  We study the degrees of freedom (DoF) regions of two-user multiple-input
multiple-output (MIMO) Z and full interference channels in this paper. We
assume that the receivers always have perfect channel state information. We
first derive the DoF region of Z interference channel with channel state
information at transmitter (CSIT). For full interference channel without CSIT,
the DoF region has been fully characterized recently and it is shown that the
previously known outer bound is not achievable. In this work, we investigate
the no-CSIT case further by assuming that the transmitter has the ability of
antenna mode switching. We obtain the DoF region as a function of the number of
available antenna modes and reveal the incremental gain in DoF that each extra
antenna mode can bring. It is shown that in certain cases the reconfigurable
antennas can bring extra DoF gains. In these cases, the DoF region is maximized
when the number of modes is at least equal to the number of receive antennas at
the corresponding receiver, in which case the previously outer bound is
achieved. In all cases, we propose systematic constructions of the beamforming
and nulling matrices for achieving the DoF region. The constructions bear an
interesting space-frequency interpretation.
