User-Centric Communications versus Cell-free Massive MIMO for 5G Cellular Networks

"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); each AP uses local channel estimates obtained from received uplink pilots and applies conjugate beamforming to transmit data to the users. The contribution of this work is twofold. First, the paper extends the cell-free approach to the case in which both the APs and the MSs are equipped with multiple antennas, proposing a beamfoming scheme that, relying on the channel hardening effect, does not require channel estimation at the MSs. Second, the cell-free massive approach is contrasted with a user-centric approach wherein each user is served only by the APs that are closest to it. Since far APs experience a bad SINR, it turns out that they are quite unhelpful in serving far users, and so, the user-centric approach, while requiring less backhaul overhead with respect to the cell-free approach, is shown here to achieve better performance results, in terms of achievable rate-per-user, for the vast majority of the MSs in the network."