WO2005107308A1

WO2005107308A1 - Mobile communications systems

Info

Publication number: WO2005107308A1
Application number: PCT/GB2005/001597
Authority: WO
Inventors: Mark Wentworth Rayne
Original assignee: Sepura Limited
Priority date: 2004-04-27
Filing date: 2005-04-27
Publication date: 2005-11-10
Also published as: GB2415332A; GB0508564D0; EP1757140A1; GB2415332B; GB0416039D0

Abstract

A communications system (1) includes an indirect access cell (2), which includes one or more voice traffic channels, a common, shared main control channel, and one or more indirect access packet data channels, and a dedicated data cell (3) that provides one or more packet data channels, but that does not offer any voice traffic channels, or include a common control channel. When a mobile station MS1 that is currently active on a data channel of the direct access data cell (3) wishes to take place in a voice call with other mobile stations MS2, MS3 and/or MS4 in the system, the mobile station MS1 is temporarily transferred from the direct access data cell (3) to the indirect access cell (2) to participate in the voice call. This is achieved by the system infrastructure sending appropriate signalling on the data channel of the cell (3). The mobile station MS1 is arranged to return automatically to its original packet data channel on the cell (3) once its participation in the voice call on the indirect access cell (2) has finished.

Description

Mobile Communications Systems The present invention relates to mobile communications systems, and in particular to such systems that include the facility to transmit packet data . As is known in the art, mobile communications systems are usually arranged on a "cellular" basis, with plural adjoining and/or overlapping cells, each serviced by a base station or base stations, providing communications services to mobile stations that fall within their coverage areas . It is becoming increasingly common for cells of mobile communications systems to provide dedicated data channels for the transmission of data, and in particular packet data, as well as providing more normal "voice" traffic channels. For example, the TETRA (TErrestrial Trunked RAdio) system supports the use of dedicated packet data channels . "Packet data" channels, as is known in the art, transmit data in discrete packets and are typically shared by a number of users that each transmit and receive their data packets on the channel. The data packets transmission can be intermittent or continuous, as desired. Typically many users transmit packets of data that are interleaved (so that each user receives a timely delivery or transmission of their data) . In such arrangements it is generally beneficial to use a single high-bandwidth packet data channel for the trunking efficiency that provides, rather than splitting users between a number of lower bandwidth packet data channels . In one such arrangement, a cell of the communications system may offer, for example, one or more voice traffic channels that can support circuit mode voice calls (circuit mode voice calls, as is known in the art, take place on traffic channels that are distinct from packet data channels and use "circuit mode" as opposed to packet data) and one or more (packet) data channels. Such a cell will typically also include a shared, common control channel or channels via which signalling, etc., to allocate mobile stations to voice or data channels in accordance with their requests is received and transmitted. In such an arrangement, the data channels supported by the cell can be considered to be "indirect" access data channels, in that mobile stations will register and authenticate on the control channel (s) of the cell, and signalling on the control channel will be used to send a mobile station to one of the data channels (which will typically, as is known in the art, be shared with one or more mobile stations) when a mobile station or the base station wishes to send (packet) data. Equally, when a data transmission has finished, the mobile station will be sent back to the common control channel by an explicit "go to channel" message sent on the (packet) data channel, or by an implicit instruction to return to the common control channel (e.g. if mobile station sees that the channel usage marker has changed (thereby implying that its call has terminated) ) . In other words, users are assigned to "traffic" channels (voice or data) by use of channel assignment messages on a shared, common control channel . Such an "indirect" access cell can provide a full range of "call" services. Thus, for example, a mobile station active on such a cell can easily participate in individual and group voice calls, send and receive SDS (short data service) or text messages, and send and receive packet data, etc. Furthermore, mobile stations may obtain any necessary time and frequency synchronisation for use on the voice or data channels of the cell from the control channel (s) of the cell. As all of the channels of the cell are served by co-located transmitters and receivers, they will all have identical coverage areas . As discussed above, it is becoming increasingly common for mobile communications systems to support (packet) data channels that have wider bandwidth than the "normal" control and (voice) traffic channels of the system, as this permits an increased data transmission rate on the data channels. This is the case, for example, for TETRA high-speed data. However, a consequence of using such higher bandwidth data channels in that they generally have lower ranges (and so cover smaller areas) than the

"normal" control and traffic channels. (This is because it is not generally practical or permitted to increase the transmitter power for the higher bandwidth data channels to compensate for their increased bandwidth, and so as the available transmitter energy is spread more thinly, they tend to have a smaller range and coverage area. ) One consequence of the reduced range of such higher bandwidth data channels is that the cells providing them will be smaller (and accordingly greater in number to cover a given geographical area) . Therefore, if it is desired to provide the channels of such data cells in an indirect access form, additional control channels may be required. However, it may not always be desirable to provide additional control channels for this purpose. One way to compensate for this would be to use "sectored" cells for the "high-bandwidth" data channels that use directional antennas so as to effectively increase the range of a single such channel. In this way, a single common control channel could be used to support several co-located high bandwidth data channels that are each arranged to cover a different sector of the area covered by the control channel . In other arrangements, it is proposed to use dedicated data cells that operate in a "direct" access manner, i.e. with registration, authentication and control signalling taking place on the data channels themselves, rather than on a common, "separate" control channel intended for that purpose, i.e. in which all signalling takes place on the (packet) data channel themselves. In such an arrangement, a mobile station would access the data channel directly, rather than being assigned there from a "control" channel. Such "dedicated" data cells would accordingly provide "high bandwidth" data channels only, and would not have any common control channel (s) or (voice) traffic channels associated with them. Such dedicated, data-only cells may be useful, for example, near the edge of existing "normal" voice and data cells, so as to augment the provision and coverage of data services at the edges of existing cells. It may also be beneficial to provide an (overlapping) independent network of such higher speed, broader bandwidth packet data direct access cells covering the same territory as the "basic" network that provides indirect access (circuit mode) voice and data services. Such networks could, e.g., be semi-independent and could even be in different frequency bands . The Applicants have recognised that mobile stations, etc., using such a dedicated data cell (or network) are likely still to wish to be able to participate in, e.g., normal voice calls within the system as and when they desire or are required to do so. However, this cannot be done simply by switching the mobile station to a voice traffic channel of the data cell, since the dedicated data-only cell will not support circuit mode voice services . The voice call could proceed using one of the dedicated packet data channels of the data cell, but voice over packet data tends to be inefficient. Also, as noted above, the data cell will tend to be lower range, which may be less efficient for voice calls. The Applicants believe therefore that it would be desirable to provide a mechanism whereby users active on dedicated, direct access data-only cells can still participate in circuit mode voice calls, etc., as and when they desire to do so. According to a first aspect of the present invention, there is provided a method of operating a mobile communications system which comprises one or more dedicated data service cells, and one or more cells providing voice call services that overlap with the coverage areas of the data service cells, the method comprising: using signalling on a data channel of a data service cell to transfer a mobile station on that cell to a channel of another cell of the communications system. According to a second aspect of the present invention, there is provided a mobile communications system which includes one or more dedicated data service cells and one or more cells providing voice call services that overlap with the coverage areas of the data service cells, the system comprising: means for using signalling on a data channel of a data service cell to transfer a mobile station on that cell to a channel of another cell of the communications system. According to a third aspect of the present invention, there is provided an apparatus for use in a mobile communications system which includes one or more dedicated data-only service cells and one or more cells providing voice call services that overlap with the coverage areas of the data service cells, the apparatus comprising: means for using signalling on a data channel of a data service cell to transfer a mobile station on that cell to a channel of another cell of the communications system. In the present invention, signalling on a data channel of a dedicated data-only cell is used to transfer a mobile station using that data cell (e.g. channel) to a channel of another cell of the communications system. This facilitates, e.g., transferring a mobile station that is transmitting or receiving data on a data-only cell to a voice call, even though there is no common control channel for the signalling for such a transfer provided in the data cell. Thus, the present invention allows a mobile station on a direct access data cell to be transferred to a, e.g., voice call on another, e.g., indirect access, cell of the communications system. Thus, according to a fourth aspect of the present invention, there is provided a method of operating a mobile communications system that includes one or more cells that provide direct access data channels, the method comprising: using signalling on a direct access data channel of the system to transfer a mobile station to another channel of the communications system. According to a fifth aspect of the present invention, there is provided a mobile communications system that includes one or more cells that provide direct access data channels, the system comprising: means for using signalling on a direct access data channel of the system to transfer a mobile station to another channel of the communications system. According to a sixth aspect of the present invention, there is provided an apparatus for a mobile communications system that includes one or more cells that provide direct access data channels, the apparatus comprising: means for using signalling on a direct access data channel of the system to transfer a mobile station to another channel of the communications system. These aspects of the invention can, as will be appreciated by those skilled in the art, include any one or more or all of the preferred and optional features of the invention discussed herein. Thus, for example, the direct access data channel is preferably on a data-only cell of the system, and the mobile station is preferably transferred to a channel on another cell of the communications system. The system also preferably supports both direct access and indirect access channels (and cells) . A particular advantage of the present invention is that because it facilitates the participation of mobile stations using direct access data channels and cells in normal voice calls, etc., and can allow mobile stations using direct access channels and cells to still have access to the services of the (indirect access) voice and data cells of the communications system, it facilitates overlaying smaller, higher bandwidth direct access data cells over the "basic" lower-speed, indirect access voice and data cells of a mobile communications system. The present invention can therefore be used to facilitate an overall enhanced communications system, that can, e.g., support higher rates of data transfer without needing to use the normal channel capacity of the (existing) voice (plus data) network. The cell or channel that the mobile station is transferred to can be selected as desired. It would, as discussed above, typically be (and preferably is) a (voice) traffic channel (preferably a circuit mode voice channel) of a cell of the system (where, e.g., the mobile station is to participate in a voice call) , but it could also be a control channel of the system, or even another data channel of the system (such as, even, another dedicated, e.g., direct access, data channel of the system) . The new channel is, as discussed above, preferably (and typically will be) on a different cell of the system. The data channel that the mobile station is transferred from can be any suitable such data channel. It is preferably one that operates in a direct access manner. It is preferably (and typically will be) a packet data service channel, and most preferably a data channel that has a relatively higher bandwidth (and hence smaller coverage area) than, e.g., normal voice channels of the system. The signalling that is used to transfer the mobile station to the new cell can be selected as desired. It preferably comprises a "go to channel" message that sends the mobile station to the new channel, e.g., to a frequency on a different, e.g., indirect access cell, of the system. This go to channel message can, as is known in the art, be piggybacked onto (combined with) other messages, such as a message containing details about the call that is being set up. It is believed that the use of such a go-to-channel message on a direct access channel may be novel and inventive in its own right. Thus, according to a seventh aspect of the present invention, there is provided a method of operating a mobile communications system that includes one or more direct access cells, the method comprising: transmitting a go to channel message on a channel of a direct access cell to instruct a mobile station to go to another channel of the communications system. According to an eighth aspect of the present invention, there is provided a mobile communications system that includes one or more direct access cells, comprising: means for transmitting a go to channel message on a channel of a direct access cell to instruct a mobile station to go to another channel of the communications system. According to a ninth aspect of the present invention, there is provided an apparatus for a mobile communications system that includes one or more direct access cells, the apparatus comprising: means for transmitting a go to channel message on a channel of a direct access cell to instruct a mobile station to go to another channel of the communications system. The signalling that transfers the mobile station to the new channel (e.g. the "go to channel" message) preferably includes all information and data that the mobile station may need to be able to communicate, e.g., participate in a call, etc., on the new channel (i.e. such that once it is on the new channel, the mobile station does not need to seek further information to be able to start communicating) . (It should be noted here that this does not preclude the need for the mobile station potentially to have to obtain information on the new cell or channel to allow it to synchronise to the new channel (although this may not be necessary if the two channels are approximately synchronised (i.e. close enough for the training sequences, or synchronisation symbols, etc., on the new channel to pull the mobile station into synchronisation with the new channel; if not, the mobile station may need to retrieve synchronisation information for the new channel from the system) ) . Thus, as well as indicating the frequency (and, e.g., timeslot(s), in the case of a TDMA (Time Division Multiple Access) system, or spreading code in the case of a CDMA (Code Division Multiple Access) system) of the new channel, the signalling on the data channel preferably also includes any appropriate system identifier, colour (scrambling) code and/or encryption information (e.g. keys or key identifier (s) ) that may be necessary to allow the mobile station to participate in the new (e.g. voice) call. (As is known in the art, many communications systems use mechanisms, such as a system identifier, scrambling code (colour code) and/or encryption to prevent terminals from being able to understand, and from responding to, signalling from another channel, cell, or system, etc. As the mobile station is being transferred to a new channel and most probably also a new cell, it will need such information to be able to communicate on the new channel . ) It is believed that including additional data, such as a colour code or encryption information in a "go to channel" message may be new and advantageous in its own right. Thus, according to a tenth aspect of the present invention, there is provided a method of operating a mobile communications system, comprising: when it is desired to transfer a mobile station of the system to another channel of the system, transmitting a channel transfer signal to the mobile station that includes the identity of the new channel, together with additional information to allow the mobile station to receive correctly communications transmitted on the new channel . According to an eleventh aspect of the present invention, there is provided an apparatus for a mobile communications system, comprising: means for, when it is desired to transfer a mobile station of the system to another channel of the system, transmitting a channel transfer signal to the mobile station that includes the identity of the new channel, together with additional information to allow the mobile station to receive correctly communications transmitted on the new channel . In these aspects of the invention, the additional information that is included in the "go to channel" message preferably includes, as discussed above, such information as may be necessary to allow the mobile station to receive and interpret correctly (decode) the signals it receives on the new channel (and equally to allow it to transmit correctly on the new channel), i.e. to allow it to participate in a call on the new channel without the need to receive any further information from the system (save for, as discussed above, any need for the mobile station to obtain information to allow it to synchronise to the new channel) . Thus the additional data preferably includes system, and preferably cell, related data for the new channel, such as details of any colour (scrambling) code and/or encryption that is to be used on the new channel. The signalling to transfer the mobile station to another channel can be initiated as desired. Thus, for example, in the case of a third party user making a call to the mobile station on the data cell, the system infrastructure would initiate the signalling to transfer the mobile station. In a particularly preferred embodiment, a mobile station on a data channel can request itself a transfer to another channel of the system, e.g., can request a voice call. In such an arrangement, the initial (call) request is preferably sent on the data channel of the data cell. In a particularly preferred embodiment of the present invention, a mobile station that has been transferred from a data channel to another channel of the communications system in accordance with the present invention can be and preferably is returned automatically (i.e. without the need for user intervention) to its previous data cell or channel on termination of its need to be present on the channel it was transferred to, for example on termination of its participation in the call on the channel it was transferred to. In a particularly preferred such arrangement, the mobile station "remembers" that it has been transferred from a data channel (cell) and returns itself automatically to that cell once its participation on the new channel has finished (e.g. it receives an end of call message (call cleardown message) and/or end of call "go to control channel" message on the new channel) . In this arrangement, the mobile station preferably stores the fact that it has come from a data channel (or cell) , preferably together with any necessary data cell and/or data channel details (such as frequency, timeslot, etc., as discussed above in relation to the message to transfer the mobile station to the new channel) , and then, if it has such an indication stored, seeks to return to the data channel once its participation in the new call, etc., has finished. The mobile station could also, e.g., record any synchronisation differences it observed between its initial and the new channel, and apply those differences in reverse when returning to its initial channe1. In such an arrangement, the mobile station could, e.g., ignore any "go to channel" instructions in a "go to channel" message terminating a call on the new channel and return directly to its stored data channel (cell) , or it could obey the "go to channel" instructions and go to the, e.g., control channel indicated, and then return itself to its stored data channel (or cell) thereafter. In the latter case, therefore, the mobile station will return to its data channel via an intermediate, e.g., control, channel. It should be noted here that the mobile station is preferably arranged to respond and act in the above manner whether the "go to channel" instructions are explicitly broadcast to the mobile station, or are as a result of an implicit or automatic channel allocation operation of the system. For example, in a TETRA system, if the present channel is removed for any reason (e.g. because there is an implicit channel allocation in signalling received by the mobile station, or the mobile station itself observes a change in the channel usage marker) , then the TETRA protocol requires a mobile station to return to the main control channel (MCCH) of the cell. In the present embodiments and arrangements of the present invention, it is preferred for the mobile station to ignore this form of channel reallocation as well, and to, instead, return to its original cell (e.g. in the manners discussed above) in the event of such an "implicit" channel reallocation. This applies to all arrangements where the mobile station is to be automatically returned to its original cell on termination of its participation in the group call. An advantage of these arrangements is that they avoid the need to include additional signalling bits in the go to channel messages (e.g. on the data channel or after the new call has finished) that are sent to the mobile station to ensure that the mobile station returns to the data channel or cell it was previously on. In another preferred embodiment, the mobile station is arranged to return automatically to its initial data channel by including appropriate commands and/or data in the signalling that transfers the mobile station from the data channel in the first place, and/or by signalling the mobile station to return to the data channel once, e.g., the call on the new channel has finished. Thus, in one preferred such arrangement, the initial signalling on the data channel to transfer the mobile station to the new channel includes an instruction to the mobile station that it can expect to return to the data cell (or channel) at the end of its participation on the new channel. This could be achieved, e.g., by setting an appropriate bit in this initial signalling (the "go to channel" message) , for example such that a parameter is included in the message sending the mobile station to the new channel, advising the mobile station that it should return to the data cell or channel when the, e.g., voice call, terminates. It is common in mobile communications systems for the system to signal to mobile stations at the end of, e.g., a group call, that they should, e.g., return to the main control channel of the cell after the call has finished. This is typically done by sending a "go to channel" message specifying the control channel at the end of the call. As is known in the art, this go to channel message can be and typically will be combined with other messages, such as an "end of call" (call cleardown) message. Thus another way to return the mobile station to the data cell or channel would be to include in this "go to channel" message an instruction that if a mobile station came from a data cell or channel, it should return to that data cell or channel, rather than the control (or other channel) otherwise indicated in the go to channel message. In this way, the go to channel message at the end of the call will tell all mobile stations that come from a data cell or channel that they can ignore the indicated channel in the go to channel message and instead return to their original data channels (or cells). This could be achieved, e.g., by providing a special field or bit in the go to channel signalling that can be set appropriately. An advantage of this arrangement is that the system infrastructure can control whether it sends a mobile station that has come from a data cell or channel back to that data cell or channel or not . Thus in another preferred embodiment, the mobile stations can be selectively automatically returned to their initial data channel or cell . It will be appreciated in these arrangements that the mobile station that was originally on the dedicated data channel is temporarily transferred to a new channel (preferably on a new cell) during the, e.g., voice call, and then returns to its original data channel after its participation in the call has terminated. It is believed that such an arrangement may be new and advantageous in its own right. Thus, according to a twelfth aspect of the present invention, there is provided a method of operating a mobile communications system that includes dedicated data channel or cell communications capability, comprising: causing a mobile station that is on a dedicated data channel or cell to transfer to another channel of the system to participate in a call or signalling on that channel; and returning the mobile station automatically to the data channel or cell once its participation in the call or signalling on the new channel has been terminated. According to a thirteenth aspect of the present invention, there is provided an apparatus for a mobile communications system that includes dedicated data channel or cell communications capability, comprising: means for causing a mobile station that is on a dedicated data channel or cell to transfer to another channel of the system to participate in a call or signalling on that channel; and means for returning the mobile station automatically to the data channel or cell once its participation in the call or signalling on the new channel has been terminated. As will be appreciated by those skilled in the art, these aspects and embodiments of the invention can include any one or more or all of the preferred and optional features of the present invention discussed herein. Thus, for example, the channel to which the mobile station is transferred is preferably on a different cell of the communications system, and, for example, the mobile station is preferably transferred to the other cell by signalling on the data channel. Furthermore, the new channel is preferably a voice traffic channel for a voice call, and, accordingly, the mobile station will return to the data channel or cell, once its participation in the voice call has terminated. Similarly, the dedicated data channel or cell is preferably a direct access such channel or cell, as discussed above. The mobile station may be returned to its initial channel or cell by the system infrastructure, or by the mobile station itself, as discussed above. In a preferred arrangement of these aspects of the invention, the mobile station can be selectively returned (whether by the system infrastructure or by the mobile station itself) to the data channel or cell once its participation in the call or signalling on the new channel has been terminated. The mobile station preferably returns to the actual data channel that it was transferred from. However, this is not necessary, and thus in some arrangements, the mobile station may, for example, instead be returned to the data cell that its original data channel is part of, but not necessarily to the same data channel in that cell. It is believed that enabling a mobile station to be transferred temporarily from its existing cell to a new cell to participate, e.g., in a call, but then returning it to its original cell (rather than, e.g., a control channel on the new cell) may have application beyond the context of such temporary transfer from data channels or cells . Thus, according to a further aspect of the present invention, there is provided a method of operating a mobile communications system, comprising: causing a mobile station that is operating on one cell of the system to transfer temporarily to another cell of the system to participate in a call or signalling on that cell; and returning the mobile station automatically to its initial cell once its participation in the call or signalling on the new cell has been terminated. According to another aspect of the present invention, there is provided an apparatus for a mobile communications system, comprising: means for causing a mobile station that is operating on one cell of the system to transfer to another cell of the system to participate in a call or signalling on that cell; and means for returning the mobile station automatically to its initial cell once its participation in the call or signalling on the new cell has been terminated. As will be appreciated by those skilled in the art, these aspects and embodiments of the invention can include any one or more or all of the preferred and optional features of the present invention discussed herein. Thus, for example, the mobile station is preferably transferred to a voice call on the new cell, and, accordingly, the mobile station will return to its initial cell once its participation in the voice call has terminated. The initial cell is preferably.a direct access cell, as discussed above (with the new cell preferably being an indirect access cell) . The mobile station may be returned to its initial cell or channel by the system infrastructure, or it may return itself, as discussed above. In a preferred arrangement of these aspects of the invention, the mobile station can be selectively returned (whether by the system infrastructure or by the mobile station itself) to its initial cell once its participation in the call or signalling on the new cell has been terminated. The mobile station preferably returns to the channel on its initial cell that it was transferred from. However, this is not necessary, and thus in some arrangements, the mobile station may, for example, instead be returned to a different channel, such as a control channel, of its initial cell. In such an arrangement, when the, e.g. voice call, has terminated, the mobile station will return to a control channel on its initial cell, i.e. a control channel on a different cell to the cell the call took place on. According to a further aspect of the present invention, there is provided a method of operating a mobile communications system, comprising: causing a mobile station that is participating in a call or signalling on one cell of the system to be transferred automatically to a channel of another cell of the system upon termination of the participation of the mobile station in the call or signalling. According to another aspect of the present invention, there is provided a mobile communications system, comprising: means for causing a mobile station that is participating in a call or signalling on one cell of the system to be transferred automatically to a channel of another cell of the system upon termination of the participation of the mobile station in the call or signalling. The data-only cells and direct and indirect access cells, etc., of the communications system that the present invention is applied to can be arranged in any appropriate manner. Thus the communications system could comprise, for example, a system that has a data-only, direct access cellular network overlaid on an indirect access cellular network (i.e. such there is in effect a separate network for each type of service) , with the networks being interconnected in an appropriate manner (and in the manner of the present invention to allow, e.g., mobile stations on the data network to make and receive voice calls on the indirect access network) . It should also be noted in this regard that while, as will be appreciated by those skilled in the art, the communications system will typically comprise a single communications network (e.g. provided by a single service provider) using the same, single communications protocol, it is also envisaged that the present invention can be applied and used over plural different communications networks, using the same or different communications protocols and standards. In the latter case, the communications system will accordingly comprise plural different communications networks that are appropriately interlinked (at least to the extent necessary to operate in accordance with the present invention) . Thus, in one preferred embodiment, the communications system will comprise a single communications network operating to a single communications protocol or standard (e.g. a TETRA network) . However, in other embodiments, the communications system may comprise plural different communications networks. These networks may all operate to the same communications protocol or standard (e.g. all be TETRA networks) , or may operate to different communications protocols and standards (e.g. there may be a TETRA and/or a GSM network, and a satellite-based communications network, etc.). In these latter cases, the data-only, and indirect or direct access, cell or cells may accordingly be on the same or different communications networks of the communications system. It will be appreciated that in order to facilitate operation in accordance with the present invention, it may be necessary for the communications system, e.g., its infrastructure, to know which, e.g., indirect access, voice cell covers the data cell of the mobile station in question (so that it can transfer the mobile station to the appropriate indirect access cell) . Thus the system preferably stores this information in an appropriate form. Alternatively or additionally, mobile stations may be arranged to monitor, e.g., periodically, the control channels of indirect access cells so as to identify the indirect access cell they are currently covered by. The mobile stations could then be arranged, e.g., to indicate on their direct access channel or cell at call request or call set-up the identity of the indirect access cell that they would prefer to use. This may require a two stage call set-up where the mobile station is being called by a third party to, e.g., allow the mobile station to be paged for the identity of its preferred indirect access cell before the channel assignment message is sent. Although the present invention has been described with reference to "direct" access data cells and "indirect" access (voice and data) cells, it is possible that "mixed" access cells may be provided (i.e. cells in which some mobile stations will treat the cell (or a channel of the cell) as though it is "direct" access and other mobile stations will treat the cell (or a channel of the cell) as an "indirect" access cell (or channel) ) . The present invention is equally applicable to such "mixed" access cells, although in such a case it may be desirable to include more "positive" signalling in the go to channel messages, for example to specify to a mobile station or stations that it should treat the allocated channel as though it is on a direct access or an indirect access cell. This could be achieved, e.g., by setting a bit or field in the "go to channel" messages for this purpose. This would then allow different mobile stations to have or be allocated a different "perception" of a channel (i.e. whether it is direct or indirect access) in the same cell. The provision of such "mixed" access cells may be beneficial in their own right, since, for example, they may allow a direct access mobile station to come and go on an indirect access system as it wishes, for example to use packet data channels on the indirect access ^• system when the direct access cell or cells are overloaded or out of range . Thus, according to a yet further aspect of the present invention, there is provided a mobile communications system in which a cell or cells of the system support both indirect and direct access to traffic channels of the cell. According to a further aspect of the present invention, there is provided a mobile communications system comprising means for selectively controlling a mobile station of the system to treat a traffic channel of the system or a cell of the system as a direct access or indirect access channel or cell. It is also believed that the provision of a mobile station that can operate on both direct and indirect access cells or channels may be advantageous. Thus, according to another aspect of the present invention, there is provided a mobile station for a mobile communications system, the mobile station comprising: means for accessing a traffic channel of the system in an indirect access manner; and means for accessing a traffic channel of the system in a direct access manner. As will be appreciated from the above, the traffic channels in these aspects of the invention can be voice or data channels (as distinct from shared, dedicated control channels) , and preferably, the indirect access channels are voice traffic channels and the direct access channels are data, preferably packet data, channels. In a particularly preferred embodiment, a, preferably packet, data channel can be treated as an indirect access data channel by some mobile stations and as a direct access data channel by other mobile stations. As will be appreciated by those skilled in the art, the present invention is applicable to mobile communications systems generally, and thus also extends to a mobile communications system, apparatus for a mobile communications system, terminals (e.g. mobile stations or base stations) for a mobile communications system, etc., incorporating or that can be operated in accordance with, any one or more or all of the aspects of the present invention discussed herein. The present invention is particularly, albeit not exclusively, suited to use in the TETRA communications system, and thus the present invention also extends to a TETRA mobile communications system, apparatus for a TETRA mobile communications system, terminals (e.g. mobile stations or base stations) for a TETRA mobile communications system, etc., incorporating or that can be operated in accordance with, any one or more or all of the aspects of the present invention discussed herein. As will be appreciated by those skilled in the art, all the described aspects and embodiments of the invention can include, as appropriate, any one or more or all of the preferred and optional features of the invention described herein. 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 data processing means. The invention also extends to a computer software carrier comprising such software which when used to operate a communications system, apparatus or terminal comprising data processing means causes in conjunction with said data processing means said system, apparatus or terminal 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. The present invention may accordingly suitably be embodied as a computer program product for use with a computer system. Such an implementation may comprise a series of computer readable instructions either fixed on a tangible medium, such as a computer readable medium, for example, diskette, CD-ROM, ROM, or hard disk, or transmittable to a computer system, via a modem or other interface device, over either a tangible medium, including but not limited to optical or analogue communications lines, or intangibly using wireless techniques, including but not limited to microwave, infrared or other transmission techniques. The series of computer readable instructions embodies all or part of the functionality previously described herein. Those skilled in the art will appreciate that such computer readable instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Further, such instructions may be stored using any memory technology, present or future, including but not limited to, semiconductor, magnetic, or optical, or transmitted using any communications technology, present or future, including but not limited to optical, infrared, or microwave. It is contemplated that such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation, for example, shrink-wrapped software, pre-loaded with a computer system, for example, on a system ROM or fixed disk, or distributed from a server or electronic bulletin board over a network, for example, the Internet or World Wide Web. A number of preferred embodiments 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 a mobile communications system that can be operated in accordance with the present invention; and Figure 2 shows schematically a "go to channel message" for use in an embodiment of the present invention. A preferred embodiment of the present invention will now be described. This embodiment will be described with reference to a TETRA mobile communications system, although, as noted above, and as will be appreciated by those skilled in the art, the present invention is not solely applicable to TETRA systems, but can be used in other mobile communications systems as well . Figure 1 shows schematically the layout of two cells in an exemplary TETRA mobile communications system 1. (In this figure, only two cells of the communications system are shown. However, as will be appreciated by those skilled in the art, a typical TETRA system will include many more overlapping and adjacent cells. ) The TETRA communications system 1 includes a "normal" indirect access TETRA cell 2, which includes, as is known in the art, one or more voice traffic channels, a common, shared main control channel, and one or more indirect access packet data channels. To use the channels of the cell 2, mobile stations (such as mobile stations MSI, MS2, MS3 and MS4 shown in Figure 1) will register and authenticate on the common, main control channel of the cell 2, and signalling on the main control channel will be used to send the mobile stations to a suitable voice traffic channel or shared packet data channel, as appropriate. At the end of its participation in a voice or data call, a mobile station will be returned to the main control channel by an explicit "go to channel" message sent on the voice or data channel, as is known in the art. Such a TETRA cell 2 with a common, shared main control channel, voice traffic channels, and indirect access packet data channels is the norm in a TETRA system, and provides the full range of TETRA services . All its channels are served by co-located transmitters and receivers, and thus have identical coverage areas. Figure 1 also shows a further cell 3 of the communications system 1. This cell is a dedicated data cell, that provides one or more packet data channels, but does not offer any voice traffic channels, or include a common control channel. Thus the cell 3 can be considered to be a "direct" access cell, as there is no common control channel associated with the cell, but rather registration, authentication and control signalling, etc., take place directly on the packet data channels of the cell 3. Thus each channel offered by the cell 3 effectively acts as a "stand-alone" channel. In the arrangement shown in Figure 1, the direct access signal cell 3 is arranged to provide wider bandwidth packet data channels than the cell 2, which accordingly permits higher data rates to be transmitted, but reduces the coverage area of the cell 3 (as shown in Figure 1) . The direct access cell 3 and the indirect access cell 2 are interconnected in a suitable manner so that the communications system 1 can keep track of the location of all mobile stations, etc., participating on calls and traffic in the two cells, and pass appropriate call requests, etc., between the two cells. The system 1 also keeps a record of which indirect access cell or cells cover any given direct access cell. The present invention relates to the situation where a mobile station, such as MSI shown in Figure 1 that is currently active on a data channel of the direct access data cell 3, is to take place in another call, such as a voice call, with other mobile stations in the system. Such a need could arise, e.g., where the mobile station MSI is called by another mobile station of the system, or where the user of the mobile station MSI wishes to call another mobile station of the system. The situation where the mobile station MSI is to participate in a voice call will be considered. The voice call could, e.g., be either an individual voice call, or a group voice call. While it would be possible to try to route the voice call via the direct access data cell 3, generally speaking voice over packet data is rather inefficient, and therefore it would be desirable to provide the direct access mobile station MSI with the ability to make a circuit mode voice using the overlapping indirect access cell 2. The present invention facilitates this, by allowing the mobile station MSI to be temporarily transferred from the direct access data cell 3 to the indirect access cell 2 to participate in the voice call. This is achieved by the system infrastructure sending appropriate signalling on the data channel of the cell 3 that the mobile station MSI is currently active on to the mobile station MSI to instruct it to transfer to a traffic channel of the indirect access cell 2. (The system "knows" that indirect access cell 2 covers the direct access cell 3, and therefore that mobile station MSI should be sent to the indirect access cell 2.) To transfer the mobile station MSI to the voice call, the system infrastructure sends an appropriate "go to channel" message in the call set-up signalling on the data channel of the direct access data cell 3 to the mobile station MSI. Figure 2 shows schematically an exemplary format for a "go to channel" message that could be sent on a packet data channel of the dedicated data cell 3 to the mobile station MSI. As shown in Figure 2, this "go to channel" message includes a number of fields. The first field 10 gives the system identifier used by the dedicated data cell 3. Field 11 contains the target group address (it is assumed in this example that the mobile station MSI is to be sent to participate in a group call of the cell 2 with the other mobile stations MS2, MS3 and MS4) . Field 12 identifies the message type, which in this case is a channel assignment (go to channe1) message . Field 13 defines the frequency of the channel that the mobile station MSI is to be transferred to, and field 14 defines the timeslots which comprise the channel in question (which in the case of a speech call would probably only be a single timeslot, but for a data call may comprise more than one timeslot) . (This is for a TDMA (time division multiple access) system; in a CDMA (code division multiple access) system, the frequency and the spreading code would be specified by fields 13 and 14 instead.) The frequency in the field 13 may be an absolute frequency, or define an offset from a predefined base frequency, as is known in the art. A convenient way to identify which timeslots are allocated in field 14 is to use a bit map . Field 15 provides the temporary system identification that the mobile station MSI will use for the duration of the call. This system identification is "temporary" in the sense that it is used temporarily by the mobile station while it is participating in the new call. However, it can be and typically will be the permanent system identification of the cell or network that the new channel is on. Field 16 gives the colour (scrambling) code to be used for the call. Field 17 includes any necessary encryption data (such as an identifier for the encryption key to be used, or even an encrypted (sealed) version of the encryption key or keys to be used (where the system thinks the mobile station does not possess the keys in use on the cell where the call is to take place) ) to allow the mobile station MSI to participate immediately in the call on the indirect access cell 2. These fields 15, 16 and 17 provide the mobile station with all the information it requires for a fast switch to the new traffic channel (i.e. such that it does not have to obtain that information from another source before it can participate in the call) . Finally, field 18 contains any necessary error correction information, such as a forward error correction code for the message. The size of the fields shown in the message of Figure 2 can, as will be appreciated by those skilled in the art, be selected as desired, and will in general depend on implementation details of the mobile communications system in question. Thus, taking the example of an incoming group call which it is desired to include the mobile station MSI in, the TETRA infrastructure will send to the mobile station MSI a "go to channel" message of the type shown in Figure 2 on the direct access packet data channel of the data cell 3 that is being used by the mobile station MSI to command the mobile station MSI to transfer directly to a traffic channel of the indirect access cell 2 to receive the call. Upon receipt of this "go to channel" message on its direct access packet data channel, the mobile station MSI will proceed to tune to the channel indicated by the frequency 13 and timeslot field 14 in the "go to channel" message, and use the indicated temporary system identifier 15, temporary colour code 16 and any provided encryption data 17 to participate in the group call on that channel . (Depending on the system protocol details, the mobile station MSI may assume that it has permission to transmit as soon as it reaches the assigned channel, or may have to wait for the cell 2 to tell it when it may start transmission.) The group call now proceeds through the indirect access cell 2. At the end of a group call, TETRA sends all participating mobile stations back to the main control channel of their current cell using a group-addressed "go to channel" message specifying the main control channel frequency. However, in the present embodiment, the mobile station MSI that has participated in the group call from a direct access packet data channel is arranged to return automatically to its original packet data channel on the cell 3 once its participation in the group call on the indirect access cell 2 has finished (e.g. because the group call has been terminated or because MSI ' s user has terminated his or her participation in the call) . This is achieved as follows . The mobile station MSI, when it receives a "go to channel" message on its packet data channel to participate in the voice call on the cell 2, stores in its memory details of its original direct access data cell 3 and of the packet data channel it was using in that cell. Then, when the mobile station MSI, receives the end of call signalling, including an explicit or implicit "go to channel" message, on the indirect access cell 2 when the group call has finished, it obeys that message and returns to the main control channel on the indirect access cell 2, but it then determines from its memory that it was previously on the direct access cell 3, and accordingly retrieves the necessary packet data channel information that it has stored and returns itself to the packet data channel on the direct access cell 3. (Alternatively, the mobile station MSI could return directly to the direct access packet data channel on the data cell 3 using the data that it has stored when it receives the group addressed "go to channel" message or implicit "go to channel" indication at the end of the call, without proceeding first to the main control channel indicated in the group-addressed "go to channel" message.) Similarly, should mobile station MSI ' s user choose to leave the group call, then when the user clears down out of the group call, mobile station MSI determines from its memory that it was previously on the direct access cell 3, and accordingly retrieves the necessary packet data channel information that it has stored and returns itself to the packet data channel on the direct access cell 3. Other arrangements to return the mobile station MSI to the packet data channel on the direct access data cell 3 could also be used. For example, the "go to channel" message at the end of the voice call on the indirect access cell 2 could include a special bit that can be set to have a meaning that any mobile station that came from a direct access cell should return to that direct access packet data channel and ignore any frequency and slot instructions in the "go to channel" message received on the indirect access cell 2. Additionally or alternatively, the original "go to channel" message sent on the direct access cell 3 (i.e. sent on the packet data channel 3 to transfer the mobile station 1 to the call on the indirect access cell 2) could include .a bit that instructs the mobile station MSI that it should return to the packet data channel on the direct access cell 3 at the end of the call on the indirect access cell 2. Although the present invention has been described above with reference to transferring the mobile station MSI to a voice call, the present invention is also applicable to the situation where it is desired to transfer the mobile station MSI to another data channel or control channel of another cell of the system 1, and in such circumstances would operate in a similar manner to that discussed above. The present embodiment has been described in the context of the cell 2 being an entirely indirect access cell, and the cell 3 being a direct access cell. However, the present invention can also be applied to situations where the communications system supports "mixed access" cells, i.e. cells for which different terminals will have a different perception of the cell or channel (i.e. some terminals will treat a channel as a direct access channel, and others may treat it as an indirect access channel) . In that case, it may be desirable in the various "go to channel" messages that facilitate the operation of the present invention to advise the receiving mobile station whether it should treat the allocated channel as though it is a direct or indirect access channel. As can be seen from the above, the present invention, in its preferred embodiments at least, permits a mobile station on a direct access packet data channel to be instructed to receive a circuit mode voice call on a channel coming from a different location (i.e. a different cell) . The mobile station is sent to the traffic channel on the different cell, and at the end of the call, it returns to its original packet data channel. In this way, mobile stations that are active on dedicated, direct access packet data channels may still participate in and make other, e.g., voice, calls in the communications system.

Claims

1. A method of operating a mobile communications system which comprises one or more dedicated data service cells, and one or more cells providing voice call services that overlap with the coverage areas of the data service cells, the method comprising: using signalling on a data channel of a data service cell to transfer a mobile station on that cell to a channel of another cell of the communications system.

2. A method of operating a mobile communications system that includes one or more cells that provide direct access data channels, the method comprising: using signalling on a direct access data channel of the system to transfer a mobile station to another channel of the communications system.

3. The method of claim 1 or 2, comprising transferring the mobile station to a voice traffic channel of a cell of the communications system.

4. A method of operating a mobile communications system that includes one or more direct access cells, the method comprising: transmitting a go to channel message on a channel of a direct access cell to instruct a mobile station to go to another channel of the communications system.

5. The method of any one of claims 1 to 4, comprising, when it is desired to transfer a mobile station of the system to another channel of the communications system, transmitting a channel transfer signal to the mobile station that includes the identity of the new channel, together with additional information to allow the mobile station to receive correctly communications transmitted on the new channel .

6. A method of operating a mobile communications system, comprising: when it is desired to transfer a mobile station of the system to another channel of the system, transmitting a channel transfer signal to the mobile station that includes the identity of the new channel, together with additional information to allow the mobile station to receive correctly communications transmitted on the new channel.

7. The method of claim 5 or 6, wherein the additional information included in the channel transfer signal includes details of a scrambling code and/or of encryption that is to be used on the new channel.

8. The method of any one of the preceding claims, comprising a mobile station on a data channel requesting a transfer to another channel of the system by sending a call request on the data channel .

9. The method of any one of the preceding claims, comprising returning a mobile station that has been transferred from a data channel to another channel of the communications system to its previous data channel on termination of its participation in a call on the channel it was transferred to.

10. A method of operating a mobile communications system that includes dedicated data channel or cell communications capability, comprising: causing a mobile station that is on a dedicated data channel or cell to transfer to another channel of the system to participate in a call or signalling on that channel; and returning the mobile station automatically to the data channel or cell once its participation in the call or signalling on the new channel has been terminated.

11. A method of operating a mobile communications system, comprising: causing a mobile station that is operating on one cell of the system to transfer temporarily to another cell of the system to participate in a call or signalling on that cell; and returning the mobile station automatically to its initial cell once its participation in the call or signalling on the new cell has been terminated.

12. A method of operating a mobile communications system, comprising: causing a mobile station that is participating in a call or signalling on one cell of the system to be transferred automatically to a channel of another cell of the system upon termination of the participation of the mobile station in the call or signalling.

13. The method of claim 9, 10, 11, or 12, wherein the mobile station can be selectively returned to its initial data channel or cell.

14. A mobile communications system which includes one or more dedicated data service cells and one or more cells providing voice call services that overlap with the coverage areas of the data service cells, the system comprising: means for using signalling on a data channel of a data service cell to transfer a mobile station on that cell to a channel of another cell of the communications system.

15. An apparatus for use in a mobile communications system which includes one or more dedicated data-only service cells and one or more cells providing voice call services that overlap with the coverage areas of the data service cells, the apparatus comprising: means for using signalling on a data channel of a data service cell to transfer a mobile station on that cell to a channel of another cell of the communications system.

16. A mobile communications system that includes one or more cells that provide direct access data channels, the system comprising: means for using signalling on a direct access data channel of the system to transfer a mobile station to another channel of the communications system.

17. An apparatus for use in a mobile communications system that includes one or more cells that provide direct access data channels, the apparatus comprising: means for using signalling on a direct access data channel of the system to transfer a mobile station to another channel of the communications system.

18. A mobile communications system that includes one or more direct access cells, comprising: means for transmitting a go to channel message on a channel of a direct access cell to instruct a mobile station to go to another channel of the communications system.

19. An apparatus for use in a mobile communications system that includes one or more direct access cells, the apparatus comprising: means for transmitting a go to channel message on a channel of a direct access cell to instruct a mobile station to go to another channel of the communications system.

20. The apparatus or system of any one of claims 14 to 19, comprising means for, when it is desired to transfer a mobile station of the system to another channel of the communications system, transmitting a channel transfer signal to the mobile station that includes the identity of the new channel, together with additional information to allow the mobile station to receive correctly communications transmitted on the new channel .

21. An apparatus for use in a mobile communications system, the apparatus comprising: means for, when it is desired to transfer a mobile station of the system to another channel of the system, transmitting a channel transfer signal to the mobile station that includes the identity of the new channel, together with additional information to allow the mobile station to receive correctly communications transmitted on the new channel .

22. The apparatus or system of claim 20 or 21, wherein the additional information included in the channel transfer signal includes details of a scrambling code and/or of encryption that is to be used on the new channel .

23. The apparatus or system of any one of claims 14 to 22, comprising: a mobile station comprising means for requesting a transfer to another channel of the communications system by sending a call request on a data channel of the communications system.

24. The apparatus or system of any one of claims 14 to 23, comprising means for returning a mobile station that has been transferred from a data channel to another channel of the communications system to its previous data channel on termination of its participation in a call on the channel it was transferred to.

25. An apparatus for use in a mobile communications system that includes dedicated data channel or cell communications capability, the apparatus comprising: means for causing a mobile station that is on a dedicated data channel or cell to transfer to another channel of the system to participate in a call or signalling on that channel; and means for returning the mobile station automatically to the data channel or cell once its participation in the call or signalling on the new channel has been terminated.

26. An apparatus for use in a mobile communications system, the apparatus comprising: means for causing a mobile station that is operating on one cell of the system to transfer to another cell of the system to participate in a call or signalling on that cell; and means for returning the mobile station automatically to its initial cell once its participation in the call or signalling on the new cell has been terminated.

27. A mobile communications system, comprising: means for causing a mobile station that is participating in a call or signalling on one cell of the system to be transferred automatically to a channel of another cell of the system upon termination of the participation of the mobile station in the call or signalling.

28. The apparatus or system of claim 24, 25, 26, or 27, wherein the mobile station can be selectively returned to its initial data channel or cell.

29. A mobile communications system in which a cell or cells of the system support both indirect and direct access to traffic channels of the cell.

30. A mobile communications system, comprising: means for selectively controlling a mobile station of the system to treat a traffic channel of the system or a cell of the system as a direct access or indirect access channel or cell.

31. A mobile station for a mobile communications system, the mobile station comprising: means for accessing a traffic channel of the system in an indirect access manner; and means for accessing a traffic channel of the system in a direct access manner.

32. A mobile communications system, an apparatus for a mobile communications system, or a terminal for a mobile communications system as claimed in or comprising the apparatus of any one of claims 14 to 31, or that can be operated in accordance with the method of any one of claims 1 to 13.

33. A computer program element comprising computer software code portions for performing the method of any one of claims 1 to 13 when the program element is run on data processing means.

34. A method of operating a mobile communications system substantially as hereinbefore described with reference to any one of the accompanying drawings .

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

36. An apparatus for use in a mobile communications system substantially as hereinbefore described with reference to any one of the accompanying drawings .

37. A mobile station for a mobile communications system substantially as hereinbefore described with reference to any one of the accompanying drawings .