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The Degrees of Freedom Region of the MIMO Interference Channel with Shannon Feedback

The two-user multiple-input multiple-output (MIMO) fast-fading interference channel (IC) with an arbitrary number of antennas at each of the four terminals is studied under the settings of Shannon feedback, limited Shannon feedback, and output feedback, wherein all or certain channel matrices and outputs, or just the channel outputs, respectively, are available to the transmitters with a finite delay. While for most numbers of antennas at the four terminals, it is shown that the DoF regions with Shannon feedback and for the limited Shannon feedback settings considered here are identical, and equal to the DoF region with just delayed channel state information (CSIT), it is shown that this is not always the case. For a specific class of MIMO ICs characterized by a certain relationship between the numbers of antennas at the four nodes, the DoF regions with Shannon and the limited Shannon feedback settings, while again being identical, are strictly bigger than the DoF region with just delayed CSIT. To realize these DoF gains with Shannon or limited Shannon feedback, a new retrospective interference alignment scheme is developed wherein transmitter cooperation made possible by output feedback in addition to delayed CSIT is employed to effect a more efficient form of interference alignment than is feasible with previously known schemes that use just delayed CSIT. The DoF region for just output feedback, in which each transmitter has delayed knowledge of only the receivers' outputs, is also obtained for all but a class of MIMO ICs that satisfy one of two inequalities involving the numbers of antennas.