Can We Rely on Channel Hardening in Cell-Free Massive MIMO?

Channel hardening makes fading multi-antenna channels behave as deterministic when there are many antennas. Massive MIMO systems utilize this phenomenon to deliver high and reliable performance from cellular access points. Recently, an alternative form of Massive MIMO has appeared: Cell-Free (CF) Massive MIMO. It is based on having many access points (APs) distributed over a large geographical area and these jointly serve all the users. Since the AP antennas are distributed, instead of co-located, it is not clear if these systems will inherit the channel hardening. In this paper, we use stochastic geometry to investigate this problem. Our results show that the amount of channel hardening is strongly affected by the number of antennas per AP and the propagation environment. To achieve channel hardening in CF Massive MIMO, it is beneficial to have multiple antennas per AP and a small path-loss exponent.