In this talk, we analyze the degrees of freedom (DoF) for K-user semi-symmetric
MxN MIMO interference channels without time or frequency extension, where the
DoF of each user may be different. The properness
and the information theoretic outer bounds of interference channels provide a
DoF upperbound for interference alignment, which is tighter than the existing
results. Simulations with the Gomadam's iterative algorithm show that the
proposed upperbound is actually feasible most of the time, and it is
conjectured
that the derived DoF is the optimal DoF.