On the Optimality of "Half the Cake" for K-User Rank-Deficient Mk x Mk Interference Channel

By introducing a novel outer bound, we find Σ_k=1^k M_k/2 degrees of freedom (half the cake per user) for a K-user multiple-inputmultiple-output (MIMO) interference channel (IC) where the cross-channels have arbitrary rank constraints, and the k^th transmitter and receiver are equipped with M_k antennas each. The result consolidates and significantly generalizes results from prior studies by Krishnamurthy et al., of rank-deficient interference channels where all users have M antennas; and by Tang et al., of full rank interference channels where the k^th user pair has M_k antennas.