A Diversity-Based Multi-Antenna Design for Comprehensive RFID Tag Reading

Wen-Jiao Liao1, Yi-Chung Li1, Hao-De Tang1, Hsin-Chin Liu1, and Chin-Chung Nien2 1National Taiwan University of Science and Technology, Taipei, Taiwan

2Industrial Technology Research Institute, Hsinchu, Taiwan

Abstract - In this work, a multi-antenna design is proposed for remotely reading RFID tags on a push cart that passing through warehouse gates. The propagation environment is challenging because the partly closed cart is made of stainless steel. Line-of-sight paths from reader antennas to RFID tags, which are attached on containers, are likely blocked by shelves on the cart. In order to provide a comprehensive reading coverage, several measures were taken to fully diversify propagation paths, eight antenna panels of different orientations, height, and polarizations are installed on poles at the two sides of the gate. Measurements with a four-port reader indicate that by installing two tags on each container, a 100% read rate can be achieved for detection of containers passing through.

Index Terms — Patch antenna, pattern diversity, spatial diversity, RFID, multipath.

1. Introduction

The adoption of radiative radio frequency identification (RFID) technology provides a convenient and cost-effective way to monitor the flow of goods, which is crucial to logistic management. Unlike the conventional barcode approach, which requires placing the reader in front of the tag, an RFID reader can retrieve information at a distance of several meters. The scanning process can therefore be automated and the speed can be substantially enhanced.

Nevertheless, the RFID reading process, which involves wave propagation, is subject to influences of the environment. For example, the unavoidable multipath propagation in an indoor environment introduces the fast fading phenomenon and results in an unstable channel.

The diversity antenna technique is devised to mitigate the aforementioned problem. It achieves a great success for indoor WLAN deployment. Commonly used diversity approaches include spatial diversity, pattern diversity [1, 2], and polarization diversity [3, 4]. By increasing the number of available propagation channel, one stands a better chance to establish a stable link [5].

The projected operation scenario of this work is monitoring of goods entering and leaving a storage room. The goods are placed in rectangular or cylindrical containers, which are stacked on a push cart as shown in Fig. 1. The RFID reader and its antennas are mounted on poles setup at the two sides of the entrance door. RFID tags are attached to the containers. Note, the push cart is made of stainless steel plates and the shelve height is nearly 20 cm, line-of-sight propagation paths from the antenna panel to the tag may be

blocked. Also, tags within stacked containers are also subject to blockage.

Fig. 1. The push cart with stainless steel shelves and

containers with RFID tags.

2. Antenna Design

The commonly used 900 MHz radiative RFID protocol is adopted in this work. The RFID reader used is the Speedway R420 four-port reader. Several passive RFID tags were tested. The tag ICs used include Impinji Monza 5, Impinj Monza R6, and NXP UCODE 7. Corresponding read sensitivities are -17.8, -20, and -21 dBm, respectively.

The element used for the proposed eight-antenna design is a homemade patch antenna as shown in Fig. 2. The air-loaded corner-truncated patch is aimed to produce a directive beam with circular polarization. The patch is elevated above a square ground by 10 mm, while the ground length is 185 mm. Other geometric parameters of the patch design are listed in Table I. Fig. 2 also shows the fabricated prototype. The patch is etched on a 0.8 mm thick FB4 slab. Plastic bolts and nuts are used as spacers. Simulated and measured reflection coefficients spectra are shown in Fig. 3(a). The 920 to 926 MHz band is fully covered. Radiation patterns on principle cuts, which are shown in Fig. 3(b), reveals that the polarization of the fabricated patch is RCP.

To maximize the diversity in propagation paths, an eight-antenna design as shown in Fig. 4 is therefore proposed. It integrates antennas of different locations, orientations, and polarizations. Since the cart contains three metal shelves, antennas should be placed on both high and low locations. Because each shelf may contain several stacked containers, antennas needed be placed on both sides of the entrance to

[ThE1-2] 2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

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reduce the probability of blockage. Note, because the reader used has only four ports, the top and bottom panels, which employ different CP types, are grouped with power dividers.

Fig. 2. Geometry and prototype of the air-loaded, corner-truncated patch antenna for 900 MHz RFID reader uses.

TABLE I Geometric parameters of the air-loaded, corner-truncated patch antenna for 900 MHz RFID reader uses. (Unit: mm)

PW 141.2 L1 14.05 W1 31.4 L3 39

PL 145.3 L2 15 W2 31

(a) Reflection coefficient,

(b) Radiation patterns on principle cuts.

Fig. 3. Comparisons of simulated and measured reflection coefficient spectra and radiation patterns on principe cuts.

3. Performance Validation

Rigorous field tests were conducted to test and improve the read rate of tags on passing-through push carts. Control programs were written for the RFID reader using the low level reader protocol (LLRP). Preliminary tests showed that with just one reader antenna panel, the read rate for the tag is merely 50%. By adopting the eight-antenna configuration, the read rate is significantly enhanced to approximately 95%. The unread tags are either placed right next to the metal

shelves or stacked below another container, which may have metal objects placed inside. To mitigate this issue, the diversity approach is also employed for tag deployment. Instead of using just one tag, two tags are placed on each container. One is placed on the top while the other is placed on the side with different orientation. With this measure, even though a few tags still missed for a cart full of densely packed container, no container is not found since at least one tag is read.

Fig. 4. The eight-antenna system installed at the storage

room entrance for RFID reader uses.

4. Conclusion

This work demonstrates that by considering characteristics of the propagation environment, nearly perfect read rate can be achieved for radiative RFID operation. For the tested scenario, with the help from the diversity scheme, which includes both the antenna deployment and tag placement, all containers on the metal push cart can be detected by the proposed eight-antenna system.

References

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[3] Y. Li, Z. Zhang, W. Chen, Z. Feng, and M. F. Iskander, “A dual-polarization slot antenna using a compact CPW feeding structure,” IEEE Antennas Wireless Propag. Lett., vol. 9, pp. 191–194, 2010.

[4] H. Li, Z. T. Miers, and B. K. Lau, “Design of orthogonal MIMO handset antennas based on characteristic mode manipulation at frequency bands below 1 GHz,” IEEE Trans. Ant. Propag., vol. 62, no. 5, pp. 2756-2766, May 2014.

[5] W.-J. Liao, B.-Y. Dai, and B.-R. Hsiao, “Effective diversity gain evaluation for large scale MIMO antenna system,” IEEE Antenna Wireless Propag. Lett., vol. 15, pp. 1394-1397, 2016.

2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

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