  Tomlinson-Harashima precoding (THP) is a nonlinear processing technique
employed at the transmit side and is a dual to the successive interference
cancelation (SIC) detection at the receive side. Like SIC detection, the
performance of THP strongly depends on the ordering of the precoded symbols.
The optimal ordering algorithm, however, is impractical for multiuser MIMO
(MU-MIMO) systems with multiple receive antennas due to the fact that the users
are geographically distributed. In this paper, we propose a multi-branch THP
(MB-THP) scheme and algorithms that employ multiple transmit processing and
ordering strategies along with a selection scheme to mitigate interference in
MU-MIMO systems. Two types of multi-branch THP (MB-THP) structures are
proposed. The first one employs a decentralized strategy with diagonal weighted
filters at the receivers of the users and the second uses a diagonal weighted
filter at the transmitter. The MB-MMSE-THP algorithms are also derived based on
an extended system model with the aid of an LQ decomposition, which is much
simpler compared to the conventional MMSE-THP algorithms. Simulation results
show that a better bit error rate (BER) performance can be achieved by the
proposed MB-MMSE-THP precoder with a small computational complexity increase.
