RF-communication system with a unit for preventing an interception of a radio communication signal transmitted between a fixed facility and a mobile station

Abstract: In a radion 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 (86) for detecting an internal signal which is transmitted between a telephone exchange (12) and the radio terminal (14) and which consists of a first and a second reception signal; these reception signals are concerned with an ordinary voice signal and a control signal, respectively. The detection circuit (86) produces a detection signal when the second reception signal is present. Responsive to the detection signal, a control circuit (94) attenuates the transmission signal (22) to prevent the interception of the second radio signal when the second reception signal is present. In this case, the control signal is used either in attenuating or switching off an echo signal which usually results from the internal signal reaching the exchange.

Description: This invention relates to a radio frequency (RF) communication system with a unit for use in a fixed facility and a mobile station, such as a mobile telephone system. A conventional mobile telephone system comprises a telephone exchange, a base station, and a plurality of mobile stations. The base station is coupled to the telephone exchange through a wire circuit and/or a radio circuit. The mobile stations are carried on automobiles and the like. A combination of the base station and the telephone exchange is herein called a fixed facility of the radio frequency communication system. The base station includes a radio terminal for transmitting and receiving a radio frequency communication signal between the radio terminal and each mobile station. The communication signal corresponds to an internal signal which is transmitted between the telephone exchange and the radio terminal and concerned with an ordinary voice signal of a frequency band between 0.3 kHz and 3 kHz. When the communication signal is transmitted between the radio terminal and a selected one of the mobile stations, the selected mobile station receives or transmits the communication signal from and to the radio terminal through a selected one of a plurality of radio channels. The conventional mobile telephone system is disadvantageous in that the communication signal is liable to reception by a third person, namely, to be intercepted by the third person. For example, the communication signal can be readily received by a radio scanner-receiver which is sold on the market. The radio scanner-receiver comprises an auto-tuning circuit capable of scanning the radio channels and of automatically detecting that one of the radio channels through which the communication signal is transmitted between the radio terminal and the selected mobile station. When only a small number of the radio channels are available, the radio scanner-receiver readily intercepts the communication signal. The interception of the communication signal can not be prevented even by transmitting an idle signal through idle ones of the radio channels. Specifically, such an idle signal sounds as a tone specific thereto and can be distinguished by the third person from the ordinary voice signal. Therefore, the third person is not confused by the idle signal. Interception of a communication signal is possible by monitoring an echo signal which occurs through the radio terminal and the telephone exchange when the communication signal is received by the radio terminal from a mobile station. An echo suppressor is included in the conventional mobile telephone system between the telephone exchange and the radio terminal and may serve to attenuate the echo signal. However, the echo suppressor is insufficient to prevent the interception. US-A-3 978 288 describes a coding method for the secret transmission of voice (speed) signals. Filling signals are inserted in breaks and interruptions of voice. Such insertion of filling signals should be distinguished from the voice. In order to distinguish the filling signals from the voice, a signal which controls the insertion of the filling signals is transmitted over a separate channel for control of a responsive device at a receiver facility to provide for suppression of the filling signals. GB-A-1 189 861 relates to a security communication system which scrambles or codes signals by the use of a TDM technique. Speech or other information is transmitted by modulating a pulse on one channel of each multiplexing cycle. Each channel is shifted in each of the multiplexing cycles. Noise information is arranged in each multiplexing cycle in addition to the speech information, i.e. the voice information is transmitted together with the noise information in each multiplexing cycle. EP-A-0 108 151 relates to a secret-speech device which inserts a secret-speech signal into a main signal as a dummy for a predetermined period of time. This secret-speech signal does not serve to prevent the main signal from being received by an unauthorized person. EP-A-0 015 041 discloses a system to avoid intentional interception (overhearing) or jamming of a third party in a mobile radio network. Specifically, pseudo noise and a significant signal are switched from one to another by a switch. It is therefore an object of the present invention to provide a unit which is useful in preventing an interception of a radio communication signal in a radio frequency communication system. This object is solved with the features of the claims. The only Fig. 1 is a detailed view of a moblie telephone system with a unit according to an embodiment of this application. According to Fig. 1 a mobile telephone system comprises a mobile station 24 and a radio terminal, a unit 70 and a telephone exchange 12 of the fixed facility. The mobile station 24 comprises a microphone 71 and a vogad amplifier 72. When a user of the mobile station 24 speaks, the microphone 71 produces an ordinary voice signal. A processing circuit 73 generates a control signal in the manner which will shortly be described. The ordinary voice signal is delivered to a superposing circuit 74 through the vogad amplifier 72. The superposing circuit 74 is for superposing the control signal on the ordinary voice signal to produce a superposed signal. The control signal is different in frequency from the ordinary voice signal. For example, the control signal may have a frequency of 3.4 kHz or 0.1 kHz. A transmitter-receiver 76 transmits the superposed signal as a radio frequency communication signal 22 to the radio terminal 14 and receives a like radio communication signal from the radio terminal 14 in the known manner. The processing circuit 73 comprises a mobile station detector 77 for detecting presence and absence of the ordinary voice signal through the vogad amplifier 72. Only when the ordinary voice signal is present, a control signal generator 78 generates the control signal. The communication signal 22 is transmitted from the mobile station 24 to the radio terminal as a combination of a first and a second radio signal. The first radio signal is concerned with the ordinary voice signal. The second radio signal represents the control signal and is superposed on the first radio signal. Responsive to the first and the second radio signals of the communication signal 22, the radio terminal 14 produces a first and a second reception signal. The first reception signal is concerned with the ordinary voice signal. The second reception signal is representative of the control signal. The radio terminal 14 sends the first and the second reception signals to a vogad amplifier 80 through a reception line 81. The unit 70 comprises a first filter circuit 83, a second filter circuit 84, and a fixed facility detector 86. Supplied with the first and the second reception signals through the vogad amplifier 80, the first filter circuit 83 cuts off the second reception signal and sends only the first reception signal to the telephone exchange 12. Similarly, the second filter circuit 84 cuts off the first reception signal and sends the second reception signal alone to the fixed facility detector 86. Responsive to the second reception signal, the fixed facility detector 86 produces the detection signal. The telephone exchange 12 comprises a hybrid transformer 91 for coupling the reception line 81 and a transmission line 92 to a communication line 93 which leads to a subscriber's substation (not shown). The transmission line 92 is for transmitting a transmission signal as the internal signal. As a rule, a part of the first reception signal appears in the transmission line 92 through the hybrid transformer 91 as an echo signal. Namely, the internal signal in the transmission line 92 consists of the transmission voice signal and the echo signal. The unit 70 further comprises an attenuator 94 which is placed in the transmission line 92 and which is controlled by the detection signal delivered from the fixed facility detector 86. The attenuator 94 is for attenuating only the echo signal and serves as a control unit. This is because the detection signal is given from the fixed facility detector 86 to the attenuator 94 only when the echo signal is returned back to the attenuator 94. As a result, only the transmission voice signal passes through the attenuator 94 and is supplied as the internal signal to the radio terminal 14. The radio terminal 14 transmits the communication signal 22 in response to the transmission voice signal. No echo signal is transmitted from the radio terminal 14. Therefore, even if the communication signal 22 is received by an unauthorized third person who uses a scanner-receiver, the communication will hardly be understood by the third person. According to the invention a switch element may be used instead of the attenuator 94 to open and close the transmission line 92 in response to the detection signal. The unit 70 may be included in the base station.

Claims: A unit for use in a fixed facility of a radio frequency communication system between a telephone exchange (12) of said fixed facility and a radio terminal (14) thereof in preventing an interception of a radio frequency communication signal (22) which is transmitted between said radio terminal (14) and a mobile station (24) of said radio frequency communication system and corresponds to an internal signal transmitted between said telephone exchange (12) and said radio terminal (14) and related to said radio frequency communication signal, characterized in that a) said mobile station (24) is adapted to send said radio frequency communication signal comprising a first and a second signal which are concerned with the ordinary voice signal and representative of a control signal, respectively, said radio terminal (14) being adapted to produce a first and a second reception signal which are concerned with said ordinary voice signal and representative of said control signal, respectively, and which appear simultaneously when said radio frequency communication signal is received by said radio terminal (14); b) said telephone exchange (12) is adapted to send, to said radio terminal (14), said internal signal consisting of said first reception signal and a transmission signal; said unit comprises : c) detecting means (86) coupled to said radio terminal (14) for detecting said second reception signal to produce a detection signal when said second reception signal is present; and d) control means (94) coupled to the telephone exchange (12) to attenuate said first reception signal from said internal signal in response to said detection signal to send said transmission signal to said radio terminal (14). A mobile station for use in radio frequency telephone communication system in conjunction with a unit according to claim 1 and to produce a first radio signal concerned with an ordinary voice signal, characterized by: processing means (73) for processing said ordinary voice signal into a control signal; and superimposing means (74) coupled to said processing means (73) for superimposing a second radio signal representative of said control signal on said first radio signal. A radio frequency communication system comprising a fixed facility and a mobile station (24) according to claim 2 and communicable with said fixed facility through a radio frequency communication signal (22) which is transmitted between said mobile station (24) and said fixed facility, wherein said fixed facility comprises: a) a radio terminal (14) for transmitting and receiving said communication signal (22); b) a telephone exchange (12) coupled to said radio terminal (14) and responsive to an input signal for producing a transmission signal sent from said telephone exchange (12) to said radio terminal (14); c) producing means responsive to said communication signal sent from said mobile station (24) for producing a first and a second reception signal which are concerned with said ordinary voice signal and representative of said control signal, respectively, and which appear simultaneously, said first reception signal being sent to said telephone exchange (12) as said input signal; d) detecting means (86) coupled to said producing means for detecting said second reception signal to produce a detection signal when said second reception signal is present; and e) control means (94) coupled to said telephone exchange (12) for attenuating the first reception signal sent through said telephone exchange (12), in response to said detection signal to send said transmission signal (22) to said radio terminal (14).

Inventors: MORISHIMA MITSUNORI, MORISHIMA, MITSUNORI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1986
Main classification: H04K3
IPC codes: H04K3, H04B7