  Intelligent reflecting surfaces (IRSs) improve both the bandwidth and energy
efficiency of wideband communication systems by using low-cost passive elements
for reflecting the impinging signals with adjustable phase shifts. To realize
the full potential of IRS-aided systems, having accurate channel state
information (CSI) is indispensable, but it is challenging to acquire, since
these passive devices cannot carry out transmit/receive signal processing. The
existing channel estimation methods conceived for wideband IRS-aided
communication systems only consider the channel's frequency selectivity, but
ignore the effect of beam squint, despite its severe performance degradation.
Hence we fill this gap and conceive wideband channel estimation for IRS-aided
communication systems by explicitly taking the effect of beam squint into
consideration. We demonstrate that the mutual correlation function between the
spatial steering vectors and the cascaded two-hop channel reflected by the IRS
has two peaks, which leads to a pair of estimated angles for a single
propagation path, due to the effect of beam squint. One of these two estimated
angles is the frequency-independent `actual angle', while the other one is the
frequency-dependent `false angle'. To reduce the influence of false angles on
channel estimation, we propose a twin-stage orthogonal matching pursuit
(TS-OMP) algorithm.
