WO2006039936A1

WO2006039936A1 - Procedure and device for jamming calls in a cellular mobile radio system

Info

Publication number: WO2006039936A1
Application number: PCT/EP2004/011414
Authority: WO
Inventors: Gilles Latouche; Frédéric Rousseau
Original assignee: Eads Secure Networks
Priority date: 2004-10-12
Filing date: 2004-10-12
Publication date: 2006-04-20

Abstract

The procedure is used to jam calls in a predefined geographic area included in a cell of a cellular mobile radio system. A jamming signal (B) is sent by means of controlled transmission means, in such a way as to jam at least partially a first logical signalling channel defined on a beacon channel. A second logical signalling channel set up on a radio signal (S) corresponding to the beacon channel, on which are transmitted information messages needed for a resource access procedure, is scanned. In the event of detection of predefined information transmitted in this second logical signalling channel, a command (ES) to stop transmission of the jamming signal (B) is generated.

Description

PROCEDURE AND DEVICE FOR JAMMING CALLS IN A CELLULAR

MOBILE RADIO SYSTEM

The present invention relates to digital cellular mobile radio communication systems.

It finds applications, in particular, in public mobile radio systems with mobile terminals such as the GSM (Global System for Mobile communications) systems defined by the ETSI (European Telecommunications Standards Institute) , and their variants in various frequency bands and for various modulations, and their GPRS (General Packet Radio Service) service extensions. It can also apply to Professional Mobile Radio systems (PMR), such as, for example, the APCO Project 25 systems, defined by the American • TIA (Telecommunications Industry Association) for the APCO (Association of Public-Safety Communications Officers) or the TETRA (TErrestrial Trunked RAdio) system defined by the ETSI. It can also be applied to air-ground communications with aircraft or satellites, such as the TFTS (Terrestrial Flight Telephone System) defined by the ETSI, or the Inmarsat and Globalstar systems. It can be applied by extension to multi-mode communication systems using mobile terminals compliant simultaneously with several radio technologies.

A cellular mobile radio system comprises an infrastructure (embodied in a routing and operation network) and mobile terminals supplied to the users. The infrastructure includes one or more base stations, which act as radio relays between the network core and the mobile terminals. Each base station covers a limited geographic area called a cell.

In most of the digital mobile radio systems, the logical traffic channels are arranged in time and/or frequency domains of at least one uplink traffic channel (that is, from mobile terminals to base station) and in time and/or frequency domains of at least one downlink traffic channel (that is, from base station to mobile terminals) .

Similarly, logical signalling channels are arranged in time and/or frequency domains of an uplink beacon channel and in time and/or frequency domains of a downlink beacon channel. In the description below, the term "beacon channel", when used alone, designates the duplex physical channel which supports, among other things, one or more logical common signalling channels, associated with a cell.

In the case of a CDMA (Code Division Multiple Access) type system, logical channels are multiplexed on physical channels in an orthogonal code base of the time-frequency space. In the case of a TDMA (Time Division Multiple Access) type system, logical channels are multiplexed into recurrent time intervals (called "slots" in the jargon of those skilled in the art) defined on particular physical channels, corresponding to frequency carriers. An FDMA (Frequency Division Multiple Access) type system can be seen here as a particular case of a TDMA type system where the multiplexing degenerates into one logical channel for each physical channel.

In the case of point-to-point calls with a predefined mobile terminal, dedicated logical channels are allocated by the base station to the call, on a resource access procedure preceding the setting up of the call, said procedure being performed on common logical channels of the cell.

In the case of broadcast mode calls or calls controlled by the network, potentially concerning a plurality of mobile terminals, dedicated or shared logical channels are allocated by the base station to said defined call, and the terminals involved are notified, by a repetitive information message, of the potential existence of this call on these logical channels and of the parameters of the access procedure associated with it.

The reader can refer to the standard GSM 05.02 for a definition of physical channels and logical signalling and traffic channels applicable to the GSM system. In other systems, such as TETRA, logical signalling and traffic channels can be multiplexed in time onto the same physical channel.

There are situations where it is desirable for the mobile terminals to be disabled. For example, the use of mobile terminals in public places such as entertainment venues by individuals disregarding silence instructions, represents a sound nuisance, both in the sending and receiving of calls (ring tones or music clips, the voice of the user) . Similarly, the fraudulent introduction of mobile terminals into prisons undermines the isolation of the inmates or the monitoring of conversations by the prison administration.

The document WO 98/18232 describes a procedure for disabling mobile terminals in a selected area, for example a bank. The procedure consists in sending a broadband jamming system in the selected area, so as to prevent communication between the base station and the mobile terminals. In operation, the jamming signal prevents the base station from communicating with the mobile terminals, so that it cannot allocate a particular communication channel to the mobile terminals. The jamming signal has a frequency band corresponding to the frequency band received by the mobile terminals, for example the 890-960 MHz (megahertz) band. Such a procedure presents the drawback of preventing all communication services, permanently and without differentiation for all the mobile terminals located in the geographic area concerned.

Now, national legislation normally dictates the need to maintain in all circumstances the availability of the emergency call service (for example, for GSM, numbers 112 in Europe, 911 in the United States of America, 999 in Great Britain, and so on) .

In certain circumstances, it may also be useful to prohibit services to all the terminals present in a prepredefined geographic area except for identified terminals or identified groups of terminals.

This is incompatible with jamming of all of the communication services for all of the mobile terminals present in a given geographic area as described in the abovementioned document WO 98/18232. This is why the use of jamming equipment is highly regulated and even often prohibited.

The document US 6,195,529 describes a procedure for blocking transmissions for mobile terminals. When a base station sends a synchronization signal to a mobile terminal, a receiver measures the received power, controls the time- and duration of transmission of a jamming signal. The transmit power of the jamming signal is adapted to the received power in such a way that the synchronization signal cannot be received or decoded or recognized by the mobile terminal. The jamming signal is derived from a pseudo-interference generator.

The drawback of such a procedure is that the decision to transmit the jamming signal is slaved to the measurement of the received power of the signal to be jammed. The jamming transmission control is sensitive to the radio link budget and can be greatly affected by a changing environment around the receive antenna, caused, for example, by fading type disturbances.

The document EP-A-I 122 897 discloses various jamming procedures . These procedures consist in detecting the transmissions from the mobile terminal. The procedure disturbs the calls to a terminal and the calls originating from that terminal by sending waves to that terminal. The transmission of the jamming can be subject to a time delay after detection of the transmissions from the mobile terminal. In an embodiment, the choice of the frequency and time characteristics of the signal to be jammed depends on previously stored system information. The stored information comprises the static parameters characteristic of each system. The jamming can therefore be limited to certain predefined logical channels, but is not therefore selective, in the sense that no condition for stopping the jamming can be generated dynamically.

There are also situations where it is desirable for the mobile terminals to remain in standby or restricted service mode, without receiving calls and without wasting their batteries by attempting to register in a cell, when they are present in a predefined area.

Besides the situations mentioned above, the use of mobile terminals in sensitive locations, ^■ such as potentially explosive areas or hospitals, requires the transmissions from mobile terminals to be limited. In particular, it is appropriate to avoid certain transient or power up to transmitted power peak conditions, which are typical in particular of certain TDMA systems. It is therefore best to avoid repeated registration transaction attempts when the radio link budget is mediocre (for example, when the best relay is remote or jammed) . This constraint should be applicable systematically to all the mobile terminals present in the area (whatever the manufacturer, type, model or age of the terminals) , without making any assumption concerning these mobile terminals, apart from their conformity to the radio interface of the system.

These requirements are incompatible with a manual action via the man-machine interface, and with any automatic procedure using a third party interface, such as described in the document EP-A-I 122 897: the drawback of such a procedure is that it necessitates such an interface on all the mobile terminal models distributed in the past or in the future and liable to pass through the predefined area.

The document FR-A-2 840 476 discloses methods of jamming radio communication with mobile terminals. A request to access the network sent by a terminal is intercepted and a selective restriction criterion is evaluated on the basis of parameters relating to the intercepted access request. A radio signal is sent to jam an acknowledgement signal sent by the network in response to the access request if the selective restriction criterion is satisfied.. Evaluating the selective restriction criterion includes determining the type of call the terminal is seeking to set up on the basis of parameters relating to the intercepted access request. For example, the call type may correspond to emergency calls, so that the selective restriction criteria can be defined in such a manner as to avoid restricting emergency calls.

In other words, the procedures disclosed in this document propose to jam only certain identified calls. These procedures however present a problem of robustness. In practice, the evaluation precedes the transmission of a jamming signal..

One object of the invention is to allow jamming of calls which is selective in the sense that it is applied to all the communication services for all the mobile terminals present in a given geographic area except for specified services and/or specified mobile terminals, identifiable by an appropriate device.

In the description that follows, the term "call" is used to mean any communication service, in particular a voice transfer, a data transfer in circuit mode, in packet mode or in short message mode, and the transmission of mobile terminal location update or mobility management information.

The invention thus proposes a method for jamming calls in a predefined geographic area included in a cell of a cellular mobile radio system. According to the procedure, a jamming signal is generated and it is sent using controlled transmission means, so as to jam at least partially certain frequency and/or time domains defining a first logical signalling channel defined on an uplink beacon channel or on a downlink beacon channel respectively associated with said cell. At the same time, a radio signal is received from the base station covering said cell or from a mobile terminal located in said geographic area, for which certain time and/or frequency domains define a second logical signalling channel on the respectively downlink or uplink beacon channel associated with said cell, on which information messages required for a resource access procedure, which precedes the setting up of a call, are transmitted with a certain repetitiveness. Said second logical signalling channel is scanned, and, in the case of detection of predefined information transmitted in said second logical signalling channel, a command is generated to stop the transmission of the jamming signal by said controlled transmission means.

Thus, the jamming signal prevents calls from being set up with the mobile terminals located in the predefined geographic area, unless a condition to stop transmission of the jamming signal is satisfied. The result is therefore the jamming selectiveness that^' is sought.

It should be noted that the method according to the present invention is more robust than that proposed in the abovementioned document FR-A-2 840 476. In practice, calls are jammed as long as no decision to stop jamming is taken.

Given the repetitiveness of the information transmitted on the signalling channels, stopping the transmission of the jamming signal which is provoked by the detection of said predefined information, then allows the resource access procedure to be conducted normally

(that is, without jamming), and therefore the call to be set up. Thus the fact that the protocol rules of the transmission of the information on the logical common signalling channels provides for the repetitive transmission of this information is used advantageously.

In other words, whereas the operation of the jammers known in the prior art consists in sending a pulsed or continuous signal, in the band used by the system or in a portion of that band, to jam the physical frequency domain signalling channels and/or the physical frequency domain traffic channels of the system, the invention profits from the structuring of the physical channels as logical channels and exploits the protocol information that governs the use of these logical channels. This means that the jamming selectiveness that is sought can be obtained.

According to one advantage of the invention, the jamming occurs before the calls are set up. It follows that, in the case of an incoming call, the mobile terminals present in the predefined geographic area will remain silent. In particular, their ringers will not sound.

According to another advantage, the jamming signal can have a bandwidth as narrow as the width of the channels in the technology concerned, since its function is to jam a single physical channel, that is, the physical signalling channel which carries the beacon channel. Furthermore, the jamming signal is sent only during certain time and/or frequency domains (time slots in TDMA) only of the physical signalling channel, since its function is to jam only a logical channel. This limits the radio frequency disturbances in the frequency spectrum, and also reduces the current consumption of the jamming signal transmission means.

Another advantage of the invention is that it can be implemented without cooperating with the network operator of the mobile radio system. The implementation of the invention is in fact completely transparent to the latter.

The selectiveness of the jamming is in particular independent of the service access control applied by the operator to classes of mobile terminals.

Another advantage of the invention is that some of its modalities act without necessarily jamming logical broadcast channels of the cell (for example, the BCCH, Broadcast Common Channel) which are useful for synchronizing, in the frequency domain and/or in the time domain, transmission of the jamming signal with the single required logical channel.

Another advantage of the invention is that it allows common or shared logical signalling channels to be jammed, while allowing selectively authorized calls to continue on dedicated logical channels, independently of the number and duration of these authorized calls. Another advantage of the invention is that reception is independent of any disturbance and coupling effect generated by the transmission of the jamming signal, in as much as the jamming and reception are separated into different frequency or time domains. In practice, in some embodiments of the invention, the first logical channel is different from the second logical channel. In other embodiments, where the first logical channel and the second logical channel are one and the same logical channel, transmission is controlled so as to jam the logical channel at least partially, which allows transmission of the jamming signal of a part of the logical channel to be separated in time from reception of a part of the logical channel. It is then possible to produce the device using a receiver subject to the engineering constraints similar to those in the mobile terminals or in the relays of the system concerned.

Because of the fact that the jamming on the downlink beacon channel is performed a priori, and that the detection of predefined information precedes any command to stop transmission of the jamming signal, the method is more robust with respect to radio frequency disturbances than the method described in the document US 6,195,529. In practice, the jamming is effective even in the case where the receive antenna is located in a fading hole. Note that in the method described in the document US 6,195,529, a jamming signal transmit power level adjustment criterion is a power criterion whereas in the procedure, the condition for stopping transmission of the jamming signal is the detection of predefined information, which requires decoding of the received signal.

In the case where the first logical channel and the second logical channel are identical and are set up on the uplink beacon channel, it is preferable to send the jamming signal only if a useful signal is received in a time window beginning at the start of a theoretical reception window of said first logical channel and having a duration dependent on the extent of the geographic area to be jammed. The duration of this time window is proportional to the propagation time between any mobile terminal in the area to be jammed and the antenna of the receiver of the jamming device, that is, proportional to the extent of the area to be jammed. In the case of the GSM, for an area to be jammed inscribed in a circle of radius 300 m (metres) , the duration of this time window is approximately the transmission duration of a symbol.

Another aspect of the invention relates to a device for implementing the method defined above. The device comprises:

- means for generating a jamming signal;

- controlled transmission means for transmitting said jamming signal, so as to jam at least partially a first logical signalling channel defined on an uplink beacon channel or on a downlink beacon channel respectively associated with said cell;

- means for receiving a radio signal originating from the base station covering said cell or from a mobile terminal located in said geographic area, for which certain time and/or frequency domains define a second logical signalling channel of the respectively downlink or uplink beacon channel associated with said cell, on which the information messages needed for a resource access procedure, which precedes the setting up of a call, are transmitted with a certain repetitiveness;

- means for scanning said second logical signalling channel, and, in the event of detection of predefined information transmitted in said second logical signalling channel, for generating a command to stop transmission of the jamming signal by said controlled transmission means. In particular in the case where said second logical signalling channel is set up on the uplink beacon channel, the device can include means for conditioning the transmission of the jamming signal by said controlled transmission means, on the reception of a useful signal in a time window beginning at the start of a theoretical reception window of said logical channel and having a duration dependent on the extent of said predefined geographic area.

It will be appreciated that, if the geographic area in which calls are to be jammed extends over a number of cells of the cellular mobile radio system, such a device must be provided for each of these cells, being programmed to send and receive at the frequency of the uplink beacon channel and/or of the downlink beacon channel respectively associated with that cell. Similarly, if this geographic area is covered by more than one cellular mobile radio system, it is essential to provide one or more similar devices, programmed to send and receive at the frequency of the uplink and/or downlink beacon channels respectively associated with the cells concerned of this system. The operation of these devices can be supervised via a centralized management unit, which manages all of a jamming system. The centralized management unit may be used to take the jamming devices put of service, for example during the intervals in a show.

Other characteristics and advantages of the invention will emerge on reading the description which follows. This is purely illustrative and should be read in light of the appended drawings in which: - Figure 1 is a diagram illustrating a frame on the RACH channel in a GSM system;

- Figure 2 is a diagram illustrating a random access request message sent in the frame of Figure 1;

- Figure 3 is a diagram illustrating a mobile radio system in which the device according to the invention is applied;

- Figure 4 is a block diagram illustrating a device according to the invention; - Figure 5a and Figure 5b are flow diagrams illustrating the steps of the method according to the invention;

- Figure 6 is a diagram illustrating a first method of implementing the partial jamming of a PCH channel or of an NCH channel in a GSM system; and,

- Figures 7a to 7h are diagrams illustrating a second method of implementing the partial jamming of a PCH channel in a GSM system.

Ways of carrying out the method are described below in an application of the invention to a GSM system. Nevertheless, a person skilled in the art will appreciate that it can be applied more generally to other types of mobile radio systems, of CDMA, TDMA or FDMA or other multiple access method types profiting from logical channels. A person skilled in the art will also appreciate that it can be applied generally equally to calls in connected mode and to calls in non- connected mode, with individual addressing or with group addressing.

In the GSM systems, which are 8th order TDMA type systems, on a physical signalling channel defined by a particular frequency associated with each base station and called the beacon frequency, logical signalling channels, called common control channels (CCCH) , are distinguished in particular. These logical channels are subdivisions of the beacon channel obtained by time- division multiplexing. It should be noted that the beacon frequency is not subject to frequency hopping, when this feature, which is provided for in the GSM standard, is implemented.

The CCCH channels comprise in particular: - a random access channel (RACH) , defined on the uplink beacon channel, on which a mobile terminal sends a message defining a random access request when .it needs to perform an operation on the network (to initiate a call, send a short message, update location data, etc.) . The RACH channel is used by the mobile terminals to report their presence to the network;

- an access grant channel (AGCH) , defined on the downlink beacon channel, on which the network transmits a response to a random access request from a mobile terminal. This response can contain a channel allocation message containing the description of a dedicated channel which is allocated to that mobile terminal in order to set up the call;

- a paging channel (PCH) , defined on the downlink beacon channel, on which the network broadcasts one or more messages containing one or more identities relating to one or more mobile terminals with which it needs to communicate (to transmit an incoming call, a short message, an authentication, etc.) . In GSM, the mobile terminal recognizing itself in an identity broadcast on the PCH channel then responds with a random access request on the abovementioned RACH channel;

- an optional channel for notifying group calls and broadcast mode calls, called a notification channel

(NCH) defined on the downlink beacon channel, on which the network broadcasts one or more messages containing information on the group calls and the broadcast mode calls accessible in the cell.

The information transmitted on these channels is transmitted with a certain repetitiveness.

The signal element transmitted within a slot is called a "burst" in the jargon of those skilled in the art. A burst is made up of groups of symbols. The length of a normal burst (that is, occupying the entire duration of the slot) corresponds to 156.25 bits.

On the PCH channels, one or more paging messages can be sent in four normal bursts. A paging message contains one or more identities relating to one or more called mobile terminals. In the case of the GSM or GPRS, these are, among other things, the IMSI identity of a subscriber, the temporary identity TMSI or P-TMSI of a subscriber, the IMEI or IMEISV identity of the equipment; in the case of TETRA, they are, among other things, the individual identity ITSI of the subscriber or the GTSI identity of a group of subscribers, or a short identity of the subscriber (ISSI) or of the group (GSSI) in the cell, or a temporary alias address ASSI.

The NCH channel operates like the PCH channel, but in relation to calls instead of mobile terminals.

As is illustrated in the diagram in Figure 1 (which is reproduced in the standard GSM 05.02), a random access request message is transmitted On the RACH channel in a single burst. Furthermore, the burst used on the RACH channel is shorter than the normal burst. In practice, the propagation time between the mobile terminal which transmits on the RACH channel and the base station depends on the distance to the mobile terminal. Now, it is necessary for the transmitted burst to be contained entirely within a slot of the base station without overlapping adjacent slots, because the propagation time is not yet compensated. The random access message comprises an 8-bit header, a 41-bit synchronization sequence, 36 encoded useful data bits, and three tail bits. In practice, the message contains only eight useful data bits which are extended to 36 encoded bits.

The diagram in Figure 2 shows the useful content of the channel access request message, that is, the abovementioned 8 uncoded bits. This diagram is reproduced in the standard GSM 04.08. As can be seen, the request contains between 2 and 5 bits which encode a random number. These are low order bits. Moreover, it contains between 3 and 6 bits which represent the requested service category. These are highest order bits.

It should be noted that in other cellular mobile radio systems, in particular in the TETRA system mentioned in the introduction, the random access request message contains bits encoding the identity of the mobile terminal instead of or in addition to the requested service category. It will be noted that the service category includes encrypted services and unencrypted services. It will be noted that, in PMR in particular, besides the individual identities of the mobile terminals, mobile terminal group identities need to be considered. It will be noted that an identity can refer, explicitly or implicitly, to hardware equipment, or to a subscriber, or to any subscriber from a group of subscribers, or to any hardware equipment out of a group of mobile terminal equipments, or to an "alias" indirectly addressing the identity of a mobile terminal or of a group of mobile terminals.

On the AGCH channel, a channel allocation message is sent on four normal bursts in GSM.

Figure 3 diagrammatically represents an example of a mobile radio system in which the procedure and the device according to the invention are applied.

This figure shows a cell 10 of a cellular mobile radio system, which can be a system conforming to the GSM standard. The cell 10 is covered by a base station 11. It is assumed that there is an area 12 included in the cell 10, in which the operation of the mobile terminals is to be prevented, except for certain identified terminals (for example incoming calls to the terminal of a defined manager or calls between the terminals of a defined group of first aiders or any person authorized to receive and/or make calls in the area 12), and/or for certain predefined communication services (for example, emergency calls from mobile terminals present in the area 12 or location updates for these terminals) .

The area 12 is for example an auditorium (theatre, cinema, etc.), a restaurant, a library, a religious establishment, etc. It will be in particular any building or location where it is desirable to prevent the sound nuisances occasioned by the use of mobile telephones. By extension, the area 12 can be any area where the making of calls is prohibited or undesirable, for example, a prison, a hospital, a confidential meeting room, a vehicle, the flight deck of an aircraft, etc.

A device 13 for jamming calls according to the invention is arranged inside the area 12. As will be detailed later, the device 13 comprises means for transmitting a jamming signal in the area 12, the effect of which is to selectively prevent the use of mobile terminals such as MT4 or MT5 located in the area 12, but not the use of those such as MTl to MT3 which are located outside the area 12.

Figure 4 shows a block diagram of a device according to the invention, such as the device 13 in Figure 3.

The device 13 comprises a receive antenna 41 and a radio receiver 42, to receive a radio signal S. It also comprises a module 43 for processing the received radio signal. The radio receiver 42 performs filtering, amplification, reception and transposition operations, which are conventional in such a receiver. The processing module 43 uses the received signal to perform the frequency and time synchronization, demodulation, any differential decoding, any demultiplexing, any dejamming, de-interleaving and channel decoding operations and in general all operations needed to recover the useful information conveyed by the received signal, and interpret it according to the specifications of the system.

The device 13 also includes a processing module 44 the function of which is to control the transmission of the jamming signal and to control the stopping of this transmission according to the invention.

The device 13 also includes a module 45 for processing the data transmitted in a jamming signal, which performs on the data to be transmitted the reciprocal operations of those performed by the processing module 43. The output of the module 45 delivers a signal, which is supplied as input to a radio transmitter 46. This in particular transposes the signal to be transmitted into the radio frequencies and, more specifically, onto the frequency of the beacon channel to be jammed. The output of the transmitter 46 drives an antenna 47 for transmitting the jamming signal B.

According to the invention, the transmitter 46 is a transmitter controlled by a signal ES controlling the transmission of the jamming signal B, generated by the processing module 44. The latter switches the signal ES to a first defined state (for example, the high logical state) to generate a command to send the jamming signal, and switches it to a second defined state (for example the low logical state) to generate a command to stop transmission of the jamming signal. When it determines a condition for stopping transmission of the jamming signal from the information contained in the received signal, the processing module 44 deactivates the transmitter 46 via the signal ES. In one example, the signal ES acts on a variable gain power amplifier, by cancelling the gain of this amplifier when the signal ES is in a defined state, corresponding to the command to stop transmission of the jamming signal.

According to the embodiments, the signal B can be transmitted at the frequency of the downlink beacon channel or that of the uplink beacon channel.

In the first case, a check is made to ensure that the jamming signal B is effective only within the area to be jammed and not outside it. This result is achieved by applying conventional radio engineering techniques, which are within the scope of those skilled in the art

(calculating the transmit power of the jamming signal

B, choosing and positioning the antenna 47 or a set of antennas such as 47 so that the jamming signal B covers all of the area 13 and not beyond it) .

In the second case, the transmission of the jamming signal B by the transmitter 46 is not permanent. It is controlled on receipt by the device 13 of a useful signal in a time window beginning at the start of a theoretical reception window of the logical channel concerned and having a duration dependent on the extent of said predefined geographic area. In other words, in the example, the control signal ES switches to the high logical state, which constitutes a command to transmit the jamming signal B, when the device receives a useful signal (that is, a signal other than noise) on the logical channel concerned, and this provided that the delay between a theoretical instant for receiving a burst on the jammed channel and an actual instant for receiving that burst is greater than a predefined time. Given the propagation delay of a burst, this can be used to limit the jamming to messages transmitted by mobile terminals geographically close to the jamming device 13. Adjusting the parameter comprising said delay adjusts the extent of the geographic area covered by the jamming device.

Naturally, the device 13 also comprises synchronization means, represented here by a module 48, for synchronizing the operation of the processing modules 43, 44 and 45. The function of the synchronization module 48 is to handle synchronization of these processing modules between themselves, and with the signals transmitted by the base station on the downlink beacon channel.

In one embodiment, the transmit power of the jamming signal B is controlled according to the strength of the radio signal received on the downlink beacon channel.

To this end, the radio receiver 42 generates an RPS signal indicating the power level of the received signal S. The RPS signal is supplied as input to the processing module 44, which consequently generates a signal EPCS controlling the power of the jamming signal

B. The EPCS signal is supplied as input to the radio transmitter 46. In one example, this signal acts on the gain control input of the abovementioned power amplifier.

In one embodiment, the modulation of the jamming signal conforms to the transmission mask of the cellular mobile radio system concerned, to comply with the rules of coexistence with the adjacent channels.

In one embodiment, the modulation of the jamming signal B is the same as that of the radio signal S received. To this end, the frequency of the radio signal S received or the frequency of the jamming signal B to be transmitted implicitly characterizes the modulation to be used by the processing module 45 to generate the jamming signal B.

In one embodiment, the modulation of the jamming signal

B used by the processing module 45 is GMSK modulation.

The antennas 41 and 47 can each form an antenna system suited to the radio engineering of the area covered by the jamming device (area to be jammed), comprising an aerial antenna or a plurality of aerial antennas, and/or a radiating cable or a plurality or radiating cables.

In one embodiment, however, the jamming signal B is sent, either towards the base station if the logical signalling channel which is jammed is defined on the uplink beacon channel, or towards the area 13 if said channel is defined on the downlink beacon channel. The antenna 47 is then a directional antenna whereas the antenna 41 is usually an omnidirectional antenna. The two antennas 41 and 47 can also be directional antennas, according to the radio engineering constraints in the area to be jammed 13.

The processing modules 43, 44 and 45 can be produced in the form of software which is run on a computer of the device (for example a microcontroller or a microprocessor) . The procedure according to the invention is run under the control of the processing means 44.

The invention proposes various embodiments exploiting the fact that the information messages needed for a resource access procedure which precedes the setting up of a call are transmitted on the logical signalling channels such as the PCH, NCH, RACH and AGCH channels.

These messages contain information relating to one or more mobile terminal identities involved in the call and/or to a service category to which the call corresponds.

The general principle of the invention is as follows: a logical signalling channel, on which such messages are transmitted, is jammed by a jamming signal which is transmitted in the frequency and/or time domains defining this channel, either permanently (once the jamming device has been initialized) or, when this channel is set up on the uplink beacon channel, only when the abovementioned condition is satisfied. Moreover, a command is given to stop transmission of the jamming signal if the transmission is detected, on this logical signalling channel or on another logical signalling channel, of predefined information which is sent by the network or by a mobile terminal, in the context of said resource access procedure, with a view to setting up a call and/or participating in an already established group call. This predefined information relates for example to the identity of a mobile terminal involved in the call, to the identity of a group of mobile terminals and/or to the service category to which the call corresponds.

Because of the repetitiveness of the transmission of said information, it is certain that information messages needed for said resource allocation procedure will be repeated after the transmission of the jamming signal has been stopped. The resource allocation procedure can then be conducted normally, that is, in the absence of jamming.

If seeking to detect said predefined information on the logical signalling channel that is jammed, this jamming can be partial, in the sense that it is enough to prevent the resource access procedure from being run, but that it must not prevent the reconstruction and detection of said information by the device.

The flow diagrams in Figures 5a and 5b illustrate the main steps of the method according to the invention. These steps are divided into two groups of steps applied in parallel by the device 13, and respectively illustrated in Figure 5a and in Figure 5b.

Thus, Figure 5a represents the steps concerning the generation of the command to stop transmission of the jamming signal B, on the assumption that the command to transmit this signal has previously been generated so that this signal is sent.

In a step 51, the radio receiver 42 receives the signal S originating from the base station covering said cell

(if the signal S is the radio signal on the downlink beacon channel associated with the cell 10) or from a mobile terminal located in the jammed area 12 (if the signal S is the radio signal on the uplink beacon channel associated with the cell 10) . As will be explained later through examples, certain time and/or frequency domains of the signal S define a logical signalling channel respectively of said downlink beacon channel or of said uplink beacon channel. It is a logical signalling channel on which information messages required for a resource access procedure, which precedes the setting up of a call, are transmitted with a certain repetitiveness.

In a step symbolized here by a test 52, the processing module 44 scans said logical signalling channel, to check whether it contains predefined information. If such information transmitted in this logical signalling channel is detected, the processing module 44 generates a command to stop transmission of the jamming signal B. In the example, this command is reflected by the transition of the signal ES from the low logical state to the high logical state.

The steps 51 to 53 are repeated on each occurrence of the logical signalling channel concerned, which is symbolized here by the fact that the step 53 and the negative conditional output of the test 52 loop to step 51.

Similarly, Figure 5b shows the steps relating to the controlled transmission of the jamming signal B.

In a step 54, the jamming signal is generated by the processing module 45, under the control of the processing module 44, when the signal B contains useful information (that is, other than noise) , or directly by the radio transmitter 46 when it is a radio signal which carries no useful information (that is, when it is only radio frequency power radiated at the frequency of the beacon channel concerned) .

If the command to stop jamming is not generated (i.e., if the signal ES is in the low logical state) , then, in a step 56, the jamming signal B is transmitted by the controlled transmitter 46. This condition is represented by the test 55. As will be explained later, the jamming signal B is used to jam at least partially a logical signalling channel defined on the uplink beacon channel or on the downlink beacon channel respectively associated with the cell 10. If, however, the command to stop jamming is generated (i.e., if the signal ES is in the high logical state) , the jamming signal B is not transmitted.

The steps 54 to 56 are repeated permanently, which is symbolized here by the fact that the step 56 and the positive conditional output of the test 55 loop to step 54. Furthermore, they are executed in parallel with the execution of the steps 51 to 53.

As will now emerge more clearly in light of exemplary embodiments of the method, the jamming signal B is designed to jam certain time and/or frequency domains of the beacon channel which define a first logical signalling channel. Furthermore, the processing means 44 are programmed to scan certain time and/or frequency domains of the beacon channel which define a second logical signalling channel.

First embodiment

According to a first embodiment, said first and second logical signalling channels are one and the same logical signalling channel. In this case, this logical signalling channel is only partially jammed by the jamming signal B so as to allow predefined information that conditions the generation of the signal ES to be detected.

More specifically, in this first embodiment, the logical signalling channel concerned is a PCH channel (paging channel) defined on the downlink beacon channel. In other words, the signal S corresponds to the downlink beacon channel. This PCH channel is structured as repetitive frames within each of which the base station can transmit one or more identities relating to one or more mobile terminals. An identity relating to one or more mobile terminals corresponds, for example, to one or more subscribers using mobile terminals, one or more groups of subscribers using mobile terminals, one or more mobile terminal equipment items. The predefined information that conditions the generation of the signal ES relates to the identity of a predefined mobile terminal or of a number of predefined mobile terminals. In other words, the command to stop transmission of the jamming signal B is generated when the identity of said predefined mobile terminal is detected or when at least one defined identity (relating to one or more authorized mobile terminals) is detected. The identities relating to one or more mobile terminals concerned are for example stored in a memory of the processing means 44 of the device 13.

In a variant of this first embodiment, the logical signalling channel concerned is an NCH channel (notification channel) defined on the downlink beacon channel. In this variant, the signal S also corresponds to the downlink beacon channel. This NCH channel is structured as repetitive frames within each of which the base station can transmit one or more identities relating to one or more group calls. The predefined information that conditions the generation of the signal ES relates to the identity relating to a group call. In other words, the command to stop transmission of the jamming signal B is generated when said identity- is detected or when at least a predefined identity (relating to one or more defined calls) is detected. The identities relating to the or each call concerned are for example stored in a memory of the processing means 44 of the device 13.

A frame of the PCH or NCH logical channel is transmitted on N bursts of rank 1 to N respectively, and the defined identity is repeated at least M+l times in M+l respective identical frames of the paging channel, where N and M are integer numbers strictly greater than unity. According to the GSM standard, N is equal to 4.

The jamming signal B is sent at instants and for time periods defined so that, on the one hand, each of said first M frames includes jammed bursts and non-jammed bursts, and on the other hand, that the distribution of the jammed bursts and of the non-jammed bursts in the frame is different for each of said M frames. More specifically, this distribution is such that:

- each of the bursts of the frame is transmitted without being jammed in at least one of said M frames; - said M frames can be associated in pairs by the fact that they respectively include at least one non- jammed burst of the same rank.

The bursts of the frames with a number greater than M are jammed or not, depending on whether the command to stop transmission of the jamming signal is generated or not.

In one example illustrated by the diagram in Figure 6, the bursts of rank 1 and 2 are jammed on a first occurrence of the frame on the PCH or NCH logical channel, the bursts of rank 2 and 3 are jammed on a second occurrence of the frame on said channel, and the bursts of rank 3 and 4 are jammed on a third occurrence of the frame on said channel. In Figure 6, each frame is represented by a rectangle made up of four squares, each square symbolizing a burst of the frame. In each frame, the jammed bursts are shaded. The rectangles are shown separated to symbolize the fact that said channel can be multiplexed with other logical channels on the downlink beacon channel.

The frames represented are denoted PCH (Mi, j) where Mi designates a defined terminal which is called by the base station, and where the number j designates the occurrence of the frame of the PCH channel concerning this mobile terminal.

Thus, the following are distinguished, in this order:

- a frame PCH(Ml,!), which is the first PCH frame transmitted by the base station to call the mobile terminal Ml;

- a frame PCH(M2,1), which is the first PCH frame transmitted by the base station to call another mobile terminal M2;

- a frame PCH (Ml,2), which is the second PCH frame transmitted by the base station to call the mobile terminal Ml; - a frame PCH(M3,1), which is the first PCH frame transmitted by the base station to call another mobile terminal M3;

- a frame PCH(M4,1), which is the first PCH frame transmitted by the base station to call another mobile terminal M4;

- a frame PCH (Ml, 3), which is the third PCH frame transmitted by the base station to call the mobile terminal Ml;

- a frame PCH (M3,2), which is the second PCH frame transmitted by the base station to call the mobile terminal M3; - a frame PCH (Ml, 4), which is the fourth PCH frame transmitted by the base station to call the mobile terminal Ml; etc.

In this example, it can be seen that the frames PCH(Ml, 1), PCH(M3,1) and PCH (M3,2) are partially jammed and, more specifically, that they are jammed during a time window corresponding to the transmission of the two bursts of rank 1 and 2 in the frame. Similarly, the frames PCH(M2,1), PCH(M4,1) and PCH (Ml, 4) are partially jammed, and, more specifically, they are jammed during a time window corresponding to the transmission of the two bursts of rank 2 and 3 in the frame. Finally, the frames PCH (Ml, 2) and PCH (Ml, 3) are partially jammed, and, more specifically, they are jammed during a time window corresponding to the transmission of the two bursts of rank 3 and 4 in the frame.

The device 13 stores the PCH frames that it receives sequentially. Since all the frames are partially jammed, no mobile terminal present in the area 12 can identify its identity. Consequently, none of them, even if its identity is contained in a frame transmitted by the base station, will recognize itself and distinguish itself by responding by transmitting an RACH frame. Consequently, no call will be set up with these terminals.

Nevertheless, the device 13 can identify the identities transmitted in the frames by proceeding as follows. It stores these frames in an appropriate memory. When it receives a new frame, it compares it with all of those already stored with a view to associating them one with the other to reconstitute a non-jammed frame containing a same mobile terminal identity from the jammed frames containing this identity. To this end, two frames are associated if they contain at least one burst of the same rank that is not jammed. Thus, for example, consider what happens as the frames containing the identity of the terminal Ml are received. The frame PCH(Ml,2) cannot be associated with the frame PCH (Ml, 1) already received, although they contain the same mobile terminal identity, because they do not have any burst of the same rank in common. Similarly, the frame PCH(Ml, 3) cannot be associated with the frame PCH (Ml, 1) already received. However, it can be with the frame PCH(Ml,2) because they have the bursts of rank 1 and 2 in common. At this stage, the device 13 cannot however reconstitute the non-jammed frame completely. When the frame PCH (Ml, 4) is received, it is associated with the frame PCH (Ml, 1) because they have the burst of rank 4 in common, and with the frames PCH (Ml, 2) and PCH(Ml, 3) because they have the burst of rank 1 in common.

In all of these four frames duly associated in pairs, the device thus has the four bursts ranked 1 to 4, non- jammed, forming a PCH frame containing the identity of one and only one mobile terminal. It is therefore capable of reconstituting this frame and deducing said identity from it. If this identity corresponds to that of a terminal which needs to be able to communicate with the network (and stored in the abovementioned memory) , the device then generates the command to stop transmission of the jamming signal B, by switching the signal ES to the low logical state for subsequent occurrences of the paging.

Through this example, it can be seen that the minimum number M of occurrences of the frame containing the identity of a mobile terminal to allow the generation of the command to stop jamming is equal to 3 (M = 3) . It will, however, be noted that it is then essential for the same frame to be transmitted again at least once in the absence of jamming, so that the resource access procedure can be completed. This is why the frame containing the identity of the mobile terminal should in fact be sent at least M+l times, with in this case M+l equal to 4 (M+l=4) .

In the preceding example, the entity on which the device 13 operates to associate the frames and reconstitute a non-jammed frame is the burst. In a variant, it can, however, operate on the symbols that each burst contains.

According to this variant, the jamming signal is sent in such a way that each of the M frames transmitted respectively on the first M occurrences of the frame is jammed for a duration corresponding to α.N times the transmission duration of a burst, where α is a real number between zero and unity (0 < a < 1) .

The distribution of the jammed symbols and of the non- jammed symbols differs from one occurrence of the frame to the next, with a certain periodicity corresponding to P frames where P is an integer number strictly greater than unity.

This distribution is such that:

- in each of said M frames, the cardinal number of the set of jammed symbols is sufficiently high for the mobile terminals not to be able to decode the frame. This is translated by a minimum value of α for a given radio technology;

- in said M frames, the cardinal number of the set of non-jammed symbols is sufficiently high to allow the frame to be decoded, taking into account the channel decoding. This enables the jamming device to have sufficient correct symbols on the M occurrences of the frame to decode the message according to the channel decoding of the technology concerned;

- said M frames can be associated in pairs by the fact that they include non-jammed symbols at identical positions in the frame.

The bursts of frames with a number greater than M are jammed or not, taking into account the discrimination applied by the information recovered above.

It should be noted that a weaker condition than the first two conditions would be that each of the symbols of the frame is transmitted without being jammed in at least one of said M frames.

This variant is illustrated by the diagrams in Figures 7a to 7h for a PCH channel. Equivalent diagrams can be derived from these for an NCH channel.

Figure 7a shows the N bursts that make up a PCH frame (with N=4) .

In the example illustrated by these figures, the number P is equal to 6. Figure 7b shows the jamming window, that is, the time window within which the jamming signal is sent, for a first occurrence of this frame. In the example, the start of said window is aligned on the start of the first burst (the leftmost) . Figures 7c to 7g show the jamming window respectively for the second, third, fourth, fifth and sixth occurrences of the frame. Because of the periodicity, the jamming window for the seventh occurrence of the frame is that shown in Figure 7b, and so on.

In the example represented, on each occurrence of the frame other than the sixth, the jamming window is offset in the same direction by a duration corresponding to an eighth of the frame duration (that is, also to half of the duration of a burst) . However, on each occurrence, the frame is jammed for a fraction of the time equal to α*N times the duration of a frame, where α is equal to 3/4 (α = 3/4) . Furthermore, the frames transmitted on each occurrence can be associated in pairs by the fact that they contain non-jammed symbols at identical positions in the frame. For example, these are the symbols transmitted during the time window T (2, 3) which can be seen in Figure 7h, for the frames transmitted on the second and on the third occurrences. Similarly, they are the symbols transmitted during the time window T (3, 4) for the frames transmitted on the third and on the fourth occurrences. There again, they are the symbols transmitted during the time window T (4, 5) for the frames transmitted on the fourth and on the fifth occurrences. And finally, they are the symbols transmitted during the time window T (1,2) for the frames transmitted on the first and on the second occurrences.

Because of the channel encoding, it should be noted that it is enough to reconstitute a sufficient portion of the burst for the demodulated and de-interleaved bursts to be decodable. It follows that the symbols transmitted in a time window of duration To can be jammed on each of the P occurrences, without that making it impossible to decode the frame. In other words, it is not essential for each symbol to be transmitted at least once without being jammed. This enables the minimum value of M to be reduced. Thus, the symbols transmitted in the time window designated To in Figure 7g (which, in the example, coincides with the burst number 3) are transmitted while being jammed on each occurrence of the frame.

Naturally, the width of the jamming window, its initial alignment (for the first occurrence of the frame) and its offset from one occurrence of the frame to the next are not limited to the examples given above. Those skilled in the art understand that the choice of these parameters will condition, in particular, the value of the minimum number M of occurrences in the frame to be able to reconstitute a complete non-jammed frame from the partially jammed frames.

It will be noted that the embodiment according to Figure 6, which is particularly suited to the case of a GSM system, is identified with the variant according to Figures 7a to Ih, by taking N = 4, P = 3, M = 3 and α=l/2. In this case, the bursts of rank 1 and 2 are jammed every (n+l)th occurrence of the frame, the bursts of rank 2 and 3 are jammed every (n+2)th occurrence of the frame, and the bursts of rank 3 and 4 are jammed every (n+3)th occurrence of the frame, where n is any integer number.

As will have been understood, this first embodiment can be used to make the jamming selective, by conditioning the stoppage of the jamming on the fact that the network is trying to call one or more authorized mobile terminals. This safeguards the possibility of being able to set up a call with these authorized terminals, in particular on incoming calls intended for them. These authorized mobile terminals are for example those of doctors, or of any person for whom it is officially recognized that they need to be able to be called at all times and in all circumstances.

Transposing the explanation above concerning the PCH ■ channel to the NCH channel takes no time at all for a person skilled in the art: the only difference is the semantic of the identities, and the content of the predefined information that conditions the generation of the command to stop transmission of the jamming signal. In particular, the NCH channel is structured as repetitive frames within which the base station transmits one or more identities relating to one or more respective calls.

Second embodiment According to a second embodiment, said first and second logical signalling channels are also one and the same logical signalling channel. In the same way as in the first embodiment described above, this logical signalling channel is only partially jammed by the jamming signal B so as to allow the predefined information that conditions the generation of the signal ES to be detected.

More specifically, the logical signalling channel concerned is here an RACH channel (random access channel) defined on the uplink beacon channel, according to a permanent recurrence and/or by dynamic allocation. In other words, the signal S corresponds to the uplink beacon channel.

Because, in this second embodiment, the logical signalling channel is a channel defined on the uplink beacon channel, the jamming signal B is transmitted towards the base station. The radiofrequency power transmitted by this signal cannot therefore be contained within the area to be jammed 12. It is therefore essential to provide additional measures to prevent the jamming signal B from disturbing transmissions on this -logical signalling channel which originate from mobile terminals located outside the area to be jammed 12. In an example, such a measure is embodied in the fact that the transmission of the jamming signal by the controlled radio transmitter 46 of the jamming device 13, is conditional on the reception of a useful signal in a time window beginning at the theoretical moment of reception of the corresponding burst by the reception device and having a duration dependent on the extent of the area 12 to be jammed. This enables the effects of the jamming to be limited to transmissions from mobile terminals geographically close to the radio receiver 42 of the jamming device 13. The concept of theoretical or actual moment of reception of a burst should be understood to mean the start the time window within which this burst should theoretically be received, or is actually- received.

In the case of the GSM, the RACH channel is structured as repetitive frames within each of which the mobile terminals can transmit a random access request containing the first bits and, in addition, second bits. The first bits encode service information (synchronization, start bits, end bits, channel encoding, etc.) . The second bits encode the requested service category. In other systems, such as the TETRA system in particular, the identity of the mobile terminal that transmits this request is encoded by said second bits, in addition to or in place of the requested service category.

In this second embodiment, the predefined information that conditions the generation of the command to stop transmission of the jamming signal B by the controlled radio transmitter 46 of the device 13, relates to at least one of a plurality of predefined service categories and/or the identity of at least one mobile terminal. Thus, the stop command is generated when a random access request corresponding to said service category and/or to the identity of said mobile terminal is detected.

Said plurality of predefined service categories can include the emergency call category. Similarly, it can include the location update category. This enables, for example, the network operator to be able to locate the mobile terminals that are located in the area to be jammed 12, despite the fact that calls with these terminals are jammed.

Preferably, the jamming signal is sent in such a way that at least some of said first bits of the random access request are jammed and that said second bits of the random access request are not jammed.

In the example of the GSM, said second bits which are not jammed are the bits referenced d(6) to d(8) in the standard GSM 05.03. In practice, it was seen above that the requested service category is encoded by at most bits 3 to 8 of the unencoded random access request message (see Figure 2 described above) . After encoding, some of these bits correspond to the bits referenced e(ll) to e(17) in the standard GSM 05.03, these bits being included in the 36 encoded bits of the RACH burst

(see Figure 1 described above) .

Similarly, said first bits which are jammed are the bits referenced d(l) and d(2) in the standard GSM 05.03 which, alone or in combination with others from the bits d(3) to d(5), encode a random number of the unencoded random access request message (see Figure 2 described above, where these bits correspond to bits 1 and 2) . After encoding, these bits correspond to the bits referenced e(0) to e(5) in the standard GSM 05.03, these bits being included in the 36 encoded bits of the RACH burst (see Figure 1 described above) .

Naturally, the above example is by no means limiting. What matters, is to jam enough bits for the RACH request not to be recognized by the base station, but to retain enough non-jammed bits to allow the device 13 of the invention to determine whether the condition to stop transmission of the jamming signal is satisfied.

As will have been understood, this second embodiment can be used to make the jamming selective, by conditioning the stopping of the jamming for the subsequent frames of the RACH channel on the fact that a mobile terminal present in the area to be jammed 12 is trying to send a resource allocation request for a predefined service (for example, setting up a call for an emergency call) , and/or that an authorized mobile terminal present in the area to be jammed 12 is trying to obtain from the network the resource allocation to set up a call (outgoing call) .

Third embodiment

According to a third embodiment, said second logical signalling channel is a random access channel (RACH) defined on the uplink beacon channel which is not jammed by the jamming signal, whereas said first logical signalling channel is an access grant channel defined on the downlink beacon channel (AGCH) which is at least partially jammed by the jamming signal. In other words, the signal S corresponds to the uplink beacon channel, and the signal B is sent on the frequency of the downlink beacon channel. It should be noted that what is important is that the AGCH channel is ^"a response channel to a random access request, that is that its jamming, at least partial, prevents the mobile terminal from interpreting it as being the response to its random access request. In practice, the mobile terminal then believes, wrongly, that its random access request has not been received by the base station.

The RACH channel is structured as repetitive frames within each of which the mobile terminals can transmit a random access request containing bits encoding the category of the requested service category and/or the identity of the mobile terminal. The statement above

(for the second embodiment) , concerning the information contained in the random access request, also applies to this third embodiment.

Similarly, the statement concerning the predefined information that conditions the generation of the signal to stop transmission of the jamming signal, also applies to this third embodiment. In practice, this third embodiment is very similar to the preceding one. The only difference lies in the fact that the jamming signal B does not affect the RACH channel defined on the uplink beacon channel, but affects the AGCH channel defined on the downlink beacon channel. This is simpler because the AGCH channel can be entirely jammed, whereas, since the RACH channel is not jammed, the detection of the predefined information conditioning the generation of the command to stop the jamming is immediate. Furthermore, since the jammed channel is a logical channel on the downlink beacon channel, it is easier to prevent unwanted jamming outside the area to be jammed 12. The device for receiving the signal S must be dimensioned as accurately as possible so as to receive only the RACHs sent in the jamming area 13. In practice, the transmit power and/or the directivity of the jamming signal B should simply be chosen such that the effect of the jamming remains contained within the area to be jammed 12, which follows from the application of the radio engineering rules that are familiar to those skilled in the art.

Other embodiments

The first, second and/or third embodiments described above can naturally be combined, according to requirements.

Concerning the second and third embodiments, the refinements below can also be provided.

Preferably, when a random access request corresponding to said predefined service category and/or to the identity of an authorized mobile terminal is ^' detected on the RACH channel, the command to stop transmission of the jamming signal is generated after the expiry of a time delay roughly corresponding to the time interval between two repetitions of the random access request, and for a duration corresponding to at least the transmission duration of that request. This time interval and this duration are parameters specific to the mobile radio system. For the GSM, they are indicated in the corresponding GSM standards.

Thus, the jamming is maintained for as long as possible, so as to affect all the mobile terminals present in the area to be jammed 12 without however affecting the resource allocation procedure for the mobile terminal having sent a request satisfying the condition to stop jamming.

In certain circumstances, for example linked to the requirements of the forces of law and order, it is also desirable for the jamming to be discrete, that is, for the user not to be aware of the existence of the jamming.

Thus, for example, it may be useful to prevent one or more mobile terminals from receiving calls, or from receiving and making calls, while giving the owner of this terminal the impression that the terminal is operating normally, in particular by allowing location updates for this terminal, so that the screen display on the terminal retains the same appearance as it would in the absence of jamming.

In this case, the jamming signal can contain a dummy random access request. To this end, the processing module 44 generates a dummy random access request message, which is processed by the module 45 and transmitted by the transmitter 46 on the RACH channel as a jamming signal.

Similarly, collisions are generated on the RACH channel, which simulates a contention situation which can occur naturally in the system when a lot of terminals in the cell are requesting resource allocation. From the point of view of the user of a terminal located in the area to be jammed 12, everything proceeds as if there were no channel available on the network. The user cannot recognize that the inability to obtain the resource allocation is the result in fact of calls being jammed in the area where he or she is located.

According to another advantage, since the jamming signal is of the same type (same radio transmission mask and same modulation) as the signals transmitted by the system, it necessarily respects the regulatory clauses on coexistence between radio systems (such as the European standard ETS 300 113 in PMR) . In practice, it is certain that any interference generated by the harmonics of the jamming signal located outside the band ^■ to be jammed respects these regulatory clauses, provided that -the mobile radio system respects them.

As a variant, the processing module 44 ensures that the jamming signal transmitted contains a synchronization sequence of the same type as a synchronization sequence for a random access request (see Figure 1) . The result is then an effect similar to that described in the preceding paragraph.

In other circumstances, it is necessary to allow a government or private spectrum monitoring entity to differentiate the jamming sources within a predefined area to decide on whether they are legal or not.

In this case, the jamming signal can contain an identification code used to identify the origin of the jamming. This identification code can be read for example by a spectrum monitoring system.

In another mode of this case, the jamming signal can contain an identification and authentication code used to identify and authenticate the origin of the jamming at a given transmission instant. The authentication can, for example, be provided by a signature associated with the identification of the jammer, generated by the processing module 45, according ^■ to a procedure conforming to the state of the art. The authentication procedure can depend also on parameters transmitted by the synchronization module 48 to the processing module 45, to avoid the reuse by an illegal jammer of a code previously transmitted by a legal jammer. This identification and authentication code can be read for example by a spectrum monitoring system.

The identification code or the identification and authentication code can be sent in the jamming signal either permanently, or periodically, or according to predefined circumstances. In the case of a non- permanent transmission, it can be sent in synchronism with radio or protocol events on the received signal S (for example, the occurrence on the frequency of the beacon channel of a frame of a logical channel or of a particular burst of a logical channel, for example the PCH, the AGCH or the RACH) , in other words, in synchronism with the reference clock of the system. As a variant, it can be sent in synchronism with a local reference clock of the jamming device 13, which in turn can be indirectly synchronized with the clock of a spectrum monitoring system.

In the case of the GSM, the jamming signal can, for example, have the same structure as the random access burst on the RACH channel (see Figure 1) and therefore carry an identification code or an identification and authentication code encoded on 36 bits.

Claims

1. Method of jamming calls in a predefined geographic area (12) included in a cell (10) of a cellular mobile radio system, comprising steps of :

- generating (54) a jamming signal (B);

- sending (56) said jamming signal using controlled transmission means, so as to jam at least partially the frequency and/or time domains defining a first logical signalling channel defined on an uplink beacon channel or on a downlink beacon channel respectively- associated with said cell;

- receiving (51) a radio signal (S) from the base station covering said cell or from a mobile terminal located in said geographic area, for which certain time and/or frequency domains define a second logical signalling channel on the respectively downlink or uplink beacon channel associated with said cell, on which information messages required for a resource access procedure, which precedes the setting up of a call, are transmitted with a certain repetitiveness; - scanning said second logical signalling channel; and

- in the case of detection (52) of predefined information transmitted in said second logical signalling channel, generating (53) a command (ES) to stop the transmission of the jamming signal by said controlled transmission means.

2. Method according to Claim 1, wherein said first and second logical signalling channels are one and the same logical signalling channel, and wherein this logical signalling channel is only partially jammed by the jamming signal so as to allow said predefined information to be detected.

3. Method according to Claim 2, wherein said logical signalling channel is a paging channel defined on the downlink beacon channel, structured as repetitive frames within each of which the base station can transmit one or more identities relating to one or more respective mobile terminals, and wherein said predefined information relates to one or more predefined identities such that said stop command is generated when at least one of said predefined identities is detected.

4. Method according to Claim 2, wherein said logical signalling channel is a notification channel defined on the downlink beacon channel, structured as repetitive frames within each of which the base station can transmit one or more identities relating to one or more respective calls, and wherein said predefined information relates to one or more predefined identities such that said stop command is generated when at least one of said predefined identities is detected.

5. Method according to any of Claims 2 to 4, wherein, a frame of said logical signalling channel being transmitted on N bursts each comprising symbols, and said predefined identity being repeated at least M+l times in M+l respective identical frames of said channel, where N and M are integer numbers strictly greater than unity, the jamming signal is sent in such a way that each of the M frames transmitted respectively on the first M occurrences of the frame is jammed for a duration corresponding to α.N times the transmission duration of a burst, where α is a real number between zero and unity (0 < α < 1), according to which the distribution of the jammed symbols and of the non-jammed symbols in a frame differs from one occurrence of the frame to the next, with a certain periodicity corresponding to P frames where P is an integer number strictly- greater than unity, and is such that for each of said M frames:

- in each of said M frames, the cardinal number of the set of jammed symbols is sufficiently high for the mobile terminals not to be able to decode the frame;

- in said M frames, the cardinal number of the set of non-jammed symbols is sufficiently high to allow the frame to be decoded, taking into account the channel decoding;

6. Method according to Claim 5, wherein each of the bursts N of the frame is transmitted without being jammed in at least one of said M frames.

7. Method according to Claim 6, wherein N = 4, P = 3, M = 3 and α = 1/2.

8. Method according to Claim 7, wherein the bursts of rank 1 and 2 are jammed every (n+l)th occurrence of the frame, the bursts of rank 2 and 3 are jammed every (n+2)th occurrence of the frame, and the bursts of rank 3 and 4 are jammed every (n+3)th occurrence of the frame, where n is an integer number.

9. Method according to Claim 2, wherein the logical signalling channel is a channel defined on the uplink beacon channel, and wherein the transmission of the jamming signal by said transmission means is controlled in the case of reception of a useful signal in a time window beginning at the start of a theoretical reception window of said logical channel and having a duration dependent on the extent of said prepredefined geographic area.

10. Method according to Claim 9, wherein the logical signalling channel is a random access channel defined on the uplink beacon channel, structured as repetitive frames within each of which the mobile terminals can transmit a random access request containing the first bits and, furthermore, the second bits encoding the category of the requested service and/or the identity of the mobile terminal, and wherein said predefined information relates to at least one of a plurality of predefined service categories and/or to the identity of at least one mobile terminal, so that the command to stop transmission of the jamming signal is generated when a random access request corresponding to said service category and/or to the identity of said mobile terminal is detected.

11. Method according to Claim 10, wherein the jamming signal is sent in such a way that at least some of said first bits of the random access request are jammed and that said second bits of the random access request are not jammed.

12. Method according to any of Claims 10 to 11, wherein the jamming signal contains a dummy random access request.

13. Method according to Claim 1, wherein said second logical signalling channel is a random access channel defined on the uplink beacon channel which is not jammed by the jamming signal, whereas said first logical signalling channel is an access grant channel defined on the downlink beacon channel which is at least partially jammed by the jamming signal, wherein, the random access channel is structured as repetitive frames within each of which the mobile terminals can transmit a random access request containing bits encoding the category of the requested service and/or the identity of the mobile terminal, and wherein said predefined information relates to at least one of a plurality of predefined service categories and/or to the identity of at least one mobile terminal, so that the stop command is generated when a random access request corresponding to said service category and/or to the identity of said mobile terminal is detected.

14. Method according to any of Claims 10 to 13, wherein, when a random access request corresponding to said predefined service category is detected, the command to stop transmission of the jamming signal is generated after the expiry of a time delay roughly corresponding to the time interval between two occurrences of the random access request, and for a duration corresponding to at least the transmission duration of that request.

15. Method according to any of Claims 10 to 14, wherein said plurality of predefined service categories includes the emergency call category.

16. Method according to any of Claims 10 to 15, wherein said plurality of predefined service categories includes the location update category.

17. Method according to any of the preceding claims, wherein the jamming signal contains a synchronization sequence of the same type as that of frames transmitted on said first logical channel.

18. Method according to any of the preceding claims, wherein the jamming signal contains an identification code used to identify the origin of the jamming.

19. Method according to any of the preceding claims, wherein the jamming signal contains an identification and authentication code used to identify and authenticate the origin of the jamming at a given transmission instant.

20. Method according to any of the preceding claims, wherein the jamming signal is sent, either towards the base station if said first signalling channel is defined on the uplink beacon channel, or towards said prepredefined geographic area if said first signalling channel is defined on the downlink beacon channel.

21. Method according to any of the preceding claims, wherein the transmit power of the jamming signal is controlled according to the strength of the radio signal received on the downlink beacon channel.

22. Method according to any of the preceding claims, wherein the modulation of the jamming signal conforms to the transmission mask of the cellular mobile radio system.

23. Device for jamming calls in a predefined geographic area (12) included in a cell (10) of a cellular mobile radio system, comprising:

- means (45) for generating a jamming signal (B);

- controlled transmission means (46) for transmitting said jamming signal, so as to jam at least partially a first logical signalling channel defined on an uplink beacon channel or on a downlink beacon channel respectively associated with said cell; - means (42) for receiving a radio signal (S) originating from the base station covering said cell or from a mobile terminal located in said geographic area, for which certain time and/or frequency domains define a second logical signalling channel of the respectively downlink or uplink beacon channel associated with said cell, on which the information messages needed for a resource access procedure, which precedes the setting up of a call, are transmitted with a certain repetitiveness; and,

- means (43, 44) for scanning said second logical signalling channel, and, in the event of detection of predefined information transmitted in said second logical signalling channel, for generating a command (ES) to stop transmission of the jamming signal by said controlled transmission means.

24. Device according to Claim 23, further including means for conditioning the transmission of the jamming signal by said controlled transmission means, on the reception of a useful signal in a time window beginning at the start of a theoretical reception window of said logical channel and having a duration dependent on the extent of said predefined geographic area.