MMSE-Based Precoding Designs for Block Diagonalization MIMO Relay Systems with Limited Feedback

This paper focuses on linear precoding designs in multi-user multiple-input multiple-output block diagonalization (BD) relay systems with limited feedback. A base station employs spatial multiplexing transmission based on quantized channel state information (CSI) obtained from a relay. Then the relay transmits the received signal to users equipped with multiple antennas. In this paper, we propose a BD precoding scheme based on minimum mean square error (MMSE) criterion, which results in a closed-form solution. We show that the proposed scheme is robust to both noise and channel uncertainties caused by quantization errors. Also, we extend the proposed scheme to the case with CSI feedback delays. Simulation results verify the effectiveness of our proposed schemes.