Degrees of Freedom Regions of Two-User MIMO Z and Full Interference Channels: The Benefit of Reconfigurable Antennas

Abstract: We study the degrees of freedom (DoF) regions of two-user multiple-input multiple-output (MIMO) Z and full interference channels in this paper. We assume that the receivers always have perfect channel state information. We first derive the DoF region of Z interference channel with channel state information at transmitter (CSIT). For full interference channel without CSIT, the DoF region has been fully characterized recently and it is shown that the previously known outer bound is not achievable. In this work, we investigate the no-CSIT case further by assuming that the transmitter has the ability of antenna mode switching. We obtain the DoF region as a function of the number of available antenna modes and reveal the incremental gain in DoF that each extra antenna mode can bring. It is shown that in certain cases the reconfigurable antennas can bring extra DoF gains. In these cases, the DoF region is maximized when the number of modes is at least equal to the number of receive antennas at the corresponding receiver, in which case the previously outer bound is achieved. In all cases, we propose systematic constructions of the beamforming and nulling matrices for achieving the DoF region. The constructions bear an interesting space-frequency interpretation.