  This paper discusses the characteristics of a space-to-space multiple-input,
multiple-output (MIMO) communication channel that distinguish it from more
common terrestrial MIMO communication channels. These characteristics imply
that the channel matrices for space communication channels have a particularly
simple structure that leads to statistical characteristics that are both
predictable and readily controllable using clusters of satellites as
distributed communication nodes. Furthermore, the extremely high cost of
launching mass into space introduces a constraint into the channel capacity
equation that leads to a spectral-efficiency vs. energy-efficiency tradeoff for
space MIMO communication that is fundamentally different from the tradeoff that
is generally considered applicable to terrestrial MIMO communication systems.
