WO2006014243A1

WO2006014243A1 - A communication unit, a cellular communication system and a method of operation therefor

Info

Publication number: WO2006014243A1
Application number: PCT/US2005/022300
Authority: WO
Inventors: Lutz Degener; Murat Cetin; Peter Hackenberg
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2004-07-02
Filing date: 2005-06-23
Publication date: 2006-02-09
Anticipated expiration: 2007-01-02
Also published as: EP1766802A4; EP1766802A1; GB2415864B; CN1993898A; GB0414824D0; KR20070026778A; GB2415864A

Abstract

A mobile communication unit for a cellular communication system (100) comprises direct mode communication means (127, 129) for communicating in a direct mode to other communication units (109) of the cellular communication system (100) and trunked mode communication means (125, 129) for communicating in a trunked mode to a serving base station (103). A user interface (119) may receive an emergency input and in response a feasibility processor (123) determines a feasibility of communicating in the trunked mode. An emergency controller (113) selects between communicating an emergency message in the direct mode and in the trunked mode in response to the feasibility. Specifically, the emergency message may be transmitted in trunked mode if this is feasible and will otherwise automatically be transmitted in direct mode. The invention is applicable to a TETRA cellular communication system.

Description

A COMMUNICATION UNIT, A CELLULAR COMMUNICATION SYSTEM AND A METHOD OF OPERATION THEREFOR

Field of the invention

The invention relates to a communication unit, a cellular communication system and a method of operation therefor and in particular, but not exclusively, to emergency call operation in a TETRA (TErrestrial Trunked RAdio) 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 mobile unit (e.g. mobile and/or portable radio, radiotelephone or the like or 'mobile station')is served via a radio communication link by the base station of the cell within which the remote unit is situated.

As a mobile 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 without the loss of service. This is known as cell reselection or handover.

Communication from a mobile 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 remote unit 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, remote unit authentication etc.

Examples of cellular communication systems include both public cellular communication systems 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 TETRA provides a number of services supporting emergency calls. For example, many TETRA remote units are provided with an easy to operate mechanism for instigating an emergency call that automatically will be routed to an emergency operations centre coupled to the fixed network.

Communication to and from a TETRA mobile unit is typically achieved through a base station. Thus, the mobile unit is in communication with a serving base station which is connected to the fixed network. This is known as Trunked Mode Operation (TMO). In TMO, the air interface communication of data is between the mobile unit and the base station. The base station is an access point for the mobile unit and the communication link from the mobile unit to the destination(s) comprises a first leg from the remote unit to the base station and a second leg from the base station to the destination(s).

For example, if the other party of a call is another mobile unit, the communication link is established through the serving base station of the other mobile unit. Thus, for each of the two mobile units an air interface communication link is formed with the respective serving base stations and a fixed network communication link is formed between these base stations through the fixed network. If the two mobile units are within the same cell, and accordingly have the same serving base station, the same serving base station supports both communication links and an internal base station communication link is formed between the two air interface links.

In trunked mode operation, the air interface communication links are supported, controlled and managed by functionality in the fixed network.

The channel allocation itself is performed by the Site Controller, which is part of the base station. Thus, resource allocation for all remote units served by a given base station is centralised allowing for a suitable and flexible allocation which reduces interference and conflicts. Furthermore, trunked mode operation efficiently utilises air interface resource and provides extended coverage and an ability to communicate with other communication units independently of the physical distance.

hi addition to TMO, TETRA provides for the possibility of mobile units communicating directly with each other over the air interface and without involving any base stations or the fixed network. This is known as Direct Mode Operation (DMO) and allows mobile units to set up and maintain a direct air interface communication link with each other. In one aspect this may allow a mobile unit to act as a repeater for another mobile unit. For example a vehicle mounted unit may act as a repeater for a mobile unit thereby improving coverage for the mobile unit.

m DMO, the air interface communication links are supported, controlled and managed by functionality in the mobile units themselves.

As DMO does not require the presence of a base station, it may enable communication between remote units in areas which are not covered by a base station, for example in many in-building environments. Furthermore, in situations where remote stations are close together, a resource efficient communication link can be set up and in particular interference may be reduced as lower transmit powers can be used.

Protocols and procedures have been standardised for initiating, supporting and terminating both DMO and TMO calls. Thus, a user of a DMO enabled mobile unit may select DMO or TMO operation before making a call. Typically, the mobile unit will be set in a default operating mode. For example, a mobile unit may be in a TMO mode wherein TMO calls can be instigated and received. However, if the user cannot make a TMO call (or receive a TMO call) because it is not within the coverage area of a base station, he may switch to DMO operation and retry instigation the call. As another example, an emergency service or the police may have a vehicle based mobile unit in a vehicle and a personal remote unit for each person. When the person leaves the vehicle, e.g. to enter a building where coverage is expected to be reduced, he may switch the personal mobile unit to DMO in order to use the vehicle based mobile unit as a repeater thereby improving coverage within the building.

As DMO and TMO provide different advantages suitable for different situations, the manual user control allows the user to select the appropriate operation for the given situation in accordance with his preferences. This may be important in some situations.

In particular, if a user instigates an emergency call, the TMO operation may provide a fast and direct access to the appropriate destinati'on for the emergency call. Furthermore, the user may enter the remote unit in DMO mode if it is considered that he is in a situation where it is likely that a communication link to another mobile unit can be established but unlikely that communication to a base station is possible.

However, although the DMO and TMO modes may improve emergency call performance, there are disadvantages associated with the conventional approach.

For example, it may be difficult for the user to determine which operational mode is the most advantageous as this may depend on characteristics which cannot easily be determined. Furthermore, the characteristics may vary significantly and may vary quickly thereby making it impractical for the user to continuously modify the operation mode to match the current conditions. In particular, evaluation of propagation conditions requiring technical insight are difficult to determine by a user without dedicated measurements. They may furthermore vary significantly with time and location thereby making it very difficult for the user to select the best operation mode. Accordingly, emergency call performance may be reduced and in particular the probability of an emergency call not being detected is increased. Furthermore, the switching between operation modes is cumbersome and time consuming. In particular, if a mobile unit uses TMO when an emergency call is instigated and the user detects that the emergency call is not successful, he may switch the mobile unit to DMO mode and repeat the emergency call. However, this requires that the user monitors the emergency call performance and manually operates the remote unit, which is particularly impractical as emergency calls are typically instigated in situations where the user's full attention is required by the emergency situation. Furthermore, the switching of operation mode is slow and will result in delay of the emergency call which may be critical.

Hence, an improved system for emergency calls in a cellular communication system would be advantageous and in particular a system allowing facilitated emergency call instigation, improved emergency call performance, increased emergency call detection, simplified user operation, reduced delay, increased flexibility and/or improved adaptation to current conditions 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 communication unit (mobile station) for a cellular communication system; the communication unit comprising: direct mode communication means for communicating in a direct mode to other communication units of the cellular communication system; trunked mode communication means for communicating in a trunked mode to a serving base station; means for determining a feasibility of communicating in the trunked mode; and an emergency call controller coupled to the direct mode communication means and the trunked mode communication means; wherein the emergency call controller is operable to select between communicating an emergency message in the direct mode and in the trunked mode in response to the feasibility. The invention may allow an automatic detection of the preferred operating mode to suit the current conditions.

The invention may allow an automatic communication of an emergency message using the best suited communication mode for the current conditions thereby improving the emergency performance and increasing the probability of a successful communication of the emergency message. No user involvement in the selection is required and user instigation of an emergency call is facilitated. Furthermore, the selection of the appropriate communication mode may be quick resulting in a faster transmission of the emergency message.

For example, if a user of the communication unit instigates an emergency call outside the coverage area of any base station, the communication unit in accordance with the invention may automatically switch to direct mode and transmit the message directly to other communication units.

The trunked mode communication is at least partly via an air interface communication link between the communication unit and the serving base station whereas the direct mode communication is at least partly via an air interface link between the communication unit and at least one other remote communication unit.

The means for determining may generate a direct feasibility indication and the selection between direct mode and trunked mode may be in response to this feasibility indication. The feasibility may e.g. be an implicit determination and the selection may be integrated with the determination of the feasibility and no separate feasibility parameter needs to be explicitly determined. For example, different operating modes or subroutines corresponding to the trunked mode and the direct mode may be executed depending on a characteristic indicative of the feasibility of operating in trunked mode. The feasibility may be a feasibility which is only a function of characteristics associated with trunked mode operation or may also e.g be dependent on characteristics associated with direct mode operation. For example, the feasibility may be a relative feasibility indicative of the relative feasibility of communicating in direct mode and in trunked mode.

The feasibility may relate to the reliability of a possible communication in trunked mode. For. example, increasing feasibility may be considered for increasing probability of a trunked mode communication meeting a given quality of service requirement.

According to an optional feature of the invention, the emergency call controller is operable to select the trunked mode when the trunked mode is feasible and to select the direct mode when the trunked mode is not feasible.

Trunked mode may be used to support emergency calls by default but an automatic back-up using direct mode may be provided for. The invention may allow for trunked mode to be prioritised with respect to direct mode thereby allowing the preferred mode to be used whenever possible. This may be advantageous in many embodiments wherein the emergency call is desired to reach a destination via the fixed network, e.g. a central emergency operations centre.

According to an optional feature of the invention, the means for determining is operable to determine the feasibility in response to a signal received from a base station.

This provides for an efficient and practical determination of a suitable feasibility in many cellular communication systems. For example, the signal may be indicative of whether the communication unit is within the coverage area of the base station and thus whether trunked mode operation can be supported. According to an optional feature of the invention, the signal is a broadcast control signal. This may provide a particularly reliable indication of the coverage area of the base station transmitting the broadcast control signal.

According to an optional feature of the invention, the signal is an acknowledge indication signal. The acknowledge indication may be an acknowledge indication associated with a transmission from the communication unit. For example, if a previous trunked mode transmission does not result in an expected acknowledge signal, this may be used as an indication of a low feasibility of communicating in trunked mode. The acknowledge indication may in particular be associated with a previous transmission of an emergency message.

According to an optional feature of the invention, the communication unit furthermore comprises user interface means for receiving an emergency input. This provides a suitable means for receiving an emergency input allowing the user to control the transmission of an emergency message without requiring explicit selection of the communication mode used. The user interface means may be simple and may for example consist in a single alarm button.

According to an optional feature of the invention, the means for determining is operable to determine the feasibility in response to the user interface receiving the emergency input; and the emergency call controller is operable to transmit the emergency message in response to the user interface receiving the emergency input.

This may provide for a simple operation with low additional complexity of the communication unit. In particular, the computational resource requirement may result if the determination and transmission is only performed when an emergency input is received. This may further ensure that the selection between the direct mode and trunked mode is based on current conditions. According to an optional feature of the invention, the emergency call controller is operable to switch the communication unit from a direct mode operation to a trunked mode operation dependent on the feasibility and in response to receiving the emergency input.

For example, the communication unit may be in a trunked idle mode and when an alarm button is transmitted, the communication unit may automatically switch to direct mode to transmit the emergency message if this cannot be achieved in trunked mode. Improved emergency operation may thus be achieved without requiring additional user invention and while allowing the communication unit to be left in trunked mode without consideration of the current conditions.

According to a second aspect of the invention, there is provided a cellular communication system comprising at least one communication unit as previously described.

According to an optional feature of the invention, the cellular communication system further comprises a plurality of communication units; each of the plurality of communication units comprising: means for receiving the emergency message when transmitted in direct mode; and means for transmitting a trunked mode emergency message to a base station in response to receiving the emergency message.

This may provide improved emergency performance in a cellular communication system. In particular, the feature may provide the characteristics of trunked mode communication to be provided to emergency calls from communication units in situations wherein trunked mode operation is not supported. For example, it may allow an emergency message to be communicated to a central emergency operations centre in the fixed network from a communication unit which is not in the coverage area of any base station. The trunked mode emergency message may be a simple emergency indication message or may e.g. comprise additional information including information received in the emergency message from the instigating communication unit, data associated with the communication unit transmitting the trunked mode emergency message and/or added information associated with the instigating communication unit. In particular, the trunked mode emergency message may be substantially identical to or comprise the original emergency message.

According to an optional feature of the invention, the plurality of communication units comprises means for operating in a combined idle mode wherein both direct mode messages and trunked mode messages can be received.

This may increase the probability of a direct mode communication link being formed as more communication units are likely to be able to receive the emergency message. The combined idle mode may for example be an idle mode dual watch mode of a TETRA cellular communication system.

The cellular communication system may in particular be a TETRA cellular communication system.

According to third aspect of the invention, there is provided a method of operation for a communication unit of a cellular communication system; the communication unit including direct mode communication means for communicating in a direct mode to other communication units of the cellular communication system and trunked mode communication means for communicating in a trunked mode to a serving base station; the method comprising the steps of determining a feasibility of communicating in the trunked mode; and selecting between communicating an emergency message in the direct mode and in the trunked mode in response to the feasibility.

These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. An embodiment of the invention will be described, by way of example only, with reference to the drawings, in which

Brief Description of the Drawings

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

FIG. 2 illustrates a method of operation for 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 100 in accordance with an embodiment of the invention.

In the embodiment of FIG. 1, a first mobile communication unit or mobile station 101 is served by a first base station 103 which also serves other mobile 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 mobile communication units of which a second communication unit or mobile station 109 is shown. The fixed network is furthermore coupled to an emergency operations centre 111 from where emergency calls are handled. The emergency operations centre 111 may for example be a centralised alarm centre for an emergency service or a law enforcement organisation from where assistance rriay be dispatched to assist in the emergency situation.

In the example, the first mobile communication unit 101 comprises functionality for communicating in trunked mode (using TETRA TMO) with the first base station 103 over a first air interface communication link 113. Similarly, the second mobile communication unit 107 comprises functionality for communicating in trunked mode (using TETRA TMO) with the second base station 107 over a second air interface communication link 115. hi addition, both the first and second mobile communication units 101, 109 comprise means for communicating directly with other communication units in a direct mode (using TETRA DMO). Hence, if conditions allow, the first and second mobile communication units 101, 109 comprise functionality for establishing a direct air interface link 117 between them.

hi the example if FIG. 1, the first communication unit 101 is attempting to initiate an emergency call intended for the emergency operations centre 111. Conventionally, the emergency call is initiated using trunked mode operation by an emergency message being transmitted to the base station 103 and then forwarded to the emergency operations centre 111. However, if the first communication unit 101 is not within the coverage area of the first base station 103, the emergency call cannot be instigated resulting in a potentially critical situation, hi this situation, the user may choose to switch to a direct mode of operation and transmit an emergency message directly to any other remote unit. However, this is cumbersome, impractical and slow and may indeed not be doable in the specific emergency situation.

hi accordance with the embodiment of FIG.1, the first mobile communication unit 101 comprises functionality for automatically selecting between direct mode operation and trunked mode operation for transmission of emergency messages and in particular comprises functionality for automatically transmitting an emergency message using direct mode if trunked mode is not adequately supported in the specific situation. Specifically, the first mobile communication unit 101 of FIG. 1 comprises a user interface 119. In the example, the user interface 119 may simply comprise an alarm button that can be pressed in an emergency situation. In the embodiment of FIG. 1, no other user activity or operation is required for the automatic transmission of a trunked mode and/or direct mode emergency message.

The user interface 119 is coupled to an emergency controller 121 which is operable to control the transmission of the emergency message. The emergency controller 121 is coupled to a feasibility processor 123 which is operable to determine a feasibility of communicating in the trunked mode of operation. The emergency controller 121 is further coupled to a trunked mode controller 125 and a direct mode controller 127. The direct mode controller 127 and trunked mode controller 125 are both coupled to a transmitter 129 which is operable to perform the required modulation, amplification etc required for transmitting radio signals in accordance with the TETRA air interface

O specifications as will be known to the person skilled in the art.

The trunked mode controller 125 is together with the transmitter 129 capable of communicating in a trunked mode with the first base station 103. Similarly, the direct mode controller 127 is together with the transmitter 129 capable of communicating in a direct mode with other communication units. Hence, in the embodiment of FIG. 1, the emergency controller 121 may control the direct mode controller 127 to transmit the emergency message in a direct mode using the direct mode controller 127 or may transmit the emergency message in a trunked mode using the trunked mode controller 125.

In use, when the user interface 119 receives an emergency input by the user pressing the alarm button, the emergency controller 121 obtains a feasibility indication from the feasibility processor 123. The feasibility indicator is indicative of the feasibility of communicating the emergency message using trunked mode. In particular, the feasibility indicator may be indicative of the feasibility of a communication being successfully established to the first base station 103 in the current conditions. In response to the feasibility indication, the emergency controller 121 selects to transmit the emergency message using either trunked mode or direct mode.

In the example, if the feasibility indicator indicates that it is likely that communication to the first base station 103 can be supported, the emergency controller 121 generates the emergency message and feeds it to the trunked mode controller 125. In response, the trunked mode controller 125 controls the transmitter 129 to transmit the emergency message in trunked mode to the first base station 103 over the trunked mode air interface communication link 113. The emergency message is then routed from the first base station 103 to the emergency operations centre 111 through the fixed network 105.

If the feasibility indicator indicates that it is unlikely that communication to the first base station 103 can be supported, the emergency controller 121 instead generates the emergency message and feds it to the direct mode controller 127. In response, the direct mode controller 127 controls the transmitter 129 to transmit the emergency message in direct mode to other remote units. In particular, the direct mode controller 127 may control the transmitter 129 to broadcast the emergency message such that it can be received by any other remote unit within range of the first mobile communication unit 101.

It will be appreciated that any suitable likelihood criterion can be used and that the specific criterion used will depend on the characteristics of the individual embodiment.

In the example, the emergency message may be received by the second mobile communication unit 109 over direct mode air interface communication link 117. In response, the second mobile communication unit 109 may alert the user of the second mobile communication unit 109 of the emergency call. In this example, the user of the second communication unit 109 is preferably presented with additional information from the emergency message. For example, the emergency message may comprise location information for the first communication unit 101 thereby allowing the user of the second mobile communication unit 109 to identify the location of the emergency situation: As another example, the message may comprise user and/or equipment identification information that can be presented to the user of the second mobile communication unit 109.

This approach may be suitable for example for police or other emergency service organisations wherein an emergency alarm may quickly alert personnel in the vicinity of the first mobile communication unit 101. Hence, in this example, if trunked mode operation is feasible, the emergency message is received at the emergency operations centre 111 from where a co-ordinated assistance operation may be controlled and instigated. This may for example include alerting other users in the vicinity of the first communication unit 101 and requesting their assistance. However, if trunked mode operation is not possible, any remote units within range and capable of direct mode operation may automatically receive the emergency call thereby providing assistance even when the emergency operations centre 111 cannot be reached through trunked mode communication.

In some embodiments, the second communication unit 109 may not only comprise functionality for receiving the emergency message in direct mode but may also comprise functionality for transmitting a trunked mode emergency message to a base station in response to receiving the emergency message. The second mobile communication unit 109 may thus receive the emergency message over the direct mode air interface communication link 117 and may in response generate a trunked mode emergency message which is transmitted over the trunked mode air interface link 115 to the second base station 107. The trunked mode emergency message is then routed from the second communication unit 109 to emergency operations centre 111. Hence, in this embodiment, the emergency operations centre 111 may receive an emergency message generated in response to the user activating the alarm button on the first communication unit 101 even in situations where the first communication unit 101 cannot use trunked mode operation. This is furthermore achievable without requiring any additional activity from the user of the first mobile communication unit 101.

The trunked mode emergency message may comprise some or all of the information of the original emergency message. For example, in some embodiments, the second communication unit 109 may simply forward the received emergency message. However, in other embodiments, the second mobile communication unit 109 may generate a new trunked mode emergency message. This new message may include additional information and may in particular comprise information related to the second communication mobile unit 109, such as an indication that the emergency message was received by the second mobile communication unit 109 and perhaps the location of the second mobile communication unit 109. This may assist and/or improve the emergency operation of the emergency operations centre 111. For example, it provides an indication that the second mobile communication unit 109 is in the vicinity of the first communication unit 101 thereby allowing the emergency operations centre 111 to instruct the user of the second mobile communication unit 109 to assist the user of the first mobile communication unit 101.

It will be appreciated that any suitable feasibility indication and algorithm or criterion for determining a feasibility may be used.

hi particular, the feasibility processor 123 may be operable to determine a feasibility indication in response to a signal received from a base station.

For example, in TETRA communication systems, base stations transmit a continuous pilot signal known as a broadcast channel. The broadcast channel is transmitted at a fixed maximum base station power and is used by remote units to determine the relative strength between base stations in order to determine which base station is the serving base station. The broadcast channel signal provides a good indication of whether communication with the corresponding base station can be supported. Thus, in a simple embodiment, the feasibility processor 123 may evaluate if a broadcast channel is received at a signal level above a given threshold. If this is the case, the feasibility indication may be set to indicate that trunked mode operation is feasible (or likely) and otherwise the feasibility indication may be set to indicate that trunked mode operation is not feasible (or unlikely). Hence, in such an embodiment, the first mobile communication unit 101 may simply evaluate if a broadcast channel signal is received with acceptable parameters, and if so may send the emergency message using trunked mode, and otherwise send the emergency message using direct mode.

hi some embodiments, more complex means of determining a feasibility of trunked mode operation may alternatively or additionally be used. In particular, the feasibility may be determined in response to an acknowledge indication signal which may be an acknowledge indication signal for acknowledging a previous trunked mode transmission. For example, if a message is transmitted from the first mobile communication unit 101 using trunked mode operation and is successfully acknowledged by the first mobile communication unit 101 or the final destination, this may be taken as an indication that trunked mode operation is currently feasible.

The trunked mode transmission may in particular be a previous transmission of an emergency message. For example, upon the activation of the alarm button, the first mobile communication unit 101 may transmit a first emergency message to the emergency operations centre 111 using trunked mode. If the emergency operations centre 111 receives the emergency message, it will transmit an acknowledge indication to the first mobile communication unit 101. A second emergency message may accordingly be transmitted using direct mode operation if no acknowledge indication is received.

It will be appreciated that the determination of the feasibility may be at any suitable time and frequency. For example, the feasibility processor 123 may continuously monitor the current conditions and may continuously update the feasibility indication to reflect the current likelihood of successful trunked mode operation. Similarly, the emergency controller 121 may select between trunked mode and direct mode at any suitable time and with any suitable frequency. For example, the emergency controller 121 may continuously select between a direct mode operation state and a trunked mode operation state in response to the feasibility. In this case, when the first communication unit 101 is in the direct mode operation state, any emergency messages to be transmitted will be transmitted in direct mode, and when the first communication unit 101 is in the trunked mode operation state, any emergency messages to be transmitted will be transmitted in trunked mode without further consideration. This may facilitate emergency processing in the first communication unit 101 as no specific evaluation of parameters is required when the alarm button is activated.

However, in other embodiments, the feasibility processor 123 determines the feasibility when the user interface receives the emergency input in the form of the alarm button being pressed. Similarly, the emergency controller 121 determines the transmission mode and generates and transmits the emergency message when the alarm button is activated. This may be more efficient in many embodiments as the first mobile communication unit 101 only needs to determine and evaluate the trunked mode feasibility in the relatively rare event that the alarm button is activated.

FIG. 2 illustrates such a method of operation for a communication unit in accordance with an embodiment of the invention. The method is appropriate for the first communication unit 101 of FIG. 1 and will be described with reference to this.

In step 201, the user of the first mobile communication unit 101 activates the alarm button causing the user interface 119 to generate an emergency alarm indication to the emergency controller 121.

Step 201 is followed by step 203 wherein the emergency controller 121 causes the feasibility processor 123 to determine a feasibility indication indicative of whether the first mobile communication unit 101 is in a situation wherein trunked mode operation can be supported. The feasibility processor 123 specifically measures the broadcast channel signal received from the first base station 103 and if the signal level is above a certain threshold it is determined that trunked mode is feasible and otherwise it is determined that trunked mode is not feasible.

Step 203 is followed by step 205 wherein the emergency controller 121 proceeds to select the mode to be used for transmitting the emergency message. If trunked mode is feasible, the emergency controller 121 selects this transmission mode and otherwise it selects the direct mode. In step 205, the emergency controller 121 furthermore generates a suitable emergency message to be transmitted, hi some embodiments, the same emergency message is generated regardless of the select transmission mode whereas in other embodiments different emergency messages may be generated in response to the selected transmission mode.

Step 205 is followed by step 207 wherein the method branches dependent on the selected transmission mode. If trunked mode operation is selected, the method continues in step 209 and if direct mode is selected the method continues in step 211.

hi step 209, the emergency message is fed to the trunked mode controller 125 which controls the transmitter 129 to transmit the emergency message in accordance with the TETRA Technical Specifications for trunked mode operation.

In step 211 the emergency message is fed to the direct mode controller 127 which controls the transmitter 129 to transmit the emergency message in accordance with the TETRA Technical Specifications for direct mode operation.

The method may iterate the steps and may in particular return to step 201 awaiting a new activation of the alarm button.

hi some embodiments, the first communication unit 101 may operate in a direct mode of operation wherein all transmissions are direct mode transmissions or in a trunked mode of operation wherein all transmissions are trunked mode transmissions, hi such an embodiment, the emergency controller 121 may not only select a transmission mode for the emergency message but may also effectively switch between operating modes. Specifically, if the emergency controller 121 determines that trunked mode operation is not feasible, it may switch the first mobile communication unit 101 to the direct mode operation wherein subsequent messages are also transmitted in direct mode. The first communication unit 101 may remain in this operational mode until the emergency controller 121 or another functional module determines that trunked mode operation is feasible and preferable.

In some embodiments, communication units may be capable of receiving transmissions either in direct mode operation or in trunked mode operation. However, in some embodiments, at least some of the communication units are capable of operating in a combined idle mode wherein both direct mode messages and trunked mode messages can be received. This may allow the communication units to support trunked mode operation in idle mode while at the same time listen for direct mode messages. Hence, the possibility of a direct mode emergency message being received by another communication unit may be increased since also trunked mode communication units can monitor for direct mode messages. In a TETRA communication system, the communication units may support an idle mode dual watch operation wherein direct mode emergency messages can be received and further processed by the communication system. In particular, the idle mode dual watch may provide synergy when the communication units are operable to receive direct mode emergency messages and transmit these in trunked mode since trunked mode processes are still maintained while monitoring for direct mode messages is enabled.

The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. However, preferably, the invention is partly implemented 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 may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention maybe 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. 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 unit for a cellular communication system; the communication unit comprising: direct mode communication means for communicating in a direct mode to other communication units of the cellular communication system; trunked mode communication means for communicating in a trunked mode to a serving base station; means for determining a feasibility of communicating in the trunked mode; and an emergency call controller coupled to the direct mode communication means and the trunked mode communication means; wherein the emergency call controller is operable to select between communicating an emergency message in the direct mode and in the trunked mode in response to the feasibility.

2. A communication unit as claimed in claim 1 wherein the emergency call controller is operable to select the trunked mode when the trunked mode is feasible and to select the direct mode when the trunked mode is not feasible.

3. A communication unit as claimed in claim 1 wherein the means for determining is operable to determine the feasibility in response to a signal received from a base station.

4. A communication unit as claimed in claim 3 wherein the signal is a broadcast control signal.

5. A communication unit as claimed in claim 3 wherein the signal is an acknowledge indication signal.

6. A communication unit as claimed in claim 1 furthermore comprising user interface means for receiving an emergency input.

7. A communication unit as claimed in claim 6 wherein the means for determining is operable to determine the feasibility in response to the user interface receiving the emergency input; and the emergency call controller is operable to transmit the emergency message in response to the user interface receiving the emergency input.

8. A communication unit as claimed in claim 6 wherein the emergency call controller is operable to switch the communication unit from a direct mode operation to a trunked mode operation dependent on the feasibility and in response to receiving the emergency input.

9. A cellular communication system comprising at least one communication unit as claimed in any of the previous claims and further comprising a plurality of communication units; each of the plurality of communication units comprising: means for receiving the emergency message when transmitted in direct mode; and means for transmitting a trunked mode emergency message to a base station in response to receiving the emergency message.

10. A cellular communication system as claimed in claim 9 wherein each of the plurality of communication units comprise means for operating in a combined idle mode wherein both direct mode messages and trunked mode messages can be received.