  In this work, we present a construction of a family of space-time block codes
for a Multi-Access Amplify-and- Forward Relay channel with two users and a
single half-duplex relay. It is assumed that there is no Channel Side
Information at the transmitters and that they are not allowed to cooperate
together. Using the Diversity Multiplexing Tradeoff as a tool to evaluate the
performance, we prove that the proposed scheme is optimal in some sense.
Moreover, we provide numerical results which show that the new scheme
outperforms the orthogonal transmission scheme, e. g. time sharing and offers a
significant gain.
