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

Abstract: In a radio frequency communication system in which a radio frequency communication signal is transmitted be­ tween a radio terminal (14) and a mobile station (24), it may be that an unauthorized third person uses a scanner-receiv­ er (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 trans­ mitted between a telephone exchange (12) and the radio terminal (14) and is concerned with an ordinary voice sig­ nal. 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 sig­ nal whenever the ordinary voice signal is absent. Alterna­ tively, the control circuit may produce a control signal while the communication signal is received from the mobile sta­ tion 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 reach­ ing the exchange.

Description: This invention relates to a unit for use in a fixed facility and a mobile station of a radio frequency communication system, 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 (speech) 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-0 015 141 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. Brief Description of the Drawing: Fig. 1 shows, together with a mobile station, a block diagram of a fixed facility of a mobile telephone system comprising a unit according to a first embodiment of this invention; Fig. 2 shows in detail, together with a radio scanner-receiver, the mobile telephone system shown in Fig. 1; and Fig. 3 is a similarly detailed view of a mobile telephone system comprising a unit according to a second embodiment of this invention. Description of the Preferred Embodiments: Referring to Fig. 1, a mobile telephone system will be described as an example of a radio frequency communication system. The mobile telephone system comprises a unit 10 according to a first embodiment of this invention. The unit 10 is used between a telephone exchange 12 for a plurality of subscribers and a plurality of base stations for different service areas. Each base station may be located remote from the telephone exchange 12 and comprises a radio terminal 14. In the manner described above, a combination of the telephone exchange 12 and the radio terminal 14 is herein called a fixed facility 16 of the mobile telephone system. An internal signal is transmitted between the telephone exchange 12 and the radio terminal 14. The internal signal is concerned with an ordinary voice signal which results from one of the subscribers. The internal signal may consist of a reception signal 18 sent from the radio terminal 14 to the telephone exchange 12 and a transmission signal 20 sent from the telephone exchange 12 to the radio terminal 14. The illustrated reception signal 18 may be specified by a reception detection signal while the illustrated transmission signal 20 may be specified by an ordinary voice signal, as will become clear as the description proceeds. The radio terminal 14 includes a transmitter-receiver (not shown) for transmitting a radio frequency communication signal 22 to a mobile station 24 and receiving a radio frequency communication signal from the mobile station. The radio terminal 14 produces the reception signal 18 in response to the communication signal 22 received from the mobile station 24. The radio terminal 14 furthermore produces the communication signal 22 in response to the transmission signal 20. The mobile telephone system further comprises a plurality of mobile stations, such as 24. In the example being illustrated, the mobile station 24 is on an automobile 26. In the manner which will be described far later in the description, the mobile station 24 includes a transmitter-receiver for transmitting and receiving the communication signal 22. The unit 10 comprises a detecting circuit 30 and a control circuit 32. In the example being illustrated, the detecting circuit 30 is coupled to the radio terminal 14 for detecting presence and absence of the reception signal 18 sent from the radio terminal 14 to the telephone exchange 12. When the reception signal 18 is not detected, a detection signal 34 is produced from the detecting circuit 30. The control circuit 32 is operable in response to the detection signal 34 and controls the transmission signal 20 sent from the telephone exchange 12 to the radio terminal 14. More specifically, the control circuit 32 comprises a signal producing unit 36 and a control unit 38. The signal producing unit 36 is for producing a dummy voice signal 40 which falls within an audio frequency band between 0.3 kHz and 3 kHz and which is variable in frequency within the audio frequency band like the ordinary audio signal, differing from an idle signal of an invariable frequency, such as 1 kHz. In this connection, the dummy voice signal may be called a quasi voice signal and be misunderstood as any meaningful information by a third person. Anyway, the third person may be confused at the dummy voice signal. The control unit 38 is coupled to the signal producing unit 36 and is responsive to the detection signal 34 and adds the dummy voice signal 40 to the transmission signal 20. The control unit 38 therefore serves as an adding unit for adding the dummy voice signal 40 to the transmission signal 20 to produce an additional inside signal which succeeds and precedes the transmission signal 20. Thus, the dummy voice signal 40 is produced instead of the transmission signal 20. In the manner which will better be described in detail, the radio terminal 14 transmits an additional radio signal in response to the additional inside signal. Production of the dummy voice signal 40 is effective to reduce a probability of an interception of the communication signal, as will later be described in detail. Referring to Fig. 2, the unit 10 illustrated in Fig. 1 will be described more in detail and is included in a bothway trunk 42 which is used between the telephone exchange 12 and the radio terminal 14 and is operable in cooperation with the telephone exchange 12 in the illustrated example. In the manner known in the art, the bothway trunk 42 is coupled to a plurality of lines, such as a set of transmission voice lines 44, a transmission detection line 46, a transmission control line 48, a reception voice line (not shown), a reception detection line 50, and a reception control line 52. When a subscriber begins to communicate with the mobile station 24, the telephone exchange 12 supplies a transmission detection signal in a known manner through the transmission detection line 46. The telephone exchange 12 furthermore supplies a transmission signal concerned with an ordinary voice signal through the transmission voice line set 44. When the mobile station 24 begins to communicate with such a subscriber, the radio terminal 14 receives the communication signal 22 and produces a reception signal concerned with an ordinary voice signal and a reception detection signal. The reception signal is sent to the reception voice line. The reception detection signal is sent to the reception detection line 50. The bothway trunk 42 further includes a transmission relay 54 and a reception relay 56. The transmission relay 54 is coupled to the transmission detection line 46 and serves as a detector for the transmission detection signal. The transmission relay 54 has a single contact point 54a. When the transmission detection signal is present and absent in the transmission detection line 46, the transmission relay 54 grounds the transmission control line 48 through the single contact point 54a and removes the ground from the transmission control line 48, respectively. The reception relay 56 is coupled to the reception detection line 50 and serves as a detector for the reception detection signal. The reception relay 56 has first through fourth contact points 56a, 56b, 56c, and 56d. When the reception detection signal is present in the reception detection line 50, the reception relay 56 grounds the reception control line 52 through the first contact point 56a. Otherwise, the first contact point 56a disconnects the reception control line 52 from ground. The second through the fourth contact points 56b to 56d serve as the above-described control unit 38. When the reception detection signal is absent and present in the reception detection line 50, the second contact point 56b places ground on the transmission control line 48 and removes the ground, respectively. When the reception detection signal is absent, the third and the fourth contact points 56c and 56d are closed to supply the dummy voice signal 40 as the additional inside signal to the set of transmission voice lines 44 from the signal producing circuit 36, with the transmission control line 48 grounded through the second contact point 56b. In Fig. 2, the transmission signal 20 is supplied to the radio terminal 14 through the transmission voice line set 44 after passing through a vogad amplifier 58. The ground is supplied to the radio terminal 14 as the transmission control signal through the transmission control line 48. The radio terminal 14 transmits the communication signal 22 in response to the transmission signal. On the other hand, the reception signal is supplied to the telephone exchange 12 through the reception voice line. The reception control signal of earth potential is supplied to the telephone exchange 12 through the reception control line 50. It is now understood that the transmission signal 20 is transmitted by the radio terminal 14 under the control of the transmission control signal of ground to the mobile station 24 as the communication signal 22 while the subscriber is talking to the mobile station 24. The communication signal 22 is also sent from the mobile station 24 to the radio terminal 14 during the talk between the subscriber and the mobile station 24. In the meanwhile, a scanner-receiver 60 may receive the communication signal 22 despite the fact that the scanner-receiver 60 is unauthorized to do so. The scanner-receiver 60 can seize each radio channel by scanning the radio channels one by one. Such scanning operation can be skipped from one channel to another on detection of an idle signal of, for example, 1 kHz. In the communication system illustrated in Fig. 2, the dummy voice signal is sent as the communication signal 22 through each of idle radio channels from the radio terminal 14 while no communication is carried out between a subscriber and the mobile station 24 through the idle radio channel. More particularly, the transmission detection signal disappears from the transmission detection line 46. The transmission relay 54 opens the contact point 54a to remove the ground from the transmission control line 48. Likewise, the reception detection signal disappears from the reception detection line 50. The reception relay 56 grounds the transmission control line 48 in place of the transmission relay 54 and furthermore sends the additional inside signal to the radio terminal 14 through the transmission voice line set 44 and the vogad amplifier 58. The radio terminal 14 therefore sends the additional radio signal in the manner described in conjunction with Fig. 1. The scanner-receiver 60 stops scanning at every idle radio channel and may come to a standstill for a while until the additional radio signal is recognized as the dummy voice signal which is meaningless at all. Recognition of the dummy voice signal must be made at every idle channel. As a result, it takes a long time to scan all of the radio channels by the scanner-receiver 60. This reduces a possibility of the interception of the communication signal 22. Referring to Fig. 3, a mobile telephone system comprises similar parts which are designated by like reference numerals. In the unit 10, the single contact point 54a of the transmission relay 54 will now be called a first contact point. In addition, the transmission relay 54 has second through fourth contact points 54b, 54c, and 54d. When the transmission detection signal is absent in the transmission detection line 46, the transmission relay 54 places ground on the transmission control line 48 through the second contact point 54b in place of the first contact point 54a. Furthermore, the dummy voice signal 40 is delivered from the signal producing unit 36 to the transmission voice line set 44 through the third and the fourth contact points 54c and 54d. The second through the fourth contact points 54b and 54d serve as the control unit 38. With this structure, the reception relay 56 is deenergized in the absence of the reception detection signal and the contact point 56a is disconnected from the ground when no communication is carried out between the mobile station 24 and the radio terminal 14. Disconnection of the contact point 56a from the ground is transmitted as the reception control signal from the contact point 56a through the reception control line 52 and the telephone exchange 12 to the transmission relay 54. Thus, the reception control signal is delivered to the transmission relay 54 as the transmission control signal to disconnect the transmission control line 48 from the first contact point 54a. In this event, the second through fourth contact points 54b to 54d are connected to the transmission control line 48 and to the transmission voice line set 44. Under the circumstances, the dummy voice signal is sent from the dummy signal producing circuit 36 to the radio terminal 14 through the transmission voice line set 44 in the manner described in conjunction with Fig. 2. Therefore, the dummy voice signal is delivered to the mobile station 24 and the scanner-receiver 60 through each idle radio channel as the additional radio signal. Thus, the additional radio signal serves to detain the scanning operation of the scanner-receiver 60 as mentioned with reference to Fig. 2. As a modification, the unit 10 may be included in the base station. The dummy voice signal may be supplied to the radio terminal 14 through a line different from that of the transmission signal.

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 thereof in preventing an interception of a radio frequency communication signal which carries a voice signal 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 communication signal, characterized by: a) detecting means (30) coupled to at least one of said telephone exchange (12) and said radio terminal (14) for detecting said internal signal to produce a detection signal indicative of presence and absence of said internal signal; b) control means (32) responsive to said detection signal for controlling said internal signal to prevent the interception of said communication signal; c) signal producing means (36) for producing a dummy voice signal; and d) adding means (38) coupled to said signal producing means (36) and responsive to said detection signal for adding said dummy voice signal to a signal sent from said telephone exchange (12) to said radio terminal (14) to produce an additional inside signal concerned with said dummy voice signal; e) said radio terminal (14) being for transmitting an additional radio signal in response to said additional inside signal to prevent the interception of said communication signal. A unit as claimed in Claim 1, said internal signal being concerned with an ordinary voice signal wherein: said detecting means (30) is coupled to said telephone exchange (12) to detect the internal signal sent from said telephone exchange (12) to said radio terminal (14) and to produce said detection signal when said internal signal is not detected. A unit as claimed in Claim 2, wherein the internal signal is added to the dummy voice signal. A unit as claimed in Claim 1, said radio terminal (14) being for receiving said communication signal from said mobile station (24) to produce a reception signal concerned with said ordinary voice signal, said internal signal consisting of the reception signal sent from said radio terminal (14) to said telephone exchange (12) and a transmission signal sent from said telephone exchange (12) to said radio terminal (14), wherein: said detecting means (30) is coupled to said radio terminal (14) to detect said reception signal and to produce said detection signal when said reception signal is not detected; and the transmission signal is added to the dummy voice signal.

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