Narrow-band interference suppression in direct sequence spread spectrum systems using a lattice IIR notch filter

This paper proposes an algorithm for the suppression of narrow-band interference in direct sequence spread spectrum (DSSS) systems, based on the open loop adaptive IIR notch filtering. The center frequency of the interference is monitored on-line by the adaptive lattice IIR notch filter in [6] or by time-frequency analysis in [3]. The power of the interference signal is also estimated from the adaptive filters. Another lattice IIR notch filter is placed in front of the receiver, the notch of which is controlled by the frequency estimate to remove the interference. However, the IIR notch filter with the zeros on the unit circle also removes the information signal at the notch frequency while removing the interference and causes data distortion. Hence, the depth of the notch should also be adjusted for the trade-off between data distortion and effective interference reduction. The objective function for adjusting the depth of the notch is defined as the overall signal to noise ratio (SNR). The SNR is expressed as a function of filter parameters and the notch depth that maximizes the SNR is found. Simulation results show that the proposed algorithm yields better performance than the existing FIR notch filter [3] and the conventional FIR LMS algorithm with very long taps [1].