A null space constraint based block Tomlinson-Harashima precoding technique for the multi-user MIMO downlink

A null space constraint based block Tomlinson-Harashima preceding technique (NS-BTHP technique) is proposed for the downlink of multi-user MIMO systems. Employing non-linear pre-processing at the base station this technique pre-eliminates multi-user interference (MUI) among different mobiles. Therefore, an equivalent point-to-point MlMO channel is realized for each mobile, and the spatial interference among the multiple data streams directed to each user (self-interference) can be eliminated using signal processing algorithms developed for point-to-point MlMO systems. The proposed nonlinear preceding approach results in an improvement of the null space constraint technique (NS technique), in which linear pre-processing is used to pre-eliminate the MUI at the base station. The advantage of the NS-BTHP technique over the NS technique is explained and demonstrated by simulations.