How narrow is narrowband? [adaptive array signal processing]

The "narrowband" or zero-bandwidth assumption is often made in the analysis of array signal processing algorithms. This paper provides both a definition and expression which precisely define the notion of narrowband The expression accurately predicts the point when the "narrowband" assumption fails for some superresolution algorithms, the Cramer Rao bound on angle estimation and the signal-to-interference-plus-noise performance of adaptive beamformers. Hence it is a useful design point for many types of sensor array systems.