WO2005104575A1

WO2005104575A1 - A communication system, a communication device and method of communicating therefor

Info

Publication number: WO2005104575A1
Application number: PCT/US2005/013209
Authority: WO
Inventors: Arvinder Singh
Original assignee: Motorola, Inc.
Priority date: 2004-04-20
Filing date: 2005-04-19
Publication date: 2005-11-03
Also published as: GB2413460A; GB0408715D0

Abstract

A communication system comprises a first communication device (101) which has a user interface (113), a subscriber information unit (117) that derives cellular communication system subscriber identity information from e.g. a SIM card and a first short range transceiver (115). The first communication device (101) communicates with a second communication device (103) over a short range air interface (111). The second communication device (103) further comprises means for communicating over an air interface (105) of the cellular communication system (107, 109). A subscriber information processor (121) receives the subscriber identity information from the first communication device (101) and a call control processor (123) establishes a communication in the cellular communication system using the subscriber identity information. A communication processor (127) then extends the communication to the first communication device over the short range air interface.

Description

A COMMUNICATION SYSTEM, A COMMUNICATION DEVICE AND METHOD OF COMMUNICATING THEREFOR

Field of the invention

The invention relates to a communication system, a communication device and method of communicating therefor and in particular to communication using cellular communication systems.

Background of the Invention

In a cellular communication system, a geographical region is divided into a number of cells each of which is served by base station. The base stations are interconnected by a fixed network which can communicate data between the base stations. A remote unit is served via a radio communication link by the base station of the cell within which the remote unit is situated.

As a remote unit moves, it may move from the coverage of one base station to the coverage of another, i.e. from one cell to another. As the remote unit moves towards a new base station, it enters a region of overlapping coverage of two base stations and within this overlap region it changes to be supported by the new base station. As the remote unit moves further into the new cell, it continues to be supported by the new base station. This is known as cell reselection or handover.

Communication from a remote unit to a base station is known as uplink, and communication from a base station to a remote unit is known as downlink. The fixed network interconnecting the base stations is operable to route data between any two base stations, thereby enabling a remote unit in a cell to communicate with a mobile station in any other cell. In addition, the fixed network may comprise gateway functions for interconnecting to external networks such as the Public Switched Telephone Network (PSTN), thereby allowing remote units to communicate with landline telephones and other communication terminals connected by a landline. Furthermore, the fixed network comprises much of the functionality required for managing a conventional cellular communication network including functionality for routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc.

Examples of cellular communication systems include both public cellular communication system such as the Global System for Mobile communication (GSM) and Professional Radio (PMR) systems such as TETRA (TErrestrial Trunked RAdio).

Currently the most ubiquitous cellular communication system is the 2nd generation communication system known as the Global System for Mobile communication (GSM). GSM uses a technology known as Time Division Multiple Access (TDMA) wherein user separation is achieved by dividing frequency carriers into 8 discrete time slots, which individually can be allocated to a user. A base station may be allocated a single carrier or a multiple of carriers. One carrier is used for a pilot signal which further contains broadcast information. This carrier is used by mobile stations for measuring of the signal level of transmissions from different base stations, and the obtained information is used for determining a suitable serving cell during initial access or handovers. Further description of the GSM TDMA communication system can be found in 'The GSM System for Mobile Communications' by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.

Currently, 3rd generation systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) wherein user separation is obtained by allocating different spreading and scrambling codes to different users on the same carrier frequency. The transmissions are spread by multiplication with the allocated codes thereby causing the signal to be spread over a wide bandwidth. At the receiver, the codes are used to de- spread the received signal thereby regenerating the original signal. Each base station has a code dedicated for a pilot and broadcast signal, and as for GSM this is used for measurements of multiple cells in order to determine a serving cell. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS), which is currently being deployed. Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in 'WCDMA for UMTS', Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.

TETRA may be used as a public cellular communication system but is designed to provide a number of features and services which are particularly suitable for private organisations or groups such as the emergency services.

For example, TETRA provides a number of features and services for managing and controlling group calls as well as for managing the membership of these groups. Other features and services provided by TETRA include, push-to-talk channel allocation, broadcast calls etc. Also, in addition to trunked mode operation wherein remote units communicate via a base station, TETRA provides for the possibility of communication which is communicated directly between remote units. This is known as Direct Mode Operation (DMO) and allows remote units to set up and maintain direct communication between these.

TETRA is a Time Division Multiple Access (TDMA) system wherein 25 kHz wide channels are further divided into four time slots which may be individually assignable. Each time slot has a duration of 14.167 msecs and four timeslots are combined into a time frame having a duration of 56.67 msecs. Each of the four time slots in a time frame may be individually allocated to the same or different remote units. Furthermore, the time frames are combined into multiframes comprising 18 time frames. Frame number 18 is reserved as a control frame wherein control information may be communicated during an active call.

It is a desire to make mobile stations as small as possible in order to increase their portability and minimise the inconvenience to the user. However, the functionality required for communicating over the air interface is complex and requires radio frequency circuits which can be difficult to reduce in size. Therefore, mobile stations tend to be relatively large and inconvenient.

Furthermore, mobile stations provide a user interface, such as speakers, microphones and displays, which requires that the mobile station is suitable for use in operating positions that provides a convenient interface to the user. For example, to make a voice call a mobile phone must typically be held by the users head to provide proximity to the microphone and the speaker. Although hands free headsets have been introduced to allow that mobile phones are used in other positions, these are additions to existing mobile phones thereby further increasing the weight, size and number of items that must be carried. In cellular communication systems, the subscription information identifying the user subscription etc is typically stored in a Subscriber Identity Module (SIM). During a call initiation or network attachment, the mobile station and the network exchanges subscriber information retrieved from the SIM. This allows the network to identify the user and thus to allow access, perform billing etc.

Typically, mobile stations allow a SIM card to be removable installed such that it can be moved from one mobile station to another. However, this is a cumbersome and inconvenient process.

As the variety and number of services provided by cellular communication systems are growing, it is increasingly desirable to have more than one subscription. For example, a user may have a connected laptop, a connected PDA and a mobile phone for voice calls. Each device requires a SIM card to access the cellular communication system and it is typically desirable to have different subscriptions for each device allowing them to be individually addressed and providing an independent selection of the subscription most suitable for the communication characteristics of each individual device.

Furthermore, each device implements the functionality required for communication in the cellular communication system thereby increasing the heat dissipation, battery drain, form factor, complexity and reducing the reliability of each device.

Hence, an improved communication system would be advantageous and in particular one allowing increased flexibility, reduced size and/or weight of mobile stations, increased freedom of use, reduced complexity of mobile stations, reduced power consumption and or reduced user inconvenience. Summary of the Invention

Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

According to a first aspect of the invention there is provided a communication system comprising: a first communication device comprising: user interface means, means for determining subscriber identity information associated with a cellular communication system, means for communicating over a short range air interface! and a second communication device comprising: means for communicating with the first communication device over the short range air interface, means for communicating over an air interface of the cellular communication system, means for receiving the subscriber identity information from the first communication device, means for establishing a communication in the cellular communication system using the subscriber identity information, and means for extending the communication to the first communication device over the short range air interface.

The invention may allow a separation between functionality for user interfacing and subscriber identity information determination and functionality for communication over the air interface of a cellular communication system. This may for example allow increased user friendliness as the first and second communication device can be optimised individually for their specific purposes. For example, the second communication device may be designed for optimal propagation performance (e.g. by designing it to be used with a suitable antenna) while the first communication device may be designed for optimal user convenience and positioning when in use. The invention may allow a reduced form factor of the first communication device and/or the second communication device.

As the subscriber identity information is received from the first communication device, the second communication device may not be limited to one specific subscription but may be used for different subscriptions. For example, a user may carry only a single second communication device which may be used to provide the cellular communication system air interface functionality for all subscriptions. As another example, the second communication device may be stationary and not carried by the user of the first communication device. Rather, the second communication device may be used by the first communication device to provide access to the cellular communication system when in proximity.

The invention may thus allow a lower complexity, higher reliability and reduced power dissipation of the first communication device. Furthermore, a flexible arrangement may be implemented where a plurality of communication devices each having a subscription may share the same transceiver means for communicating over the cellular communication air interface.

The short range air interface typically has a range up to a few hundreds meters and may in some embodiments have a range of a maximum of around 10 to 20 meters.

According to a preferred feature of the invention, the second communication device further comprises: means for receiving second subscriber identity information from a third communication device, means for establishing a second communication in the cellular communication system using the second subscriber identity information; and the means for extending is simultaneously operable to extend the second communication to the second communication device over the short range air interface.

The second communication device may simultaneously support communication for more than one subscription thereby allowing reduced complexity, reduced cost and increased reliability of the communication devices. Furthermore, the form factor of the first and third communication device may be reduced as the air interface functionality may be reused.

According to a preferred feature of the invention, the second communication device further comprises: means for storing the subscriber identity information, means for receiving paging messages over the air interface of the cellular communication system,' and means for sending an alert message to the first communication device over the short range air interface in response to receiving a paging message matching the stored subscriber identity information.

This may allow a low complexity and efficient means of receiving an incoming call to the first communication device.

According to a preferred feature of the invention, the first communication device comprises means for transmitting a communication initialisation message to the second communication device and the means for establishing the communication is operable to establish the communication in response to receiving the communication initialisation message.

This may allow a low complexity and provide efficient means of initiating and estabhshing a communication from the first communication device. The communication may for example be a voice call, a data call or a packet data session.

According to a preferred feature of the invention, the means for establishing the communication is operable to perform an attach procedure in accordance with a standard attach procedure of the cellular communication system using the subscriber identity information.

Specifically, the second communication device may perform the same attach procedure as any other cellular communication device thereby reducing or obviating the requirement for any changes or modifications to the network of the cellular communication system.

According to a preferred feature of the invention, the means for determining subscriber identity information comprises means for receiving a removable subscriber identity unit and for extracting the subscriber identity information from the removable subscriber identity unit. Preferably the removable subscriber identity unit is a Subscriber Identity Module (SIM) of the cellular communication system. This provides for increased flexibility and a compatibility with other communication devices of the cellular communication system.

According to a preferred feature of the invention, the first communication device comprises means for transmitting a desired transmission protocol indication to the second communication device,' and the means for communicating over the air interface of the cellular communication is operable to select a transmission protocol in response to the transmission protocol indication.

This allows increased flexibihty and provides enhanced control for the first communication device and possibly the user. The second communication device is preferably operable to use different transmission protocols thereby providing compatibility with a plurality of services and/or systems. The transmission of the transmission protocol indication provides for a simple means of dynamically customising the operation of the second communication unit to the characteristics and requirements of the first communication device.

According to a preferred feature of the invention, the transmission protocol indication comprises an identification of a service in the cellular communication system. This allows increased flexibility and provides enhanced control for the first communication device and possibly the user to select the desired service.

According to a preferred feature of the invention, the first communication system is operable to transmit the subscriber identity information in response to a detection of the first communication device entering an active operating state.

This provides for a simple and effective means of automatically interfacing the first and second communication device. For example, the first communication device may automatically transmit the subscriber identity information when it is switched on, when a user starts talking, when it is placed in an operation position etc.

According to a preferred feature of the invention, the first communication device is comprised in a headset. This is particularly suitable for voice calls and may provide a low complexity headset which does not require built in functionality for communicating over the air interface. Furthermore, the headset can be used with different units operable to communicate over the air interface of the communication system and does not require a dedicated cellular transceiver. According to a preferred feature of the invention, the first communication device is a wearable communication device. The first communication device may for example be attachable to or comprised in clothes, shoes, belts etc. The invention thus provides for a separation of functionality for communicating over a cellular communication system and for user interfacing which may allow a form factor and/or design that makes it practical for implementation as a wearable item.

According to a preferred feature of the invention, the second communication device is a wearable communication device. The second communication device may for example be attachable to a comprised in clothes, shoes, belts etc. The invention thus provides for a separation of functionality for communicating over a cellular communication system and for user interfacing which may allow a form factor and/or design that makes it practical for implementation as a wearable item. . A particular advantage is that functionality for communicating over the air interface may be worn by a user thereby reducing the inconvenience to the user of carrying the communication devices.

According to a preferred feature of the invention, the short range air interface is an air interface of an ad-hoc network. For example, the short range air interface may be implemented in an IEEE 801.11a, IEEE 801.11b or IEEE 801.11g ad-hoc network as standardised by the Institute of Electrical and Electronic Engineers (IEEE). This may provide a particularly suitable and advantageous means. of estabhshing a short range communication between the first and second communication device in some embodiments. In particular, it may be advantageous when the second communication device is used to provide small coverage areas wherein ad-hoc cellular communications may be set up from the first communication device as and when it can detect a second communication device.

According to a preferred feature of the invention, the short range air interface is a Bluetooth short range air interface. This may provide a particularly suitable and advantageous means of establishing a short range communication between the first and second communication device in some embodiments. In particular, it may be advantageous when the second communication device is carried by a user together with the first communication device.

The cellular communication system may be a 3^rd generation cellular communication system, such as UMTS, and/or may be a GSM cellular communication system (including GPRS and EDGE systems) and/or may be a TETRA cellular communication system.

According to a second aspect of the invention there is provided a communication device comprising: means for communicating with a first communication device over a short range air interface; means for communicating over an air interface of a cellular communication system,' means for receiving a subscriber identity information associated with the cellular communication system from the first communication device; means for establishing a communication in the cellular communication system using the subscriber identity information,^' and means for extending the communication to the first communication device over the short range air interface.

According to a first aspect of the invention there is provided a method of communicating in a communication system comprising the steps of: in a first communication device performing the steps of: providing a user interface, determining subscriber identity information associated with a cellular communication system, communicating over a short range air interface; and in a second communication device performing the steps of: communicating with the first communication device over the short range air interface, communicating over an air interface of the cellular communication system, receiving the subscriber identity information from the first communication device, establishing a communication in the cellular communication system using the subscriber identity information, and extending the communication to the first communication device over the short range air interface.

These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

Brief Description of the Drawings

An embodiment of the invention will be described, by way of example only, with reference to the drawings, in which

FIG. 1 illustrates a communication system in accordance with an embodiment of the invention.

Detailed Description of a Preferred Embodiment of the Invention

The following description focuses on an embodiment of the invention applicable to a GSM cellular communication system but it will be appreciated that the invention is not limited to this application but may be applied to many other communication systems including for example a 3^rd generation cellular communication system or a TETRA cellular communication system.

Furthermore, the description will focus on the example of voice communication but it will be appreciated that this is merely provided as an example and that the invention is equally applicable to other communications including data communication and packet communication.

FIG. 1 illustrates a communication system in accordance with an embodiment of the invention. The communication system comprises a first communication device 101 and a second communication device 103. The second communication device 103 comprises functionality for communicating with a GSM cellular communication system. Specifically, the second communication device 103 is operable to communicate over the GSM air interface 105 with a GSM base station 107. The GSM base station 107 is coupled to a cellular network 109 as is well known to the person skilled in the art.

The first and second communication devices 101, 103 communicate with each other over a short range air interface 111. Specifically, the range of the short range air interface may be less than the cell sizes of the cellular communication system. For example, a GSM cellular communication system typically has cell sizes of more than one kilometre radius. The short range air interface 111 preferably supports communications up to a range of less than 1 kilometre. In some embodiments, the range of the short range air interface 111 is limited to a few hundred meters, a few tenths of metres or even to a few metres. In the described embodiment, the short range air interface is a Bluetooth(TM) air interface. Bluetooth (TM) tjφically supports communication up to a maximum distance of around 10- 20 meters between communication devices. In the described embodiment, the second communication device 103 is thus capable of communicating both over the GSM air interface 105 as well as the Bluetooth(TM) air interface 111 whereas the first communication device 101 is operable to communicate only over the Bluetooth(TM) air interface 111. Furthermore, the second communication device 103 is capable of supporting a communication of the first communication device 101 in the cellular communication system. Thus, the second communication device 103 can provide cellular communication system services to a communication device that does not itself comprise functionality for communicating over the GSM air interface.

In more detail, the first communication device 101 comprises a user interface 113 which receives input from a user and presents received signals to the user. In the specific example of voice communication, the first communication device 101 comprises a microphone and a speaker and specifically the first communication device 101 may be a headset comprising at least one microphone and at least one speaker.

In other embodiments, the user interface may for example comprise a display and/or a keyboard and the first communication device 101 may for example be comprised in (or be a) PDA or a computer.

In the first communication device 101, the user interface 113 is coupled to a first short range transceiver 115. The first short range transceiver 115 comprises the required functionality for communicating over the Bluetooth(TM) air interface. The first short range transceiver 115 is thus operable to receive data from the user interface 113 and to transmit it to the second communication device 103 as well as to receive data from the second communication device 103 and to feed that to the user interface 113. The functionality required and desired for communication over the Bluetooth(TM) air interface is well known to the person skilled in the art and will for brevity and clarity not be described in further detail here.

In addition, the first communication device 101 comprises a subscriber information unit 117 which determines subscriber identity information associated with the cellular communication system. In the described embodiment, the subscriber information unit 117 comprises a means for receiving a removable subscriber identity unit and for extracting the subscriber identity information from the removable subscriber identity unit.

In particular, the subscriber information unit 117 comprises a GSM SIM (Subscriber Information Module) card reader which is capable of reading a removable SIM card in order to retrieve the GSM subscriber information stored on the card. It will be appreciated that for other cellular communication systems other subscriber identity information or SIM cards may be used.

The subscriber identity information typically includes a subscriber identity and may for example also comprise information which is specific to the subscription such as a home network identification, billing information, service accessibility information etc.

The subscriber information unit 117 is coupled to the first short range transceiver 115 and may feed the subscriber identity information to this for communication over the Bluetooth(TM) air interface to the second communication device 103.

The second communication device 103 comprises a second short range transceiver 119 which communicates with the first communication device

101 over the Bluetooth(TM) air interface 111. The second short range transceiver 119 is coupled to a subscriber information processor 121 which is operable to receive the subscriber identity information transmitted from the first communication device 101 and received by the second short range transceiver 119.

The subscriber information processor 121 is coupled to a call control processor 123 which is operable to establishing a communication in the cellular communication system using the subscriber identity information received from the subscriber information processor 121. For this purpose, the call control processor 123 is coupled to a cellular transceiver 125 which is controlled by the call control processor 123.

Specifically, the call control processor 123 is operable to control the cellular transceiver 125 to initiate a new communication in the cellular communication system using the received subscriber identity information or to detect an incoming call to the subscriber identity of the subscriber identity information and to set up the call with the cellular communication system in response thereto.

The second communication device 103 further comprises a communication processor 127 which is coupled to the second short range transceiver 119 and the cellular transceiver 125. When a communication has been established by the call control processor 123, the communication processor 127 is operable to extend the cellular communication to the first communication device over the short range air interface.

In a simple embodiment, the communication processor 127 may simply receive user data of the cellular communication from the cellular transceiver 125 and feed this to the second short range transceiver 119 which transmits it to the first communication device 101. Similarly, the communication processor 127 may receive user data from second short range transceiver 119 and feed this to the cellular transceiver 125 for transmission to the GSM base station.

In more complex embodiments, the communication processor 127 may additionally or alternatively perform other processing such as converting received user data before transmitting it on the other air interface.

Hence, the embodiment enables a first communication device 101 comprising a user interface to be very simple and accordingly have reduced cost, reduced size, reduced weight, reduced power dissipation and increased reliability.

Furthermore, a second communication device 103 may provide cellular communication functionality for one or more user interface units. This may allow an overall reduced complexity and form factor. For example, only a single unit with functionality for communicating with the GSM communication system is needed and the remaining communication devices will require only functionality for communicating over a Bluetooth(TM) air interface. Hence, a general common GSM communication device may be used with one or more subscriber specific communication devices.

As the second communication device 103 does not need to provide a user interface, more design freedom is achieved. Furthermore, a more flexible arrangement is obtained where for example the second communication device 103 may be used in positions and locations which are not suitable for interfacing to a user. This may also allow for improved performance in the cellular communication system for example by the second communication device 103 being optimised for propagation characteristics of the cellular air interface. In the described embodiment, an incoming communication to the first communication device 101 from the cellular communication may be established as follows.

The call control processor 123 is preferably capable of storing subscriber identity information and in particular a subscriber identity. Furthermore, the call control processor 123 in this embodiment monitors all paging messages transmitted by the GSM base station 107. For each received paging message, the call control processor 123 compares the identity of the subscriber identity in the paging message with the stored subscriber identity. If a match is found, the call control processor 123 controls the communication processor 127 to transmit an alert message to the first communication device 101 over the Bluetooth(TM) air interface. Upon receiving the alert message, the first communication device 101 proceeds to initiate in preparation for the call.

In particular, the first communication device 101 may generate an audio alert such as a ring tone to indicate the incoming communication to the user. If the user accepts the call, an accept message may be transmitted from the first communication device 101 to the second communication device 103. In response, the second communication device 103 proceeds to acknowledge the paging message and to setup a GSM communication using the standard GSM procedure for setting up an incoming communication as is well known to the person skilled in the art.

In the described embodiment, an outgoing communication from the first communication device 101 through the cellular communication may be established as follows.

The first communication device 101 may for example instigate a communication in the cellular communication system in response to a user input. When the user input is received, the first communication device 101 generates an initialisation message which is sent to the second communication device 103 over the Bluetooth(TM) air interface 111. When the second communication device 103 receives the initialisation message, the call control processor 123 proceeds to setup a cellular communication using the standard GSM setup procedure. The call control processor 123 specifically sets up the communication using the subscriber identity information received from the first communication device 101. The subscriber identity information may specifically be communicated from the first communication device 101 to the second communication device 103 in the initialisation message.

As the setup procedure uses the subscriber identity information, the communication is from the GSM communication systems side identical to a communication with a conventional mobile station. The GSM communication system does not have any information that a non- subscriber specific GSM communication device is used or that the communication further includes a second short range air interface. Rather the arrangement is indistinguishable from a conventional mobile station from the GSM communication systems point of view and no modifications are required in the GSM cellular communication system.

In accordance with the embodiment, the communication system may comprise a plurality of communication devices similar to the first communication device 101. In particular the communication system may comprise a third communication device (not shown) also comprising a user interface, a subscriber information unit and a short range transceiver. Similarly, to the first communication device 101, the third communication device transmits subscriber identity information to the second communication device 103. In this embodiment, the subscriber information processor 121 is also operable to receive the subscriber identity information from the third ( communication device and the call control processor 123 is operable to establish a second communication in the cellular communication system using the second subscriber identity information in the same way as a communication may be established for the first communication device 101.

In addition, the communication processor 127 is operable to extend the second communication to the third communication device. Specifically, the communication processor 127 is operable to receive user data for the third communication device from the cellular transceiver 125 and to transmit to the third communication device through the second short range transceiver 119. Similarly, the communication processor 127 may receive data from the third communication device and transmit it over the GSM air interface 105 using the cellular transceiver 125.

Preferably, the second communication device 103 is capable of supporting the communication to the first communication device 101 and to the third communication device at the same time. This is feasible due to the multi user aspect of both the cellular air interface and the short range air interface. Specifically, both GSM and Bluetooth(TM) use Time Division Multiple Access (TDMA) to allow multiple communications.

Hence, multiple communications with more than one subscriber (and possibly more than one communication device) may be supported simultaneously by the second communication device 103. This may provide further flexibility and allow for increased user service.

In some embodiments, the second communication device 103 is operable to operate in accordance with a plurahty of different transmission protocols. For example, the second communication device 103 may be capable of operating on a plurality of different cellular communication systems. In particular, the second communication device 103 may comprise communication functionality supporting 3^rd generation communication systems such as UMTS, standard GSM, TETRA and GPRS/EDGE services.

In the embodiment, the first communication device 101 may select a desired transmission protocol such as a preferred cellular communication system and it may accordingly transmit a transmission protocol indication to the second communication device 103. In response, the second communication device 103 may use the selected cellular communication system for communication thereby allowing the first communication device 101 to effectively operate as a fully functional remote unit of the selected communication system.

As a specific example, the second communication device 103 may be operable to communicate with a UMTS and a GSM cellular communication system. The first communication device 101 has a SIM card comprising a subscription for both a UMTS and GSM communication system. When setting up a voice call, the user of the first communication device 101 may prefer to use the GSM communication system as this will typically have a lower cost. However, when setting up a data call, the user may prefer to use the UMTS cellular communication system as this provides for a higher data rate.

In this example, a voice call may be instigated by the first communication device 101 generating an initialisation message comprising the subscriber identity information and a transmission protocol indication identifying the GSM cellular communication system. In response, the second communication device 103 proceeds to set up a voice call on the GSM communication system. Similarly, a data call may be instigated by the first communication device 101 generating an initialisation message comprising the subscriber identity information and a transmission protocol indication identifying the UMTS cellular communication system. In response, the second communication device 103 proceeds to set up a voice call on the UMTS communication system. If no UMTS communication system is available, the second communication device 103 may set up data call on the GSM communication system, and if no GSM communication system is available, the second communication device 103 may set up data call on the UMTS communication system

In some embodiments, the transmission protocol indication may be an indirect indication. For example, an initialisation message for a voice call may be seen as a transmission protocol indication for the GSM communication system and an initialisation message for a data call may be seen as a transmission protocol indication for the UMTS communication system.

Alternatively or additionally, the transmission protocol identification may comprise an identification of a preferred service in the cellular communication system. For example, a UMTS communication system typically provides different services at different cost and the transmission protocol identification may identify a specific service. In response, the second communication device 103 sets up the required service in the cellular communication system.

The indication of the desired service may be direct or indirect. For example, the transmission protocol identification may indicate a specific quahty of service parameter such as a desired data rate, delay and/or error rate and the second communication device 103 may set up a corresponding service. It will be appreciated that the subscriber identity information can be transmitted from the first communication device 101 to the second communication device 103 over the short range air interface in any suitable way and at any suitable time(s). For example, the subscriber identity information may be transmitted at regular intervals, in response to inquiry messages from the second communication device 103 or when a given event occurs.

In some embodiments, the first communication device 101 is operable to transmit the subscriber identity information in response to a detection of it entering an active operating state. For example, whenever the first communication device 101 is switched on, a message comprising the subscriber identity information may be transmitted to the second communication device 103.

In other embodiments more complex means of detecting an operating state may be used such as a detection of an operating position. For example, a mechanical switch may detect a physical orientation of the device and the subscriber identity information may be transmitted whenever the switch detects a change from an orientation associated with a non- active state to an orientation associated with an active state.

An advantage of the separation between the first communication device 101 and the second communication device 103 is that both may be reduced in size and weight. Furthermore, the separation of the subscriber identity information from the functionality for communication over the air interface allows for the second communication device 103 to be a general unit that is not specific to the individual subscriber and which may support a plurality of different subscriptions. This allows increased design freedom and in particular allows increased design freedom for the physical implementation of the communication devices. It also allows increased flexibility in the use of the communication device.

In some embodiments, the first communication device 101 and/or the second communication device 103 are wearable devices that may be worn on a person. In particular, the communication devices may be comprised in clothing or footwear. For example, the second communication device 103 may be built into a belt buckle, a coat or a pair of boots. This will facilitate carrying of the communication device and allow for the user to always have cellular communication means with him. As the battery consumption associated with communicating over the cellular air interface is typically higher than for communicating over a short range air interface, the batteries and thus the size and weight of the second communication device 103 is typically larger than for the first communication device 101. Hence, in some embodiments, a large common communication unit may be carried in a convenient and unobtrusive fashion while communication unit interfacing with the user may be made smaller, lighter and suitable for operation in a convenient operating position. For example, the first communication device 101 may be comprised in headset.

It will be appreciated that although the above description focuses on a Bluetooth(TM) short range air interface, any suitable short range air interface may be used. For example, in some embodiments the short range air interface may be part of an ad-hoc network such as an IEEE 802.11 ad hoc network. In such an embodiment, the first communication device 101 may continually scan for other ad-hoc network devices, and if one is detected information is exchanged to determine if the new network device has functionality for communicating with a cellular communication system, i.e. whether it comprises the functionality of the second communication device 103. If so, the first communication device 101 may use the new network device to communicate over the cellular communication system.

This embodiment, may completely remove the requirement for the user to carry any functionality for communicating on a cellular air interface. Hence, only a very small and low cost device needs to be carried.

The invention can be implemented in any suitable form including implementation of functionality in hardware, software, firmware or any combination of these. However, preferably, the invention is implemented partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality of each communication device may be implemented in a single unit, in a plurality of units or as part of other functional units.

Although the present invention has been described in connection with the preferred embodiment, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. In the claims, the term comprising does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. Thus references to "a", "an", "first", "second" etc do not preclude a plurality.

Claims

1. A communication system comprising: a first communication device comprising: user interface means, means for determining subscriber identity information associated with a cellular communication system, means for communicating over a short range air interface,' and a second communication device comprising: means for communicating with the first communication device over the short range air interface, means for communicating over an air interface of the cellular communication system, means for receiving the subscriber identity information from the first communication device, means for establishing a communication in the cellular communication system using the subscriber identity information, and means for extending the communication to the first communication device over the short range air interface.

2. A communication system as claimed in claim 1 wherein the second communication device further comprises: means for receiving second subscriber identity information from a third communication device,^' means for establishing a second communication in the cellular communication system using the second subscriber identity information,' and wherein the means for extending is simultaneously operable to extend the second communication to the second communication device over the short range air interface.

3. A communication system as in claim 1 wherein the second communication device further comprises: means for storing the subscriber identity information; means for receiving paging messages over the air interface of the cellular communication system! and means for sending an alert message to the first communication device over the short range air interface in response to receiving a paging message matching the stored ' subscriber identity information.

4. A communication system as in claim 1 wherein the first communication device comprises means for transmitting a communication initialisation message to the second communication device, and the means for establishing the communication is operable to establish the communication in response to receiving the communication initialisation message.

5. A communication system as in claim 1 wherein the means for establishing the communication is operable to perform an attach procedure in accordance with a standard attach procedure of the cellular communication system using the subscriber identity information.

6. A communication system as in claim 1 wherein the means for determining subscriber identity information comprises means for receiving a removable subscriber identity unit and for extracting the subscriber identity information from the removable subscriber identity unit.

7. A communication system as in claim 1 wherein the first communication device comprises means for transmitting a desired transmission protocol indication to the second communication device; and the means for communicating over the air interface of the cellular communication system is operable to select a transmission protocol in response to the transmission protocol indication.

8. A communication system as in claim 1 wherein the first communication system is operable to transmit the subscriber identity information in response to a detection of the first communication device entering an active operating state.

9. A communication system as in claim 1 wherein the first communication device is at least one of a) comprised in a headset and b) a wearable communication device.

10. A communication system as in claim 1 wherein the short range air interface is at least one of a) an air interface of an ad-hoc network and b) a Bluetooth(TM) short range air interface.