An efficient method for combining multi-user MIMO tomlinson-harashima precoding with user selection based on spatial orthogonality

Combining multi-user multiple-input and multiple-output (MU-MIMO) Tomlinson-Harashima precoding (THP) with semiorthogonal user selection (SUS) yields both space diversity and multi-user diversity benefits; however, it entails a significant computational effort. To overcome this limitation while retaining system-level performance, this paper proposes an efficient method for combining MU-MIMO THP with SUS. The proposed method is inspired by the fact that the ordering calculations in both THP and SUS are generally based on spatial orthogonality. Particularly, the ordering result of SUS is fully reused for the ordering in THP, thereby eliminating the ordering calculation in the latter. Through a system-level performance evaluation, the proposed method is compared to a traditional method with the ordering process, thereby demonstrating the effectiveness of the proposed method. Numerical results obtained via computer simulations show that although the proposed method achieves almost the same system capacity as the traditional MU-MIMO THP method with ordering, the computational complexity of the proposed method decreases substantially with an increase in the number of antennas.