Reading Collocated RFID Tags

Abstract

One of the ultimate commercial objectives of RFID systems is the ability to read, and charge for, all of the tagged goods in a standard supermarket shopping cart by simply pushing the cart through an instrumented aisle. Such a system would speed the progress of customers through checkout areas and reduce operational costs. The solution to this problem can be thought of as the holy grail of RFID technology. It has many engineering issues that make it difficult. First, the RF environment inside a shopping cart is particularly challenging. The product packaging in the cart is made of a wide variety of materials that include metal cans and aluminized plastics that reflect and shield the interrogation signals. Furthermore, some of the products contain water, and plastics, that may absorb RF signals in the microwave band. To complicate matters further, all of the products are in close proximity to each other and in random configurations. RFID tags attached to these products will sometimes be poorly orientated with respect to the reader’s antenna, thereby making RF communication difficult. In addition, tag antennas are typically flat to enable them to be embedded in labels, but if orientated edge-on to the reader the tag will likely not receive enough energy to power up. These specific problems are discussed in more detail in Section 10; however, even if the RF reading environment for a group of RFID tags is ideal, it is still an engineering challenge to design readers that can successfully query multiple collocated tags, and accurately determine all of their IDs in a short period of time.