WO2000035232A1

WO2000035232A1 - Mobile radio communications system

Info

Publication number: WO2000035232A1
Application number: PCT/GB1999/004126
Authority: WO
Inventors: Peter Howard
Original assignee: Simoco International Limited
Priority date: 1998-12-08
Filing date: 1999-12-08
Publication date: 2000-06-15
Also published as: GB2344725A; GB9929052D0; GB9826985D0; AU1668000A; GB2344725B

Abstract

A mobile radio system comprising a radio infrastucture (10) with base stations (11, 12, 13), and a switching and management infrastructure (15). When mobile station (MS1) wishes to set up a full-duplex point to multipoint call to the call group of mobile stations (MS2-MS6), it makes an individual call to a multipoint gateway group subscriber identity which corresponds to the call group. When the radio network infrastructure (10) receives such a call request, it checks the availability of those mobile stations associated with the gateway group subscriber identity (i.e. mobile stations (MS2-MS6)), and, if sufficient are available, then attempts to establish duplex individual calls with as many of the associated mobile stations as possible. Once the individual duplex calls have been successfully established, the group call proceeds with each group member talking as desired and a selected audio signal from one of the mobile stations being retransmitted to the remaining mobile stations to which the full-duplex calls have been established to enable them to listen in.

Description

Mobile Radio Communications System

The present invention relates to the establishment of calls in a mobile radio communications system and in particular to the establishment of point to multipoint radio calls in such a system.

Many users of mobile radio communications systems make extensive use of point to multipoint group calls.

In such calls, members of a group of users of the system can listen into and participate in calls to all other members of the group simultaneously. Such group calls are usually half-duplex in nature (i.e. there can be audio transmission in both directions, but only in one direction at any given time) . In such an arrangement each party in the group will typically have a half- duplex connection to the fixed radio network (such that they can only transmit to or receive from the fixed network at any given time, and select which they want to do usually by pressing their push-to-talk (PTT) button) . Only one party in the group can talk at any given time, and the remaining members of the call group all listen in on their respective radio channel . Recently, mobile radio communications systems, such as the TETRA (TErrestrial Trunked RAdio) system and cellular mobile phone systems, have been developed which permit full duplex calls (i.e. with audio transmission in both directions simultaneously) between individual users (i.e. point to point calls) . In such an arrangement each user receives on one radio channel and transmits on another radio channel, simultaneously.

Some of these mobile radio systems, such as the TETRA system, that permit full duplex point to point calls, also support point to multipoint calls (i.e. group calls) but those point to multipoint calls are usually constrained to be half-duplex in nature in the standard system implementation (such that any request for a call to a predetermined group identity would cause a half-duplex group call to be set up) . This is the case for the TETRA system. However, the Applicants have recognised that some users of mobile radio systems such as the TETRA system, will require the establishment of open channel, all informed groups with hands-free operation. This will require point to multipoint full -duplex operation. According to a first aspect of the present invention, there is provided a method of establishing a call in a mobile radio communications system which includes a system infrastructure for establishing and routing calls in the system and a plurality of call terminating stations, and in which system a group subscriber identity which has associated therewith a plurality of the call terminating stations has been defined, the method comprising: the system infrastructure, in response to a call terminating station making a call request using the defined group subscriber identity, attempting to establish an individual full-duplex connection between the system infrastructure and each of the call terminating stations associated with the group subscriber identity, and then retransmitting the audio signal received from one of the full-duplex connections established to the remaining call terminating stations of the group to which full-duplex connections have been established. According to a second aspect of the present invention, there is provided a mobile radio communications system, comprising: a plurality of call terminating stations; and a system infrastructure, the system infrastructure comprising means for storing a group subscriber identity which has associated therewith a plurality of the call terminating stations; and means for, in response to a call terminating station making a call request using the group subscriber identity, attempting to establish an individual full-duplex connection between the system infrastructure and each of the call terminating stations associated with the group subscriber identity, and then retransmitting the audio signal received from one of the full-duplex connections established to the remaining call terminating stations of the group to which full- duplex connections have been established. In the present invention, a group subscriber identity is defined which, when used by a call terminating station to denote a desired group of call terminating stations to be called, causes the mobile radio system infrastructure to try to establish full- duplex individual connections (i.e. calls) to all members of the group and to then transmit the audio signal from one or more of the established full-duplex connections to the remaining group members to which a full-duplex connection has been established. In this way a full-duplex point to multipoint group call is effectively set up as a combination of plural individual point to point full-duplex calls. In other words, each individual call terminating station is effectively participating in an individual full-duplex call with the system infrastructure, rather than a group call. This avoids the need for the call terminating stations to operate in a full-duplex mode within the radio system (e.g. TETRA) standard group call set-up, and thus permits, for example, unmodified TETRA compliant terminals to participate in full-duplex point to multipoint calls.

The radio system infrastructure will typically comprise the fixed part of the radio network or system, such as base stations or sites, switching and management infrastructure, etc, as is known in the art. However, it can also be a suitably equipped mobile station, if desired. The call terminating stations of the mobile radio communications system may be any stations of the communications system at which a call terminates (i.e. a subscriber party of the communications system, as distinct from intermediate stations of the communications system such as base stations or sites of a fixed radio communications network) . Thus, as well as mobile stations of the mobile radio communications system, the call terminating stations could include a fixed line station such as a user of the public switched telephone network (PSTN) or a despatcher having a hard wired connection to the communication system. Typically the call terminating stations will include plural mobile radio stations which are in radio communication with the system infrastructure (e.g. fixed radio network).

The group subscriber identity can be associated with the call terminating stations of the group as desired. It will typically be associated with the individual subscriber identities of each of the call terminating stations in the group, as this is a convenient way of identifying those call terminating stations which will be attempted to be included in the full-duplex multipoint call to be the specified duplex group subscriber identity. Preferably more than one group subscriber identity in accordance with the present invention is defined for the radio system, so as to permit group calls in accordance with the present invention to different groups of call terminating stations. The group subscriber identities and group memberships would typically be predetermined, but could also be defined in use of the radio system, if desired.

The establishment of the individual full -duplex connections or calls can be performed as desired, and should typically be performed in the normal manner for the mobile radio communications system in question.

In a preferred embodiment the group call is only established and proceeded with if a minimum number of full-duplex individual calls is successfully established to the group members .

The retransmitted audio signal to the group members should be such that each group member can listen to the speech of the other group members as the call proceeds. Thus, for example, where the system infrastructure can combine audio received signals from more than one call terminating station, it could do so and rebroadcast the combined audio signals appropriately, such that each call terminating station plays back a single combined audio signal to its user.

However, in a particularly preferred embodiment, only a selected single one (and only one) of the audio signals received from the call terminating stations is retransmitted by the infrastructure to the remaining call terminating stations (i.e. to all the call terminating stations except for the one from which the audio signal originates) to which the full-duplex connections have been established. This avoids the need for any adding or combining of the (plural) received audio signals, which could be difficult to do and/or, particularly for example in the case of a digital radio system and digital signals, require decoding and re-encoding of, and/or converting of, the received audio signals which could, for example, lead to a loss of quality.

Thus according to a third aspect of the present invention, there is provided a method of operating a mobile radio communications system, comprising: establishing a group call between plural call terminating stations of the system in which it is possible for two or more of the call terminating stations participating in the call to transmit simultaneously; during the call, selecting, at any one time, for retransmission to the remaining group members, the transmitted audio signal from only a single one of the call terminating stations that are transmitting; and retransmitting only the selected single one of the audio signals being transmitted by the group members to the remaining group members . According to a fourth aspect of the present invention, there is provided an apparatus for controlling a call in a mobile radio communications system, comprising: means for establishing a group call between plural call terminating stations of the system in which it is possible for two or more of the call terminating stations participating in the call to transmit simultaneously; means for, during the call, selecting, at any one time, for retransmission to the remaining group members, the transmitted audio signal from only a single one of the call terminating stations that are transmitting; and means for retransmitting only the selected single one of the audio signals being transmitted by the group members to the remaining group members.

The selection of which audio signal to retransmit to the group can be performed as desired. It is preferably performed in accordance with one or more predetermined selection criteria, and preferably is such that individual users do not have transmit requests to transmit to the system (as this permits hands-free operation) . The selection function would normally be performed by the system infrastructure, e.g. in the fixed radio network. In a particularly preferred selection arrangement, the signal from each participating call terminating station is analysed to see if it appears to contain speech. The speech detection can be carried out in any suitable manner known in the art , such as by measuring the average energy in the (decoded, if necessary) audio signal over a short period of time (e.g. 0.3 seconds), with, e.g., the signal being considered to contain speech if its average energy exceeds a predetermined threshold energy level. Alternatively, more sophisticated ways to detect speech could be employed, such as by looking for particular features characteristic of speech in the audio signals. If only one signal appears to contain speech, then that signal alone is retransmitted to each participating call terminating station (except for the one from which it originates) . If more than one call terminating station is transmitting speech at the same time, the single signal to retransmit is selected appropriately, e.g. on the basis of further predetermined selection criteria.

It is believed that this method of selecting one of a plurality of audio signals and in particular one of a plurality of received radio signals containing audio signals to retransmit is advantageous in its own right. Thus, according to a fifth aspect of the present invention, there is provided a method of selecting a single audio signal to transmit from a plurality of received audio signals, comprising: determining which of the received audio signals contains speech; if only one of the received audio signals contains speech, selecting that audio signal alone for transmission; if more than one of the received audio signals contains speech, selecting for transmission a single one of those audio signals containing speech on the basis of one or more predetermined selection criteria. This aspect of the invention also extends to an apparatus for carrying out the selection (which apparatus could for example, comprise a base station or other site of a fixed radio network infrastructure) . Thus according to a sixth aspect of the present invention, there is provided an apparatus for selecting a single audio signal to transmit from a plurality of received audio signals, comprising: means for determining which of the received audio signals contains speech; and selection means for, if only one of the received audio signals contains speech, selecting that audio signal alone for transmission, and if more than one of the received audio signals contains speech, selecting for transmission a single one of those audio signals containing speech on the basis of one or more predetermined selection criteria. These aspects of the present invention are, as will be appreciated from the above, particularly suitable for selecting a single audio signal to retransmit to a call group during a group call in a mobile radio communications system. The predetermined audio signal selection criteria, e.g. for use if more than one audio signal appears to contain speech, or otherwise, could, for example, be based on the priority status of the respective audio signal originators e.g. call terminating stations, the length of time that each station has been transmitting speech (e.g. such that the station that has been transmitting the longest is selected) , or a combination thereof, etc. These criteria provide a mechanism whereby the incoming signal to be retransmitted can be selected without relying on requests to transmit from participating call terminating stations.

In a preferred embodiment other call terminating stations can join the ongoing group call. This can conveniently be achieved by the new call terminating station calling the relevant group subscriber identity, in response to which a full-duplex individual connection would be set up to the calling station, over which the audio signal from the group call would be retransmitted. The group call of the present invention can be cleared and terminated as desired. For example, the overall group call can be cleared and terminated when the calling station (i.e. call originator) attempts to clear the call, and/or after a predetermined time limit associated with the call has expired. Preferably, individual call terminating stations can also clear their individual participation in the call without affecting the rest of the group (i.e. without terminating the ongoing group call) , for example by clearing their individual call .

As will be appreciated by those skilled in the art, a group call set up in accordance with the present invention can be restricted to a single cell or base station of a mobile radio communications system, or may extend over one or more cells of the mobile radio communications system, such that one or more base sites are involved in setting up and handling the call . In any event, it is probable that at least one base station will be involved in a full-duplex connection with a mobile station which originated the call, and full- duplex connections with two or more second mobile stations participating in the group call . Thus, according to a yet further aspect of the present invention, there is provided a radio communications station for use in a mobile radio communication system which includes a plurality of mobile stations, the communications station comprising: means for establishing full-duplex radio communication between the station and a first mobile radio station of the radio communications system; means for establishing full-duplex radio communication between the station and two or more other mobile stations of the radio communication system; and means for transmitting selectively the audio signal received from one of the said mobile stations substantially simultaneously to the other ones of said mobile stations via said full-duplex radio communication connections.

This radio communications station would typically be a fixed base station of the radio communication system, but this is not essential. It could also comprise a suitably equipped mobile station, such as a mobile radio or phone (whether vehicle mounted or hand portable) . The methods in accordance with the present invention may be implemented at least partially using software e.g. computer programs. It will thus be seen that when viewed from further aspects the present invention provides computer software specifically adapted to carry out the methods hereinabove described when installed on data processing means, and a computer program element comprising computer software code portions for performing the methods hereinabove described when the program element is run on a computer. The invention also extends to a computer software carrier comprising such software which when used to operate a radio system or radio station comprising a digital computer causes in conjunction with said computer said system or station to carry out the steps of the method of the present invention. Such a computer software carrier could be a physical storage medium such as a ROM chip, CD ROM or disk, or could be a signal such as an electronic signal over wires, an optical signal or a radio signal such as to a satellite or the like. It will further be appreciated that not all steps of the method of the invention need be carried out by computer software and thus from a further broad aspect the present invention provides computer software and such software installed on a computer software carrier for carrying out at least one of the steps of the methods set out hereinabove .

A preferred embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows schematically an apparatus suitable for use with the present invention; and

Figure 2 shows schematically a call set up in accordance with the present invention.

Figure 1 shows an apparatus capable of operating in accordance with the present invention. This apparatus is included in the mobile radio system infrastructure . It could for example be part of the fixed radio network and switching and management infrastructure of a TETRA radio system.

The apparatus comprises a data structure or database 1 which stores, inter alia, a set of one or more defined predetermined group subscriber identities

(in the TETRA system, ITSIs (individual TETRA subscriber identities) defined as being assigned to a duplex multipoint gateway.

Each of these duplex multipoint gateway group subscriber identities has associated with it (and stored in the database) a list of individual call terminating station identities (e.g. TETRA ITSIs) . These individual call terminating station identities identify those call terminating stations of the mobile radio system which will be attempted to be included in a duplex multipoint call to the specified duplex multipoint gateway group subscriber identity in the database.

The arrangement is such that the set of group subscriber identities and list of individual subscriber identities associated with each group subscriber identity can be modified, for example to permit additions to and/or deletions from the set of list.

The database 1 also stores associated with each group subscriber identity a predetermined minimum acceptable number of participants for a call to that duplex multipoint gateway group subscriber identity. It can also store information on whether individual group members are essential to the group call (i.e. such that the call should not proceed if that member is not present or available) and the maximum permitted duration for calls to that particular group, if desired.

The data stored in the database 1 is controlled and accessed by the radio network management system 7.

The apparatus also includes a call control unit 2, which controls the setting up and maintaining of the group call . The control unit is arranged such that when a call request is made to a subscriber identity that is a member of the set of duplex multipoint gateway group subscriber identities stored in the database 1, it will cause the system to attempt to establish an individual full-duplex call (i.e. connection) between the system infrastructure and each of the call terminating stations whose individual subscriber identity is in the list associated with the requested full-duplex multipoint gateway group subscriber identity. This can be done with reference to the databases 6 of other radio systems which can be checked if appropriate to see if group members are already in other calls or are deregistered.

The call control unit 2 can also, optionally, determine for example on the basis of the number of successfully established individual calls, and/or which individual calls have been established and the minimum acceptable number of participants and/or required participants for the call stored in the database 1, whether the full -duplex multipoint call should be proceeded with. The control unit 2 is also responsible for clearing the individual call set up to any participating terminal when requested to do so by that terminal , and for clearing all associated individual calls when the calling party (i.e. the call originating station) clears the call, or when the call has exceeded any appropriate predetermined call limit timer.

The apparatus further comprises an audio signal selection unit 3, which selects which single one of the audio signals received by the infrastructure from the call terminating stations participating in the group call should be retransmitted to the remaining stations in the call . This is done by means of a speech arbitration in unit 4 which determines which of the incoming radio signals 8 from the group members contains speech and controls a speech routing unit 5 to select one of the speech signals for retransmission on the basis of predetermined arbitration criteria, such as the priority status of the participating call terminating stations, which can if appropriate be stored in the database 1 and accessed therefrom by the audio signal selection unit 3. Figure 2 shows schematically a mobile radio system operating a point to multipoint full duplex call in accordance with the present invention. The system comprises a fixed radio network or infrastructure 10, comprising in this example three base stations 11, 12, 13, and a switching and management infrastructure 15 which controls the setting up and maintaining of calls in the system, linked by land lines 14. The switching and management infrastructure 15 includes the database, call control unit and audio signal selection unit of Figure 1, although this is not essential and that apparatus could be located elsewhere in the fixed radio network.

The system includes plural mobile stations MS1-MS6. These can be mobile radios, cellular phones, etc and can be vehicle mounted or hand portable, as desired. In the present example mobile stations MS1-MS6 comprise a duplex multipoint group in accordance with the present invention. Figure 2 shows a duplex group call set up to this group. Thus mobile stations MSI and MS2 have individual full-duplex radio connections to base station

11, mobile station MS3 has an individual full-duplex radio connection to base station 12 and mobile stations MS4 , MS5 and MS6 have individual full-duplex radio connections to base station 13. Consider now the setting up of the call in accordance with the method of the present invention illustrated in Figure 2. Consider, for example, that the user of mobile station MSI wishes to set up a full -duplex point to multipoint call to the rest of the group. Mobile station MSI would therefore make an individual call to the multipoint gateway group subscriber identity which corresponds to the set (i.e. group) of call terminating stations with which it wishes to communicate .

When the radio network infrastructure 10 receives such a call request for a call to a duplex multipoint gateway group subscriber identity, it checks the availability of those call terminating stations associated with the gateway group subscriber identity (in this case mobile stations MS2-MS6) , and, if sufficient are available, then attempts to establish duplex individual calls with as many of the associated call terminating stations as possible.

Once the individual duplex calls have been successfully established (as shown in Figure 2), the group call proceeds with each group member talking as desired and a selected audio signal from one of the call terminating stations being retransmitted to the remaining call terminating stations to which the full- duplex calls have being established to enable them to listen in. The speech selection unit 3 of the fixed radio network infrastructure determines the routing of the audio signals. The speech selection unit 3 firstly analyses the incoming audio signals (e.g. in a TETRA system the incoming TETRA encoded speech stream) from each of the participating call terminating stations and determines which of them is likely to contain speech. For example, in a TETRA system it is possible to determine whether an incoming TETRA encoded audio stream is likely to contain speech without transcoding the stream, because the encoded speech contains amplitude information. Thus the infrastructure can analyse the amplitude of each incoming audio stream to discover which of the parties in the call is speaking. Alternatively, the infrastructure could look for and analyse the speech vectors encoded in the TETRA speech frames to determine which audio signals contain speech. If only one incoming radio signal contains speech, then that signal is routed on the downlink (i.e. base station to mobile station link) to each participating call terminating station, except for the one from which it originates. If more than one incoming signal contains speech, the speech selection unit 3 selects one of the signals to distribute on the basis of predetermined selection criteria, such as the priority of access of the relevant call terminating stations, the duration of speech signal from each station (e.g. the length that they have been talking) , for example if the relevant participants have equal priority status, or a combination thereof, and routes the selected signal to the downlinks of each participating call terminating station (i.e. those call terminating stations to which individual full-duplex calls have been successfully established) , except for the one from which it originates .

Selecting and retransmitting a single speech signal in this way is believed to be preferable to combining or adding all the audio signals together to make a composite signal for retransmission. This is because, for example, in the TETRA system, combining the signals would require the system to decode the signals, mix them and then re-encode the resultant signal for retransmission. Re-encoding the signal would require a relatively large amount of processing power, and decoding and re-encoding speech for a second time as this technique would require will tend to degrade the speech quality. Once the group call has been established, it proceeds as described above, until such time as it is cleared or terminated, for example by the call originator clearing the call at which point all the individual calls are cleared, and the duplex multipoint call is terminated. The duplex multipoint call may also, if desired, be terminated if a call limit timer associated with the call expires. Whilst the call is ongoing individual call terminating stations or participants (except the call originator) can attempt to clear their call. The system in response to such an attempt would clear the individual call established with that participant, but the overall group call will remain ongoing.

Further call terminating stations can join the group call by making a call to the appropriate duplex multipoint gateway group subscriber identity while a call is already in progress.

It can be seen from the above that the present invention provides a mechanism for establishing and maintaining point to multipoint circuit mode radio calls in a way that permits full-duplex mode operation within signalling systems that normally preclude this mode of operation. For example, it avoids the need for modification to the TETRA specification or the functionality of TETRA terminals, because the call setup, maintenance and clear down process is just as for a normal individual duplex call. In its preferred embodiments, the present invention also avoids the need to mix the audio signal from participants in a duplex multipoint call by performing arbitration on and selection of the incoming audio signals. Although the above embodiment has been described with specific reference to the TETRA system, the present invention is, as will be appreciated by those skilled in the art, equally applicable to other forms of mobile radio systems which can support full duplex individual calls.

Claims

1. A method of establishing a call in a mobile radio communications system which includes a system infrastructure for establishing and routing calls in the system and a plurality of call terminating stations, and in which system a group subscriber identity which has associated therewith a plurality of the call terminating stations has been defined, the method comprising: the system infrastructure, in response to a call terminating station making a call request using the defined group subscriber identity, attempting to establish an individual full-duplex connection between the system infrastructure and each of the call terminating stations associated with the group subscriber identity, and then retransmitting the audio signal received from one of the full -duplex connections established to the remaining call terminating stations of the group to which full-duplex connections have been established.

2. The method of claim 1, wherein the call is only established and proceeded with if at least a predetermined number of full-duplex individual calls is successfully established to the group members.

3. The method of claim 1 or 2 , wherein only a selected single one of the audio signals received from the call terminating stations is retransmitted by the infrastructure to the remaining call terminating stations to which the full-duplex connections have been established.

4. A method of operating a mobile radio communications system, comprising: establishing a group call between plural call terminating stations of the system in which it is possible for two or more of the call terminating stations participating in the call to transmit simultaneously; during the call, selecting, at any one time, for retransmission to the remaining group members, the transmitted audio signal from only a single one of the call terminating stations that are transmitting; and retransmitting only the selected single one of the audio signals being transmitted by the group members to the remaining group members.

5. The method of claim 3 or 4 , comprising selecting which audio signal to retransmit in accordance with one or more predetermined selection criteria.

6. The method of claim 3, 4 or 5 , wherein the selection process comprises analysing the audio signal received from each participating call terminating station to see if it appears to contain speech, and, if only one signal appears to contain speech, retransmitting that signal alone to each remaining participating call terminating station, or, if more than one call terminating station appears to be transmitting speech at the same time, selecting from those signals the single signal to retransmit on the basis of further predetermined selection criteria.

7. A method of selecting a single audio signal to transmit from a plurality of received audio signals, comprising: analysing each of the received audio signals to see if they appear to contain speech; if only one of the received audio signals appears to contain speech, selecting that audio signal alone for transmission; if more than one of the received audio signals appears to contain speech, selecting for transmission a single one of those audio signals appearing to contain speech on the basis of one or more predetermined selection criteria.

8. The method of claim 5, 6, or 7 , wherein the predetermined selection criteria comprise the priority status of the respective audio signal originators, and/or the length of time that each audio signal originator has been transmitting speech.

9. The method of any one of claims 1 to 6 , or of claim 8 when dependent upon claim 5 or 6 , wherein other call terminating stations of the system can join the ongoing call.

10. The method of claim 9, wherein another call terminating station can join the ongoing call by calling the relevant group subscriber identity, in response to which the system infrastructure attempts to establish a full-duplex individual connection to that new call terminating station, and then retransmits an audio signal from the call over that established full-duplex connection.

11. The method of any one of claims 1 to 6 , or of claim 8 when dependent upon claim 5 or 6 , or of claim 9 or 10, comprising clearing and terminating the call when the call originating station attempts to clear the call, and/or after a predetermined time limit associated with the call has expired.

12. The method of any one of claims 1 to 6, or of claim 8 when dependent upon claim 5 or 6 , or of claim 9, 10 or

11, wherein individual call terminating stations can clear their individual participation in the call without terminating the ongoing call .

13. The method of any one of claims 1 to 6 , or of claim 8 when dependent upon claim 5 or 6 , or of claim 9, 10, 11 or 12, wherein individual call terminating stations can transmit speech signals during the call without first having to make a request to transmit to the system infrastructure .

14. A mobile radio communications system, comprising: a plurality of call terminating stations; and a system infrastructure, the system infrastructure comprising means for storing a group subscriber identity which has associated therewith a plurality of the call terminating stations; and means for, in response to a call terminating station making a call request using the defined group subscriber identity, attempting to establish an individual full-duplex connection between the system infrastructure and each of the call terminating stations associated with the group subscriber identity, and means for then retransmitting the audio signal received from one of the full-duplex connections established to the remaining call terminating stations of the group to which full-duplex connections have been established.

15. The system of claim 14, wherein the means for retransmitting the audio signal received from one of the full-duplex connections established to the remaining call terminating stations of the group to which full- duplex connections have been established comprises means for retransmitting only a selected single one of the audio signals received from the call terminating stations to the remaining call terminating stations to which the full-duplex connections have been established.

16. An apparatus for controlling a call in a mobile radio communications system, comprising: means for establishing a group call between plural call terminating stations of the system in which it is possible for two or more of the call terminating stations participating in the call to transmit simultaneously; means for, during the call, selecting, at any one time, for retransmission to the remaining group members, the transmitted audio signal from only a single one of the call terminating stations that are transmitting; and means for retransmitting only the selected single one of the audio signals being transmitted by the group members to the remaining group members .

17. The system of claim 15 or 16, wherein the system infrastructure comprises means for selecting which single audio signal to retransmit in accordance with one or more predetermined selection criteria.

18. The system of claim 15, 16 or 17, comprising means for analysing the audio signal received from each participating call terminating station to see if it appears to contain speech, and means for, if only one signal appears to contain speech, retransmitting that signal alone to each remaining participating call terminating station, or, means for, if more than one call terminating station appears to be transmitting speech at the same time, selecting from those signals the single signal to retransmit on the basis of further predetermined selection criteria.

19. An apparatus for selecting a single audio signal to transmit from a plurality of received audio signals, comprising: means for analysing each of the received audio signals to see if they appear to contain speech; and selection means for, if only one of the received audio signals appears to contain speech, selecting that audio signal alone for transmission, and if more than one of the received audio signals appears to contain speech, selecting for transmission a single one of those audio signals appearing to contain speech on the basis of one or more predetermined selection criteria.

20. The system of claim 17, 18, or 19, wherein the predetermined selection criteria comprise the priority status of the respective audio signal originators, and/or the length of time that each audio signal originator has been transmitting speech.

21. The system of any one of claims 14 to 18, or of claim 20 when dependent upon claim 17 or 18, wherein the system infrastructure further comprises means for, in response to another call terminating station not participating in the call calling the relevant group subscriber identity, attempting to establish a full- duplex individual connection to that new call terminating station, and means for then retransmitting an audio signal from the call over that established full-duplex connection.

22. The system of any one of claims 14 to 18, or of claim 20 when dependent upon claim 17 or 18, or of claim 21, wherein the system infrastructure further comprises means for clearing and terminating the call when the call originating station attempts to clear the call, and/or after a predetermined time limit associated with the call has expired.

23. A radio communications station for use in a mobile radio communication system which includes a plurality of mobile stations, the communications station comprising: means for establishing full-duplex radio communication between the station and a first mobile radio station of the radio communications system; means for establishing full-duplex radio communication between the station and two or more other mobile stations of the radio communication system; and means for transmitting selectively the audio signal received from one of the said mobile stations substantially simultaneously to the other ones of said mobile stations via said full-duplex radio communication connections .

24. The apparatus of claim 23, wherein the means for transmitting selectively the audio signal received from one of the said mobile stations comprises: means for analysing whether each of the received audio signals appears to contain speech; and selection means for, if only one of the received audio signals appears to contain speech, selecting that audio signal alone for transmission, and if more than one of the received audio signals appears to contain speech, selecting for transmission a single one of those audio signals appearing to contain speech on the basis of one or more predetermined selection criteria.

25. A computer program element comprising computer software code portions for performing the method of any one of claims 1 to 13 when said program element is run on a computer.

26. A method of establishing a call in a mobile radio communications system substantially as hereinbefore described with reference to any one of the accompanying drawings .

27. A method of selecting an audio signal to transmit from a plurality of received audio signals substantially as hereinbefore described with reference to any one of the accompanying drawings.

28. A mobile radio communications system substantially as hereinbefore described with reference to any one of the accompanying drawings .

29. An apparatus for selecting an audio signal to transmit from a plurality of received audio signals substantially as hereinbefore described with reference to any one of the accompanying drawings .

30. A radio communications station substantially as hereinbefore described with reference to any one of the accompanying drawings .