Multiuser Transmit Precoding based on Outdated Interference Alignment

The closed loop transmission schemes for multi user MIMO systems are usually sensitive to the delays in the feedback of channel state information (CSI). We present a 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 last transmission is aligned at the desired receiver with the intra-cell interference subspace of current transmission. Due to this fact, the alignment is not perfect for current transmission and causes leakage interference in the desired signal space. However, we show that with the help of proper receiver design, we can minimize this leakage interference and get substantial gains even with outdated alignment. 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.