ADAPTIVE FILTER

Abstract: An adaptive filter (1) may be realised by weighting the outputs from a tapped delay line ( ,W). This, however, allows nulls to corrupt a wanted signal spectral region. A fixed(non-adaptive) filter (2) coupled between the signal input and an input of means (3) for summing the weighted outputs serves to allow nulls only to be formed in the skirt region of the fixed filter response thus preserving a wanted signal.

Description: This invention relates to adaptive filters such as may be used in adaptive array signal processing arrangements and filtering means including such adaptive filters. Sinusoidal interference corrupting a signal is typically eliminated with a fixed notch filter tuned to the frequency of the interference. A very narrow notch is usually desired in order to filter out the interference without distorting the signal. However, when the frequency of interference is unknown or drifts with time then the best solution is one which identifies the frequency or frequencies of interference and adapts one or more notches to cancel it. This may be achieved by means of an adaptive notch filter. In a broad RF band, any interfering carrier, modulated or unmodulated, can usually be considered to be narrow band, and an adaptive notch filter will be able to cancel it. However, the wanted signal is generally indistinguishable from the interference and will also be cancelled. Adaptive notch filters which involve combinations of adaptive filters are disclosed by D V Bhaskar et al in an article entitled "Adaptive notch filtering for the retrieval of sinusoids in noise" IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. ASSP-32, No. 4, August 1984, p. 791-802, IEEE, New-York, US. French Patent Application FR-A-2550670 relates to filters for signalling systems and involves combinations of non-adaptive filters. These documents comprise relevant prior art for the present invention. According to the present invention there is provided a filtering means comprising an adaptive filter including a tapped delay line coupled to a signal input, means for weighting the outputs of the tapped delay line and means for summing the weighted outputs, characterised by combination of the adaptive filter with a non-adaptive filter, coupled between the signal input and a respective input of the summing means, having a passband including a frequency of interest and serving to prevent the formation of nulls at the frequency of interest. Embodiments of the invention will now be described with reference to the accompanying drawings, in which Fig.1 illustrates schematically a constrained adaptive filter according to the present invention; Fig.2 illustrates more specifically a constrained adaptive filter according to the present invention; Fig.3 illustrates an alternative way of drawing the constrained adaptive filter circuitry of Fig 2, and Fig.4 illustrates a typical transfer function of a constrained adaptive filter according to the present invention. Most receiver designs include a front end bandpass filter to remove interference from outside the working band. The nature of communication links requires that the strongest possible signals in the adjacent channels are filtered such that they cannot cause interference to the weakest receivable signals in the channel in use. Adjacent channel interference can be removed in the intermediate frequency (IF) stage of a super heterodyne receiver effectively, but very strong interference can corrupt signals before the IF stage by forcing pre-amplifiers and other active components into limiting. The purpose of a front end bandpass filter is to increase the receiver's tolerance to strong "out of band" interference without affecting "in band", and potentially wanted, signals and it follows that the sharpest cut-off is desirable. A sharp cut-off is commonly achieved with a multiple pole filter and due to the need for high "Q" sections this is constructed in large, metal resonant cavities, which are heavy, inconvenient and out of place in a solid state design. As the interference may be intermittent and is rarely present on more than a few frequencies, an adaptive filter that recognises the passband of the wanted signal rather than just cancelling it along with the unwanted signals is highly desirable. A basic adaptive notch filter may be realised by weighting the outputs from a tapped delay line and summing the weighted outputs but, as mentioned above, this approach allows nulls in the filters frequency response to enter and corrupt the wanted spectral region. When a reference for the interference is available it is known that it can be adaptively filtered to match the interfering signals as closely as possible, allowing them to be subtracted out to leave the wanted signals. In a front end filter application as in Fig. 1, a suitable reference is an attenuated copy of the incoming signals. The adaptive filter 1 is arranged to provide no more than unity gain and it follows that no cancellation can occur at the point of subtraction. If the interference in the primary input could be attenuated by as much or more than the reference then it could be cancelled. As "out of band" signals are the interference, a suitably chosen bandpass filter 2 will allow their cancellation without affecting "in band" signals. The advantage of this method is that whilst a bandpass filter is still required, a sharp cut-off is no longer needed because the filter's purpose is only to identify "out of band" interference for the adaptive filter to cancel. The sharp cut-off is effectively provided by the notches, reducing the number of poles required. The adaptive filter of Fig. 1 may be configured as a transversal filter with variable weights at the taps, see Fig. 2 which shows an n tap transversal filter with passband protection i.e. a constrained adaptive filter. A basic version of such a filter may be drawn as illustrated in Fig. 3. The delays τ₁ to τ n-1 provide a phase gradient at the taps, weights W₂ to W n , which affect amplitude and phase, can control the frequencies at which destructive interference takes place. Each tap and associated weight provide a tunable notch. The delays are required because, mathematically, two weights in parallel could be replaced by one, and a delay between the taps ensures that the phase at each tap is a different function of frequency. The signal at the summing point 3 in Fig. 2 or Fig. 3 represents the error between the adapted reference and the main signal path. The filter is made adaptive by creating a control loop to minimise the error by controlling the weights accordingly, this is conventional and is achieved by means of a suitable control algorthm. Thus by adding a fixed (non-adaptive) filter to the normal adaptive notch filter, nulls are only allowed to be formed in the skirt region of the fixed filter response, that is the spectral location of the nulls is constrained. This is illustrated in Fig. 4 which shows a typical transfer function of a constrained adaptive filter according to the present invention. The coupler 4 can be adjustable as shown in Fig. 4. The value of coupling will then be adjustable and will determine the extent to which nulls can approach the wanted signal frequency.

Claims: 1. A filtering means comprising an adaptive filter (1), including a tapped delay line (τ) coupled to a signal input, means (W) for weighting the outputs of the tapped delay line (τ), and means for summing (3) the weighted outputs, characterised by combination of the adaptive filter (1) with a non-adaptive filter (2), coupled between the signal input and a respective input of the summing means, having a passband including a frequency of interest and serving to prevent the formation of nulls at the frequency of interest. 2. A filtering means as claimed in claim 1 characterised by including means for controlling the applied weights whereby to form nulls at interference frequencies outside of said passband. 3. A filtering means as claimed in claim 1 or claim 2 characterised in that the non-adaptive filter (2) allows the formation of nulls only in the skirt region of the non-adaptive filter response. 4. A filtering means as claimed in any one of the preceding claims, characterised by a coupler (4) for coupling the input signal to the tapped delay line (τ) and wherein the amount of coupling is adjustable.

Inventors: SEARLE, JEFFREY GRAHAM
Applicants: STC PLC
Application year: 1987
Main classification: H03H21
IPC codes: H03H21