Many situations can be envisioned in which it is desirable for government bodies such as law enforcement agencies to prevent the use of cellular telephones within a certain geographical area. For example, during a situation of civil unrest such as riots, it may be desirable for the police or other similar government bodies to prevent the function of cellular telephones within a certain section of a city.
Other cases where it may also be desirable to prevent the function of cellular networks in a defined geographical area might be, for example, before a police raid on a building. Another example of a problem area which could be addressed by area-selective prevention of cellular telephony is cellular telephones which have been smuggled into prisons, which is beginning to constitute a major problem.
Traditionally, the prevention of cellular telephony within an area has been approached much in the same manner as traditional electronic warfare. This means that traditional solutions have been used which are mainly aimed at transmitting a jamming signal, usually noise, on the relevant frequencies, so as to “drown” the information in noise, and to thus create a signal-to-noise ratio which prevents the user terminals in the targeted area from detecting the “payload” signal.
A problem with the technique described above, i.e. so called “brute force” jamming, is that this is a method which consumes a great deal of energy, thus making the equipment heavy and not very portable. In addition, since noise signals are transmitted with a relatively high output power at well known frequencies, it can be quite easy for a skilled user to detect that the system is being tampered with.
Document FR 2 858 742 describes a procedure and a device for tampering with cellular communications which addresses some of the problems described above. However, the device uses a great deal of processor power since it relies on a complex algorithm and tries to identify certain signals on a more or less constant basis.
Furthermore, since there are temporary radio base stations that may control the communication system to prevent the use of mobile stations such as cellular telephones within a certain geographical area, there is a need for a mobile station that can be used in spite of the appearance of such a temporary base station. This is a concern for said bodies since it is desirable for said bodies to have the possibility to allow communication for designated mobile stations within the controlled area at the same as non-designated mobile stations cannot operate in the controlled area.
The document FR 2858742 describes a mobile station that may be used in a controlled area, but a problem with this mobile station is that the user terminal has to order the temporary radio base station to stop interfering with a certain dedicated channel and to allow communication on the dedicated channel. This step is not a desirable solution since the existing system has to be amended in order to allow communication to and from the mobile station. Another disadvantage is that a non-designated mobile station may easily be modified so that it can order the temporary base station to allow communication on a dedicated channel.
There is thus a need for a more simple temporary base station and a more simple mobile station that can be used in an existing communication system, and a method for using said devices.