  Performance of reliable communication over a coherent slow fading channel at
high SNR is succinctly captured as a fundamental tradeoff between diversity and
multiplexing gains. We study the problem of designing codes that optimally
tradeoff the diversity and multiplexing gains. Our main contribution is a
precise characterization of codes that are universally tradeoff-optimal, i.e.,
they optimally tradeoff the diversity and multiplexing gains for every
statistical characterization of the fading channel. We denote this
characterization as one of approximate universality where the approximation is
in the connection between error probability and outage capacity with diversity
and multiplexing gains, respectively. The characterization of approximate
universality is then used to construct new coding schemes as well as to show
optimality of several schemes proposed in the space-time coding literature.
