To determine whether an RF-identification (RFID) system should be deployed, the cost of the RFID tags is often taken into account. If the price of the RFID system is lowered, RFID would thus be a fit for more applications. In this vein, researchers have looked into developing chip-less RFID tags with no application-specific integrated circuit (ASIC). At the University of Hong Kong, for example, a chipless ultra-wideband (UWB) RFID system has been proposed by Y.F. Weng, S.W. Cheung, T.I. Yuk, and L. Liu.
The tags employed by this system are both printable and uniplanar. For example, the chipless tag comprises two UWB monopole antennas, which are connected by coplanar waveguide (CPW) transmission line. The RFID tag’s ID is represented by a spectral signature in the UWB frequency range. It is created by a multi-resonator embedded in the CPW circuitry. To simplify the detection process, the detection of the tag’s ID only uses the amplitude of the spectral signature.
To reduce the mutual coupling between the uplink and downlink signals, the RFID reader uses two Vivaldi antennas. One transmits a vertically polarized signal while the other receives a horizontally polarized signal. To further reduce mutual coupling, a copper plate at the reader separates the uplink and downlink signals. Using these two proposed methods, the researchers have reduced mutual coupling by 20 dB.
By relying on UWB instead of narrowband RFID technology, it also is possible to overcome limitations like small coverage area, insufficient ranging resolution for accurate localization, sensitivity to interference, and scarce multiple-access capability. According to measured results of the system in an anechoic chamber, it can achieve a reading range of at least 30 cm—way beyond the 10 cm reported for a similar system. See “Design of Chipless UWB RFID System Using a CPW Multi-Resonator,” IEEE Antennas And Propagation Magazine, Feb. 2013, p. 13.