Compared with traditional methods that employ inertial sensors or wireless sensors, device-free approaches do not require that people carry devices, and they are considered a useful technique for indoor navigation and posture recognition. However, few existing methods can detect the trajectory and movements of humans at the same time. In this study, we propose a scheme called PADAR for addressing these two problems simultaneously by using passive radio frequency identification (RFID) tags but without attaching them to the human body. The idea is based on the principle of radio tomographic imaging, where the variance in a tag’s backscattered radio frequency signal strength is influenced by human movement. We integrated a commodity off-the-shelf RFID reader with a two-dimensional phased array antenna and a matrix of passive tags to evaluate the performance of our scheme. We conducted experiments in a simulated indoor environment. The experimental results showed that PADAR achieved an accuracy of over 70%.

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Real-time location system (RLS) based on RFID is an effective indoor positioning system. The battery-free and low cost UHF passive tags can be attached on almost any objects, which are recognized as the best medium to achieve high precision ranging and positioning for large-scale objects. This paper proposes an indoor range measurement based on multifrequency phase difference of arrival (MF-PDoA) using UHF RFID passive tags and discusses the measurement principle, experiment implementation, and results evaluation in detail. After a theoretical overview of MF-PDoA range measurement principle, it introduces an experimental prototype under EPC C1G2 standard for range measurements. Both our prototype and a commercial off-the-shelf RFID reader have been used to verify the measurement method. We propose a Kalman filter and weighting method to process the measuring data. Experiment results indicate that, in a real environment, this method can effectively improve the ranging accuracy, which lays a foundation to extend the proposed measurement into two to three dimensions indoor object positioning.

In RFID systems, how to detect the position precisely is an important and challenging research topic. In this paper, we propose a range-free 2D tag localization method based on phased array antenna, called PATL. This method takes advantage of the adjustable radiation angle of the phased array antenna to scan the surveillance region in turns. By using the statistics of the tags’ number in different antenna beam directions, a weighting algorithm is used to calculate the position of the tag. This method can be applied to real-time location of multiple targets without usage of any reference tags or additional readers. Additionally, we present an optimized weighting method based on RSSI to increase the locating accuracy. We use a Commercial Off-the-Shelf (COTS) UHF RFID reader which is integrated with a phased array antenna to evaluate our method. The experiment results from an indoor office environment demonstrate the average distance error of PATL is about 21 cm and the optimized approach achieves an accuracy of 13 cm. This novel 2D localization scheme is a simple, yet promising, solution that is especially applicable to the smart shelf visualized management in storage or retail area.

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