YIN ETAL:FOCUS AND SHOOT:EXPLORING AUTO-FOCUS IN RFID TAG IDENTIFICATION TOWARDS A SPECIFIED AREA 899 i00 90 30 80 03.5 82.5 24 8 18 一P 20 2 10 05 nmun品vel:I(m) 7.5 0.5 10 1.5 (a)pvs. (b)T vs. (c)A vs.I (d)Pie vs. Fig.15.=60%,tag identification with different tag placement. 100 100 90 迟70 3.5 28 品60 50 525 250 24 MnPw 4000 30 18 05 Mirimum() Minimum nevm) D.5 Minimum ervtI(m) Mirim) (a)p vs.I (b)T vs.I (c)入vs.l (d)Pu vs.I Fig.16.=60%,tag identification with target tags in left area. 100 100 一EP EP E29 370 50 PID 30 20 ◆一EPW 0 20 18 40 Nn80rotir&2reneg2s:u 20 0 照er f2u (a)p vs.u (b)T vs.u (c)入vs.u (d)P vs.u Fig.17.a=60%,tag identification towards two-dimensional space our solutions try to select the optimal power to identify the better performance than the baseline solutions.When tags,as shown in Fig.15d. 1=1m,PID reduces the execution time by 47 percent com- pared to MaxPw,as shown in Fig.16b.PID and PIA do not 10.7 Changing the Location of Target Tags identify too many interference tags as MaxPw does,as In this experiment,we change the left area in Fig.1 as the shown in Fig.16c.Because our solutions try to select the opti- specified area.The interference tags in the center area are in mal power to identify the target tags,as shown in Fig.16d. front of the antenna.We vary the minimal distance l If we want to further improve the coverage ratio and between target tags and interference tags.The distance reduce the misreading ratio,we can use multiple readers between the target tags and the antenna is farther than [321,[33]to joint identify the target tags.For example,the that between the interference tags and the antenna.There- target tags are located in the left side of antenna Al,while fore,the coverage ratio decreases,while the misreading in the right side of antenna A2.Then,we can take the inter- ratio increases,as shown in Figs.16a and 16c.However, section of the identified tags from Al and A2 as the target based on Fig.4b,the target tags are in the major interro- tags,while eliminating the other tags.In future,we will con- gation region,while the interference tags are mostly in sider how to schedule the multiple readers and improve the minor interrogation region.Due to the different performan- efficiency of our solutions with multiple readers. ces in major minor interrogation regions,our solutions can basically satisfy the requirement p>a,as shown in 10.8 Extension for Tag ldentification Towards Two- Fig.16a.When l=1.5m,the coverage ratios of PID and PIA Dimensional Space are a little less than the requirement.Because the distance In order to evaluate the performance of our solutions in between the target tags and the antenna is about 1.8 m, two-dimensional space,we conduct the following experi- which is too large,according to Table 3.Even using the ment in the bookshelf.We deploy the books (tags)as Fig.8 maximal power,the coverage ratio is only about 61 percent. shows.The books located in the center of the bookshelf are In such a scenario,it is appropriate to decrease the require- target tags,while the tags around the target tags are interfer- ment of a.Nevertheless,our proposed solutions still have ence tags.We vary the number of interference tags u to in theour solutions try to select the optimal power to identify the tags, as shown in Fig. 15d. 10.7 Changing the Location of Target Tags In this experiment, we change the left area in Fig. 1 as the specified area. The interference tags in the center area are in front of the antenna. We vary the minimal distance l between target tags and interference tags. The distance between the target tags and the antenna is farther than that between the interference tags and the antenna. There￾fore, the coverage ratio decreases, while the misreading ratio increases, as shown in Figs. 16a and 16c. However, based on Fig. 4b, the target tags are in the major interro￾gation region, while the interference tags are mostly in minor interrogation region. Due to the different performan￾ces in major / minor interrogation regions, our solutions can basically satisfy the requirement r  a, as shown in Fig. 16a. When l ¼ 1:5 m, the coverage ratios of PID and PIA are a little less than the requirement. Because the distance between the target tags and the antenna is about 1:8 m, which is too large, according to Table 3. Even using the maximal power, the coverage ratio is only about 61 percent. In such a scenario, it is appropriate to decrease the require￾ment of a. Nevertheless, our proposed solutions still have better performance than the baseline solutions. When l ¼ 1m, PID reduces the execution time by 47 percent com￾pared to MaxPw, as shown in Fig. 16b. PID and PIA do not identify too many interference tags as MaxPw does, as shown in Fig. 16c. Because our solutions try to select the opti￾mal power to identify the target tags, as shown in Fig. 16d. If we want to further improve the coverage ratio and reduce the misreading ratio, we can use multiple readers [32], [33] to joint identify the target tags. For example, the target tags are located in the left side of antenna A1, while in the right side of antenna A2. Then, we can take the inter￾section of the identified tags from A1 and A2 as the target tags, while eliminating the other tags. In future, we will con￾sider how to schedule the multiple readers and improve the efficiency of our solutions with multiple readers. 10.8 Extension for Tag Identification Towards Two￾Dimensional Space In order to evaluate the performance of our solutions in two-dimensional space, we conduct the following experi￾ment in the bookshelf. We deploy the books (tags) as Fig. 8 shows. The books located in the center of the bookshelf are target tags, while the tags around the target tags are interfer￾ence tags. We vary the number of interference tags u to in the Fig. 15. a ¼ 60%, tag identification with different tag placement. Fig. 16. a ¼ 60%, tag identification with target tags in left area. Fig. 17. a ¼ 60%, tag identification towards two-dimensional space. YIN ET AL.: FOCUS AND SHOOT: EXPLORING AUTO-FOCUS IN RFID TAG IDENTIFICATION TOWARDS A SPECIFIED AREA 899