  Ubiquitous cell-free massive MIMO (multiple-input multiple-output) combines
massive MIMO technology and user-centric transmission in a distributed
architecture. All the access points (APs) in the network cooperate to jointly
and coherently serve a smaller number of users in the same time-frequency
resource. However, this coordination needs significant amounts of control
signalling which introduces additional overhead, while data co-processing
increases the back/front-haul requirements. Hence, the notion that the "whole
world" could constitute one network, and that all APs would act as a single
base station, is not scalable. In this study, we address some system
scalability aspects of cell-free massive MIMO that have been neglected in
literature until now. In particular, we propose and evaluate a solution related
to data processing, network topology and power control. Results indicate that
our proposed framework achieves full scalability at the cost of a modest
performance loss compared to the canonical form of cell-free massive MIMO.
