  Multiple-antenna systems is a key technique to serve multiple users in future
wireless systems. For low energy consumption and hardware complexity we first
consider transmit symbols with constant magnitude and then 1-bit
digital-to-analog converters. We propose precoding designs which maximize the
minimum distance to the decision threshold at the receiver. The precoding
design with 1-bit DAC corresponds to a discrete optimization problem, which we
solve exactly with a branch-and-bound strategy. We alternatively present an
approximation based on relaxation. Our results show that the proposed
branch-and-bound approach has polynomial complexity. The proposed methods
outperform existing precoding methods with 1-bit DAC in terms of uncoded bit
error rate and sum-rate. The performance loss in comparison to infinite DAC
resolution is small.
