  In order to gain new insights into MIMO interference networks, the optimality
of $\sum_{k=1}^K M_k/2$ (half the cake per user) degrees of freedom is explored
for a $K$-user multiple-input-multiple-output (MIMO) interference channel 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. The broader outcome of this work is
a novel class of replication-based outer bounds for arbitrary rank-constrained
MIMO interference networks where replicas of existing users are added as
auxiliary users and the network connectivity is chosen to ensure that any
achievable scheme for the original network also works in the new network. The
replicated network creates a new perspective of the problem, so that even
simple arguments such as user cooperation become quite powerful when applied in
the replicated network, giving rise to stronger outer bounds, than when applied
directly in the original network. Remarkably, the replication based bounds are
broadly applicable not only to MIMO interference channels with arbitrary
rank-constraints, but much more broadly, even beyond Gaussian settings.
