Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W16/00—Network planning, e.g. coverage or traffic planning tools; Network deployment, e.g. resource partitioning or cells structures
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W72/00—Local resource management
  • H04W72/04—Wireless resource allocation

Definitions

• the present invention relates to mobile communications systems, and in particular to such systems that include the facility to transmit packet data .
• 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 .
• cells of mobile communications systems 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.
• the TETRA TErrestrial Trunked RAdio
• Packet data channels 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.
• many users transmit packets of data that are interleaved (so that each user receives a timely delivery or transmission of their data) .
• 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.
• 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
• packet 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.
• 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.
• a mobile station if mobile station sees that the channel usage marker has changed (thereby implying that its call has terminated) ) .
• 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.
• 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.
• 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.
• 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 .
• 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.
• 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.
• the data cell will tend to be lower range, which may be less efficient for voice calls.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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)
• 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.
• 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.
• 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.
• 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.
• 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 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) .
• 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.
• 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 .
• 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 .
• 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) .
• 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.
• the system infrastructure would initiate the signalling to transfer the mobile station.
• 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.
• the initial (call) request is preferably sent on the data channel of the data cell.
• 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.
• 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) .
• 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.
• 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.
• 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.
• the TETRA protocol requires a mobile station to return to the main control channel (MCCH) of the cell.
• MCCH main control channel
• 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.
• 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.
• 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 .
• the mobile stations can be selectively automatically returned to their initial data channel or cell .
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• a mobile station may have application beyond the context of such temporary transfer from data channels or cells .
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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) .
• the communications system will comprise a single communications network operating to a single communications protocol or standard (e.g. a TETRA network) .
• 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.
• the communications system e.g., its infrastructure
• the system preferably stores this information in an appropriate form.
• 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.
• 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.
• 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.
• 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.
• a mobile station for a mobile communications system 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Figure 1 shows schematically a mobile communications system that can be operated in accordance with the present invention
• 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.
• 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.
• 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.
• 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.
• a mobile station 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.
• 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.
• 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.
• each channel offered by the cell 3 effectively acts as a "stand-alone" channel.
• 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.)
• 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.
• 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) .
• 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.
• 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) .
• 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.
• 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.
• the mobile station MSI 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 .
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.)
• 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.
• 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.
• 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
• 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.
• 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) .
• 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.
• 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.

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  • 2005-04-27 EP EP05737911A patent/EP1757140A1/de not_active Withdrawn
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