Moving Tag Detection via Physical Layer Analysis for Large-Scale RFID Systems Chuyu Wang,Lei Xie,Wei Wang,Tao Xue,Sanglu Lu State Key Laboratory for Novel Software Technology,Nanjing University,China Email:{wangcyu217,txue@dislab.nju.edu.cn,fIxie,ww,sanglu@nju.edu.cn Abstract-In a number of RFID-based applications such as essential to devise a moving tag detection scheme to detect the logistics monitoring,the RFID systems are deployed to monitor motion status of all tags.In this way,we only need to focus on a large number of RFID tags.They are usually required to tracking the moving tags,saving a lot of effort which might track the movement of all tags in a real-time approach,since the tagged-goods are moved in and out in a rather frequent approach. be wasted in tracking stationary tags. However,a typical cycle of tag inventory in COTS RFID system According to the prior art,in order to track the moving tags usually takes tens of seconds to interrogate hundreds of RFID in the monitoring area,two schemes are essentially required, tags.This hinders the system to track the movement of all tags i.e..a fast tag inventory scheme to interrogate tags,and an in time.One critical issue in such type of tag monitoring is to effective positioning scheme to detect the motion status of the efficiently distinguish the motion status of all tags,i.e..stationary or moving.According to the motion status of different tags,the tags.For the tag inventory in RFID systems,polling protocols state-of-art localization schemes can further track those moving [3,4]have been proposed to improve the time efficiency. tags,instead of tracking all tags.In this paper,we propose a However,they still take tens of seconds to interrogate RFID real-time approach to detect the moving tags in the monitoring tags in a real environment,when the cardinality of tags is area,which is a fundamental premise to support tracking the movement of all tags.We achieve the time efficiency by decoding more than several hundreds.The inefficiency is primarily due collisions from the physical layer.Instead of using the EPC ID to the waste of the collision slots,which usually occupy a fairly which cannot be decoded in collision slots,we are able to extract large proportion in the overall time slots.Hence,even schemes two kinds of physical-layer features of RFID tags,i.e.,the phase based on the pooling protocols [5,6]can efficiently detect the profile and the backscatter link frequency,to distinguish among moving tags based on the statistical signal information.but different tags in different positions.By resolving the two physical- also suffer from the effect of collision slots.Recent research layer features from the tag collisions,we are able to derive the motion status of multiple tags simultaneously,and greatly works consider to redesign new protocols or modify the EPC improve the time-efficiency.Experiment result shows that our C1G2 protocols to make use of the collision slots for better solution can accurately detect the moving tags while reducing time efficiency [7-9].However,they have not yet considered to 80%of inventory time compared with the state-of-art solutions. detect the motion status of the tags.Some protocols [3,10]are Index Terms-RFID,Collision Decoding,Tag Inventory also proposed to detect the missing tags via the physical-layer symbols.But they cannot be used to detect the moving tags I.INTRODUCTION since a moving tag can still exist in the monitoring area instead With the rapid proliferation of RFID-based applications, of missing.For the positioning scheme of RFID systems,the RFID tags have been deployed into various applications in state-of-the-art localization schemes [11,12]usually locate the increasingly large numbers.For example,in the application of tags one by one,and the time delay of localizing a unique logistics monitoring,there are usually more than hundreds of object is up to several hundreds of milliseconds.When dealing goods attached with RFID tags in the monitoring area.Since with more than hundreds of RFID tags it is impossible to the tagged-goods are moved in and out in a rather frequent concurrently locate all tags in a real time approach. approach,the RFID systems are usually required to track the In this paper,we propose a real-time approach to detect movement of all tags in a real-time approach.However.a the moving tags in large scale RFID systems,which is a typical round of tag inventory in COTS RFID system usually fundamental premise to support tracking the movement of takes tens of seconds to interrogate hundreds of RFID tags all tags.Since a missing tag must be moved first,it can [1,2].This greatly hinders the system to track the movement also be simplified as a moving tag detection problem.In of all tags in time.One critical issue in such type of tag our problem,we achieve the time efficiency by effectively monitoring is to efficiently distinguish the motion status of all decoding collisions from the physical layer.Instead of using tags,i.e.,stationary or moving.For the"moving"tags,we can the EPC ID,which cannot be decoded in collision slots,we leverage the state-of-art localization schemes to track them;for are able to extract two kinds of physical-layer features of the "stationary"tags,we do not need to track them any more, RFID tags,i.e.,the phase profile and the backscatter link since they are supposed to be statically placed in a specified frequency,to distinguish the tags in different positions.These position.In most situations,the stationary tags occupy a rather physical-layer features serve as fingerprints of tag identities large proportion while the moving tags occupy only a small and positions.By resolving the two physical-layer features proportion in regard to a certain moment.Therefore,it is from the tag collisions,we are able to derive the motionMoving Tag Detection via Physical Layer Analysis for Large-Scale RFID Systems Chuyu Wang, Lei Xie, Wei Wang, Tao Xue, Sanglu Lu State Key Laboratory for Novel Software Technology, Nanjing University, China Email: {wangcyu217, txue}@dislab.nju.edu.cn, {lxie, ww, sanglu}@nju.edu.cn Abstract—In a number of RFID-based applications such as logistics monitoring, the RFID systems are deployed to monitor a large number of RFID tags. They are usually required to track the movement of all tags in a real-time approach, since the tagged-goods are moved in and out in a rather frequent approach. However, a typical cycle of tag inventory in COTS RFID system usually takes tens of seconds to interrogate hundreds of RFID tags. This hinders the system to track the movement of all tags in time. One critical issue in such type of tag monitoring is to efficiently distinguish the motion status of all tags, i.e., stationary or moving. According to the motion status of different tags, the state-of-art localization schemes can further track those moving tags, instead of tracking all tags. In this paper, we propose a real-time approach to detect the moving tags in the monitoring area, which is a fundamental premise to support tracking the movement of all tags. We achieve the time efficiency by decoding collisions from the physical layer. Instead of using the EPC ID, which cannot be decoded in collision slots, we are able to extract two kinds of physical-layer features of RFID tags, i.e., the phase profile and the backscatter link frequency, to distinguish among different tags in different positions. By resolving the two physical￾layer features from the tag collisions, we are able to derive the motion status of multiple tags simultaneously, and greatly improve the time-efficiency. Experiment result shows that our solution can accurately detect the moving tags while reducing 80% of inventory time compared with the state-of-art solutions. Index Terms—RFID, Collision Decoding, Tag Inventory I. INTRODUCTION With the rapid proliferation of RFID-based applications, RFID tags have been deployed into various applications in increasingly large numbers. For example, in the application of logistics monitoring, there are usually more than hundreds of goods attached with RFID tags in the monitoring area. Since the tagged-goods are moved in and out in a rather frequent approach, the RFID systems are usually required to track the movement of all tags in a real-time approach. However, a typical round of tag inventory in COTS RFID system usually takes tens of seconds to interrogate hundreds of RFID tags [1, 2]. This greatly hinders the system to track the movement of all tags in time. One critical issue in such type of tag monitoring is to efficiently distinguish the motion status of all tags, i.e., stationary or moving. For the “moving” tags, we can leverage the state-of-art localization schemes to track them; for the “stationary” tags, we do not need to track them any more, since they are supposed to be statically placed in a specified position. In most situations, the stationary tags occupy a rather large proportion while the moving tags occupy only a small proportion in regard to a certain moment. Therefore, it is essential to devise a moving tag detection scheme to detect the motion status of all tags. In this way, we only need to focus on tracking the moving tags, saving a lot of effort which might be wasted in tracking stationary tags. According to the prior art, in order to track the moving tags in the monitoring area, two schemes are essentially required, i.e., a fast tag inventory scheme to interrogate tags, and an effective positioning scheme to detect the motion status of the tags. For the tag inventory in RFID systems, polling protocols [3, 4] have been proposed to improve the time efficiency. However, they still take tens of seconds to interrogate RFID tags in a real environment, when the cardinality of tags is more than several hundreds. The inefficiency is primarily due to the waste of the collision slots, which usually occupy a fairly large proportion in the overall time slots. Hence, even schemes based on the pooling protocols [5, 6] can efficiently detect the moving tags based on the statistical signal information, but also suffer from the effect of collision slots. Recent research works consider to redesign new protocols or modify the EPC C1G2 protocols to make use of the collision slots for better time efficiency [7–9]. However, they have not yet considered to detect the motion status of the tags. Some protocols [3, 10] are also proposed to detect the missing tags via the physical-layer symbols. But they cannot be used to detect the moving tags since a moving tag can still exist in the monitoring area instead of missing. For the positioning scheme of RFID systems, the state-of-the-art localization schemes [11, 12] usually locate the tags one by one, and the time delay of localizing a unique object is up to several hundreds of milliseconds. When dealing with more than hundreds of RFID tags it is impossible to concurrently locate all tags in a real time approach. In this paper, we propose a real-time approach to detect the moving tags in large scale RFID systems, which is a fundamental premise to support tracking the movement of all tags. Since a missing tag must be moved first, it can also be simplified as a moving tag detection problem. In our problem, we achieve the time efficiency by effectively decoding collisions from the physical layer. Instead of using the EPC ID, which cannot be decoded in collision slots, we are able to extract two kinds of physical-layer features of RFID tags, i.e., the phase profile and the backscatter link frequency, to distinguish the tags in different positions. These physical-layer features serve as fingerprints of tag identities and positions. By resolving the two physical-layer features from the tag collisions, we are able to derive the motion