The present invention relates to an RF interference cancellation system for a fixed or mobile radio.

Many fixed and mobile radio platforms have a number of RF transmitters and receivers where the associated antennas are in close proximity. In most cases, the antennas are omni-directional as the direction in which the signal is being transmitted to or received from is unknown. The close proximity and omni- directional properties of the antennas inevitably mean that there can be significant levels of mutual coupling between them, eg, at 30MHz, the coupling can be 15dB or stronger on typical mobile platforms. Therefore, a 50W transmitter could be coupling 1 .6W into a receiver leading to significant desensitisation, total blocking or even damage.

There are known techniques for dealing with RF interference, particularly a cancellation technique as will be described below.

Such systems are effective when working to cancel interference from a narrow band signal, such as from a communications radio, with the signal power over a narrow frequency range controlling the circuit providing cancellation and defining frequencies at which maximum cancellation occurs. However, such systems work less well when being used to cancel a broadband signal that may be coming from a device such as a high power broadband radio source. In such circumstances, the radio signal power may be spread over several 10s of megaHertz and the circuit minimises the total power over that frequency range. Whilst this has some benefit, it can hinder quite significantly reliable wanted communication and is not particularly useful in providing an appropriately high level of cancellation at a specific frequency or narrower range of frequencies, such as those that will be used by a narrow band communications radio. The present invention seeks therefore to improve RF interference cancellation systems, particularly when trying to deal with compensation for broadband interference signals. According to the present invention there is provided a radio interference cancellation system, the system comprising a sample coupler arranged to be connected, in use, to a transmit antenna acting as a source for radio interference;

a residue coupler arranged to be connected, in use, between a receiving antenna and a radio receiver;

a pair of narrow band filters associated with the outputs of the sample coupler and residue coupler respectively, for filtering the output of the sample coupler and the residue coupler and feeding the filtered output to a correlator; a correlator for receiving the respective filtered signals and outputting a signal to a vector modulator;

a vector modulator for receiving the output of the correlator and modulating the received signal from the sample coupler to provide a cancellation signal;

a cancellation coupler for receiving the cancellation signal and applying it to the path between the receive antenna and the residual coupler, such that, in use, a cancellation signal is applied to the signal received by the receive antenna and passed to the radio receiver to minimise the interference effects of any unwanted RF signal coupling in a frequency range defined by the bandwidth of the narrow band filters. .

With the present invention in its broadest configuration, cancellation can be provided within a narrow frequency range at which narrow band radio communication is required to ensure optimum operation of a radio and remote communication even when significant broadband interference is present.

The present invention may further include a filter positioned between the output of the cancellation circuit and the receiving radio to reduce the potential for saturation of the radio or for it to be driven into a distortion mode. Such a filter may either be at a fixed frequency, or can be tuneable so as to set its frequency at the point of maximum cancellation by a controller. This reduces the potential for saturation, and in the latter case the received frequency can then be set to any desired frequency and the centre frequency of the filter will track the frequency of maximum cancellation to improve further the operation of the radio.

An example of the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a schematic circuit diagram showing a prior art interference cancellation circuit;

Figure 2 is a schematic circuit diagram showing a first example interference cancellation circuit according to the present invention; and

Figure 3 is a schematic circuit diagram showing a further example circuit according to the present invention. Referring to figure 1 , a known RF cancellation circuit is shown. This circuit is a single channel narrow band RF interference cancellation circuit which is configured to remove narrow band interference coupled from a transmitter 1 to a receiver 2. This circuit operates by taking a sample of the interfering signal from the transmitter 1 via a directional coupler 3 inserted between the transmitter 1 and its antenna 4. The sampled signal is passed through a cancellation module 5 which comprises a vector modulator 6 which adjusts the amplitude and phase of the sampled signal. The output of the vector modulator 6 is injected into a cancellation coupler 7 inserted between a receiving antenna 8 and the receiver 2. A residue coupler 9 is inserted between the cancellation coupler and the receiver 2 to monitor the residual signal after cancellation. Residue coupler 9 drives a negative feedback loop 10 to adjust the vector modulator 6 to minimise the residual power, therefore maximising the cancellation of the transmitted interfering signal. The negative feedback loop 10 is represented by the loop in the figure and generally comprises, in sequence, the residual coupler 9, an amplifier 1 1 , correlator 12, low pass filter 13 and further amplifier 14. As mentioned above, when the circuit is cancelling a narrow band signal such as that which might be from a communication radio system, the signal power over a narrow frequency range controls the circuit and defines the frequency at which maximum cancellation occurs. As mentioned above also, whilst this circuit has many benefits, it does not operate well when dealing with a broad band signal, in terms of providing a high level of cancellation at a specific frequency or in a narrow frequency range such as those required by a narrow band communications radio.

Figure 2 shows a first example of the present invention that seeks to improve the prior art circuit shown in figure 1 . In this figure components that correspond generally to those in the prior art of figure 1 are numbered identically.

In this digital domain example of the present invention, narrowband digital filters are implemented within the digital signal processing block 23 on the residue and sample inputs to the correlator 12. These filters ensure that the controlling signal provided from the correlator 12 to the vector modulator 6 maximises the cancellation at the centre frequency of the matched narrowband digital filters. It will be appreciated by a person skilled in the art that if a digital approach is taken, then anti-aliasing filters are necessary and provided as the filters 20, 21 with their characteristics being defined by the sampling frequency used to sample the correlator input waveforms. In this case a pair of analogue to digital convenors 22 feed the digital processing block 23 which outputs to a digital to analogue convenor 24. The invention could also be implemented as an equivalent analogue technique in which case the ADC and DAC components are not present. Employment of a digital approach allows however for exact matching of the filter characteristics which improves the level of cancellation over a narrow band. It will also be appreciated by a person skilled in the art that additional cancellation circuits can be provided to insert further cancellation frequencies, in the form of additional "notches" in the frequency characteristics of the system. In such a circumstance N-way splitters 24 and N-way combiners 25 are provided to take signals received from a number of sources to combine cancellation and residual signals.

The operation of the above configuration will now be described in relation to two particular scenarios, the first being the presence of low power broadband interfering signals, and the second being the situation of high power signals into the receiver 2.

In a first scenario the power in any interfering signal, together with any coupling between the source and the radio antennas, is sufficiently weak that the received power does not overdrive the front end of the radio 2 and cause compression and harmonic and intermodulation distortion. In this scenario a typical antenna coupling figure would be weaker than 35dB. However radio desensitisation occurs at the level of "noise like" interfering power in the receiver channel. In such a situation radio sensitivity can be restored by producing a narrow band null in the manner described with respect to the module and system of figure 2.

In the second scenario, where there is a high power interfering signal, the cancellation module and system of figure 2 will reduce interfering noise over the receive channel frequency, but the total received power either side of the receive frequency may well be sufficient to drive the radio 2 into saturation, because of the relatively wideband front end of the receiving radio 2. This occurs when the antenna coupling between the source and the radio antennas is relatively strong, perhaps in the region of 20dB or stronger. This scenario can be overcome by attempting to remit interference over a wider bandwidth in the configuration of figure 2. However, an alternative approach is shown in figure 3, in which a narrowband band pass filter is provided between the output of the cancellation module 5 and the input to the radio 2. In figure 3, again, components that are equivalent to those in earlier figures are numbered with the same numbering. In figure 3 a filter, preferably a tuneable bandpass filter 30, is positioned adjacent to coupler 9 and the radio receiver 2 and in the path of the receive antenna 8 of the system of figure 3. The filter 30 may be set to filter at a fixed frequency or programmed by a controller 31 for operation at the same frequency as the cancellation module 5. With this configuration it is possible to vary the frequency at which narrow band compensation is provided dependent upon the receive channel. However, by also providing additional filtering at a wider range given by filter 30 around that central selected narrow band, it is possible to reduce the power received by the radio receiver 2 at either side of the receiving channel frequency to reduce the likelihood of the receiving radio 2 being driven into saturation and distortion. As will be appreciated from the above, with the system of the present invention it is possible to provide a simple and easily produced and controlled cancellation module which improves significantly unwanted coupling issues between a transmit antenna and a receive antenna, and also enables communication to continue through an interfering signal at a desired received frequency. In one example it is also possible to ensure that saturation and distortion in the received radio is minimised whilst also cancelling the negative effects of a broadband interfering signal and/or narrow band coupling.