  Cell-free massive MIMO (CF-mMIMO) provides wireless connectivity for a large
number of user equipments (UEs) using access points (APs) distributed across a
wide area with high spectral efficiency (SE). The energy efficiency (EE) of the
uplink is determined by (i) the transmit power control (TPC) algorithms, (ii)
the numbers, configurations, and locations of the APs and the UEs, and (iii)
the propagation channels between the APs and the UEs. This paper investigates
all three aspects, based on extensive (~30,000 possible AP locations and 128
possible UE locations) channel measurement data at 3.5 GHz. We compare three
different TPC algorithms, namely maximization of transmit power (max-power),
maximization of minimum SE (max-min SE), and maximization of minimum EE
(max-min EE) while guaranteeing a target SE. We also compare various antenna
arrangements including fully-distributed and semi-distributed systems, where
APs can be located on a regular grid or randomly, and the UEs can be placed in
clusters or far apart. Overall, we show that the max-min EE TPC is highly
effective in improving the uplink EE, especially when no UE within a set of
served UEs is in a bad channel condition and when the BS antennas are
fully-distributed.
