Achievable DoF for 2-User MIMO Relay Interference Channels with Outdated Channel State Information

In this paper, we investigate the achievable degree of freedom (DoF) of a fully connected 2-user multiple- input multiple-output (MIMO) relay interference channel with M antennas at each transmitter and N antennas at each receiver when channel state information (CSI) is not available at transmitters and a delayed version of the CSI of direct links from the transmitters to the receivers is known at the relay. We propose a two-phase transmission scheme and obtain different sum-DoF gains (inner bound) under various configurations. We found the achievable sum-DoF values for different antenna configurations at the relay node for the MIMO relay interference channels. The results show that if the relay node has more than 2M antennas, our proposed scheme can achieve the outer bound when the instantaneously CSI is known at the relay and at the receivers. This result implies that with the help of a relay with enough antennas, the requirement for instantaneous CSI can be relaxed to the delayed CSI. Moreover, when compared to the MIMO interference channel with no relay, the presence of relay with delayed global CSI can boost the DoF performance even when there is no CSI at the transmitters.