Multiuser transmit precoding based on outdated interference alignment

Downlink closed loop transmission schemes for multi user MIMO systems are usually sensitive to delays in the feedback of channel state information (CSI). We present a novel and simple transmit precoding scheme which exploits the delayed interference state information (INTSI) and the concept of interference alignment (IA). We design the precoding such that the inter-cell interference subspace from the previous transmission is aligned at the desired receiver with the intra-cell interference subspace of the current transmission. Although the alignment is not perfect for the current transmission, we show that with the help of proper receiver design, one can minimize the leakage interference caused by outdated information and still achieve good performance. We evaluate our scheme with the help of extensive system level simulations and provide a comparison with state of the art MU-MIMO transmit precoding schemes in terms of mean user rates. The simulation results show that our proposed scheme provides considerable gains over the other transmission schemes.