signal are provided separately. The far-end echo canceller then transmits the pseudo far-end echo signal generated by the delay means for delaying the transmission signal by the delay time of the far-end echo signal and the built-in adaptive filter means from the transmission signal via the delay means. and a phase tracking means for following the phase of the end echo signal and applying it to the combining means for removing the far end echo.

[Operation] In this invention, the far-end echo canceller and the near-end echo canceller are separated, and the number of taps of each adaptive filter means is reduced.In addition, the far-end echo canceller has a far-end echo A delay means for delaying the transmitted signal by a time corresponding to the signal delay time is provided to compensate for the delay time of the far-end echo signal.Furthermore, a phase tracking means is provided to compensate for the delay time of the far-end echo signal. The frequency offset, which is a time-varying element, is compensated for.

Embodiment J An embodiment of the present invention will be described in detail below with reference to FIG. m1.

Here, FIG. 1 is a block diagram showing a modem having an echo canceller device of this embodiment, and FIG. 3 is a block diagram showing the configuration of the near-end echo canceller.
jl is a block diagram showing the configuration of a far-end echo A-Yancer.

In FIG. 1, the modem includes a transmitter 10, a receiver 20,
A near-end echo canceller 30 connected between the transmitting unit] 0 and the receiving unit 20, a far-end echo canceller 40 similarly connected between the transmitting unit 10 and the receiving unit 20, and
It is composed of 50 hybrids.

The transmitter 10 includes a code converter 1-1, a roll-off filter 12, a modulator 1.3, and a digital/analog conversion circuit 1-.
4 and transmission filter]-5. The output code S from the code converter]1 is subjected to waveform shaping processing taking into account the frequency characteristics of the line by the roll-off filter]-2, then modulated by the modulator 13, and converted into an analog signal by the digital/analog conversion circuit 14. After being converted into a signal, it is filtered through the transmission filter 15 and output to the line via the hybrid 50.

However, since there is signal loopback in the hybrid 50, this transmission mode j1 signal is input to the receiving section 20 as a near-end echo signal.

On the other hand, the transmitted modem signal from the four-wire terminal also passes through the hybrid 51- and then goes to the far-end hybrid 52 in the other station.
The signal loops around and is input to the receiving section 20 as a far-end echo signal together with the other party's transmission modem signal.

Therefore, the received signal contains not only the far-end signal of the other party's modem but also the near-end echo signal and the far-end echo signal.

The receiving section 20 includes a receiving filter 21, an analog/digital conversion circuit 22, a roll-off filter 23, an adder 24,
Hilbert transform filter 25, adder 26, demodulator 27
and a code converter 28.

In the receiving section 20, the received signal is filtered by a receiving filter 21 to remove noise components fS, and then converted to a digital signal by an analog/digital conversion circuit 22, and then converted to a digital signal by a roll-off filter 23. After being subjected to waveform shaping processing considering the frequency characteristics of the signal, the signal is input to the adder 24. The adder 24 is given the pseudo near-end echo signal yn' generated by the near-end echo canceller 30, and the adder 24 subtracts and synthesizes the pseudo near-end echo signal yn' with the received signal. The near-end echo signal contained in the received signal is removed. Therefore, the adder 24 constitutes a part of the near-end echo canceller 30.

In fact, the near-end echo signal is removed first because it has a much higher level than the far-end echo signal.

The received signal that has been subjected to near-end echo signal removal processing by the adder 24 is subjected to Hilbert transformation through the Hilbert transform filter 25, and is converted into a received signal (complex vector signal) consisting of an in-phase component and a quadrature component. This becomes the input to the adder 26. The adder 26 also receives the pseudo far-end echo signal yf' generated by the far-end echo canceller 40, and the adder 26 subtracts and synthesizes the pseudo far-end echo signal y1' with the received signal. In this case, the far-end echo signal included in the received signal is removed. Therefore, the adder 26 constitutes a part of the far-end echo canceller 40.

In this embodiment, the far-end echo canceller 4 (-
) is equipped with a configuration to compensate for the influence of frequency offset (), which is a time-varying element of the far-end echo signal, as will be described later.In order to easily perform the compensation, a Hilbel l-transform filter 25 is inserted. There is.

Therefore, it can be said that the Hilbert transform filter 25 also constitutes a part of the far-end echo canceller 4 [-].

The received signal from which the far-end echo signal has been removed by the adder 26, or more precisely, the in-phase component output from the adder 26, is supplied to the demodulator 27, where it is demodulated, and then code-converted by the converter 28. Then, the code that the other station tried to transmit is regenerated.

In this embodiment, as described above, the near-end echo canceller 30 and the far-end echo canceller 40 are provided at the male IJ.

This is due to the following reasons. Since the near-end echo signal is due to loopback that occurs in the hybrid 50 within the own station, it has a higher level, a smaller delay time, and a shorter delay time than the far-end echo signal that occurs due to loopback in the hybrid 52 of the other station. The difference is that it remains almost constant even if the partner station changes, and that delay time fluctuations (frequency offset) are not a problem. Therefore, rather than catching and removing these echo signals uniformly, it is more appropriate to remove them individually according to the characteristics of the echo signals.3 Therefore, as mentioned above, near-end echo A structure for removing the signal and a structure for removing the far-end echo signal are separately provided.

Next, the near-end echo canceller 30 will be described in detail.

As shown in FIG. 1, the near-end echo canceller 30 includes a pseudo near-end echo generation filter section 3, a modulator 32,
It is composed of a demodulator 33 and a coefficient control section 34.

FIG. 3 shows this near-end echo canceller 30 in more detail, and in particular, the pseudo near-end echo generation filter section 31 is shown in 11° r detail. The pseudo near-end echo generation filter unit 31 includes an in-phase orthogonal converter 60, a plurality of adaptive filters 61-a to 6Ld (four in the figure), and a timing adjustment switch 62. There is.

Note that a plurality of adaptive filters 61. A to 61 (1) and the timing adjustment switch 62 form a so-called sub-canceller configuration.

Since the transmitted signal wraps around the received signal and becomes a near-end echo signal, a pseudo near-end echo signal yn' is formed from the transmitted signal. That is, the transmission code S from the code converter 11 of the transmitter 10 is given to the near-end echo canceller 30.

In the near-end echo canceller 30, an in-phase orthogonal converter 60 having a table structure, for example, first converts the transmission code S into a multi-line vector signal consisting of an in-phase component and an orthogonal component.

This converted signal is transmitted to each adaptive filter 61a1.61I),
6tc and 61d, respectively.

Each adaptive filter 6 ]-a, 611), 6 ], c
, 61d are each configured with, for example, a transversal filter] 1-, and perform convolution processing (filtering processing) using coefficients (different for each filter) given from the coefficient control unit 34.

Here, each adaptive filter 61-a, 6 ] 1:), 6
1C261,d differs in its processing value and imitation by one sample of the transmitted signal. In addition,
In fact, the number of adaptive filters is required to be equal to the number of samplings included in one baud rate I of the transmission code, but in this embodiment, for the sake of simplicity of illustration and explanation, the number of adaptive filters is different from the actual number shows four adaptive filters.

Each adaptive filter 61-a, 6]-IT), 61c, 6
The output signal 1d is applied to the corresponding input terminal of the timing adjustment switch 62. The timing adjustment switch 62 is
The input terminals are switched cyclically with the sampling relay (.). The pseudo near-end echo signal yn selected via the timing adjustment switch 62
modulated by the final pseudo near-end echo signal yn′
-L is applied to the adder 24 as described above.

The error signal en remaining in the received signal from which the near-end echo signal has been removed by the adder 24 is input to the demodulator 33 and demodulated, and the demodulated signal is sent to the coefficient controller 3 in the form of a complex vector I signal. given to.

The coefficient control unit 34 generates coefficients according to the input error signal and controls each adaptive filter 61-a, 6lb, 61-
c, 61d. adaptive filter 6], a~61
d, timing adjustment switch 62, adder 24, demodulator 33, coefficient control section 34, and adaptive filter 61 a to 6
Through loop processing consisting of 1d, the coefficient control unit 34 generates coefficients so that the error signal en is minimized.

Next, the far-end echo canceller 40 will be described in detail.

Far-end echo canceller 40 includes modulators 41 . , a pseudo far-end echo generation filter section 42 , a phase follower 43 , a coefficient control section 44 , and a phase control section 7
It consists of 15. FIG. 4 shows this far-end echo A-yancer 40 in more detail, and particularly shows the pseudo far-end echo generation filter section 42 in detail,
ing. The pseudo far-end echo generation filter unit 42 includes a bulk delay 70, an in-phase orthogonal converter 71, a plurality of adaptive filters 72a to 72d (four in the figure), and a timing adjustment switch 73. There is.

Note that the plurality of adaptive filters 72a to 72d and the timing adjustment switch 73 form a so-called sub-canceller configuration.

Since the transmitted signal loops into the received signal at the hybrid 52 of the partner station and becomes a far-end echo signal, the far-end echo canceller 40 also forms a pseudo far-end echo signal yf' from the transmitted signal. That is, the transmission code S from the code converter 11 of the transmitter 10 is given to the far-end echo canceller 40.

In the far-end echo canceller 40 , the transmit signal S is taken in by the modulator 41 , modulated, and then applied to the bulk delay 70 .

The bulk delay 70 is configured with a shift I register, for example, and functions as a variable delay circuit without performing a filter operation.The delay time due to the bulk delay 70 is as follows.
It is determined during a series of connection procedures between modems. In a series of connection procedures between modems, commands, responses, etc. are exchanged, and through this procedure, for example, the approximate distance between the two modems can be detected, and the delay time can be set accordingly. The transmitted signal delayed by the bulk delay 70 is transmitted to, for example, an in-phase orthogonal converter 7 having a table structure.
1. given to.

The reason why the bulk delay 70 is provided is that it almost compensates for the delay time of the far-end echo signal, so that the configuration of the adaptive filters 72a to 72d can be simplified and the convergence can be accelerated. This is because it can be done. Incidentally, while the delay time of the near-end echo signal is almost constant, the delay time of the far-end echo signal varies depending on the partner station. Delay 70 is required.

The transmission signal whose delay time of the far-end echo signal has been substantially compensated for by the bulk delay 70 is provided to an in-phase orthogonal converter 71 . The in-phase orthogonal converter 71 has a table configuration, for example, and converts an input signal into a complex vector signal consisting of an in-phase component and an orthogonal component.

This complex vector signal is transmitted to each adaptive filter 72a, 721.
It is input to each of 72c and 72d.

Each of the adaptive filters 72a, 72b, 72c, and 72d is configured with a transversal filter, for example, and performs convolution processing (filtering processing) using coefficients given by the coefficient control unit 44 (different for each filter).
Do this.

Here, each adaptive filter 72a, ? 2b, 72C172
d is such that the processing timing differs by one sample print delay of the transmission signal. Note that, in practice, the number of adaptive filters must be equal to the number of samplings included in one baud rate of the transmission code, but in the case of this embodiment, the number of adaptive filters is different from the actual number to simplify illustration and explanation.
adaptive filters are shown.

1.6 The output signal of each adaptive filter 72a, 721), 72c, 72d is given to the corresponding input terminal of the timing adjustment switch 73. The timing adjustment switch 73 is used to cyclically switch the input terminals at the sampling rate. The signal selected via this timing adjustment switch 73 is given to the phase follower 43 as a pseudo far-end echo signal yf.

The phase follower 43 is for compensating for the influence of frequency offset, which is a time-varying element of the far-end echo signal. The phase follower 43 varies the phase of the signal from the timing adjustment switch 73 according to the control signal from the phase controller 45 (by adding a pseudo offset corresponding to the frequency offset) to produce a final pseudo far-end echo. The signal yr' is applied to the adder 26 as described above.

The far end echo signal is removed by this adder 26 and l-
The error signals e and f remaining in the received signal are processed by the coefficient controller 4.
4 and phase controller 45.

The coefficient control unit 44 generates coefficients according to the input error signal ef and controls each adaptive filter 72a, 72Ill, 7
2c, 72d. adaptive filters 72a to 72d,
Through a loop process including the timing adjustment switch 73, the adder 26, the coefficient control section 44, and the adaptive filters 72a to 72d, the coefficient control section 44 generates coefficients so that the error signal ef is minimized.

The phase control unit 45 operates according to the input error signal ef:
A phase control signal is generated and provided to the phase follower 43. The phase control section 45 generates a phase control signal through a loop process formed by the phase follower 43, the adder 26, the phase control section 45, and the phase follower 43 so that the error signal ef is minimized.

The received signal from which the far-end echo signal has been removed by the adder 26 in this way is, in fact, its in-phase component, sent to the demodulator 27 as described above! After being extracted and demodulated,
The code converter 284 converts the code and reproduces the code that the other station tried to transmit.

1- According to the embodiment described above, the following effects can be obtained.

(1) The near-end echo canceller 30 and the far-end echo canceller 40 are provided separately. Therefore, the filter length of the pseudo echo generation phase filter can be shorter than that of a conventional echo canceller, and the convergence time can be shortened. , coefficient control can be easily executed appropriately, and cancellation accuracy can be improved.

(2) Delay time of far end echo signal bulk delay 70
Since it is compensated by
The filter J in the far-end echo canceller 40 can be shortened, and from this point as well, effects such as shortening of the convergence time can be obtained.

(3) Since the phase follower 43 is provided to track the frequency offset, which is a time-varying element of the far-end echo signal,
The convergence time is shortened, and even if there is a large-level far-end echo signal with a frequency offset, it is possible to reduce the deterioration of the echo cancellation characteristics. In addition, bulk delay 70
The effect is significant in relation to

It should be noted that the above embodiment is merely an embodiment of the present invention, and various changes can be made without departing from the gist of the present invention. For example, the pseudo near-end echo generation filter section 31
A modulator 32 may be provided on the 0 input side. Of course, the configuration of the adder 24 and the like is also changed accordingly.

Furthermore, although not mentioned in the above, the initial coefficients for generating pseudo echoes, etc. may be determined in the training mode, and then controlled in accordance with the echo signal included in the received signal.

[Effects of the Invention] As described above, according to the present invention, a far-end echo signal canceler and a near-end echo signal canceler are separately provided, and the far-end echo signal canceler is provided with a delay means and a phase tracking means. Since the delay time and delay time fluctuation of the far-end echo signal are compensated for, it is possible to realize an echo canceller device with good echo cancellation characteristics including the convergence time.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a modem including an embodiment of a full-duplex modem 7-canceller device according to the present invention;
The figure is a block diagram showing a conventional echo canceller device, FIG. 3 is a block diagram showing the detailed configuration of the near-end echo canceler in FIG. 1, and FIG. 4 is a block diagram showing the detailed configuration of the far-end echo canceler in FIG. 1. It is a diagram. DESCRIPTION OF SYMBOLS 10... Transmitter, 20... Receiver, 24... Adder for near-end echo removal, 25... Hilbert transform filter, 26... Adder for far-end echo removal, 30... Near-end echo canceller, 40... Far-end echo canceller, 41... Modulator, 42... Pseudo far-end echo generation filter section, 43... Phase follower, 44... Coefficient control section, 45... Phase control unit, 70... Bulk delay, A, 7]・In-phase orthogonal converter, 72 a・~72 d・
...Adaptive filter, 73...Timing adjustment switch.

Claims (1)

[Claims] An echo canceller device for a full-duplex modem provided between a transmitter and a receiver of a two-wire full-duplex modem, comprising a near-end echo canceler for removing a near-end echo signal and a far-end echo. A far-end echo canceller for removing the signal is provided separately, and the far-end echo canceller includes a built-in delay means for delaying the transmitted signal by the delay time of the far-end echo signal, and a built-in delay means for delaying the transmitted signal by the delay time of the far-end echo signal. An echo canceler for a full-duplex modem, comprising phase tracking means for causing the pseudo far-end echo signal generated by the adaptive filter means to follow the phase of the far-end echo signal and applying the same to the far-end echo removal synthesis means. Device.