Null Space Precoding for Downlink Network MIMO Systems with No CSI

Multicell coordination increases the cell-edge users´ throughput in cellular wireless communication if the inter-cell interference (ICI) is mitigated properly. In this paper, we use the null space concept to design precoders and decoders to remove ICI for cell-edge users in the downlink of a network MIMO system. We propose two novel methods that are able to eliminate ICI assuming no channel state information (CSI) is available at the basestations. Calculating signal-to-interference-ratio (SINR), we show that our proposed methods reaches to higher SINR in comparison to the block diagonalization (BD) with full CSI. By simulations, we show our methods outperform the BD method with full CSI in terms of BER since the SINR is higher for our proposed methods. We also show that to obtain most of sum-rate provided by coordination strategy, it is not necessary to have global coordination and having four cells coordinating with each other is enough to reach more than 85 percent of maximum achievable sum-rate.