  Recently, the so-called cell-free Massive MIMO architecture has been
introduced, wherein a very large number of distributed access points (APs)
simultaneously and jointly serve a much smaller number of mobile stations
(MSs). A variant of the cell-free technique is the user-centric approach,
wherein each AP just decodes the MSs that it receives with the largest power.
This paper considers both the cell-free and user-centric approaches, and, using
an interplay of sequential optimization and alternating optimization, derives
downlink power-control algorithms aimed at maximizing either the minimum users'
SINR (to ensure fairness), or the system sum-rate. Numerical results show the
effectiveness of the proposed algorithms, as well as that the user-centric
approach generally outperforms the CF one.
