AU3785100A

AU3785100A - Process and circuit arrangement for echo compensation in a telecommuncations system with non-linear transmission links

Info

Publication number: AU3785100A
Application number: AU37851/00A
Authority: AU
Inventors: Christian Georg Gerlach
Original assignee: Alcatel CIT SA; Alcatel SA
Current assignee: Alcatel Lucent SAS
Priority date: 1999-06-18
Filing date: 2000-06-01
Publication date: 2000-12-21
Also published as: EP1069752A3; CN1278699A; EP1069752A2; DE19928045A1; JP2001036438A

Description

P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: 'Process and circuit arrangement for echo compensation in a telecommunications system with non-linear transmission links' The following statement is a full description of this invention, including the best method of performing it known to us: FI IP;Yi)CANATPl12( l1Il71f1i2 2 CE00370671.7 Process and circuit arrangement for echo compensation in a telecommunications system with non-linear transmission links Field of the invention The present invention relates to a process for suppressing an echo in a telecommunications system with non-linear transmission sections and a circuit for implementing such a method.

Background of the invention In modern telecommunications systems speech quality is an important quality criterion.

For a subscriber, speech intelligibility, tone, naturalness, completeness of the speech transmission and the two-way communication facility, for example, are features for subjectively-experienced speech quality. Objectively speaking, the transmission quality is essentially determined by the control of disturbing echoes which occur as a result of undesirable coupling between a receiving path and a transmitting path and which, in the case of propagation times above 50 ms, are perceived as very disturbing. Echoes occur, for example, as line echoes at 2-wire/4-wire junctions of hybrid circuits as a result of electrical coupling between the transmitting path and the receiving path and/or the acoustic coupling between loudspeaker and microphone in a radio telephone, especially if a less than optimum handsfree device is present.

It is known that the effect of echoes can be largely eliminated by means of echo compensators, see R. Wehrmann et al: Signalverarbeitungsverfahren zur Verbesserung der Sprachkommunikation iber Freisprecheinrichtungen. Der Fernmelde-Ingenieur (Signal Processing Method for Improving Speech Communication via Handsfree Devices. The Telecommunications Engineer), Verlag flir Wissenschaft und Leben Georg Heidecker GmbH, Erlangen, 48th Year, October 1994, 10/94 pages 27-28. The transfer function of a real telecommunications system can be simulated with maximum possible accuracy by means of an echo compensator. The signal arriving at the subscriber then passes through both the real system and the system simulated by the echo compensator and the output signal of the echo compensator is then subtracted from the echo-affected signal of the real system, so that consequently the echo is compensated to a large degree.

Such types of echo compensators are realised with Finite Impulse Response Filters, or FIR filters for short, whose effect depends on the precise specification of the filter coefficients, CE00370671.7 see DE-A-4430189. A very reliable distinction therefore has to be made between echoes and local noise, between echoes from reflectors some considerable distance away and echoes from reflectors located in the vicinity, as well as between received signal waveforms that can be measured with adequate precision, such as sufficiently strong broadband signals, for example, and received signal waveforms that are unsuitable for measurements, such as sinusoidal continuous tones, for example.

Furthermore, circuits are known which attenuate the echoes to such a degree that they are no longer noticeable to a subscriber. To achieve this a compander is employed, with which transmitted signals above a reference value are compressed to a standard level, in so far as they have the reference level, are amplified and, in so far as they are below the reference value, attenuated. At the same time, the transfer characteristic of the compander is externally controlled by a coupling factor between transmitting path and receiving path and a control signal from the received speech signal of the remote speaker, which is determined by the noise level. See DE-A-196 11548.

e 15 To improve the echo suppression and to improve the specification of the filter coefficients, the use of a so-called background filter in addition to a main filter is known. For this, the signal differences due to each filter, between echo-affected signal and filter output signal are compared to each other and a control signal is derived from the result of comparison to improve the setting of the filter coefficients. See EP-A-0 872 962.

20 The solutions as described have certainly proved successful in the case of linear transfer functions if echo signal and input signal of the digital filter are correlated. However, they are .0o°°i S unsuitable for telecommunications systems with non-linear transmission links which always occur when encryption devices and/or highly-compressed speech coders/decoders are present in the transmission path, as is the case of mobile radio systems conforming to the GSM (Global System for Mobile Communication) standard or the AMPS (Advanced Mobile Phone System) standard. In order to achieve a high speech quality comparable to that of fixed networks, measures for echo compensation or echo suppression also have to be implemented in these cases. To achieve this, known solutions can certainly be employed directly in the terminals of the mobile subscribers, but there are considerable numbers of terminals in use, in which no or ineffectual echo compensation measures or echo attenuation measures are provided and the retrofitting of such measures would mean unjustifiably high outlay, so that measures can be implemented only in the network itself.

CE00370671.7 Summary of the invention Resulting from this, an object is to specify a process and a circuit arrangement for echo compensation in a non-linear telecommunications system, in particular in a mobile radio system conforming to the GSM standard.

According to a first aspect of the present invention there is provided a process for suppressing an echo caused by acoustic or electrical coupling between the transmission paths in a telecommunications system with non-linear transmission sections consisting of a speech coder and a speech decoder, in particular in a mobile radio system, that contains a mobile station a base station a transcoder unit and a functional unit for controlling the base station as well as a mobile switching centre wherein a device for suppressing the echo is arranged in the transcoder unit and the non-linear portion of the echo path is simulated with the aid of Sfunctional units present in the telecommunications system.

Preferably the non-linear portion of the echo path is simulated in the transcoder unit by using a speech decoder S 15 It is also preferable that the device for suppressing the echo is controlled in relation to a bad frame signal.

oooo According to a second aspect of the present invention there is provided a circuit arrangement for implementing the process according to Claim 1, wherein in the transcoder unit the input of a downlink speech decoder is connected to the line carrying the downlink datastream ,and that the output of the downlink speech decoder is connected to the device for suppressing the echo.

Preferably an FIR filter or a compressor/expander is used as the device for suppressing the echo.

The essence of the invention is that echo compensation circuits known from the fixed network are combined with non-linear circuits from a telecommunications network having speech coding devices for simulating the echo path and existing signals are used for control.

Brief description of the drawings The invention is now explained by means of an exemplary embodiment. In the associated drawing: CE00370671.7 Fig. 1 shows a block diagram of a first variant of the circuit arrangement according to the invention, and Fig. 2 shows a block diagram of a second variant of the circuit arrangement according to the invention.

Detailed description of the embodiments The main modules of a mobile radio system conforming to the GSM standard are illustrated in Fig. 1. These include a radio telephone 1 or mobile station MS, a base station 2 or base transceiver station BTS, a transcoder unit 36 or transcoder rate adaptation Unit TRAU, which here, by way of an example, is arranged in a functional unit 3 or base station controller BSC for controlling the base stations, and a mobile switching centre 4 or MSC. In the following the modules are denoted by their English abbreviations. The MS 1 contains a loudspeaker 13, a microphone 14 and a coder 11 for speech in the transmitting direction, or uplink, as well as a decoder 12 for speech in the receiving direction, or downlink. The BTS 2, includes an uplink channel decoder 21 and a downlink channel coder 22. In the TRAU 36 there is an uplink 15 decoder 31, a downlink coder 32 and now additionally an echo compensator 34 and a downlink decoder 33. As an alternative to the arrangement of the TRAU 36 in the BSC 3 illustrated in Figs. 1, 2, the TRAU 36 can also be arranged between BSC 3 and MSC 4 or in the MSC 4. The loudspeaker signal x(n) of the MS 1 is generated as output signal of the downlink decoder 33 by means of the downlink decoder 33, from the downlink datastream 51 present in the 20 TRAU 36, the loudspeaker signal x(n) being delayed by the time T with respect to the output signal The correlatable part of the echo stemming from the acoustic coupling 15 between .00.0: 0 loudspeaker 13 and microphone 14 can now be compensated by the echo compensator 34 using this output signal where the echo compensator 34 can be realised by an FIR filter.

Advantageously, the bfi (bad frame indicator) signal which is present in the TRAU 36 anyway can be employed to improve the adaptation of the FIR filter. The calculation of the filter coefficients is thus interrupted, for example, if a poor-quality uplink channel is indicated by the bfi signal. Based on the uplink datastream 52 it is possible to simulate the effect of the nonlinearities in the uplink channel and to refine echo compensation by means of an uplink coder, not shown here, of substantially identical construction to that of the uplink coder 11 in the MS 1. It should of course be taken into account that the complexity of a coder is roughly ten times greater than the complexity of a decoder. For a cost-effective solution it is therefore sensible to CE00370671.7 model the effect of the non-linearities in the downlink channel and make the setting of the echo compensator 34 dependent on the bfi signal of the uplink channel.

The advantages as described will be realised in a similar manner by using a compander, as described in the publication DE-A 196 115 48 cited at the outset, or by using the combination of a compander with an FIR filter. The circuit arrangement using such an echo attenuation element 35 is illustrated in Fig. 2. In this case it is also possible to model the non-linearities of the uplink channel with known circuits and existing signals.

The echo compensation can be further improved if further quality data are available, for example for the transmission quality in the downlink channel. Such data are present in the TRAU 36 when GSM-AMR (adaptive multi rate) coders and decoders are used in the future.

Basically, it is also possible to arrange an echo compensator in the MSC 4. In this case, .ooooi S° in order to simulate the non-linearities to obtain a correlatable signal for the echo compensator, a cascade circuit comprising a downlink coder and a downlink decoder is required. This is therefore a solution which is considerably more expensive than the solution according to the 15 invention.

All in all, a cost-effective solution for echo compensation at the network level in mobile *radio networks is stated by the invention.

It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of S the invention.

The foregoing describes embodiments of the present invention and modifications, obvious to those skilled in the art can be made thereto, without departing from the scope of the present invention.

The applicant does not concede that the prior art discussed in this specification forms part of the common general knowledge in the art at the priority date of this application.

Claims

1. Process for suppressing an echo caused by acoustic or electrical coupling between the transmission paths in a telecommunications system with non-linear transmission sections consisting of a speech coder and a speech decoder, in particular in a mobile radio system, that contains a mobile station a base station a transcoder unit and a functional unit for controlling the base station ,as well as a mobile switching centre wherein a device for suppressing the echo is arranged in the transcoder unit and the non-linear portion of the echo path is simulated with the aid of functional units present in the telecommunications system.

2. Process according to Claim 1, wherein the non-linear portion of the echo path is simulated in the transcoder unit by using a speech decoder

3. Process according to Claim 1, wherein the device for suppressing the echo is controlled in relation to a bad frame signal.

4. Circuit arrangement for implementing the process according to Claim 1, wherein in the transcoder unit the input of a downlink speech decoder is connected to the line carrying 1 15 the downlink datastream and that the output of the downlink speech decoder is connected to the device for suppressing the echo.

5. Circuit arrangement according to Claim 4, wherein an FIR filter is used as the device for suppressing the echo.

6. Circuit arrangement according to Claim 4, wherein a compressor/expander is used as the device for suppressing the echo.

7. A process for suppressing an echo substantially as hereinbefore described with reference to the accompanying drawings.

8. A circuit arrangement substantially as hereinbefore described with reference to the accompanying drawings. Dated this 1st day of June 2000 ALCATEL by its attorneys FREEHILLS CARTER SMITH BEADLE