  Radio-frequency wireless energy transfer (WET) is a promising technology to
realize wireless-powered underground sensor networks (WPUSNs) and enable
sustainable underground monitoring. However, due to the severe attenuation in
harsh underground soil and the tight energy budget of the underground sensors,
traditional WPUSNs relying on the channel state information (CSI) are highly
inefficient, especially in massive WET scenarios. To address this challenge, we
comparatively assess the feasibility of several state-of-the-art CSI-free
multi-antenna WET schemes for WPUSNs, under a given power budget. Moreover, to
overcome the extremely low WET efficiency in underground channels, we propose a
distributed CSI-free system, where multiple power beacons (PBs) simultaneously
charge a large set of underground sensors without any CSI. We consider the
position-aware K-Means and the position-agnostic equally-far-from-center (EFFC)
approaches for the optimal deployment of the PBs. Our results evince that the
performance of the proposed distributed CSI-free system can approach or even
surpass that of a traditional full-CSI WET strategy, especially when adopting
an appropriate CSI-free scheme, applying the advisable PBs deployment approach,
and equipping the PBs with an appropriate number of antennas. Finally, we
discuss the impact of underground parameters, i.e., the burial depth of devices
and the volumetric water content of soil, on the system's performance, and
identify potential challenges and research opportunities for practical
distributed CSI-free WPUSNs deployment.
