By introducing a novel outer bound, we find a sufficient condition for optimality of ∑_(k=1)~K M_k/2 degrees of freedom (half the cake per user) for a K-user multiple-input-multiple-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.
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