  Downlink scheduling schemes are well-known and widely investigated under the
assumption that the channel state is perfectly known to the scheduler. In the
multiuser MIMO (broadcast) case, downlink scheduling in the presence of
non-perfect channel state information (CSI) is only scantly treated. In this
paper we provide a general framework that addresses the problem systematically.
Also, we illuminate the key role played by the channel state prediction error:
our scheme treats in a fundamentally different way users with small channel
prediction error ("predictable" users) and users with large channel prediction
error ("non-predictable" users), and can be interpreted as a near-optimal
opportunistic time-sharing strategy between MIMO downlink beamforming to
predictable users and space-time coding to nonpredictable users. Our results,
based on a realistic MIMO channel model used in 3GPP standardization, show that
the proposed algorithms can significantly outperform a conventional
"mismatched" scheduling scheme that treats the available CSI as if it was
perfect.
