Behavioral Modeling and Simulation of Antennas :

Efficient simulation of wireless systems requires the development of antenna models compatible with microelectronic tools. This article describes a first behavioral antenna model using the VHDL-AMS language. Moreover, we present a complete behavioral RFID system model using this antenna model. Finally, several system simulation results demonstrate the interest of this approach. Introduction For many years, microelectronic market has been a constant growing market. Predicted averaged growth rate keeps on high. This market is also a very competitive one. Thus, to conquer new application fields, designers must reduce the time-tomarket. This implies generating a first-time right design. Moreover, to reduce costs and optimize performances (size, power...), systems reach very high level integration. Success of System-on-Chip (SoC) or System-in-Package (SiP), which are embedding on the same chip or package several different digital and analog cores microprocessors, memories, filters, amplifiers confirms this major trend. Furthermore, progress of microelectronic processes allows the integration of new hybrid components. Hybrid meaning that these components deal with non-electrical signals : for example, Micro-Electro-Mechanical Systems (MEMS) or RF components. Obviously, the integration of these new hybrid components increases designs complexity. Designers need new models compatible with classical microelectronic tools and adapted to microelectronic design flow. In this context of hybrid system integration, top level design verification of complete integrated Radio-Frequency (RF) wireless communication systems, involving analog and digital parts, is a major task. A hot topic concerns simulation tools for these RF systems. In fact, the coexistence of a low baseband signal and a high carrier frequency involves high simulation time. This problem is clearly addressed by RF simulation engines (Eldo-RF, Spectre-RF) specialized in non-linear system simulations involving for example mixers or switches [1]. Due to this new class of tools, efficient simulation of a complete integrated communication system is possible [2] [3] [4]. Nevertheless, there still is a lack of RF component models : few works concern RF signal propagation modeling or electromagnetic effects modeling [5]. Moreover, this deficiency particularly concerns RF antennas modeling. Of course, without antenna models, complete functional validation of wireless communication systems is made impossible. Considering this problem, this paper presents a first behavioral antenna model using VHDL-AMS, the mixed-signal IEEE 1076.1 standard modeling language [6]. In the following part, a behavioral antenna model is described. Then, in the third part, to validate this model and to demonstrate its interest, we propose a complete behavioral model of a RadioFrequency Identification (RFID) system. A RFID system is described and major RFID design issues are discussed. In the last part, several simulation results concerning these RFID issues are briefly analyzed. Behavioral modeling of an antenna A. Transmitting mode An antenna is “a means for radiating and receiving radio waves”. According to [7], in the transmitting mode, an antenna system can be represented by a Thevenin circuit equivalent as shown in Fig. 1. In this figure, the antenna is represented by an impedance ZA given by : X j R R Z A r L A + + = (1) Radiating element is symbolized by a radiation resistance Rr and an imaginary part XA. RL represents both the conduction and the dielectric losses of the antenna. The source to which the antenna is connected is represented by an ideal generator Vg having its own internal complex impedance. The radiation power delivered by the antenna is the power collected by resistance Rr and it is given by :