RFID Localization using Planar Antenna Arrays with Arbitrary Geometry. Mohammed J Abedin and Ananda S Mohan. Centre for Health Technologies. Faculty of ...
RFID Localization using Planar Antenna Arrays with Arbitrary Geometry Mohammed J Abedin and Ananda S Mohan Centre for Health Technologies Faculty of Engineering and IT University of Technology Sydney (UTS), Australia. {mabedin, ananda}@eng.uts.edu.au Introduction RFID is emerging as the key technology for wireless identification and localization for assets, humans etc. Each RFID tag contains a code that uniquely identifies the object and can be queried by a wireless reader. RFID tags come in two types: active and passive tags. Active tags are equipped with their own batteries and are housed with the transmitter and antenna to communicate with readers at a designated frequency. Passive RFID tags are battery free and identify themselves by producing backscattered signals when the incident energy from nearby readers (query) impinges on them. In general, an RFID tag can only report its identity but not its location. Self-locating capability requires the use of reference battery-powered tags which could be a bottleneck for medium to large-scale deployments[1]. It is reported that putting a number of active, reference, RFID tags in an environment may even cause significant interference and can undermine the receipt of backscattered signal from weaker tags leading to inaccurate location estimation. Thus, the availability of battery-free, distributed localization system for RFID tags using multiple antennas can be very advantageous. Generally, RFID tags are deployed in wireless environments where the presence of multipath propagation can cause errors in positioning. Thus, it is of interest to investigate the RFID localization performance in multipath scenarios using arrays of reader antennas located in the radiating near field of tags. Zhang et al [2] applied a direction of arrival estimation method to obtain angular information to localize passive RFID tags. However, they did not consider the effects of multipath. A more accurate location of tags can be achieved when both range and angular coordinates are available. In this paper, we present a method of localizing RFID tags located in a Ricean multipath environment using a planar arbitrary array of reader antennas. By employing the well known MUSIC algorithm, we estimate the range and angular coordinates of the tags. When many reader antennas are positioned closely together, the mutual coupling between them affects the accuracy of localization. Assuming that the reader array is formed of thin half-wave dipoles, we present a technique to compensate the effects of mutual coupling for improving the accuracy of localization[3]. Signal Structure We consider that the signals received at an arbitrary reader array arrive from passive tags which are located within radiating near field of the array. In this case, the coupling between tag and reader antennas and the field distribution around them can be affected. To simplify, we assume that the tag antenna to be a small dipole so that it would not perturb the field of the reader antenna [4]. Nonetheless, we included the mutual coupling between reader antennas within the array to estimate the location parameters. We also assume that the array elements are arbitrarily placed in a two dimensional plane with reference to a known centroid. We consider a Ricean fading environment with the presence of LOS signal between reader and the tag.
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Once the tag receives the RF signal transmitted by the reader transmitter, it produces a backscattered response by changing its input impedance states. Generally, the impedance states are high and low, and both states present different radar cross sections. This variable impedance state creates amplitude shift keying (ASK) or phase shift keying (PSK) modulated signal. Here, we assume that the antennas to be perfectly impedance matched and incoming signals at the reader to be similar to the radiation from dipole sources. We also assumed that the tag and reader antennas are within the receiving range of each other and a LOS is present at the reader along with multipath. Now, the signal x(t) at the m-th reader antenna can be obtained after removing the carrier given by xm (t ) = [αa (r ,φ ) + h(t )]s (t ) + nm (t ) (1) where denotes the strength of the line of sight of modulated signal which is returned from tag, h(t) includes the channel effect and nm(t) is additive white Gaussian noise at mth antenna and s(t) is the signal envelope. We fixed the elevation of tag at =90° so that the tag and the reader array to be co-planar. Assuming that the tag antenna and reader antennas are thin dipoles oriented parallel to Z-axis, the array steering vector can be expressed as e − jβρ k ( m ) e − jβR1 ( m ) e − jβR2 ( m ) a(rk , φ k ) = + − 2 cos( βL) (2) ρ k ( m) R1 (m) R2 ( m ) 2 dm d ρ k (m) = rk (1 + 2 − 2 m cos φ k ) − 1 rk rk
2 2 R1 (m) = sqrt ( ρ k (m) − L )
(3) (4)
R2 (m) = sqrt ( ρ k (m) 2 − L2 ) (5) where denotes the wave number, L denotes length of the tag antenna, and R1, R2 are the distances from two ends of k-th dipole tag to m-th reader array, and dm is the distance measured from reference point to m-th reader array.
RFID Localization
We employ MUSIC algorithm [5] to estimate the location for passive RFID tags using arbitrary arrays of reader antennas. We assume that K tags radiate uncorrelated signals on to an M-element reader array. For an arbitrary array, the error due to calibration and mutual coupling among the elements can degrade the localization performance. We also assume perfect calibration, but the effects due to mutual coupling are incorporated by calculating the mutual coupling matrix using the standard EMF method. We then use a compensation technique at the reader [3]that require the knowledge of array output. To compensate for the mutual coupling, first we estimate the location of k uncorrelated tags without considering mutual coupling (ideal case) at the reader array. For this, the signal covariance matrix was formed at the reader given by 1 N R x = x(t )x H (t ) (6) N n =1 The eigen value decomposition of Rx was carried out to find out the signal and noise subspaces which are orthogonal to each other. The locations (azimuth and range) of radiating sources can be found by using a 2-D MUSIC [5]. MUSIC algorithm exploits the orthogonality between the signal and noise subspaces but mutual couplings of antenna array
destroy the orthogonality. Consequently the performance of estimation algorithm is deteriorated. Therefore, compensation is necessary to recover the localization performance. However, when the mutual coupling matrix is incorporated in the ideal MUSIC algorithm, the equation for pseudo-spectrum is modified as a H (r ,φ )a(r ,φ ) (7) Pmod = H a (r ,φ ) H UU H a(r ,φ ) To compensate for the mutual coupling, a compensation matrix should be determined accurately to adopt into (7) during localization. The compensation matrix can be calculated by using an optimization method. Here we calculate by using the standard Genetic algorithm. Our objective function is the correlation between ideal MUSIC pseudo-spectrum and the one given by (7). The genetic algorithm runs until the value of correlation coefficient approaches to unity. Incorporation of compensation matrix in (7) allows proper determination of location information in the presence of mutual coupling. Results and Discussion We have considered a ten-element array at the reader which is positioned arbitrarily in a 2D plane shown in Fig. 1(a) with respect to the reference. The array elements are taken to be thin vertical dipoles and are considered to be perfectly calibrated so that the array phase centre is assumed to be known. To model a RFID tag, another vertical thin, small dipole is considered in the radiating near-field of the reader array. The operating frequency is 900MHz. A Ricean fading channel is considered with fixed number of multipath. A signal to noise ratio of 30 dB is considered. For simulation, the elevation angle of the tag is set to be 90°. The collision between readers and tags has been ignored. The mutual coupling is calculated using standard EMF method. The correlation coefficient for compensation is chosen to be greater than or equal to 0.8. Fig 1 (b) shows the location parameters calculated using ideal MUSIC pseudospectrum without incorporating mutual coupling. Figs. 2(a) show the MUSIC pseudospectrum after compensating mutual coupling effect. The root mean square error (RMSE) performance of estimated results without and with mutual coupling effect in a non-fading channel as well as in a Ricean fading channel (without mutual coupling) are presented in Fig. 2(b) where an arbitrary planar array is considered at the reader. Conclusions The results indicated that the MUSIC algorithm can effectively be applied with an arbitrary reader antenna array for the localization of passive RFID tags located in its near-field. Also, results demonstrate that the localization can overcome the ambiguities introduced by the mutual coupling through introduction of our compensation technique. Acknowledgements: The work reported in this paper is supported in part by the Australian Research Council through a discovery project grant: DP 0346540.
References [1]
[2]
L. M. Ni, Y. Liu, Y. C. Lau, and A. P. Patil, "Landmarc: Indoor location sensing using active RFID," in IEEE Conference on Pervasive Computing and Communications, 2003, pp. 407 – 415. Y. Zhang, M. G. Amin, and S. Kaushik, "Localization and tracking of passive RFID tags based on direction estimation," International Journal of Antennas and Propagation, vol. 2007, p. 9 pages, 2007.
[3]
[4] [5]
T. Huang and A. S. Mohan, "Effects of array mutual coupling on near-field DOA estimation," in IEEE (CCECE 2003) Canadian Conference on Electrical and Computer Engineering 2003, pp. 1881-1884. P. V. Nikitin, K. V. S. Rao, and S. Lazar, "An overview of near field UHF RFID," in IEEE International Conference on RFID, 2007, pp. 167-174. Y.-D. Huang and M. Barkat, "Near field multiple source localization by passive sensor array," IEEE Trans. Antennas Propag., vol. 39, pp. 968-975, Jul. 1991. Figures
Y (x1,y1) r1 dm
r2
Passive RFID tag
X
(xm,ym) m-th reader array
(a)
(b)
Figure1. (a) Near-field radiating RFID localization scenario with an arbitrary antenna array of 10-dipole elements and two tags are to be located at (x1=0.6, y1=0.3) and (x2=1.2, y2=-0.15). (b) Estimated pseudospectrum of two passive RFID tags in the near field.
(a)
(b)
Figure 2: MUSIC pseudo spectra for a 10-element arbitrary planar array of dipoles. A single tag is located at (x=1.27, y=0.55). (a) Pseudo-spectrum after using GA based mutual coupling compensation method, (b) Root mean square error (RMSE) performances of estimated azimuth and range of tag.
