The Interference Channel with MIMO Relay: Degrees of Freedom

This paper investigates the degrees of freedom of the interference channel in the presence of a dedicated MIMO relay. The relay is used to manage the interference at the receivers. We pose a fundamental question: What benefits are achieved by exploiting the direct links from the sources to the receivers and whether a two-hop strategy suffices considering the prelog factor in the capacity formula? Using a number of hybrid encoding strategies and power allocation policies, we obtain non-asymptotic achievable sum-rates, subsequently leading to achievable degrees of freedom. The results are generalized from a two-user to a K-user network. Our main result is that only K/2 degrees of freedom are achievable in an interference channel with MIMO relay, assuming global channel knowledge at the relay but not at other nodes. Thus, appropriate signaling in a two-hop scenario captures the degrees of freedom gains without the need for the direct links. We also investigate the case where the relay (unlike other nodes) has access to abundant power, showing that when sources have power P and the relay is allowed power proportional to O(P-2), the full degrees of freedom K are available to the network.