Sufficient excitation and stable reduced-order adaptive IIR filtering

A perturbed error system is used to describe the parameter and output error behaviors of reduced-order application of adaptive identifier/filters such as the hyperstable adaptive recursive filter (HARF). Given satisfaction of a sufficient excitation condition, this error system is shown to exhibit a bounded-input, bounded-state (BIBS) property. This implies that, despite order insufficiency, the output and parameter estimates of HARF (and similar adaptive identifier/filter algorithms) remain bounded.