  Conventional MU-MIMO techniques, e.g. Linear Zero-Forced Beamforming (LZFB),
require sufficiently accurate channel state information at the transmitter
(CSIT) in order to realize spectral efficient transmission (degree of freedom
gains). In practical settings, however, CSIT accuracy can be limited by a
number of issues including CSI estimation, CSI feedback delay between user
terminals to base stations, and the time/frequency coherence of the channel.
The latter aspects of CSIT-feedback delay and channel-dynamics can lead to
significant challenges in the deployment of efficient MU-MIMO systems. Recently
it has been shown by Maddah-Ali and Tse (MAT) that degree of freedom gains can
be realized by MU-MIMO even when the knowledge of CSIT is completely outdated.
Specifically, outdated CSIT, albeit perfect CSIT, is known for transmissions
only after they have taken place. This aspect of insensitivity to CSIT-feedback
delay is of particular interest since it allows one to reconsider MU-MIMO
design in dynamic channel conditions. Indeed, as we show, with appropriate
scheduling, and even in the context of CSI estimation and feedback errors, the
proposed MAT scheme can have performance advantages over conventional MU-MIMO
in such scenarios.
