Interference-free relay transmission without CSI-S for the MIMO two-hop interference channel

In this paper, we consider the multiple-input-multiple-output (MIMO) two-hop interference channels where two source-destination pairs having M antennas wish to communicate via two intermediate relays, each equipped with M antennas. For such a channel, we prove that 2M - 1 degrees of freedom (DoF) are achievable without any knowledge of the channel state information at the sources (CSI-S). The key to showing this result is a new interference-free relay transmission method, which is inspired by interference neutralization.