WO1995033351A1

WO1995033351A1 - Fixed cellular terminal device providing a two-wire telecommunications service

Info

Publication number: WO1995033351A1
Application number: PCT/EP1995/001988
Authority: WO
Inventors: Antonio Javier Morcillo Martinez; Antonio Martinez Navarro
Original assignee: Alcatel Standard Electrica S.A.; Alcatel N.V.
Priority date: 1994-05-31
Filing date: 1995-05-26
Publication date: 1995-12-07
Also published as: ZA953851B; ES2101633A1; ES2101633B1; AU2615995A

Abstract

To access a mobile communications network and provide basic telecommunications services of the public switching network for an ordinary telephone terminal (TT2) connected to it. This terminal is derived from a GSM mobile cellular terminal, employing four-wire transmission over the whole path, including a two-wire conversion circuit (H) connected between the transmitting (TP) and receiving (RP) pairs and the subscriber line (SL) of the telephone terminal (TT2). It also includes an echo canceller (EC) situated before the two-wire conversion circuit (H), in order to subtract part of a first signal (FS) directed towards the telephone terminal (TT2) from a second signal (SS) coming from the latter and in this way compensate part of the echo produced mainly by the telephone terminal (TT2) and by the two-wire conversion circuit (H).

Description

FIXED CELLULAR TERMINAL DEVICE PROVIDING A TWO-WIRE TELECOMMUNICATIONS SERVICE

OBTECT OF THE INVENTION

This invention concerns a fixed cellular terminal for use in cellular mobile communications networks that provide speech and/or data services to terminals mainly of the telephone type, specially indicated for those areas where there is still no fixed communications infrastructure, or this is already in a state of saturation and it is planned to implement an operational "island" where this communications service will be offered. Such cases are found in rural areas, for example.

The fixed cellular terminals are basically very similar to the cellular mobile terminals, which work like a four-wire terminal: two for the transmitting path and two for the receiving path, mutually independent from each other. The purpose of this invention is to provide an interface that performs the four-to-two wires conversion, and vice versa, in fixed cellular terminals derived from mobile terminals, in a manner adequate to facilitate thereby a two-wire subscriber line for the connection of ordinary type telephone and/or telematic terminals. This invention is of particular application in terminals used in long delay communications networks, such as GSM type cellular mobile networks. BACKGROUND TO THE INVENTION

In recent years the proliferation of mobile services based on the development of new standards has produced a series of alternative uses that extend beyond that for which they were initially developed.

Thus, for example, in certain areas of low subscriber density, the installation of a fixed infrastructure would be costly and uneconomic. For this reason, it is difficult to envisage these areas enjoying a basic telephone service, at least in the same form as that provided in large cities. To resolve such problems, various technologies are employed, based mainly on radio access, which greatly simplify the connection.

One of the alternatives that most expects to be widely accepted according to the article "APPLICATION OF MOBILE TECHNOLOGY IN THE ACCESS NETWORK" by J.M. Garcia Aguilera, published in the Workshop Proceedings of the 23rd European Microwave Conference, . i .

Madrid, September 10th, 1993, pages 124 and 125, is based on already existing mobile network infrastructures, whereby they can support simultaneously mobile communications services, with all their characteristics associated with the mobility concept, and "fixed" services but having cellular radio access.

This means that in areas where there is a cellular mobile system already installed, of the GSM type for example, operators can offer subscriber lines very rapidly, as long as the cell capacity, mainly with respect to the number of available channels, so permits. To this end, development work has gone ahead into what are termed "fixed cellular terminals" which, basically, are cellular mobile terminals, in this case of the GSM type, that normally employ directional antennas and which normally have their mobility functions removed. In other words, the network control system knows that the terminal in question is always going to be found in the same cell and, consequently, the processing that it will require will be more straightforward than that of a standard mobile terminal.

In addition, use is normally made of directional antennas which, firstly permits a greater range than the cellular mobile coverage zone and, secondly interferences induced in other receivers and received from other transmitters are less.

The GSM cellular mobile system sets up full duplex bidirectional communications effected by means of equivalent four-wire trunks. This four- wire connection is implemented fully up to the final audio stage of the cellular mobile terminal, that is, the loudspeaker and the microphone, by means of perfectly isolated wire pairs, in contrast to ordinary telephone terminals of the basic telephone service for which a two-wire conversion is made in order to transmit and receive simultaneously over a subscriber line and subsequently perform the four-wire conversion in the terminal set in order to separate the signals corresponding to the two directions, one of which will be directed towards the loudspeaker and the other will be coming from the microphone.

To adapt both equipments, the fixed cellular terminal and the telephone terminal, it is common practice to use, for example, passive circuits with hybrid transformers or else active circuits as shown in patent application WO 90/13962, in which use is mentioned of an electronic bridge circuit that converts the balanced two-wire audio signal coming from a telephone terminal into unbalanced four-wire audio signals for transmitting and receiving audio signals in a bidirectional telephone system, commonly described as duplex. Nevertheless, it should be noted that for the GSM standard, the maximum transmission time allowed in the two directions (send and receive) between the subscriber terminal and the point of interconnection between the GSM network and the public switched network is 180 ms, as can be observed in the standard GSM 03.50 of the ETSI. On the other hand, according to Recommendation G.131 of the CCITT, which shows echo tolerance curves for networks with long delay, the attenuation of the sonority index has to have a very high value, which is very difficult to achieve when conversions are made from four-to-two wires, which is the case of the present invention. Thus, the technical problem resides in implementing an interface that performs the conversion to two-wire communications from four-wire communications, as is the case with cellular mobile terminals of the GSM type, which is completely transparent for the subscriber and meets the established standards, especially those referring to maximum tolerable values of echo.

CHARACTERISATION OF THE INVENTION

This invention finds its main application in fixed cellular terminals used to obtain access to the public network via a switched mobile communications network and provide, in this way, the basic telephone service to a two-wire telephone terminal connected to it. The fixed cellular terminal, derived from a cellular mobile terminal, operates with four-wire connections. Consequently, this terminal has an interface that includes a two- wire conversion circuit that is connected on one side to the audio transmit and receive pairs of the fixed cellular terminal and, on the other side, to the subscriber line pair to which the telephone terminal is connected.

In this way the four-to-two wires conversion is achieved, permitting the direct connection of the telephone terminal with the fixed cellular terminal. The interface is characterised in that it also comprises an echo canceller in the four-wire part of the link and prior to the two-wire conversion circuit, which subtracts at least part of a first signal intended for the telephone terminal, from a second signal coming from this terminal and compensates, in this way, part of the echo produced mainly in the two-wire conversion circuit and in the telephone terminal.

The aforementioned echo canceller consists of a digital filter of the FIR type, the coefficients of which are determined when a link is first established by measuring the pulse response in the echo channel. That is, an electrical signal is generated in the form of a narrow pulse and sent over the transmit pair, and the measurement of the obtained signal, which corresponds to the echo channel response, is then obtained in the receive pair. Once the filter coefficients have been obtained, these can remain constant as the characteristics of the echo channel can normally be considered to be constant for this type of situation.

Even so, in order to compensate for possible variations in the echo channel, a slow adaptation algorithm is applied to the filter mentioned in order to modify its coefficients so that the echo is cancelled, insofar as possible, even with variations in time of the echo channel response.

As a consequence of the application of this invention, it is possible to perform the four-to-two wires conversion of signals in the voice frequency band, between terminals that operate with four-wires and two-wire telephone terminals. It offers particular advantages in the case of four-wire terminals that form part of a communications network with a long delay, such as digital cellular mobile networks, of the GSM type, in which the effect of echo is especially pernicious to good communications. BRIEF FOOTNOTES TO THE FIGURES A fuller explanation of the invention can be found in the description given based on the figures below in which:

- figure 1 shows, in schematic form, a telephone connection between two terminals, one fixed connected directly to the public switched network, and the other mobile that has access to the latter by means of a cellular type public mobile switched network;

- figure 2 shows a similar arrangement, but in this case the mobile terminal has been replaced by a fixed cellular terminal to which is connected an ordinary type telephone terminal;

- figure 3 is a block diagram of a GSM terminal for cellular mobile communications, from which a fixed cellular terminal has been derived; - figure 4 shows, in basic form, the classic solution for performing the conversion from four-to-two wires, whereby it is possible to connect a two- wire standard telephone terminal;

- figure 5 shows, in schematic form, the solution offered so that the previous interface satisfies the existing recommendations with respect to return echo characteristics, and

- figure 6 shows the layout of an FIR filter of the type employed in the echo canceller of the preceding figure.

DESCRIPTION OF A PREFERRED IMPLEMENTATION There are many telecommunications networks that permit voice and /or data communications services to be offered between subscribers connected to them. Of these, that with greatest application is the public switched telephone network which can offer, among other features, the basic telephone service whereby a subscriber is connected via a two-wire line, to a local exchange where a conversion is made to four wires. From this point, and up to the subscriber line interface at the other end, communications always take place on a four-wire equivalent circuit.

Figure 1 shows, schematically, the connection between a subscriber to the basic telephone service via a two-wire telephone terminal TT1 and the public switched telephone network PSN. For this, it is necessary to have a

^■ two-to-four wires conversion circuit H known as a hybrid coil, or simply a hybrid. This hybrid carries out the separation of the transmit and receive signals on two one-way pairs for the direction of subscriber to exchange, while in the other direction, exchange to subscriber, it combines them onto a single bidirectional pair. From this point, the public switched network

• always works with four wires.

In the case of a subscriber to a public land mobile communications network PLMN, like the GSM cellular mobile network, the connection between the mobile cellular terminal MCT and the said network is achieved, as can be seen in the right-hand part of figure 1, by means of a base station system BSS, in which the whole communications process takes place using four equivalent wires up to the loudspeaker and microphone of the mobile cellular terminal MCT.

When a subscriber to the public switched network PSN wishes to set up a communications link with another in the GSM network, the interconnection is also made with four wires through an interface termed POI, or Point Of Interconnection.

The GSM network also contemplates in its mobile services switching centre, also termed the MSC, an electric echo canceller EEC, present on the receive pair of the POI, in order to suppress possible echo coming from the public switched network terminal, the effect of which is extremely pernicious for the mobile terminal because the GSM network has a long inherent delay.

Figure 2 shows the connection of a basic telephone service subscriber to the public switched network via a two-wire telephone terminal TTl and a subscriber also to the basic telephone service via cellular access. The infrastructure is basically the same, but the mobile cellular terminal MCT, of figure 1, has been replaced by a fixed cellular .erminal FCT to which is attached a telephone terminal TT2. The network operator considers this kind of access as a fixed access, for which reason billing, numbering, etc., must be completely compatible with the former.

Figure 3 shows the configuration of a GSM type cellular terminal, in which there is an antenna ANT, a radio subsystem RSS, a synthesiser subsystem SSS, a baseband subsystem BBSS, a control subsystem CSS, and an interface subsystem IFSS. The baseband subsystem BBSS works with the two transmit and receive digital channels separately, operating in a four- wire equivalent mode, in which one of the channels is connected to the microphone MF and the other to the loudspeakei LS. In this situation, the only source of echo to be considered is the acoustic echo produced by the coupling between the loudspeaker and the microphone MF and the value of ■ which, except for the handsfree applications, is insignificant.

Figure 4 shows, in schematic form, the main modifications made to a mobile cellular terminal MCT in order to turn it into a fixed cellular terminal FCT, in which is included a four-to-two wires conversion circuit H, such as a hybrid, the two-wire output of which is the subscriber line SL to which the telephone terminal TT2 is connected. In this case the echo phenomenon is produced as follows: the fixed cellular terminal FCT recovers a first signal FS generated at the other extremity of the link and sent to the telephone terminal TT2 over a receive pair RP, this first signal FS passing through a two-wire conversion circuit H in which it suffers a determined attenuation. The telephone terminal TT2 transmits a signal TS that is basically the actual signal generated in its microphone plus the return echo of the first signal FS produced in the two-to-four wires conversion circuit that exists in the actual telephone terminal TT2 in order to separate the signals, one destined for the loudspeaker and the other coming from the microphone. The signal coming from the telephone terminal TT2 reaches the two-wire conversion circuit H and, after suffering the corresponding attenuation, is applied to the transmit pair TP on which is transmitted a second signal SS consisting of the signal coming from the telephone terminal TS and an echo signal ES produced by the actual two-wire conversion circuit H of the fixed cellular terminal FCT.

Figure 5 shows, in accordance with the invention, the fixed cellular terminal FCT which includes an echo canceller EC to suppress the echo produced previously in the two conversion circuits, both that of the fixed cellular terminal FCT for the conversion from four-to-two wires, and that of the telephone terminal TT2 for the conversion from two-to-four wires.

The echo canceller EC is formed by a digital filter of finite impulse response, termed a FIR filter, which has n coefficients, Cl, C2,..., Cn, that, respectively, multiply the n consecutive samples of the filter input signal Xn that corresponds to the first signal FS present on the receive pair RP. The sum signal Yn of the results of the multiplications is the output of the filter, which is subtracted from the second signal SS present on the transmit pair TP coming from the two-wire conversion circuit H, in order to be applied to the transmit part of the GSM terminal.

The echo canceller also receives both the first signal FS directed towards the telephone terminal TT2 and the second signal SS coming from

' the latter, so that means of control (not shown) modifie the values of the coefficients of the aforementioned FIR filter and, by means of a minimum squares adaptation algorithm, termed LMS, the filter response is similar to the response of the global echo channel and therefore compensates the echo produced by the receive pair RP on the transmit pair TP.

As the echo channel, for a determined link, varies very slowly, it is preferable to have the value of the coefficients also evolving very slowly. The consequence of this is that great precision can be achieved with respect to the cancellation values that have to be of the order of 46 dB to meet all the CCITT Recommendations for tolerable echo with a delay of 180 ms. The use of a slow adaptation algorithm has, on the other hand, the inconvenience that if the initial values of the digital filter coefficients are not close to the true values that it ought to have, the time necessary for these to converge to the optimum value would be too long, and therefore the subscriber may at first hear a very strong echo that would last for a certain time until it was gradually attenuated, something that could result very annoying for the subscriber. To avoid this, the echo canceller initially measures the echo channel response whenever the subscriber lifts the handset of the telephone terminal TT2. In this way certain initial coefficients are found which, even if not sufficiently exact, are similar to those that should be finally obtained; consequently, the process of adaptation, even if occurring slowly, does not require too much time to produce an echo cancellation that satisfies the CCITT Recommendations mentioned in the state of the art. The measurements of the echo channel are obtained from its actual response to a sufficiently narrow signal generated, in digital form, by the control means in the receive pair RP and prior to the two-wire conversion circuit H and to the digital-to-analogue converter DAC. The measurements are made on the transmit pair TP, after the analogue-to-digital converter ADC, when taking samples of the value of the second signal SS. During this process the output signal Yn of the digital filter is null.

To prevent the audio signals that can be picked up by the microphone from being interpreted as part of the echo channel impulse response, the impulse response measurement is made not once but is repeated up to 15 times at intervals of 12 ms, whereby the components of the measured signal that are due to sounds captured by the microphone can be considered to be uncorrelated. Consequently the average of these 15 readings is taken to be the echo channel impulse response.

In some situations and with the object of simplifying the complexity of the echo canceller, it can be sufficient to obtain the initial values of the digital filter coefficients calculated according to the readings of the echo channel impulse response. For this, it is not necessary to apply any adaptation algorithm, but to perform a more accurate measurement of the echo channel response on those occasions when this is sufficient. As a drawback, the subscriber would be left unconnected for a few tenths of a second, however this would not normally be noticeable as it would normally correspond, at most, to the time it takes him to lift the handset of the telephone terminal TT2 to his ear.

Claims

1.- FIXED CELLULAR TERMINAL which has access to a mobile communications network in order to provide a two-wire telephone terminal

(TT2) connected to it with telecommunications services and which incorporates a two-wire conversion circuit (H) connected between the transmit (TP) and receive (RP) pairs and the subscriber line (SL) to which the telephone terminal (TT2) is connected, in order to perform in this way a four-to-two wires conversion; this interface being characterised in that it also includes an echo canceller (EC) in the four-wire part of the link and prior to the two-wire conversion circuit (H), which subtracts at least part of a first signal (FS) destined for the telephone terminal (TT2) from a second signal (SS) coming from the latter and in this way compensates part of the echo produced mainly in the two-wire conversion circuit (H) of the actual fixed cellular terminal (FCT) as well as in the telephone terminal (TT2) connected to the subscriber line (SL).

2.- TERMINAL according to claim 1, characterised in that the echo canceller (EC) is constituted by a digital filter the coefficients (Cl, C2,..„, Cn) of which are determined when first setting up a telephone connection between this terminal and a second subscriber, and subsequently remaining constant while the connection lasts.

3.- TERMINAL according to claim 2, characterised in that the digital filter is of the adaptive type, such that once the initial values of the filter coefficients have been determined, these vary in accordance with a slow adaptation algorithm in order to adapt to variations in the echo channel.

4.- TERMINAL according to claim 1, characterised in that the coefficients (Cl, C2, .., Cn) of the digital filter are obtained by measuring the impulse response of the echo channel, by generating an electrical signal in the form of a narrow pulse on the receive pair (RP) and measuring the signal obtained on the transmit pair (TP).

5.- TERMINAL according to claim 4, characterised in that, in order to determine the values of the initial coefficients of the digital filter, an average is taken of a number of readings of the echo channel impulse response, and in that these readings are made under the same conditions but delayed one from the other by a certain interval of time.