On the degrees of freedom of MIMO X channel with delayed CSIT

The multiple-input multiple-output (MIMO) Gaussian X channel in i.i.d. fading environment and with delayed channel state information at transmitters (delayed CSIT) is considered. It is assumed that each transmitter has M antennas and each receiver has N antennas. New achievable results on the sum degrees of freedom (DoF) of this channel are provided and shown to be tight for all possible values of M and N except for 1/2 <; N/M <; 4/3. It is noteworthy that for certain values of M and N, the channel DoF coincides with the DoF of the broadcast channel obtained by assuming perfect transmitter cooperation.