  Intelligent reflecting surface (IRS) can bring significant performance
enhancement for wireless communication systems by reconfiguring wireless
channels via passive signal reflection. However, such performance improvement
generally relies on the knowledge of channel state information (CSI) for
IRS-associated links. Prior IRS channel estimation strategies mainly estimate
IRS-cascaded channels based on the excessive pilot signals received at the
users/base station (BS) with time-varying IRS reflections, which, however, are
not compatible with the existing channel training/estimation protocol for
cellular networks. To address this issue, we propose in this paper a new
channel estimation scheme for IRS-assisted communication systems based on the
received signal power measured at the user, which is practically attainable
without the need of changing the current protocol. Specifically, due to the
lack of signal phase information in power measurements, the autocorrelation
matrix of the BS-IRS-user cascaded channel is estimated by solving equivalent
matrix-rank-minimization problems. Simulation results are provided to verify
the effectiveness of the proposed channel estimation algorithm as well as the
IRS passive reflection design based on the estimated channel autocorrelation
matrix.
