The Degrees of Freedom Region of the MIMO Interference Network

The layered two-hop, two-unicast multi-input, multi-output (MIMO) interference network consists of two transmitters, two relays, and two receivers with the first and the second hop networks between transmitters and relays, and between relays and receivers, respectively, both being Gaussian MIMO interference channels. The degrees of freedom (DoF) region is established in the general case in which there are an arbitrary numbers of antennas at each of the six terminals. It is shown that the DoF region coincides with the min-cut outer bound for both time-varying or fixed-channel coefficients. The min-cut bound is shown to be achievable via a concatenated communication scheme that consists of linear vector space joint beamforming (that includes zero forcing and signal alignment) at all terminals as the inner precoding scheme and point-to-point coding and aligned interference neutralization as the outer precoding scheme.