Instantaneous Relaying: Feasibility Conditions for Interference Neutralization

In this letter, we study the feasibility of interference neutralization (IN) for a two-user interference channel (IC) with a multi-input-multi-output (MIMO) instantaneous relay. With this channel model, Lee et al. show that instantaneous relaying is a promising approach to neutralize interference and increase the degrees of freedom (DoF). However, only the scenario where each node has the same number of antennas has been studied yet, for general antenna configuration at the relay, the maximum achievable DoF remains unknown. Assuming linear transceivers with an amplify-and-forward (AF) relay, we first present the necessary condition for generalized IN. Then, we derive the necessary and sufficient conditions for pure IN and coordinated IN. The derived sufficient conditions give rise to the minimum relay antenna configuration to support interference-free transmission. Our proof throws lights on how to use the relay resources to neutralize the interference efficiently.