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Top1. Introduction
The research of wireless position technology originates from the military application in the World War II. As is well known, GPS (Global position system) is the most mature system for outdoor localization. However, suffering from the buildings structure and indoor environment, GPS cannot work for obtaining the signal successfully in the indoor environment (Lhuillier, 2012). Nowadays, there are many requirements for accurate locating a target, such as in the airport, and we need to real-time track luggage.
With the development of Internet of things, RFID (Radio Frequency Identification) technology has been employed in many industries (Chen, Cheng & Huang, 2013; Kundra, Dureja & Bhatnagar,2014; Zhu, Chen & Wang, 2014; Sakurai, 2011; Yamagiwa, Murakami & Uehara, 2011; Ishida, Ando, Uchida & Shibata, 2016; Ito, Hirakawa, Arai & Shibata, 2016; Cha & Kim, 2014), such as logistics, ID cards, retails, and in recent years, many sensors based system have used RFID to automatically collect information (Xia, Yun, Yu, Yin, Wang & Bu, 2014); (Zhang, Yang & Zhao, 2016); (Khan, Saad, & Aalsalem, 2013; Zhang & Minier, 2013; Wang, 2015). RFID has also been employed in the indoor localization system. In addition, there are other indoor position technologies (Rida, Liu, Jadi, Algawhari & Askourih, 2015; Zhao, Mojrzisch & Wallaschek, 2013), such as infrared position, Bluetooth position, and ultrasonic position. The most preponderant advantage of RFID-based indoor position system is that RFID has low cost, better positioning effect, and higher accuracy. Many technologies have been proposed for RFID indoor localization. One is the frequently-used technology, which consists of triangulation location, fingerprinting position method, and near space method. A lot of companies and universities have get many research achievements at RFID indoor localization area, such as Microsoft’s RADAR System, AcroScout’s Mobileview System, Ekahua’s Position Engine System and SpotON System of UW. The present RFID indoor localization technologies are divided into two varieties: (1) Triangulation method: this method uses multiple spots cooperation for position with distance, angle and related elements. (2) Scene perception method: this method locates object with advanced research on monitoring area or real-time reaction of wireless radio signal in the whole monitoring area.
Triangulation method composes of time-based method, antenna angle-based method, and radio feature-based method. Firstly, time-based method locates target tag using time or time difference, which includes TOA and TDOA (Azzouzi, Cremer, Dettmar, Kronberger & Knie, 2011). As shown in Figure 1, TOA takes three anchor nodes for reference points, which describes three possible cycle areas among the target node and reference nodes. TDOA method has the similar theory as TOA, which uses the distance difference between the anchor nodes and target node for indoor position. Secondly, AOA uses the angle of target node relative to anchor node for indoor position. In this method, every anchor node can get the angle of target node or anchor nodes so as to get the target localization (Montaser & Moselhi, 2014; Wang, Amin & Zhang, 2006). Thirdly, radio feature of RFID can be used for triangulation method, called DAH (Differential Augmented Hologram), which divides the monitoring area into many grids, and gets the most possible grid, where the radio feature is closest to the theory value (Liu, Yang, Lin, Guo & Liu, 2014; Yang, Chen, Li, Xiao, Li & Liu, 2014; Shangguan & Jamieson, 2016; Shangguan, Yang, Liu, Zhou & Liu, 2015).
Figure 1. Time-based triangulation method