  In a massive MIMO system with large arrays, the channel becomes spatially
non-stationary. We study the impact of spatial non-stationarity characterized
by visibility regions (VRs) where the channel energy is significant on a
portion of the array. Relying on a channel model based on VRs, we provide
expressions of the signal-to-interference-plus-noise ratio (SINR) of conjugate
beamforming (CB) and zero-forcing (ZF) precoders. We also provide an
approximate deterministic equivalent of the SINR of ZF precoders. We identify
favorable and unfavorable multi-user configurations of the VRs and compare the
performance of both stationary and non-stationary channels through analysis and
numerical simulations.
