  We characterize the total degrees of freedom (DoF) of the full-rank MIMO X
channel with arbitrary number of antennas at each node. We elucidate that the
existing outer bound is tight for any antenna configuration and provide
transmit and receive filter designs that attain this outer bound. The proposed
achievable scheme exploits channel extensions in terms of both, symbol and
asymmetric complex signaling when the communication is carried out over a
constant channel case, and is also applicable to time varying channels. The
proposed scheme represents a general framework for the derivation of the total
DoF of any two-by-two multiuser channels. Furthermore, the rank-deficient MIMO
channels case is naturally addressed, and it is shown that the total DoF of the
interference (IC) and MIMO X channels are in general superior to the full rank
MIMO case
