  Interference alignment allows multiple users to share the same frequency and
time resource in a wireless communications system. At present, two performance
bounds, in terms of degree of freedom, have been proposed. One is for
infinite-dimension extension and the other is for MIMO systems. This paper
provides an understanding of the MIMO bound by examining its proofs and shows
that it does not apply to a more practical case: MIMO-OFDM. Several approaches
are proposed in searching for DoF bounds for systems such as finite-dimension
time extension and MIMO-OFDM systems.
