  In this paper, we investigate the capacity of a multiple-input
multiple-output (MIMO) optical intensity channel (OIC) under per-antenna peak-
and average-intensity constraints. We first consider the case where the average
intensities of input are required to be equal to preassigned constants due to
the requirement of illumination quality and color temperature. When the channel
graph of the MIMO OIC is strongly connected, we prove that the strongest
eigen-subchannel must have positive channel gains, which simplifies the
capacity analysis. Then we derive various capacity bounds by utilizing linear
precoding, generalized entropy power inequality, and QR decomposition, etc.
These bounds are numerically verified to approach the capacity in the low or
high signal-to-noise ratio regime. Specifically, when the channel rank is one
less than the number of transmit antennas, we derive an equivalent capacity
expression from the perspective of convex geometry, and new lower bounds are
derived based on this equivalent expression. Finally, the developed results are
extended to the more general case where the average intensities of input are
required to be no larger than preassigned constants.
