Patent GB-1020953-A (GB)
IPC: H04W76/02
Abstract: 1,020,953. Mobile telephone systems. STANDARD TELEPHONES & CABLES Ltd. Dec. 4, 1962 [Dec. 4, 1961], No. 45802/62. Heading H4K. In a system for connecting up mobile stations to base stations over carrier frequency links, the transmitters and receivers at the stations being adjustable in frequency to permit two-way communication over any one of the links, each transmitter is provided with voice frequency signalling means which transmits V.F. signals one at a time in sequence over the line, and the establishment of a connection between one of the base stations and one, or a group of the mobile stations is achieved in response to a sequence of V.F. signals, the establishment of communication being distinguished by the transmission of a continuous tone over the link. The invention is described in relation to a system having base stations interconnected via a main exchange network, the base stations being in radio communication with the mobile stations. These latter may effect communication to any base station within range and are capable of searching for one of a number of radio channels marked free by a characteristic tone sent thereover from the base station. On such a channel being found the mobile station then transmits to the base station, over a carrier frequency differing from the received frequency by a fixed amount, a sequence of V.F. signals representing digital values corresponding to the wanted number &c. In the system described each digital value is represented by one of ten frequencies F0 . . . F9 and two further frequencies F11, F12 are used to mark a channel free or busy. As shown in Fig. 3, the top sequence relates to a call from the mobile station to the base. The latter is normally emitting frequency F11 and when push-button B is pushed in mobile station number 35 the latter emits frequency F3. When this is received at base the latter also emits F3 which enables the mobile station to emit F5 which is also repeated back. This causes the mobile station to emit F12 which is also repeated back. The mobile station has thus identified itself for metering purposes, and is now enabled to send the digits of the wanted subscriber, here assumed to be 41. These are then transmitted by pushing appropriate buttons and each time an acknowledgement signal F3 is returned which restores the push-button and causes F12 to be emitted to the base station and returned therefrom until the next push-button is depressed. The call is thus set up and during conversation F12, the " busy " frequency, is sent in both directions. In order to discriminate against false signalling conditions due to interference from other bases, each base may impose random interruptions on the signal sent out therefrom, which causes the signals returned from the mobile station to be interrupted in the same way, so that the base station receiving these signals may make a comparison to tell whether the signals concerned are intended for it. A relay circuit for producing the interruptions of the busy signal for identification purposes in which the periods of the break are determined by the particular phase of an A.C. signal at the moment of breaking of a relay contact feeding the A.C. signal on to a capacitor, is described with reference to Fig. 4 (not shown), a recognition circuit being incorporated which breaks the connection if the signals returned from the mobile station are not of the same periodicity as the transmitted signal. On completion of a call button N is pushed to cause the " free " frequency, to be emitted to and returned from the base station. The lower portion of the drawing illustrates a call to the mobile station 41. The frequency F4 is first sent out and this is acknowledged by F4 from all mobile stations having this first digit. Frequency F1 is then sent out and this resets all mobile stations equipment except the called one which reverts F1. Ringing is then effected by the base station sending F12, F1 in repeated succession. When the mobile station answers it emits F12 which stops the ringing and the base station then emits F12. Conversation and release then takes place as before. In the case of a " group " call to all stations having the first digit 4 then the call is signalled from the base station by alternate F4 and F12 signals of such periodicity as not to be confused with normal ringing signals.