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Experimental results and EMC considerations on RFID location systems

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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.
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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.
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... Note that tag range in passive UHF RFID systems can also be severely affected by various types of interference, such as tag-on-tag, reader-on-tag, reader-onreader, and interference from external EMI sources, etc. All these subjects deserve separate discussions and have been covered in literature [61,[80][81][82][83][84][85][86][87][88]. ...
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In this paper, we discuss antennas and propagation aspects in current passive UHF RFID systems. We consider a "reader-tag-reader" link and concentrate on each part of it: reader antennas, propagation channel, and tags. We include channel modeling equations and support our discussion with experimental measurements of tag performance in various conditions. We also provide a comprehensive literature review.
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RFID location systems for indoor and outdoor positioning are a promise for the future, even the performances of these systems are affected by many factors. We identified here that for a system working in the RF 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 results. 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 object. In such conditions, there are small chances for this kind of systems to be used for high precision indoor applications requiring more than several tens of centimetres accuracy. The results obtained from the measurements we made in open area test sites are more promising, more than 93 percent of total result were not affected by notable errors. For speed measurement of mobile objects by using RFID systems, we may conclude there are many aspects to solve before such systems may be used in commercial applications. Despite the precision for both passive and active transponders positioning is in the range of 10-30 centimetres for methods based on the time of arrival and angle of arrival, the performances obtained for speed measurements are not good enough when a large number of mobile objects are simultaneously in range. For a single transponder or a reduced number of transponders and small speeds, bellow 40 km/h, the speed measurement errors were below 30 %. For better speed measurement results we must combine the use of a RFID system for reading IDs and transponder internal memory contents with a classical radar system and process the results in a software interface. 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.
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