  In this letter, we propose a practical non-linear energy harvesting model and
design a resource allocation algorithm for simultaneous wireless information
and power transfer (SWIPT) systems. The algorithm design is formulated as a
non-convex optimization problem for the maximization of the total harvested
power at energy harvesting receivers subject to minimum required
signal-to-interference-plus-noise ratios (SINRs) at multiple information
receivers. We transform the considered non-convex objective function from
sum-of-ratios form into an equivalent objective function in subtractive form,
which enables the derivation of an efficient iterative resource allocation
algorithm. In each iteration, a rank-constrained semidefinite program (SDP) is
solved optimally by SDP relaxation. Numerical results unveil a substantial
performance gain that can be achieved if the resource allocation design is
based on the proposed non-linear energy harvesting model instead of the
traditional linear model.
