  The degrees of freedom (DoF) of the $K$-user multiple-input multiple-output
(MIMO) interference channel are studied when perfect, but delayed channel state
information is available at the transmitter side (delayed CSIT). Recent works
have proposed schemes that achieve increasing DoF values, but at the cost of
long communication delays. This work proposes three linear precoding
strategies, formulated in such a way that the achievable DoF can be derived as
a function of the transmission delay, thus elucidating its achievable DoF-delay
trade-off. All strategies are based on the concept of interference alignment,
and built upon three main ingredients: delayed CSIT precoding, user scheduling,
and redundancy transmission. In this respect, the interference alignment is
realized by exploiting delayed CSIT in order to align the interference at the
non-intended receivers along the space-time domain. Finally, the latter part of
this work settles that all the proposed strategies work also for constant
channels, except for SISO. In such a case, the schemes can be made feasible by
resorting to asymmetric complex signaling concepts. This conclusion removes the
time-varying channels assumption unnecessarily common along all the literature
on delayed CSIT.
