  MIMO systems are considered as most promising for wireless communications.
However, with an increasing number of radio front ends the corresponding energy
consumption and costs become an issue, which can be relieved by the utilization
of low-resolution quantizers. In this study we propose an optimal precoding
algorithm constrained to constant envelope signals and phase quantization that
maximizes the minimum distance to the decision threshold at the receivers using
a branch-and-bound strategy. The proposed algorithm is superior to the existing
methods in terms of bit error rate. Numerical results show that the proposed
approach has significantly lower complexity than exhaustive search.
