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Cell-free massive MIMO is a variant of multiuser MIMO and massive MIMO, inwhich the total number of antennas $LM$ is distributed among the $L$ remoteradio units (RUs) in the system, enabling macrodiversity and joint processing.Due to pilot contamination and system scalability, each RU can only serve alimited number of users. Obtaining the optimal number of users simultaneouslyserved on one resource block (RB) by the $L$ RUs regarding the sum spectralefficiency (SE) is not a simple challenge though, as many of the systemparameters are intertwined. For example, the dimension $\tau_p$ of orthogonalDemodulation Reference Signal (DMRS) pilots limits the number of users that anRU can serve. Thus, depending on $\tau_p$, the optimal user load yielding themaximum sum SE will vary. Another key parameter is the users' uplink transmitpower $P^{\rm ue}_{\rm tx}$, where a trade-off between users in outage,interference and energy inefficiency exists. We study the effect of multipleparameters in cell-free massive MIMO on the sum SE and user outage, as well asthe performance of different levels of RU antenna distribution. We provideextensive numerical investigations to illuminate the behavior of the system SEwith respect to the various parameters, including the effect of the systemload, i.e., the number of active users to be served on any RB. The results showthat in general a system with many RUs and few RU antennas yields the largestsum SE, where the benefits of distributed antennas reduce in very densenetworks.

Optimal User Load and Energy Efficiency in User-Centric Cell-Free Wireless Networks