RFID Tags Localization along an axis using a Tunable Near-Field Focused
`Circular-Phase Array Antenna
`
`R. Siragusa1, P. Lemaître-Auger1, A. Pouzin1 and S. Tedjini1
`
`1Laboratoire de Conception et d’Intégration des Systèmes, Grenoble INP-Esisar, 50 Barthélémy de Laffemas, BP54,
`26902 Valence, France. Email: pierre.lemaitre-auger@esisar.grenoble-inp.fr
`
`
`
`
`
`Abstract
`
`A novel concept for RFID tag localization using a tunable near-field focused circular-phase array antenna
`
`working at 5.8 GHz is presented. It serves as the reader antenna and focuses the power into a small region, in the tag
`vicinity. By scanning the focal spot along one axis and monitoring the differential scattered power by a tag, its position
`along the axis is easily computed with good accuracy. This simple localization scheme is well adapted for specific
`localization scheme, for example for objects placed over a conveyor belt.
`
`1. Introduction
`
`In less than a decade, radiofrequency identification (RFID) has known an incredible growth, it is now a well
`
`known technology used in many industrial applications [1]. To reach the efficiency it has today, the research effort first
`focused on the tag design to keep it unaffected by the environment and on the base station antenna, often designed to
`radiate with a high gain in a static direction. But more recently, for security reasons and under the industrial needs, an
`exciting and new research RFID topic appeared: indoor tag localization [2, 3].
`
`
`
`Several tag localization techniques exist for two dimensions (2-D) and fewer for 3-D. All of them use more than
`one reader antenna [3, 4]. Several different algorithms are proposed, some requiring initial calibration with special
`reference tags. Each technique is often well adapted to a particular situation. Best accuracy obtained up to now is on the
`order of 15cm [5]. In all those developments, final cost of a system is always an important key factor in its
`implementation in industry.
`
`
`
`In [6], the authors proposed a tunable near-field focused phased array antenna (FPAA) for RFID systems
`working at 5.8 GHz. This system provides a dynamically tunable focal length thanks to two phase shifters. In this work,
`we proposed a novel concept for tag localization: delivering power only close to a tag, where it is needed. We will show
`that by doing this, in simple situation like a conveyor belt, this lead to a simple and potential efficient localization
`system. It uses passive tag, two phase shifters and classical envelope detection.
`
`
`
`The theory of passive RFID systems is first presented. Then, the overview of the system and the localization
`algorithm is detailed.
`
`
`2. Theory of Passive RFID Systems
`
`A schematic view of an RFID system is presented in Fig. 1 (a) which shows the two principal components: the
`
`reader, also called the base station, and the tag. The reader is the active part of the system: it sends power and
`information to the tag which is totally passive in most RFID systems. Thanks to the chip included in the tag, the load
`connected to the tag’s antenna is switch between two different impedances: Z1 and Z2. The reflected power is thus
`modified at each switch, producing a modulation of the backscattered power. At first, the reader sends a continuous
`signal to deliver power to the passive tag. Then, to start the communication, an information frame is sent by the reader
`as it is shown in Fig. 1(b). Finally, if the power received by the tag is greater than the low-threshold power, called the
`activation power, the tag responds and the communication between the two elements begins.
`
`
`
`From the Friss equation, the power reradiated and scattered by the tag and delivered to the load of the reader,
`PL_reader is calculated with the following equation [7, 8]:
`2*
`
`⋅
`=
`(cid:65)Γ−
`
`(1)
`
`,
`
`(cid:65)(cid:65)
`
`Z
`Z
`
`+−
`
`*
`
`aa
`
`Z
`Z
`
`,
`
`=Γ
`*
`(cid:65)
`
`P
`L
`
`_
`
`reader
`
`A
`Re
`_
`
`A
`e
`
`_
`
`tag
`
`1
`
`S
`
`i
`
`G
`tag
`π
`2
`d
`4
`
`978-1-4244-6051-9/11/$26.00 ©2011 IEEE
`
`RFC - Exhibit 1016
`
`1
`
`

`
`where Ae_R and Ae_tag are the effective area of the reader antenna and the tag antenna respectively, Γ(cid:65) is the modified
`reflection coefficient that takes into account the tag-load and the tag-antenna impedances, Z(cid:65) and Za [9], Si is the power
`density seen by the tag, Gtag is the tag gain and d is the distance between the tag and the reader antenna. This equation
`assumes that the reader antenna and its load are matched and that the polarizations of both antennas are also matched.
`The differential scattered power received at the reader load, ΔPr, is easily obtained from (1) and is:
`(
`)
`( Γ−−
`)
`G
`2*
`2*
`tag
`=
`Δ
`Γ−
`⋅
`
`A
`P
`A
`S
`
`
`
`
`1
`1
`
`e
`Re
`R
`2
`1
`_
`π
`2
`d
`4
`
`i
`
`(2)
`
`_
`
`tag
`
`
`
`Figure 1 (a) Passive RFID system. (b) Schematic representation of an RFID communication.
`
`3. RFID Tag Localization System
`
`3.1 General Overview
`
`The general overview of the reader localization system is schematically presented on the Fig. 2. It consists of
`
`an FPAA which focuses the incident power into a small region around the focal point [6]. The FPAA produces an E-
`field linearly polarized. The FPAA is made with 24 dipole antennas radially oriented and equally placed on three
`concentric rings of radii: 10, 30, and 50cm. The working frequency is 5.8 GHz. The antennas are fed with identical
`current magnitude but with a phase which is proportional to the off-axis distance. The condition imposed on the
`different phase delays is that each quasi-isotropic waves produced by individual antennas must interfere constructively
`at the focal point. The focalization distance is tuned inside a delimited range along the propagation axis (the z-one) with
`the help of phase shifters. Thanks to the circular dispositions of antennas, few phase shifters are required: only two in
`the present case.
`
`
`Figure 2 Schematic representation of the FPAA, its feeding network, and of the tag localization algorithm sub-system.
`Each antenna is fed with a coaxial cable connected to a 1:8 power divider [6].
`
`The focal spot power density, Si, can be approximated by the following empirical function:
`(
`)
`−
`2
`f
`z
`r
`2
`2
`−
`−
`(
`)
`(
`)f
`(
`)
`(3)
`,
`
`=
`fw
`w
`Ae
`e
`zrS
`,
`FD
`B
`i
`where A is a proportionality constant, r is the off-axis distance, f is the focal length and wB(f) and wFD(f) are the lateral
`and the longitudinal half-width power at 1/e2 respectively. From measurements published in [6], empirical formulas
`were found for the latter:
`
`2
`
`
`
`2
`
`

`
`
`
`(
`fw
`B
`(
`w
`FD
`
`)
`f
`
`f
`−
`
`.
`
`(4)
`
`=
`.0
`551
`)
`=
`f
`30
`
`+
` (in m)in f cm; 2569
`.0
`
`
`
`
`
`
`
`
`
`
`
`
`
` 3.1 (in m)in f cm;
`
`3.2 The Tag Localization Principle
`
`The localization technique proposed here is meant to find the z-position of an RFID tag with respect to the
`
`reader. The physical principle of localization is the following. Due to the focusing nature of the FPAA, there is
`sufficient power to wake up the tag only in the vicinity of the focal point. In a coarse description, this region could be
`assimilated to an ellipsoid, later called the power-ellipsoidal volume (PEV). If a tag is outside the PEV, the reader do
`not return modulated power to the reader. Inversely, when the tag is inside the PEV, the tag power is above the
`activation power and a scattered modulated signal is now received by the reader. A first rough estimate of the tag z-
`, where f is the focal length of the FPAA and FD is the effective focal depth
`=
`±
`f
`FD
`ztag
`position, ztag, is therefore:
`defined as the distance from the focal spot center for which the tag activation power is reached. FD is proportional to
`wFD, and, in fact, defines the front and back limits of the PEV.
`
`
`
`This precision can be improved by performing a scan of the focal spot. This can easily be done with simple
`dedicated electronics added to the reader. Its role is double: 1) to feed the electronic phase shifters for the scan process
`from zmin to zmax, and 2) to monitor the magnitude of the scattered differential power, ΔPr, received by the reader. For a
`tag positioned at ztag, ΔPr, is null until the PEV reaches the tag. From that moment, the tag will start responding to the
`reader with a low magnitude ΔPr because the activation power is just above the threshold limit. This situation is
`illustrated in Fig. 3(a). For increasing values of f, the power density seen by the tag will increase rapidly until it reaches
`a maximum. From equation (2), we see that ΔPr will also increase up to a maximum which occurs when f = ztag. This
`second situation is illustrated in Fig. 3(b). For f > ztag, the magnitude of ΔPr will rapidly decrease until f reaches z3, the
`position for which the PEV leaves the tag, see Fig. 3(c). Above that point, activation power is not reached at the tag and
`ΔPr becomes null once again. A schematic representation of ΔPr and of its absolute magnitude during the scan is
`represented in Fig. 4. The tag actual z-position thus corresponds to the focal spot position for which the maximum
`magnitude of ΔPr is recorded. With the scan, the precision of the measurement is improved. It depends on the signal-to-
`noise-ratio (SNR) of the whole RFID system.
`
`
`Figure 3 Three different situations encountered during the scan of the focal spot along z. The tag position is ztag. Focal
`length is: (a) z1 < ztag, (b) ztag, (c) z2 > ztag. Schematic graph of the backscattered power for the three focal lengths.
`
`For this new localization concept, a simple situation was assumed: the tags are quasi-static during the scanning
`process and they are all placed at a height such that the PEV will indeed pass through them. It is also assumed that at all
`times, only one tag is present in the power-ellipsoid volume.
`
`
`
`Finally, it is important to mention that the above discussion is qualitative and relies on equation (2) which is
`valid in the far-field approximation: effective areas are indeed defined for plane waves [7]. In the present situation, the
`tag cannot be considered as illuminated with a single plane wave. The exact power that will be received cannot be
`simply calculated from (2). However, only relative value of the power is pertinent in the localization technique. Also,
`any field can be decomposed on the basis of plane-waves and evanescent fields [10]. The latter do not carry power and
`equation (2) can be considered valid and evaluated for each plane-wave of the decomposition. The total power can be
`obtained by summation of all those contributions. Very small biases come from the decomposition because of the static
`
`
`
`3
`
`

`
`position of the tag during the scan. Even though the effective areas and Gtag are functions of the plane-wave directions,
`the plane-wave spectrum will change very slowly during the scan process at a given ztag.
`
`
`Figure 4 Schematic representation of the differential scattered power by the tag and measured at the reader load and of
`the envelope magnitude of the signal. The tag z-position corresponds to the envelope maximum value.
`
`5. Conclusion
`
`A novel concept of an RFID tag localization was proposed. It is based on a reader dynamic Focused Circular-
`
`Phase Array Antenna capable of rapidly modifying its focal length. It was shown that a scan of the focal spot leads to
`tags localization along the z-axis. More analysis of the system is underway.
`
`6. References
`
`[1] K. Finkenzeller, RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification.,
`Second ed.: John Wiley & Sons, 2003.
`[2] M. Bouet and A. L. dos Santos, "RFID tags: Positioning principles and localization techniques," in Wireless Days,
`2008. WD '08. 1st IFIP, 2008, pp. 1-5.
`[3] T. Sanpechuda and L. Kovavisaruch, "A review of RFID localization: Applications and techniques," in Electrical
`Engineering/Electronics, Computer, Telecommunications and Information Technology, 2008. ECTI-CON 2008.
`5th International Conference on, 2008, pp. 769-772.
`[4] C. Hekimian-Williams, B. Grant, L. Xiuwen, Z. Zhenghao, and P. Kumar, "Accurate localization of RFID tags
`using phase difference," in RFID, 2010 IEEE International Conference on, 2010, pp. 89-96.
`[5] K. Chawla, G. Robins, and L. Zhang, "Object localization using RFID," in Wireless Pervasive Computing
`(ISWPC), 2010 5th IEEE International Symposium on, 2010, pp. 301-306.
`[6] R. Siragusa, P. Lemaitre-Auger, and S. Tedjini. "Tunable Near-Field Focused Circular-Phase Array Antenna for
`5.8 GHz RFID Applications," Antennas
`and Wireless Propagation Letters,
`IEEE,
`(DOI:
`10.1109/LAWP.2011.2108632).
`[7] C. A. Balanis, Antenna Theory: Analysis and Design., Third ed. Hoboken, N.J.: Wiley-Interscience, 2005.
`[8] K. V. Seshagiri Rao, P. V. Nikitin, and S. F. Lam, "Antenna design for UHF RFID tags: a review and a practical
`application," Antennas and Propagation, IEEE Transactions on, vol. 53, pp. 3870-3876, 2005.
`[9] A. Pouzin, A. L. Perrier, T. P. Vuong, J. Perdereau, L. Dreux, and S. Tedjini, "Measurement of RCS and reading
`range of UHF tags in free space and with wood support," in Advanced Technologies for Communications, 2008.
`ATC 2008. International Conference on, 2008, pp. 262-265.
`[10] J. W. Goodman, Introduction To Fourier Optics., Second ed.: McGraw-Hill, 1996.
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