This article has been accepted for publication in a future issue of this journal,but has not been fully edited.Content may change prior to final publication.Citation information:DOI 10.1109/TMC.2019.2907244.IEEE Transactions on Mobile Computing Probing into the Physical Layer:Moving Tag Detection for Large-Scale RFID Systems Chuyu Wang,Student Member,IEEE,Lei Xie,Member,IEEE,Wei Wang,Member,IEEE, Yingying Chen,Senior Member,IEEE,Tao Xue,and Sanglu Lu,Member,IEEE Abstract-Logistics monitoring is a fundamental application that utilizes RFID systems to manage numerous tagged-objects.Due to the frequent rearrangement of tagged-objects,a fast RFID-based tracking approach is highly desired for accurate logistics distribution. However,traditional RFID systems usually take tens of seconds to interrogate hundreds of RFID tags,not to mention the time delay involved to locate all the tags,which severely prevents from in-time tracking.To address this issue,we reduce the problem domain by first distinguishing the motion status of the tagged-objects,i.e.."stationary"or"moving",and then tracking the moving objects with the state-of-the-art localization schemes,which significantly reduces the efforts of tracking all the objects.Toward this end,we propose a moving tag detection mechanism,which achieves the time efficiency by exploiting the useless collision signal in RFID systems.In particular,we extract two kinds of physical-layer features(namely phase profile and backscatter link frequency)from the collision signal received by the USRP to distinguish tags at different positions.We further develop the Graph Matching(GM)method and Coherent Phase Variance(CPV)method to detect the moving tagged-objects.Experiment results show that our approach can accurately detect the moving objects while reducing 80%inventory time compared with the state-of-art solutions. Index Terms-RFID,Collision Decoding,Tag Inventory 1 INTRODUCTION WimdutryD.ml h -300 Decode collision signal 3600 in I-Q plane ployed in increasingly large numbers to facilitate the smart 3800 management.For example,in the logistic monitoring,there are usually more than hundreds of objects attached with 200 RFID tags in the monitoring area.Due to the frequent 400 Recovery rearrangement of the tagged-objects,the RFID systems are 4500 500 4 )e)1 Inphase required to track the movement of all tags timely to prevent Backscatter link frequency Phase profiles extraction the target objects from mistakenly rearranging.However, extraction a Commercial-Of-The-Shelf (COTS)RFID system usually Fig.1.Illusion of extracting physical-layer features from collision signal. takes tens of seconds to interrogate hundreds of RFID based schemes 3-5,14]leverage the Frame-Slotted- tags [1],[2],not to mention the time delay to track all the Aloha (FSA)protocol to identify the tags,which usually tags.This severely hinders the system from tracking the takes tens of seconds to interrogate hundreds of RFID tags in movement of tagged-objects in time.Since only some of the real RFID systems.The main cause of such time inefficiency objects are moved at a certain moment,to reduce the efforts is the waste of the collision slots,which usually occupy a of tracking all the objects,one possible solution is to first large proportion of the overall time slots.Recently,some identify the motion status of the objects,i.e.,"stationary"or emerging work try to make use of the collision slots to "moving",and then only track the"moving"objects.For the improve time efficiency of tag inventory [6]-[9]and further stationary"objects,since they are presumed to be statically detect the missing tags from the collision signals [3],[10]. placed in a specific location,we do not need to track them. However,different from a missing tag,a moving tag can For the "moving"objects,we can leverage the existing still be interrogated by the reader,thus these methods localization techniques to track them.Since the "moving" are not suitable to detect the moving tags.In addition, objects only occupy a small part of the total number,we can for the positioning schemes,the state-of-the-art localization save a lot of time by only focusing on tracking the moving schemes [11]-[13]usually locate the tags one by one,and objects,instead of wasting time in tracking the stationary they usually take up to several hundreds of milliseconds to objects,which makes it possible to perform the fast large- locate a unique tag.Therefore,it is difficult to concurrently scale monitoring. locate all tags timely using existing solutions,when dealing To track the moving tags in the monitoring area,exist- with hundreds of tags. ing studies [3]-[14]usually involve two steps,i.e.,a fast In this paper,we propose a fast moving tag detection tag inventory scheme to interrogate tags,and an effective scheme for large-scale RFID systems,which works as a positioning scheme to determine the motion status of the fundamental premise to support the tracking applications tags,which would be hard to perform large-scale moving of tagged-objects.The main idea is to extract the physical- RFID monitoring in a timely manner.Specifically,for the layer features of each tag from the collision signal to achieve tag inventory schemes in RFID systems,traditional polling- the time efficiency.Fig.1 uses the collision signal of three 1536-1233(c)2018 IEEE Personal use is permitted,but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.1536-1233 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TMC.2019.2907244, IEEE Transactions on Mobile Computing 1 Probing into the Physical Layer: Moving Tag Detection for Large-Scale RFID Systems Chuyu Wang, Student Member, IEEE, Lei Xie, Member, IEEE, Wei Wang, Member, IEEE, Yingying Chen, Senior Member, IEEE, Tao Xue, and Sanglu Lu, Member, IEEE Abstract—Logistics monitoring is a fundamental application that utilizes RFID systems to manage numerous tagged-objects. Due to the frequent rearrangement of tagged-objects, a fast RFID-based tracking approach is highly desired for accurate logistics distribution. However, traditional RFID systems usually take tens of seconds to interrogate hundreds of RFID tags, not to mention the time delay involved to locate all the tags, which severely prevents from in-time tracking. To address this issue, we reduce the problem domain by first distinguishing the motion status of the tagged-objects, i.e., “stationary” or “moving”, and then tracking the moving objects with the state-of-the-art localization schemes, which significantly reduces the efforts of tracking all the objects. Toward this end, we propose a moving tag detection mechanism, which achieves the time efficiency by exploiting the useless collision signal in RFID systems. In particular, we extract two kinds of physical-layer features (namely phase profile and backscatter link frequency) from the collision signal received by the USRP to distinguish tags at different positions. We further develop the Graph Matching (GM) method and Coherent Phase Variance (CPV) method to detect the moving tagged-objects. Experiment results show that our approach can accurately detect the moving objects while reducing 80% inventory time compared with the state-of-art solutions. Index Terms—RFID, Collision Decoding, Tag Inventory ✦ 1 INTRODUCTION WITH the rapid proliferation of IoT (Internet of Things) industry, RFID, as a key technology, has been de￾ployed in increasingly large numbers to facilitate the smart management. For example, in the logistic monitoring, there are usually more than hundreds of objects attached with RFID tags in the monitoring area. Due to the frequent rearrangement of the tagged-objects, the RFID systems are required to track the movement of all tags timely to prevent the target objects from mistakenly rearranging. However, a Commercial-Of-The-Shelf (COTS) RFID system usually takes tens of seconds to interrogate hundreds of RFID tags [1], [2], not to mention the time delay to track all the tags. This severely hinders the system from tracking the movement of tagged-objects in time. Since only some of the objects are moved at a certain moment, to reduce the efforts of tracking all the objects, one possible solution is to first identify the motion status of the objects, i.e., “stationary” or “moving”, and then only track the “moving” objects. For the “stationary” objects, since they are presumed to be statically placed in a specific location, we do not need to track them. For the “moving” objects, we can leverage the existing localization techniques to track them. Since the “moving” objects only occupy a small part of the total number, we can save a lot of time by only focusing on tracking the moving objects, instead of wasting time in tracking the stationary objects, which makes it possible to perform the fast large￾scale monitoring. To track the moving tags in the monitoring area, exist￾ing studies [3]–[14] usually involve two steps, i.e., a fast tag inventory scheme to interrogate tags, and an effective positioning scheme to determine the motion status of the tags, which would be hard to perform large-scale moving RFID monitoring in a timely manner. Specifically, for the tag inventory schemes in RFID systems, traditional polling￾Decode collision signal in I-Q plane Backscatter link frequency Phase profiles extraction extraction Decode Recovery Fig. 1. Illusion of extracting physical-layer features from collision signal. based schemes [3]–[5], [14] leverage the Frame-Slotted￾Aloha (FSA) protocol to identify the tags, which usually takes tens of seconds to interrogate hundreds of RFID tags in real RFID systems. The main cause of such time inefficiency is the waste of the collision slots, which usually occupy a large proportion of the overall time slots. Recently, some emerging work try to make use of the collision slots to improve time efficiency of tag inventory [6]–[9] and further detect the missing tags from the collision signals [3], [10]. However, different from a missing tag, a moving tag can still be interrogated by the reader, thus these methods are not suitable to detect the moving tags. In addition, for the positioning schemes, the state-of-the-art localization schemes [11]–[13] usually locate the tags one by one, and they usually take up to several hundreds of milliseconds to locate a unique tag. Therefore, it is difficult to concurrently locate all tags timely using existing solutions, when dealing with hundreds of tags. In this paper, we propose a fast moving tag detection scheme for large-scale RFID systems, which works as a fundamental premise to support the tracking applications of tagged-objects. The main idea is to extract the physical￾layer features of each tag from the collision signal to achieve the time efficiency. Fig. 1 uses the collision signal of three