  Block diagonalization (BD) is an attractive technique that transforms the
multi-user multiple-input multiple-output (MU-MIMO) channel into parallel
single-user MIMO (SU-MIMO) channels with zero inter-user interference (IUI). In
this paper, we combine the BD technique with two deterministic vector
perturbation (VP) algorithms that reduce the transmit power in MU-MIMO systems
with linear precoding. These techniques are the fixed-complexity sphere encoder
(FSE) and the QR-decomposition with M-algorithm encoder (QRDM-E). In contrast
to the conventional BD VP technique, which is based on the sphere encoder (SE),
the proposed techniques have fixed complexity and a tradeoff between
performance and complexity can be achieved by controlling the size of the set
of candidates for the perturbation vector. Simulation results and analysis
demonstrate the properness of the proposed techniques for the next generation
mobile communications systems which are latency and computational complexity
limited. In MU-MIMO system with 4 users each equipped with 2 receive antennas,
simulation results show that the proposed BD-FSE and BD-QRDM-E outperforms the
conventional BD-THP (Tomlinson Harashima precoding) by 5.5 and 7.4dB,
respectively, at a target BER of 10^{-4}.
