4 Yafeng Yin et al. power.We uniformly deploy 72 tags on the wall and the distance between two adjacent tags is 20cm,as shown in Fig.2(b).The new identified tags may not be in the interrogation region's boundary.We cannot distinguish a tag's position only by adjusting the power.In regard to a tag,Fig.2(c)shows that if a tag can be identified with a low power,then it definitely can be identified by a larger power.Usually,the large power can increase the number of identified tags. 32 -◆Rotate antenna:e, ■25.7Bm0.26.74Bm 30 28 一-Rotate tag8 3 24 30 025 320 崔 1401630Ad3607500 0123色o品1品eg10111213 16.7187207227307287307 (a)Minimum power vs.an-(b)Distribution of identi-(c)Identified tag IDs vs. gles fied tags vs.powers Dowers 100 Poer157d日m 20 60 10 0510o6品2303 -1.5-15g98l1520 10 209s 050 60 (d)Coverage ratio vs.dis-(e)Scanning range vs.tag(f)Number ofidentified tags tances densities vs.tag sizes Fig.2.Observations from the realistic experiments 3.3 Vary the distance between the tags and the antenna As the distance between the tags and the antenna increases,the reading per- formance decreases.Besides,when the distance is fired,the marimum coverage ratio has an upper bound,whatever the reader's power is.We vary the distance d from 0.5m to 3.5m.Fig.2(d)shows as d becomes larger,the number of identified tags decreases.When the distance is small (eg.d <1.5m),the reading perfor- mance is relatively good.However,when the distance and the number of tags are fixed,the coverage ratio has an upper bound.For example,when d=1.5m and n=55,the maximum coverage ratio is 78%.Fortunately,some applications (eg.sampling inspection)just needs the coverage ratios meet the constraint in- stead of achieving 100%.However,when considering the high coverage ratio,the antenna should not be placed far away from the tags 3.4 Effect of the tag size The tag size can affect the effective interrogation region.However,it has little effect on the number of identified tags.We uniformly deploy the tags in a row with length 4m and vary the number of tags (20,40,60,80).As shown in Fig. 2(e),given a fixed power(30.7dBm),as the tag size increases,the effective in- terrogation region decreases.Therefore,when the tag size in the specified area4 Yafeng Yin et al. power. We uniformly deploy 72 tags on the wall and the distance between two adjacent tags is 20cm, as shown in Fig. 2(b).The new identified tags may not be in the interrogation region’s boundary. We cannot distinguish a tag’s position only by adjusting the power. In regard to a tag, Fig. 2(c) shows that if a tag can be identified with a low power, then it definitely can be identified by a larger power. Usually, the large power can increase the number of identified tags. 0 15 30 45 60 75 90 14 16 18 20 22 24 26 28 30 32 Angle (q) Power (dBm) Rotate antenna: T r Rotate tag: T t (a) Minimum power vs. an￾gles 0 1 2 3 4 5 6 7 8 9 101112 13 0 1 2 3 4 5 6 7 Column number Row number 25.7dBm 26.7dBm (b) Distribution of identi- fied tags vs. powers 16.7 18.7 20.7 22.7 24.7 26.7 28.7 30.7 0 5 10 15 20 25 30 35 40 45 50 55 Power (dBm) Tag ID (c) Identified tag IDs vs. powers 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 20 40 60 80 100 Distance: d (m) Coverage Ratio (%) (d) Coverage ratio vs. dis￾tances -2 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 0 5 10 15 20 25 Scanning range (m) Tag density (tags/m) (e) Scanning range vs. tag densities 10 20 30 40 50 60 0 5 10 15 20 Tag size Number of identified tags Power: 15.7dBm Power: 16.7dBm Power: 17.7dBm Power: 18.7dBm (f) Number of identified tags vs. tag sizes Fig. 2. Observations from the realistic experiments 3.3 Vary the distance between the tags and the antenna As the distance between the tags and the antenna increases, the reading per￾formance decreases. Besides, when the distance is fixed, the maximum coverage ratio has an upper bound, whatever the reader’s power is. We vary the distance d from 0.5m to 3.5m. Fig. 2(d) shows as d becomes larger, the number of identified tags decreases. When the distance is small (eg. d ≤ 1.5m), the reading perfor￾mance is relatively good. However, when the distance and the number of tags are fixed, the coverage ratio has an upper bound. For example, when d = 1.5m and n = 55, the maximum coverage ratio is 78%. Fortunately, some applications (eg. sampling inspection) just needs the coverage ratios meet the constraint in￾stead of achieving 100%. However, when considering the high coverage ratio, the antenna should not be placed far away from the tags. 3.4 Effect of the tag size The tag size can affect the effective interrogation region. However, it has little effect on the number of identified tags. We uniformly deploy the tags in a row with length 4m and vary the number of tags (20, 40, 60, 80). As shown in Fig. 2(e), given a fixed power (30.7dBm), as the tag size increases, the effective in￾terrogation region decreases. Therefore, when the tag size in the specified area