TRUNKED RADIO SYSTEMB-ackground of the InventionThis invention relates to a trunked radio system comprising a central station and at least two remote units, each remote unit having its own identification code and means for transmitting a channel request signal, the central unit having means for transmitting a channel assignment signal, and the remote units having means for initiating transmission and reception on the channel on receipt of the channel assignment signal.
Summary of the Prior ArtTrunked systems in general comprise a base station transmitter which is shared among different groups of users. When one user in a group requests access to a channel (by pressing the push-to-talk button), that channel is assigned to the particular user's group. The central unit transmits a channel assignment signal, which is recognised by the users in the group and all the respective remote units in that group switch into an operating mode.
In existing trunked systems, such as Motorola MRTU or MIR 5000 (type 1 trunking), any user is able to initiate a transmission once the channel has been assigned to the group. Of course, it will normally be the user that pressed his PTT that will be the one that makes the first transmission, however there is the possibility of collisions when two users simultaneously request access to the channel and a channel assignment signal is transmitted by the central and received by both units.
In order to minimise these collisions, a time consuming time slice has been used in the past, however collisions can still occur on a statistical basis.
Summary of the InventionIt is an object of the present invention to provide improved data transmission in a trunked radio system.
Accordingly the system of the invention is characterised in that the channel request signal includes the identification code of the remote unit requesting the channel; the channel assignment signal includes the identification code of the remote units to which the channel is assigned; and each remote unit has means for initiating transmission on receipt of a channel assignment signal which includes its identification code and for initiating reception only on receipt of a channel assignment signal which does not include its identification code.
Accordingly, individual voice channel transmit assignment signals are sent to individual remote units and each knows whether the channel assignment signal has been transmitted in response to a request from that unit.
The invention has the advantages of more efficient use of transmission time, minimising air time usage and increasing message throughput.
Brief Description of the DrawingsFigure 1 shows a central station of a trunked radio system and two remote units in accordance with the invention.
Figure 2 shows the exchange of signals between the central and one of the remotes of Figure 1 andFigure 3 provides a table showing some of the improvements in the invention over prior art systems.
Detailed Description of the Preferred EmbodimentReferring to Figure 1, there is shown a central station 10 of a trunked radio system and two remote units 11 and 12 of one particular group of users sharing the various radio channels supported by the central 10. There are other groups of users which are not shown and the channels supported by the central unit 10 are shared in time between the various groups of users. Each remote unit 11 and 12, and the central 10 has its own unique trunking ID. In contrast, in prior art systems, there would be only two IDs, one for the group of remote units and one for the central.
The operation of the system in accordance with the preferred embodiment of the invention is as follows. Referring to Figure 2, two situations are described. In each case, the unit on the left hand side of the figure originates the transmission and the unit on the right hand side responds.
In the first case, the central unit initiates a transmission by making a polLing request, which includes the ID of unit 11 which is being polled. If unit 11 wishes to transmit, it responds to this request by issuing a polling response, which includes the ID of remote unit 11. On receipt of the polling response, central unit 10 issues a control command which also includes the ID of remote unit 11. This control command is in effect a channel assignment signal.
In response to this signal, remote unit 11 is activated for reception and transmission. In the meantime, all other units in the group (e.g.
unit 12) receive this control command, recognise it as a channel assignment signal and enter an active mode, in which they are active for reception, but not for transmission. As shown by the dotted line in the figure, the unit 11 may issue a control response to acknowledge receipt of the control command.
In the case where the remote unit initiates transmission (e.g.
through pressing of the PTT), that unit transmits a contention message. This message is in effect a request for channel assignment and includes the ID of the remote unit 11. The central unit 10 receives this message and, if the centrals channel allocation algorithm so allows, it transmits an acknowledgement message to the remote unit 11. This acknowledgement message includes the ID of the remote unit 11. On receipt of this message, the remote unit 11 recognises its ID and enters an active mode, in which it is active for transmission and reception. Simultaneously, all other units in the same group as the remote 11 (e.g. unit 12) receive the acknowledgement message, recognise it as relating to a unit in that group, and enter an active mode for reception only. In order for one of these units to transmit, it must undergo the same procedure.
In accordance with the procedure described above, responsibility for collision avoidance is given to the transmission originator (the central unit 10 in the case of a polling request or control command or the remote unit 11 in the case of contention messages). To achieve this collision avoidance, the procedure described makes use of an identification code, which is unique toeach remote unit, and a channel assignment signal permittingtransmission. This results in an order of magnitude reduction inchannel access time, as shown in the table of Figure 3. In that table,the central column shows the polling access time, and the contentiontime for the prior art, and in the right hand column the corresponding times for the system of the invention.
It will be appreciated that the polling request, polling response, control command, contention message and acknowledgement message signals identify the unit making or requesting the channel and at the same time identify the group of units making or requesting the channel. The particular unit ID can be considered as a sub-address of the group address.
The above description has been given by way of example only, and modifications of detail can be made within the scope of the invention.