User-Centric 5G Cellular Networks: Resource Allocation and Comparison with the Cell-Free Massive MIMO Approach

TL;DR

This paper compares user-centric and cell-free massive MIMO architectures in 5G networks, proposing beamforming schemes and power allocation strategies that improve user rates and reduce backhaul overhead.

Contribution

It introduces a beamforming scheme for multi-antenna CF massive MIMO without channel estimation at MSs and demonstrates the superiority of UC approach in practical scenarios.

Findings

UC approach achieves higher per-user rates for most MSs.

UC approach requires less backhaul overhead than CF.

Proposed power strategies optimize data rate and fairness.

Abstract

Recently, the so-called cell-free (CF) 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). The paper extends the CF 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. We contrast the CF massive MIMO approach with a user-centric (UC) approach wherein each MS is served only by a limited number of APs. Since far APs experience a bad SINR, it turns out that they are quite unhelpful in serving far users, and so, the UC approach, while requiring less backhaul overhead with respect to the CF 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. Furthermore, in the paper we propose two power allocation strategy for the uplink and downlink, one aimed at maximizing the overall data-rate and another aimed at maximizing system fairness.