Coordinated Transmission of Interference Mitigation and Power Allocation in Two-User Two-Hop MIMO Relay Systems

This paper considers coordinated transmission for interference mitigation and power allocation in a correlated two-user two-hop multi-input multioutput (MIMO) relay system. The proposed transmission scheme utilizes statistical channel state information (CSI) (e.g., transmit correlation) to minimize the cochannel interference (CCI) caused by the relay. To this end, it is shown that the CCI can be represented in terms of the eigenvalues and the angle difference between the eigenvectors of the transmit correlation matrix of the intended and CCI channel, and that the condition minimizing the CCI can be characterized by the correlation amplitude and the phase difference between the transmit correlation coefficients of these channels. Then, a coordinated user-scheduling strategy is designed with the use of eigen-beamforming to minimize the CCI in an average sense. The transmit power of the base station and relay is optimized under separate power constraint. Analytic and numerical results show that the proposed scheme can maximize the achievable sum rate when the principal eigenvectors of the transmit correlation matrix of the intended and the CCI channel are orthogonal to each other, yielding a sum rate performance comparable to that of the minimum mean-square error-based coordinated beamforming which uses instantaneous CSI.