  We establish the optimal diversity-multiplexing (DM) tradeoff of coherent
selective-fading multiple-access multiple-input multiple-output (MIMO) channels
and provide corresponding code design criteria. As a byproduct, on the
conceptual level, we find an interesting relation between the DM tradeoff
framework and the notion of dominant error event regions which was first
introduced in the AWGN case by Gallager, IEEE Trans. IT, 1985. This relation
allows to accurately characterize the error mechanisms in MIMO fading
multiple-access channels. In particular, we find that, for a given rate tuple,
the maximum achievable diversity order is determined by the error event that
dominates the total error probability exponentially in SNR. Finally, we show
that the distributed space-time code construction proposed recently by Badr and
Belfiore, Int. Zurich Seminar on Commun., 2008, satisfies the code design
criteria derived in this paper.
