Theoretical Considerations on the Impact of Nearby Tags on the Wireless Power Transfer From the Reader to the Target Tag in Passive UHF RFID

Radio frequency identification (RFID) technology, and in particular the passive UHF (ultra-high frequencies) standard, is developing strongly with the emergence of cognitive sensor networks and especially the Internet-of-Things. A significant indicator of this evolution is that tags are increasingly equipped with new capabilities: in addition to the identification functionality, there are, for example, those of sensor or actuator. In this context, the energy requirement at the tag level, known as augmented tag, is clearly increasing. Consequently, on the one hand, reading distances can be limited (the energy received is shared between the different components of the augmented tag) and on the other hand, to overcome the problem, semi-passive solutions (on-board battery to power the active electronic circuits) are proposed at the expense of the decisive advantage of the passive nature of the tags. It is therefore necessary to find alternative solutions to optimize the energy transfer between the reader and the tags. The concept proposed here is to target a particular tag, the one being queried, and to exploit the tags in its vicinity as reflectors in order to converge a maximum of power transmitted by the reader on the “target” tag. The paper uses an adapted mathematical formalism to evaluate the total power that can be received by a target tag as a function of the tags in its neighborhood: the impact of their relative position and of the load impedance of the reflector tags is analyzed and illustrated by simulations.