An active interference cancellation apparatus for controlling duplex operation of signals to be transmitted and received, the apparatus comprising: a receiver chain comprising a received signal input node; a transmitter chain comprising a transmitter chain tap node and a transmitter output node; an auxiliary transmitter chain having an auxiliary chain input node operably coupled to the transmitter chain tap node, the auxiliary transmitter chain comprising an adaptive filter unit and a cancellation signal output, the cancellation signal output being operably coupled to the received signal input node of the receiver chain; the apparatus being characterised by: a coefficient processing unit comprising an interpolation processor, the adaptive filter unit being operably coupled to the receiver chain via the coefficient processing unit; wherein the interpolation processor is arranged to cooperate with the coefficient processing unit; the transmitter chain is arranged to generate, when in use, a first waveform comprising a first plurality of frequency-separated signals; the auxiliary transmitter chain is arranged to generate, substantially contemporaneously with the transmitter chain generating the first waveform, second waveform comprising a second plurality of frequency-separated signals and apply the second waveform at the received signal input node; the receiver chain is arranged to receive a composite waveform comprising a waveform coupled from the transmitter output node to the received signal input node and the second waveform from the auxiliary transmitter chain; and the first waveform does not comprise any signals required for determination of equaliser coefficients of the adaptive filter unit that coincide in frequency with the second plurality of frequency-separated signals of the second waveform. a receiver chain comprising a received signal input node; a transmitter chain comprising a transmitter chain tap node and a transmitter output node; an auxiliary transmitter chain having an auxiliary chain input node operably coupled to the transmitter chain tap node, the auxiliary transmitter chain comprising an adaptive filter unit and a cancellation signal output, the cancellation signal output being operably coupled to the received signal input node of the receiver chain; the apparatus being characterised by: a coefficient processing unit comprising an interpolation processor, the adaptive filter unit being operably coupled to the receiver chain via the coefficient processing unit; wherein the interpolation processor is arranged to cooperate with the coefficient processing unit; the transmitter chain is arranged to generate, when in use, a first waveform comprising a first plurality of frequency-separated signals; the auxiliary transmitter chain is arranged to generate, substantially contemporaneously with the transmitter chain generating the first waveform, second waveform comprising a second plurality of frequency-separated signals and apply the second waveform at the received signal input node; the receiver chain is arranged to receive a composite waveform comprising a waveform coupled from the transmitter output node to the received signal input node and the second waveform from the auxiliary transmitter chain; and the first waveform does not comprise any signals required for determination of equaliser coefficients of the adaptive filter unit that coincide in frequency with the second plurality of frequency-separated signals of the second waveform. a coefficient processing unit comprising an interpolation processor, the adaptive filter unit being operably coupled to the receiver chain via the coefficient processing unit; wherein the interpolation processor is arranged to cooperate with the coefficient processing unit; the transmitter chain is arranged to generate, when in use, a first waveform comprising a first plurality of frequency-separated signals; the auxiliary transmitter chain is arranged to generate, substantially contemporaneously with the transmitter chain generating the first waveform, second waveform comprising a second plurality of frequency-separated signals and apply the second waveform at the received signal input node; the receiver chain is arranged to receive a composite waveform comprising a waveform coupled from the transmitter output node to the received signal input node and the second waveform from the auxiliary transmitter chain; and the first waveform does not comprise any signals required for determination of equaliser coefficients of the adaptive filter unit that coincide in frequency with the second plurality of frequency-separated signals of the second waveform.