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Title:
ECHO CANCELLATION DEVICE WITH MOBILE CROSSTALK CONTROL
Document Type and Number:
WIPO Patent Application WO/2004/054127
Kind Code:
A1
Abstract:
This device for cancelling an echo generated by a telecommunication unit located on a first end of a connection, comprises: a first adaptive filtering unit (36; 56) outputting an echo estimate, controlled by a first error signal (E¿MCC1?; E¿MCC2?); a delay estimation unit (44; 54) outputting an echo delay estimate, adpated to determine a shift between a signal (Y') transmitted towards the first end an orginating from a second end of the connection and the first error signal (E¿MCC2?); a delaying unit (46;57), for delaying by the echo delay estimate the signal (Y') transmitted towards the first end, the output of the delaying unit being connected to an input of the first adaptive filtering unit (36, 56); and an echo cancellation unit (38, 58), for subtracting the estimate of the echo from a signal (Xin) received from the telecommunication unit.

Inventors:
TOPALOVIC DRAGAN (IE)
Application Number:
PCT/IB2002/005697
Publication Date:
June 24, 2004
Filing Date:
December 09, 2002
Export Citation:
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Assignee:
ERICSSON TELEFON AB L M (SE)
TOPALOVIC DRAGAN (IE)
International Classes:
H04B3/23; H04B3/54; H04M9/08; (IPC1-7): H04B3/23; H04M9/08
Foreign References:
US5896452A1999-04-20
US4970715A1990-11-13
Other References:
MARQUES P A C ET AL: "A DSP based long distance echo canceller using short length centered adaptive filters", ACOUSTICS, SPEECH, AND SIGNAL PROCESSING, 1997. ICASSP-97., 1997 IEEE INTERNATIONAL CONFERENCE ON MUNICH, GERMANY 21-24 APRIL 1997, LOS ALAMITOS, CA, USA,IEEE COMPUT. SOC, US, 21 April 1997 (1997-04-21), pages 1885 - 1888, XP010226510, ISBN: 0-8186-7919-0
ERIKSSON A ET AL: "ERICSSON ECHO CANCELLERS A KEY TO IMPROVED SPEECH QUALITY", ON - ERICSSON REVIEW, ERICSSON. STOCKHOLM, SE, vol. 73, no. 1, 1996, pages 25 - 33, XP000584598, ISSN: 0014-0171
ERIKSSON A ET AL: "MOBILE CROSSTALK CONTROL-ENHANCING SPEECH QUALITY IN DIGITAL CELLULAR NETWORKS", ON - ERICSSON REVIEW, ERICSSON. STOCKHOLM, SE, no. 2, 1998, pages 83 - 89, XP000754990, ISSN: 0014-0171
Attorney, Agent or Firm:
Rinuy, Santarelli (Paris, Paris, FR)
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Claims:
CLAIMS
1. An echo cancellation device in a telecommunications network, for cancelling an echo generated by first telecommunication means located on a first end of a connection, characterised in that it comprises: first adaptive filtering means (36; 56), controlled by a first error signal (EMCC1 ; EMCC2) which is the difference between a signal (Xin) received from said first telecommunication means and a signal output by said first adaptive filtering means (36; 56), said first adaptive filtering means outputting an estimate of said echo; delay estimation means (44; 54), adapted to determine a shift between a signal (Y') transmitted towards said first end and originating from a second end of said connection and said first error signal (EMCC2), said delay estimation means outputting an estimate of the delay of said echo; delaying means (46; 57), for delaying by said echo delay estimate said signal (Y') transmitted towards said first end, the output of said delaying means being connected to an input of said first adaptive filtering means (36; 56); and echo cancellation means (38; 58), for subtracting said estimate of said echo from said signal (Xin) received from said first telecommunication means.
2. An echo cancellation device according to claim 1, characterised in that it comprises further adaptive filtering means (40), controlled by a further error signal (Et) that is used for estimating the delay only during a start phase of the echo cancellation process, said further error signal (Et) being the difference between said signal (Xin) received from said first telecommunication means and a signal output by said further adaptive filtering means (40), said further adaptive filtering means (40) receiving as an input said signal (Y') transmitted towards said first end, and in that during the start phase, said delay estimation means (44; 54) are adapted to determine a shift between said signal (Y') transmitted towards said first end and said further error signal (Et).
3. An echo cancellation device according to claim 2, characterised in that said delay estimation means (44; 54) are adapted to calculate the cross correlation of said first error signal (EMCC2) and said signal (Y') transmitted towards said first end and, during the start phase, to calculate the cross correlation of said further error signal (Et) and said signal (Y') transmitted towards said first end.
4. An echo cancellation system in a telecommunications network, wherein first telecommunication means are located on a first end of a connection, and second telecommunication means are located on a second end of said connection, said echo cancellation system being characterised in that it comprises an echo cancellation device according to any of the preceding claims.
5. A telecommunications network, characterised in that it comprises an echo cancellation system according to claim 4.
6. A telecommunications network according to the preceding claim, characterised in that the link to said first telecommunication means comprises a cellular or a satellite link.
Description:
ECHO CANCELLATION DEVICE WITH MOBILE CROSSTALK CONTROL The present invention relates to an echo cancellation device with mobile crosstalk control.

An echo canceller is a device that is commonly used in telephony systems to suppress or remove echoes in long distance traffic. For example, in cellular Public Land Mobile Networks (PLMNs), echo cancellers are used in Mobile services Switching Centres (MSCs) to suppress or remove echoes in speech traffic. Echo cancellers are also used in mobile radiotelephones and "handsfree"telephone equipment to compensate for acoustical or mechanical echoes.

In principle, a digital mobile radiotelephone handset should not generate echoes, because there is a duplex connection to the handset and no hybrid, and therefore, a priori no source of echo. In practice, however, many such mobile phones generate echoes that originate from acoustical or mechanical crosstalk in the handset. This type of echo is annoying to users, especially if the system operator has elected to raise the downlink signal levels.

Raising the downlink signal levels has become an accepted practice, since many customers have complained about the low output levels from the mobile handsets'speakers.

The echo path for a digital mobile phone is non-linear and time- varying, due to the use of two speech coder/decoder (codec) pairs and radio interfaces in the transmission path. Additionally, the level of the echo in a digital mobile phone is much lower than that from a PSTN (Public Switched Telephone Network). For example, the specification for the digital cellular Global System for Mobile Communications (GSM) requires an Echo Return Loss (ERL) of 46 dB (for the mobile phones) measured for pure tones of level 0 dBm in the 300- 3,400 Hz band. However, the ERL can be lower if signals other than pure tones are used for the measurements, but suppression levels of about 40 dB can still be expected. In other words, the quantisation noise appears to be a considerable source for the echo path non-linearity.

Many solutions have been proposed for echo cancellation.

For example, document EP-A-0 746 133 describes the use of a long- term average value of filter coefficients in order to estimate the delay of the echo, whereas document WO-A-96 27951 discloses the use of calculation of FFT (Fast Fourier Transformation) as a one-step delay value estimate.

However, these solutions are very complicated and make real-time implementation difficult, because they are processor time-consuming.

In document US-A-6 064 873, a better solution is proposed, but it still suffers from complexity. Besides, no detail is given about mobile crosstalk control. Furthermore, this solution does not give the possibility of fine-tuning of delay from the mobile network side.

Figure 1 shows a schematic view of a conventional network echo canceller with mobile crosstalk control. A plurality of such echo cancellers may form an Echo Canceller Pool (ECP) and may be integrated in a digital switching system.

The signal Xin originating from the far end side, which may be for example a cellular or a satellite link, is input in a Mobile Crosstalk Control (MCC) unit 10. The MCC unit 10 also receives as input the output of a Non- Linear Processor (NLP) 12. On the near end, which may for example correspond to a PSTN, a hybrid unit 14 converts the 4-wire line to a 2-wire line.

The hybrid unit 14 receives as input the output of the MCC unit 10 and outputs a signal Yin. A Double Talk Detector (DTD) unit 16 is connected in parallel with the hybrid unit 14. The output of the MCC unit 10 is also input in a Delay (D) unit 18, whose output is supplied to a Finite Impulse Response (FIR) filter 20.

The FIR filter 20 is adaptive, since it is controlled by an error signal E, which is the output of a subtraction unit 22. The subtraction unit 22 computes the difference between the signal Yin originating from the near end and the output of the FIR filter 20. The output of the subtraction unit 22 is also input in the NLP 12.

This echo canceller is simpler than the devices proposed by the above-mentioned prior art documents. However, it does not make it possible to obtain an adaptive value of the echo delay.

The present invention aims at overcoming the above-mentioned drawbacks.

To this end, the present invention provides an echo cancellation device in a telecommunications network, for cancelling an echo generated by first telecommunication means located on a first end of a connection, remarkable in that it comprises: first adaptive filtering means, controlled by a first error signal which is the difference between a signal received from the first telecommunication means and a signal output by the first adaptive filtering means, the first adaptive filtering means outputting an estimate of the echo; delay estimation means, adapted to determine a shift between a signal transmitted towards the first end and originating from a second end of the connection and the first error signal, the delay estimation means outputting an estimate of the delay of the echo; delaying means, for delaying by the echo delay estimate the signal transmitted towards the first end, the output of the delaying means being connected to an input of the first adaptive filtering means; and echo cancellation means, for subtracting the estimate of the echo from the signal received from the first telecommunication means.

Thus, thanks to the present invention, real-time implementation is simple. Moreover, the echo signal delay value is estimated in an adaptive fashion. The invention also makes it possible to carry out a fine-tuning of the echo signal delay value. Furthermore, the echo signal level value is estimated in an adaptive way.

According to a particular feature, the echo cancellation device comprises further adaptive filtering means, controlled by a further error signal that is used for estimating the delay only during a start phase of the echo cancellation process, the further error signal being the difference between the signal received from the first telecommunication means and a signal output by the further adaptive filtering means, the further adaptive filtering means receiving as an input the signal transmitted towards the first end, and during the

start phase, the delay estimation means are adapted to determine a shift between the signal transmitted towards the first end and the further error signal.

It makes it possible to obtain more quickly an estimate of the coefficients of the further adaptive filtering means during the start phase.

According to a particular feature, the delay estimation means are adapted to calculate the cross-correlation of the first error signal and the signal transmitted towards the first end and, during the start phase, to calculate the cross-correlation of the further error signal and the signal transmitted towards the first end.

The calculation of the cross-correlation is simple and makes real-time implementation easy.

With the same object as above, the present invention also provides an echo cancellation system in a telecommunications network, wherein first telecommunication means are located on a first end of a connection, and second telecommunication means are located on a second end of the connection, said echo cancellation system comprising an echo cancellation device as succinctly described above.

This echo cancellation system has the same advantages as the echo cancellation device.

With the same object as above, the present invention also provides a telecommunications network, comprising an echo cancellation system as succinctly described above.

This telecommunications network has the same advantages as the echo cancellation device.

According to a particular feature of the network, the link to the above- mentioned first telecommunication means comprises a cellular or a satellite link.

Since, in such links, delays are long and time-varying, in particular because of changing radio transmission conditions, the invention is especially valuable.

Other features and advantages of the present invention will appear upon reading the following detailed description of preferred embodiments, given by way of non-limiting examples.

The description refers to the accompanying drawings, in which: - Figure 1, already described, illustrates schematically a conventional echo canceller device with mobile crosstalk control ; - Figure 2 illustrates schematically the differences in the echo path of acoustical crosstalk and PSTN echo; - Figure 3 shows schematically an echo cancellation device according to the present invention, in a particular embodiment where all entities that operate during a start phase of the echo cancellation process are shown; - Figure 4 shows schematically how the delay of the echo is estimated, according to the present invention in a particular embodiment; and - Figure 5 shows schematically an echo cancellation device according to the present invention, in a particular embodiment, where only the entities that operate after the end of the start phase of the echo cancellation process, during a fine-tuning phase, are shown.

In the following, the expressions"far end"and"near end"are not intended to be limiting, other than to denote the opposite ends of a path for two- way communication.

The task of mobile crosstalk control is exacting. It should cancel acoustical crosstalk echo from the cellular telephone, but it should not degrade speech quality if echo is not present and it should not introduce clipping into or distort the incoming speech signal.

From the viewpoint of the switching system, as shown in Figure 2, there are many characteristics that distinguish the echo that originates from the acoustical crosstalk in a mobile digital handset from network echo.

Figure 2 shows a Mobile Station (MS) 24 connected to the Base Station System (BSS) 26 of a cellular network. A Mobile Switching Centre (MSC) 28 is connected to an echo canceller pool 30. The arrows input in and output from the MSC 28 and the echo canceller pool 30 represent the path of the signals coming from and returned to the BSS 26 (on the left of the MSC 28) and coming from and returned to a Local Exchange (LE) of the Public Switched Telephone Network (PSTN) 32. The LE contains a hybrid unit H for converting

the 4-wire line to a 2-wire line. A Stationary Telephone ST 34 is connected to the LE.

The arrow above the MS 24 and the BSS 26 represents the path of the acoustical crosstalk and the arrow below the PSTN 32 and the ST 34 represents the path of the PSTN echo. Only minor signal disturbances occur in the echo path of the network echo canceller. However, the echo path of the echo canceller that controls acoustical crosstalk includes radio transmission.

This gives rise to fundamental differences in the characteristics of the mobile echo path, in comparison with the network echo.

The delay in the handset echo is long, since radio transmission requires coding and interleaving. The actual echo delay may vary, depending, by way of non-limiting examples, on the handset, system hardware, extra signal processing equipment, and the routing of the call.

The characteristics of the mobile echo path are liable to vary rapidly and frequently due to changes in the position of the handset. The characteristics may also be affected by bit errors in the radio transmission, handover, or discontinuous transmission, for example. The MS echo delay can for instance vary from 120 to 320 ms. The duration of the MS echo is generally shorter than that of the PSTN echo, and its level is generally lower.

Figure 3 shows a particular embodiment of an echo cancellation device in accordance with the present invention, which, on the basis of the above-mentioned differences between the echoes originating from a mobile terminal and from a fixed network, handles the two kinds of echoes separately, and which is used during a so-called start phase of the echo cancellation process.

A connection in a telecommunications network is considered between a far end and a near end, comprising respectively first and second telecommunication means. The link to the first telecommunication means, that are located on the far end comprise a cellular or a satellite link, whereas the second telecommunication means, that are located on the near end are assumed to be fixed telecommunication means. The start phase takes place at

the beginning of the connection between the near end and the far end, and has a predetermined duration.

For cancelling the echo originating on the near end side, a conventional echo canceller 300 identical to the one described earlier with reference to Figure 1 is used, comprising a non linear processor 12, a double talk detector unit 16, a delay unit 18, a finite impulse response filter 20 and a subtraction unit 22.

For cancelling the echo originating on the far end side, according to the present invention, the echo canceller comprises an echo cancellation device 302 having a first adaptive filtering unit FMCC1 36, that outputs an estimate of the echo originating from the far end. The FMCC1 unit 36 is controlled by a first error signal EMCC1, which is the difference between a signal Xin received from the first telecommunication means and a signal output by the FMCC1 unit 36.

The calculation of this difference is carried out by a subtraction unit 38. The first error signal EMCC1 is used for adjusting the coefficients of the filter FMCC1 36.

The echo cancellation device 302 also comprises a further adaptive filtering unit Ft 40. The Ft unit 40 receives as an input a signal Y'transmitted towards the far end and originating from the near end. The Ft unit 40 is controlled by a further error signal Et, which is the difference between the signal Xin and a signal output by the Ft unit 40. This difference is calculated by a further subtraction unit 42. The further error signal Et is used for adjusting the coefficients of the filter Ft 40. It is to be noted that the filter Ft 40 can be designed so as to compute only a subset of coefficients, e. g. one coefficient out of ten, in order to supply a rough estimate of the echo, during the start phase of the echo cancellation process.

The echo cancellation device 302 also comprises a delay estimation unit C 44, outputting an estimate of the delay of the echo originating from the far end. The C unit 44 receives as a first input the signal Y', and receives as a second input the further error signal Et, and is adapted to determine a shift between the signal Y'and the further error signal Et.

The echo cancellation device 302 also comprises a delaying unit DELAY 46, for delaying the signal Y'by the echo delay estimate output by the C

unit 44. The output of the DELAY unit 46 is connected to an input of the FMCC1 unit 36.

The subtraction unit 38 acts as an echo cancellation unit by subtracting the echo estimate output by the FMCC1 unit 36 from the signal Xin.

The echo delay is estimated by the C unit 44 for example during the near end talk, which is detected by the DTD unit 16.

The echo delay is estimated as shown in Figure 4. The cross- correlation of the further error signal Et and the signal Y'is calculated in a plurality of intervals of time such as the intervals between two samples, and the cross-correlation values thus obtained are stored in a random access memory 460 of the DELAY unit 46. A comparator 462 then compares all stored values and determines the maximum cross-correlation value, which corresponds to the echo delay. Indeed, when the peak of the far end echo signal occurs, the cross-correlation between the further error signal Et and the signal Y'originating from the near end is maximal.

During the determination of this echo delay, all coefficients of the FMCC1 unit 36 are equal to 0, so that there is no influence of this filter. Once the delay estimation is performed, the FMCC1 unit 36 starts calculating the value of the filter coefficients.

As previously mentioned, the echo signal originating from the far end is shorter and has a lower level than the echo signal originating from the PSTN side. As a consequence, the number of segments of adaptive filters with transversal structure that are used for estimating the echo signal from the far end is significantly smaller than that used in the conventional echo canceller 300 on the PSTN side. In addition, the number of segments used in the Ft unit 40 can be much smaller than that used in the FMCC1 unit 36.

The normalised LMS (NLMS, Normalised Least Mean Square) algorithm can be applied to both adaptive filters. For example, both filters can therefore be implemented through the use of a single Digital Signal Processor (DSP).

Figure 5 shows a particular embodiment of an echo cancellation device 502 in accordance with the present invention, which corresponds to a

fine-tuning phase of the echo cancellation process and in which less entities operate than during the start phase shown in Figure 3.

For cancelling the echo originating on the near end side, a conventional echo canceller 300 identical to the one shown in Figure 3 and described earlier with reference to Figure 1 is used, comprising a non linear processor 12, a double talk detector unit 16, a delay unit 18, a finite impulse response filter 20 and a subtraction unit 22.

For cancelling the echo originating on the far end side, according to the present invention, the echo canceller comprises an echo cancellation device 502 having a single adaptive filtering unit FMCC2 56, that outputs an estimate of the echo originating from the far end. At the end of the start phase of the echo cancellation process, the coefficients of the FMCC2 unit 56 are initialised with the coefficients of the Ft unit 40, obtained at the end of the start phase.

The FMCC2 unit 56 is controlled by an error signal EMCC2, which is the difference between a signal Xin received from first telecommunication means, located on the far end, and a signal output by the FMCC2 unit 56. The calculation of this difference is carried out by a subtraction unit 58. The error signal EMCC2 is used for adjusting the coefficients of the filter FMCC2 56.

The echo cancellation device 502 also comprises a delay estimation unit C 54, outputting an estimate of the delay of the echo originating from the far end. The C unit 54 receives as a first input a signal Y'transmitted towards the far end and originating from the near end, and receives as a second input the error signal EMCC2, and is adapted to determine a shift between the signal Y' and the error signal EMCC2, for example by calculating the cross-correlation of the error signal EMCC2 and the signal Y'in a similar fashion as described above with reference to Figure 4.

The echo cancellation device 502 also comprises a delaying unit DELAY 57, for delaying the signal Y'by the echo delay estimate output by the C unit 54. The output of the DELAY unit 57 is connected to an input of the FMCC2 unit 56.

The subtraction unit 58 acts as an echo cancellation unit by subtracting the echo estimate output by the FMCC2 unit 56 from the signal Xin.

The echo delay is estimated by the C unit 54 for example during the near end talk, which is detected by the DTD unit 16.