Projection-Based Partitioned MiLAC Architecture for MU-MIMO Precoding

Microwave linear analog computers (MiLACs) are reconfigurable microwave networks capable of realizing linear transformations directly in the analog electromagnetic domain, reducing the need for repeated digital matrix operations during transmission. This paper proposes a projection-based Sub-MiLAC precoding architecture for low-complexity multiuser multiple-input multiple-output (MU-MIMO) downlink transmission. The proposed approach partitions the MiLAC into independent user-specific analog subnetworks and embeds projection operators into the MiLAC Schur-complement formulation to suppress inter-user interference. Under the ideal subnetwork-decoupling assumption \(C^{(i, j)}=0\), the resulting precoder preserves the interference-nulling structure of block diagonalization (BD) while avoiding repeated high-dimensional digital factorizations. Numerical results show that the proposed Sub-MiLAC precoder achieves spectral efficiency close to fully digital BD and improves symbol error rate performance compared with global MiLAC-designed ZFBF and regularized ZFBF precoders. These results indicate that Sub-MiLAC architectures are a promising route toward scalable, energy-efficient, and analog-assisted MU-MIMO transmission.