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2010 IEEE
654 IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, . 57, . 3, MARCH 2010
Abstract—SAW tags were invented more than 30 years ago,
but only today are the conditions united for mass application
of this technology. The devices in the 2.4-GHz ISM band can
be routinely produced with optical lithography, high-resolution
radar systems can be built up using highly sophisticated, but
low-cost RF-chips, and the Internet is available for global ac-
cess to the tag databases. The “Internet of Things,” or I-o-T,
will demand trillions of cheap tags and sensors. The SAW tags
can overcome semiconductor-based analogs in many aspects:
they can be read at a distance of a few meters with readers
radiating power levels 2 to 3 orders lower, they are cheap,
and they can operate in robust environments. Passive SAW
tags are easily combined with sensors. Even the “anti-collision”
problem (i.e., the simultaneous reading of many nearby tags)
has adequate solutions for many practical applications.
In this paper, we discuss the state-of-the–art in the devel-
opment of SAW tags. The design approaches will be reviewed
and optimal tag designs, as well as encoding methods, will be
demonstrated. We discuss ways to reduce the size and cost of
these devices. A few practical examples of tags using a time-
position coding with 10
6
different codes will be demonstrated.
Phase-coded devices can additionally increase the number of
codes at the expense of a reduction of reading distance.
We also discuss new and exciting perspectives of using ultra
wide band (UWB) technology for SAW-tag systems. The wide
frequency band available for this standard provides a great
opportunity for SAW tags to be radically reduced in size to
about 1 × 1 mm
2
while keeping a 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.
I. I
I
this paper, we briefly review the current status of the
development of radio frequency identification (RFID)
tags based on SAW technology. We mainly discuss the tag
devices, omitting issues related to the reader design and
the corresponding signal processing issues.
The first RFID systems appeared already during World
War II for identification of airplanes. However, it is only
now that the technical conditions are right for widespread
use of RFID. The 2 key issues for RFID technology are the
number of different codes that can be stored on a tag and
the possibility of transferring and communicating infor-
mation. Because of the ongoing progress of semiconductor
technology, mass production of such devices at a low cost
has become possible. Micro- and nanometer lithographic
technology enables the fabrication of very small tags with
a chip size on the order of 1 mm and smaller, operat-
ing in the GHz-range, where sufficiently wide frequency
bands are available. These industrial, scientific, and medi-
cal (ISM) frequency bands can be used without licensing
when using a limited radiated power.
The wide frequency bands finally allow for a practically
infinite number of different codes to be written and read
at microsecond time intervals. The omnipresent internet,
intranet, and similar communication networks enable the
processing of databases and development of smart systems
that use the information automatically read from RFID
tags. The dramatic development of mobile phones, which
only combine a transmitter with a receiver, both used in
radio communications for a century by now, was based on
exactly the same 2 reasons: first, the development of tech-
nology enabling the use of high and wide frequency bands
which support a large number of subscribers, and second,
computer databases with high-speed data links enabling
fast communication. The type of RFID tag introduced
in this paper, the surface acoustic wave (SAW) tag, is
similar to RF SAW filters that are widely used in mobile
phones. SAW tags and SAW filters use basically the same
technology.
RFID tags will be omnipresent. Below is a small list of
possible applications:
Traffic control of vehicles, wagons, ships, etc. Č
Identification of containers, pallets, bags in airports, Č
etc.
Individual goods control and inventory in stocks, Č
shops, etc.
Tracing of animals and products of animal origin Č
Tracking of wild animals, marking of trees in forests, Č
etc.
Access to buildings, parking, restricted areas, com- Č
puters, etc.
Ambient assisted living for the disabled and the el- Č
derly
Identification of parts, equipment, machines, and cars Č
assembled on conveyer lines
Tracing of dangerous and explosive substances Č
Security and guard services Č
These applications will demand trillions of tags per
year, which may result in an industry larger than the SAW
industry of today. Mass application of tags was predicted
for the first time by C. Hartmann many years ago [1].
II. A P RFID T
Both semiconductor integrated circuit (IC)-based and
SAW-based RFID tags use no transceiver stage to gener-
Review on SAW RFID Tags
Victor P. Plessky, Senior Member, IEEE, and Leonhard M. Reindl, Member, IEEE
(Invited Paper)
Manuscript received May 21, 2009; accepted November 23, 2009.
V. P. Plessky is with GVR Trade SA, Bevaix, Switzerland (e-mail:
victor.plessky@gmail.com).
L. M. Reindl is with the Institute for Microsystems Technology, Uni-
versity Freiburg, Germany.
Digital Object Identifier 10.1109/TUFFC.2010.1462
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