Review on SAW RFID tags

SAW-tags were invented more than 30 years ago, but only nowadays the conditions are united for mass-application of this technology: the devices in 2.4 GHz ISM band can be routinely produced with optical lithography and Internet is available for global access to the tag databases. Perspectives of the ldquoInternet of Things, I-o-Trdquo will demand trillions of cheap tags and sensors. The SAW-tags overcome semiconductor based analogues in many aspects: they are true passive devices, can be ldquoreadrdquo at a few meter distance with readers radiating 2 to 3 orders lower power levels, they are cheap and can operate in robust environment. Passive SAW tags are easily combined with sensors. Even the ldquoanti-collisionrdquo problem (simultaneous reading of many tags situated nearby) has solutions sufficient for many practical applications. In this paper we discuss state-of-the-art in development of SAW-tags. The design approaches will be reviewed - and the most optimal tag designs are demonstrated, as well as encoding methods. We discuss the ways to reduce size and cost of these devices. A few practical examples of tags with simple time-position coding will be demonstrated with 10⁶ different codes. Phase coded devices allow further radical increase of the number of codes, the price to pay being reduction of the reading distance. We also discuss exciting perspectives of using Ultra Wide Band (UWB) technology emerging now for communication applications in SAW-tag systems. Wide frequency band available in this standard is a great opportunity for SAW-tags to radically reduce their size to about 1times1 mm² keeping practically infinite number of possible different codes. Finally, the reader technology will be discussed as well as detailed comparison made between SAW-tags and IC-based semiconductor device.