  Massive multi-user multiple-input multiple-output (MU-MIMO) wireless systems
operating at millimeter-wave (mmWave) frequencies enable simultaneous wideband
data transmission to a large number of users. In order to reduce the complexity
of MU precoding in all-digital basestation architectures, we propose a
two-stage precoding architecture that first performs precoding using a sparse
matrix in the beamspace domain, followed by an inverse fast Fourier transform
that converts the result to the antenna domain. The sparse precoding matrix
requires a small number of multipliers and enables regular hardware
architectures, which allows the design of hardware-efficient all-digital
precoders. Simulation results demonstrate that our methods approach the
error-rate of conventional Wiener filter precoding with more than 2x reduced
complexity.
