  This paper studies the capacity of a general multiple-input multiple-output
(MIMO) free-space optical intensity channel under a per-input-antenna
peak-power constraint and a total average-power constraint over all input
antennas. The focus is on the scenario with more transmit than receive
antennas. In this scenario, different input vectors can yield identical
distributions at the output, when they result in the same image vector under
multiplication by the channel matrix. We first determine the most
energy-efficient input vectors that attain each of these image vectors. Based
on this, we derive an equivalent capacity expression in terms of the image
vector, and establish new lower and upper bounds on the capacity of this
channel. The bounds match when the signal-to-noise ratio (SNR) tends to
infinity, establishing the high-SNR asymptotic capacity. We also characterize
the low-SNR slope of the capacity of this channel.
