  Massive multiple-input multiple-output (MIMO) is envisioned to offer
considerable capacity improvement, but at the cost of high complexity of the
hardware. In this paper, we propose a low-complexity hybrid precoding scheme to
approach the performance of the traditional baseband zero-forcing (ZF)
precoding (referred to as full-complexity ZF), which is considered a virtually
optimal linear precoding scheme in massive MIMO systems. The proposed hybrid
precoding scheme, named phased-ZF (PZF), essentially applies phase-only control
at the RF domain and then performs a low-dimensional baseband ZF precoding
based on the effective channel seen from baseband. Heavily quantized RF phase
control up to $2$ bits of precision is also considered and shown to incur very
limited degradation. The proposed scheme is simulated in both ideal Rayleigh
fading channels and sparsely scattered millimeter wave (mmWave) channels, both
achieving highly desirable performance.
