Achieve the Degrees of Freedom of K-User MIMO Interference Channel with a MIMO Relay

This paper investigates the problem of achieving the degrees of freedom (DoF) of a K-user multiple-input-multipleoutput (MIMO) interference channel with a MIMO relay, where each transmitter and receiver is equipped with M antennas. We propose a two-hop relay aided transmission scheme based on the idea of interference alignment to achieve the full DoF KM/2. The scheme requires global channel state information (CSI) at the MIMO relay only while other nodes only need to have their local CSI. We also study the number of antennas needed at the MIMO relay to achieve interference alignment. In particular, we find that using just linear processing at the relay, the minimum number of antennas needed at the relay for each transmitter-receiver pair to achieve the maximum M/2 DoF per channel use is (K-1)M, less than what is needed for decode-and-forward approach and all other reported approaches in existing literatures.