Interference Alignment With Asymmetric Complex Signaling on MIMO X Channels

It is known that with conventional spatial interference alignment, i.e., linear interference alignment without symbol extension, the achievable Degrees of freedom (DoF) of MIMO X channels is no more than ⌊D_outer⌋, where D_outer denotes the outer bound of DoF of MIMO X Channels. In this paper, we propose a spatial interference alignment and zero-forcing framework in combination with asymmetric complex signaling, which can achieve the DoF of 1/2⌊2D_outer⌋ for arbitrary number of antennas on each terminal. Note that 1/2⌊2D_outer⌋ is equal to or larger than ⌊D_outer⌋. The result shows that the technique of asymmetric complex signaling, which was originally proposed for single-antenna systems, can be useful for MIMO channels, in particular when symbol extensions are not allowed.