Patent GB-1387438-A (GB)
Title: Mobile radio communication system
IPC: H04W84/08
Filed: 1972-03-28
Abstract: 1387438 Radio signalling TELEVERKETS CENTRALFORVALTNING 28 March 1972 [29 March 1971] 14393/72 Heading H4L In a system for radio communication between a base station and a plurality of mobile stations using a common carrier wave, a number of control units EA 1  to EA are connected, for example, by the normal telephone system, to the base station, Fig. 1. Each control unit communicates with its own group of mobile stations and on activating the apparatus EA 1  &c. a corresponding mark is registered in a store M 1 . If the common channel is unoccupied a device A coupled to the store M 1  energizes a tone transmitter TS which modulates the transmitter FS via a duplexer G with a tone code representative of the corresponding group of mobile units. The device A also energizes a switch V so that the control unit EA 1 , for example, is connected to the transmitter FS via duplexer G. The tone code also sounds in the control unit to indicate that the radio channel is vacant and the connection is being established and tone generator TS modulates the transmitter FS with a special signal SI 1  which indicates to all mobile units that the channel is occupied. If the channel is occupied when the mark is registered at M 1 , a queue registering device K 1  energizes a speech time limiter B 1  whereby TS modulates the transmitter with a signal notifying the engaged units that the connection will be disconnected after a predetermined time, after which the next unit in the queue is connected. The code transmitted by FS contains a group tone code specific to the mobile units within a group, a clearing signal which is the same for the units in all groups and a code specific to each mobile unit within a group. At each mobile unit, Fig. 2, the output of receiver MM is supplied to a tone receiver TMM which responds to the first tone of the received code and is then switched via register R to receive the next tone until the complete group code has been received. The register R unmutes the amplifier LF and device TMM is tuned to respond to the clearing signal. Receipt of the signal SI 1  will energize a holding circuit H to operate an indicator I 1  and the clearing signal followed by the calling signal for the specific mobile unit will disconnect all but that unit. In the called unit an indicator I 2  will be operated. When a mobile unit is to transmit, the switch S/M on the hand-set is operated to switch on the transmitter MS and a tone transmitter TSM will modulate the transmitter with a special signal SI 2 . At the central station this signal received at FM is detected at TM whereby speech signals from the mobile unit will reach the appropriate control unit via G and V. When transmission from the mobile unit stops, the switch SM at the mobile unit is operated to connect the receiver MM and the register R is held in its set position via holding circuit H. The end of signal SI 2  also causes the tone receiver TM at the central station to operate a queue registering device K 2  to cause tone generator TS to modulate the transmitter FS with a queue inviting signal. The mobile unit presses a call button ANR if the channel is unoccupied whereby transmitter MS is connected and the tone transmitter sends a first group code signal identifying the group to which it belongs. This signal is received at the central station and registered in a store M 2  connected to device A which causes transmitter TS to transmit the group code signal and a clearing signal. The group code signal puts all the mobile units in that group in the receive position and the clearing signal disconnects all except the calling unit. If the channel is occupied when the call is made, a queue button KÍ is operated and the queue inviting signal is received at MM whereby tone receiver TMM will energize tone generator TSM via register R and the queue button to modulate MS so that a first group code signal is transmitted. This signal received at TM, Fig. 1, causes a mark to be registered in store M2. Since the channel is occupied this via device K 1  causes the speech limiter B 1  to disconnect the present conversation after a predetermined time. The register R at the calling station maintains that station in readiness until a connection is effected. Two or more mobile units may communicate if a button BB is operated whereby TSM transmits a signal SI 3 . When this is received at TM, Fig. 1, a connection is effected so that the received voice signals are re-transmitted via FS. The calling unit informs by voice which mobile unit is required and at this unit the button BB is also operated and the connection established. A button SL in the mobile units will reset the indicator I 2  when the conversation is finished. A speech limiter B 2  at the base station will disconnect a connection a predetermined time after the conversation has ceased and a speech limiter B 3  will break the connection after a predetermined time if a call is not answered at the control unit. Timing at the base station is controlled by a clock generator C and a call charge device S is provided.