  Channel estimation is challenging for the reconfigurable intelligence surface
(RIS) assisted millimeter wave (mmWave) communications. Since the number of
coefficients of the cascaded channels in such systems is closely dependent on
the product of the number of base station antennas and the number of RIS
elements, the pilot overhead would be prohibitively high. In this letter, we
propose a cascaded channel estimation framework for an RIS assisted mmWave
multiple-input multiple-output system, where the wideband effect on
transmission model is considered. Then, we transform the wideband channel
estimation into a parameter recovery problem and use a few pilot symbols to
detect the channel parameters by the Newtonized orthogonal matching pursuit
algorithm. Moreover, the Cramer-Rao lower bound on the channel estimation is
introduced. Numerical results show the effectiveness of the proposed channel
estimation scheme.
