Downlink Cell-Free Massive MIMO With Pilot Contamination

This paper shows that performance evaluations and system designs using the use-and-then-forget (UatF) bound are inappropriate when cell-free massive multiple-input multiple-output (CF-mMIMO) systems are not in the environment with the channel hardening. This work also reveals that the pilot contamination further impairs the channel hardening and makes the issue with the UatF bound more exacerbated. Motivated by the technical problem, we introduce a new bound on the achievable rate of downlink CF-mMIMO systems. It will be demonstrated that the proposed bound allows one to design a better CF-mMIMO system via extensive performance comparisons between designed systems based on the proposed bound and the UatF bound.