  We develop a deterministic ab-initio model for the input-output relationship
of a multiple-input multiple-output (MIMO) wireless channel, starting from the
Maxwell equations combined with Ohm's Law. The main technical tools are
scattering and geometric perturbation theories. The derived relationship can
lead us to a deep understanding of how the propagation conditions and the
coupling effects between the elements of multiple-element arrays affect the
properties of a MIMO channel, e.g. its capacity and its number of degrees of
freedom.
