Method for controlling a communications channel shared by several stations

i

PROCESS FOR CONTROL OF A COMMUNICATION CHANNEL

SHARING BY SEVERAL STATIONS, AND SENDING / RECEPTION STATION FOR THE IMPLEMENTATION OF THE PROCESS

The present invention relates to the field of telecommunications, and in particular that of professional mobile radiocommunications.

Most professional mobile radiocommunications systems use the principle of "trunking", that is to say that the network has an infrastructure which manages the use of radio resources, for the duration of the communications. The network allocates resources to communications, and controls terminal access to the air interface.

There are also systems without “trunking”, in which mobile terminals can communicate in direct mode or in relayed mode, that is to say via repeaters. In general, voice communications are of the walkie-talkie type, with a discipline of PTT for channel access. The terminal listens to the frequency of the channel to determine whether it has the right to transmit when the user actuates a PTT (“push-to-talk”) button with which the terminal is provided. The right to transmit is granted if the channel is free or occupied by a communication on which the terminal has priority.

In the latter type of system, telephone-type call-off-hook (FOACSU: "full off-air call set-up") services are not supported. Indeed, in the absence of a base station or repeater managing the connection and access to the radio channel, a communication can be interrupted by an external user, during a silence on the channel due to the shutdown of the radio channel. voice transmission.

An object of the present invention is to provide a way of supporting call-off-hook type services in radiocommunications systems without “trunking”.

The invention thus proposes a method for controlling a channel shared by several transmission / reception stations, comprising at least one phase of use of the channel by a station participating in a communication, during which said station transmits information. of traffic on the channel, immediately followed by a phase of conservation of the channel by said station, during which said station transmits signaling information authorizing each other station participating in the communication to in turn transmit traffic information on the channel. This phase of conservation of

-2 channel by said station is continued, as long as the communication is in progress, until another phase of use of the channel by a station participating in the communication.

A transmitting / receiving station is master of the channel when it transmits traffic signals (typically voice). In the case where the station is a mobile terminal, when its owner stops speaking, the station nevertheless continues to transmit the signaling information on the radio resource used by the communication. This allows the slave stations participating in the communication to speak, and prevents other stations, not concerned by the communication, from intruding into believing that the channel is free.

The signaling information, transmitted at least between the speech exchanges, can comprise data identifying a set of stations authorized to participate in the communication, and / or data indicating a priority level of the communication, and / or a reference of identification of the communication. By receiving this information, a transmitting / receiving station different from the master station can determine whether it has the right to transmit on the radio resource, that is to say whether it participates in the communication (of the point-to-point type). point or group call) or if it has a pre-emptive right of the channel. The station which starts transmitting then becomes the new master station, and the previous master station becomes a slave station (unless it is this station which has taken over the floor).

Another aspect of the invention relates to a transmission / reception station on a shared channel, implementing the above method and comprising:

means for using the channel in master mode during a communication, arranged to send traffic information on the channel;

means for maintaining the channel in master mode during a communication, arranged to alternately transmit signaling information on the channel and listen to the channel, said signaling information authorizing each other station participating in the communication to transmit in turn traffic information on the channel;

- reception means in slave mode to receive traffic and signaling information sent in master mode by a

-3 other station participating in the communication; and

control means for activating the means for using the channel in master mode in response to a request for use when signaling information authorizing the transmission of traffic information has been received by the receiving means in slave mode, for activate the means for maintaining the channel in master mode in response to a deactivation of the means for using the channel in master mode, and to activate the receiving means in slave mode in response to the reception of signals from another station participating in the communication by the means for maintaining the channel in master mode listening to the channel.

Other features and advantages of the present invention will appear in the following description of non-limiting embodiments, with reference to the accompanying drawings, in which:

- Figure 1 is a diagram illustrating a radiocommunication system to which the invention can be applied;

FIG. 2 and 3 are diagrams showing examples of frame structures that can be used in a method according to the invention; and

FIG. 4 is a block diagram of a transmission / reception station according to the invention.

FIG. 1 schematically shows a professional radio communication system 1 providing telephony services to holders of mobile terminals (MS) 2-5. The system comprises radio transmission / reception stations comprising mobile terminals 2-5, possibly repeaters 6 serving to increase the radio coverage of the system, and possibly interfaces 7 serving as gateways with one or more switches 8 of external networks ( for example switched network or cellular network) to allow telephone calls between the holders of mobile terminals 2-5 and external subscribers.

The fixed stations 6, 7 of the system 1 do not necessarily offer a network service to supervise the radio communications in progress. Access to the channel is controlled by the various stations involved in the communication, one of them being a master station, and the other or others of the slave stations. During a communication, the assignment of the role of master station is carried out according to the speeches by the speakers.

In the illustration of figure 1, mobile terminal 2 is the station

-4master at the instant considered, in a communication also involving the mobile terminals 3 and 4 in direct mode.

By way of example, each mobile terminal 2-5 has a PTT (“press-to-talk”) button that the user actuates to speak. In response to this actuation, the terminal evaluates whether it can access the channel supporting the communication. If he cannot, speaking is refused. In the case of a gateway 7, the role of actuating the PTT button is played by the detection of a speech signal to be relayed from the switch 8.

By way of example, in the particular case of a frequency division multiple access (FDMA) system, each physical channel used to support a communication consists of a frequency available to the system. In the event that a repeater 6 intervenes, the latter can apply a transposition of this frequency to avoid interference problems between the radio signals that it transmits and those that it receives.

FIG. 2 shows an example of a frame structure adopted in a phase of use of the channel by a master station transmitting traffic information during a voice communication in an FDMA radio system according to FIG. 1. In this example, superframes of 1440 ms follow one another on the frequency considered. This superframe is divided into four parts of the same duration, each comprising an interval of 320 ms for the transmission of traffic information (voice signals) on a TCH traffic channel, and an interval of 40 ms for the transmission of information. signaling.

Every second 40 ms interval is devoted to a logical CS / FS signaling channel from the master station to the slave stations participating in the communication. This logical channel carries a synchronization pattern known a priori to the stations of the system and serving to synchronize them on the frame structure generated by the master station. The CS (“channel status”) information informs stations other than the master station about the occupation status of the channel, and notably includes:

- the identity or address in the system of the transmitting station (master);

- the address of the destination station (in the case of point-to-point communication), or in the case of a group call, the addresses of the destination stations, which can be concatenated into a group address;

- a reference number of the current call;

- identification of system 1;

- 5 - the priority level of the current call;

- the state of the channel.

FS (“forward signaling”) information provides signaling provided in the system from the master station to the other station (s) involved in the communication in progress.

Within the 40 ms interval dedicated to the CS / FS logical channel, transmission can be performed in the following order:

- FS information, intended for stations already synchronized;

- the synchronization pattern allowing these stations to continue synchronization and the other stations to acquire it;

- CS information that the synchronized stations can decode to determine if a communication is taking place concerning them and to what extent they can transmit on the channel.

The other 40 ms time intervals of the superframe according to FIG. 2 are used for the reception of information by the master station. This information falls under two logical channels:

- RS ("reverse signaling") providing the signaling provided in the system to the master station from the other station (s) involved in the communication in progress, and for example allowing the slave stations to respond to interrogations formulated by the master station in FS signaling or to signal their entry or withdrawal from a group call;

- Tl (“terminal interrupt”) allowing system terminals, which may be outside the communication in progress, to request access to the channel for emergency calls. The logical channel T1 is also listened to by the slave stations which are not transmitting on the logical channel RS.

FIG. 3 shows an example of a frame structure that can be used in a call establishment phase in the radio system of FIG. 1. The same structure can be adopted when a voice communication is in progress in a phase of maintenance of the call. channel by the master station, i.e. after the master station has stopped transmitting traffic information and before another station starts transmitting (thus becoming the new master station using the channel). In this example, 160 ms frames follow one another on the considered frequency, and are each divided into four 40 ms intervals. The first interval contains information of type CS

-6 as described previously, accompanied by the synchronization pattern. The third interval is also provided to be able to contain information of the CS type. The second and fourth intervals belong to the aforementioned logical channel T1.

In the establishment phase, the station which initiates the call (master) generates the frame structure according to FIG. 3, and sends in the first 40 ms interval information CS identifying the station (s) called, the coding of the state of the channel corresponding to a call establishment. Each called station (slave) detecting the synchronization pattern then has the third 40 ms interval of the frame to respond to the calling station. If a single CS-type information exchange is not sufficient to establish communication in accordance with the call protocol used, a dialogue can take place between the stations concerned, the master station using the first intervals of the frames and the slave stations the third intervals for transmitting signaling information, the state of the indicated channel representing the progress of the call establishment procedure (ringing, off-hook, etc.). Stations can also use FS / RS type signaling. Once the establishment phase is complete (the called party has picked up), any one of the stations participating in the communication takes the floor and becomes master of the channel by generating the frame structure according to FIG. 2.

In the channel conservation phase by a master station having ceased transmitting on the logical channel TCH, this master station transmits the CS information in the first 40 ms interval of the frame according to FIG. 3, with coding of the state of the channel indicating to the slave stations involved in the communication that they are authorized to take the channel. By detecting this CS information, the slave stations synchronize themselves on the frame structure of figure 3. If one of them takes the channel (actuation of the PTT button), it transmits a first block of information CS in the third 40 ms interval of the frame and it becomes master by adopting the frame structure according to figure 2 The master station keeping the channel listens during the third 40 ms interval of the frame. When it detects therein the information CS transmitted by the other station taking the channel, it gives up the status of master station now held by this other station. If it is the owner of the master station keeping the channel who takes the floor first, this station remains master of the channel.

-7The first block of information CS transmitted by the station authorized to take the channel could also be in the second or fourth 40 ms intervals since the master station is listening during these intervals.

The stations external to the communication detect the synchronization pattern accompanying the information CS in the first 40 ms interval of the frame according to FIG. 3, thus noting the occupation of the channel. They can possibly decode the CS information which follows in the first interval, which allows them to check whether they can enter the communication (station targeted by a group call which was not previously participating in the communication; this is the "function". late entry ”also supported by the CS information when the channel is being used by the master station) or if they have a channel pre-emption right taking into account the level of priority indicated. Otherwise, they are prevented from accessing the channel, except for transmitting information T1 in the event that they have to make an emergency call.

A block diagram of a transmission / reception station is shown in FIG. 4. The antenna 10 of the station is connected to its radio stage 11 corresponding to the analog part of the radio station.

For the reception part, the baseband signal delivered by the radio stage 11 is supplied to a synchronization unit 12 and to a demodulator 13. The unit 12 searches for the synchronization patterns of the received signal, in particular the patterns of. synchronization accompanying the CS information. The demodulator 13, synchronized by the unit 12, estimates the symbols transmitted from the baseband signal, and supplies these estimated symbols to the processing circuits 14 of the station.

A unit 15 controls the status, master or slave, of the transmission / reception station, by delivering an M / E bit to a frame generator 16 which delivers sequencing signals conforming to the frame structure according to FIG. 2 or 3. Unit 15 also controls generator 16 with a W bit to indicate to it that the frames must be according to figure 2 (W = 1, phase of use of the channel) or according to figure 3 (W = 0, phase call establishment or channel conservation). When the station is a master terminal (for example M / E = 1), the frame structure is generated with reference to a local clock. When the station is a slave (M / E = 0), the frame structure is generated in accordance with the synchronization information acquired by

-8unit 12 from the received signal.

The frame generator 16 controls the radio stage 11, the modulator 19 and the demodulator 13 to place the station in transmission or in reception in the appropriate time intervals according to the frame structure in force.

The frame generator 16 also ensures the sequencing of the circuits 14 for processing the received signal, and that of the circuits 17 for processing the signals to be transmitted. FIG. 4 illustrates in each block 14, 17 the circuits 20-23, 24-27 for processing the logical channels mentioned with reference to FIGS. 2 and 3.

When the station is master, the bit M / E = 1 delivered by the unit 15 controls the switches 18 in the position M shown in FIG. 4. In the use phase of the channel, the transmitted signal, supplied to the floor radio 11 by the modulator 19, comes from the circuits 20, 21 for processing the TCH traffic logical channel and the CS / FS signaling logical channel, clocked by the frame generator 16. In the remaining 40 ms intervals of the frame according to FIG. 2, the circuits 24, 25 of the master station listen to the logical channels RS and T1 to receive the signals possibly transmitted by the other stations. In the call establishment or channel conservation phase, the signal transmitted in the first 40 ms intervals of the frames comes from the only circuit 21 for processing the logical channel CS / FS, clocked by the frame generator 16. In the three other 40 ms intervals of the frame according to FIG. 3, the circuits 24, 25 of the master station listen to the logical channels CS / RS and T1 to receive the signals possibly transmitted by the other stations.

When the station is a slave, the switches 18 are controlled in position E of FIG. 4. In the phase of use of the channel by another station, the frame generator 16 activates at the appropriate times the circuit 22 for processing the logical channel RS for the transmission part and the circuits 25-27 for processing the logical channels T1, CS / FS and TCH for the reception part, in accordance with the frame structure of FIG. 2 generated by the master station. In the phase of call establishment or channel conservation by another station, the frame generator 16 activates the processing circuit 22 of the CS / RS logical channel at the appropriate times for the transmission part and the processing circuits 25, 26. logical channels T1 and CS / FS for the reception part, in accordance with the frame structure of FIG. 3 generated by the master station.

The frame generator 16 also controls the circuit 23 of

-9treatment in transmission on the logical channel T1 in the case of terminals external to the communication in progress (switches 18 in the position E) to enable them to possibly place an emergency call. Otherwise, the processing circuit 25 for reception on the logical channel T1 is activated in the corresponding intervals to allow the station to pick up a possible emergency call.

Terminals according to FIG. 4 can communicate with each other in walkie-talkie mode. When the user of a terminal according to FIG. 4 wishes to access the channel for transmitting voice signals, he actuates the button w PTT 30 with which the terminal is provided. Its terminal, which is not master of the channel at this time, then examines whether the carrier is busy, listening to the channel long enough to pick up at least one time slot of the CS / FS logical channel. In the absence of occupancy detection, the terminal determines that no other station is master of the channel, and it itself becomes the master to transmit the user's signals. It can also become a master if the channel is occupied by a communication on which the user has a right of preemption (which can be determined on the basis of the priority level of the communication indicated in the CS information). The channel is released when the user has finished speaking. The system thus provides telephony services similar to those offered in traditional systems without "trunking".

Advantageously, the system also allows communications of the call type with off-hook, between two or more mobile terminals in direct or repeated mode. One or more of the terminals involved in such a communication can also be external terminals connected via a gateway 7. The call is first established as indicated above. At the end of this establishment phase and until the end of the call (the penultimate station hangs up), there is always one of the radio stations involved which is the master of the channel.

While a station transmits a voice signal on a carrier, it is master of the corresponding frequency channel. When it stops transmitting on the logical channel TCH without the communication being terminated, it remains the master and its generator 16 generates the frame structure according to FIG. 3. The circuits 21 are controlled so that the master station continues to transmit on the logical signaling channel CS, by providing at least the information CS and the synchronization pattern as explained previously. This

-10 prevents any intrusion on the channel of a terminal not concerned by the communication in progress and not having a right of preemption. On the other hand, the master station keeping the channel goes into reception in the 40 ms time intervals where it does not send the CS information.

The state of the channel encoded in the information CS sent in the channel conservation phase indicates to the slave stations involved in the communication that they can take the channel. If the user of such a station actuates his PTT button 30, this station becomes the master of the channel and begins to transmit signals on the logical channels CS / FS and TCH. To ensure that the synchronization pattern transmitted on the CS / FS logical channel will be detected by the previously master station, the new master station shifts its transmission so that the first synchronization pattern falls within the third 40 ms time slot of the frame previously transmitted in the channel conservation phase (at a pinch in the second or fourth). In response to the detection of signals transmitted on the channel by another station, in particular of the synchronization pattern, the station which was master becomes a slave and is synchronized with the frame structure of the new master station.

In the embodiment illustrated by FIG. 4, the reception processing circuits 26 on the logical channel CS / FS deliver a bit V, the activation of which indicates that the slave station is authorized to take the channel. This bit is determined by analyzing the CS information sent by the master station. It is activated if the state of the channel indicates that the master station has stopped transmitting on the logical channel TCH (channel conservation phase) and if the slave station identifies itself as participating in the communication on the basis of the addresses included. in CS information. It can also be activated if the terminal finds that it can exercise a right of pre-emption of the channel taking into account the level of priority indicated. This V bit is supplied to the control unit 15 to validate the master status when the user actuates the PTT button 30. On the other hand, the unit 15 sends the W bit to the frame generator 16 which, in the process of being set. 'call, indicates whether one of the stations in communication is using the channel (W = 1) or not.

When the station is master (M / E = 1) without occupying the logical channel TCH (W = 0), the control of the radio stage 11, of the modulator 19 and of the demodulator 13 by the frame generator 16 is modified to place the receiving station in the second, third and fourth

- 11 times of 40 ms of each frame according to figure 3. Detection of the synchronization pattern by unit 12 when M / E = 1 and W = 0 then switches the station to the slave state (M / E = 0).

In an alternative embodiment, the change of master station of the channel during communication could result from a dialogue (handshake) between the station previously master and that which requires to become so.

The call with off-hook takes place in a similar way when a repeater 6 is interposed between the master terminal 2 and the slave terminal 5, as illustrated in dotted lines in FIG. 1. The repeater 6 reflects the frame structure w generated by the terminal. master 2, with carrier frequency transposition, to terminal 5, which also enables the latter to be synchronized. Thus, with regard to the frequency channel used between terminal 2 and repeater 6, terminal 2 is the master station transmitting the voice signal and generating the frame structure. On the other frequency channel used between terminal 5 and repeater 6, it is repeater 6 which, relaying terminal 2, plays the role of master station transmitting voice signals using the frame structure on which it s. 'is previously synchronized. The repeater 6 can be considered as comprising two back-to-back transmission / reception stations, one operating on the primary frequency on which it receives the signals from the master terminal, and the other on the transposed frequency on which it retransmits them, the difference with the case of the direct mode described above being that the terminals must switch their frequency for the logical channel RS.