Improved Tomlinson-Harashima precoding for the downlink of multiple antenna multi-user systems [mobile radio applications]

In this paper, the Tomlinson-Harashima precoding (THP) algorithm is studied for use on the downlink of multiple antenna multi-user systems. Based on the minimum mean-square error (MMSE) criterion, a new THP solution is derived, which provides significant performance improvement as compared to the zero-forcing (ZF) THP discussed in the literature. Ordering of the rows of the channel matrix is found to be important to the THP system performance. We reveal how this ordering reflects on the changing of system structure and prove the "best-first" ordering method is optimal for the ZF THP (or MMSE THP) in the minimax noise (or error) variance sense. Simulation results are used to show the performance advantage attained by the MMSE THP and the optimal ordering.