  Accurate channel estimation is essential for achieving the performance gains
offered by reconfigurable intelligent surface (RIS)-aided wireless
communications. A variety of channel estimation methods have been proposed for
such systems; however, none of the existing methods takes into account the
effect of synchronization errors such as carrier frequency offset (CFO). In
general, CFO can significantly degrade channel estimation performance of
orthogonal frequency division multiplexing (OFDM) systems. Motivated by this,
we investigate the effect of CFO on channel estimation for RIS-aided OFDM
systems. Furthermore, we propose a joint CFO and channel impulse response (CIR)
estimation method for these systems. Simulation results demonstrate the
effectiveness of our proposed method, and also demonstrate that the use of
time-domain rather than frequency-domain estimation in this context results in
an improvement in the mean-squared error (MSE) performance of channel
estimation as well as a significantly lower overall computational complexity.
