  Cell-free massive MIMO is a variant of multiuser MIMO and massive MIMO, in
which the total number of antennas $LM$ is distributed among the $L$ remote
radio units (RUs) in the system, enabling macrodiversity and joint processing.
Due to pilot contamination and system scalability, each RU can only serve a
limited number of users. Obtaining the optimal number of users simultaneously
served on one resource block (RB) by the $L$ RUs regarding the sum spectral
efficiency (SE) is not a simple challenge though, as many of the system
parameters are intertwined. For example, the dimension $\tau_p$ of orthogonal
Demodulation Reference Signal (DMRS) pilots limits the number of users that an
RU can serve. Thus, depending on $\tau_p$, the optimal user load yielding the
maximum sum SE will vary. Another key parameter is the users' uplink transmit
power $P^{\rm ue}_{\rm tx}$, where a trade-off between users in outage,
interference and energy inefficiency exists. We study the effect of multiple
parameters in cell-free massive MIMO on the sum SE and user outage, as well as
the performance of different levels of RU antenna distribution. We provide
extensive numerical investigations to illuminate the behavior of the system SE
with respect to the various parameters, including the effect of the system
load, i.e., the number of active users to be served on any RB. The results show
that in general a system with many RUs and few RU antennas yields the largest
sum SE, where the benefits of distributed antennas reduce in very dense
networks.
