  The degrees of freedom (DoF) region of the two-user MIMO (multiple-input
multiple-output) interference channel is established under a new model termed
as hybrid CSIT. In this model, one transmitter has delayed channel state
information (CSI) and the other transmitter has instantaneous CSIT, of incoming
channel matrices at the respective unpaired receivers, and neither transmitter
has any knowledge of the incoming channel matrices of its respective paired
receiver. The DoF region for hybrid CSIT, and consequently that of
$2\times2\times3^{5}$ CSIT models, is completely characterized, and a new
achievable scheme based on a combination of transmit beamforming and
retrospective interference alignment is developed. Conditions are obtained on
the numbers of antennas at each of the four terminals such that the DoF region
under hybrid CSIT is equal to that under (a) global and instantaneous CSIT and
(b) global and delayed CSIT, with the remaining cases resulting in a DoF region
with hybrid CSIT that lies somewhere in between the DoF regions under the
instantaneous and delayed CSIT settings. Further synergistic benefits accruing
from switching between the two hybrid CSIT models are also explored.
