I. Introduction
After several years of extensive academic and industrial research, massive multiple input multiple output (mMIMO) has reached its mature stage and become one of the key technologies in fifth generation (5G) networks [1]. While 5G base stations equipped with mMIMO can provide higher levels of spectral efficiency (SE) compared to previous technologies, the inter-cell interference including pilot contamination is still present in the system and may become a bottleneck due to network densification. Inter-cell interference may particularly affect the performance of cell-edge users [2], [3], and thereby the overall system performance and fairness. To suppress inter-cell interference and provide uniformly great service to every user in the system, an alternative network infrastructure was proposed under the name of cell-free mMIMO [4]–[6].