The degrees of freedom region of the MIMO interference channel with delayed CSIT

The degrees of freedom (DoF) region of the 2-user MIMO interference channel (IC) is studied under fast fading and the assumption of delayed channel state information at transmitters (CSIT) wherein the receivers know all channel matrices perfectly and the transmitters acquire this knowledge with a certain delay. An outer bound to the DoF region is first derived. This bound is then shown to be tight for all possible values of the number of antennas at the four terminals. This is done by developing interference alignment based achievability schemes. A comparison of the DoF region under the delayed CSIT assumption is made with that of the idealistic perfect CSIT assumption where perfect and global channel knowledge (without delay) is available at the transmitters (and receivers) on the one hand and with the DoF region of the conservative no CSI assumption on the other, where CSI is available at the receivers but not at all at the transmitters.