In order to generate a first-time right design, system level modeling and simulation is a major step. Thus, optimization of integrated wireless systems requires the development of behavioral models of the RF link (antenna-medium-antenna) compatible with the microelectronic design tools. Firstly, this paper shows the need of such models within the Top-Down design flow of an UHF RFID system. Then, a behavioral model of a narrow band far field RF link is presented and validated. This model is associated with a new behavioral model of an RFID tag and finally, several examples of tag-antenna matching optimization simulations are presented in order to demonstrate the interest of our approach.