  Optical intelligent reflecting surface (OIRS) has attracted increasing
attention due to its capability of overcoming signal blockages in visible light
communication (VLC), an emerging technology for the next-generation advanced
transceivers. However, current works on OIRS predominantly assume known channel
state information (CSI), which is essential to practical OIRS configuration. To
bridge such a gap, this paper proposes a new and customized channel estimation
protocol for OIRSs under the alignment-based channel model. Specifically, we
first unveil OIRS spatial and temporal coherence characteristics and derive the
coherence distance and the coherence time in closed form. Next, to achieve fast
beam alignment over different coherence time, we propose to dynamically tune
the rotational angles of the OIRS reflecting elements following a geometric
optics-based non-uniform codebook. Given the above beam alignment, we propose
an efficient joint space-time sampling-based algorithm to estimate the OIRS
channel. In particular, we divide the OIRS into multiple subarrays based on the
coherence distance and sequentially estimate their associated CSI, followed by
a spacetime interpolation to retrieve full CSI for other non-aligned
transceiver antennas. Numerical results validate our theoretical analyses and
demonstrate the efficacy of our proposed OIRS channel estimation scheme as
compared to other benchmark schemes.
