  The performance of the automatic repeat request-dynamic decode and forward
(ARQ-DDF) cooperation protocol is analyzed in two distinct scenarios. The first
scenario is the multiple access relay (MAR) channel where a single relay is
dedicated to simultaneously help several multiple access users. For this setup,
it is shown that the ARQ-DDF protocol achieves the optimal diversity
multiplexing tradeoff (DMT) of the channel. The second scenario is the
cooperative vector multiple access (CVMA) channel where the users cooperate in
delivering their messages to a destination equipped with multiple receiving
antennas. For this setup, we develop a new variant of the ARQ-DDF protocol
where the users are purposefully instructed not to cooperate in the first round
of transmission. Lower and upper bounds on the achievable DMT are then derived.
These bounds are shown to converge to the optimal tradeoff as the number of
transmission rounds increases.
