  It is expected that the number of wireless devices will grow rapidly over the
next few years due to the growing proliferation of Internet-of-Things (IoT). In
order to improve the energy efficiency of information transfer between small
devices, we review state-of-the-art research in simultaneous wireless energy
and information transfer, especially for relay based IoT systems. In
particular, we analyze simultaneous information-and-energy transfer from the
source node, and the design of time-switching and power-splitting operation
modes, as well as the associated optimization algorithms. We also investigate
the potential of crowd energy harvesting from transmission nodes that belong to
multiple radio networks. The combination of source and crowd energy harvesting
can greatly reduce the use of battery power and increase the availability and
reliability for relaying. We provide insight into the fundamental limits of
crowd energy harvesting reliability based on a case study using real city data.
Furthermore, we examine the optimization of transmissions in crowd harvesting,
especially with the use of node collaboration while guaranteeing
Quality-of-Service (QoS).
