UNIT FOR PREVENTING AN INTERCEPTION OF A RADIO COMMUNICATION SIGNAL TRANSMITTED BETWEEN A FIXED FACILITY AND A MOBILE STATION

摘要

In a radio frequency communication system in which a radio frequency communication signal is transmitted between a radio terminal (14) and a mobile station (24), it may be that an unauthorized third person uses a scanner-receiver (60) to intercept the communication signal. In order to prevent such an interception, the unit comprises a detecting circuit (30) for detecting an internal signal which is transmitted between a telephone exchange (12) and the radio terminal (14) and is concerned with an ordinary voice signal. The detection circuit produces a detection signal when the internal signal is absent. Responsive| to the detection signal, a control circuit (32) controls the internal signal to prevent the interception, as by sending a dummy voice signal whenever the ordinary voice signal is absent. Alternatively, the control circuit may produce a control signal while the communication signal is received from the mobile station and consequently while the internal signal is sent from the radio terminal to the exchange. In this event, the control signal is used either in attenuating or switching off an echo signal which usually results from the internal signal reaching the exchange.