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Experimental results and EMC considerations on
RFID location systems
Eugen COCA and Valentin POPA
University "Stefan cel Mare" of Suceava, Department of Electrical Engineering and Computer Science
13, Universitatii Street, Suceava, 720229 ROMANIA
{eugen.coca, valentin.popa}@usv.ro
Abstract-Many efforts where made in the last years in order to
develop new techniques for mobile objects identification, location
and tracking. Radio Frequency Identification (RFID) systems are
a possible solution to this problem. There are many different
practical implementations of such systems, based on the use of
radio waves from low frequencies to high frequencies. In this
paper we present a short review of existing RFID systems, and an
in depth analysis of one commercial system, the RFID RADAR.
The results are from experiments performed in real life conditions.
Also, this paper offers important EMC information regarding the
use of high frequency RFID system.
I. INTRODUCTION
Radio Frequency Identification (RFID) Systems are based
on radio frequency (RF) tags and RF readers. Tags are made up
around a microchip containing a small memory. Modern tags
have memory capacities of several tens of kilobits or more. RF
readers are microprocessor based systems.
RFID systems may be divided in two main categories:
passive and active. In passive systems the power supply needed
by the tags is assured by a small antenna, located near the chip.
This antenna captures the RF energy from the reader and uses
it to power the logic circuits. Active systems use tags with on-
board batteries power sources and can support more
sophisticated electronics with more data storage capacity, data
processing capabilities and / or interfaces to external sensors.
Many applications require precise location information for
objects or persons.
II. RFID LOCATION SYSTEMS
Location of mobile objects becomes of great interest in the
last years and will be in the period to come. There are many
applications where precise positioning information is desired:
goods and assets management, supply chain management,
point of interest (POIs), proximity services, navigation and
routing inside buildings, emergency services as defined by the
E911 recommendations in North America and EU countries,
etc.
There are numerous outdoor solutions, based mainly on GPS
but there are also so-called inertial systems (INS). Solution
based on cellular phone networks signals are another good
example of outdoor positioning service. For GPS based
solution the precision of location is dictated by a sum of factors,
almost all of them out of user control. Inertial systems can
provide continuous position, velocity and orientation data that
are accurate for short time intervals but are affected by drift
due to sensors noise [1], [2]. For indoor environments the
outdoor solutions are, in most of the practical situations, not
applicable. The main reason is that the received signal, affected
by multiple path reflections, absorptions and diffusion, is too
weak to provide accurate location information. This introduces
difficulties to use positioning techniques applied in cellular
networks (time of arrival, angle of arrival, observed time
reference, etc.) in order to provide accurate location
information inside buildings or isolate areas.
Indoor positioning systems should provide the accuracy
desired by the context-aware applications that will be installed
in that area. There are three main techniques used to provide
location information: triangulation, scene analysis and
proximity [3, 4, 5, and 6]. These three techniques may be used
separately or jointly.
Indoor positioning systems may be divided into three main
categories. First of all there are systems using specialized
infrastructure, different from other wireless data
communication networks. Second, there are systems based on
wireless communication networks, using the same
infrastructure and signals in order to obtain the location
information. Third, there are mixed system, that use both
wireless networks signals and another sources to achieve the
goal.
There are many implementations, we mention here several
of them having something new in technology and / or the
implementation comparing with previous systems [6, 7, 8, 9,
10, 11, 12, 13, 14, and 15]:
- Active Badge is a proximity system that uses infrared
emission of small badges mounted on the moving objects. A
central server receives the signals and provides location
information as the positions of the receivers are known;
- Cricket system from MIT which is based on "beacons"
transmitting an RF signal and an ultrasound wave to a receiver
attached to the moving object. The receiver estimates the it's
position by listening to the emissions of the beacons based on
the difference of arrival time between the RF signal and the
ultrasound wave;
- MotionStar is a magnetic tracker system which use
electromagnetic sensors to provide position information;
- MSR Easy Living uses computer vision techniques to
recognize and locate objects in 3D;
- MSR Radar uses both triangulation based on the
attenuation of the RF signal received and scene analysis;
- Pinpoint 3D-iD which uses the time-of-flight techniques
for RF emitted and received signals to provide position
information;
- Pseudolites are devices emulating the GPS satellite signals
for indoor positioning;
- RFID Radar which used RF signals
- SmartFloor utilizes pressure sensors integrated in the floor.
The difference of pressure created by a person movement in
the room is analyzed and transmitted to a server which
provides the position of that person;
- SpotON is a location technology based on RF signals. The
idea is to measure on the fixed receivers the strength of the RF
signals emitted by the tags mounted on moving objects to be
located.
III. THE RFID RADAR POSITIONING SYSTEM
The RFID Radar system used for the tests and evaluation
was made by Trolley Scan [24], the version we had was the
"Development version". The main unit is built around a
development board from Microchip. The antenna system is
composed by three patch panel antennas, one for signal
generation and two for receiving the signals from the
transponders. As stated in the RFID Radar handbook, the
processor inside the system is able to make calculation to
determine the positions of up to 50 tags in a range of 50 meters.
Both RFID Radar and RFID system functions are available to
the user, only one of the two selectable by software. The RFID
radar measures the path length for the signals traveling from
the transponder to the reader to determine the distance. By
comparing the two signals the reader is able to determine the
angle of arrival of the signals from the transponder.
Transponders are either passive (Ecochiptag 500 µWatts
transponders) or active. We used for the tests two types of
active transponders Claymore long-range Ecotag and Stick
long-range Ecotag and one type of passive transponder. All
long-range active transponders use a Lithium battery to supply
the chip.
IV. PERFORMANCE EVALUATION BASED ON
EXPERIMENTAL RESULTS
We made a series of test during several days, in different
environment conditions and using different positions for the
tags. Before starting the measurement session the receiver
itself must be calibrated using, as recommended by the
producer, an active tag. The tag was positioned in the center in
front of the antenna system at 9 m distance. The operation is
mandatory as the cables length introduces delays in the signal
path from the antenna to the receiver. We made a calibration
for every site we made the measurements, in order to
compensate the influence of antenna, cables and receiver
positions.
For the tests we used all three types of tags provided (two
type active and one passive). The batteries voltages where
checked to be at the nominal value before and after every
individual test in order to be sure the results where not affected
by the low supply voltage. During all the tests we used a
spectrum analyzer to measure the electric field strength in the
test area, in a frequency interval from 75 MHz up to 3 GHz.
The screen captures saved on the spectrum analyzer internal
memory where downloaded in a computer after each set of
tests.
For the first set of tests we used a real laboratory room, with
a surface of about 165 square meters (7.5 meters x 22 meters).
There where several wooden tables and chairs inside, but we
did not changed their positions during the experiment. The
antenna system was mounted about 1.4 meters height above the
ground on a polystyrene stand, with no objects in front. All
tags where placed at the same height, but their position where
changed in front of the antenna. We used a notebook PC to run
the control and command software.
We present only the relevant results of the tests and
conclusions, very useful for future developments of this kind of
location systems. For the first result presented we used two
long range tags, one Claymore (at 10 meters in front of the
antenna) and one Stick type (at 5 meters) - Figure 1.
A1 A2 A3
RFID
Radar
PC
Tag 2
5 m
Tag 1
5 m
Test setup no. 1
Figure 1. Test setup for distance measurement fro two tags - one at 5 m and the
second at 10 m in front of the antenna
As we might see in Figure 2, the positions for each
individual tag reported by the system where not enough stable
in time. We run this measurement for several times using the
same spatial configuration for all elements. The test presented
here was made for duration of 4 hours. Analyzing the
numerical results, we find out the for 65 % of cases for the tag
located at 5 meters the position was reported with an error less
than 10 % and for 47 % of cases the results where affected by
the same error for the tag located 10 meters in front of the
antenna.
Figure 2. Results for 2 active tags placed on 5 meters and 10 meters
respectively, in front of the antenna system in a room
The second setup was the same in respect of location of the
measurement, but one tag was moved more in front of the
antenna system, at a distance of 20 meters. The results are
practically the same regarding the position dispersion. Only in
about 35 % of all measurements for the tag situated at 20
meters the results where with an error less than 10 %.
Figure 3. Test setup for distance measurement fro two tags - one at 5 m and the
second at 20 m in front of the antenna
Figure 4. Results for 2 active tags placed on 5 m and 20 m respectively, in
front of the antenna system in a room
The measurements for the third case presented here where
made in an open area, with no obstacles between the antenna
system and the tags, using a tag placed at 10 meters in from of
the antenna. The results obtained (Figure 5) are much better
than the results from the measurements done in the laboratory.
In this case (Figure 3) about 6 % of the measured distances
where affected by an error more than 10%.
A1 A2 A3
RFID
Radar
PC
Tag 2
5 m
Tag 1
15 m
Test setup no.2
Figure 5. Results for 1 active tag placed on 10 meters in front of the antenna
system in an open-area site
V. EMC MEASUREMENTS
The RFID location system is supposed to use a central
frequency of 870.00 MHz with a bandwidth of 10 kHz. The
frequency was chosen in order to be outside the GSM 900 band
used in Europe (880.0 MHz - 915.0 MHz / 925.0 MHz - 960.0
MHz). As we might see in the capture from the spectrum
analyzer (see Figure 6), the electric field strength, at distance
of 20 m in front of the reader antenna, is about 1.2 V/m, a
value sufficiently low to be in accordance with the EMC safety
levels in Europe and in the US.
Figure 6. Electric field magnitude at 20m distance in front of the antenna
There are also visible, above the RF noise floor, the
emissions from the GSM base stations, located at about 600
meters from the laboratory the tests where made.
Figure 7. Electric field magnitude at 3m distance in front of the antenna
Problems appear right in front and very close to the antenna
system. In Figure 7 we show the field strength at a distance of
3 meters in front of the antenna. At this distance the emission
level is about 39 V/m, a value high enough to worry. At about
30 cm near the emission antenna the filed was about 200 V/m,
the maximum value the spectrum analyzer could measure.
Regarding the bandwidth of the signal, we observe to be in
the range of 10 to 25 kHz, small enough not to produce
interference with other radio spectrum users. If many such
devices are to be used simultaneously, on different central
frequencies, there will be no problem if the spacing between to
channel will be as low as 30 kHz.
VI. CONCLUSIONS
RFID location systems for indoor and outdoor positioning
are a promise for the future [16, 17, 18, 19 and 20]. The
performances of these systems are affected by many factors.
We identified here that for a system working in the band near
900 MHz, the objects interposed between the antenna system
and the tags to be located may have a great influence in terms
of accuracy of the measurement.
In closed areas multiple reflection paths may disturb the
measurement systems, a percent of only 40 to 60 of total
measurements are enough accurate to locate an objects. In such
conditions, there are small chances for this kind of systems to
be used for high precision applications.
The results obtained in open area test sites are more
promising, more than 93 percent of total result where not
affected by big errors. Regarding the EMC aspects of this
RFID location system, we may say, based on measurements
presented here, that the electric field are high enough not to use
this system indoors at distances less than 5 meters, if humans
are present on a regular basis in that area. For applications in
open areas, like access control for auto vehicles and many
similar others, this kind of systems are very good.
REFERENCES
[1] P.Y. Gillieron, D. Buchel, I. Spassov, et al., “Indoor navigation
performance analysis", Proceedings of the 8th European Navigation
Conference (GNSS ’04), Rotterdam, The Netherlands, May 2004.
[2] P.Y. Gillieron and B. Merminod, "Personal navigation system for indoor
applications", Proceedings of the 11th IAIN World Congress, Berlin,
Germany, October 2003.
[3] J. Hightower and G. Borriello, “A survey and taxonomy of location
systems for ubiquitous computing,” Tech. Rep. UWCSE 01-08-03,
University of Washington, Washington, DC, USA, August 2001
[4] F. Evennou and F. Marx, "Advanced Integration of WiFi and Inertial
Navigation Systems for Indoor Mobile Positioning", EURASIP Journal
on Applied Signal Processing, vol. 2006.
[5] L. M. Ni, Y. Liu and A. P. Patil, "LANDMARC: Indoor Location Sensing
Using Active RFID", Proceedings of the First IEEE International
Conference on Pervasive Computing and Communications (PerCom'03),
pp. 407-415, 2003.
[6] N. B. Priyantha, A. Chakraborty, and H. Baladrishnan, “The cricket
location-support system” in Proceedings of the 6th Annual International
Conference on Mobile Computing and Networking (MOBICOM ’00), pp.
32–43, Boston, Mass, USA, August 2000.
[7] F. Van Diggelen and C. Abraham, “Indoor GPS Technology,” CTIA,
Dallas, Tex, USA, May 2001.
[8] A. Borrelli, C. Monti, M. Vari, and F. Mazzenga, “Channel models for
IEEE 802.11b indoor system design,” in IEEE International Conference
on Communications, vol. 6, pp. 3701–3705, Paris, France, June 2004.
[9] K. Finkenzeller, "RFID Handbook: Fundamentals and Applications in
Contactless Smart Cards and Identification," John Wiley & Sons, New
York, NY, USA, 2nd edition, 2003.
[10] P. Bahl, N. Venkata, N. Padmanabhan, and A. Balachandran,
“Enhancements to the RADAR user location and tracking systems,”
Microsoft Research Technical Report MSR-TR-2000-12, February 2000.
[11] R. J. Fontana, “Recent Applications of Ultra Wideband Radar and
Communications Systems”, http://www.multispectral.com.
[12] G. Wolfle, P. Wertz, and F. M. Landstorfer, “Performance, accuracy and
generalization capability of indoor propagation models in different types
of buildings,” in Proceedings of 10th IEEE International Symposium on
Personal, Indoor and Mobile Radio Communications (PIMRC ’99),
Osaka, Japan, September 1999.
[13] Damiano De Luca, Franco Mazzenga, Cristiano Monti, and Marco Vari,
“Performance Evaluation of Indoor Localization Techniques Based on
RF Power Measurements from Active or Passive Devices,” EURASIP
Journal on Applied Signal Processing, vol. 2006, Article ID 74796, 11
pages, 2006. doi:10.1155/ASP/2006/74796
[14] Nabil R. Yousef, Ali H. Sayed, and Nima Khajehnouri, “Adaptive
Subchip Multipath Resolving for Wireless Location Systems,” EURASIP
Journal on Applied Signal Processing, vol. 2006, Article ID 25431, 16
pages, 2006. doi:10.1155/ASP/2006/25431
[15] A. H. Sayed, A. Tarighat, and N. Khajehnouri, “Network-based wireless
location: challenges faced in developing techniques for accurate wireless
location information,” IEEE Signal Processing Magazine, vol. 22, no. 4,
pp. 24–40, 2005.
[16] FCC Docket no. 94-102, “Revision of the commission's rules to ensure
compatibility with enhanced 911 emergency calling,” Tech. Rep. RM-
8143, July 1996
[17] C. Drane, M. Macnaughtan, and C. Scott, “Positioning GSM telephones,”
IEEE Communications Magazine, vol. 36, no. 4, pp. 46–54, 1998.
[18] B. C. Barker, J. W. Betz, and J. E. Clark, et al., “Overview of the GPS M
code signal,” in CDROM Proceedings of the ION National Meeting;
Navigating into the New Millennium, Anaheim, Calif, USA, January
2000.
[19] J. W. Betz, “The offset carrier modulation for GPS modernization,” in
Proceedings of the National Technical Meeting of the Institute of
Navigation (ION-NTM '99), pp. 639–648, San Diego, Calif, USA,
January 1999.
[20] J. W. Betz and D. B. Goldstein, “Candidate designs for an additional civil
signal in GPS spectral bands,” Technical Papers, MITRE, Bedford, Mass,
USA, January 2002.
[21] G. A. McGraw and M. Braasch, “GNSS multipath mitigation using high
resolution correlator concepts,” in Proceedings of the National Technical
Meeting of the Institute of Navigation (ION-NTM '99), pp. 333–342, San
Diego, Calif, USA, January 1999
[22] R. E. Játiva and J. Vidal, “First arrival detection for positioning in mobile
channels,” in Proceedings of the 13th IEEE International Symposium on
Personal, Indoor and Mobile Radio Communications (PIMRC '02), vol. 4,
pp. 1540–1544, Lisbon, Portugal, September 2002
[23] A. R. Jimenez, F. Seco, R. Ceres, and L. Calderon, “Absolute Localization
using Active Beacons: A survey and IAI-CSIC contributions,” Institute
for Industrial Automation, CSIC Madrid.
[24] http://www.trolleyscan.com
[25] http://www.cisco.com
[26] http://www.alientechnology.com
