Modulation of signals in rapidly-updated adaptive filters: theory, mitigation, and applications

This paper addresses the technique of rapidly updating the weights of an adaptive filter to cancel nonstationary interference. This method is used, for example, in radar systems that receive coherent multipath interference, known as "hot clutter" or "terrain scattered jamming". We develop a framework for understanding how this rapid weight updating (inadvertently) modulates signals. Two principal types of modulation are studied: random modulations induced by finite-length training sets and non-random modulations induced by the coherence of the interference. Our framework is then used to assess the impact of these modulations on the overall performance of various well-known jammer multipath mitigation algorithms. Several techniques for controlling this modulation are proposed.