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Wireless Energy Transfer (WET) is a promising solution for powering massiveInternet of Things deployments. An important question is whether the costlyChannel State Information (CSI) acquisition procedure is necessary for optimumperformance. In this paper, we shed some light into this matter by evaluatingCSI-based and CSI-free multi-antenna WET schemes in a setup with WET in thedownlink, and periodic or Poisson-traffic Wireless Information Transfer (WIT)in the uplink. When CSI is available, we show that a maximum ratio transmissionbeamformer is close to optimum whenever the farthest node experiences at least3 dB of power attenuation more than the remaining devices. On the other hand,although the adopted CSI-free mechanism is not capable of providing averageharvesting gains, it does provide greater WET/WIT diversity with lower energyrequirements when compared with the CSI-based scheme. Our numerical resultsevidence that the CSI-free scheme performs favorably under periodic trafficconditions, but it may be deficient in case of Poisson traffic, specially ifthe setup is not optimally configured. Finally, we show the prominentperformance results when the uplink transmissions are periodic, whilehighlighting the need of a minimum mean square error equalizer rather thanzero-forcing for information decoding.

CSI-free vs CSI-based multi-antenna WET for massive low-power Internet of Things