  Spatial wideband effects are known to affect channel estimation and
localization performance in millimeter wave (mmWave) massive multiple-input
multiple-output (MIMO) systems. Based on perturbation analysis, we show that
the spatial wideband effect is in fact more pronounced than previously thought
and significantly degrades performance, even at moderate bandwidths, if it is
not properly considered in the algorithm design. We propose a novel channel
estimation method based on multidimensional ESPRIT per subcarrier, combined
with unsupervised learning for pairing across subcarriers, which shows
significant performance gain over existing schemes under wideband conditions.
