WO2001076300A1

WO2001076300A1 - Method of and apparatus for setting up calls in a mobile communication system

Info

Publication number: WO2001076300A1
Application number: PCT/GB2001/001450
Authority: WO
Inventors: Mark Wentworth Rayne
Original assignee: Simoco International Limited
Priority date: 2000-03-30
Filing date: 2001-03-30
Publication date: 2001-10-11
Also published as: AU4434501A; GB2366146A; GB0007812D0; GB0108032D0

Abstract

In order to establish a call in a mobile communication system, a call request is sent by a mobile station or a controller to a system infrastructure. The infrastructure then determines, at step 3, whether or not the call request is for a geographically restricted or localised call. If it is detected that the call request is for a localised call, i.e. there is a call area defining message in the call request, the call is processed according to steps 9 to 19. In particular, an area to which the call is to be restricted is defined with reference to the location of a selected mobile station and called mobile stations within the defined call area are connected to the call.

Description

METHOD OF AND APPARATUS FOR SETTING UP CALLS IN A MOBILE

COMMUNICATION SYSTEM

This invention relates to the field of setting up or establishing calls in a mobile communication system and, in particular, to a method of and an apparatus for establishing or setting up calls in a limited geographical area within an area covered by a mobile communication system.

In some mobile communication systems, such as the GSM-R and TETRA (TErrestrial Trunked RAdio) systems, a group call facility is provided in which a user may call a group of users in a single call. When a group call request is made, all the members of the group are generally connected to the call . This allows a message to be broadcast to the group of users simultaneously or an open communications channel to be maintained between the group of users. Virtual private networks within the communication system can therefore be set up as may be useful, for example, when there is some commercial or logistical link between a group of users.

Mobile communication systems typically have an infrastructure arranged, as is known in the art, in a cellular fashion with each cell being served by a radio base site.

In GSM-R, for example, when a request for a group call is made, the infrastructure makes a list of users who are members of the called group. The infrastructure locates each user on the list and sets up a connection with each user via the radio base site currently serving the user. However, this is an inefficient method as individual calls have to be set up with each member of the group call via the respective base sites.

In contrast, the TETRA system sets up a single call to a called group on receipt of a group call request . The call is, for example, forwarded to all the base sites of the system and users belonging to the group simply have the call forwarded to them from whichever base site is currently serving them. Alternatively, in a more sophisticated and frequency efficient method, the TETRA system may maintain a list of base sites serving members of a group and only forward the group call to those base sites .

However, the communication network will typically cover a very wide area using a multiplicity of interconnected radio base sites and it may therefore be desirable to limit the participants in a group call to users in a more restricted geographical area than the area covered by the entire communication network. For example, where a call only concerns members of the group who are within a particular geographical area, it may not be desired to disturb other group members, and it may be a waste of communication system resources and incur unnecessary call charges to call members of a group that are outside that area. Furthermore, it may be useful to define the group 'itself as the users within a particular geographical area. It can therefore be desirable to be able to limit the members of a call group or recipients of a call to those users who are within a particular geographical area.

Methods for limiting the geographical area within which a call is set up have been suggested. In one known such method, described in US Patent No. 5633913, the user defines a geographical area to which the call is to be restricted with reference to fixed geographical locations, such as grid references or base site locations, and transmits information including the selected area as part of the call request.

In some circumstances it may be desired to send a group call to a group of users who extend over a restricted geographical area but who are moving together. This could be the case, for example, where the users are following a moving sports event (such as the Tour de France) or are guarding a moving cargo. In such circumstances, ev-en though the same group of users is being called, the desired geographical area for the call will move, possibly during the call itself.

It would be possible in the known method described above for the (calling) user frequently or continually to redefine the desired call area with reference to new fixed locations in order for the moving users to be contacted or remain connected to the call. However, the Applicants have recognised that this places an additional burden on the (calling) user which may be inconvenient, particularly if they don't know which way the group is moving. Thus, the Applicants believe that the known techniques for restricting the location area of a call are not particularly suitable where the desired area for the call is moving, particularly if the area is moving quickly.

According to a first aspect of the present invention, there is therefore provided a method of connecting mobile stations to a call in a mobile communication system comprising: defining an area to which the call is to be restricted with reference to a mobile reference point; and connecting one or more mobile stations within the defined area to the call.

Also, according to a second aspect of the present invention, there is provided an apparatus for connecting mobile stations to a call in a mobile communication system comprising: means for defining an area to which the call is to be restricted with reference to a mobile reference point; and means for connecting one or more mobile stations within the defined area to the call.

The present invention thus provides an arrangement for restricting the area within which a call is set up to a particular geographical area, i.e. that defined with reference to the mobile reference point . As the area to which the call is restricted is linked to a mobile reference point, i.e. a reference point that is able to move (and typically will be moving) , the area for the call is able to move with, or be adapted to, the motion of reference point, without the need for a user to continually redefine the area for the call. The present invention is therefore believed to provide an efficient technique for setting up a geographically restricted call, in particular when the call area is moving . As the call is restricted to a defined area, mobile stations outside the defined call area are preferably not connected to (i.e. excluded from) the call. Thus, for example, call charges are not incurred by the system trying to connect to the call mobile stations outside of the area (and, e.g., to whom the call does not therefore relate) . Similarly, communications resources (such as available bandwidth) is not wasted by trying to connect the call to undesired users, i.e. those outside the call area. In operation of the present invention, as in the prior art system, the identity of the other mobile station (s) or user(s) to be called and the geographical area to which the call is to be restricted are first identified and the communication system can then attempt to connect the call to the identified called mobile station(s), but only if that/those mobile station(s) are within the indicated geographical area.

Thus, according to a third aspect of the present invention, there is provided a method of requesting a call in a mobile communications system, which call is to be restricted to a particular geographical area covered by the system, the method comprising: transmitting the identity of one or more mobile stations of the system to which the call is to be established; and transmitting the identity of a mobile reference point in relation to which a restricted geographical area for the call is to be defined.

Also, according to a fourth aspect of the present invention, there is provided a method of establishing a call in a mobile radio communications system which is to be restricted to a particular geographical area covered by the system, and which system comprises one or more mobile stations and a system infrastructure for establishing and routing communications between the mobile stations, the method comprising: the system infrastructure receiving a request for a call to one or more mobile stations of the system, which call is to be restricted to a particular geographical area and, in response to the call request, the system infrastructure attempting to connect the called mobile station (s) to the call only if they are within the indicated geographical area, wherein the geographical area to which the call is to be restricted is defined with reference to a mobile reference point.

According to a fifth aspect of the present invention, there is provided calling apparatus for requesting a call in a mobile communications system, which call is to be restricted to a particular geographical area covered by the system, the apparatus comprising:^' means for transmitting the identity of one or more mobile stations of the system to which the call is to be established; and means for transmitting the identity of a mobile reference point in relation to which a restricted geographical area for the call is to be defined. According to a sixth aspect of the present invention, there is provided a mobile radio communications system for establishing a call which is to be restricted to a particular geographical area covered by the system, the system comprising: one or more mobile stations and a system infrastructure for establishing and routing communications between the mobile stations; wherein the system infrastructure comprises means for attempting, in response to a request for a call to one or more mobile station (s) of the system which is to be restricted to a particular geographical area, to connect the called mobile station (s) to the call only if they are within the indicated geographical area, and wherein the geographical area to which the call is to be restricted is defined with reference to a mobile reference point .

Thus, the caller can request a geographically restricted call by identifying the mobile station (s) to be called (such as by indicating a call group to be called) and identifying a mobile reference point to be used to define the geographical area to which the call is restricted. The infrastructure of the mobile radio communications system can then connect the called mobile stations within the area defined with reference to the mobile reference point to the call .

For example, the system may locate a mobile station to be included in the call and open a communications channel or channels between the base station in whose coverage area the mobile station is located and the mobile station. Alternatively, a mobile station to be included in the call may be instructed to listen on or share a communication channel of the base station in whose coverage area it is located with other mobile stations to be included in the call, which channel may be dedicated to the call .

The call may be requested by a dispatch operator of the system (e.g. a control unit of the infrastructure of the mobile communication system) . Alternatively, the call may be requested by a mobile station, or another user of the mobile communication system connected to the system infrastructure, for example, by a hardwired (e.g. PSTN) connection or the like. Thus, the calling apparatus might comprise a hardwired unit, e.g. control unit, of the system infrastructure, a mobile unit of the communication system, or a user connected by wire to the system infrastructure .

The call can include both mobile station and fixed stations (e.g. dispatchers). Where fixed stations are included in the call, they may be excluded from the geographical restriction (particularly where they have initiated the call) , but the called mobile stations will typically be restricted to the geographical area.

The system infrastructure would typically comprise the fixed radio network, i.e. multiple fixed radio base sites connected in a fixed network along with appropriate control systems and switches. However the system infrastructure may equally comprise, entirely or in part, any number of mobile stations adapted to perform any of the functions of the system infrastructure. For example, in a TETRA Direct Mode system, mobile radio units may act as "gateways" which relay communications between a standard TETRA network and mobile units operating in Direct Mode out of range of the standard TETRA network. These "gateways" might comprise part or all of the system infrastructure in the above aspects of the invention. Likewise, reference to "base stations" below is intended to include such "gateways" or similar parts of a radio network that forward calls to mobile units.

The call request may be transmitted in a variety of ways. For example, the transmission of the identity of one or more mobile stations of the system to which the call is to be established may comprise the transmission of the identification code or serial number of the mobile stations to be called. Alternatively, the identity may comprise identification of a group of mobile units (e.g. a "group address") which may identify a group of mobile units, the membership list of which is maintained by the system infrastructure .

Similarly, the transmission of the identity of the mobile reference point might typically be part of the transmission of the mobile units to be called. For example, the "group address" might include a predefined mobile reference point, known to the system infrastructure, such that the "group address" identifies both the mobile units to be called and the mobile reference point to be used. Alternatively, the identity of the mobile reference point might be transmitted separately, for example as part of an earlier identification of the geographical area to which later calls are to be restricted. This could, for example, be done as a specially addressed TETRA short data service message. The identity of the mobile reference point could also be prestored in the infrastructure, by, e.g. a system management terminal or despatcher terminal for the duration of some event, e.g. a military convoy.

Thus, the mobile reference point indication in the call request could be a specific indication in the call request, or, alternatively, it could, for example, be indicated implicitly by the identities of the other mobile station (s) being called (for example where a call group has a predefined mobile reference point already associated with it) . It could also be communicated separately, preferably in advance.

The mobile reference point with reference to which the area of the call is defined can be selected as desired. It preferably has some particular significance to the call being made, as this can provide a convenient way of ensuring that the call covers the desired geographical area. For example, the mobile reference point may be a vehicle or person of particular significance to the call being set up. The area for the call can then move automatically with that vehicle or person.

In a preferred embodiment the mobile reference point is a user of the mobile communication system. In particular, the mobile reference point may be a mobile station of the mobile communication system. This is particularly convenient where the mobile communication system already stores information regarding the position of its users or mobile stations as the position of the reference user or mobile station, and hence the mobile reference point, may easily be obtained.

In one such embodiment, the reference user is a member of a group of users to be connected to the call, although this is not essential. This provides a way of conveniently enabling the area for a group call to track a call group as it moves since, by defining the area for the call with reference to one of the users to be connected to the call, the call area can track the movement of that user and thus of the call group.

In one preferred such embodiment, the reference user is the user requesting the call to be set up. This can, for example, conveniently allow a user to make a call to users only within an area specific to his/her own position, for example within a given radius of his/her position, at any given time.

The area for the call would normally be defined with reference to the location of the mobile reference point. This allows the location of the area of the call to change as the location of the mobile reference point changes .

The location of the mobile reference point can be defined as accurately as desired. It could, for example, be defined in terms of its latitude and longitude or other appropriate coordinates where those are known or can be determined (e.g. by detecting the location of the mobile reference point using position detection means, such as a Global Positioning System (GPS) apparatus) . Alternatively, where a less accurate location definition is acceptable, the location of the mobile reference point could, for example, be defined with reference to the (location of) the base station of the system in whose coverage area the reference point is currently located (e.g. where the mobile reference point is a mobile station of the system, the base station currently serving that mobile station) .

The former arrangement has the advantage of tracking the location of the mobile reference point more accurately, and therefore allowing movement of the desired call area to be carried out more accurately. It also allows locations outside the range of any base station of the system to be used, which may be particularly useful in systems that are operating by direct user to user communication (i.e. not via base stations of the system) , such as in a TETRA Direct Mode arrangement.

The latter arrangement may be particularly convenient where the mobile communication system logs the position of mobile stations of the system by noting the base station that the mobile station is using or last used, as this may reduce system complexity. The area for the call may be defined (with reference to the location of the mobile reference point) in various ways. In one example, it can be defined as the coverage area of the base station whose coverage area the mobile reference point is located in, or the coverage area of that base station and all its neighbouring base stations . These are particularly straightforward ways of defining the area and are useful when it is desirable to minimise system complexity. Alternatively, as in US-A-5633913 , for example, the area can be defined using one or more identified locations, sometimes with information regarding the shape or dimensions of the area. The location of the mobile reference point of the present invention may be substituted for one or more of the locations used to define the area in the technique of US-A-5633913, thereby allowing (in accordance with the present invention) the area to be redefined automatically as the mobile reference point moves, rather than only on re-definition by a user as in US 5633913.

In a particularly preferred embodiment, the area for the call is defined by locating a geometrical shape with reference to the location of the mobile reference point. Preferably, a plurality of shapes are predetermined, from which the shape to use can be selected. In such an arrangement a plurality of predetermined shapes, each having a corresponding shape identification, may be stored by the system (e.g. in a database linked to the system infrastructure and, optionally, in mobile stations of the system) , and users can then make call requests including the shape identification for the selected shape and the identification of the selected mobile reference point with reference to the location of which the shape is to be located. The system then determines the location of the reference point and locates the shape with reference to that location to define the call area.

In such an arrangement, the geometrical shape is preferably defined in terms of its dimensions with reference to an origin which is mapped to the location of the mobile reference point to locate the shape and define the call area. The origin may be located at the centre of the shape, or it could be at some other position in the shape.

The particular shapes may be selected as desired. In particularly preferred embodiments, the shape is a particular range or distance in any direction from the origin (i.e. a circle of given radius with the location of the mobile reference point at its centre) , or a line of given length, with the origin at a particular position along the line, such as at its centre or at one of its ends.

In order for the desired area to be properly defined it may be necessary to define an orientation at which the shape should be located. This is of course, not required when the shape is a circle, but may be important when the shape is a straight line for example. The shape may therefore be orientated as desired. For example, the shape may be orientated at a particular angle with reference to the Earth's latitude and longitude. This enables the shape to be located accurately and consistently to define the call area. In another example, the shape may be orientated at a particular angle with reference to the direction of movement of the mobile reference location. A particularly preferred such shape and orientation is a line orientated perpendicular to the direction of movement of the mobile reference point. This sweeps out an area either side of the reference point as it moves, and may be useful, for example, where it is desired to provide coverage for a group of users that tend to assume a formation relative to the direction of travel of a user, such as dispersed along a direction of travel of a user or parallel to a route.

In a preferred embodiment, the orientation of the shape may be defined or selected by a user. In such an arrangement, a user may include orientation data specifying the orientation of the shape in their call request (as well as the identity of the desired shape and mobile reference point as appropriate) . Orientation data may also or instead be predetermined and stored in a database. Users could then make call requests including, for example, the identification of the desired predetermined orientation that they wish to use.

The orientation data may comprise, for example, an indication of whether the shape should be orientated at a fixed angle with respect to the Earth's latitude and longitude, or with respect to the direction of movement of the mobile reference point.

More generally, the applicants have therefore recognised that it may be preferable to define the area, additionally or alternatively, with reference to the movement of the mobile reference point. This allows, for example, the shape to be located (e.g. orientated) or the defined area to be altered to take account of the direction or speed of movement of the reference point. Thus, the system can ensure that users behind, in front or to either side of the reference point are selectively included or not included in the defined area by, for example, locating, distorting or extending the area as desired with reference to the movement of the reference point . This arrangement could be used in combination with a technique that uses fixed geographical reference points, such as that described in US-A-5633913, to, for example vary the orientation of a given shape relative to the fixed geographical reference points in accordance with the movement (e.g. direction of travel) of a mobile reference point .

The geometric shape itself, once "located" and "oriented" as desired, could define entirely the area of the call, such that users within the located area covered by the shape are called, but any users outside the area covered by the shape are not . This may be appropriate, for example, when the exact location of all the users is known sufficiently accurately that the users within the area defined by the shape can be determined straightforwardly.

In another arrangement, the area for the call could be defined with reference to the communication coverage areas of base stations of the system whose communication coverage areas lie within the area covered by the located shape. This may be particularly convenient where it is readily possible to determine the base station a user is using or last used, but the exact location of a user is more difficult to determine. In such an arrangement, the call area could be defined as the area of coverage of all those base stations whose coverage lies entirely within the located shape, i.e. base stations providing coverage all of which is within the located shape. However, in a preferred such arrangement, the call area is defined as the area of coverage of all those base stations whose coverage lies at least partly within the located shape, i.e. all base stations providing at least some coverage within the located shape. This latter arrangement is a particularly flexible system, as the geometrical shape can then, for example, be a line or point, if desired, and the defined call area is effectively the combined coverage areas of the base stations whose coverage areas the line or point is located in.

The area for the call would typically be defined with reference to the current location of the mobile reference point. However, in certain circumstances, it may be desirable to define the area with reference to a previous location of the reference point. This could be used to, for example, cause the defined area to lag behind the current location of the reference point, such that, for example, when the reference point is a user of the mobile communication system, the defined area would be generally behind the user.

The previous location, with reference to which the area may be defined, could, for example, be defined in terms of a relative, preferably variable, distance or time behind the current location of the mobile reference point. Alternatively, the previous location could be defined as the location of the reference point at an actual, absolute (previous) time. Thus, for example, the mobile reference point's location twenty minutes previously could be used to locate the shape and define the area. In that case, the defined call area will lag behind the present location of the mobile reference point by twenty minutes and effectively trace the movement of the mobile reference point with a twenty minute time delay. This may be useful, for example, where the mobile reference point is a vehicle in a convoy which tends to run ahead of the group of users it is desired to call.

The previous location to use is preferably indicated in the call request, for example by including in the call request a "lag factor" representing, e.g. the relative time previously at which the location of the mobile reference point is to be considered. Predetermined "lag factors" may be stored in a data base and identified by a user in a call request. In a further embodiment, it may be desirable to define the call area with reference to more than one location of the reference point. For example, both the present and one or more previous locations of the reference point may be used to define the call area. Thus, where a shape is located to define the area, the shape may be located with reference to each desired location of the reference point, the defined area being that of all the located shapes. This could be used to, for example, cause the defined area to take the form of a trail behind the present location of the reference point .

In another preferred embodiment, more than one, and preferably .only two, mobile reference points are used to define the call area. For example, one or more geometric shapes may be located with reference to the locations of the mobile reference points. This could be used to allow a previously defined call area to remain in the area defined for the call until a second mobile reference point enters the previously defined call area, thereby, for example, forming a trail. It could be used to have the defined call area extend between two users, for example in a convoy following the same route. This could be achieved by, for example, defining the extent of the call area as being between a lead and a tail vehicle of a convoy. This arrangement should be a more accurate way of providing a "trail", and so may be particularly useful where it is desired to restrict the call "footprint" to the absolute minimum for security, efficiency or cost purposes.

The use of more than one mobile reference point to define the call area may also be applicable where it is desired to include multiple moving areas in a single group call, e.g. two convoys following the same path but spaced apart from each other.

In the present invention, each time a new location is detected for the mobile reference point, the call area will be redefined. When that occurs, any area that was previously within the call area but is no longer within the redefined call area could be removed immediately from the call area, if desired (and any called users in that area told to terminate their participation in the call, if desired) .

However, in one preferred embodiment of the present invention, the no longer covered call area is not removed immediately from the call, but instead, the previously covered area is arranged to remain as part of the area defined for the call for a selected, e.g. predetermined, period of time after it is no longer part of the current call area. In this way the defined call area can be arranged to include previously defined areas, thereby, for example, causing the call area to trail behind the present location of the mobile reference point .

This feature may be particularly useful when it is desired to cover an area defining a route used by a group of users, such as a delivery fleet or bus company. In this case, the predetermined time period may be set at a large value such that the defined call area covers the whole route used by the reference user.

Preferably, a user can indicate, e.g. in their call request, the time period for which the call areas no longer covered should remain within the call area. A selection of predetermined time periods may be stored in a data base and a call request may simply identify the desired time period.

Particularly where a call may have a long duration, it is preferred that the way in which the call area is defined may be changed during a call, for example, by a user defining one or more new parameters such as shape, mobile reference, orientation, etc, with which the area is to be defined during the call, and the call then continuing with the call area redefined using the new parameter (s) . This allows a call to remain established for long periods of time and the way in which the call area is defined to be changed from time to time during the call.

In a particularly preferred embodiment, the mobile reference point, with reference to which the call area is defined, may be changed during a call. This allows, for example, when the mobile reference point is a person or vehicle of particular significance to the call for example, that person or vehicle to be changed during the call to allow for shift changes or the like. The means for carrying out methods in accordance with the present invention may comprise pure hardware means such as discrete components or hardwired logic gates. Alternatively, the methods 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 data transmitting apparatus comprising a digital computer causes in conjunction with said computer said system to carry out the steps of the method of the present invention. Such a computer software carrier could be physical storage media 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 may further be appreciated that not all steps of the methods of the invention need to 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.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which :- Fig. 1 is a flow diagram illustrating a method of establishing a call in a mobile communications system in accordance with the present invention;

Fig. 2 is an illustration of a shape defining message for use in the method of Fig. 1; Fig. 3 is a first example of a call area defined in accordance with the present invention; and

Fig. 4 is a second example of a call area defined in accordance with the present invention.

As is known in the art, in order to establish a call in a mobile communication system, a call request is sent by a mobile station to a system infrastructure, for example via a base station of the system. The call request is then processed and the calling mobile station connected to other (called) mobile stations of the system to establish the call. Calls may also be initiated by a controller, at the volition of the controller or due to a call request from outside the system, in which case the controller creates a call request similar to that sent by a calling mobile station.

Figure 1 illustrates one embodiment of a call set-up process in accordance with the present invention. When a base station, controller or other designated call processing device of the mobile communication system receives a call request, the system infrastructure processes that call request by initiating, at step 1, a call processing routine.

At step 2 , it is determined whether or not the call request relates to a group call. In this example, if the call request does not relate to a group call, the call may be established by any method known in the art and the details of this are not therefore described here. However, it should be understood that the present invention is not only applicable to group calls, but to any call requiring a restricted call area including where only one other mobile station or user of the system is called.

At step 3, it is determined whether or not the call

I request is for a geographically restricted or localised call. In this example, it is determined whether or not a call is a localised call by the presence or absence of a call area defining message in the call request. If the group call is not a localised (geographically restricted) call, i.e. there is no call area defining message in the call request, the system proceeds to process the call in a conventional manner for a group call. (In which case, at step 4, a list of base stations providing service to each of the members of the group call is created and, at step 5, the group call is connected via these base stations to the members of the group defined in the call request. The call then proceeds until it is detected, at step 6, that the call has ended, at which point the call is terminated at step 7 and the call processing routine is exited from at step 8.)

If it is detected that the call request is for a localised call, i.e. there is a call area defining message in the call request, the call is processed according to steps 9 to 19. Figure 2 shows an embodiment of a call area defining message in accordance with the present invention. The message contains a message header 20, locating radio identity 21 and shape identity 22. Optionally it also contains a lag factor 23, trail duration 24 and orientation indication 25. As mentioned above, the message header 20 is used to allow detection of whether or not the call request is for a localised call. In addition, the header 20 indicates the start of the call area defining message in the call request. The locating radio identity 21 identifies the reference mobile station that is to be used as the mobile reference point to define the call area. The shape identity identifies the reference geometrical shape (and its size where appropriate) that is to be located to define the call area. This might, for example, be a circle of a given radius. The lag factor 23, trail duration 24 and orientation indication 25 all relate to further features that are used, in this example, to define the call area, and are described below.

At step 9, in Figure 1 the call area defining message is identified using the header 20 and the locating radio identity 21 is read from the message. The identity of reference mobile station is then stored as the mobile reference point to be used to define the call area and the present location of the reference mobile station is found and stored in order that it can be used to locate the shape defining the call area. In this example, the location of the reference mobile station is found by identifying the base station of the mobile communication system presently in use by the mobile station, which is generally stored in the system as is known in the art. In another example, the location may be determined by a Global Positioning System (GPS) system, or some such position determining system.

At step 10, the call area is defined and a list of base stations or sites presently serving called mobile stations within the defined call area is created. (In an alternative embodiment, the list could include all base stations of the system within the defined call area.) First, the shape identity 22 is read from the shape defining message to identify the shape definition desired to be used to define the call area. In this example, the shape identity 22 defines a geometrical shape which is to be located with reference to the location of the mobile reference point to define the call area. In another example, the shape identity 22 may simply be the index address of a shape definition stored by a database of the system and, on reading the shape identity 22, the system may retrieve the shape definition using the index address .

Once the shape definition has been determined, the call area is defined by locating the defined shape using the stored present location of the mobile reference point. The shape definition contains an origin which is mapped to the location of the mobile reference point.

In addition, any orientation indication 25 is read from the call area defining message. Orientation data will generally not be present when the defined shape is a circle, as such data is not required for a circle. However, where the defined shape must be orientated to be properly located to define the call area, orientation data is included in the call area defining message. In this example, the orientation indication 25 comprises an indication of whether the orientation of the shape should be fixed relative to the Earth's latitude and longitude, or relative to the direction of movement of mobile reference point (obtained, for example, by interpolating previous stored locations) . An axis used to define the shape, e.g. the y-axis of a Cartesian co-ordinate system is then fixed parallel to the Earth's longitude, or the direction of movement of the mobile reference point according to the orientation indication 25.

The system also reads any lag factor 23 which is present in the call area defining message. The lag factor 23 comprises a time or distance behind the present location of the mobile reference point, and thereby indicates a previous location of the mobile reference which is actually to be used as the location for locating the shape instead of the current position of the mobile reference point. For example, if the lag factor was 10 minutes, then the (stored) location of the mobile reference point from 10 minutes previously would be used to locate the shape defining the call area, with the result that the shape (call area) lags behind the present location of the mobile reference point by 10 minutes .

Once the shape has been located, the call area is defined by determining which base stations of the mobile communication system have coverage that extends into the area of the located shape. Those base stations are then stored as a 'site' list, and their coverage areas defines the actual call area.

At step 11, the system connects called mobile stations presently being served by base stations on the 'site' list to the group call. In this example, this is achieved by only forwarding the call to base stations that are on the 'site' list of selected base stations.

The system then enters a call control phase in which the reference mobile station is monitored to see whether it moves position. In this example, at step 12, the reference mobile station is monitored to see when it moves to be serviced by a new base station. In another example, the location of the reference mobile station may be monitored by checking the actual position of the reference mobile station using a global positioning system (GPS) or the like at regular intervals.

When movement is detected the shape is re-located using the new location of the reference mobile station along with any orientation indication 25 or lag factor 23. The call area is thus redefined by updating the selected base station ^"'site' list. At step 13, any base stations that come within the area defined by the located shape that were not previously within that area are added to the 'site' list.

When the area defined by the located shape moves, any base stations that were previously within that area, but are now no longer within the area defined by the located shape, may be removed from the 'site' list and no longer form part of the call area. Alternatively, those base stations could be allowed to remain within the call area for a selected time period after they are no longer covered by the area defined by the relocated shape before they are removed from the 'site' list. The initial call request preferably therefore includes a trail duration 24 indicating the time period that such base stations are to remain as part of the call area once they no longer fall within the located shape. Where the call request includes a trail duration, the system starts a trail timer when a base station is no longer covered by the located shape, which trail timer counts down from an initial value corresponding to the trail duration value 24 from the shape defining message. The given base station then remains as part of the call area until the trail timer has expired. So, if no trail duration is set, or the trail duration is 0 minutes, the base stations are removed from the 'site' list and therefore from the defined call area virtually straightaway. However, where the trail duration has some value, e.g. 20 minutes, base stations no longer in the area defined by the located shape remain within the defined call area for 20 minutes. The defined call area would then map out a trail extending behind the locating mobile unit (see also Figure 4) .

The system thus checks at step 14 , whether the trail timers of any base stations have expired. If so, at step 15, any base stations whose trail timer has expired is removed from the selected base station ('site') list. In an alternative embodiment in which the defined call area is maintained between calls, the base stations whose trail timer has expired might only be removed from the list when a call terminates.

At step 16, it is checked whether or not the 'site' list- of selected base stations (i.e. the defined call area) has changed. If so, at step 17, the call is re- routed to the newly selected base stations (i.e. to the called mobile stations within the newly defined call area) . Trail timers are also started (at step 18) for any base stations no longer having coverage in the area of the newly located shape . At step 19, the system determines whether or not the call has been cleared, e.g. terminated by all of the mobile stations or by a controlling mobile station, etc. If so, the system proceeds to terminate the call, at step 7 and 8, in the usual way. If not, the system returns to step 12, at which it is determined whether or not the locating radio unit has moved.

Figure 3 shows an example of call areas defined in accordance with the present invention. Figure 3 is a map of idealised base station coverage areas 26. In the example shown in Figure 3, the shape defined for the call area is a circle 27 of radius R (which is typically a few kilometres, e.g. 30 km), and base stations that have some degree of coverage within the located circle 27 are included in the call area. Thus when the mobile reference point is at location 28, the circle 27 is located as shown in Figure 3 with its centre at the point 28 and the call area is defined as those base stations that have any coverage within the so-located circle 27, i.e. the shaded area A in Figure 3. For another call, after the mobile reference point has moved to position 29, the circle 27 is located as shown in Figure 3 to define a new coverage area B . Figure 4 illustrates the situation where the mobile reference point moves during the call and a trail duration has been set to a period of time long enough for several previous base stations no longer covered by the call area defining shape to remain in the call area for a certain time period. In this example, a circle is again used to define the geographical area of the localised call, but the radius of the circle has been set to zero, such that the shape is therefore effectively a point and therefore the "current" call area is effectively the coverage area of the base station in whose coverage area the mobile reference point is currently located. Figure 4 illustrates the situation where the mobile reference point has moved from position 30 to a current location at position 40 during the call. At previous positions 33 to 39, the trail timers for the base stations have not yet expired and the active call area therefore comprises the coverage areas of the base stations at all positions 33 to 40, i.e. the shaded area in Figure 4. However, the trail timers for the base stations at positions 30 to 32 have expired and although these locations are along the route of the mobile reference point, the base stations at these locations are no longer in the active call area.

Claims

1. A method of connecting mobile stations to a call in a mobile communication system comprising: defining an area to which the call is to be restricted with reference to a mobile reference point; and connecting one or more mobile stations within the defined area to the call .

2. The method of claim 1, wherein the mobile reference point is a vehicle or a person involved in or related to the call.

3. The method of claim 1 or claim 2, wherein the mobile reference point is a mobile station of the mobile communication system.

4. The method of any one of the preceding claims, wherein the mobile reference point is a mobile station to be connected to the call .

5. The method of any of claims 1 to 3 , wherein the mobile reference point is a mobile station requesting the call to be set up.

6. The method of any one of the preceding claims , wherein the call area is defined with reference to the location of the mobile reference point.

7. The method of claim 6, wherein the location of the mobile reference point is defined as the location of a base station of the system in whose coverage area the mobile reference point is currently located.

8. The method of any one of the preceding claims, wherein the call area is defined as the coverage area of a base station of the system in whose coverage area the mobile reference point is located.

9. The method of any one of claims 1 to 7 , wherein the call area is defined as the coverage area of a base station of the system in whose coverage area the mobile reference point is located and the coverage area of all of its neighbouring base stations.

10. The method of any one of claims 1 to 7, wherein the call area is defined by locating a geometrical shape with reference to the location of the mobile reference point .

11. The method of claim 10, wherein the shape to use is selected from a plurality of stored predetermined shapes .

12. The method of claim 10 or claim 11, wherein the shape is defined in terms of its dimensions with reference to an origin which is mapped to the location of the mobile reference point to locate the shape and define the call area.

13. The method of claim 12, wherein the shape is a particular range or distance in any direction from the origin.

14. The method of any one of claims 10 to 12, wherein the shape is a line of given length.

15. The method of any one of claims 10 to 14, wherein the shape is located with a particular orientation.

16. The method of any one of claims 10 to 15, wherein the shape is orientated at a particular angle with reference to the Earth's latitude and longitude.

17. The method of any one of claims 10 to 15, wherein the shape is orientated at a particular angle with reference to the direction of movement of the mobile reference point .

18. The method of any one of claims 10 to 17, wherein the call area is defined as the communication coverage areas of base stations of the system whose communication coverage areas lie within the area covered by the located shape.

19. The method of claim 18, wherein the call area is defined as the area of coverage of all those base stations whose coverage lies entirely within the located shape.

20. The method of claim 18, wherein the call area is defined as the area of coverage of all those base stations whose coverage lies at least partly within the located shape.

21. The method of any one of the preceding claims, wherein the call area is defined with reference to the movement of the mobile reference point .

22. The method of any one of the preceding claims, wherein the call area is defined by distorting or extending an area or shape with reference to the movement of the reference point .

23. The method of any one of the preceding claims, wherein the call area is defined with reference to the current location of the mobile reference point.

24. The method of any one of the preceding claims, wherein the call area is defined with reference to a previous location of the reference point.

25. The method of claim 24, wherein the previous location, with reference to which the call area is defined, is a relative distance or time behind the current location of the mobile reference point.

26. The method of any one of the preceding claims, wherein the call area is defined with reference to more than one location of the reference point.

27. The method of any one of the preceding claims, wherein the call area is defined with reference to both the present and one or more previous locations of the reference point .

28. The method of claim 26 or claim 27, wherein a shape is located with reference to each location of the reference point and the call area is defined as the area covered by all of the located shapes.

29. The method of any one of the preceding claims, wherein each time a new location is detected for the mobile reference point, the call area is redefined.

30. The method of claim 29, wherein the previously defined call area is not removed immediately from the presently defined call area, but the previously defined call area remains as part of the presently defined call area for a predetermined period of time.

31. The method of any one of the preceding claims, wherein the mobile reference point, with reference to which the call area is defined, can be changed during the call.

32. The method of any one of the preceding claims, wherein the call to which the mobile stations are connected is requested by a mobile station of the mobile communication system.

33. The method of any^"one of the preceding claims, wherein a mobile station of the system makes a request for the call, the request comprising: transmitting the identity of one or more mobile stations of the system to which the call is to be established; and transmitting the identity of a mobile reference point in relation to which a restricted geographical area for the call is to be defined.

34. A method of making a call request in a mobile communications system, which call is to be restricted to a particular geographical area covered by the system, the method comprising: transmitting the identity of one or more mobile stations of the system to which the call is to be established; and transmitting the identity of a mobile reference point in relation to which a restricted geographical area for the call is to be defined.

35. The method of claim 33 or claim 34, wherein the transmission of the identity of the one or more mobile stations of the system to which the call is to be established comprises the transmission of the identification code or serial number of the mobile stations to be called.

36. The method of claim 33 or claim 34, wherein the transmission of the identity of the one or more mobile stations of the system to which the call is to be established comprises the transmission of a group address identifying a group of mobile units, the membership list of which is maintained by the system infrastructure .

37. The method of claim 36, wherein the group address includes a predefined mobile reference point .

38. The method of any one of the preceding claims, further comprising: a system receiving the or a request for the call, which call is to one or more mobile stations of the system, and, in response to the call request, the system infrastructure attempting to connect the called mobile stations to the call only if it/they are within the defined area.

39. A method of establishing a call in a mobile radio communications system which is to be restricted to a particular geographical area covered by the system, and which system comprises one or more mobile stations and a system infrastructure for establishing and routing communications between the mobile stations, the method comprising: the system infrastructure receiving a request for a call to one or more mobile stations of the system, which call is to be restricted to the particular geographical area and, in response to the call request, the system infrastructure attempting to connect the called mobile station (s) to the call only if they are within the particular geographical area, wherein the geographical area to which the call is to be restricted is defined with reference to a mobile reference point.

40. The method of any one of claims 33 to 39, wherein a plurality of stored shapes each have a corresponding shape identification and are stored by the system and the call request includes the shape identification for the selected shape.

41. The method of any one of claims 33 to 40, wherein a plurality of predetermined orientations, each having a corresponding orientation identification, are stored by the system and the call request includes the orientation identification for the selected shape.

42. The method of any one of claims 33 to 41, wherein a previous location of the mobile reference point with reference to which the call area is to be defined is indicated in the call request.

43. The method of claim 42, wherein the call request includes a lag factor representing a relative time previously at which the location of the mobile reference point is to be considered as the previous location.

44. The method of claim 43 , wherein plural predetermined lag factors are stored in a database and the lag factor to use is identified in the call request .

45. The method of claims 33 to 44, wherein a selected time period for which a previously defined call area is not removed from the presently defined call area, but remains as part of the presently defined call area is included in the call request.

46. Computer software specifically adapted to carry out the method of any one of claims 1 to 45 when installed on data processing means.

47. An apparatus for connecting mobile stations to a call in a mobile communication system comprising: means for defining an area to which the call is to be restricted with reference to a mobile reference point ; and means for connecting one or more mobile stations within the defined area to the call.

48. The apparatus of claim 47, wherein the mobile reference point is a mobile station of the mobile communication system.

49. The apparatus of claim 47 or claim 48, wherein the call area is defined with reference to the location of the mobile reference point .

50. The apparatus of any one of claims 47 to 49, wherein the location of the mobile reference point is defined as -the location of a base station of the system in whose coverage area the mobile reference point is currently located.

51. The apparatus of any one of claims 47 to 50, wherein the call area is defined by locating a geometrical shape with reference to the location of the mobile reference point.

52. The apparatus of claim 51, wherein the shape is located with a particular orientation.

53. The apparatus of any one of claims 47 to 52, wherein the call area is defined with reference to the movement of the mobile reference point .

54. The apparatus of any one of claims 47 to 53, wherein the call area is defined with reference to both the present and one or more previous locations of the reference point .

55. The apparatus of any one of claims 47 to 54, wherein a previously defined call area is not removed immediately from the presently defined call area, but the previously defined call area remains as part of the presently defined call area for a predetermined period of time.

56. A calling apparatus for requesting a call in a mobile communications system, which call is to be restricted to a particular geographical area covered by the system, the apparatus comprising: i means for transmitting the identity of one or more mobile stations of the system to which the call is to be established; and means for transmitting the identity of a mobile reference point in relation to which a restricted geographical area for the call is to be defined.

57. A mobile radio unit comprising the calling apparatus of claim 56.

58. A control unit of a communication system comprising the calling apparatus of claim 56.

59. A mobile radio communications system for establishing a call which is to be restricted to a particular geographical area covered by the system, the system comprising: one or more mobile stations and a system infrastructure for establishing and routing communications between the mobile stations; wherein the system infrastructure comprises means for attempting, in response to a request for a call to one or more mobile station (s) of the system which is to be restricted to a particular geographical area, to connect the called mobile station (s) to the call only if they are within the indicated geographical area, and wherein the geographical area to which the call is to be restricted is defined with reference to a mobile reference point .

60. The mobile radio communication system of claim 59, wherein the system infrastructure comprises a fixed radio network .

61. The mobile radio communications system of claim 59, wherein the system infrastructure comprises, entirely or in part, any number of- mobile stations adapted to perform any of the functions of the system infrastructure.

62. An apparatus substantially as described with reference to any one of the accompanying drawings .

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

64. A method substantially as described with reference to any one of the accompanying drawings .