  We study the asymptotic outage performance of incremental redundancy
automatic repeat request (INR-ARQ) transmission over the multiple-input
multiple-output (MIMO) block-fading channels with discrete input
constellations. We first show that transmission with random codes using a
discrete signal constellation across all transmit antennas achieves the optimal
outage diversity given by the Singleton bound. We then analyze the optimal
SNR-exponent and outage diversity of INR-ARQ transmission over the MIMO
block-fading channel. We show that a significant gain in outage diversity is
obtained by providing more than one bit feedback at each ARQ round. Thus, the
outage performance of INR-ARQ transmission can be remarkably improved with
minimal additional overhead. A suboptimal feedback and power adaptation rule,
which achieves the optimal outage diversity, is proposed for MIMO INR-ARQ,
demonstrating the benefits provided by multi-bit feedback.
