## 2 State of the Art
The type of reader in question, in addition to its main function of detecting RFID tags and communicating with them in its interrogation zone, checks that it has a region, or diameter, where such communication can be established between the reader and the tag. It is also responsible for activating, powering up (if necessary), and structuring the communication sequence with the tag to establish the data transfer operation. The RFID tag is the simplest component of the system, which can take on various shapes and sizes. Its purpose is to identify the item or object on which it is placed using its unique ID. It consists of an integrated circuit and an antenna, which are physically connected to its chip. The integrated circuit is the essential component of the tag, as it is responsible for transmitting information when it is activated [17]. RFID tags can be categorised according to their power and communication capacity. On the one hand, a passive type of tag has no built-in power, using its incident radio wave as a modulated backscatter signal (a technique commonly used in RFID systems in which incompatible loads are used to reflect the incident radio-frequency signal back with amplitude and/or phase modulation) and needs to be within range of an antenna to be operational. On the other hand, an active-type tag has built-in power that can transmit signals and provide a higher degree of localisation accuracy, but it requires power sources and is, therefore, more expensive to install and maintain [18]. For product traceability, it may in many cases only be necessary to know when a product or asset has entered and left specific areas (in the context of industrial production processes), something that can be achieved using passive RFID tags on products and a reader antenna in those areas that require monitoring [19]. In turn, when using RFID tags, it is important to consider which tags to use and where on the products they are placed, since certain materials (particularly metal and liquids) can cause interference. This can be avoided by using the most appropriate tags for each case. You can then consider the following distinction (in terms of the activation mode required) between the two types of labels [18]:  
* In passive tags, the antenna energises and activates data transmission;
* On active labels, the antenna only activates data transmission. Returning to the RFID reader devices, this component is the most complex because it is responsible not only for reading the information transmitted by the tags but also for communicating with external agents, i.e., transmitting the data received to the respective servers [20]. In turn, some of the obstacles to the adoption of this technology include the following aspects: the phenomenon of electromagnetic interference, in the sense that automatic identification and data collection through RFID, for its success, also depends on the type of objects in which the tags are inserted, where objects with metallic or liquid content may absorb the radio-frequency energy emitted by the reader, causing shorter transmission ranges or even the product not being identified; and, in terms of sustainability, the non-recycling and non-reuse of the tags may be a condition to be taken into consideration, so that if the tags are integrated into the object to be identified, it may not be possible to use them again, and the non-reuse of electronic waste (which includes these tags) may constitute a serious environmental risk [21]. In addition, the fact that the technology under study uses different operating frequencies in different parts of the world could make interconnection and compatibility between equipment significantly more difficult [22]. In this context, the received signal strength indicator (RSSI) is a measure of the power received from the return signal of an RFID tag when interrogated by a reader. The RSSI is an important but often misunderstood property of a radio-frequency identification system [22].