Focus and Shoot:Efficient Identification Over RFID Tags 355 Kinect Identifying the tags Identifying the tags (3D Camera Smart Car Car (a)PID (b)PIA Fig.6.System prototypes work in the realistic environments Table 1.Upper bound of a d(m) 0.5 1.0 1.5 40 100%100%90% 80 95%85%65% 120 89%81%63% the requirement of coverage ratio (a=60%).Because the power is too small to activate the majority of the tags.When we identify the tags with the maximum power (MaxPw)or our proposed solutions (PID and PIA),the coverage ratios are all larger than 60%(p>a),which satisfy the requirement.As mentioned in Sect.2.2,the coverage ratio must be satisfied.Therefore,the solution MinPw is invalid and we ignore it in the following comparisons. 7.3 Execution Time T Figure8 shows the execution time of each solution.Our solutions PID and PIA have better performances than MaxPw.Because PID and PIA only focus on the target tags in S.MaxPw identifies all the tags in the interrogation region, including a lot of interference tags.Usually,PID has a better performance than PIA,due to the use of a 3D camera.In Fig.8(a),(b),the difference in execution time between PID,PIA and MaxPw is small.This is because the tag size is relatively small.When the tag size becomes large,our proposed solutions become 30 00 05 10 15 0 to 15 20 :d im) mber o时int切en网tg家9 (a)p vs.d (b)p vs.I (c)p vs.s (d)p vs.u Fig.7.a =60%,Coverage ratioFocus and Shoot: Efficient Identification Over RFID Tags 355 (a) PID (b) PIA Fig. 6. System prototypes work in the realistic environments Table 1. Upper bound of α n d (m) 0.5 1.0 1.5 40 100% 100% 90% 80 95% 85% 65% 120 89% 81% 63% the requirement of coverage ratio (α = 60 %). Because the power is too small to activate the majority of the tags. When we identify the tags with the maximum power (MaxPw) or our proposed solutions (PID and PIA), the coverage ratios are all larger than 60 % (ρ ≥ α), which satisfy the requirement. As mentioned in Sect. 2.2, the coverage ratio must be satisfied. Therefore, the solution MinPw is invalid and we ignore it in the following comparisons. 7.3 Execution Time T Figure 8 shows the execution time of each solution. Our solutions PID and PIA have better performances than MaxPw. Because PID and PIA only focus on the target tags in S. MaxPw identifies all the tags in the interrogation region, including a lot of interference tags. Usually, PID has a better performance than PIA, due to the use of a 3D camera. In Fig. 8(a), (b), the difference in execution time between PID, PIA and MaxPw is small. This is because the tag size is relatively small. When the tag size becomes large, our proposed solutions become (a) ρ vs. d (b) ρ vs. l (c) ρ vs. s (d) ρ vs. u Fig. 7. α = 60 %, Coverage ratio