Interference alignment with diversity for the 2 × 2 X-Network with three antennas

Interference alignment is known to achieve the maximum sum DoF of 4M/3 in the 2 × 2 X-Network (i.e., two-transmitter (Tx) two-receiver (Rx) X-Network) with M antennas at each node, as demonstrated by Jafar and Shamai. Recently, an Alamouti code based transmission scheme, which we call the Li-Jafarkhani-Jafar (LJJ) scheme, was proposed for the 2×2 X-Network with two antennas at each node. This scheme achieves a sum degrees of freedom (DoF) of 8/3 and also a diversity gain of two with fixed finite constellation inputs. This work first proposes a new STBC for a three transmit antenna single user MIMO system. Building on this STBC, we extend the LJJ scheme to the 2×2 X-Network with three antennas at each node. As in the LJJ scheme and unlike the Jafar-Shamai scheme, only local channel knowledge is assumed at each Tx. It is shown that the proposed scheme achieves the maximum possible sum DoF of 4. A diversity gain of 3 is also guaranteed when fixed finite constellation inputs are used.