Self-Interference Cancellation for MIMO Full-Duplex Downlink Systems: A Constrained MMSE-Based Precoding Approach

This paper introduces a new spatial domain-based self-interference cancellation (SIC) precoding method named constrained minimum mean square error (C-MMSE) for a undersized massive multiple-input multiple-output (mMIMO) full-duplex (FD) system, where the FD receiver (RX) only receives a single signal of interest (SOI). To tackle this, the main idea is to translate the commonly used singular value decomposition (SVD)-based null-space projection approach, which is unfeasible in our considered system model, into an optimization problem under MMSE criterion, where additional constraints are introduced to perform SIC. Theoretical derivation of the C-MMSE precoder is presented, followed by performance comparison with conventional MMSE precoding, where no constraints are added for SIC. We theoretically show that the C-MMSE scheme outperforms the conventional one in terms of SIC, and allows the FD system to work under an almost interference-free environment. Additionally, we also assess the performance of the proposed method under imperfect channel state information (CSI), to further evaluate the robustness of our spatial precoder in more realistic conditions. We show that the C-MMSE precoder outperforms MMSE in terms of interference suppression ratio (ISR), even under CSI imperfection. Additionally, the C-MMSE achieves the same spectral efficiency (SE) as a hypothetical perfect SIC in a wide range of SNRs.