  Wireless networks can be self-sustaining by harvesting energy from
radio-frequency (RF) signals. Building on classic cognitive radio networks, we
propose a novel method for network coexisting where mobiles from a secondary
network, called secondary transmitters (STs), either harvest energy from
transmissions by nearby transmitters from a primary network, called primary
transmitters (PTs), or transmit information if PTs are sufficiently far away;
STs store harvested energy in rechargeable batteries with finite capacity and
use all available energy for subsequent transmission when batteries are fully
charged. In this model, each PT is centered at a guard zone and a harvesting
zone that are disks with given radiuses; a ST harvests energy if it lies in
some harvesting zone, transmits fixed-power signals if it is outside all guard
zones or else idles. Based on this model, the spatial throughput of the
secondary network is maximized using a stochastic-geometry model where PTs and
STs are modeled as independent homogeneous Poisson point processes (HPPPs),
under the outage constraints for coexisting networks and obtained in a simple
closed-form. It is observed from the result that the maximum secondary
throughput decreases linearly with the growing PT density, and the optimal ST
density is inversely proportional to the derived transmission probability for
STs.
