Besides,some localization systems are conducted on IV.UNDERSTANDING THE UNDERLYING REGULARITIES robots[12]13],which is used to locate automatically.Deyle IN RFID SYSTEM et al.propose a system to grasp objects by a robot[14].And We conduct several experiments to study the features of Aditya et al.propose a system to index and locate all the RFID.which reveal several original findings in realistic en- objects in the room assisted with a robot[15].Sherlock is vironment.In our experiments,we use Alien-9900 RFID the closest work among these to our own.But it focuses on reader,Alien-9611 linear antenna and ALN-9662 passive locating all the objects in the room,which results in a lot of RFID tag.Without loss of generality,we use the RSSI value time delay.And their theoretical model for localization is not directly got from the Alien-API which ranges from 0 to accurate in reality,which reduces their locating accuracy. 65535 and the value can be converted to value in dBm as: III.PROBLEM DESCRIPTION RSS(dBm)=10*lg(RSSI/65536*2000) In the experiments,we fix the reader and deploy the antenna A.Problem Formulation facing the tag with 1.2m high from the floor.We attach In our system,we consider a typical scenario,i.e.,locating our RFID tag on real objects with the same height.Our the object from a specific initial point without the help of any experiments are conducted in realistic environment,which reference points.It can be widely applied to general scenarios, exposes the performance of passive tag dealing with influence e.g.,locating the misplaced keys in the room,or locating the like multipath effect and path loss. desired good in the warehouse.By means of RFID technology, A.RSSI Varies according to Power and Distance locating the object can be considered as locating the attached target tag with an RFID reader.We use the RFID reader to When the tag-antenna distance exceeds a certain threshold, scan the target field and approach to locate according to the RSSI varies little as the distance increasing.We deploy the RSSI value. tag in front of the antenna,and query the tag with different In this work,we assign the localization accuracy and the transfer powers and different tag-antenna distances.As shown in Fig.1,RSSI value varies a lot when we fix the distance. localization duration as our performance metrics.Specifically, As RSSI falls below 3000,the RSSI variation provides little we define the object's real position is (xtag,ytag)and the reader's initial position is(reader,yreader),which represents discrimination as the distance increasing,which influences the accuracy severely.On the other hand,the curve provides an the position of the reader.We move the reader according to RSSI.When the RSSI value reaches a certain threshold. approach to roughly estimate the distance. we estimate relative position according to the historical data. For small tag-antenna distance,RSSI decreases as the Specifically,the performance metrics are defined as follows: transfer power increases.When we focus on the relationship between the transfer power and the RSSI,we plot Fig.I in localization accuracy:The distance between the real another way,as shown in Fig.2.According to the figure,when position and the estimate position,i.e.,(tag-zreader)2+ the tag-antenna distance is less than 1m.RSSI at 30.7dBm is (ytag-yreader)2,should be as small as possible. smaller than RSSI at lower power.High transfer power obtains localization duration:We consider both navigation time smaller RSSI when dealing with near field communication. and query times as metrics for localization duration This observation is contrary to intuition that high transfer because of robot consuming time power would not perform worse than lower power.Therefore, it is essential for accuracy to consider power stepping when B.Motivation and Challenges narrowing the tag-antenna distance. Based on the above understanding,we design a new lo- RSSI swings little when we fix all the parameters.We calization system using only RFID technology.This work measure the standard deviation of RSSI every 100 samples is different from the previous localization systems based on with the same parameters,and plot the CDF of the swing rate RFID,e.g.,LANDMARC[6],whose principal method mainly of RSSI(std/mean).As shown in Fig.3,RSSI varies within a relies on the reference tags.We focus on how to locate the certain rate with high probability.It guarantees sampling RSSI desired tag from a specific initial point based on RFID mainly. can provide similar information as repetition collection This is a typical scenario in realistic life and it is not practical to deploy reference tags everywhere or collect fingerprint from B.RSSI Varies according to the Rotation of Reader Antenna different places. As the angle changes,RSSI varies in different speed at In our problem,there are several challenges to be solved:different tag-antenna distance.But RSSI always reaches a (a)RSSI got from the reader varies a lot,meaning we need peak when the tag is in front of the antenna and reaches a study the RSSI variation pattern through realistic experiments.small peak behind the antenna.We study the features of the (b)Even if our antenna is directed,it can identify tags in a antenna for accurate localization.We experiment on 5 typical large range,which may reduce the accuracy.(c)There are distances,then we rotate the antenna as Fig.5 continuously. no reference tags or fingerprints which can provide extra The result is shown in Fig.4,and we note that RSSI variation information.New approaches should be used to locate the pattern submits to quadratic function.Besides,the antenna can object.(d)How to combine the information we get to satisfy identify the tag behind it if the tag-antenna distance is small the high demand in accuracy and time-delay. enough,which can confuse the orientation decision.Besides, some localization systems are conducted on robots[12][13], which is used to locate automatically. Deyle et al. propose a system to grasp objects by a robot[14]. And Aditya et al. propose a system to index and locate all the objects in the room assisted with a robot[15]. Sherlock is the closest work among these to our own. But it focuses on locating all the objects in the room, which results in a lot of time delay. And their theoretical model for localization is not accurate in reality, which reduces their locating accuracy. III. PROBLEM DESCRIPTION A. Problem Formulation In our system, we consider a typical scenario, i.e., locating the object from a specific initial point without the help of any reference points. It can be widely applied to general scenarios, e.g., locating the misplaced keys in the room, or locating the desired good in the warehouse. By means of RFID technology, locating the object can be considered as locating the attached target tag with an RFID reader. We use the RFID reader to scan the target field and approach to locate according to the RSSI value. In this work, we assign the localization accuracy and the localization duration as our performance metrics. Specifically, we define the object’s real position is (xtag, ytag) and the reader’s initial position is (xreader, yreader), which represents the position of the reader. We move the reader according to RSSI. When the RSSI value reaches a certain threshold, we estimate relative position according to the historical data. Specifically, the performance metrics are defined as follows: • localization accuracy: The distance between the real position and the estimate position ,i.e.,(xtag−xreader) 2+ (ytag − yreader) 2 , should be as small as possible. • localization duration: We consider both navigation time and query times as metrics for localization duration because of robot consuming time. B. Motivation and Challenges Based on the above understanding, we design a new lo￾calization system using only RFID technology. This work is different from the previous localization systems based on RFID, e.g., LANDMARC[6], whose principal method mainly relies on the reference tags. We focus on how to locate the desired tag from a specific initial point based on RFID mainly. This is a typical scenario in realistic life and it is not practical to deploy reference tags everywhere or collect fingerprint from different places. In our problem, there are several challenges to be solved: (a) RSSI got from the reader varies a lot, meaning we need study the RSSI variation pattern through realistic experiments. (b) Even if our antenna is directed, it can identify tags in a large range, which may reduce the accuracy. (c) There are no reference tags or fingerprints which can provide extra information. New approaches should be used to locate the object. (d) How to combine the information we get to satisfy the high demand in accuracy and time-delay. IV. UNDERSTANDING THE UNDERLYING REGULARITIES IN RFID SYSTEM We conduct several experiments to study the features of RFID, which reveal several original findings in realistic en￾vironment. In our experiments, we use Alien-9900 RFID reader , Alien-9611 linear antenna and ALN-9662 passive RFID tag. Without loss of generality, we use the RSSI value directly got from the Alien-API which ranges from 0 to 65535 and the value can be converted to value in dBm as: RSS(dBm) = 10 ∗ lg(RSSI/65536 ∗ 2000) In the experiments, we fix the reader and deploy the antenna facing the tag with 1.2m high from the floor. We attach our RFID tag on real objects with the same height . Our experiments are conducted in realistic environment, which exposes the performance of passive tag dealing with influence like multipath effect and path loss. A. RSSI Varies according to Power and Distance When the tag-antenna distance exceeds a certain threshold, RSSI varies little as the distance increasing. We deploy the tag in front of the antenna, and query the tag with different transfer powers and different tag-antenna distances. As shown in Fig. 1, RSSI value varies a lot when we fix the distance. As RSSI falls below 3000, the RSSI variation provides little discrimination as the distance increasing, which influences the accuracy severely. On the other hand, the curve provides an approach to roughly estimate the distance. For small tag-antenna distance, RSSI decreases as the transfer power increases. When we focus on the relationship between the transfer power and the RSSI, we plot Fig. 1 in another way, as shown in Fig. 2. According to the figure, when the tag-antenna distance is less than 1m, RSSI at 30.7dBm is smaller than RSSI at lower power. High transfer power obtains smaller RSSI when dealing with near field communication. This observation is contrary to intuition that high transfer power would not perform worse than lower power. Therefore, it is essential for accuracy to consider power stepping when narrowing the tag-antenna distance. RSSI swings little when we fix all the parameters. We measure the standard deviation of RSSI every 100 samples with the same parameters, and plot the CDF of the swing rate of RSSI(std/mean). As shown in Fig. 3, RSSI varies within a certain rate with high probability. It guarantees sampling RSSI can provide similar information as repetition collection. B. RSSI Varies according to the Rotation of Reader Antenna As the angle changes, RSSI varies in different speed at different tag-antenna distance. But RSSI always reaches a peak when the tag is in front of the antenna and reaches a small peak behind the antenna. We study the features of the antenna for accurate localization. We experiment on 5 typical distances, then we rotate the antenna as Fig. 5 continuously. The result is shown in Fig. 4, and we note that RSSI variation pattern submits to quadratic function. Besides, the antenna can identify the tag behind it if the tag-antenna distance is small enough, which can confuse the orientation decision