On Degrees of Freedom Region of Three-User MIMO Interference Channels

Degrees of freedom (DoF) region of three-user interference channels is studied in this paper. The outer-bound of sum DoF of three-user interference channels was derived based on subspace alignment chain, but the complete DoF region remains unknown. In this paper, an outer-bound of DoF region of three-user interference channels is first derived. Then, a linear interference alignment scheme based on spatial beamforming is proposed that can achieve all integer DoF inside the region. As a result, the region can be seen as both the sufficient and necessary condition for the feasibility of linear interference alignment based on spatial beamforming in three-user interference channels. Moreover, the achievability of fractional DoF inside the outer bound DoF region is also addressed with the proposed beamforming scheme in combination with symbol extension.