  This paper studies the setup of a multiple-relay network in which $K$
half-duplex multiple-antenna relays assist in the transmission between
a/several multiple-antenna transmitter(s) and a multiple-antenna receiver. Each
two nodes are assumed to be either connected through a quasi-static Rayleigh
fading channel, or disconnected. We propose a new scheme, which we call
\textit{random sequential} (RS), based on the amplify-and-forward relaying. We
prove that for general multiple-antenna multiple-relay networks, the proposed
scheme achieves the maximum diversity gain. Furthermore, we derive
diversity-multiplexing tradeoff (DMT) of the proposed RS scheme for general
single-antenna multiple-relay networks. It is shown that for single-antenna
two-hop multiple-access multiple-relay ($K>1$) networks (without direct link
between the transmitter(s) and the receiver), the proposed RS scheme achieves
the optimum DMT. However, for the case of multiple access single relay setup,
we show that the RS scheme reduces to the naive amplify-and-forward relaying
and is not optimum in terms of DMT, while the dynamic decode-and-forward scheme
is shown to be optimum for this scenario.
