In this paper, the authors are proposing a way to maximize the DC power collected in the case of a wireless power transfer (WPT) scenario. Three main aspects are taken into account: the RF (radio frequency) source, the propagation channel and the rectifier as the main part of the energy collecting circuit. This problem is formulated as a convex optimization one. Then, as a first step towards solving this problem, a rectifier circuit was simulated by using Keysight's ADS software and, by using a classical model identification strategy i. e. Vector Fitting (VF) algorithm, the state-space model of the passive parts of this rectifier were extracted. In order to verify the extracted model, S 11 input reflection coefficients and DC output voltages of the original circuit and the state-space model are compared.