  Wireless rechargeable sensor networks (WRSNs) consisting of sensor nodes with
batteries have been at the forefront of sensing and communication technologies
in the last few years. Sensor networks with different missions are being
massively rolled out, particularly in the internet-of-things commercial market.
To ensure sustainable operation of WRSNs, charging in a timely fashion is very
important, since lack of energy of even a single sensor node could result in
serious outcomes. With the large number of WRSNs existing and to be existed,
energy-efficient charging schemes are becoming indispensable to workplaces that
demand a proper level of operating cost. Selection of charging scheme depends
on network parameters such as the distribution pattern of sensor nodes, the
mobility of the charger, and the availability of the directional antenna. Among
current charging techniques, radio frequency (RF) remote charging with a small
transmit antenna is gaining interest when non-contact type charging is required
for sensor nodes. RF charging is particularly useful when sensor nodes are
distributed in the service area. To obtain higher charging efficiency with RF
charging, optimal path planning for mobile chargers, and the beamforming
technique, implemented by making use of a directional antenna, can be
considered. In this article, we present a review of RF charging for WRSNs from
the perspectives of charging by mobile charger, harvesting using sensor nodes,
and energy trading between sensor nodes. The concept of a pivot cluster head is
introduced and a novel RF charging scheme in two stages, consisting of charging
pivot cluster heads by a mobile charger with a directional antenna and charging
member sensor nodes by pivot cluster heads with directional antennae, is
presented.
