  Concurrent link communications built on multi-antenna systems have been
widely adopted for spatial resource exploitation. MIMA-MAC, a classical MIMO
MAC protocol utilizing concurrent link scheme, is able to provide superior link
throughput over conventional single link MAC (under certain isolated link
topologies). However, when utilizing rich link adaptation functions in MIMO
systems, there exists a non-ignorable probability that MIMA-MAC's throughput
will be lower than that of single link scheme (such probability is dominated by
the statistics of instantaneous link topology and channel response). Inspired
by this critical observation, and for adapting to various link topologies, this
paper will present a novel MAC design that can adaptively switch between single
or concurrent link scheme. With the aim of absolutely outperforming the single
link MAC, here our optimization criterion is to guarantee a throughput result
that is either better than or at least equal to single link MAC's counterpart.
To highlight the design rationale, we first present an idealized implementation
having network information perfectly known in a non-causal way. Then for
realistic applications, we further develop a practical MAC implementation
dealing with realistic system impairments (distributed handshaking and
imperfect channel estimation). Simulation results validate that link throughput
in our MAC is higher than or equal to single link MAC's counterpart with
minimized outage probabilities. And for ergodic link throughput, our proposed
MAC can outperform the single link MAC and MIMA-MAC by around 20{%}-30{%}.
