We show that the 3 user MT × MR MIMO interference channel where each transmitter is equipped with MT and each receiver is equipped with MR antennas has min (M/2−1/k, N/2+1/k) degrees of freedom (DoF) per user normalized by time, frequency, and space dimensions, where N = max(MT, MR), M = min(MT, MR), k = [M/N−M]. While the information theoretic DoF outer bound is established for every M, N value, the achievability, relying only on linear interference alignment, is established in general subject to a normalization with respect to spatial-extensions, i.e., the scaling of the number of antennas at all nodes. In the absence of spatial extensions, we can also show through examples how essentially the same alignment scheme may be applied over time or frequency extensions. The central new insight to emerge from this work is the notion of subspace alignment chains as DoF bottlenecks. The subspace alignment chains are instrumental both in identifying the extra dimensions provided by a genie to a receiver for the DoF outer bound, as well as constructing the optimal interference alignment schemes. In addition, our results also settle the question of feasibility of linear interference alignment for the 3 user MT × MR MIMO interference channel, for all values of MT, MR.
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