  Optical wireless communication (OWC) using intensity-modulation and
direct-detection (IM/DD) has a channel model which possesses unique features,
due to the constraints imposed on the channel input. The aim of this tutorial
is to overview results on the capacity of IM/DD channels with input-independent
Gaussian noise as a model of OWC channels. It provides the reader with an entry
point to the topic, and highlights some major contributions in this area. It
begins with a discussion on channel models and how this IM/DD Gaussian channel
model comes about, in addition to an explanation of input constraints. Then, it
discusses the capacity of the single-input single-output channel, its
computation, and capacity bounds and asymptotic capacity results. Then, it
extends the discussion to the multiple-input multiple-output setup, and reviews
capacity bounds for this channel model. Finally, it discusses multi-user
channels modelled as a broadcast channel (downlink) or a multiple-access
channel (uplink), with their associated capacity bounds.
