## 1 Introduction  
Applications using passive RFID (Radio Frequency Identification) technology, especially in the UHF (Ultra-High-Frequency) spectrum, are encountered in many sectors of operation nowadays. It is especially the identification and localization of objects and people in indoor space [1; 2; 3; 4], logistics and inventory [5; 6; 7] or the monitoring of pipeline defects [8] in order to improve performance and efficient productivity and to increase service quality. An RFID system typically consists of a reading device, i.e., an RFID reader with a set of antennas and RFID tags that store information about a product or service. A larger number of readers and tags together form a complex system for identifying, tracking and monitoring objects bearing RFID tags. In this paper, the focus is on passive RFID communication in the UHF band 860-960 MHz, where the energy to activate the RFID tag is obtained from the reader device. Thus, the reading area depends mainly on the power radiated by the reader, the sensitivity of the tags, the orientation of the antennas and the shape of the area of operation of the system [9]. For passive UHF RFID systems, a difficult problem is to design the optimal placement of these RFID systems so that the desired area is completely covered by the signal. The tags have to be activated by the reader, and at the same time, there is no unwanted interference between the reader and the tags, which could lead to service degradation and reduced quality or error conditions. Another problem is the operation of the RFID system in the UHF band, where influences related to the environmental properties are typical and can have a strong impact on the performance of the system [10]. Furthermore, when designing the system, the parameters of the reader, antenna and tags must be taken into account, as well as the practical implementation (cable lengths, availability of electricity, etc.) [11]. Last but not least, other telecommunications technologies that can be operated in a given location in the same frequency spectrum should be considered to avoid undesired interference when designing an RFID system [11]. For the design of the RFID system, in terms of the accuracy of the results, physical measurement or on-site experimentation, which allows for obtaining accurate values of the measured parameters, appears to be suitable. From the physical measurements, we can mention, for instance, the determination of the required power (strength of the electromagnetic field) of the antennas of RFID readers by placing the tag in the space and measuring their detectability by the reader. The values of the RSSI (Received Signal Strength Indicator) parameter of the tag response at a constant power of the RFID reader can be recorded in order to determine the reading range of RFID UHF readers or antennas [11]. However, this procedure seems to be inappropriate in terms of time and money consumption. As a suitable solution, the use of simulation tools allow the placement of RFID components in 2D or 3D space. The simulation allows the implementation of various scenarios, such as the prediction of tag coverage (detectability), the effect of signal attenuation on obstacles, reflections from surfaces and others [12]. Such simulations can then help, for instance, in tuning the performance of RFID readers and thus in estimating the signal range for tag activation. The results of these simulations can have comparable predictive value to physical measurements and can thus provide a basis for physical implementation. Over the last decades, several methods have been proposed to simulate the propagation of electromagnetic waves ranging from extremely low-frequency (ELF) [13] to terahertz waves [14]. The authors of article [15] divide these methods into three categories: empirical methods, deterministic methods (full-wave simulation) and ray tracing.