2D localisation using SAW-based RFID systems: a single antenna approach.

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Int. J. Radio Frequency Identification Technology and Applications, Vol. 1, No. 4, 2007

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Copyright © 2007 Inderscience Enterprises Ltd.













2D localisation using SAW-based RFID systems:
a single antenna approach
Darmindra D. Arumugam,*
Vijayakrishnan Ambravaneswaran,
Amar Modi and Daniel W. Engels
Department of Electrical Engineering,
University of Texas at Arlington,
Arlington, TX 76019-0016, USA
E-mail: darumugam@uta.edu
E-mail: vijayakrishnan.ambravaneswaran@uta.edu
E-mail: amar.modi@uta.edu
E-mail: dengels@uta.edu
*Corresponding author
Abstract: This paper presents a novel Real-Time Localisation System (RTLS)
based upon 2.45 GHz Surface Acoustic Wave (SAW) Radio Frequency
Identification (RFID) systems. The system utilises a novel localisation method
that combines the angular rotation of the RFID reader’s antenna system with
the inherent Time-of-Flight (TOF) distance measurement capabilities of the
SAW RFID system. The system design rests upon the sound physical
fundamentals of electromagnetic radiation and SAW operation. The system
was implemented and empirically evaluated. It was determined to provide
accurate 2-Dimensional (2D) location of the SAW tag that is within 3.17 cm of
the actual location for a reader to tag range of up to 10 m.
Keywords: Radio Frequency Identification; RFID; Surface Acoustic Wave;
SAW; localisation; real-time localisation; Real-Time Localisation System;
RTLS; Single-Antenna Single-Reader Approach; SASR.
Reference to this paper should be made as follows: Arumugam, D.D.,
Ambravaneswaran, V., Modi, A. and Engels, D.W. (2007) ‘2D localisation
using SAW-based RFID systems: a single antenna approach’, Int. J. Radio
Frequency Identification Technology and Applications, Vol. 1, No. 4,
pp.417–438.
Biographical notes: Darmindra D. Arumugam is a Researcher with the Texas
Radio Frequency Innovation and Technology Center at the University of Texas
at Arlington. He is a Graduate Research Assistant and a Graduate Student in the
Department of Electrical Engineering at UTA. He received a BSc and an MSc
in Electrical Engineering, and is currently pursuing his Doctoral degree at
UTA. His current research is in the area of Radio Frequency Identification
(RFID) system design, applications of RFID, RFID antennae design,
system-level modelling of RFID systems, energy harvesting, wireless sensor
networks and intelligent objects.
Vijayakrishnan Ambravaneswaran received a BE in Electrical and Electronics
Engineering from Anna University, India in 2005. He is presently a graduate
student at UT Arlington. He is affiliated with NanoFAB and the Texas RF
Innovation and Technology Center. His research interests include RFID-based
devices for MEMS applications, biological application of MEMS.

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Amar Modi is a Systems Engineer for Lockheed Martin Missiles and Fire
Control. He is tasked with technical planning, system integration, verification
and validation, cost and risk and supportability and effectiveness analyses for
total systems. Currently, he is working on the Patriot Advanced Capability
(PAC-3) Missile Programme and ensures the logical and systematic conversion
of customer or product requirements into total systems solutions that
acknowledge technical, schedule and cost constraints. He received a BSc in
Electrical Engineering from University of Texas at Arlington and is currently
pursuing an MSc in Electrical Engineering at UTA.
Daniel W. Engels received a PhD in Electrical Engineering and Computer
Science from the Massachusetts Institute of Technology in 2000. He is an
Associate Professor at the University of Texas at Arlington and the Founding
Director of the Texas Radio Frequency Innovation and Technology Center
headquartered at UTA. His research interests are in the areas of low-cost
wireless communication systems with special emphasis on passive RFID
systems and in virtual environments augmented by reality.

1 Introduction
Recently, there has been a significant interest within the Radio Frequency Identification
(RFID) field for initiatives using Surface Acoustic Wave (SAW) based RFID tags. SAW
devices are a radically different RFID implementation when compared to the more
common silicon-based RFID systems. The key differences between these two
technologies are due to the physical characteristics of the substrate materials used to
create the tags and the use, or lack thereof in SAW tags, of functional elements like
transistors, capacitors and diodes. SAW tags utilise a piezoelectric substrate such as
lithium niabate and commonly have one metal layer deposited upon them to create a
functioning tag. Silicon-based tags, however, utilise silicon substrates and multiple layers
of n-type, p-type, poly and metal materials to create devices and a functioning tag. The
similarities in these systems lie in the fact that they both require that their tags
communicate with the reader via radio frequency communications, and the tags carry and
communicate an identifier that is used to identify the object to which the tag is affixed.
In SAW-based RFID systems, the reader sends out a signal spike or ramp in the
allotted frequency band, such as the 2.45 GHz Industrial, Scientific and Medical (ISM)
band. The SAW tag receives this electromagnetic signal and converts it into a mechanical
SAW by way of an Inter-Digital Transducer (IDT). The SAW propagates across the
surface of the piezoelectric substrate away from the IDT. Partial reflectors, created from
the deposited metal and located at precise distances from the IDT, partially reflect the
mechanical surface wave on the substrate. A unique sequence of waves propagating
towards the IDT is created by these reflectors. The IDT converts these waves into an RF
signal, effectively transmitting the tag’s data to the reader (Hartmann, 2002). The unique
RF pattern communicated by the tag, which is a sequence of reflections of the reader’s
sent signal, is received by the reader and decoded to reveal the unique identity of the tag
(Hartmann, 2002).
The physical operating characteristics and communication capabilities of SAW RFID
systems enable the distance of the tag from the reader’s antenna to be accurately
determined by the Time-of-Flight (TOF) of the communication signals. This inherent

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feature of SAW RFID systems, which does not exist for any active or passive
silicon-based RFID system, make them well suited for use in low-cost Real-Time
Location Systems (RTLSs).
In this paper, we present a novel SAW RFID enabled RTLS method and system that
utilises a systems approach in solving the fundamental need for 2-Dimensional (2D)
localisation systems: accuracy. Our method is founded upon the physics of
electromagnetic radiation and the operation of SAW RFID systems. We review the
physics of piezoelectric materials, the piezoelectric effect and SAW RFID systems in
Section 2. And, in Section 3 we examine the fundamental design parameters that impact
the function of SAW RFID tags. Our novel RTLS system merges a SAW RFID system
using a single reader antenna with current Radio Detection and Ranging (RADAR)
techniques. By using the angular rotation of the SAW reader’s antenna, our novel
method is shown to allow 2D localisation for the first time using a single antenna with a
SAW RFID system. In Section 4, we examine the physical foundations for using
the angular rotation of the antenna system in conjunction with the SAW RFID read
attempts. In Sections 5 and 6, we detail and analyse the experimental method and
model used to evaluate our RTLS system. The detection algorithm is analysed for
accurate localisation in Section 7. In Section 8 we present the experimental test setup
used to examine the system. Section 9 presents our discussions regarding results obtained
from the experimentation of the RTLS system. We draw the relevant conclusions in
Section 10.
2 Background
The means of detection employed by SAW-based RFID systems involve the changing of
physical properties of the signal using a transduction mechanism. This is accomplished
by converting the RF signal to an acoustic wave and then back into an electrical signal
(Hartmann, 2002). This conversion is done by means of an IDT, which is deposited onto
the piezoelectric substrate and is connected to an antenna for the RF signal detection and
radiation. In addition to coupling to the incident electromagnetic radiation, the IDT
allows the coupling of this radiation onto the substrate. In doing so, the IDT serves as an
effective coupling mechanism between the electromagnetic signal and the mechanical or
acoustical signal on the substrate. There exist a constant called the coupling coefficient
that describes the effectiveness of the conversion between the electrical and mechanical
signals. The coupling coefficient K
and bandwidth. A greater value would typically contribute in higher energy transfer and
wider bandwidths (Julian et al., 2002). The coupling coefficient is determined by the
design and implementation of the IDT.
Figure 1 presents the typical systems view of the SAW-based RFID system. Notice
that the interrogator, or reader, sends an interrogation signal that is reflected back by the
SAW tag. This response is detected as a read and can be associated to a specific tag
based on the reflector sequence for the given tag. SAW tags are programmed during the
manufacturing process and will not allow user manipulation of the tag ID. The following
section describes the details of piezoelectric materials that are important to the operation
of SAW RFID tags.

2 (%) gives a good indication of the energy transfer

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Figure 1 The passive SAW-based RFID system

2.1 Piezoelectric materials
In a piezoelectric crystal, the crystal remains electrically neutral because the positive and
negative charges are symmetrically distributed. During the application of mechanical
stress, the symmetry is disturbed causing charge asymmetry. This charge asymmetry
generates a voltage across the piezoelectric material. The piezoelectric effect is based
on the relationship of energy transfer from the electrical to mechanical domain
and vice versa, that is, a relationship between Gauss’ law and Hooke’s law (Dunn and
Whatmore, 2001). This relationship between the electrical and mechanical properties is
governed by the following equations, where
(Arumugam et al., 2007).
, ,
i j m = …
1, ,6
and
, ,1,2,3
k l n =

E
ijij kik
S s Td E
=+
(1)
T
lnl lmmn
D d TE
ε=+
(2)
Here S, D, E and T are the strain, dielectric displacement, electric field and stress,
respectively, and
,

ki
d and
ln
ε are the elastic compliances, the piezoelectric constants
and the dielectric permittivity, respectively (Arumugam et al., 2007). Thus, the voltage
generated by this effect is directly dependant on the force (mechanical stress) applied to
the piezoelectric substrate (Arumugam et al., 2007). The direction in which the stress is
applied is important because application of stress on one side or direction
will generate more voltage in that direction/side (Dunn and Whatmore, 2001).
Thus, in a SAW-based RFID system, it is this relationship of stress and strain that
governs acoustic wave generation and conversion to electromagnetic radiation
through the IDT.
E
ijs
T
2.2 SAW-based RFID systems
The SAW-based RFID system consists of five major components: namely, the reader, the
tag, the reader antenna, the tag antenna and the information subsystem utilising the
information captured by the reader. The tag and the reader must be designed to
accommodate the working of the SAW-based system (Arumugam et al., 2007). Figure 2
is a diagram graphically presenting the operation of SAW-based RFID systems. These
systems rely on the conversion of RF waves into nano-scaled mechanical or acoustic
waves (Hartmann, 2002).

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Figure 2 Operational principles of SAW RFID system

Source: Hartmann (2002).
The acoustic waves propagate along the piezoelectric surface in the form of Rayleigh
waves. Metal strips deposited on the piezoelectric substrate in a unique fashion act as
tiny reflectors, sending part of the acoustic wave in the reverse direction back to the IDT
while letting sufficient energy in the wave continue on its path (Arumugam et al., 2007).
A complete series of reflections creates a unique train of pulses (based on the deposited
reflectors on the substrate) that will be converted into EM waves by the IDT. These
waves are then detected by the reader and identified based on a specific identification
algorithm (Arumugam et al., 2007). The tag antenna design used for this paper is a patch
antenna as shown in Figure 3.
Figure 3 The SAW tag

Source: Arumugam et al. (2007).
The Model 501 SAW RFID reader manufactured by RFSAW Inc. and shown in Figure 4
is the reader used in our experimental analysis. The operating frequency for the RFSAW
system is 2.45 GHz, an ISM frequency band. This reader has a 1000 (cycles/second) tag
read speed for data collection. The reader antenna used for our experimental analysis is
the HG2418P manufactured by Hyperlink Technologies. This device has a horizontal
beam width of 22º, vertical beam width of 17º and other electrical specifications as given
in Table 1. Figure 5 shows the gain patterns for the vertical (a) and horizontal
(b) polarisation gains. Here, we note that the vertically polarised gain is narrower than
the horizontal polarised gain plot, and is the polarisation form that is used throughout this