Successive optimization Tomlinson-Harashima precoding (SO THP) for multi-user MIMO systems

Multi-user multiple-input multiple-output (MU MIMO) systems have the advantage of combining the high capacity achievable with MIMO processing and the benefits of space division multiple access. Previously proposed techniques that use the channel state information at the transmitter to improve the performance of the downlink suffer from a capacity loss due to a zero multi-user interference (MUI) constraint or they have to allow some MUI to improve the system performance. In this paper we propose a combination of one linear precoding technique named successive optimization (SO) and a nonlinear precoding technique, Tomlinson-Harashima precoding (THP). It uses all of the subspaces available in a MU MIMO system and can completely eliminate multi-user interference. We show that SO THP provides a very high capacity and a better BER performance than similar minimum mean-square-error (MMSE) THP precoding techniques at low SNR especially for the users equipped with multiple antennas. SO THP is also less sensitive to channel estimation errors than MMSE THP precoding. Thereby, we minimize the capacity loss due to the MUI cancellation and we reduce the complexity of the receiver since there is no MUI in the system.