XIE er al:MANAGING RFID DATA:CHALLENGES.OPPORTUNITIES AND SOLUTIONS 1307 TABLE VIII The deployment of tags:If the tags are densely deployed,the CHALLENGES AND OPPORTUNITIES FOR RFID LOCALIZATION AND incident power from the reader can be fully diluted among the ACTIVITY SENSING tags,and the signal interference and energy absorption can be Mechanism Challenges Opportunities aggravated among the tags.For a specified tag,if the incident RFID lo- The unstable communi- The low cost and battery-free calization cation in the backscat- property make large scale de. angle of the power parallels the tag's plane (also known as ter channel makes accu- ployment of reference tags pos- the tag antenna's plane),then it is impossible for the tag to rate localization hard to sible.The sensitivity to ambien backscatter a strong enough signal.Hence,the incident power achieve. environment contributes to ac. should be perpendicular to the tag's plane as much as possible. curately depict the differences among various positions. Table IX illustrates the impact of these critical factors on the Activity Complex activities can- It is possible to implemen system performance metrics,including the scanning range, sensing not be accurately per device-free activity sensing by read throughput and energy consumption. ceived in a real-time ap exploring the sensitivity to the proach due to the limited surrounding mobile environ- perception in RFID sys ment in RFID system. B.Experimental Findings tems. Since the above factors are ubiquitous in the realistic settings,it is very difficult to achieve good performance VI.PERFORMANCE TUNING IN REALISTIC SETTINGS according to the algorithms and optimized parameters derived from ideal settings.Therefore,in recent years a lot of re- As a new emerging technology,RFID has been widely searchers have paid attention to the performance tuning for explored from various aspects by a lot of researchers in recent RFID systems in realistic settings.In order to depict the impact years.However,most former research work mainly consider of communication error on the anti-collision performance of optimized design of algorithms and protocols under a rela- Gen2 RFID system,Kawakita et al.provide experiment results tively ideal transmission environment,and evaluate the perfor- to prove that the bit errors in physical layer greatly reduce the mance through simulations.As a matter of fact,a number of overall performance [86.Buettner et al.examine the perfor- physical factors in realistic transmission environment,like path mance of EPC Class-1 Gen-2 UHF RFID systems in realistic loss,energy absorption and signal interference,have brought settings,and identify factors that degrade overall performance great unreliability to normal operations in RFID systems. and reliability with a focus on the physical layer [87].They Therefore,it is essential to consider the performance tuning find that physical layer considerations have a significant im- in realistic settings,such that the performance of algorithms pact on reader performance,and that this is exacerbated by and protocols can be guaranteed for realistic applications. a lack of integration between the physical and MAC layers. Aroor el al.use a simple,empirical,experimental approach A.Critical Factors for System Performance to evaluate the performance of the commercially available In realistic settings,a lot of factors can impact the perfor- RFID systems [88].By varying the reading distance between mance of RFID systems,in the following we elaborate those the reader and the tags.they examine various performance critical factors which are closely correlated with the system parameters in various environments (including free space, performance. near-water and near-metal environment).Ramakrishnan et al. The reader's power:If the reader's power is too small,then provide comprehensive performance benchmarks for passive the effective scanning range is reduced.In this situation,some UHF RFID systems [89],which can effectively depict the tags cannot be activated as the incident power is below the reading efficiency of RFID systems in realistic settings.Jeffery activation threshold,or the reader cannot successfully resolve et al.conduct experiments in realistic settings and find that the backscattered signal as the backscattered signal noise ratio within each reader's detection range,a large difference exists is rather low.If the reader's power is too large,then the in reading performance [90].Specifically,within each readers effective scanning range is increased.In this situation,the detection range,there are two distinct regions:the major backscattered signal strength of some tags is greatly increased, detection region and the minor detection region.The major amplifying the signal interference among tags. detection region is the area directly in front of the reader. The path loss,multi-path effect,and energy absorption:All giving high detection probability,and the minor detection the three issues can lead to the signal attenuation in RFID region extends from the end of the major detection region systems,which greatly impacts both the forward link trans- to the edge of the detection range,where the read ratio drops mission(from the reader to tag)and reverse link transmission off to zero at the end of the detection range.Xie et al.show (from the tag to reader). the first comprehensive experimental study on mobile RFID The signal interference:As multiple readers are convention- reading performance based on a relatively large number of ally deployed in RFID applications,there exist three kinds tags [23].By making a number of observations regarding of interference in RFID systems:(1)tag-tag interference: the tag reading performance,they have a few important new multiple tags respond to the reader simultaneously and cause findings from the experiments.For example,the probabilistic collisions at the reader,(2)reader-tag interference:response backscattering is a ubiquitous phenomenon of the RFID sys- from the tag to a reader is "drowned"by the signal from tem in realistic settings,which has an important effect on the another reader,(3)reader-reader interference:signal from reading performance.Besides,it is not wise to blindly increase multiple readers collide at a tag.The above interference can the readers power for tag identification,which can degrade cause bit errors during data transmission,thus reducing the the overall performance including the effective throughput and reading efficiency of RFID systems. energy consumption.XIE et al.: MANAGING RFID DATA: CHALLENGES, OPPORTUNITIES AND SOLUTIONS 1307 TABLE VIII CHALLENGES AND OPPORTUNITIES FOR RFID LOCALIZATION AND ACTIVITY SENSING Mechanism Challenges Opportunities RFID lo￾calization The unstable communi￾cation in the backscat￾ter channel makes accu￾rate localization hard to achieve. The low cost and battery-free property make large scale de￾ployment of reference tags pos￾sible. The sensitivity to ambient environment contributes to ac￾curately depict the differences among various positions. Activity sensing Complex activities can￾not be accurately per￾ceived in a real-time ap￾proach due to the limited perception in RFID sys￾tems. It is possible to implement device-free activity sensing by exploring the sensitivity to the surrounding mobile environ￾ment in RFID system. VI. PERFORMANCE TUNING IN REALISTIC SETTINGS As a new emerging technology, RFID has been widely explored from various aspects by a lot of researchers in recent years. However, most former research work mainly consider optimized design of algorithms and protocols under a rela￾tively ideal transmission environment, and evaluate the perfor￾mance through simulations. As a matter of fact, a number of physical factors in realistic transmission environment, like path loss, energy absorption and signal interference, have brought great unreliability to normal operations in RFID systems. Therefore, it is essential to consider the performance tuning in realistic settings, such that the performance of algorithms and protocols can be guaranteed for realistic applications. A. Critical Factors for System Performance In realistic settings, a lot of factors can impact the perfor￾mance of RFID systems, in the following we elaborate those critical factors which are closely correlated with the system performance. The reader’s power: If the reader’s power is too small, then the effective scanning range is reduced. In this situation, some tags cannot be activated as the incident power is below the activation threshold, or the reader cannot successfully resolve the backscattered signal as the backscattered signal noise ratio is rather low. If the reader’s power is too large, then the effective scanning range is increased. In this situation, the backscattered signal strength of some tags is greatly increased, amplifying the signal interference among tags. The path loss, multi-path effect, and energy absorption: All the three issues can lead to the signal attenuation in RFID systems, which greatly impacts both the forward link trans￾mission (from the reader to tag) and reverse link transmission (from the tag to reader). The signal interference: As multiple readers are convention￾ally deployed in RFID applications, there exist three kinds of interference in RFID systems: (1) tag-tag interference: multiple tags respond to the reader simultaneously and cause collisions at the reader, (2) reader-tag interference: response from the tag to a reader is “drowned” by the signal from another reader, (3) reader-reader interference: signal from multiple readers collide at a tag. The above interference can cause bit errors during data transmission, thus reducing the reading efficiency of RFID systems. The deployment of tags: If the tags are densely deployed, the incident power from the reader can be fully diluted among the tags, and the signal interference and energy absorption can be aggravated among the tags. For a specified tag, if the incident angle of the power parallels the tag’s plane (also known as the tag antenna’s plane), then it is impossible for the tag to backscatter a strong enough signal. Hence, the incident power should be perpendicular to the tag’s plane as much as possible. Table IX illustrates the impact of these critical factors on the system performance metrics, including the scanning range, read throughput and energy consumption. B. Experimental Findings Since the above factors are ubiquitous in the realistic settings, it is very difficult to achieve good performance according to the algorithms and optimized parameters derived from ideal settings. Therefore, in recent years a lot of re￾searchers have paid attention to the performance tuning for RFID systems in realistic settings. In order to depict the impact of communication error on the anti-collision performance of Gen2 RFID system, Kawakita et al. provide experiment results to prove that the bit errors in physical layer greatly reduce the overall performance [86]. Buettner et al. examine the perfor￾mance of EPC Class-1 Gen-2 UHF RFID systems in realistic settings, and identify factors that degrade overall performance and reliability with a focus on the physical layer [87]. They find that physical layer considerations have a significant im￾pact on reader performance, and that this is exacerbated by a lack of integration between the physical and MAC layers. Aroor el al. use a simple, empirical, experimental approach to evaluate the performance of the commercially available RFID systems [88]. By varying the reading distance between the reader and the tags, they examine various performance parameters in various environments (including free space, near-water and near-metal environment). Ramakrishnan et al. provide comprehensive performance benchmarks for passive UHF RFID systems [89], which can effectively depict the reading efficiency of RFID systems in realistic settings. Jeffery et al. conduct experiments in realistic settings and find that within each reader’s detection range, a large difference exists in reading performance [90]. Specifically, within each readers detection range, there are two distinct regions: the major detection region and the minor detection region. The major detection region is the area directly in front of the reader, giving high detection probability, and the minor detection region extends from the end of the major detection region to the edge of the detection range, where the read ratio drops off to zero at the end of the detection range. Xie et al. show the first comprehensive experimental study on mobile RFID reading performance based on a relatively large number of tags [23]. By making a number of observations regarding the tag reading performance, they have a few important new findings from the experiments. For example, the probabilistic backscattering is a ubiquitous phenomenon of the RFID sys￾tem in realistic settings, which has an important effect on the reading performance. Besides, it is not wise to blindly increase the readers power for tag identification, which can degrade the overall performance including the effective throughput and energy consumption