  In a cell-free (CF) massive MIMO architecture a very large number of
distributed access points (APs) simultaneously and jointly serves a much
smaller number of mobile stations (MSs); a variant of the cell-free technique
is the user-centric (UC) approach, wherein each AP just decodes a reduced set
of MSs, practically the ones that are received best. This paper introduces and
analyzes the CF and UC architectures at millimeter wave (mmWave) frequencies.
First of all, a multiuser clustered channel model is introduced in order to
account for the correlation among the channels of nearby users; then, an uplink
multiuser channel estimation scheme is described along with low-complexity
hybrid analog/digital beamforming architectures. Interestingly, in the proposed
scheme no channel estimation is needed at the MSs, and the beamforming schemes
used at the MSs are channel-independent and have a very simple structure.
Numerical results show that the considered architectures provide good
performance, especially in lightly loaded systems, with the UC approach
outperforming the CF one.
