Selected Papers

Asset Tracking System Using Long-life
Active RFID Tags
Hitoshi Hayashi†, Toshimitsu Tsubaki, Tomoaki Ogawa,and Masashi Shimizu
Abstract
We have developed an asset tracking system using long-life active RFID (radio frequency identification) tags that reports the position of an asset when required. To determine the location of a tag attached
to an asset indoors, such as in stores, warehouses, and offices, we use a high-precision location systemthat is not greatly affected by environmental factors. Experimental results show that the root-meansquare error between the estimated and actual tag positions measured at 77 points is 2.9 m, indicatingthat this locating system is applicable not only to asset tracking but also to other ubiquitous services thatutilize the position of an item or a person indoors.

1. Introduction
RFID (radio frequency identification) tags are usedin various fields. For example, the tag system† NTT Network Innovation Laboratories
Yokosuka-shi, 239-0847 Japan
E-mail: hayashi.hitoshi@lab.ntt.co.jp

schematically illustrated in Fig. 1 is actually working
in many stores. This system comprises gates andRFID tags attached to items or merchandise. Items aand b have tags a and b, respectively. If either item
passes through the gate before the tag has been disabled by the cashier, the system generates an alarm.
This system uses passive RFID tags [1]. A passive
RFID tag receives radio waves generated from theItem a

Tag a

Tag b

Item bGate

Fig. 1. Passive RFID tag system using gates.

Vol. 1 No. 9 Dec. 2003

19Selected Papers

gate, modulates them, and emits return signals. If thegate receives the return signals transmitted from thetag, it sounds an alarm because the presence of thetagged item at that location is unauthorized. The passive RFID tag system in combination with a gate is a
low-maintenance system because the tag (or transmitting station) does not require a power source.
However, the communication range is limited to a
few tens of centimeters, so it is unsuitable for longerrange applications.
In contrast, the active RFID tag system (Fig. 2) is
known as a long-range tag system [2], [3]. Here, apower source is provided to each of the tags 1–6 toextend the communication range. In general, theactive RFID tag system uses a frequency bandassigned to a specific low power, and can communicating in a range up to several tens of meters. However, this active RFID tag system only has a function for
determining the presence or absence of tags 1 through6 in the communication areas a and b of the receiving
stations a and b, respectively. If the conventional
active RFID tag system is used to estimate the position of each tag, the position estimation error exceeds
the communication range. In order to improve thepositioning accuracy, the transmission power of thetag must be lowered or the sensitivity of the receiving
station must be reduced, while increasing the numberof receiving stations, to narrow the area covered byeach receiving station. Thus, this is not applicable toan asset tracking system, which reports the positionof the asset only when required, to reduce the cost.
To overcome the above problems of the active
RFID tag system, locating systems of the type illustrated in Fig. 3(a) have been proposed; the most wellknown example of this is the Global Positioning System (GPS) [4], [5]. The system shown in Fig. 3(a)includes a transmitting station T1, three or more
receiving stations A1-A3, and a center station communicating with the receiving stations. The transmitting station transmits a signal containing its identification code and the current time (i.e., transmissiontime) at a predetermined time interval using radiowaves. Every time a receiving station receives thesignal from the transmitting station, it transmits thereceived signal together with the reception time andits identification code to the center station. The center station calculates the distance between the transmitting station and each receiving station based on
the information provided by the receiving stations,and estimates the position of the transmitting station.
More precisely, the center station determines the signal propagation time from the transmission time to
the reception time and calculates the distance
between the transmitting station and each receiving
station by multiplying the propagation time by thepropagation speed of radio waves. Then, the centerstation estimates the position of the transmitting station by triangulation relative to the receiving stations.
Figure 3(b) illustrates another known locating system [6]. This system includes a transmitting station
T1 and three or more receiving stations A1-A3. The
transmitting station generates and transmits radio signals during the positioning operation. Each receiving
station reports the measured intensity of the received
signal to the center station. The center station calculates the distance between the transmitting stationand each receiving station from the signal intensity,and estimates the position of the transmitting station

Receiving station a

Receiving station b

Communication
area aCommunication
area b

Tag 1

Tag 3

Tag 4

Tag 2

Tag 6
Tag 5

Fig. 2. Conventional active RFID tag system.

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Receiving station
A1

Time of reception

Time of transmission
Center station

Transmitting
station T1
Time of reception

Time of reception

Receiving station
A3

Receiving station
A2(a) System using transmission time.

Receiving station
A1

Intensity of received signal

Center station

Transmitting
station T1
Intensity ofreceived signal

Intensity ofreceived signal

Receiving station
A2

Receiving station
A3(b) System using signal strength.
Fig. 3. Previously proposed locating systems.

based on its relative position to each receiving station.
Although positioning based on GPS may be effective outdoors, it is unsuitable indoors. Even if a position is estimated using amplitude information, inmany cases the relationship between the distance andintensity of the received signal does not obey Friis’formula, which is expressed by
4πd ,
L = 20 × log10  λ Vol. 1 No. 9 Dec. 2003

(1)

where L denotes the propagation loss, d the distance,and λ the wavelength.
The reason Friis’ formula does not work for indoor
propagation is that the receiving station may beblocked from line-of-sight transmission, or local
fluctuations in intensity of the received signal may
occur due to the influence of reflected waves.
This paper presents an asset tracking system using
long-life active RFID tags. In this system, we used alocating system for determining the location of a21

Selected Papers

transmitting station attached to an item or a person,
and more particularly a locating system for efficiently determining the positions of a number of transmitting stations indoors, such as in stores, warehouses,and offices, with high precision without being subjected to much influence from environmental factors.
This locating system is applicable not only to asset
tracking but also to other ubiquitous services that utilize the position of an item or a person indoors.

Receiving station(u1,v1)
R1
R1

R2e51

e52

2. Our locating system
Figure 4 schematically illustrates an example of
our locating system, and Fig. 5 illustrates its transmitting and receiving stations. The locating system
includes transmitting stations (T1-T8), receiving stations (R1-R4), a server connected to the receiving stations and functioning as a data management unit, anda positioning computer connected to the server. The
server and positioning computer function as the center station in Fig. 3. The locating system also includes(u2,v2)

Position of tag
Past record
Environmental coefficient

R2

T7
User terminal ad51

d52

T6

User terminal b
LAN
User terminal c

T5 (x5,y5)Target tag ID
Transmittingstation
d53
T3

d54

e53

(u4,v4)
e54

T8

R3

T4

Positioning computer

R4LANServer(u3,v3)

Received data

Fig. 4. Our locating system.
Receiving station

Transmitting station

Microcontroller

ID signal
generator

ControlID signal
TransmitterReceiverIDsignal

Anti-collision
determination unit

ID signalControlMicrocontroller

Fig. 5. Structures of the transmitting and receiving stations used in the locating system.

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user terminals a-c connected to the positioning computer. These components are connected to one another via a local area network (LAN). In the example
shown in Fig. 4 the receiving stations (R1-R4) are
fixed, and their positions are known. T 1-T 4 areattached to R1-R4, respectively, so they can be regarded as having the same positions as them. The positions of T5-T8 are unknown. The transmitting station
has a microcontroller, a transmitter, and an ID signalgenerator. The ID signal generator periodically generates an ID signal containing a unique ID of thattransmitting station.
The receiving station has a microcontroller, areceiver, and an anti-collision determination unit. The
receiver receives signals and measures their intensities. The receiver then passes the received signals
onto the anti-collision determination unit which reads(or extracts) the identifiers from the received signals.
The receiving station reports the received signal
intensities and the corresponding identifiers together
with time stamps to the server. The server records andstores each of the intensities in association with thecorresponding identifier and time stamp. Timestamps may be created by the server when the server
receives signal information from the receiving station.
The positioning computer estimates the position oftransmitting station T5 (in the example shown in Fig.
4) using information about this transmitting station
stored in the server. The estimate is also stored in theserver. The user can obtain the position of the transmitting station T5 by inputting the identifier of thistransmitting station through the user terminal to beretrieved in the server. The positioning computer
determines a correcting formula defining a relationship between signal intensity and distance to accurately estimate the position of either a transmitting orreceiving station indoors. The first correcting formula is a corrected Friis’ formula using correcting coefficients including environmental coefficients. Since
the distance and hence the positional coordinates of atransmitting station are estimated based on the actually measured value taking into account the correcting factors, the positioning accuracy is improved evenindoors.
The algorithm using a corrected Friis’ formula (thatis, the first correcting formula) is explained in detailbelow. For simplicity, the explanation below uses
two-dimensional coordinates, but the positioning
computer actually estimates positions using threedimensional (spatial) coordinates.

Vol. 1 No. 9 Dec. 2003

2.1 Algorithm of corrected Friis’ formula
If the known position of the jth receiving station is(uj, vj) and the unknown position of the ith transmitting station is (xi, yi), then the distance between thestations is expressed:
dij = ( xi − u j )2 + ( yi − v j )2 .

(2)First, we correct Friis’ formula using correcting
coefficients S1 and S2 on the assumption that signal
intensity decreases logarithmically with distance.
The corrected formula considering interference fromenvironmental factors is expressed aseij = S1 × log10 (dij) + S2 ,(3)

where eij denotes the intensity of the signal transmitted from i received by j.
The solutions for two unknowns (S1 and S2) thatminimize the error can be obtained by minimizing
estimation function “q” given by Eq. (4).
rn tnq = ∑ ∑ (eij − S1 log10 ( dij ) − S2 )2 ,(4)j =1 i =1

where rn denotes the number of receiving stations atknown positions and tn denotes the number of transmitting stations at known positions. In the example inFig. 4, both rn and tn are 4, so there are 16 (4 × 4)
terms in the equation. Consequently, two unknowns(S1 and S2) can be determined. To clarify the explanation, the unknowns are marked with an arc above
the symbols in Eq. (4).
Next, the estimated distance kij derived from themeasured intensity eij is expressed askij = 10(eij − S2)/S1 .

(5)

Now, the position of the target (unknown) transmitting station “i” is determined by minimizing estimation function hi expressed byrn(hi = ∑ 10 ij
j =1( e − S2 )/ S1− ( xi − u j )2 + ( yi − v j )2) . (6)2The unknowns in Eq. (6) are also marked with an arc
above the symbols for clarification. Using thismethod, the position of an unknown transmitting station can be estimated accurately even if the number of
fixed-position receiving stations is not so large.
The estimated position of the transmitting station isstored in the server. As has been explained, to check23Selected Papers

the position of a target transmitting station, the user
(or the manager) simply asks the server via the LAN
by inputting the identifier of the target transmitting
station through the user terminal. The positioning
computer repeatedly estimates and updates the positions of transmitting stations using update information.
3. Application for asset management
Our prototype asset management application using
long-life active RFID tags (shown in Fig. 6) is basedon commercially available equipment. We chose the300-MHz frequency band to avoid interference fromwireless LANs. We use on-off keying (OOK) as themodulation method. Each tag is 4 cm × 2 cm × 0.5 cm
and weighs 6 grams. An ID is sent out every sevenseconds; this interval gives a battery life of about
three years. The readers have an anti-collision function, which can handle signals from about 400 tags ata time.
In the prototype system we adapted commerciallyavailable readers to increase their sensitivity, and wedeveloped software to handle the position and location information by linking to a database. The communication range is about 20 m (line of sight) or
about 10 m indoors. Figure 7(a) shows the layout of
the experimental laboratory and position estimation
results. Receivers are fixed at positions P1-P5. The
positions of tags set at each lattice point are estimat-

ed using the information supplied from thesereceivers. Each square in the graph is a unit area on
the floor with a length of 1.0 m. In Fig. 7(b), the rootmean-square (RMS) error between estimated andactual tag positions measured at 77 (11 × 7) lattice
points is 2.9 m. By executing the positioning computer operation flow shown in Fig. 4, we could estimate the positions of multiple tags with high accuracy taking into account the environmental factors.
Figure 8 shows an example of asset tracking. We
can access this database directly from the Web, andwe can search for individual assets: the room they arein, their positions in that room, and the movement
history of each asset. Including image data in the4 cm × 2 cm × 0.5 cm

Fig. 6. Photograph of long-life active RFID tag.

Size of grid: 1.0 m × 1.0 m(1, 7)

Annotation:
P1–P5 indicate the positions of the receivers.

P1(11, 7)
P2

Partition made of aluminum(Height: 1.9 m)

P5Transmittingstation

Receiving station

P3

P4(1, 1)(11, 1)

Fig. 7(a). Layout of experimental laboratory.

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Selected Papers(1, 7)(11, 7)

9-10
8-9
7-8
6-7
5-6
4-5
3-4
2-3
1-2
0-1

P5

(1, 1)

Error distribution [m](11, 1)RMS error between estimated and actual tag positionsmeasured at 77 (11 × 7) lattice points is 2.9 m

Fig. 7(b). Results of estimating tag positions.
Position in the room
Room in whichasset exists

History ofmovement

Photograph of asset

Fig. 8. Example of tracking of assets.

database lets us find the assets more easily. Furthermore, this system can email the owner to inform
him/her when assets are moved from room to room orwhen none of the readers can receive signals from thetag, or it can email the administrator to inform
him/her when the network experiences trouble.

Tom A. Scharfeld, Massachusetts Institute of Technology, for their encouragement and fruitful discussion.
References[1]

4. Acknowledgments[2]

We thank Professor Hideki Mizuno, Tokai University, Doctor Hirohito Suda, NTT DoCoMo Inc., andVol. 1 No. 9 Dec. 2003

K. V. S. Rao, “An overview of backscattered radio frequency identification system (RFID),” 1999 Asia Pacific Microwave Conference
Digest, Singapore, Vol. 3, pp. 746-749, 30 Nov. 1999.
J. R. Tuttle, “Traditional and emerging technologies and applicationsin the radio frequency identification (RFID) industry,” 1997 Radio
Frequency Integrated Circuits Symposium Digest, Denver, U.S.A.,

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[3]

[4]

[5]

[6]

pp. 5-8, Jun. 1997.
M. Kossel, H. Bebedickter, and W. Baechtold, “An active tagging
system using circular polarization modulation,” 1999 IEEE MTT-SDigest, Anaheim, U.S.A., Vol. 4, pp. 1595-1598, Jun. 1999.
G. Lachapelle, B. Townsend, H. Gehue, and M.E. Cannon, “GPS versus Loran-C for vehicular navigation in urban and mountainousareas,” Proceedings of the IEEE-IEE Vehicle Navigation and Information Systems Conference, Ottawa, Canada, pp. 456-459, Oct.
1993.
J. Werb and C. Lanzl, “Designing a positioning system for finding
things and people indoors,” IEEE Spectrum, Vol. 35, No. 9, pp. 7178, Sep. 1998.
J. Hightower and G. Borriello, “Location systems for ubiquitous computing,” IEEE Computer Mag., Vol. 34, No. 8, pp. 57-66, Aug. 2001.

Hitoshi Hayashi
Senior Research Engineer, Wireless Systems
Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in electronics engineering from the University ofTokyo, Tokyo in 1990, 1992, and 2000, respectively. In 1992, he joined the NTT Radio Communication Systems Laboratories, Yokosuka,
Japan. He is currently with NTT Network Innovation Laboratories, Yokosuka, Japan, where heis engaged in the development of wireless communication systems. From 2000 to 2001, he wasalso involved in wireless systems research at theMassachusetts Institute of Technology (MIT),U.S.A., as a Visiting Scientist. He is a member ofIEEE and the Institute of Electronics, Information and Communication Engineers (IEICE). He
received the 1998 Young Engineer Award fromIEICE.

Toshimitsu Tsubaki
Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.S. and M.S. degrees in geophysics from Tohoku University, Sendai, Miyagi
in 1994 and 1996, respectively. In 1996, hejoined the NTT Wireless Systems Laboratories,Yokosuka, Japan. He is currently with NTT Network Innovation Laboratories, Yokosuka, Japan.
Since joining NTT, he has been engaged in R&D
of broadband wireless access systems and RFID
tagging systems. He is a member of IEICE.

Tomoaki Ogawa
Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.A. degree in environmental
information and Master degree in media and governance from Keio University, Kanagawa in
1996 and 1998, respectively. After working at
NTT Wireless Systems Laboratories from 1998
to 1999, he joined NTT Network Innovation
Laboratories where he has been working on wireless ad-hoc networking and local positioning
systems. He is a member of IEICE.

Masashi Shimizu
Senior Research Engineer, Supervisor, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E. and M.E. degrees inmechanical engineering from Keio University,Yokohama, in 1986 and 1988, respectively. In
1988, he joined NTT Wireless Systems Laboratories, Yokosuka, Japan. Since then, he has been
engaged in research on the pointing control for
deployable space antennas and surface error
compensation through feed distributions control.
His recent interest focuses on active RFID and its
applications. He is a member of IEICE.

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