text: abstract—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 cellfree 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 channelindependent 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.