  In order to minimize electric grid power consumption, energy harvesting from
ambient RF sources is considered as a promising technique for wireless charging
of low-power devices. To illustrate the design considerations of RF-based
ambient energy harvesting networks, this article first points out the primary
challenges of implementing and operating such networks, including
non-deterministic energy arrival patterns, energy harvesting mode selection,
energy-aware cooperation among base stations (BSs), etc. A brief overview of
the recent advancements and a summary of their shortcomings are then provided
to highlight existing research gaps and possible future research directions. To
this end, we investigate the feasibility of implementing RF-based ambient
energy harvesting in ultra-dense small cell networks (SCNs) and examine the
related trade-offs in terms of the energy efficiency and
signal-to-interference-plus-noise ratio (SINR) outage probability of a typical
user in the downlink. Numerical results demonstrate the significance of
deploying a mixture of on-grid small base stations (SBSs)~(powered by electric
grid) and off-grid SBSs~(powered by energy harvesting) and optimizing their
corresponding proportions as a function of the intensity of active SBSs in the
network.

Networking and Internet Architecture