  In a cell-free massive MIMO architecture a very large number of distributed
access points simultaneously and jointly serves a much smaller number of mobile
stations; a variant of the cell-free technique is the user-centric approach,
wherein each access point just serves a reduced set of mobile stations. This
paper introduces and analyzes the cell-free and user-centric architectures at
millimeter wave frequencies, considering a training-based channel estimation
phase, and the downlink and uplink data transmission phases. First of all, a
multiuser clustered millimeter wave channel model is introduced in order to
account for the correlation among the channels of nearby users; second, an
uplink multiuser channel estimation scheme is described along with
low-complexity hybrid analog/digital beamforming architectures. Third, the
non-convex problem of power allocation for downlink global energy efficiency
maximization is addressed. Interestingly, in the proposed schemes no channel
estimation is needed at the mobile stations, and the beamforming schemes used
at the mobile stations are channel-independent and have a very simple
structure. Numerical results show the benefits granted by the power control
procedure, that the considered architectures are effective, and permit
assessing the loss incurred by the use of the hybrid beamformers and by the
channel estimation errors.
