Towards Optimal Energy Efficiency in Cell-Free Massive MIMO Systems

Motivated by the ever-growing demand for green
wireless communications and the advantages of cell-free (CF)
massive multiple-input multiple-output (mMIMO) systems, we
focus on the design of their downlink (DL) for optimal energy
efficiency (EE). To address this fundamental topic, we assume
that each access point (AP) is deployed with multiple antennas
and serves multiple users on the same time-frequency resource
while the APs are Poisson point process (PPP) distributed, which
approaches realistically their opportunistic spatial randomness.
Relied on tools from stochastic geometry, we derive a lower
bound on the DL average achievable spectral efficiency (SE).
Next, we consider a realistic power consumption model for CF
mMIMO systems. These steps enable the formulation of a tractable
optimization problem concerning the DL EE, which results in
the analytical determination of the optimal pilot reuse factor,
the AP density, and the number of AP antennas and users that
maximize the EE. Hence, we provide useful design guidelines for
CF mMIMO systems relating to fundamental system variables
towards optimal EE. Among the results, we observe that an
optimal pilot reuse factor and AP density exist, while larger
values result in an increase of the interference, and subsequently,
lower EE. Overall, it is shown that the CF mMIMO technology
is a promising candidate for next-generation networks achieving
simultaneously high SE and EE.