GB2205468A

GB2205468A - Modems and echo cancellers

Info

Publication number: GB2205468A
Application number: GB08712824A
Authority: GB
Inventors: Raymond Taylor; Christopher Arthur Butler
Original assignee: ATLANTIC NETWORK SYSTEMS PLC
Current assignee: ATLANTIC NETWORK SYSTEMS PLC
Priority date: 1987-06-01
Filing date: 1987-06-01
Publication date: 1988-12-07
Also published as: GB8712824D0

Abstract

An echo canceller, for use in a modem, comprises means 32-37 for producing a signal T'X(n) which simulates the echo from a distant end of a line 26 or network of a signal TX(n) transmitted by the modem. A second order control loop 38, 45, 50 compares the simulated echo signal with the actual echo received by the modem and alters the phase PHI n/T of the simulated echo signal so as to reduce the phase difference between the simulated echo signal and the actual echo. The simulated echo signal is cancelled from the signal RX (n) received by the modem so as to provide a clean data signal E (n). This is intended to reduce distant echoes with phase roll. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO MODEMS AND ECHO CANCELLERS The present invention relates to modems for transmitting data to and receiving data from a transmission path, and to echo cancellers for such modems.

Modems operating in full duplex transmit and receive data on a single two-wire line connection, for instance a Private Wire or via a Public Switched Telephone Network. In the transmission mode, the modem modulates a carrier with the data, after possible encoding, for instance to produce a frequency shift keyed signal which is transmitted to the line connection via a hybrid. In the reception mode, the modem receives the modulated signal and separates it from the transmitted signal by means of the hybrid. The separated signal is then demodulated and decoded to reveal the received data.

Because the modem uses a common two-wire line connection for transmission and reception, the signal received by the modem is contaminated by the signal transmitted by the modem via various mechanisms.

Separation between the transmitted and received signals depends on the efficiency or balance of the hybrid and inevitably some of the transmitted signal will leak into the reception side downstream of the hybrid. Also, because of abrupt impedance changes and mismatches in the line connection, the transmitted signal will be reflected back to the modem at various points along the communication path so that the modem receives "echoes" of its transmitted signal.

In order to improve the utility of modems, it is necessary to remove these contaminating signals. It is known to do this by echo cancellation, and a known type of digital echo canceller is shown in Figure 1 of the accompanying drawings.

The transmitted signal TX(n) and received signal RX(n) are supplied from the respective ports of a hybrid (not shown) within the modem. The transmitted signal is supplied to the input of a delay line comprising a local delay line, a bulk delay, and a distant delay line connected in series. The local delay line comprises a number of series connected delay elements 1 providing incremental delays such that the total delay is at least as large as the delay of the last echo of significant amplitude produced at the local end of the line connection. The bulk delay 2 produces a delay not greater than the time between the arrival of the last echo of significant amplitude from the local end and the arrival of the first echo from the distant end of the line connection. This delay is generally between 0 and 800 milliseconds.The distant delay line comprises delay elements 3 similar to the elements 1 and provides a delay such that the total delay of the delay line is at least as large as the delay of the last echo of significant amplitude produced at the distant end.

Each tapping point is supplied to one input of a multiplier 4O ...., 4m whose other input receives a coefficient αo ...., αm, respectively, from means 5 for performing an algorithm to update the coefficients. The outputs of the multipliers are added together by means 6 and the result C(n) forms a correction signal which is subtracted from the received signal RX(n) at 7 to provide an echo cancelled output E(n) which forms the output of the echo canceller and which is also used as the input for the algorithm performed in the means 5.

Usually, all of these functions are implemented in software and performed in one or more microprocessors and associated hardware.

During an initial or training mode, the modem transmits a test signal and the algorithm at 5 adjusts the coefficients αo, ...., αm so as to minimise the signal E(n) (in the absence of transmissions from the distant end of the line connection). In this mode, the values of the coefficients are allowed to change relatively rapidly so as to train the echo canceller for operation. In the steady state mode after the training mode, the coefficient values are allowed to change much more slowly so that the operation of the echo canceller is not upset by normal operation of the modem.

This type of echo canceller operates effectively under many conditions, but is ineffective when distant echoes with phase roll are encountered. The reason for this is as follows.

In modern communications networks, communication channels are multiplexed together when transmission over a large distance is necessary, for instance via inter-continental cables or satellite links. A common form of multiplexing is frequency division multiplexing as illustrated in simplified diagrammatic form in Figure 2 of the accompanying drawings.

A local line connection LL is connected to a hybrid 10 having ports connected to the input of a modulator 11 and to the output of a demodulator 12. A local oscillator 13 supplies a carrier wave to the modulator 11 and demodulater 12 so that the transmitted signal is raised in frequency to a predetermined frequency band and a received signal in the predetermined frequency band is demodulated down to the baseband. The output of the modulator 11 and the input of the demodulator 12 are connected to a communication path 15 such as a cable or satellite link. A distant line connection LD is likewise connected to the path 15 by a corresponding arrangement comprising a hybrid 17, a modulator 18, a demodulator 19, and a local oscillator 20.

The local oscillators 13 and 20 produce carrier waves of the same frequency and, for instance, comprise crystal-controlled oscillators of high stability.

However, these crystal oscillators are not necessarily synchronised to each other and, as a result. the phase and frequency of the oscillator 20 drift with respect to the phase and frequency of the oscillator 13. Thus, when a signal is transmitted from the local end, the echo from the distant end, i.e. from the hybrid 17 and the line connection LD connected thereto, has a phase which varies continuously.

The rate of change of the phase of the distant echoes is such that, in known echo cancellers of the type shown in Figure 1, the coefficients of multiplication cannot change value sufficiently quickly to give effective echo cancellation. Moreover, the control loop of the echo canceller is a first order control loop so that there is a residual phase difference between the distant echoes and the attempt by the echo canceller to cancel these echoes.

According to a first aspect of the invention, there is provided an echo canceller for a modem. comprising means for producing a simulated echo signal from a transmitted signal. means for reducing the phase difference between the simulated echo signal and a received echo, and means for cancelling the simulated echo signal from the received echo.

Preferably, the means for reducing the phase difference includes means for calculating the difference between the simulated echo signal multiplied by the integral of the received echo and the received echo multiplied by the integral of the simulated echo signal to provide a measure of the phase difference between the simulated echo signal and the received echo.

Preferably. the output of the calculating means is connected to averaging means for forming an average value signal sign determining means for providing a signal indicating the sign of the calculated difference is provided between the calculating means and the averaging means.

Preferably, the means for reducing the phase difference includes a second order feedback control loop.

According to a second aspect of the invention, there is provided a modem including an echo canceller according to the first aspect of the invention.

Preferably, the modem includes a modulator for modulating a first carrier wave with a data signal to be transmitted and the means for producing the simulated echo signal includes means for delaying the data signal to be transmitted and means for modulating a second carrier wave with the delayed data signal to be transmitted. Preferably, the phase of the second carrier wave is controlled by the means for reducing the phase difference.

The invention will be further described, by way of example, with reference to Figure 3 of the accompanying drawings, which illustrates diagrammatically part of a modem including an echo canceller constituting a preferred embodiment of the invention.

The modem comprises an encoder 21 for receiving input data to be transmitted. The encoder, which is optional but which is normally present, performs any suitable encoding on the input data, such as "trellis coding". The output of the encoder 21 is supplied, via a low pass filter 22, to the modulation input of a modulator 23 which modulates a carrier wave with the band-limited data signal. The output of the modulator 23 is connected to a digital/analogue convertor 24 whose output is connected to an input port of a hybrid 25.

The hybrid is connected to a single two-wire line connection 26 operating in full duplex. Thus, the signal TX(n) to be transmitted is converted into analogue form by the convertor 24 and is supplied via the hybrid to the line 26.

Signals received from the line 26 are supplied to an output port of the hybrid 25 and thence to an analogue/ digital convertor 27 which provides the digital received signal RX(n).

The remainder of the modem shown in Figure 3 forms the echo canceller. The output of the modulator 23 is supplied to the input of convolution means 28 which performs the same function as the local delay line and associated multipliers and summing means shown in Figure 1. The output signal of the convolution means 28 is thus a representation of the near end echo of the signal transmitted from the modem, and this signal is supplied to summing means 29 and subtracting means 30. The output RX(n) of the convertor 27 is supplied to the subtracting means 30 and to subtracting means 31, whose other input receives the correction signal C(n) from the summing means 29. The output of the subtracting means 31 comprises the echo cancelled signal E(n) of the echo canceller and is also supplied to means 32 for performing an algorithm for updating the multiplier coefficients.

The echo canceller further comprises means 33 for providing a bulk delay to the encoded input data signal. The output of the means 33 is connected via a low pass filter 34 to the modulation input of a modulator 35 similar to the modulator 33 but with an input 36 for controlling the phase of the carrier wave.

The output of the modulator 35 is connected to the input of convolution means 37 corresponding to the distant delay line and associated multipliers and summing means of Figure 1. The coefficients of multiplication for both convolution means 28 and 37 are supplied by the update algorithm means 32.

The output of the convolution means 37 comprises a stimulated far end echo signal and is supplied to the summing means 29.

The output of the subtracting means 30 essentially comprises the received far end echo. Thus, the simulated echo signal and the received echo signal are supplied to the respective inputs of calculating means 38. which comprises integrators 39 and 40, multipling means 41 and 42, and subtracting means 43. The calculating means 38 therefore produces at its output a signal equal to the difference between the received echo multiplied by the integral of the simulated echo signal and the simulated echo signal multiplied by the integral of the received echo. The output signal of the calculating means 38 is supplied to means 44 for producing a signal corresponding to the sign of the output of the calculating means 38.This sign signal is supplied to the input of averaging means 45 which comprises multiplying means 46 for multiplying the sign signal by a coefficient k, summing means 47, and an integrator 48, and multiplying means 49 for multiplying by the coefficient (l-k). The average signal produced by the averaging means 45 is supplied to the input of a second order control loop 50 whose output is connected to the carrier phase control input 36 of the modulator 35. The control loop comprises multipliers 51 and 52 for multiplying by coefficients a and ss, respectively, summing means 53 and 54, and integrators 55 and 56.

With the exceptions of the convertors 24 and 27 and the hybrid 25, everything illustrated in Figure 3 operates in the digital domain. Although in theory these parts could operate in the analogue domain, this is normally impractical, although it is possible for a mixture of digital and analogue circuitry to be used.

In particular, although it is impractical for the convolution means 28 and 37 and the update algorithm means 32 to be analogue, any of the other parts of the modem shown in Figure 3 could be implemented using analogue techniques.

Although the digital domain parts could be implemented using discrete digital circuitry, it is preferred for these parts to be implemented in software by means of one or more microprocessors and associated har2are. Thus, the diagram of Figure 3 is to be understood as illustrating the functions performed by the software and hardware, although this diagram could equally well represent a discrete or semi-discrete digital implementation.

In practice, separate microprocessors are used for the convolution means 28 and 37 in order to achieve the necessary speed of operation.

In terms of operation, the echo canceller may be notionally divided between its function of cancelling near end or local echoes and of cancelling far end or distant echoes. Local echo cancellation is performed essentially in the same way as in the known echo canceller illustrated in Figure 1. Thus, the algorithm means 32 supplies, on the basis of the echo cancelled signal E(n), multiplier coefficients to the convolution means 28 which simulates the local echoes by its output signal which, via the summing means 29, is subtracted from the received signal RX(n). A similar subtraction is performed in the subtracting means 30, except that the distant echoes are not cancelled in the subtracting means 30.

Distant echo cancellation is performed as follows.

The encoded date signal is delayed in the delaying means 33 by a delay period not greater than the time taken for the first echo of significant amplitude to return to the modem from the far end of the line connection 26. The low pass filter 34 is identical to the low pass filter 22 and the modulator 35 produces a modulated signal TX' (n) corresponding to transmitted signal TX(n) except in phase. The algorith means 32 supplies multiplier coefficients to the convolution means 37 so that its output signal simulates the distant echoes. The local and distant echoes are summed in summing means 29 and are cancelled from the received signal RX(n) to give the echo cancelled signal E(n).

The calculating means 38 forms the instantaneous difference between the received distant echoes multiplied by a unit delayed version of the simulated distant echoes and the simulated distant echoes multiplied by a unit delayed version of the received distant echoes, which difference is a measure of the phase difference between the simulated and received distant echoes. If the simulated and distant echoes are thought of as the axes of a Cartesian co-ordinate system, then the calculating means 38 calculates the area of a loop formed by the tip of a vector whose co-ordinates are the simulated and distant echoes, and the sign of this area, which depends on whether the loop is traced clockwise or anticlockwise, represents the direction of phase shift and is extracted by the sign means 44.The averaging means 45 averages the sign signal and produces an output signal corresponding to the average area within the loop or contour. This is supplied to the second order control loop 50 which produces the phase control signal for the carrier wave of the modulator 35.

The distant echo cancellation system is thus a feedback look with second order control such that the phase difference between the simulated and received distant echoes is substantially zero, once this loop has locked to the phase difference. The echo canceller is thus able to perform accurate cancellation of both the near end or local echoes. which do not suffer from phase roll, and the far end or distant echoes. which do suffer from phase roll.

Various modifications may be made within the scope of the invention. For instance, the sign means 44 and the averaging means 45 could be omitted. Also.

instantaneous area could be computed from the output of the converter 27 and the signal E(n) if partial sums are not available in the particular echo canceller used.

Claims

1. An echo canceller for a modem, comprising means for producing a simulated echo signal from a transmitted signal, means for reducing the phase difference between the simulated echo signal and a received echo signal, and means for cancelling the simulated echo signal from the received signal.

2. An echo canceller as claimed in claim 1, in which the means for reducing the phase difference includes means for calculating the difference between the simulated echo signal multiplied by the integral of the received echo signal and the received echo signal multiplied by the integral of the simulated echo signal to provide a measure of the phase difference between the simulated echo signal and the received echo signal.

3. An echo canceller as claimed in claim 2, in which the output of the calculating means is connected to averaging means for forming an average value signal.

4. An echo canceller as claimed in claim 3, in which sign determing means for providing a signal indicating the sign of the calculated difference is provided between the calculating means and the averaging means.

5. An echo canceller as claimed in any one of the preceding claims, in which the means for reducing the phase difference includes a second order feedback control loop.

6. A modem including an echo canceller as claimed in any one of the preceding claims.

7. A modem as claimed in claim 6, including a modulator for modulating a first carrier wave with a data signal to be transmitted, the means for producing the simulated echo signal including means for delaying the data signal to be transmitted and means for modulating a second carrier wave with the delayed data signal.

8. A modem as claimed in claim 7, in which the phase of the second carrier wave is controlled by the means for reducing the phase difference.

9. A modem substantially as hereinbefore described with reference to and as illustrated in Figure 3 of the accompanying drawings.