WO2010005348A1

WO2010005348A1 - Switching between modes of operation for mobile communications

Info

Publication number: WO2010005348A1
Application number: PCT/SE2008/050840
Authority: WO
Inventors: Hans Hannu; Jesper Eman; Mats Folke; Peter De Bruin
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2008-07-07
Filing date: 2008-07-07
Publication date: 2010-01-14

Abstract

The invention relates to mobile communications, and more particularly to switching between a network mode of operation and a direct mode of operation. A method, a portable server, a mobile communications device, a computer program product, and a communications network are disclosed. A communication between a first mobile communication device and at least one additional mobile communications device is initiated via the network mode of operation. Switching between the network mode of operation and the direct mode of operation is performed in response to a trigger event.

Description

Switching between modes of operation for mobile communications

Technical Field

The invention relates to mobile communications, and more particularly to switching between a network mode of operation and a direct mode of operation.

Background

The mobile communications industry is continuously seeking to offer new services and new ways to improve how users communicate. The introduction of packet switched voice communication in cellular networks such as Voice over IP (VoIP), Push-to-Talk over Cellular (PoC) and Multimedia Telephony (MMTeI) has enabled the possibility to create such new services. Such technology permits Advanced Voice Group Communication (AVGC) services which allow voice calls within pre-configured or dynamically set up talk groups. These services permit simultaneous distribution of voice communication from one member talking to all other group members listening. Typically, floor control is required to permit only one speaker to talk at a time to the other group members. Such floor control is typically provided by a Push-To-Talk (PTT) method of communicating whereby a button on a mobile telephone implements a way of conversing in a half-duplex mode to switch from voice reception mode to voice transmission mode.

Certain user groups, for example emergency services such as the police, the fire service, and the ambulance services typically use dedicated Public Mobile Radio (PMR) or

Public Safety Radio (PSR) systems to communicate with one another. Such radio systems have been in use for many decades but there is a need to improve them. Communications for the emergency services have stringent operational requirements such as short set-up times, service coverage and availability. Such short set-up times may be, for example, the time taken from a request to talk until permission to talk is granted. An ability to prioritise between users and groups of users, and to handle very large user groups is also important. These requirements set the standard by which any substitute for the traditional analogue radio based systems must meet or exceed.

Usually a mobile communication device communicates with other mobile communication devices via a Network Mode of Operation (NMO) whereby calls are routed via a mobile base station and a cellular network. It is known to allow mobile communication devices within radio range of one another to communicate via a Direct Mode of Operation (DMO) using AVGC. This permits calls to be made directly between mobile communication devices without the need for calls to be placed via the cellular network. The known AVGC method is manually activated whereby a user must decide to initiate a call via the DMO.

A problem associated with the known AVGC method is that the decision to communicate via the DMO or NMO may be slow or inefficient due to the requirement for manual intervention. Such a way of communicating is unsuited to the requirements of the emergency services, or to provide a sufficiently high quality of service to the general consumer market. A further problem associated with using the known AVGC method relates to the synchronisation of DMO communication for many users which may be required by the emergency services, or other user groups.

Summary

An object of the present invention is to provide a way of improved switching between different modes of operation of mobile communications devices whilst reducing or minimising the above-mentioned problems.

According to a first aspect of the invention there is provided a method of switching between a network mode of operation and a direct mode of operation. The method comprising initiating a communication between a first mobile communication device and at least one additional mobile communications device via the network mode of operation. The method further comprising automatically switching from the network mode of operation to the direct mode of operation in response to a trigger event.

Such a method has the advantage of providing an automated process to switch to a direct mode of operation based on a set criteria. The decision regarding switching is taken away from the user which has the advantage of providing an improved level of communication. Furthermore a higher level of service is provided by such a method because the direct mode of operation allows users to communicate seamlessly even where there is limited network coverage, for example, in building basements. This advantage is provided by the automated switching to the direct mode of operation. Preferably the method further includes automatically switching from the direct mode of operation to the network mode of operation in response to the trigger event. This has the advantage of switching back to the network mode of operation at an appropriate time, so that the decision to switch is taken away from the user which allows the level of service to be maintained.

Preferably the method includes defining the trigger event as exceeding or falling below a preselected threshold of a predefined parameter.

Preferably the method includes operating the direct mode of operation via the first mobile communications device or the at least one additional mobile communications device operating as a relay device. Preferably the method further includes switching between the network mode of operation and the direct mode of operation via the relay device. This has the advantage of allowing the decision regarding switching of a plurality of mobile communications devices to a direct mode of operation or a network mode or operation to be synchronised and conducted centrally by the relay device. Such a relay device may also increase the range of network coverage.

Preferably the method further includes notifying users of the first mobile communications device or the at least one additional mobile communication device that communication is via the direct mode of operation or the network mode of operation. Such notification may further assist with synchronising the switching to the NMO or the DMO for a group of users. Preferably the selecting of which mode of operation is to be used is performed by the first mobile communications device, the at least one additional mobile communications device, or a network device.

The method may further include arranging communication via a network mode of operation and a direct mode of operation in response to the trigger event.

According to second aspect of the invention there is provided a portable server for switching between a network mode of operation and a direct mode of operation. The server comprising a trigger device to monitor a predefined parameter and to determine a trigger event. The portable server being arranged to permit at least one mobile communications device to communicate via the network mode of operation or the direct mode of operation. The portable server being operable to permit switching between the network mode of operation and the direct mode of operation in response to the trigger event.

According to a third aspect of the invention there is provided a mobile communications device for switching between a network mode of operation and a direct mode of operation. The mobile communications device being arranged to initiate a communication to at least one additional mobile communications device via the network mode of operation. The mobile communications device arranged to automatically switch from the network mode of operation to the direct mode of operation in response to a trigger event. According to a fourth aspect of the invention there is provided a computer program product operable to perform a method according to the first aspect, or operable to control a portable server according to the second aspect, or operable to control a mobile communications device according to the third aspect.

According to a fifth aspect of the invention there is provided a communications network configured to operate using a method according to the first aspect, or using a portable server according to the second aspect, or operable to control a mobile communications device according to the third aspect, or arranged to implement a computer program product according to the fourth aspect.

It will be appreciated that any preferred or optional features of one aspect of the invention may also be preferred or optional feature of other aspects of the invention.

Brief Description of the Drawings

Other features of the invention will be apparent from the following description of preferred embodiments shown by way of example only with reference to the accompanying drawings, in which;

Figure 1 shows a cell of a mobile communications system with mobile devices communicating via a network mode of operation; Figure 2 shows the cell of Figure 1 and a handover between a network mode of operation and a direct mode of operation according to an embodiment of the invention;

Figure 3 shows an alternative example to the embodiment of Figure 2 in a multi-storey building;

Figure 4 illustrates alternatives decision to change between DMO and NMO taken by a mobile device or a mobile network according to embodiments of the invention;

Figure 5 shows a flow diagram illustrating a method according to an embodiment of the present invention;

Figure 6 shows the cell of Figure 1 with a communications server operating as a relay device for direct mode of operation according to an embodiment of the invention; and

Figure 7 shows the cell of Figure 1 with a mobile device operating as a relay device for direct mode of operation according to an alternative embodiment of the invention.

Detailed Description

Figure 1 shows a cell of a mobile communications system, generally designated 10, having mobile devices 12, 14, 16, 18, 20 communicating via a Network Mode of Operation (NMO). The cell 10 has a mobile communications transceiver 22 in communication with a wider communications network (not shown) which is operable to provide mobile communications to the mobile devices 12, 14, 16, 18, 20 within a perimeter 24 of the cell 10 which indicates the extent of the network coverage provided by the mobile communications transceiver 22. The NMO is illustrated by arrows 11 between each mobile device 12, 14, 16, 18, 20 and the mobile communications transceiver 22.

Figure 2 shows the cell 10 of Figure 1 and a handover between the NMO and a Direct Mode of Operation (DMO) according to an embodiment of the invention. Features in common to the arrangement of Figure 1 are shown with like reference numerals. In Figure 2 the DMO, also known as a Trunked Mode of Operation (TMO), is illustrated by arrows 25 between the mobile devices 12, 14, 16, 18, 20. The DMO of Figure 2 has been switched from the NMO of Figure 1 in response to a trigger event 26.

In the embodiment of Figure 2 the trigger event 26 is the mobile device 16 moving towards the perimeter 24 of the cell 10 thereby creating a reduced mobile signal strength 28 when compared to a typical mobile signal strength 29 of one of the other mobile devices 12, 14, 18, 20. It will be appreciated that the reduced mobile signal strength 28 may be defined by a threshold mobile signal strength 27 such as up to 40% of a full mobile signal strength, or up to 20% of the full mobile signal strength as required. It will further be appreciated that should the mobile device 16 move away from the perimeter 24 of the cell 10 such that the mobile signal strength increases above the threshold 27, the communication may be switched back to NMO. The switching between DMO and NMO may be initiated by one of the mobile devices 12, 14, 16, 18, 20 or by the communications network. If the switching is initiated by the communications network this may be undertaken by a network node such as a Radio Network Controller, a Base Station Controller, or Base Transceiver Station indicated generally at 27. A trigger device 23 is also shown which may be used to determine the trigger event. Whilst the trigger device 23 is shown on the network side it may alternatively, or in addition, be on the mobile device 16.

The trigger event 26 is intended to initiate an automated process that does not require manual intervention to switch to DMO or NMO. Once the trigger event 26 has occurred the switch to DMO or NMO is initiated such that full communication via DMO or NMO occurs substantially simultaneously. It is estimated that such triggering could occur in times as short as 200ms or shorter. It is also intended that switching between DMO and NMO is performed before communication becomes unavailable e.g. due to poor signal strength thereby securing the best possible communication available between users. It will be appreciated that the mobile devices 12, 14, 18, and 20 may still be able to communicate via the NMO in other call sessions, but when communicating with the mobile device 16 communication via the DMO is required.

Figure 2 also shows an optional feature whereby one of the mobile devices 12 is operating as a relay device which is in communication with the mobile communications transceiver 22 indicated by an arrow 23. This feature will be discussed in greater detail with reference to Figures 6 and 7 below.

Figure 3 shows an alternative example to the embodiment of Figure 2 in a multi-storey building, generally designated 30. Features in common to the arrangement of Figure 2 are shown with like reference numerals. In Figure 3 the building 30 has a basement 32 below ground level 34, and an upper floor level 36. One mobile device 14 is shown on the upper floor level 36, and another communications device 16 is shown in the basement 32. In the arrangement of Figure 3 the trigger event for switching from the NMO to the DMO is the mobile device 16 moving into the basement 32 thereby cutting connection with the transceiver 12 as shown at 38, or creating the reduced mobile signal strength 28 when compared to a typical mobile signal strength 29 of one of the other mobile device 14. The advantage of switching to DMO in response to the trigger event is that it allows a group of emergency service personnel, for example firemen, who may be inside the building 30 to communicate with each other even when the coverage via the NMO is insufficient. Should the mobile device 16 move out of the basement 32 and back into the building 30, the communication may switch to NMO. Alternatively, the communication may switch to being via NMO and DMO in parallel as indicated at 32, 39. It will be appreciated that the mobile device 16 may be inside a particular cell and the overall coverage area, but out of communication via the NMO due to being in the basement, or shadowed by large buildings.

Figure 4 illustrates alternative decisions to change between DMO and NMO taken by a mobile device or a mobile network according to embodiments of the invention. Like features to Figure 1 are shown with like reference numerals. In Figure 4 the decision to switch between DMO and NMO may be based on many different trigger events that can be initiated by the mobile device 16 indicated at 50, or by the network indicated at 52. It will be understood that such switching by the network 52 may be undertaken by a network node such as a Radio Network Controller, a Base Station Controller, or Base Transceiver Station shown at 27. In Figure 4 it is envisaged that the trigger event may be defined by a predefined parameter which exceeds or falls below a preselected threshold. In relation to network initiated switching to DMO one such predefined parameter may relate to an amount of data in a network transmit buffer 54 of a Base Station Controller 27, which may be generally applicable to a mobile cell 10 operating a High Speed Downlink Packet Access (HSDPA) standard. In this arrangement the network continuously receives Transmit Power Control (TPC) bits from the mobile device 16 during data transmission in the downlink direction. These TPC bits state whether the network should increase or decrease a transmit power of the downlink data, which is dependant on the type of transport format used. For example, if the power of the BSC 27 is not sufficient to remain in contact with the mobile device 16 when the mobile device 16 moves away from the transceiver 12, a smaller transport format may be used to increase the transmit power of the downlink data. Such a smaller transport format than is required for the data transmission may increase the amount of data in the network transmit buffer 54. A threshold level 56 of the amount of data in the network transmit buffer 54 is set so that the switching to DMO can be initiated if the amount of data increases beyond the threshold level 56.

Another such predefined parameter in relation to network initiated switching to communication via DMO may relate to a difference between a target Signal-to- Interference Ratio (SIR) and an estimated SIR, shown at 51. When communicating via the NMO the network continuously estimates the uplink SIR. An Outer Loop Power Control (OPLC) algorithm increases the SIR target when a Block Error target (BLER) is not reached. Furthermore, the outer loop power control decreases the target SIR when the BLER is reached. If the network increases the target SIR but the difference between the estimated SIR and target SIR increases, the mobile device 16 may not have sufficient power. Accordingly, if the difference between the estimated SIR and the target SIR increases beyond a certain preselected threshold this may be used as the trigger for network-initiated switching to communication via DMO.

Another predefined parameter in relation to network initiated switching to communication via DMO may relate to using a number of retransmissions of a data channel associated with a size of a transport block 53. When communicating via the NMO a state of the art OLPC algorithm for a network operating a High Speed Uplink Packet Access (HSUPA) standard increases the SIR target based on the number of retransmissions of the Enhanced Dedicated Physical Data Channel (E-DPDCH). Accordingly, if the number of retransmissions relative to the size of the transport block 53 used increases beyond a certain preselected threshold this may be used as the trigger for network-initiated switching to communication via DMO.

Another predefined parameter in relation to network initiated switching to communication via DMO may relate to an amount of power used per Transmission Time Interval (TTI). Cellular systems using Wideband Code Division Multiple Access (WCDMA) and High Speed Packet Access (HSPA) standards have a number of control signalling protocols and event triggered signalling that may be used. The TTI is the time required to send a transport block 53 which might be, for example, between 2 - 10ms. If the communication via NMO deteriorates the network may use an increased power 55 to send the transport block. Accordingly, if the power per TTI to send the same number of bits of data increases above a certain threshold, for example between 100 - 40OmW, this may be used as the trigger for network-initiated switching to communication via DMO. This trigger may be implemented via control signalling protocols or event triggered signalling in WCDMA or HSPA.

Another alternative for network initiated switching to communication via DMO may relate to defining the trigger event as failing to provide location update information, or a pager message response 61. Under normal operation of communication via NMO the mobile device 16 sends location update messages when it moves between cells, and also at predefined time intervals to make the network aware of its location and its existence. If the network does not receive the location update messages it may send a pager message intended for the mobile device 16. If a response is not received the mobile device 16 may have lost connection to the network or be turned off. Accordingly, another trigger for network-initiated switching to communication via DMO may be the lack of location update messages and/or a missing pages response.

In relation to the lack of location update messages and/or a missing pager response being used as the trigger for network-initiated switching to communication via DMO it will be appreciated that the mobile device 16 may be switched on but inactive. In this scenario the mobile device 16 does not receive or transmit any data. The network may then poll the mobile device 16 by transmitting a location update message or a pager message. The poll could be done either automatically by the BSC 27 or be initiated by the network operator either if radio conditions are expected to be severe or if communication becomes very important such as communication in the event of an emergency. If a response is not received, the network will determine that the mobile device 16 has lost connection to the network, or may be turned off, or may have entered communication via DMO. In any of these scenarios the network or network operator may then try to reach the mobile device 16 via another mobile device that is expected to be nearby. This other mobile device may then operate as a relay device as described below with reference to Figures 6 and 7.

In Figure 4, regarding the decision to switch between DMO and NMO initiated by the mobile device 16 indicated at 50, one predefined parameter may use a Negative Acknowledgement (NACK) Response 57 sent from the mobile device 16. In a network operating a High-Speed Downlink Shared Channel (HS-DSCH) standard a Hybrid Automatic Repeat-request (HARQ) scheme is utilised for increased system efficiency. In this arrangement the mobile device 16 transmits Acknowledgement (ACK) responses and Negative Acknowledgement (NACK) responses on the uplink High-Speed Dedicated Physical Control Channel (HS-DPCCH) depending on whether data has successfully been received or not. Accordingly, a possible trigger for mobile device 16 initiated switching to communication via DMO may be the number of NACKs received above a certain predetermined threshold.

Another predefined parameter for mobile device 16 initiated switching to communication via DMO may relate to a power level 59 of a control channel of the mobile device 16. According to the Third Generation Partnership Project (3 GPP) standard of HSUPA, the power of the data channel, i.e. E-DPDCH, may be reduced so that the control channels, such as DPCCH and E-DPCCH, can have sufficient power. Accordingly, another trigger for mobile device 16 initiated switching to communication via DMO may be the power limitation of the E-DPDCH. Such a power limitation may be defined as a threshold, and would have the advantage of being straight forward to implement.

Another predefined parameter for mobile device 16 initiated switching to communication via DMO may relate to Broadcast Control Channel (BCCH) signal strength. The BCCH is a downlink channel that is broadcast over the whole cell area and contains specific parameters required by the mobile device 16 so that it can identify the network and gain access to it. Typical information relating to the cell includes a Location Area Code (LAC) and a Routing Area Code (RAC). Accordingly, another trigger for mobile device 16 initiated switching to communication via DMO may be decreasing BCCH signal strength below a threshold.

In the different switching arrangements described with reference to Figure 4 one or more of the mobile devices 12, 14, 16, 18, 20 or the network may be arranged to periodically monitor the pre-defined parameter, such as the mobile signal strength, to establish whether the communication should be switched from DMO to NMO. It is envisaged that such periodic monitoring would be undertaken as long as the communication is provided via DMO. It is further envisaged that such periodic monitoring may include monitoring to establish whether the communication should be switched from NMO to DMO. Another predefined parameter for switching to communication via DMO may relate to an availability of channel resources. A typical cell operating a Global System for Mobile communication (GSM) may have thirty time slots available for calls that are provided by four transceivers. A typical user might require between one half and two time slots for a call depending on various parameters such as signal strength, voice quality etc. If there are more users than available time slot then a shortage of channel resources might occur. Accordingly, another trigger for mobile device 16 initiated switching to communication via DMO may relate to an availability of channel resources below a threshold.

Figure 5 shows a flow diagram illustrating a method according to an embodiment of the present invention, generally designated 60. The method includes initiating a communication between a first mobile communication device 16 and at least one additional mobile communications device 12, 14, 18, 20 via the NMO as shown at 62. Monitoring of a predefined parameter is then performed to determine if it falls below a threshold as shown at 64. If the predefined parameter falls below the threshold a notification is sent to indicate switching to communication via the DMO as shown at 66. A further notification is sent upon successfully switching to communication via the DMO as shown at 68. An indication that the communication mode has changed is then provided as shown at 70. Monitoring of the predefined parameter is then performed again to determine if it exceeds the threshold as shown at 72. If the predefined parameter exceeds the threshold a notification is sent to indicate switching to communication via the NMO as shown at 74. Upon successfully switching to communication via the NMO a further notification is sent as shown at 76. An indication that the communication mode has changed is then provided as shown at 78. The method may then switch between communication via the NMO or DMO by repeating the method 60 shown in Figure 5. Monitoring of the threshold of the predefined parameter shown at 64 and 72 to determine if the predefined parameter exceeds or falls below the threshold represents a trigger event to automate switching between the NMO and the DMO.

It will be appreciated that to avoid repeated switching back and forth between the NMO and the DMO at close to the threshold, it may be necessary to implement the threshold over a range instead of at a specific value. Alternatively, the threshold may be implemented as two thresholds close to one another, such that switching to DMO happens at one threshold, and switching to NMO happens at the other threshold. A further alternative may be implemented by measuring the predefined parameter over a period of time to determine an average value, which could be compared with the threshold. This may also assist in introducing a delay or time lag if required between switching between communication via the NMO and the DMO at close to the threshold.

Figure 6 shows the cell of Figure 1 with a communications server operating as a relay device for communication via DMO according to an embodiment of the invention, generally designated 80. Features common to the embodiment of Figure 1 are shown with like reference numerals. In Figure 6 a public safety vehicle, such as a fire emergency vehicle, is provided with a communications server, i.e. a PoC server 82, which is arranged to operate as a relay device for mobile communication devices 83, 84, 85 within a cell 86 so that they can communication via NMO or DMO. The mobile devices 83, 84, 85 are carried by fire fighters in a building and may have a poor connection to a wider communications network 88 due to the building. Since the vehicle 82 is most likely to be outdoors there is an improved possibility of network coverage. Furthermore the vehicle 82 has an improved power supply when compared to hand-held devices which further increases the likelihood that it will remain in communication with the network, and be able to remain in contact with the mobile devices 83, 84, 85. The PoC server 82 has a trigger device 87 to determine the trigger event.

The PoC server 82 is operable to act as the relay point between the mobile devices 83, 84, 85 and the wider communications network 88, as shown at 90. When one of the mobile devices 83, 84, 85 determines that it can no longer maintain communication via the NMO it may switch to DMO using the PoC server 82. This may be achieved by the vehicle 82 continuously listening to a predetermined frequency channel, such as a Random Access Channel (RACH), or to a predefined set of mobile frequencies/channels 81. If one of the mobile devices 84, 85, 86 for example mobile device 84, decides that communication via DMO is necessary it performs a general broadcast on the RACH, or alternatively on all of the predefined set of mobile frequencies/channels 81, in order to get in touch with the PoC server 82. The PoC server 82 contains various security credentials of the mobile devices 83, 84, 85 which are associated with it i.e. the mobile devices belonging to a particular fire station, or a fire department, or other group of users. The mobile device 84 then identifies and authenticates itself with the PoC server 82 which in turn, using the stored security credentials of the mobile devices 84, 85, 86, operates transparently to allow the mobile devices 84, 85, 86 to communicate with the wider communications network 88 if required, as shown at 90. The PoC server 82 and the mobile devices 84, 85, 86 then negotiate a DMO channel to communicate on.

It will be appreciated that the PoC server 82 does not need to know that the mobile device 84 has switched to communication via DMO because it is merely operating as a relay device. Whereas the mobile device 84 is not required to inform the PoC server 82 before communicating via the DMO, in an alternative arrangement the mobile device 84 requests the PoC server 82 to contact it, for example, 30 seconds in the future. If an answer is not subsequently received by the PoC server 82 an alarm is sent to the other mobile devices 85, 86 to contact the mobile device 84. This has the advantage of reducing the possibility that one of the mobile device 83, 84, 85 is lost during handover from NMO to DMO. Once communication has been restored with the mobile device 84 the alarm is cancelled.

If the mobile device 84 is unable to establish communication with the PoC server 82, the mobile device 84 is arranged to make a general emergency call on several frequencies. If the mobile device 84 manages to contact another NMO/DMO gateway, for example, one of the mobile devices 85, 86, which does not contain the identity of the mobile device 84, the identity is transferred to the PoC server 82 so that the mobile device 85, 86 can perform the relaying. This arrangement may breach security of the network, and should only be used in an emergency. Once the emergency is over the identity should be revoked. The mobile device 84 then re-installs its security credential in order to establish communication directly with the PoC server 82. In an alternative embodiment the PoC server 82 is arranged to communicate with a BSC 94 from another cell 96 instead of direct communication with the wider network 88 as shown at 90. This arrangement is intended for when the PoC server 82 cannot make direct contact with the wider communications network 88. In this arrangement the BSC 94 if the cell 96 operates as a Network-to-Network Interface (NNI) between the PoC server 82 and the wider network 88. The advantages of the arrangements shown in Figures 6 are the increased network coverage provided to the mobile devices 84, 85, 86 and the reduced possibility that one of these mobile devices will be out of contact or lost during switching between NMO and DMO.

Figure 7 shows the cell of Figure 1 with a mobile device operating as a relay device for communication via the DMO according to an alternative embodiment of the invention, generally designated 100. Features in common to the arrangement of Figure 1 are shown with like reference numerals. Figure 7 shows a mobile device 102 which has moved out of the perimeter 24 of the cell 100 such that operation via NMO has been lost, as shown at 104. Once communication via NMO has been lost the mobile device 102 establishes communication with the mobile device 16 via the DMO. The mobile device 16 is still in communication via the NMO. This is achieved in a similar manner to the arrangement of Figure 6 whereby the mobile device 102 makes a general broadcast on the RACH. The mobile device 16 then operates as a relay device in a similar manner to the PoC server 82 of Figure 6. The advantage of providing coverage via the mobile device 16 operating as a relay device is that the network coverage is increased. The relay device arrangements of Figures 6 and 7 may be used with the trigger of an availability of channel resources, described with reference to Figure 4, to implement switching to communication via DMO for a group of users. In this embodiment if there are many users in a cell trying to communicate via the NMO such that the channel resources are insufficient to cope with the demand, the group of users may switch to communication via DMO using the relay device to maintain contact with the wider network 88.

Another feature of the embodiments described above are the various notification events, such as a notification message 40 shown in Figure 3, that are transmitted to inform other users, or the network operator, that communication is being performed via DMO or NMO. A notification message may be sent from the mobile device 12, 14, 16, 18, 20 or the network depending on whether triggering is performed by DMO or NMO as required. If the notification message is sent by the network this may be sent from an operator of the network, or from a network node such as a Radio Network Controller, a Base Station Controller, or Base Transceiver Station indicated generally at 27 in Figure 2. The notification events may include a message to indicate that communication is about to be changed to DMO or NMO, and a further message when the change to DMO or NMO has been successfully implemented. Once DMO or NMO has been successfully implemented the mobile device 12, 14, 16, 18, 20 is arranged to emit a sound, or to provide a visual indication, such as a symbol on the display of the mobile device that the communication is via DMO or NMO. Such notifications may be required because the DMO has different operating characteristics, for example bit rate, when compared to the NMO connection and accordingly certain functionality may not suited for use via DMO.

In an alternative arrangement communication is provided via both NMO and DMO at the same time in response to the trigger event. Such a dual mode has the advantage that a user is more likely to remain in communication. In this arrangement an internal correlation of the mobile devices operating both modes of communication is required to ensure that dual transmission and reception is performed substantially at the same time.

A message may also be sent to the other mobile devices 12, 14, 18, 20 and the network operator when a switch to DMO is triggered which informs the other mobile devices 12,

14, 18, 20 that they should switch to the dual mode.

Under the dual way of communicating it is also envisaged that messages are periodically sent to the other mobile devices 12, 14, 18, 20 via the DMO. The other mobile devices 12, 14, 18, 20 are then arranged to listen for such messages via the DMO with an interval that guarantees that they will be detected. If such a message is detected by one of the mobile devices 12, 14, 18, 20 the other members of the group are arranged to be automatically informed so that they all communicate via both DMO and NMO. As an alternative to this arrangement a message is sent from the mobile device 16 with a certain interval via the NMO for as long as the mobile device 16 is in contact with the network. When the mobile device 16 switches to the DMO the message no longer reaches the other mobile devices 12, 14, 18, 20 which means that they should switch to DMO or to the dual mode. It will be understood by those skilled in the art that a standardised protocol for PoC signalling may be used to convey the required information such as Session Initiation Protocol (SIP), Real-time Transport Protocol (RTP), Real-time Transport Control Protocol (RTCP), and Talk Burst Control Protocol (TBCP).

The above embodiments may be applied to any cellular network such as those operating GSM, Enhanced Data Rates of GSM Evolution (EDGE), Wideband Code Division Multiple Access (WCDMA), High Speed Packet Access (HSPA) or Long Term Evolution (LTE), although the embodiments are not limited to these standards.

Claims

1. A method of switching between a network mode of operation and a direct mode of operation (60) comprising; initiating a communication between a first mobile communication device and at least one additional mobile communications device via the network mode of operation (62); and automatically switching from the network mode of operation to the direct mode of operation (66) in response to a trigger event (26).

2. A method according to claim 1 and further including automatically switching from the direct mode of operation to the network mode of operation in response to the trigger event (26).

3. A method according to claim 1 or claim 2 and further including defining the trigger event as exceeding or falling below a preselected threshold of a predefined parameter

(64, 72).

4. A method according to claim 3 and further including using a mobile signal strength (28) as the predefined parameter.

5. A method according to claim 3 and further including using a transmitter buffer level (56) as the predefined parameter.

6. A method according to claim 3 and further including using a Signal to Interference Ratio (51) as the predefined parameter.

7. A method according to claim 3 and further including using a number of retransmissions of a data channel associated with a size of a transport block (53) as the predefined parameter.

8. A method according to claim 3 and further including using an amount of power (55) used per Transmission Time Interval as the predefined parameter.

9. A method according to claim 3 and further including using a Negative Acknowledgement response (57) sent from the first mobile communications device or the at least one additional mobile communications device as the predefined parameter.

10. A method according to claim 3 and further including using a power level of a control channel (59) of the first mobile communications device or the at least one additional mobile communications device as the predefined parameter.

11. A method according to claim 3 and further including using an availability of channel resources (81) as the predefined parameter.

12. A method according to claim 1 or 2 and further including defining the trigger event as the first mobile communications device or the at least one additional mobile communications device leaving a mobile cell (26).

13. A method according to claim 1 or 2 and further including defining the trigger event as the first mobile communications device or the at least one additional mobile communications device failing to provide location update information, or a pager message response (61).

14. A method according to any preceding claim and further including operating the direct mode of operation via the first mobile communications device or the at least one additional mobile communications device operating as a relay device (16, 82).

15. A method according to claim 14 and further including switching between the network mode of operation and the direct mode of operation via the relay device (16, 82).

16. A method according to claim 14 or 15 and further including using the relay device to monitor at least one mobile frequency channel (81).

17. A method according to any of claims 14 - 16 and further including using a portable server (82) as the relay device.

18. A method according to claim 17 and further including performing a broadcast from the portable server to the first mobile communications device and the at least one additional mobile communications device via at least one mobile frequency channel (81).

19. A method according to any preceding claim and further including notifying users of the first mobile communications device or the at least one additional mobile communication device that communication is via the direct mode of operation or the network mode of operation (66, 74).

20. A method according to claim 19 and further including notifying the users by transmitting periodic messages (40).

21. A method according to claim 20 wherein notifying the users is performed by the omission of said periodic messages (40).

22. A method according to any preceding claim and further including performing communication via the direct mode of operation in parallel (32, 39) with the network mode of operation.

23. A method according to any preceding claim and further including the first mobile communications device, the at least one additional mobile communications device, or a network device selecting which mode of operation is to be used.

24. A method according to any preceding claim and further including using a trigger device (23) to determine the trigger event (26).

25. A method according to any preceding claim and further including arranging communication via a network mode of operation and a direct mode of operation in response to the trigger event (26).

26. A portable server (82) for switching between a network mode of operation and a direct mode of operation comprising a trigger device (86) to monitor a predefined parameter and to determine a trigger event (26), wherein the portable server is arranged to permit at least one mobile communications device to communicate via the network mode of operation or the direct mode of operation, the portable server operable to permit switching between the network mode of operation and the direct mode of operation in response to the trigger event (26).

27. A mobile communications device (12, 14, 16, 18, 20, 82) for switching between a network mode of operation and a direct mode of operation comprising arranging the mobile communications device to initiate a communication to at least one additional mobile communications device via the network mode of operation (62), and automatically switching from the network mode of operation (66) to the direct mode of operation in response to a trigger event (26).

28. A computer program product operable to perform a method according to any of claims 1 - 25, or operable to control a portable server of claim 26, or operable to control a mobile communications device of claim 27.

29. A Communications network configured to operate using a method according to any of claims 1 - 25, or using a portable server of claim 26, or operable to control a mobile communications device of claim 27, or arranged to implement a computer program product according to claim 28. diments are not limited to these standards.