A Diversity-Based Multi-Antenna Design for Comprehensive RFID Tag Reading Wen-Jiao Liao 1 , Yi-Chung Li 1 , Hao-De Tang 1 , Hsin-Chin Liu 1 , and Chin-Chung Nien 2 1 National Taiwan University of Science and Technology, Taipei, Taiwan 2 Industrial 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 191