## 2 Research Background  
### Introduction to RFID  
RFID is the generic term for information-identifying systems using radio-waves. Unlike the barcode system, RFID identifies tag information through non-contact recognition and is attracting attention as a next-generation technology to replace the barcode system. The RFID system is composed of an interrogator, RFID tags, and a server computer (i.e., middleware). The main purpose of the interrogator is requesting the identification of each tag in the reading area (i.e., recognition area), where the interrogator corresponds to an RFID reader connected with an antenna. The RFID reader connected with an antenna generates radio-waves toward the tags periodically or continuously to scan them and interprets radio-wave signals as meaningful information. This periodicity for generating radio-waves is typically referred to as the recognition interval and is interpreted as how often the RFID reader generates radio-waves to scan the tags for a specific time (times/sec.). The tag (transponder) is an electronic component that includes an electronic circuit and an ultra-small built-in antenna and can store user-defined information in its mounted circuit, which can then be exchanged through an external antenna. An RFID tag supports a larger set of unique IDs than a barcode and can incorporate additional data, such as manufacturer and product type. Moreover, it can measure environmental factors, such as temperature. The antenna transmits and receives data between the tags and the RFID reader and activates or deactivates the tags by sending a radio signal to them, whereas the server computer processes the data identified by the tags. An RFID system can freely change the tag data and simultaneously recognize a large number of tags, but it cannot be modulated and is reusable [19; 20; 21]. RFID tags can be divided into two types, passive and active, depending on the power resource requirement. Active tags require a power source, either by being connected to a powered infrastructure or by using energy stored in an integrated battery. One example of an active tag is the transponder attached to an aircraft that identifies its national origin. However, the batteries make the cost, size, and lifetime of active tags impractical for the retail industry. By contrast, the lifetime of a passive tag is limited by the stored energy in relation to the number of read operations the device must undergo. Nonetheless, the passive tag is of interest because it does not require batteries or maintenance, and it also has an indefinite operational life and is small enough to fit onto a practical-sized adhesive label. In sum, the active tag has its own built-in power resource, which allows for its own processing and for being distanced from the RFID reader, but the operation time of the power resource is limited and the cost is high. Meanwhile, the passive tag is operated by the electric field from the antenna without the need for a direct power supply, which is advantageous, as the lifespan is semi-permanent, and the cost is lower than that of the active tag. However, its recognition distance is short. Of note, this research defines the RFID active type in terms of active tag-utilizing systems and the RFID passive type in terms of passive tag-utilizing systems to avoid any confusion. The tags are classified into low frequency, high frequency, ultra-high microwave, and microwave depending on the radio-frequency band. The tags have been applied to various fields depending on their radio-frequency band. For example, 125 and 134 kHz tags are used in livestock product distribution management, process automation, and access cards, whereas 13.56 MHz tags are used for transportation and credit cards. Meanwhile, 433.92 MHz tags are classified as active tags and are usually applied to container terminals and real-time location tracking systems, whereas 860-960 MHz tags are applied to logistics distribution and material and process management and have a wide range of uses at low cost.