  In this work, we consider the uplink of a scalable cell-free massive MIMO
system where the users are served only by a subset of access points (APs) in
the network. The APs are physically grouped into predetermined "cell-centric
clusters", which are connected to different cooperative central processing
units (CPUs). Given the cooperative nature of the considered communications
network, we assume that each user is associated with a "virtual cluster", that,
in general, involves some APs belonging to different cell-centric clusters.
Assuming the maximum-ratio-combining at the APs, we propose a user-association
procedure aimed at the maximization of the sum-rate of the users in the system.
The proposed procedure is based on the Hungarian Algorithm and exploits only
the knowledge of the position of the APs in the network. Numerical results
reveal that the performance of the proposed approach is not always better than
the alternatives but it offers a considerably lower backhaul load with a
negligible performance loss compared to full-cell free approaches.
