  In this paper, we propose a wirelessly powered Internet of Things (IoT)
system based on the cell-free massive MIMO technology. In such a system, during
the downlink phase, the sensors harvest radio-frequency (RF) energy emitted by
the distributed access points (APs). During the uplink phase, sensors transmit
data to the APs using the harvested energy. Collocated massive MIMO and
small-cell IoT can be treated as special cases of cell-free IoT. We derive the
tight closed-form lower bound on the amount of harvested energy, and the
closed-form expression of SINR as the metrics of power transfer and data
transmission, respectively. To improve the energy efficiency, we jointly
optimize the uplink and downlink power control coefficients to minimize the
total transmit energy consumption while meeting the target SINRs. Extended
simulation results show that cell-free IoT outperforms collocated massive MIMO
and small-cell IoT both in terms of the per user throughput for uplink, and the
amount of energy harvested for downlink. Moreover, significant gains can be
achieved by the proposed joint power control in terms of both per user
throughput and energy consumption.
