Channel Hardening and Favorable Propagation in Cell-Free Massive MIMO with Stochastic Geometry

Cell-Free (CF) Massive MIMO is an alternative topology for Massive MIMO networks, where a large number of single-antenna access points (APs) are distributed over the coverage area. There are no cells but all users are jointly served by the APs using network MIMO methods. In prior work, it has been claimed that CF Massive MIMO inherits the basic properties of cellular Massive MIMO, namely channel hardening and favorable propagation. In this paper, we evaluate if one can rely on these properties when having a realistic stochastic AP deployment. Our results show that the level of channel hardening depends strongly on the propagation environment and there is generally little hardening, except when the pathloss exponent is small. We further show that using 5–10 antennas per AP, instead of one, we can greatly improve the hardening. The level of favorable propagation is affected by the propagation environment and by the distance between the users, where only spatially well separated users exhibit favorable propagation. The conclusion is that we cannot rely on channel hardening and favorable propagation when analyzing and designing CF Massive MIMO networks, but we need to use achievable rate expressions and resource allocation schemes that work well in the absence of these properties. Some options are described in this paper.