Wireless charging through directed radio frequency
(RF) waves is an emerging technology that can be used to replenish
the battery of a sensor node, albeit at the cost of data communication
in the network. This tradeoff between energy transfer and
communication functions requires a fresh perspective on medium
access control (MAC) protocol design for appropriately sharing
the channel. Through an experimental study, we demonstrate how
the placement, the chosen frequency, and number of the RF energy
transmitters impact the sensor charging time. These studies are
then used to design aMAC protocol called RF-MAC that optimizes
energy delivery to sensor nodes, while minimizing disruption to
data communication. In the course of the protocol design, we
describe mechanisms for (i) setting the maximum energy charging
threshold, (ii) selecting specific transmitters based on the collective
impact on charging time, (iii) requesting and granting energy
transfer requests, and (iv) evaluating the respective priorities of
data communication and energy transfer. To the best of our knowledge,
this is the first distributed MAC protocol for RF energy
harvesting sensors, and through a combination of experimentation
and simulation studies, we observe 300% maximum network
throughput improvement over the classical modified unslotted
CSMA MAC protocol.