Joint Power Allocation and Constellation Rotation in Alamouti coded Interference Alignment in X-Channel with imperfect CSI

Interference Alignment (IA) with Alamouti coding is an innovative method to increase the multiplexing gain and the diversity gain, simultaneously, in wireless Multiple-Input Multiple-Output (MIMO) systems. The main limitation of this method is requiring Perfect Channel State Information (CSI) to perform the beamforming on transmitters and receivers signals. Contrary to the acceptable performance of Alamouti coding in perfect CSI conditions, its efficiency is severely reduced in imperfect CSI conditions. In order to overcome this shortcoming, this paper discusses about the effect of imperfect CSI on a two user MIMO X-channel which uses interference alignment together with Alamouti coding. Also, a closed form solution is extracted for the received signal in an imperfect CSI scenario. Our analysis shows that when the variance of the CSI error is increased, the Bit Error Rate (BER) of system increases linearly. Also, we propose a joint Power Allocation (PA) and constellation rotation algorithm to improve the performance of system in an imperfect CSI scenario. Computer simulations show a desirable improvement in BER by using PA, constellation rotation and joint of them.