  This work studies both limited sensitivity and nonlinearity of far field RF
energy harvesting observed in reality and quantifies their effect, attempting
to fill a major hole in the simultaneous wireless information and power
transfer (SWIPT) literature. RF harvested power is modeled as an arbitrary
nonlinear, continuous, and non-decreasing function of received power, taking
into account limited sensitivity and saturation effects. RF harvester's
sensitivity may be several dBs worse than communications receiver's
sensitivity, potentially rendering RF information signals useless for energy
harvesting purposes. Given finite number of datapoint pairs of harvested
(output) power and corresponding input power, a piecewise linear approximation
is applied and the statistics of the harvested power are offered, as a function
of the wireless channel fading statistics. Limited number of datapoints are
needed and accuracy analysis is also provided. Case studies include duty-cycled
(non-continuous), as well as continuous SWIPT, comparing with industry-level,
RF harvesting. The proposed approximation, even though simple, offers accurate
performance for all studied metrics. On the other hand, linear models or
nonlinear-unlimited sensitivity harvesting models deviate from reality,
especially in the low input power regime. The proposed methodology can be
utilized in current and future SWIPT research.
