MMSE Approaches to Multiuser Spatio-Temporal Tomlinson-Harashima Precoding

We investigate Tomlinson-Harashima precoding (THP) for multiuser systems. Since we aim at simple receivers, the feedforward and feedback filters are located at the transmitter. The filters are derived based on minimum mean square error (MMSE) criteria, with (zero-forcing THP) and without (Wiener THP) additional constraints. For flat fading and frequency selective scenarios, we modify the well known linear transmit filter optimizations to yield the optimizations for the respective THP approaches. The resulting feedback filters directly depend on the feedforward filters which are computed successively. Inspired by this observation, we include a suboptimum ordering strategy for spatial THP similar to vertical Bell Laboratories space time (V-BLAST) at the receiver. Moreover, we incorporate a suboptimum latency time optimization for temporal THP. Simulation results reveal that THP is particularly advantageous in conjunction with higher-order modulation schemes and for high signal to noise ratio (SNR).