  Wireless Energy Transfer (WET) is a promising solution for powering massive
Internet of Things deployments. An important question is whether the costly
Channel State Information (CSI) acquisition procedure is necessary for optimum
performance. In this paper, we shed some light into this matter by evaluating
CSI-based and CSI-free multi-antenna WET schemes in a setup with WET in the
downlink, and periodic or Poisson-traffic Wireless Information Transfer (WIT)
in the uplink. When CSI is available, we show that a maximum ratio transmission
beamformer is close to optimum whenever the farthest node experiences at least
3 dB of power attenuation more than the remaining devices. On the other hand,
although the adopted CSI-free mechanism is not capable of providing average
harvesting gains, it does provide greater WET/WIT diversity with lower energy
requirements when compared with the CSI-based scheme. Our numerical results
evidence that the CSI-free scheme performs favorably under periodic traffic
conditions, but it may be deficient in case of Poisson traffic, specially if
the setup is not optimally configured. Finally, we show the prominent
performance results when the uplink transmissions are periodic, while
highlighting the need of a minimum mean square error equalizer rather than
zero-forcing for information decoding.
