Patent GB-1103105-A (GB)
Title: Improvements in or relating to radiotelephony communication systems
Inventor: BLACKBURN PETER ALAN
IPC: H04B7/005
Filed: 1963-09-20
Abstract: 1,103,105. Radio signalling. PLESSEY-U.K. Ltd. 4 Nov., 1964 [20 Sept., 1963], No. 37199/ 63. Heading H4L. In a radio communication system a number of independently movable subscriber stations, each having a transmitter and a receiver, communicate via a relay station, separate information modulation channels of a first common carrier frequency being transmitted by the subscriber stations, and separate information modulation channels of a second common carrier frequency being transmitted by the relay station. At each subscriber station a generator of standard output level is provided from which pilot tone is transmitted in an individual modulation channel associated with the information modulation channel. There is also provided at each subscriber station an automatically adjustable attenuator which is operable by control signals from the relay station, and which equally affects the transmitted levels of the information modulation channel and of the pilot tone modulation channel. At the relay station the strength of an incoming pilot tone signal is assessed, and any necessary control signals are transmitted to the subscriber station concerned on an individual control tone modulation channel associated with the appropriate information modulation channel of the second common carrier frequency. The control signals cause operation of the attenuator at the subscriber station until the strength of the pilot tone received at the relay station approaches a pre-determined value. As described, each subscriber station transmitter is fitted with a series attenuator adjustable in equal ratio steps of both of 9dB and 3dB each. A narrow channel (for example, 50 c/s. wide) is made available by filters in either the sub-audio region or in the guard band between an adjacent channel for each subscriber station and for each co-operating section of the relay station. In these channels are sent pilot signals from the subscriber stations which are of a set strength on the input sides of the attenuators, and control signals from the relay station. Before a subscriber station is switched on, its attenuator interposes maximum attenuation but thereafter, it begins decreasing in 9dB steps. At a predetermined " recognition level " a " change " signal is sent out by the relay station and the attenuation then decreases in 3dB steps. When the level of the pilot signal received at the relay station closely approaches a pre-determined standard, a " stop " signal is sent, and the attenuator ceases stepping. If the level falls, the relay station sends an " increase " signal which causes the attenuator to re-commence stepping so that the attenuation again decreases in 3dB steps, whereas if it rises a " decrease " signal is sent, to produce the reverse effect. The signals in the auxiliary channels from both stations are in the form of short (30 milli-second) pulses and are sent in successive pairs, the first being a reference pulse, to reduce the chances of false operation. As the relay station only sends three kinds of signals after the reference pulse, that is, a pulse in phase with it, or in antiphase with it, or no pulse, the same kind of signal is used for " change " and " increase " and for " stop " and " decrease ". The pilot signals from the subscriber station each comprise a reference pulse followed by a pulse of opposite phase, and the received amplitude is obtained from them at the relay station and compared with a reference value. It is stated that the maximum time lag between demanding and receiving a channel is 1À44 seconds and that the maximum rate of change of signal level achievable during operation is 30dB in 1À2 seconds. The coding and control apparatus is made the same for all stations and may be changed in function from " relay " to " subscriber " by operation of a switch. An emergency switch is provided at each subscriber station so that the attenuator may be placed at its minimum value of attenuation. An alarm may be provided at a relay station to indicate malfunction of a loop, by integration of the output of the comparator which operates on the pilot signal received from a subscriber station.