How many degrees of freedom can be achieved for mutually interfering MIMO broadcast channels?

In this paper, we consider homogeneous mutually interfering multiple-input multiple-output broadcast channels(MI-MIMO-BC). It is investigated that maximally achievable spatial degrees of freedom(DoF) for MI-MIMO-BC with constant channel coefficients. Assuming that there are two base station(BS) M transmit antennas, and each mobile stations(MS) has N receive antennas, it is shown that maximally min(M + N - 1, 2M) DoF can be achieved. We formulate zero intra- and inter-cell interference equations as function of the precoders and decoders. By checking solvability of these equations as multivariate polynomial equations, an achievable DoF upper bound can be obtained. To achieve the DoF upper bound, at least M + N - 1 MSs in each cell when M + N is an even number or M + N - 1/2 MSs when M + N is an odd number are required. Our analysis is verified by numerical results.