Space-orthogonal Scheme for IRSs-aided Multi-user MIMO in mmWave/THz Communications

The sum-rate maximization for intelligent reflecting surfaces (IRS)-aided multi-user MIMO is a recent open problem. The challenge lies in the coefficient designs for reflecting phase shifts (at the IRS) and precoder/decoders (at the BS/users). By imposing two additional constraints, i.e., 1) each IRS only serves one user, 2) no interference exists between users, this paper proposes a novel space-orthogonal scheme for multiple IRSs-aided multi-user MIMO in millimeter wave (mmWave) and terahertz (THz) communications. Based on a new zero-interference criterion, we can successively find high-quality solutions for the IRSs' phase shifts and precoder/decoders one by one. Specifically, we first propose a null-space singular value decomposition (SVD) approach to determine a part of the precoder/decoders. Then, two solutions are developed for IRSs' phase shifts, namely, the segment matching (SM) and the phase iterative evolution (PIE) solutions. Finally, the remanent part of the precoder/decoders are calculated by SVD with water-filling under the zero-interference constraint. Numerical results demonstrate the effectiveness and superiority of our proposed scheme.