  In this work we explicitly provide the first ever optimal, with respect to
the Zheng-Tse diversity multiplexing gain (D-MG) tradeoff, cooperative
diversity schemes for wireless relay networks. The schemes are based on
variants of perfect space-time codes and are optimal for any number of users
and all statistically symmetric (and in some cases, asymmetric) fading
distributions.
  We deduce that, with respect to the D-MG tradeoff, channel knowledge at the
intermediate relays and infinite delay are unnecessary. We also show that the
non-dynamic selection decode and forward strategy, the non-dynamic amplify and
forward, the non-dynamic receive and forward, the dynamic amplify and forward
and the dynamic receive and forward cooperative diversity strategies allow for
exactly the same D-MG optimal performance.
