  Power consumption of multi-user (MU) precoding is a major concern in
all-digital massive MU multiple-input multiple-output (MIMO) base-stations with
hundreds of antenna elements operating at millimeter-wave (mmWave) frequencies.
We propose to replace part of the linear Wiener filter (WF) precoding matrix by
a finite-alphabet WF precoding (FAWP) matrix, which enables the use of
low-precision hardware that consumes low power and area. To minimize the
performance loss of our approach, we present methods that efficiently compute
FAWP matrices that best mimic the WF precoder. Our results show that FAWP
matrices approach infinite-precision error-rate and error-vector magnitude
performance with only 3-bit precoding weights, even when operating in realistic
mmWave channels. Hence, FAWP is a promising approach to substantially reduce
power consumption and silicon area in all-digital mmWave massive MU-MIMO
systems.
