Continuous Scanning with Mobile Reader in RFID Systems:an Experimental Study Lei Xiet,Qun Lit,Xi Chent,Sanglu Lut,and Daoxu Chent tState Key Laboratory of Novel Software Technology,Nanjing University,China +College of William and Mary,Williamsburg,VA,USA Ixie @nju.edu.cn,liqun @cs.wm.edu,hawkxc@163.com,(sanglu,cdx@nju.edu.cn ABSTRACT of tags in a realistic scenario is much longer than the time In this paper,we show the first comprehensive experimen- computed for free space,as shown in our experiments.In ad- tal study on mobile RFID reading performance based on dition,RFID readers have a wide range of power selections a relatively large number of tags.By making a number e.g.,the Alien-9900 reader has a maximum power 30.7dBm of observations regarding the tag reading performance,we which is 30 times larger than the minimum power 15.7dBm. build a model to depict how various parameters affect the There is no guideline,however,in selecting a suitable power. reading performance.Through our model,we have designed Therefore,we aim to design an efficient solution to contin- very efficient algorithms to maximize the time-efficiency and uous scanning problem for a mobile RFID reader based on energy-efficiency by adjusting the reader's power and mov- experimental study ing speed.Our experiments show that our algorithms can Although there have been some experimental studies on reduce the total scanning time by 50%and the total energy reading performance in a stationary RFID system [4,1,7, consumption by 83%compared to the prior solutions. the previous studies have the following limitations.First, previous experiments were usually conducted in a small s- Categories and Subject Descriptors cale (fewer than 20 tags),which does not capture the com- plication for a large number of tags.Second,previous work C.2.1 [Network Architecture and Design:Wireless Com- has been focused on reading performance in a close to free munication space scenario.In reality,path loss,multi-path effect and mutual interference are common and have a big impact to R- Keywords FID reading process.Third,previous work mainly examined how factors such as distance,coding scheme and frequency RFID;Realistic Settings;Algorithm Design;Experimental affect reading performance.Very important factors,i.e.,the Study:Model reader's power and tag density,were neglected.Therefore, the previous work does not give a model for RFID reading 1.INTRODUCTION process in a realistic and large scale setting;in particular,it Mobile RFID reading performance is critical to a num- does not include the power and tag density.Indeed,before ber of applications that rely on mobile readers.Scanning we started our work,there was no realistic model which can books in a library or a bookstore,tracking merchandises in guide us in designing an efficient tag identification solution a store,all require a mobile reader to be used for continu- in our setting. ous scanning over the tags attached to the physical goods We have,thus,conducted comprehensive measurements and assets.The mobile reader moves continuously to scan a over a large number of tags in realistic settings by varying large number of tags effectively compensating for its limited various parameters.Surprisingly,we have a few important reading range.In those types of mobile reader systems,two new findings from the experiments.For example,we have performance metrics are highly pertinent:time efficiency to found that the probabilistic backscattering is a ubiquitous reduce the total scanning time,and energy efficiency to re- phenomenon of the RFID system in realistic settings,i.e., duce the total power consumption.Unfortunately,there is during every query cycle each tag randomly responds with no realistic model to characterize the performance for mo- a certain probability,which has an important effect on the bile RFID reading for a large scale setting.The factors that reading performance.This observation is contrary to the affect the mobile reading performance are very complicat- previous belief that tags respond to a reader with either ed.For example,the actual scanning time for a number probability 1 or 0.We have also found it is not wise to blind- ly increase the reader's power for tag identification,which can degrade the overall performance including the effective Permission to make digital or hard copies of all or part of this work for personal or throughput and energy consumption.These findings are es- classroom use is granted without fee provided that copies are not made or distributed sential to improving reading performance for a mobile RFID for profit or commercial advantage and that copies bear this notice and the full cita system.Most importantly,we can(1)model the patterns tion on the first page.Copyrights for components of this work owned by others than of reading a large number of tags by giving a probabilis- ACM must be honored.Abstracting with credit is permitted.To copy otherwise,or re- tic model to capture the major and minor detection region, publish,to post on servers or to redistribute to lists,requires prior specific permission and/or a fee.Request permissions from permissions@acmorg. and(2)model how the reading power and tag density affect MobiHoc'13,July 29-August 1,2013.Bangalore,India the reading performance by proving an empirical mapping. Copyright2013ACM978-1-4503-2193-8/13/07S15.00.Continuous Scanning with Mobile Reader in RFID Systems: an Experimental Study Lei Xie†, Qun Li‡, Xi Chen†, Sanglu Lu†, and Daoxu Chen† †State Key Laboratory of Novel Software Technology, Nanjing University, China ‡College of William and Mary, Williamsburg, VA, USA lxie@nju.edu.cn, liqun@cs.wm.edu, hawkxc@163.com, {sanglu,cdx}@nju.edu.cn ABSTRACT In this paper, we show the first comprehensive experimen￾tal study on mobile RFID reading performance based on a relatively large number of tags. By making a number of observations regarding the tag reading performance, we build a model to depict how various parameters affect the reading performance. Through our model, we have designed very efficient algorithms to maximize the time-efficiency and energy-efficiency by adjusting the reader’s power and mov￾ing speed. Our experiments show that our algorithms can reduce the total scanning time by 50% and the total energy consumption by 83% compared to the prior solutions. Categories and Subject Descriptors C.2.1 [Network Architecture and Design]: Wireless Com￾munication Keywords RFID; Realistic Settings; Algorithm Design; Experimental Study; Model 1. INTRODUCTION Mobile RFID reading performance is critical to a num￾ber of applications that rely on mobile readers. Scanning books in a library or a bookstore, tracking merchandises in a store, all require a mobile reader to be used for continu￾ous scanning over the tags attached to the physical goods and assets. The mobile reader moves continuously to scan a large number of tags effectively compensating for its limited reading range. In those types of mobile reader systems, two performance metrics are highly pertinent: time efficiency to reduce the total scanning time, and energy efficiency to re￾duce the total power consumption. Unfortunately, there is no realistic model to characterize the performance for mo￾bile RFID reading for a large scale setting. The factors that affect the mobile reading performance are very complicat￾ed. For example, the actual scanning time for a number Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full cita￾tion on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or re￾publish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from permissions@acm.org. MobiHoc’13, July 29–August 1, 2013, Bangalore, India. Copyright 2013 ACM 978-1-4503-2193-8/13/07 ...$15.00. of tags in a realistic scenario is much longer than the time computed for free space, as shown in our experiments. In ad￾dition, RFID readers have a wide range of power selections, e.g., the Alien-9900 reader has a maximum power 30.7dBm, which is 30 times larger than the minimum power 15.7dBm. There is no guideline, however, in selecting a suitable power. Therefore, we aim to design an efficient solution to contin￾uous scanning problem for a mobile RFID reader based on experimental study. Although there have been some experimental studies on reading performance in a stationary RFID system [4, 1, 7], the previous studies have the following limitations. First, previous experiments were usually conducted in a small s￾cale (fewer than 20 tags), which does not capture the com￾plication for a large number of tags. Second, previous work has been focused on reading performance in a close to free space scenario. In reality, path loss, multi-path effect and mutual interference are common and have a big impact to R￾FID reading process. Third, previous work mainly examined how factors such as distance, coding scheme and frequency, affect reading performance. Very important factors, i.e., the reader’s power and tag density, were neglected. Therefore, the previous work does not give a model for RFID reading process in a realistic and large scale setting; in particular, it does not include the power and tag density. Indeed, before we started our work, there was no realistic model which can guide us in designing an efficient tag identification solution in our setting. We have, thus, conducted comprehensive measurements over a large number of tags in realistic settings by varying various parameters. Surprisingly, we have a few important new findings from the experiments. For example, we have found that the probabilistic backscattering is a ubiquitous phenomenon of the RFID system in realistic settings, i.e., during every query cycle each tag randomly responds with a certain probability, which has an important effect on the reading performance. This observation is contrary to the previous belief that tags respond to a reader with either probability 1 or 0. We have also found it is not wise to blind￾ly increase the reader’s power for tag identification, which can degrade the overall performance including the effective throughput and energy consumption. These findings are es￾sential to improving reading performance for a mobile RFID system. Most importantly, we can (1) model the patterns of reading a large number of tags by giving a probabilis￾tic model to capture the major and minor detection region, and (2) model how the reading power and tag density affect the reading performance by proving an empirical mapping