EP1772026A1

EP1772026A1 - A cellular communication system, a communication unit and a method of call initiation therefor

Info

Publication number: EP1772026A1
Application number: EP05769452A
Authority: EP
Inventors: Gottlieb Schwarz
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2004-07-16
Filing date: 2005-07-08
Publication date: 2007-04-11
Also published as: GB2416279A; GB0415936D0; WO2006019630A1; GB2416279B; EP1772026A4

Abstract

In a cellular communication system (100), a first communication unit (101) comprises an association memory (115) storing dynamic associations between user identifiers and communication unit identifiers. A user interface (111) may receive a target user identifier. The user interface (111) is coupled to an association controller (113) which determines a target communication unit identifier as a communication unit identifier associated with a user identifier matching the target user identifier. A transmission controller (117) then initiates communication to a second communication unit (109) using the target communication unit identifier. The invention facilitates the use of user identifier addressing in a communication system based on communication unit identifier addressing. The association memory (115) is updated by the first communication unit (101) receiving association messages comprising association information from other communication units. The association messages may be transmitted and generated in response to association request messages transmitted from the first communication unit (101).

Description

A CELLULAR COMMUNICATION SYSTEM, A COMMUNICATION UNIT AND A METHOD OF CALL INITIATION THEREFOR

Field of the invention

The invention relates to a cellular communication system, a communication unit and a method of call initiation therefor and in particular, but not exclusively, to call initiation in a TETRA cellular communication system.

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 Mobile Radio (PMR) systems such as the TETRA (TErrestrial Trunked RAdio) cellular communication system.

Specifically, although TETRA may be used as a public cellular communication system, it^is designed to provide a number of features and services that are particularly suitable for private organisations or groups such as for example 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. Thus, TETRA provides services for making group calls wherein e.g. a voice message is quickly and efficiently relayed to a specific group of remote units. TETRA furthermore provides features for managing the membership of different groups, priorities of different groups etc.

Also, TETRA provides for both acknowledged and non-acknowledged communication. For acknowledged services, the call setup comprises the call originator receiving an acknowledgement indicating that the destination(s) has been reached. However, for non-acknowledged services, no acknowledgement is received from the destination.

Other features and services provided by TETRA include a high level of security which may be optimised for specific purposes, 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 directly between

remote units without involving the fixed network. This is known as Direct Mode Operation (DMO) and allows remote units to set up and maintain direct communication, hi one aspect this may allow a remote unit to act as a repeater for another remote unit. For example a vehicle mounted unit may act as a repeater for a portable unit thereby improving coverage for the portable unit.

Currently, when a mobile station of a TETRA communication system wishes to communicate to one or more receiving mobile stations, a protocol is used in which codes giving the identities of the transmitting and receiving mobile stations are included in call initiation signalling. This code is known as the ID address of the mobile station, the ISSI (Individual Short Subscriber Identity) or, in its full TETRA form, the ITSI (Individual TETRA Subscriber Identity). Thus, conventionally communication in TETRA is controlled and routed in accordance with identifiers associated with the mobile stations themselves rather than identifiers associated with the users of the mobile stations.

However, for communication systems such as TETRA, there is a desire to communicate by targeting a mobile station user rather than the mobile station equipment. For example, in some scenarios, a pool of mobile stations is shared between a group of users. The individual user will select a random mobile station from the pool when required and will log on using a personal user identity. Thus, when contacting the individual user, other users do not have any knowledge of the specific mobile station that is currently used. Therefore, in these situations it is desirable to be able to specify the user to be called rather than the mobile station.

A known proposed method for satisfying this requirement involves each mobile station user having a user identifier in the form of an identity number uniquely defining the user. The identity number is referred to as a RUN (Registered User Number). The RUN may include one or more portions which define the user's organisation and/or organisational unit, e.g. police and particular police force. A user wishing to communicate with a target user may indicate the target user by entering the target user's RUN. This may for example be found from a directory of RUNs corresponding to user names. A signal including the

target user's RUN is sent to the fixed network (known as the SwMI (Switching and Management Infrastructure) in a TETRA system). The SwMI then translates the target user's RUN into the ID address of the mobile station that the target user is currently logged onto, so that the SwMI can signal the appropriate mobile station with the incoming communication.

The need for the SwMI to perform this translation in order to forward the transmissions to the appropriate mobile stations has certain associated disadvantages.

For example, the system requires that a central (possibly distributed) server is implemented in the fixed network and that this server is accessed for each call. This may increase complexity and may result in an increased number of messages over the air interface and/or in the fixed network thereby reducing the capacity of the communication system.

Furthermore, the central server must be continuously updated to reflect the current situation which is a complex and resource demanding process. Also, the requirement for additional functionality of the fixed network typically increases cost, hi particular, the introduction of a server to an existing TETRA communication system is complex, costly and requires exhaustive testing and verification to ensure full compatibility with the existing system.

Also, a centralised approach may reduce the reliability of the system as any failure of the server is likely to affect a large number of mobile stations. Such failure may even affect mobile stations not relying on user identities.

In addition, in certain situations translation by the fixed network is not possible. For example, when a base station is operating in the condition known as base station 'fallback', the fixed network is not fully operational. Also, in DMO, there is no fixed network involved at all and therefore DMO using user identification cannot be supported.

Hence, an improved system would be advantageous and in particular a system allowing increased flexibility, increased functionality, improved reliability, reduced complexity, facilitated introduction to existing systems, improved user service, reduced cost, improved support for direct mode operation and/or improved performance would be advantageous.

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 cellular communication system supporting a plurality of communication units wherein a First communication unit comprises: storage means comprising dynamic associations between user identifiers and communication unit identifiers; means for receiving a target user identifier; means for determining a target communication unit identifier as a communication unit identifier associated with a user identifier matching the target user identifier; and means for initiating a communication to a second communication unit using the target communication unit identifier.

The first and/or second communication unit may be subscriber units of the cellular communication system such as e.g. wireless user equipment, mobile stations or communication terminals. The second communication unit may be permanently or semi-permanently associated with the second communication unit identifier. Specifically, the second communication unit identifier may be allocated to the second communication unit or to a subscriber identity module used in the second communication unit. The means for receiving the target user identifier may be user interface means. For example, the target user identifier may be a user identifier for a user to be called entered through an alphanumeric keyboard of the first communication unit. The storage means may comprise associations in the form of a table matching each stored user identifier to a single communication unit identifier.

The invention may facilitate calls to a specific user based on user identifiers in a cellular communication system using communication unit identifier addressing. Locally held associations between user identifiers and communication unit identifiers may allow a simple means of supporting user identifier addressing. The user identifier addressing may for example be added to an existing cellular communication system without requiring any modifications to the fixed network. No server or data base is necessary in the fixed network. The invention may allow a low cost support of user identifier addressing. In particular, user identifier addressing may be introduced simply by introducing additional functionality to the remote communication units. Also, maintaining the associations between user identifiers and communication unit identifiers in local storage means may allow user identifier addressing to be supported even for direct mode communication and/or where all or some functionality of the fixed network is unavailable.

In particular, a more flexible and dynamic system for user addressing in a communication unit identifier based cellular communication system may be facilitated or enabled in some embodiments. Also, signalling may be reduced in many embodiments as there is no requirement for additional signalling to servers or data bases comprised in the fixed network. This may in particular reduce the traffic load in the fixed network thereby potentially increasing the capacity of the cellular communication system as a whole and reducing costs associated with the fixed network.

The means for initiating the communication to the second communication unit may follow standard procedures of the cellular communication system based on communication unit identifier call setup. Hence, increased compatibility with existing equipment and standards may be provided in many embodiments.

According to an optional feature of the invention, the first communication unit further comprises means for receiving an association message comprising an indication of an association between a first user identifier and a first communication unit identifier; and means for updating the associations between user identifiers and communication unit identifiers in response to the first association.

This provides for an efficient means of modifying the associations stored in the storage means, hi particular, it may allow the storage means to be dynamically updated to reflect changes in associations between user identifiers and communication unit identifiers. For example, the associations may be dynamically updated when a message is received indicating that a new user having the first user identifier is using the communication unit having the first communication unit identifier. The associations may for example be updated by modifying a current association of the first user identifier to be with the first communication unit identifier, by modifying a current association of the first communication unit identifier to be with the first user identifier or by storing a new association between the user identifier and the communication unit identifier. The association message may for example be transmitted to the first communication unit from a base station or may be transmitted directly from a remote communication unit.

According to an optional feature of the invention, the first user identifier is associated with a current user of the second communication unit and the first communication unit identifier is associated with a communication unit identifier of the second communication unit. This may allow the first communication unit to receive information allowing it to access the second communication unit based only on an indication of the target user identifier. An efficient means of determining the appropriate communication unit identifier of the communication unit associated with the target user identifier is thus provided.

According to an optional feature of the invention, at least one of the plurality of communication units comprises means for transmitting the association message. The remote communication units may transmit the association message thereby providing an efficient system wherein the remote communication units may establish and maintain local associations between user identifiers and communication unit identifiers without the involvement of the fixed network.

The at least one of the plurality of communication units maybe the second communication unit. This provides for a simple and efficient system wherein individual communication

units may transmit the local association between their communication unit identifier and the user identifier of the user currently using the communication unit.

According to an optional feature of the invention, the at least one of the plurality of communication units comprises user interface means for receiving a user identifier input comprising the first user identifier. This provides for a practical, flexible, efficient and/or low complexity way of determining the user identifier. The user interface means may for example comprise an alphanumeric keyboard on which the user enters his user identifier. This may for example be a part of a standard log-on procedure when a new user is provided with a communication unit. The input provided by the user may not necessarily be the user identifier which is used by the cellular communication system but may for example be an input allowing this to be determined. For example, the user may enter his name and a look-up may be performed to find a user identifier for this name.

According to an optional feature of the invention, the at least one of the plurality of communication units is operable to transmit the association message in response to receiving the user identifier input.

This may provide for an efficient communication of association messages. Typically, a change of user is associated with a new user input and a new association message may be transmitted whenever there is a change of user. For example, the at least one communication unit may transmit a new association message whenever a new user logs-on.

According to an optional feature of the invention, the at least one of the plurality of communication units is operable to transmit the association message if the first user identifier does not match a stored user identifier.

The stored user identifier may be a user identifier of the previous user. Accordingly, in some embodiments an association message may only be transmitted if a new user identifier is received whereas if the same user identifier is received no association message is

received. This may reduce the number of association messages being transmitted and thus the communication system load and the required processing in other communication units, hi particular, the stored user identifier may have been stored prior to a power down. Hence, on power up the communication unit may determine if the same user logs-on and if so may suppress the association message. According to an optional feature of the invention, the first communication unit is operable to transmit an association request message. This may allow the first communication unit to actively request association information and allows association information to be obtained in response to local characteristics and parameters of the first communication unit.

According to an optional feature of the invention, the first communication unit is operable to transmit the association request message in response to a detection of an invalid association in the storage means between a user identifier and a communication unit identifier.

This provides a practical and efficient way of managing, updating and maintaining the associations of the storage means. An invalid association may for example be an existing association which is considered invalid according to a suitable invalidity criterion. Alternatively or additionally, a detection of a missing association for a user identifier or a communication unit identifier may be considered an invalid association.

Optionally, the communication unit may comprise means for setting designating an association as invalid in response to a message received from another communication unit.

According to an optional feature of the invention, the invalid association is an invalid association associated with the target user identifier. This may provide a simple and efficient system. For example, the first communication unit may simply generate an association request message if an invalid association (including the absence of an association) is found for the target user identifier.

According to an optional feature of the invention, the cellular communication system comprises means for broadcasting the association request message to a plurality of communication units. This may increase the probability of the association request message reaching a communication unit that can provide a desired response. The broadcast may e.g. be made directly from a remote communication unit or may be via the fixed network. For example, the association request message may be broadcast from a plurality of base stations. The broadcast may be a broadcast to all communication units or may be to a particular set or group of communication units. For example, if a given group of communication units belong to the same pool, the association request message may be transmitted only to that group of communication units.

According to an optional feature of the invention, the association request message comprises an indication of a second association between a user identifier associated with the first communication unit and a communication unit identifier associated with the first communication unit; and at least one of the plurality of communication units comprises means for storing the second association.

The association request message may comprise information of the current association between user identifier and communication unit identifier applicable to the first communication unit. This information may be used by receiving communication units to update their local storage means of associations. Hence, the association request message may allow update of other communication units without requiring more or dedicated messages to be transmitted.

According to an optional feature of the invention, the at least one of the plurality of communication units comprises means for transmitting the association message in response to receiving the association request message. This provides for an efficient system for updating association information.

According to an optional feature of the invention, the first user identifier is associated with the at least one of the plurality of communication units and the first communication unit identifier is associated with the at least one of the plurality of communication units. An association messages may be transmitted from a communication unit comprising information of the association between a user identifier and a communication unit identifier for that communication unit. This provides for a simple and efficient system for updating user information.

Additionally or alternatively, the association message may comprise information related to the associations between user identifiers and communication unit identifiers for other communication units thereby improving the distribution of association information between the plurality of communication units.

According to an optional feature of the invention, the association request message comprises an indication of the target user identifier; and the at least one of the plurality of communication units is operable to transmit the association message if the target user identifier matches the first user identifier.

This may provide for a simple and efficient system for distributing association information. For example, the association request message may specify a user identifier for which association information is sought. If the at least one of the plurality of communication units has this information, it will generate and transmit an association message.

According to an optional feature of the invention, the at least one communicating unit is operable to transmit the association message to the first communication unit on a point to point communication link. This allows an efficient communication of the association message without requiring shared resource to be used.

According to an optional feature of the invention, the second communication unit comprises second storage means comprising associations between user identifiers and communication unit identifiers.

According to an optional feature of the invention, a second user identifier and a second communication unit identifier are associated with the first communication unit, and the first communication unit comprises means for including an indication of a third association between the second user identifier and the second communication unit identifier in a call initiation message; and the second communication unit comprises means for receiving the call initiation message, and means for storing the second association in the second storage means.

The call initiation message may thus provide information which can be used by receiving communication units to update their locally stored associations. Hence, the call initiation message may allow updating of other communication units without requiring more or dedicated messages to be transmitted. The call initiation message may be any suitable message involved in setting up a call.

According to an optional feature of the invention, the second communication unit comprises: means for receiving a call initiation message comprising a second communication unit identifier associated with the first communication unit; and means for determining a calling communication unit identifier as a user identifier associated with a communication unit identifier matching the second communication unit identifier.

The second communication unit may determine the user identifier of the calling first communication unit from a call initiation message that does not comprise the user identifier of the first communication unit. This may for example allow the user identifier of the calling first communication unit to be determined in cellular communication systems wherein user identifiers are not included in call messages.

The second communication unit may for example comprise means for presenting the user identifier of the first communication unit to a user. E.g. the second communication unit may display the user identifier of the calling user on a display of the communication unit. The cellular communication may optionally be a TETRA cellular communication system. At least one of the user identifiers may be a Registered User Number (RUN). At least one of the communication unit identifiers may be an ID address such as an Individual Short Subscriber Identity (ISSI) or an Individual TETRA Subscriber ^l Identity (ITSI).

According to a second aspect of the invention, there is provided a communication unit for a cellular communication system; the communication unit comprising: storage means comprising dynamic associations between user identifiers and communication unit identifiers; means for receiving a target user identifier; means for determining a target communication unit identifier as a communication unit identifier associated with a user identifier matching the target user identifier; and means for initiating a communication to a second communication unit using the target communication unit identifier.

According to a third aspect of the invention, there is provided method of call initiation for a cellular communication system, the method comprising the steps of: storing dynamic associations between user identifiers and communication unit identifiers; receiving a target user identifier; determining a target communication unit identifier as a communication unit identifier associated with a user identifier matching the target user identifier; and initiating a communication to a second communication unit using the target communication unit identifier.

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 block schematic a cellular communication system in accordance with an embodiment of the invention;

FIG. 2 illustrates a block schematic a communication unit 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 TETRA cellular communication system but it will be appreciated that the invention is not limited to this application but may be applied to many other cellular communication systems.

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

In the embodiment of FIG. 1, a first communication unit 101 is served by a first base station 103 which also serves other communication units (not shown). The base station 103 is coupled to a fixed network 105, known as a SwMI (Switching and Management Infrastructure) for a TETRA communication system. The fixed network 105 is further coupled to other base stations 107 of which one is shown. The second base station 107 supports a plurality of TETRA communication units of which a second communication unit 109 is shown.

In the example, the first communication unit 101 is attempting to initiate a call. In TETRA communication system, call initiation and destination addressing is performed based on the identity of the communication unit, i.e. based on the ID address such as the Individual Short Subscriber Identity (ISSI) or the Individual TETRA Subscriber Identity (ITSI). Conventionally, a communication is established by the user of the calling communication unit entering a phone number or other identifier of the destination communication unit. In a TETRA system, the calling communication unit conventionally transmits a call instigation

message that carries the ISSI or ITSI of the communication unit to be called. This is used by the SwMI to alert the desired communication unit.

However, in many situations it is preferable for one user to be able to call a specific user rather than a communication unit. For example, in many situations, a group of

communication units is shared between different users and the communication unit which at a given time is used by a specific user will vary.

Accordingly, the first communication unit 101 of FIG. 1 comprises functionality for allowing a call to be instigated in response to a target user identifier.

Specifically, the first communication unit 101 of FIG. 1 comprises a user interface 111. In the example, the user interface 111 comprises an alphanumeric keyboard and display by means of which the user of the first communication unit 101 may provide a user input. The user interface 111 is operable to generate a target user identifier from the user input. In some embodiments, the user may directly input the target user identifier while in other embodiments the user input may be used to determine the target user identifier. For example, the user may enter a name of the intended user which may be translated into a target user identifier for example using a look-up table.

The user interface 111 is coupled to an association controller 113. The association controller 113 is furthermore coupled to an association memory 115. The association memory 115 comprises associations between user identifiers and communication unit identifiers. The associations between user identifiers and communication unit identifiers are dynamically updated to reflect changes and variations in the system. As a specific example, the association memory 115 may comprise a look-up table having a number of entries corresponding to specific user identifiers. For each user identifier entry, one corresponding communication unit identifier is stored. The communication unit

identifier stored for each user identifier is the identifier of the communication unit currently used by the user with that user identifier. The stored communication unit identifiers are dynamically modified to reflect changes in the allocations of communication units to users.

The user interface means 111 feeds the target user identifier to the association controller 113 and in response the association controller 113 accesses the association memory 115

using the target user identifier. If the association memory 115 has a valid user identifier entry matching the target user identifier, the corresponding communication unit identifier is retrieved.

Thus, the association controller 113 determines a target communication unit identifier corresponding to the communication unit which according to the entries in the association memory 115 is currently used by the user with the target user identifier. In the embodiment of FIG. 1, the target communication unit identifier is a TETRA E) address such as the ISSI or ITSI.

The association controller 113 is coupled to a transmission controller 117 which is fed the target communication unit identifier. When receiving the target communication unit identifier, the transmission controller 117 proceeds to initiate a communication to the communication unit with the target communication unit identifier. In the example of FIG. 1, the transmission controller 117 initiates a call setup procedure in accordance with the TETRA Technical Specifications. Specifically, call initiation messages are transmitted using the ITSI or ISSI communication unit identifiers. The transmission controller 117 is coupled to a transmitter 119 which is operable to transmit the call initiation messages over the TETRA air interface to the first base station 103. As the call initiation messages comprise the ITSI or ISSI of the communication unit used by the user to be called, the base stations and the fixed 5 network is capable of addressing and routing the messages to the desired communication unit using a standard

TETRA call setup procedure and protocols. In the specific example, it is assumed that the target user identifier is currently associated with the second communication unit 10 109 and a call between the first communication unit and the second communication unit may be set up using standard TETRA call setup procedures as is well known in the art.

The embodiment of FIG. 1 thus provides a low complexity means of supporting user 15 identification addressing and calling. The calling user can simply enter the user identifier (or a user input allowing this to be determined) and the communication unit will automatically determine the corresponding communication unit identifier and initiate a call to the appropriate communication unit. The embodiment provides for this functionality without necessitating any changes to the fixed network (including 0 the base stations) or the existing call setup procedures. Hence, reduced cost and increased reliability may be achieved.

In a system using a local storage of associations between user identifiers and communication unit identifiers, it is desirable that the associations as accurately as 5 possible reflect the current situation and the current association between communication unit identifier and user identifier.

Embodiments utilising various techniques for dynamically maintaining and updating the locally stored dynamic associations will be described in the following. 0 FIG. 2 illustrates a communication unit in accordance with an embodiment of the invention. The communication unit maybe the first communication unit 101 of FIG. 1 and will be described with reference to this.

In addition to the user interface means 111, the association controller 113, the association memory 115, the transmission controller 117 and the transmitter 119 already described, the first communication unit 101 of FIG. 2 also comprises a receiver 121 which is operable to receive messages transmitted from the first base station 103.

hi particular, the receiver 121 may receive association messages comprising an indication of an association between a user identifier and a communication unit identifier. As a specific example, the association message comprises a user identifier and the communication unit identifier of the communication unit used by that user.

The receiver 121 is coupled to an association update controller 123 which is further coupled to the association memory 115. When the receiver 121 receives an association message it feeds this to the association update controller 123. hi response, the association update controller 123 updates one or more associations stored in the association memory 115. For example, if an association message is received comprising an indication of a first user identifier currently being associated with a first communication unit identifier, the association update controller 123 creates an entry in the association memory 115 for the first user identifier indicating that it is associated with the first communication unit identifier. If an entry already exists for the first user identifier in the association memory 115, the stored corresponding communication unit identifier is overwritten by the first communication unit identifier.

Accordingly, the association update controller 123 will, in response to receiving association messages, update the association memory 115 to reflect the current associations between users and communication units. The association memory 115 may thereby be updated automatically without involving the user of the first communication unit.

The association messages may be received from any suitable source and may comprise any suitable association information. For example, the association message may be transmitted from a central server in the fixed network or may be transmitted from other communication units.

For example, communication units may at regular intervals transmit association messages comprising information indicating which user identifier is currently logged in to the individual communication unit. The receiver 121 may in this case frequently receive

association messages from different communication units. For each new association message, the association update controller 123 updates the association memory 115 which is accordingly automatically and dynamically updated to reflect the current allocation of communication units to users.

As a specific example, the second communication unit may transmit an association message which comprises the communication unit identifier of the second communication unit as well as the user identifier of the user currently using the second communication unit. When receiving this message, the association is stored in the association memory 115. When the user of the first communication unit seeks to transmit to the user which, unbeknownst to the user of the first communication unit, is currently using the second communication unit, he may simply enter the user identifier of this user. The association controller 113 will then retrieve the association that was received in the association message from the second communication unit and accordingly set the target communication unit identifier to the communication unit identifier of the second communication unit.

The association messages maybe transmitted to the first communication unit 101 in any suitable form and by any suitable means. For example, the association messages may be transmitted or broadcast to other communication units directly from the individual communication units using direct mode operation techniques. Additionally or alternatively, the association message may be distributed through the fixed network and transmitted from base stations. This transmission may for example be a broadcast transmission using TETRA broadcast channels. It is desirable to broadcast the association messages in many embodiments as the association messages typically will be relevant for many communication units.

In the embodiment of FIG. 2, the first communication unit 101 furthermore comprises an association request controller 125 coupled to the association memory 115 and the transmission controller 117.

The association request controller 125 is operable to generate and cause the transmission controller 117 to transmit an association request message. The association request message constitutes a request for association information. The association request message is in the example transmitted or broadcast to other communication units either directly or through the fixed network and the base stations. In a simple embodiment, any communication unit that receives an association request message will transmit an association message comprising association information. Thus in response to transmitting an association request message, the first communication unit may receive a number of association messages which may be used to update the association memory 115.

The first communication unit 101 may determine the need to transmit an association request message in accordance with any suitable criteria. For example, it may simply transmit an association request message at regular intervals thereby ensuring that new association information is received at a regular rate.

However, in more advanced embodiments, the association request controller 125 causes an association request message to be transmitted selectively in response to characteristics of the associations stored in the association memory 115. In some such embodiments, the association request controller 125 may cause an association request message to be transmitted when an invalid association between a user identifier and a communication unit identifier is detected in the association memory 115.

An invalid association may be considered invalid in accordance with any suitable invalidity criterion. For example, associations may be time stamped when stored in the association memory 115 and an association may be considered invalid if it is older than a certain threshold. As another example, an association may simple be considered invalid if there is no corresponding association stored in the association memory 115. For example, an association for a given user identifier may be considered invalid if there is no entry in the association memory 115 corresponding to that user identifier or if there is an entry but no corresponding communication unit identifier. Similarly, an association for a given

communication unit identifier may be considered invalid if there is no entry in the association memory 115 corresponding to that communication unit identifier or if there is an entry but no corresponding user identifier.

In some embodiments, the association request controller 125 may for example at regular intervals scan the association memory 115 to detect invalid associations. However, in other embodiments, invalid association detection may be part of the call initiation process. In particular, the invalid association may be an association associated with the target user identifier. In one such embodiment, the association request controller 125 may be operable to cause an association request message to be transmitted if there is no valid entry for the target user identifier which the user is intending to call. Thus, if a user attempts to make a call to a specific user identifier and the association memory 115 comprises no indication of the required communication unit identifier, the association request controller 125 will proceed to generate and transmit an association request message in order to obtain this information. In some embodiments the communication unit may comprise functionality for designating an association as invalid in response to receiving a message from another communication unit. For example, an existing association may be marked as invalid, if a de-association indication is received from another communication unit. The de- association indication may be initiated by log-off of the user of the other communication unit which in response may send the de-association indication prior to switching off the radio. Thus the de-association indications is an indcation from a communication unit that the previous user is no longer associated with the communication unit.

In some embodiments, the association request message may be transmitted directly to a specific destination such as a specific database or server, a specific communication unit or a group of communication units. However, in the embodiment of FIG. 2, the association request message is broadcast in order to increase the probability of receiving the desired information. The transmission and/or broadcast of the association request message may be

a direct communication to other communication units using direct mode operation techniques or may be transmitted via the base stations and possibly the fixed network.

hi the embodiment of FIG. 2, the association request controller 125 is operable to generate an association request message which comprises an indication of the association between a user identifier of the user of the first communication unit 101 and the communication unit identifier of the first communication unit. Hence, any communication unit receiving the association request message is not only receiving a request for information but is also provided with information of which user is currently using the communication unit requesting the information. Accordingly, the receiving communication unit may update a local database of associations to reflect that the first communication unit is associated with the given user identifier. By using the message for dual purposes, improved distribution of association information is achieved without requiring additional resources for transmitting this information.

In some embodiments the association requests messages may be generated using a priority and/or predefined sort algorithm. For example, a frequently called target user may be identified and association request messages may be transmitted to this user.

FIG. 3 illustrates a communication unit which comprises functionality for generating and distributing association information. The communication unit may for example be the second communication unit 109 of FIG. 1 and will be described with reference to this.

The second communication unit 109 comprises a user interface 127. In the example, the user interface 127 comprises an alphanumeric keyboard by means of which the user of the second communication unit 109 may provide a user input. The user interface 127 is from the user input operable to generate a local user identifier that indicates a user identifier of the user currently using the second communication unit 109. hi particular, the user interface 127 may be used by users to log on to the second communication unit 109. The log-on may be performed by the user entering their user identifier which is subsequently stored as the local user identifier, hi some embodiments; the user may directly input the

user identifier while in other embodiments the user input may be used to determine the local user identifier. For example, the user may enter a name which may be translated into a user identifier for example using a look-up table.

The user interface 127 is coupled to an association transmission controller 129 which is operable to generate and transmit an association message comprising an indication of the association between the local user identifier and the communication unit identifier of the second communication unit 109. The association transmission controller 129 is coupled to a transmitter 131 which is operable to transmit the association message. In some embodiments, the transmitter may be operable to transmit the association message directly to other communication units using direct mode operation techniques. Additionally or alternatively, the transmitter 131 may be able to transmit the association message to the serving base station 107 from where it is routed to the appropriate base station(s) for transmission to the intended recipient. For example, the second communication unit 109 may transmit the association message to the second base station 107 from where it is routed to the first base station 103 through the fixed network 105. The first base station 103 then transmits the association message to the first communication unit 101.

It will be appreciated that the association transmission controller 129 may generate and transmit an association message in accordance with any suitable algorithm. For example, the association transmission controller 129 may simply transmit the association message at regular intervals.

However, in some embodiments, the association transmission controller 129 may alternatively or additionally be operable to transmit an association message whenever a new user input is received by the user interface 127. Hence, the association transmission controller 129 may cause a new association message to be transmitted whenever a new user logs on the second communication unit 109. The association message may therefore specifically be transmitted when a change in the association between a local user identifier

and the communication unit identifier of the second communication unit 109 changes. This will provide for a fast update of associations at other communication units while reducing the number of association messages transmitted thereby reducing the resource usage.

In some embodiments, a new user identifier is compared to the previous user identifier and an association message is transmitted only if the new user identifier is different from the previous. This may in particular be advantageous in situations where the previous user identifier is stored when the second communication unit 109 is switched off. When powering up, a new log-on procedure is required by the user. However, if the user has not changed, the same user identifier will be received and the current associations will be correct. Hence, there is no need for transmitting a new association message and the association transmission controller 129 therefore does not proceed to generate and transmit an association message.

In the embodiment of FIG. 3, the second communication unit 109 furthermore comprises a receiver 133 which is operable to receive messages from other communication units. In particular, the receiver 133 may receive association request messages from other communication units.

The receiver 133 is coupled to an association response controller 135 which is fed any association request messages received. The association response controller 135 evaluates the received association request message to determine if the requested association information is available at the second communication unit 109 and if so it causes the association transmission controller 129 to transmit an association message comprising the required association information.

As a specific example, the second communication unit 109 may simply be operable to transmit information of the current association between the current local user identifier and the communication unit identifier of the second communication unit 109. hi this example, the association response controller 135 may receive an association request message from the first communication unit requesting information related to a given user identifier.

The association response controller 135 will compare the given user identifier to the local user identifier, and if they match it proceeds to cause an association message to be sent indicating that the user identifier is currently associated with the communication unit identifier of the second communication unit 109. Upon receiving this message, the first communication unit 101 may update its association memory 115 and may subsequently proceed to make a call to the user identifier using the communication unit identifier of the second communication unit 109.

The transmission of the association message may in this example be transmitted directly to the first communication unit 101 on a point to point communication link. This may minimise the resource usage as broadcast or other point to multipoint communications tend to be more resource demanding.

In the embodiment, the second communication unit 109 of FIG. 3 further comprises a second association memory 137 which comprises associations between user identifiers and communication unit identifiers. This local association memory 137 may for example be used by the second communication unit 109 to instigate calls to other users in the same way as described for the first communication unit 101.

The second association memory 137 is coupled to a second association controller 139 which is further coupled to the receiver 133. In the example, the receiver 133 may receive messages comprising association information and feed these to the second association controller 139 which accordingly updates the association information stored in the second association memory 137.

In particular, association request messages may, as previously described, comprise association information for example relating to the communication unit transmitting the association request message. This information may be extracted and included in the second association memory 137.

As another example, the receiver may receive a call initiation message which comprises association information. For example, the first communication unit 101 may when initiating a call to the second communication unit 109 include information of the user identifier currently associated with the first communication unit 101. This information may be extracted by the second association controller 139 and may be used to update the second association memory 137. Accordingly, the first communication unit 101 may subsequently be able to call the user of the first communication unit 101 based only on the user identifier. This may facilitate and improve the distribution of association information without increasing the number of messages that must be transmitted.

hi some embodiments, the association information may be used to provide an indication of the identity of the calling user. For example, when receiving a call initiation message comprising the communication unit identifier of the originating communication unit, but not the user identifier of the calling user, the second association controller 139 of the second communication unit 109 may access the second association memory 137 based on the received communication unit identifier. The corresponding user identifier may be retrieved from the second association memory 137 and may be presented to the user of the second communication unit 109 for example by displaying the calling user identifier on a display 141 of the second communication unit 109. Hence, the association information may be used to provide an enhanced user experience.

It will be appreciated that in some embodiments, communication units may send association messages comprising information of the local association between communication unit identifiers and user identifiers only. However, in other embodiments, communication units may also send association information related to other communication units. For example, a communication unit may simply include the full contents of the local association memory in the association message. This may facilitate distribution of association information but will typically result in increased resource consumption associated with the transmission of association messages.

It will be appreciated that in many embodiments, communication units may preferably comprise both some or all of the functionality described with reference to the communication unit of FIG. 2 and some or all of the functionality described with reference to the communication unit of FIG. 3.

The described functionality may thus allow user identifier addressing in a communication system relying on communication unit addressing for routing and identification. A local translation is performed in the individual units in response to locally stored associations between user identifiers and communication unit identifiers. The associations may dynamically and automatically be maintained and updated to reflect changes in the communication system thereby allowing a flexible, user friendly and high performance implementation.

The invention can be implemented in any suitable form including. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors.

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, hi 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, hi addition, singular references do not exclude a plurality. Thus references to "a", "an", "first", "second" etc do not preclude a plurality.

Claims

1. A cellular communication system supporting a plurality of communication units wherein a first communication unit comprises: storage means comprising dynamic associations between user identifiers and communication unit identifiers; means for receiving a target user identifier; means for determining a target communication unit identifier as a communication unit identifier associated with a user identifier matching the target user identifier; and means for initiating a communication to a second communication unit using the target communication unit identifier.

2. A cellular communication system as claimed in claim 1 wherein the first communication unit further comprises means for receiving an association message comprising an indication of an association between a first user identifier and a first communication unit identifier; and means for updating the associations between user identifiers and communication unit identifiers in response to the first association.

3. A cellular communication system as claimed in claim 2 wherein the first user identifier is associated with a current user of the second communication unit and the first communication unit identifier is associated with a communication unit identifier of the second communication unit.

4. A cellular communication system as claimed in claim 3 wherein at least one of the plurality of communication units comprises means for transmitting the association message.

5. A cellular communication system as claimed claim 3 wherein the at least one of the plurality of communication units comprises user interface means for receiving a user identifier input comprising the first user identifier.

6. A cellular communication system as claimed in claim 4 wherein the at least one of the plurality of communication units is operable to transmit the association message a) in response to receiving the user identifier input or b) if the first user identifier does not match a stored user identifier.

7. A cellular communication system as claimed in claim 1 comprising means for broadcasting the association request message to a plurality of communication units.

8. A cellular communication system as claimed in claim 1 wherein the association request message comprises an indication of a second association between a user identifier associated with the first communication unit and a communication unit identifier associated with the first communication unit; and at least one of the plurality of communication units comprises means for storing the second association.

9. A cellular communication system as claimed in claim 2 wherein the first user identifier is associated with the least one of the plurality of communication units and the first communication unit identifier is associated with the least one of the plurality of communication units.

10. A cellular communication system as claimed in claim 1 wherein a second communication unit comprises second storage means comprising associations between user identifiers and communication unit identifiers.