Interference alignment with diversity for the 2 × 2 x-network with four antennas

Abstract

A transmission scheme based on the Alamouti code, which we call the Li-Jafarkhani-Jafar (LJJ) scheme, was recently proposed for the 2 × 2 X-network [i.e., two-transmitter (T_x) two-receiver X-network] with two antennas at each node. This scheme was claimed to achieve a sum degrees of freedom (DoF) of 8/3 and also a diversity gain of two when fixed finite constellations are employed at each T_x. Furthermore, each T_x. required the knowledge of only its own channel unlike the Jafar-Shamai scheme which required global CSIT to achieve the maximum possible sum DoF of 8/3. In this paper, we extend the LJJ scheme to the 2 × 2 X-network with four antennas at each node. The proposed scheme also assumes only local channel knowledge at each T_x. We prove that the proposed scheme achieves the maximum possible sum DoF of 16/3. In addition, we also prove that, using any fixed finite constellation with appropriate rotation at each T_x, the proposed scheme achieves a diversity gain of at least four.