Controlling talk group activity in a communications system

(71) Applicant(s)

Motorola Israel Limited (Incorporated in Israel)

Kremenetski Street, Tel Aviv 67899, Israel (72) Inventor(s)

Amiram Gozes (74) Agent and/or Address for Service

Hugh Christopher Dunlop

Motorola, European Intellectual Property Operation, Jays Close, Viables Industrial Estate, BASINGSTOKE, Hampshire, RG22 4PD, United Kingdom (51) (52) (56)

INT CL⁵

H04Q 7/04, H04B 7/00

UK CL (Edition M) H4KKY4T

Documents Cited

WO 91/15932 A (58) Field of Search

UK CL (Edition K) H4K KYR KYX INT CL⁵ H04B,H04Q ONLINE DATABASES: WPI (54) Controlling talk group activity in a communications system (57) A communications system 10 covering a plurality of cells 11,12, comprises a controller 13, for administering control of the system, and a plurality of communications devices A-J each having a transceiver for transmitting request messages to and receiving control messages from the controller 13. The communications devices A-J comprise registered communication devices that are registered with the controller 13 as being associated with a talk group and scanning communication device receptive to at least one talk group. The controller has a data base containing information relating registered communication devices to a current cell in which they are located, a receiver responsive to the request messages and a transmitter for transmitting control messages. The controller 13 allocates voice channels to cells participating in a talk group call. When a talk group request message is received by the controller, a first control message OSW is transmitted to cells containing registered communications devices associated with the talk group. The first control message assigns a voice channel for the talk group in the cell. A second control message TGCA OSW is transmitted to all other cells indicating that the talk group call has been established for the talk group.

FIG.1 — PRIOR ART — 10

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At least one drawing originally filed was informal and the print reproduced here is taken from a later filed formal copy.

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SUBSCRIBER RADIO

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MONITOR CC OF REMOTE SITE I

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HAS'

USER REQUESTED TALKGROUP CALL FOR TALKGROUP a ?

YES

HAS ³⁴

ISW FOR

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HAS

OSW FOR TALKGROUP a

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OSW FOR TAKKGROUP a BEEN RECEIVED. 9

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NO

62^-100 HAS

TGCA OSW FOR TALKGROUP a BEEN

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60YES

TUNE TO VC AND START TRANSMISION .64

TRANSMISSION TERMINATED 9

Tyes

NO

IS RADIO ON DISPATCH MODE AFFILIATED TO

TALKGROUP a 9

RADIO ON SCAN MODE AND SCANNING

FOR TALKGROUP a?

MESSAGE TRUNKING OR TRANSMISSION TRUNKING 9

YES

<td> \ YES</td><td rowspan="2"> MESSAGE TRUNKING</td>

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<td colspan="2"> TUNE TO VOICE CHANNEL!-™ AND ACTIVATE SPEAKER f^/U</td>

<td colspan="2"> ΙΑΙΙ^^ΝΟ . TERMINATED -</td>

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ZONE CONTROLLER

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MONITOR DATA FROM [SITE CONTROLLER] SUBSCRIBER RADIO

SEND ISW FOR TALKGROUP «

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REJECT TALKGROUP L₄₄REQUEST Γ⁴⁴

IDENTIFY ALL SITESiX}

IN WHICH THERE IS

REGISTERED AT LEAST ONE TALKGROUP a

MEMBER .47

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SEND CHANNEL GRANT OSfl FOR TALKGROUPfl TO SUBSCRIBER MAKING REQUEST

SEND CHANNEL TO OTHERI

SUBSCRIBERS AFFILIATEDl TO TALKGROUP a '58

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SEND TGSA OSW TO ALL |X| AND OR WHICH VOICE CHANNEL AVAILABLE

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56I

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FIG.3

-1A Communications System.

Background to the Invention.

This invention relates, in general, to the monitoring of talk group activity in a communications system and is particularly, but not exclusively, applicable to the controlled monitoring of talk group activity in a trunked radio system, such as the Smartzone™ trunked radio system manufactured by Motorola Inc.

Summary of the Prior Art.

A Smartzone™ trunked radio system 10, as shown in Fig. 1, is a network of several remote trunking systems 11, 12 (termed remote sites) which are each connected to a central computer, usually referred to as a Zone Controller 13, which controls the operation of remote sites 11, 12. [Smartzone™ is a trade mark of Motorola Inc.]. Typically, a zone controller may be responsible for up to 50 of these remote sites, all of which are dispersed throughout a wide geographic area. Specifically, the zone controller 13 administers data/voice and control signal transmission 14 between remote sites. Moreover, voice messages, transmitted using a voice networking methodology, are relayed through an audio switch 15, controlled by the zone controller 13, from a first remote site to a second remote site, such that the voice message is heard as a dispatch call in the second remote site. The zone controller 13 is coupled, typically through a microwave link 18, to a remote site controller 16, 17 located in each remote site. It is the site controller 16, 17 that directly controls subscriber radio A-J operation in a particular remote site, and it is the site controller 16, 17 that interfaces with the zone controller 13. Control channel repeaters 18, 19 and voice channel repeaters 20-23 interface subscriber radios A-J to the site controller 16, 17 and the zone controller respectively. A subscriber radios A-J may operate in a trunking mode as long as it remains in range of a trunking system operative in the zone of a Smartzone™ system.

-2The most desirable attribute of a Smartzone™ system is the dynamic remote site assignment capability therein. The zone controller maintains a data base of all active subscriber radios A-J within zone 10 and, in addition, a current talk group identification (ID) x, y, z for each subscriber radio A-J. When a subscriber radio A-Q initiates a talk group call in a specific remote site 11-12, the zone controller 13 identifies all other sites 11, 12 in which there is at least one member of that talk group x, y, z located therein, and directs that a voice channel be assigned, by the site controller, in those other identified sites 11,12 thereby allowing each member of that talk group to receive that call. In order to distinguish between subscriber radios in different talk groups, a subscript will be used to indicate an association, e.g. A« represents subscriber radio A affiliated to talk group a. Furthermore, a subscriber radio may have the ability to ‘scan’ a talk group to which it is not strictly affiliated,

i.e. the scanning subscriber radio is able to monitor a call directed towards talk group β irrespective of whether the Tadio is registered with the zone controller 13 as belonging to talk group β . In order to distinguish a scanning radio from a normal talk group member, a scanning radio will be labelled with an additional superscript β

indicating the talk group that it may scan, e.g. H represents subscriber radio H affiliated to talk group a and able to scan talk group β. It will be appreciated that a subscriber radio may be able to scan more than one talk group, although a subscriber radio may only ever be affiliated to one talk group at a time. The talk groups that a subscriber radio may scan are retained in a scan list.

A control channel repeater contains a radio frequency (RF) transceiver which continuously transmits data, sent from the zone controller 15, to all the subscriber radios A-E, F-J operating in a remote site 11, 12. The transceiver also relays data sent to the remote site controller 16, 17 from subscriber radios A-E, F-J in its site 11, 12. Data transmitted from the site controller 16, 17 is encoded in a special format, and designated as an Outbound Signalling Word (OSW). In a similar manner, data received by the site controller 16, 17 is also encoded in a special format, and

-3designated the arbitrary label Inbound Signalling Word (ISW). Each remote site 11,12 further comprise voice channel repeaters 20-23, each having one or more RF transceivers, that relay audio signals from a subscriber radio C_y to subscriber radios D_y having the same talk group affiliation in that particular site 11. When a voice channel repeater re-transmits a received audio signal, the audio signal is usually combined in some form of handshaking protocol. Subscriber radios A-J typically comprise a transceiver and are capable of operating as a two-way radio in accordance with techniques known to one skilled in the art. Moreover, the subscriber radios A-J are capable of synthesising any control channel or voice/data channel frequencies so as to be able to send and receive control and data/voice messages respectively thereon.

A talk group scan operation initiated by a subscriber radio has, typically, the following methodology. All subscriber radios A-E, F-J in a remote site 11, 12 listen to a control channel, broadcast by a particular remote site controller 16, 17, and decode OSW messages transmitted thereon. It will be appreciated by one skilled in the art that the operation of a remote site controller 16, 17 is to provide a means for interfacing the subscriber radios in a remote site to the zone controller 13. More specifically, the primarily operation of a site controller is to allocate available voice channels in its site and to adjust transmission protocols, such as the conversion of data from the zone controller to OSW’s destined for the subscriber radios A-J. As such, a site controller does not contribute to the decision making processes of the Smartzone™ operating system, i.e. in determining which sites talk group a radios are located. Therefore, for the sake of simplicity, information flow through the site controller 16, 17 has been substantially neglected in this application since the channel allocation by a site controller, and operating protocol conversion thereby, would be apparent to one skilled in the art.

If subscriber radio A, for example, wishes to initiate a talk group dispatch call to talk group x, subscriber radio A sends an ISW to the zone controller, via site controller 16, identifying itself and requesting that a talk group call to talk group x be established. In response, zone controller 13 acknowledges the request by

-4transmitting a call grant. Site controller 16 will then assigning a voice channel for that particular talk group in that particular site, i.e. a dedicated voice channel for subscriber radios A_x and B_x. Zone controller 13 simultaneously directs all other subscriber and scanning radios in talk group x, i.e. subscriber radios F_x and G_x, located in other remote sites, to move to an available voice channel. Subscriber radio A_x begins transmitting a voice message whilst all radios responsive to its talk group x activate their speakers and receive the voice message as a dispatch call. Scanning radio E_z also moves to the assigned voice channel in remote site 11 in order to monitor the talk group x call. Subscriber radios C_y, D_y and H_y and z

scanning subscriber radio Jy continue to monitor the control channel of their respective remote site 11, 12. At some time later, subscriber radio C_y wishes to initiate a talk group call to talk group y. Subscriber radio C_y sends an ISW to the zone controller 13 which acknowledges the request by sending an OSW and assigning, through the site controller, a second voice channel to remote site 11. Subscriber radios D_y moves to this second voice channel frequency and receives the voice messages thereon. Subscriber radio H_y, along z

with scanning radio J^, also receive the talk group y call in accordance with the aforementioned methodology. Since the zone controller 13 is multi-tasking, operation of talk group x is unaffected by this additional talk group y call. When subscriber radio A_xceases transmission to the other members of its talk group, all subscriber radios listening or scanning talk group x return to monitor the control channel of their respective remote sites. At this point, a subscriber radio which had been participating in the talk group x call, and which was a scanning radio for talk group y, would decode OSW’s emanating from the zone controller 13, and move to the voice channel assigned to talk group y. The scanning radio would activate its speaker and listen to the call sent by subscriber radio C_y.

-5Unfortunately, a scenario exists wherein a subscriber radio located in remote site 13 and registered with talk group y (as indicated by the subscript) wishes to scan talk group z and subsequently listen into any calls directed to this talk group. There is, in fact, a high probability that there is no subscriber radio A-H registered to talk group z in its remote site 12. Therefore, if a

X subscriber radio E , operating in remote area 11 and assigned to talk il group z, initiates a call, the zone controller 13 will not allocate, through control of site controller 17, a voice channel to remote site 12 because no subscriber radio assigned to talk group z is located within that remote site. Without the provision of a voice channel for z talk group z in remote site 12, scanning subscriber radio is X unaware of the call initiated by subscriber radio E_z.

It can be appreciated that there is a requirement in the art for a trunked radio system with an operating methodology which allows a subscriber to monitor call activities of talk groups to which neither the subscriber nor any other subscriber located in the subscriber's remote site are assigned.

Summary of the Invention.

This invention addresses at least some deficiencies which prevail in the prior art described above. In accordance with a first aspect of the present invention there is provided a communications system covering a plurality of cells, comprising a plurality of communications devices, dispersed throughout said plurality of cells, each having a transceiver for transmitting request messages to and receiving control messages from a controller. The plurality of communications devices comprise registered communication devices that are registered with the controller as being associated with a talk group and scanning communication device receptive to at least one talk group. The communications system further comprises a controller for administering control thereof. The controller has a

-6data base containing information relating registered communication devices to a current cell in which they are located, a receiver responsive to said request messages and a transmitter for transmitting control messages. The controller further comprises processing means having first means for providing a first control message for instructing both said registered communications devices associated with said specific talk group and said scanning communications devices scanning said specific talk group to receive a first voice channel and thereby participate in a talk group call.

The processing means further comprises second means that provides a second control message for indicating that the talk group call has been established for the specific talk group and third means for directing that the first control message be transmitted to all cells containing registered communications devices associated with said specific talk group. Fourth means, contained in the processing means, directs that the second control message be transmitted to all other cells. The first and second control messages are transmitted when said receiver receives a first request message, transmitted from a registered communications devices, requesting that a talk group call for a specific talk group be established.

In the first aspect of the present invention, the second control message is only transmitted to those other cells in which a voice channel is available. The scanning communications devices each further comprise means for providing an affiliation request message for directing the controller to register a scanning communications device receptive to said specific talk group as a registered device for said specific talk group and means for directing said affiliation request message be transmitted to the controller. The processing means further comprises fifth means for providing a third control message for instructing said scanning communications devices receptive to said specific talk group to receive a voice channel and thereby participate in said talk group call and sixth means for directing that said third control message be transmitted. The means for directing said affiliation request message directs said affiliation request message in response to the reception of said second control message and said sixth means directs said third control message to

-Ί all cells from which an affiliation request message was received. Furthermore, the means for directing said affiliation request message directs said affiliation request message after a random time delay. Additionally, the processing means further comprises: seventh means for providing an up-date control message for indicating that said talk group call has been established for said specific talk group and eighth means for directing that said up-date control message be transmitted to cells not currently involved in said talkgroup call, wherein said up-date control message is transmitted periodically. In addition, the up-date message is only transmitted to those cells not currently involved in said talkgroup call and in which a voice channel is available.

In a second aspect of the present invention, there is provided a controller for a trunked radio system. The trunked radio system comprises radio coverage areas served by base stations, registered radio units and scanning radio units, arranged in talkgroups and adapted for communicating with the controller through the base stations. The controller comprises means for correlating registered radio units with talk groups to which those units belong, means for correlating registered radio units with areas where those units are located at a given time, means for inputting an instruction to set up a talk group communication between radio units that belong to the same talk group, means, responsive to said means for inputting, for sending a first command to those base stations serving areas in which registered radio units that are registered as belonging to the talk group are located, and means for sending a second command to base stations serving all other areas, wherein the second command requests that a scanning radio unit, receptive to said talk group, affiliate itself with the controller. The first command instructs the base stations to assign a first voice channel to said talk group. Moreover, the second message is only transmitted to those other areas in which a voice channel is available.

In accordance with a third aspect of the invention, there is provided a method of assigning voice channels in a communications system comprising a plurality of coverage areas, a controller, registered radio units and scanning radio units. The method

-8comprises the steps of correlating registered radio units with talk groups to which those units belong, correlating registered radio units with coverage areas where those units are located at a given time, receiving a request, at the controller (13), from a registered communications unit for a talk group call to be established for a particular talk group, identifying those coverage areas in which registered radio units, belonging to that particular talk group, are located, issuing a first voice channel grant to all coverage areas identified, thereby setting up a talk group communication between radio units that belong to the particular talk group and issuing, from the controller to all other coverage areas not identified, a control message indicating that a talk group call for the particular talk group has been established. The method further comprises the steps of sending a response to the controller from a scanning radio unit, receptive to the particular talk group and located in a coverage area to which said control message was issued, indicating that the scanning radio unit is to be registered, with the controller, as a registered radio unit for said particular talk group in the coverage area in which it is located and receiving the response at the controller and issuing a second voice channel grant to all coverage areas from which a response is received, thereby extending the talk group communication to coverage areas in which the scanning radio units receptive to that particular talk group are located.

Furthermore, the method comprises the steps of periodically sending, from the controller, an up-date control message to all coverage areas not currently participating in the talk group communication, whereby said up-date message indicates that a talk group call for the particular talk group has been established, and sending a response to the controller from a scanning radio, receptive to that particular talk group, indicating that the scanning radio unit is to be registered, with the controller, as a registered radio unit for said particular talk group in the coverage area in which it is located. Additionally, the method comprises the step of receiving the response at the controller and issuing a third voice channel grant to all coverage areas from which a response is received, thereby extending the talk group communication to coverage areas in which

-9said scanning radio units receptive to that particular talk group are located. In this further aspect, the sending of a response from the scanning radio is delayed for a random time. Moreover, the step of determining may be preceded by the step of determining whether the request to establish the particular talk group is valid and, if invalid, refusing the request.

An exemplary embodiment of the invention will now be described with reference to the accompanying drawings.

Brief Description of the Drawings.

Fig. 1 illustrates a prior art trunked radio system suitable for implementation with a preferred embodiment of a methodology in accordance with present invention.

Fig. 2-3 illustrate a flow diagram of a scanning methodology implemented in a trunked radio system constructed in accordance with the present invention.

Detailed Description of a Preferred Embodiment.

With reference to the flow diagram of Figs. 2 and 3, there is shown, in accordance with a preferred embodiment of the present invention, a scanning methodology for a Smartzone™ trunked radio system. As was previously highlighted, exact interfacing operation of a remote site controller 16, 17 has been neglected since it is the interaction between subscriber radios, in a multiple remote sites 11-12, and the zone controller 13 that is significant.

All subscriber radios A-E, F-J in a remote site 11, 12 monitor 30 a designated control channel therein and determine whether or not a user has requested 32 a talk group call for talk group a, i.e whether a user has pressed a PTT on the subscriber radio A-E, F-J.

If a talk group call has been requested for talk group a, where a defines any one of talk groups x, y or z, the subscriber radio (the requesting radio) A-J sends 36 an ISW to the zone controller 13, if it has not already done so 34, requesting that a talk group call for talk

-10group a be set up. The zone controller 13 has, in the mean time, been monitoring 38,40 ISW ‘request’ information from the subscriber radios, as relayed from site controllers 16 and 17 in an appropriate data format. If no ISW’s have been received, the zone controller continues to monitor for ISW requests and remains in this monitoring loop. If an ISW request has been received 41, the zone controller determines 42, by reference to its data base, whether the requesting subscriber radio is affiliated to talk group a and hence whether or not the request should be granted. If the request is refused 44, the zone controller returns to monitoring 38,40 ISW ‘request’ information and the subscriber radio returns to START on the flow chart. In the event that the subscriber radio A-J is allowed to communicate 45 with talk group a, the zone controller 13 identifies 47 all remote sites {X} in which there is at least one member of talk group a registered. At 48, the zone controller 13 determines whether voice channels are available at all sites {X}. In the negative 50, the zone controller informs the requesting subscriber radio that a sufficient number of voice channels are unavailable 52. The zone controller 13 loops through a wait period 52 and then returns to 48. Once voice channels have become available 54 at all sites {X}, the zone controller sends 56, 58 a channel grant OSW for talk group a to the subscriber radio making the ISW request and to all other subscribers affiliated to talk group a. In the remote site containing the subscriber radio that requested the talk group a call, it will be appreciated that this channel grant OSW 56, 58 may be a single command. However, the requesting subscriber radio responds to the issuance 60 of the OSW in a different manner from that of any other subscriber radio affiliated to the same talk group a and located in either the same or a different remote site 62.

The requesting radio tunes its transceiver to a voice channel (allocated by the remote site controller responsible for the control thereof) and begins transmission 64. Transmission continues for as long as a PTT, for example, is activated. If the transmission has ceased 68, the requesting radio can either return to START, and thereby re-commence the procedure, or move to a receive mode 70;

-11the decision based on the type of trunking methodology employed.

If the trunking protocol is transmission trunking the former is selected, if the trunking protocol is message trunking the latter is selected. It will be appreciated by one skilled in the art that message trunking refers, in this sense, to whether a subscriber radio wishes to continue to receive the talkgroup call. Therefore, the subscriber radio may enter the received mode, whereby the recievre is tuned to receive the voice channel and the speaker is activated. Furthermore, a subscriber radio may subsequently revert back to the transmission mode (not illustrated) and, consequentially, de-activte its speaker. When the talk group call finally terminates 72, all subscriber radios in talk group a return to START. It will further be appreciated that the term “tune” should, in this context, be construed as meaning that the transceiver moves to the voice channel frequency in order to transmit/receive the talk group call or, alternatively, that the subscriber radio is already transmitting or receiving at the voice channel frequency and, therefore, the subscriber radio should remain locked to this frequency.

All subscriber radios A-J, located in remote sites (X), receive 62 channel grant OSW’s from the zone controller 13 and interrogate their present operating state. First, each subscriber radio determines 72 if it is affiliated to talk group a and currently able to respond to the talk group a call, i.e. whether it is in a dispatch mode c.f. a privacy mode, for example. In the affirmative, the subscriber adjusts its transceiver to receive the voice channel allocated by the site controller 16, 17. In the negative 74, the subscriber radio determines 76 whether it is in a scan mode and scanning for talk group a. In the affirmative 78, the scanning subscriber radio adjusts its transceiver to receive the voice channel allocated by the site controller 16, 17. Therefore, it will be appreciated that as soon as the channel grant OSW has been sent to all {X} sites containing talk group a radios, all affiliated talk group a radio and scanning radios scanning talk group a respond/participate in the talk group a call. If the response to 76 is negative, the subscriber radio returns to START.

-12Returning to the identification 47 of sites {X} containing at least one talk group a member registered in the data base of the zone controller 13. Clearly, at this instant, the zone controller, by implication, has identified all sites { X} currently not containing subscriber radios affiliated to talk group a. This group {X} does not exclude subscriber radios wishing to scan talk group a. The zone controller sends 80 a Talk group Call Activity Outbound Signalling Word (TGCA OSW) to all of those sites having unused voice channels. Any subscriber radio which scans 82 talk group a waits a random delay time and then responds to the TGCA OSW by sending 84 a Talk group Call Activity Inbound Signalling Word (TGCA ISW) for talk group a. In the case where either no TGCA ISW is received for the {X} sites, i.e. no subscriber radio wishes to scan talk group a, the subscriber radios return to START and monitor the control channel. The zone controller 13 acknowledges the received TGCA ISW by directing 86 that an available voice channel be assigned thereto. Alternatively, the present invention provides for an up-date to be sent to remote sites not initially addressed by the TGCA OSW. After step 80, the zone controller times-out and sends an up-date at 94-98 in order to overcome this difficiency. Steps 94-98 will be descirbed in detail below. Furthermore, the zone controller 13 registers the scanning subscriber radio to talk group a. When a scanning subscriber radios receives 88 a channel grant OSW from the [site controller 16, 17] zone controller 13, the scanning subscriber radio adjusts (tunes) 90 its transceiver to the assigned voice channel and participates in the talk group a call by activating its speaker. When the talk group a call terminates 92, the scanning subscriber radio returns to START and monitors the control channel of its present site.

During the duration of a talk group a call, the zone controller 13 may continue to search for an available channel at { X] sites. If after a predetermined time 94 the talk group a call is still active, the zone controller may send 98 a periodic TGCA OSW up-date thereby possibly allowing further scanning radios to participate in the talk group a call. However, if the talk group a call has

-13terminated before an available voice channel is identified, a scanning radio located in a remote sites not registered as containing a subscriber radio assigned to talk group a is oblivious to the talk group a call and the scanning subscriber radio monitors 40 the designated control channel for its present remote site. Finally, in summary, a subscriber radio operates in a main loop which comprises the steps of monitoring the control channel 30, determining whether a user has requested a talk group a call 32, determining whether an OSW has been received 60, 62 for its affiliated talk group or for a talk group that it is contained in its scan list and determining 100 whether a TGCA OSW has been received for a talk group that is contained in its scan list.

In general, whenever a talk group call is initiated, the zone controller 13, via a remote site controller 16-17, sends a special and solitary TGCA OSW to any remote site in which channel assignment is not required and a voice channel is available, thereby informing scanning radios of the presence of a talk group call. Moreover, the TGCA OSW is transmitted a number of times according to the amount of OSW traffic on a control channel in any particular remote site. In the preferred embodiment, the TGCA OSW is transmitted between 1 and 4 times. Furthermore, a typical structure for the TGCA OSW format is a 16-bit identification (ID) field followed by a 1-bit I-bit identification and a 10-bit status field. More specifically, the ID field comprises a 12-bit talk group identification code and a 4-bit call type which classifies the mode of call, e.g. an emergency call or a multiple talk group call. The I-bit field defines whether or not the call is to be conducted on a private basis whereby only 1 or 2 user are allowed access thereto. Finally, the status field defines the status of a call e.g. an allocated voice channel frequency, whether or not the call has been rejected, etc. In the preferred embodiment, an OSW is encoded into a frame, termed a WFT frame, having a transmission time of approximately 23.33ms. The WFT frame corresponds to an 84-bit word transmitted at 3600 baud and is comprised from 57-bits of encryption and the 27-bit OSW.

When a scanning subscriber radio wishes to participate in a talk group call to which no voice channel has been automatically

-14as signed, the scanning subscriber radio waits a random amount of time, defined in terms of WFT frames, before sending the TGCA ISW. The time is in the range of 1 WFT frame to 43 WFT frames (1 second). If the zone controller 15 fails to respond to the TGCA ISW within 60 WFT frames (~1.4 seconds) the scanning subscriber radio will re-transmit the TGCA ISW after the aforementioned delay. During long lasting talk group calls, the zone controller 15 may periodically re-transmit the TGCA OSW. The rate at which this re-transmission occurs is subject to current OSW traffic in a particular site and may vary, typically, between once every 15 seconds and once every minute.

It will be appreciated that a scanning subscriber radio may be required to scan more than one talk group, say α, β and γ, wherein none of the talk groups has priority. Therefore, if the subscriber radio responds to a TGCA for talk group β, there is no requirement to re-configure itself to scan for either of the other two talk groups. Alternatively, a scanning subscriber radio could have a prioritised scan order whereby on termination of, say, a talk group β call, the subscriber radio would re-affiliate itself to the highest priority talk group, say, talk group γ.

The affect of the proposed invention on Smartzone™ trunked radio system operation and performance is considered below. First, additional control channel loading resulting from TGCA OSW transmission is addressed. Currently, the maximum number of remote sites in a Smartzone™ trunked radio system is 50. On average, there are 10 voice channels allocated to each remote site and, therefore, 500 voice channels within the entire Smartzone™ system. Assuming that an average of 4 remote sites participate in any one talk group call, then, by ignoring private calls, repeater malfunctions, etc., the average number of talk group calls cannot exceed 125. If there are 10 active talk group calls in an average site, the average number of calls that should be informed using the TGCA ISW is 125-10=115. By assuming an average talk group call time of 15 seconds (which includes responses and repeater hold time), the average rate at which a TGCA OSW needs to be transmitted is 115^-15=7.66 OSW’s per second (assuming that TGCA

-15OSW transmission is only required once). Since the total OSW transmission rate is 42.85 OSW's per second, the increase in OSW transmission is 7.66-5-42.85=0.178 (orl7.8%). By assuming that 50% of all calls last for more than 15 seconds, 10% last for more than 30 seconds and a negligible number last for more than 45 seconds, the re-transmission of TGCA OSW every 15 seconds accounts for an increase of (7.66 x 0.5) + 7.66 x 0.1) = 4.6 OSW's per second. In the case of maximum voice activity in a Smartzone™ trunked radio system, control channel loading increases by 17.8%. If we take into consideration the transmission of additional TGCA OSW’s, the control channel loading increases to 28.5%. It will be appreciated that if there are fewer remote sites in the system or fewer talk group calls initiated therein, the OSW loading is reduced and the initial TGCA OSW's could be re-transmitted and, additionally, a TGCA OSW update sent at as higher rate as the system would allow.

The affect of the proposed invention on TGCA ISW loading in a Smartzone™ trunked radio system is set out below. When two or more subscriber radios of an FM trunked radio system attempt to send an ISW at the same time, interference may result. Moreover, if the strengths of the received signals are substantially identical, a receiver is not able to decode the ISW information. This phenomenon is known as ISW collision. If N subscriber radios respond to a TGCA OSW by sending an TGCA ISW after a random WFT period in the range 1 to M, the probability of exactly one radio sending its TGCA ISW after the first WFT is:

&lt;(M-1P&gt; M (N-l) x N = 77 I^x /(Μ-Ι)^ = Pl

The probability that either more than one subscriber radio or no subscriber radio has sent an ISW after I periods of the WFT is 1-Pi; independent of I. The probability that there was no single ISW transmission up to K periods of the WFT is (l-Pi)^K, which indicates that there have been K successive failures to send an ISW. If it is assumed that there are 130 subscriber radios operating in a scan

-16mode of a particular site and that each talk group is scanned by one of the subscriber radios, then 43 radios should respond simultaneously to the same TGCA OSW. In the preferred embodiment of the present invention M=43, the probability of sending a single TGCA ISW after 1 WFT is

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iy\(43-l) = 0.372 = Pi

The probability of not succeeding in sending a TGCA ISW after, say, 22 periods of WFT is (1-0.372)22 = 3.56E-5

Assuming that the probability of at least one subscriber radio sending an ISW at any other time, where the ISW has been sent in response to a task other than a TGCA OSW, is 50% and, as a consequence, that P2= 0.5. The probability that exactly one subscriber radio sends a TGCA ISW after 1 WFT period and with no other interference generated from ordinary ISW response is

Pl(l-P2) = 0.372 x 0.5 = 0.186.

The probability of not succeeding in sending a single TGCA ISW after K WFT periods arising from ISW collisions is:

(1 - Ρι(1-Ρ₂))^κ·

The probability of not succeeding in sending a single TGCA ISW after, say, 43 WFT periods is (1 - 0.186)43 = 1.43E-4

For P2 = 0.9, the probability of not succeeding in sending a single TGCA ISW after, say, 43 WFT periods is

-17(1 - (0.372 x 0.1))43 = 0.195 = 19.5%

It can be appreciated from the above calculations that the probability of sending a decodable TGCA ISW for 43 radios which are simultaneously scanning the same talk group in a sequence of 43 WFT periods is 99.99% for a 50% loading of other ISW words or 80.5% for a 90% loading of other ISW words. Therefore it can be appreciated that the above algorithm guarantees a high probability of success.

For a Smartzone™ trunked radio system having 10 voice channels allotted to every site and an average call duration of 15 seconds, it can be concluded that, at very most, an ISW request is made, and hence a call is started, every 1.5 seconds. Furthermore, the majority of these requests will be normal ISW requests and not TGCA ISW requests. With such a site load, all repeaters will be busy and no TGCA ISW’s will be transmitted. Moreover, if one assumes that no more than one third of all ISW request are TGCA ISW requests then, in the preferred embodiment, a TGCA ISW request occurs at a rate of approximately one every 5 seconds. In conclusion, the random delay before TGCA ISW request in combination with the relatively large number of radios requesting the same service results in a high probability of success at a first attempt to acquire a voice channel in response to a TGCA OSW. However, the ISW control channel overhead is an increase in loading of -20%.

With regard to response time and call activity loss calculations, since the TGCA ISW requests are uniformly distributed over the range of 1-43 WFT periods, an average request will be, in the limit, received by a site controller from a solitary subscriber radio after 22WFT (-0.5 seconds). Furthermore, with the addition of (a) a delay between the start of the call and the TGCA OSW; (b) the response time of the zone controller and the remote controller; and (c) the time needed for the subscriber radio to move to the allocated voice channel and activate its speaker; the scanning subscriber radio may miss between 1 and 2 seconds of a talk group call. If a call lasts for, typically, 15 seconds, this equates to a lose of between 15%-20%.

-18Moreover, if the TGCA ISW has to be re-transmitted, an additional 60 WFT frames (~1.4 seconds) are lost.

It can be appreciated that an invention so designed and described produces the novel advantages of a trunked radio system having a control methodology capable of determining the existence of scanning subscriber radios in the trunked radio system and assigning a voice channel therefor. Moreover, the methodology provides for the assignment of the voice channel only in the event that the scanning subscriber radio is not automatically assigned a

0 suitable voice channel which allows the scanning subscriber radio to monitor a talk group call set up in a particular talk group. It will, of course, be understood that the above description has been given by way of example only and that modification in detail, such as the use of redundancy loops to ensure that either an ISW or OSW has been

5 sent 34 or received, may be made in the scope of the invention.