Patent GB-1214082-A (GB)
Title: Duplex mobile radiotelephone system
IPC: H04W68/00
Filed: 1968-04-11
Abstract: 1,214,082. Automatic exchange systems. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 11 April, 1968 [14 April, 1967], No. 17581/68. Heading H4K. In a method of establishing duplex radio connections between any one of a number of mobile stations and at least one fixed base station (which may be connected to a wired exchange system) the radio speech channels are arranged in groups, idle channels being marked by a signal identifying the group to which it belongs, all connections initiated from a fixed station are established by signalling on an additional (signalling only) radio channel, the number of the wanted mobile and the speech channel to be used, all connections initiated from a mobile station are established by signalling over a free speech channel the identity of the calling mobile and the wanted number and all signals are stored in registers at the transmitting end and transmitted in voice frequency from using the same frequencies for traffic in both directions. As described the signalling technique employed is frequency shift signalling and only two modulation frequencies are employed in the whole system. It is however suggested that other voice frequency signalling systems such as 2 out of 5 could be used. The mobile equipment is detailed. Each mobile automatically tunes or is manually switched to the signalling channel when not in use for a call. Call initiated from fixed station.-The fixed station initiates a call to a mobile by sending alternately the modulation frequencies on the calling channel in accordance with the number of the wanted mobile. This is followed by a further modulation designating the free speech channel to be used. The called mobile recognizes its own number and automatically switches to the specified channel on which it transmits on answer tone and receives a ringing signal for triggering in annunciator. If the call is not answered in a predetermined time the base station alternates the modulation frequencies in accordance with a clearing signal on the speech channel and the mobile station responds returning to its original state ready for a further call. If however the call is answered by lifting the receiver a start tone is returned to the fixed station causing the ringing tone to cease which in turn terminates the start tone and speech may commence. Call initiated from mobile station.-Before lifting the receiver the mobile subscriber sets up the wanted number in a number switch and selects a group of speech channels by setting another selector switch. When the receiver is lifted the mobile apparatus tests the channels in that group in turn until it finds one on which the idle signal is being transmitted. The mobile transmits a continuous tone on this channel and when the fixed station responds by replacing the idle signal by a continuous tone, transmits by alternating tones its own identity and the identity of the wanted subscriber. The fixed station re-transmits the number of the calling mobile which has the effect of releasing any other mobile which may have simultaneously selected that channel when it was free. The connection can then be completed presumably to a wired subscriber. Interconnection between mobiles is not described. Release of the connection.-After conversation either party may release the connection by replacing his handset. When the mobile subscriber replaces his handset a tone is returned over the speech channel to the base station which returns the clearing signal i.e. a coded sequence of the two tones recognizable by the mobile equipment.