  Although the multi-antenna or so-called multiple-input multiple-output (MIMO)
transmission has been the enabling technology for the past generations of
radio-frequency (RF)-based wireless communication systems, its application to
the visible light communication (VLC) still faces a critical challenge as the
MIMO spatial multiplexing gain can be hardly attained in VLC channels due to
their strong spatial correlation. In this paper, we tackle this problem by
deploying the optical intelligent reflecting surface (OIRS) in the environment
to boost the capacity of MIMO VLC. Firstly, based on the extremely near-field
channel condition in VLC, we propose a new channel model for OIRS-assisted MIMO
VLC and reveal its peculiar ``no crosstalk'' property, where the OIRS
reflecting elements can be respectively configured to align with one pair of
transmitter and receiver antennas without causing crosstalk to each other.
Next, we characterize the OIRS-assisted MIMO VLC capacities under different
practical power constraints and then proceed to maximize them by jointly
optimizing the OIRS element alignment and transmitter emission power. In
particular, for optimizing the OIRS element alignment, we propose two
algorithms, namely, location-aided interior-point algorithm and log-det-based
alternating optimization algorithm, to balance the performance versus
complexity trade-off; while the optimal transmitter emission power is derived
in closed form. Numerical results are provided, which validate the capacity
improvement of OIRS-assisted MIMO VLC against the VLC without OIRS and
demonstrate the superior performance of the proposed algorithms compared to
baseline schemes.
