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

TL;DR

This paper investigates the presence of channel hardening and favorable propagation in realistic stochastic deployments of Cell-Free Massive MIMO, finding these properties are less reliable than previously assumed, especially with single antennas per AP.

Contribution

It provides a realistic evaluation of channel hardening and favorable propagation in CF Massive MIMO with stochastic AP deployment, challenging prior assumptions.

Findings

Channel hardening is limited to specific conditions like small pathloss exponent.

Using 5-10 antennas per AP improves channel hardening.

Favorable propagation occurs mainly among well-separated users, less generally.

Abstract

Cell-Free (CF) Massive MIMO is an alternative topology for future wireless 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. Prior works have 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 channel hardening only appears in special cases, for example, when the pathloss exponent is small. However, by using 5--10 antennas per AP, instead of one, we can substantially improve the hardening. Only spatially well-separated users will exhibit favorable propagation, but when adding more antennas and/or reducing the pathloss exponent, it becomes more likely for favorable propagation to occur. 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 also in the absence of these properties. Some options are reviewed in this paper.