On the degrees of freedom of the 3-user rank-deficient MIMO interference channels

We study the achievable degrees of freedom (DoF) of the 3-user multiple-input multiple-output (MIMO) interference channel (IC), with possibly rank-deficient channel matrices. A two-layered linear processing scheme is proposed, with the inner layer zero-forcing part of the interfering links by applying a technique known as change of basis, and the outer layer performing interference alignment for the remaining interfering links. The achievable DoF of the proposed scheme is derived. In many special cases of the problem considered in this paper, our proposed two-layered scheme is DoF optimal.