WO1997038543A1

WO1997038543A1 - A squelch in an analogue mobile communication network

Info

Publication number: WO1997038543A1
Application number: PCT/FI1997/000217
Authority: WO
Inventors: Markku Vimpari; Kari Reponen; Jaakko Sippola
Original assignee: Nokia Telecommunications Oy
Priority date: 1996-04-10
Filing date: 1997-04-09
Publication date: 1997-10-16
Also published as: AU2510797A; FI961567A0; SE9803421D0; SE9803421L; FI961567A

Abstract

The invention relates to a squelch control arrangement in an analogue mobile network. The invention is characterized by comprising a first information activity detector (23) for monitoring the information activity of an uplink transmission from a mobile station (MS); a second information activity detector (24) for monitoring the information activity of a downlink transmission; and a squelch control unit (25), responsive to said first and second information activity detectors (23, 24), for controlling a squelch switch (21). The invention also relates to a method of controlling the squelch in an analogue mobile system.

Description

A SQUELCH IN AN ANALOGUE MOBILE COMMUNICATION NETWORK

FIELD OF THE INVENTION

The invention relates to a squelch control arrangement in an analogue mobile communication network. The invention also relates to a method of controlling a squelch in an analogue mobile system.

BACKGROUND OF THE INVENTION

Discontinuous Transmission, or DTX, is a method in which transmission from a mobile phone to the radio path can be interrupted for the duration of any pauses occurring in the speech. The aim is to reduce the power consumption of a transmitter, which is essential to a mobile telephone

Discontinuous transmission is previously known in connection with digital mobile systems. In the GSM, for example, a mobile station monitors the speech activity of a signal transmitted from the mobile station to a base station and interrupts the transmission from the mobile station to the radio path when the signal lacks speech information. In order that a receiving party would not hear the termination of the transmission as unpleasant complete silence, comfort noise parameters are transmitted from the mobile station at certain intervals The comfort noise parameters are utilized in generating^ comfort noise, which simulates background noise, in a speech decoder at the receiving end

Fig. 1 shows, by way of example, a simplified block diagram of an analogue mobile system NMT (Nordic Mobile Telephone). In Fig 1 , the fixed mobile network of the system comprises a Mobile Telephone Exchange MTX and Base Stations BS. A mobile station MS communicates over the radio path with a base station BS. The direction of transmission from the base station BS to the mobile station MS over the radio path is called downlink, and from the mobile station MS to the base station BS, uplink. A mobile telephone exchange MTX typically controls the operation of a plurality of base stations BS. Base station control is handled by Base Station Signalling Equipment BSE contained in the mobile telephone exchange. The mobile telephone exchange MTX communicates with a Public Switched Telephone Network PSTN and other mobile telephone exchanges The operation of the NMT system is monitored by an Operation and Maintenance Centre OMC.

In analogue mobile systems, the transmission from a mobile station to the radio path can also be interrupted by the mobile station for the duration of any pauses occurring in the speech. The problem is then the unpleasant, loud background noise passed to the receiving party. Since in analogue radio systems data transmission cannot be implemented as short burst-form transmissions like in digital mobile systems, an analogue mobile station cannot send comfort noise to the receiving party during the pauses occurring in the transmission. On the network side of an analogue radio system, it would be possible to use a squelch in order to avoid unpleasant background noise when no speech is present in an uplink transmission from a mobile station MS received at a base station BS. The problem then, however, is that some sounds or even whole words are lost at the beginning of the speech before it has been detected that speech activity has started and before the squelch has been switched off.

BRIEF DESCRIPTION OF THE INVENTION

The object of the invention is to implement a squelch in an analogue mobile system without impairing transmission of information.

A new type of squelch control is achieved by a control arrangement according to the invention, which is characterized by comprising a first information activity detector for monitoring the information activity of an uplink transmission received from a mobile station; a second information activity detector for monitoring the information activity of a downlink transmission to the mobile station; and a squelch control unit, responsive to said first and second information activity detectors, for controlling a squelch switch.

The invention also relates to a method of controlling a squelch in an analogue mobile system, the method being characterized by monitoring the information activity of an uplink transmission from a mobile station; monitoring the information activity of a downlink transmission to a mobile station; and controlling the use of the squelch on the basis of both the uplink and downlink information activity.

The invention is based on the idea that transmission of a message on a mobile connection is typically interactive. When information activity is monitored in both directions of transmission, it is possible to anticipate the start of uplink information activity and to prepare the network for the start of the uplink transmission.

An advantage of a squelch like this is that no unpleasant noise is passed to a receiving party when a mobile station stops the uplink transmission. Another advantage of a squelch according to the invention is that a transmission from a mobile subscriber is forwarded to a receiving party in the most complete form possible from the very beginning of the message. in the invention, squelch control is implemented by monitoring the information transmission activity of uplink and downlink signals. The squelch is switched on and the on-state is maintained, when the downlink signal contains information, such as speech or data, and the uplink signal does not contain anything but noise. The squelch is switched off, and the off-state is maintained, when the downlink signal does not contain anything but noise. Further, in the case of dual transmission (e.g. double talk), when both the mobile station and the other party are transmitting information simultaneously, the squelch is switched off and the off-state is maintained for as long as the uplink signal contains information.

BRIEF DESCRIPTION OF THE FIGURES In the following the invention will be described in greater detail with reference to the attached drawings, in which

Fig. 1 is a simplified block diagram of an NMT mobile system, Fig. 2 is a block diagram showing base station signalling equipment BSE according to the invention, Fig. 3 is a flow chart of a preferred embodiment of a method according to the invention, and

Figs. 4A - 4H illustrate, by way of example, situations during a speech connection in a mobile system.

DETAILED DESCRIPTION OF THE INVENTION The present invention can be applied in connection with any analogue mobile system. In particular, the invention is suitable for use in analogue radio systems in which the system does not directly support the use of discontinuous transmission in a mobile station. In the following, the invention will be described in greater detail, by way of example, in connection with an NMT mobile system. Fig. 1 shows the simplified structure of the NMT network described above.

The functionality of a squelch according to the invention can be implemented, for example, by a squelch control arrangement arranged in connection with a mobile telephone exchange MTX or a base station BS. We shall now describe a preferred embodiment of the invention with reference to Fig. 2. Fig. 2 is a block diagram of base station signalling equipment BSE according to the invention, showing only the components that are essential to the invention. The base station signalling equipment of Fig. 2 comprises a squelch switch 21 , which switches the transmission from a mobile station MS received at a base station BS or, for example, comfort noise generated by a noise generator 27, to a mobile telephone exchange MTX for further processing. When the signal path from the base station BS to the mobile telephone exchange MTX is disconnected by the squelch switch 21 , the receiving subscriber in the fixed network (PSTN) may interpret the silence (due to missing speech and noise) to mean that the call has been disconnected. To avoid this impression, it is possible to send the receiving party a signal containing comfort noise when the uplink signal is lacking.

In a first audio-level-measuring unit 23, the audio level in an uplink signal of a mobile station MS received from a base station BS is measured, and the signal strength is compared with a predefined threshold value T1 at voice frequencies. A signal strength that is below threshold value T1 is interpreted to indicate lack of speech/data activity, i.e. the signal received from the base station contains nothing but noise or normal background noise. When a signal is stronger than or equal to threshold value T1 , the signal can be interpreted to contain information, such as speech or data, or very strong noise, which is generated when the mobile station MS has stopped the transmission. To identify the strong noise, the characteristic frequency response of the uplink signal may be compared, for example, with a pre-set reference response. When the characteristic frequency response of the uplink signal differs from the reference response, the signal contains information and a measuring result M1 is set to indicate the speech/data activity of the uplink signal. Otherwise, measuring result M1 is set to indicate lack of speech/data activity. Measuring result M1 is passed to a squelch control unit 25. Correspondingly, in a second audio-level-measuring unit 24, the audio level of a downlink signal supplied from a mobile telephone exchange MTX through a base station BS to a mobile station MS is measured, and the signal strength is compared with a predefined threshold value T2 at voice frequencies. When the signal is stronger than or equal to threshold value T2, the downlink signal is interpreted to contain information, such as speech or data. A signal strength below threshold value T2 is interpreted to indicate lack of speech/data activity. Measuring result M2 is passed to the squelch control unit 25, the result indicating the speech/data activity of the downlink signal or lack of same in relation to threshold value T2. Threshold value T2 is preferably equal to threshold value T1.

On the basis of the measuring results M1 and M2 obtained from the audio-level-measuring units 23, 24, the squelch control unit 25 controls the squelch switch 21 to switch the uplink signal from the base station BS to the mobile telephone exchange MTX or to switch comfort noise from the noise generator 27 to the mobile telephone exchange MTX. When measuring result M2 indicates lack of speech/data activity in the downlink signal, the squelch control unit 25 controls the switch 21 to switch the base station signal to the mobile telephone exchange MTX, independently of measuring result M1. If the base station signal has already been switched to the mobile telephone exchange, the control signal is interpreted as a command to maintain the connection. When measuring result M2 indicates the presence of speech/data activity in the downlink signal and measuring result M1 simultaneously indicates lack of speech/data activity in the uplink signal, the squelch control unit 25 controls the switch 21 to switch the signal of the noise generator 27 to the mobile telephone exchange MTX or, if the signal of the noise generator has already been switched to the mobile telephone exchange, to maintain the present state of the switch 21. When both measuring signals M1 and M2 indicate that speech/data is being transmitted, the squelch control unit 25 controls the switch 21 to switch the base station signal to the mobile telephone exchange MTX or maintain the base station signal connection, if it already exists. If necessary, the audio-level-measuring units of the above- described preferred embodiment can also be replaced with other types of information activity detectors. In the present invention, normal background noise is not regarded as information.

Fig. 3 is a flow chart illustrating a preferred embodiment of a squelch method according to the invention. In the block diagram of Fig. 2, the squelch is switched on when the switch 21 switches the signal of the noise generator 27 to the mobile telephone exchange. The squelch is switched off when the switch 21 switches the signal obtained from the base station BS to the mobile telephone exchange MTX. In block 31 of Fig. 3, it is monitored whether the downlink signal sent by the mobile telephone exchange MTX through the base station BS to the mobile station MS contains information, such as speech. If the downlink signal contains speech, it is monitored whether the uplink signal sent by the mobile station MS and received from the base station BS contains speech (block 33). If the uplink signal does not contain speech, the squelch is switched on (block 35). If the squelch is already on, the on-state is maintained. If the uplink signal as well as the downlink signal contains speech, i.e. in a dual speech situation in which both parties of a conversation are speaking simultaneously, the squelch is switched off (block 37). If the squelch is already off, the off-state is maintained. When it is detected in block 31 that the downlink signal does not contain speech, the squelch is switched off or the off-state is maintained (block 37).

The signal diagrams of Figs. 4A-4H illustrate, by way of example, various situations during a conversation on a mobile call connection. The situations described below are only intended to illustrate the idea of the invention. The invention is thus not limited to the situations discussed. Fig. 4A illustrates a situation in which the downlink signal at first contains speech. As the speech activity of the downlink signal is terminated, the speech activity of the uplink signal is started with a short delay. In the squelch facility according to the invention, the squelch is switched off after the termination of the downlink signal speech activity, even though the uplink signal speech activity has not started yet. When the uplink signal speech activity is started, the squelch is already off, and so the switching off of the squelch does not result in loss of sounds at the beginning of the speech message. The situation corresponds to a normal conversation in which the participants take turns to speak. In such a situation, the method of the invention makes it possible to anticipate when the speech activity of a mobile subscriber will be started.

Fig. 4B illustrates a situation in which the uplink signal at first contains speech. Shortly after the uplink signal speech activity has been terminated, the downlink signal speech activity is started. In the invention, the squelch is not switched on until the downlink signal speech is started, since a mobile subscriber is then assumed to concentrate on listening to the downlink signal, and in the case of discontinuous transmission, the transmitter of the mobile station is switched off. During the speech activity of the downlink signal, the on-state of the squelch is maintained, whereby no unpleasant background noise is passed to the subscriber in the fixed network. If, however, the mobile subscriber starts the speech activity before the downlink signal speech activity has been terminated, the squelch is switched off immediately after detecting uplink signal speech activity, as shown in Fig. 4C. The switching off of the squelch may here cause some sounds to be lost at the beginning of the uplink signal transmission.

Fig. 4D illustrates the termination of a double talk situation as mobile subscriber stops the speech activity. Since the downlink signal still contains speech, the squelch is switched on to avoid extra noise.

Figs. 4E and 4F illustrate situations in which the downlink signal does not contain any speech. The squelch is then maintained in the off-state independently of the speech activity of the uplink signal. When anticipatory discontinuous transmission is used in the mobile station MS, the transmitter of the mobile station is on when the downlink signal does not contain any speech. Instead of disturbing noise, normal background noise is then passed from the mobile station MS to the mobile telephone exchange MTX.

In Figs. 4G and 4H, the uplink signal does not contain any speech activity. The squelch control is then totally dependent on the speech activity of the downlink signal. In Fig. 4G, the downlink signal at first contains nothing but noise and the squelch is maintained in the off-state. As the downlink signal speech activity is started, the squelch is switched on. In Fig. 4H, the squelch is on until the speech activity of the downlink signal is terminated. The squelch is then switched off to wait for an uplink signal response.

The squelch control according to the invention is at its most advantageous when it is used together with discontinuous transmission in which a mobile station transmission is controlled on the basis of both the uplink and the downlink information activity. The squelch control, which is similar to mobile station transmission control, is then able to anticipate when the transmitter of a mobile station is started. Even unpleasant noise bursts can be avoided, as the squelch control arrangement detects the information activity of a downlink signal slightly before the mobile station does.

The drawings and the description thereof are only intended to illustrate the idea of the invention. The squelch of the invention may vary in its details within the scope of the claims. Although the invention is described above mainly in connection with speech and/or data transmission, it can also be used in connection with transmission of other kind of information.

Claims

1. A squelch control arrangement in an analogue mobile communication network, characterized by comprising a first information activity detector (23) for monitoring the information activity of an uplink transmission received from a mobile station (MS); a second information activity detector (24) for monitoring the information activity of a downlink transmission to the mobile station (MS); and a squelch control unit (25), responsive to said first and second information activity detectors (23, 24), for controlling a squelch switch (21).

2. A squelch control arrangement according to claim 1, characterized in that the information activity detectors (23, 24) for monitoring the information activity of the uplink and downlink transmissions are arranged to measure the audio levels of said transmission signals and to compare the audio levels measured with predefined threshold values.

3. A squelch control arrangement according to claim 1 or 2, characterized in that the control arrangement is arranged to switch the squelch off and maintain the off-state when the downlink transmission does not contain any information activity or when both the downlink and the uplink transmissions contain information activity, and switch the squelch on and maintain the on-state when the downlink transmission contains information activity but the uplink transmission does not contain any information activity.

4. A squelch control arrangement according to claim 1, 2 or 3, characterized by also comprising a noise generator (27) for generating comfort noise on a speech connection when the squelch is in the on-state.

5. A squelch control arrangement according to claim 1, 2, 3 or 4, characterized by being located in base station signalling equipment (BSE) of a mobile telephone exchange (MTX).

6. A squelch control arrangement according to claim 1, 2, 3 or 4, characterized by being located in connection with the base station

(BS).

7. A method of controlling the squelch in an analogue mobile system, characterized by monitoring the information activity of an uplink transmission from a mobile station (MS); monitoring the information activity of a downlink transmission to the mobile station; and controlling the use of the squelch on the basis of both said information activities.

8. A method according to claim 7, characterized in that the steps of monitoring the information activity of the uplink and downlink transmissions comprise the steps of: measuring the audio levels of the uplink and downlink transmissions, and comparing the audio levels measured with predefined threshold values.

9. A method according to claim 7 or 8, characterized by switching the squelch off and maintaining the off-state when the downlink transmission does not contain any information activity or when both the downlink transmission and the uplink transmission contain information activity, and switching the squelch on and maintaining the on-state when the downlink transmission contains information activity but the uplink transmission does not contain any information activity.

10. A method according to claim 7, 8 or 9, characterized by generating noise on a telephone line of a receiving party when the squelch is in the on-state.