Background of the Invention In South Africa the GSM and DCS cellular telephone communication networks operate on frequency channels which are spaced 200 kHz apart. In order to inhibit communication between a cellular telephone and a base station of the network, sufficient electromagnetic energy must be generated on each of these frequency channels. It is preferable that electromagnetic energy be generated continuously on each of these frequency channels and only on the frequency channels on which the networks operate.

Summary of the Invention According to the invention, there is provided a method of disabling operation of a cellular telephone in a particular zone, the method including the following steps, in any order generating a deterministic interference signal so that carriers are generated continuously on every frequency channel on which the cellular telephone operates, wherein the telephone operates on frequency channels spaced apart by fixed intervals ; and transmitting the interference signal in the zone thereby to inhibit communication between the cellular telephone and a base station.

The interference signal may include a waveform segment having at least one pulse per interference signal period, wherein the same waveform segment is repeated after every period of the interference signal, so that the carriers are generated without generating carriers on frequencies on which the telephone does not operate.

The method may include the step of selecting the period of the ---interference-signal-to ensure that the carriers-of--the~-interference-signal-are frequencially spaced the same as the channels on which the cellular telephone operates. The period may be selected at 5 microseconds so that the carriers are spaced by 200 kHz.

The method may include the step of selecting a maximum pulse width of the interference signal to ensure that a bandwidth of the interference signal is at least the same as a bandwidth in which the cellular telephone operates. The maximum pulse width may be selected to ensure that the bandwidth of the interference signal is 200 MHz.

The interference signal may be a pseudo-noise signal wherein the maximum pulse width of the interference signal and the period of the interference signal are both selectable whereas the rest of the waveform segment is determined by an underlying deterministic process wherein the pulse sequence is known.

Further according to the invention, there is provided a cellular telephone disabling device for disabling a cellular telephone in a particular zone, the device including a signal source for generating a deterministic interference signal so that carriers are generated continuously on every frequency channel on which the cellular telephone operates, wherein the telephone operates on frequency channels spaced apart by fixed intervals ; and transmitter means operable to transmit the interference signal in the zone thereby to disable operation of the telephone.

The interference signal may include a waveform segment having at least one pulse per interference signal period, wherein the same waveform segment is repeated after every period of the interference signal, so that the carriers are generated without generating carriers on frequencies on which the telephone does not operate.

The period of the interference signal may be selected to ensure that the carriers of the interference signal are frequencially spaced the same as the channels on which the cellular telephone operates. The period may be 5 microseconds to ensure that the carriers are spaced by 200 kHz.

The maximum pulse width of the interference signal may be selected to ensure that a bandwidth of the interference signal is at least the same as a bandwidth in which the cellular telephone operates. The maximum pulse width may be selected to ensure that the bandwidth of the interference signal is 200 MHz.

The interference signal may be a pseudo-noise signal wherein the maximum pulse width of the interference signal and the period of the interference signal are both selectable whereas the rest of the waveform segment is determined by an underlying deterministic process wherein the pulse sequence is known.

Still further according to the invention, there is provided a deterministic interference signal which includes carriers on every frequency channel on which the cellular telephone operates, wherein the telephone operates on frequency channels spaced apart by fixed intervals.

The signal may include a waveform segment having at least one pulse per signal period, wherein the same waveform segment is repeated after every period of the signal, so that the carriers are generated without generating carriers on frequencies on which the telephone does not operate.

The period of the signal may be selected to ensure that the carriers of the signal are frequencially spaced the same as the channels on which the cellular telephone operates. The period may be 5 microseconds to ensure that the carriers are spaced by 200 kHz.

The maximum pulse width of the signal may be selected to ensure that a bandwidth of the-signal-is-at-least the same as a bandwidth in which the-cellular telephone operates. The maximum pulse width may be selected to ensure that the bandwidth of the signal is 200 MHz.

The interference signal may be a pseudo-noise signal wherein the maximum pulse width of the interference signal and the period of the interference signal are both selectable whereas the rest of the waveform segment is determined by an underlying deterministic process wherein the pulse sequence is known.

Detailed Description of the Invention The invention will now be described, by way of non-limiting example, with reference to the following drawings, in which Figure 1 shows a block diagram of a cellular telephone disabling device in accordance with the invention; Figures 2 and 3 show graphical representations of a pseudo-noise signal, in the time and frequency domains respectively, in accordance with the invention; and Figures 4 and 5 show graphical representations of a pulse train signal, in the time and frequency domains respectively in accordance with the invention.

In South Africa, telephones using the GSM or DCS networks operate at frequencies spaced by fixed intervals 44 of 200 kHz. The extended GSM band includes frequencies between 925 MHz and 960 MHz spaced by fixed intervals of 200 kHz and the DCS band includes frequencies between 1805 MHz and 1880 MHz spaced by fixed intervals of 200 kHz.

In the drawings, reference numeral 10 generally indicates a cellular telephone disabling device for disabling a cellular-telephone in a particular zone.

Typically, the device 10 is located within a bank or the like. The device 10 includes a signal source 12 for generating a deterministic interference signal 40 (shown in Figures 2 and 3) so that carriers 42 are generated continuously on every frequency channel on which the cellular telephone operates. The telephone operates on frequencies spaced apart by fixed intervals 44.

The signal source 12 includes a reference frequency generator 14 which generates a reference frequency which is in turn used to centre a carrier source at the centre of the GSM or DCS down-link channels. The signal source 12 also includes a carrier source generator in the form of a synthesised or PLL source 16 for generating the carrier source for accurate placement in either the GSM or DCS down-link channels.

The signal source 12 further includes a pattern generator 18 for generating a waveform pattern so that carriers 42 are spaced 200 kHz apart with a bandwidth of 200 MHz. A mixer or alternatively an up-converter 20 is used to mix the waveform pattern up to the GSM or DCS bands, or to up-convert the waveform pattern to the GSM or DCS bands respectively.

The signal source 12 also includes a first filter 22 to filter out all out of band signals which are not on the frequency channel on which the telephone operates. These signals may have arisen from non-linearity in the mixer or up- converter 20.

A variable gain amplifier 24 is used to amplify the power of the signal outputted by the first filter 22. The amplifier 24 is adjustable by means of a gain adjuster 26 to vary the power of the signal which in turn determines the size of the zone in which the telephone is disabled. A second filter 28 is used to filter out unwanted harmonics caused by the amplifier 24 thereby outputting the interference signal 40.

The device 10 also includes-transmitter means-in-the form of an antenna 30 operable to transmit the interference signal 40 in the zone thereby to disable operation of the telephone.

Figures 2 and 3 show an interference signal 40, in the form of a pseudo- noise signal in one embodiment of the invention, in the time and frequency domains respectively. The maximum pulse width 52 of the waveform segment 46 of the signal 40 and the period 50 of the signal 40 are both selectable whereas the rest of the waveform segment 46 is determined by an underlying deterministic process wherein the pulse sequence is known and generated by the pattern generator 18. The amplitude generated by the pattern generator 18 is fixed.

Figures 4 and 5 show an interference signal 40, in the form of a pulse train signal in another embodiment of the invention, in the time and frequency domains respectively.

In Figures 2 to 5, the interference signal 40 includes a waveform segment 46 having at least one pulse 48 over an interference signal period 50 in the time domain. The same waveform segment 44 is repeated after every period 50 of the interference signal 40, so that the carriers 42 are generated in the frequency domain without generating carriers on frequencies on which the telephone does not operate.

The period 50 of the interference signal 40is selected to ensure that the carriers 42 of the interference signal 40 are frequencially spaced the same as the channels on which the cellular telephone operates. The period 50 is selected at 5 microseconds to ensure that the carriers 42 are spaced by fixed intervals 44 of 200 kHz.

The maximum pulse width 52 of the waveform segment 46 is selected to ensure that the bandwidth of the interference signal 40 is 200 MHz which-is more than the bandwidth in which the cellular telephone operates.

In use, a cellular telephone within the zone will receive a telephone signal from a base station and the interference signal 40 from the cellular telephone disabling device 10. The telephone will only be able to demodulate signals from the base station if the signals received from the base station are stronger than the interference signal received from the disabling device 10. Accordingly, the telephone will not be operable as long as it is located within the zone. The fact that the signal transmitted by the base station may be on any one of the frequency channels on which the cellular telephone operates necessitates that all the frequency channels on which the cellular telephone operates be inhibited continuously.

The Applicant believes it to be advantageous that a cellular telephone disabling device generates and transmits an interference signal so that carriers are generated continuously on every frequency channel on which a cellular telephone operates without generating carriers or noise on frequencies on which the telephone does not operate. This improves the efficiency of the device as opposed to when white noise or sweeping signals are used to disable a cellular telephone thereby removing the need for high-power output amplifiers. This also has the effect that little or no tuning of the device is required as the carriers are placed only on every frequency channel on which the cellular telephone operates.