On the Degrees-of-Freedom for Relay-Aided MIMO Interference Channels With Partial and Delayed CSI

Abstract

This letter shows that a multi-antenna relay using only partial and delayed channel state information (CSI) helps mitigate interference effects for multi-input multi-output interference channels when no CSI is available at transmitters. To this end, we present a novel relay-aided interference alignment, which comprises of two phases: 1) side-information learning and 2) overheard equation swapping via a relay. Leveraging the proposed scheme, we show that the achievable sum-degrees of freedom (sum-DoF) is equal to 2min(R, M)N/(min(R, M) + N) if min(R, M) > N where M, N, and R are the number of antennas at transmitter, receiver, and relay, respectively. One major implication of this result is that using the outdated CSI at the relay is sufficient to attain the optimal sum-DoF achieved with perfect CSI knowledge, provided that the number of antennas at the relay is larger than that of transmit and receive antennas.