Inter-Relay Interference Mitigation for AF-based Successive Relay Systems

The achievable performance of successive relay system will significantly degrade, if severe inter-relay interference (IRI) exist. In this paper, two beamforming methods are proposed for mitigating the IRI inflicted by multiple-input multiple-output (MIMO) assisted two-path successive relay nodes. More specifically, we exploit the additional degree of freedom offered by the spatial domain during the alternate transmission process across consecutive time slots in the relay system. The first method aims to maximize the source-to-destination signal-to-interference-plus-noise ratios (SINR) of separate transmission paths with the aid of a minimum variance (MV) beamformer under the high SINR condition. The second method constitutes a near-optimum beamformer design attempts to minimize the sum of mean squared errors (MSEs) at the destination node by exploiting the mutual coupling of SINRs received from the different transmission paths. Numerical results prove the benefits of our proposed beamforming schemes, especially under the presence of high IRI.