  The characteristics of a space-to-space multiple-input, multiple-output
(MIMO) communication channel that distinguish it from terrestrial MIMO
communication channels are summarized and discussed primarily from an
information-theoretic viewpoint. The implications of these characteristics for
the design and application of future space-based communication systems are also
discussed, and it is shown that in general, either energy-efficient or
spectrally-efficient communication in space can only be achieved using a
distributed MIMO architecture.
