354 Y.Yin et al. N={ID,ID2,...,ID}from the boundary.Otherwise,it increases the power by AP and gets An new tag IDs.Everytime,it should make sure that Ans >0, which indicates the new tag IDs are not from the area S.If Ans=0,the antenna keeps rotating away from S.PIA repeats the above process until nt ne.Then, the antenna has rotated Aor degree.After that,the antenna rotates to the opposite direction (right)and works in the same way.It rotates A0r.degree to the right side..If△9，r,>△9，n,it indicates the boundary in the right side is farther than the left one,then the reader terminates the process.Otherwise,it obtains N,={IDi,ID2,,IDnn}.The reader compares△f,and△9，to find the nearer boundary with the smaller angle,and gets the new set N of interference tags.If△er,=△9r,Nb=NUNr.If△9rm<△a,r,N%=i.If A >A0r,,No Nr.No is used for power stepping. The values of parameters in PIA are equal to those in PID.In regard to A0, in PIA,we set△0，=30°.Based on Fig.2,when0,∈[75°，90]，the reader undoubtedly has good performance.Therefore,when A0,30,each tag can be requested in the region with0,∈[75°，90]. 7 Performance Evaluation We evaluate the performance of each solution in the realistic environments.The experimental facilities are the same as those used in the observations.The execu- tion time,coverage ratio,and misreading ratio are used for performance metrics. In the experiments,each book is attached with an RFID tag,and the tag ID is 96 bits.The books are randomly deployed in three boxes and the distribution of the tag IDs are unknown.Each box is placed on a tablet chair with a height of 0.5m,as shown in Fig.6.PID uses a 3D camera,while PIA does not.The antenna is deployed on the smart car,which is controlled by the program and can rotate with the antenna flexibly.The antenna faces towards the tags to be identified.The specified area here is the center box,which is the target bor,while the other two boxes are interference bores.The distance between the target box and the antenna is d.The minimum distance between the interference box and the target box is l.s and u respectively represent the number of target tags (in target box)and the number of interference tags(in interference boxes).We verify the values of the parameters d,l,s,u to evaluate the performance of each solution.We set d 1m,l=1m,s=80,u 70 by default. 7.1 Upper Bound of a As mentioned in Sect.2.2,when the distance d,the number of tags n are fixed, we can determine the value of a.In Table 1,we give the upper bound of a under different conditions.We set a=60%for the following experiments by default. 7.2 Coverage Ratio p Constraint We first investigate the coverage ratio p of each solution,as shown in Fig.7.We can observe that scanning with the minimum power (MinPw)can not achieve354 Y. Yin et al. Nl = {ID 1,ID 2,...,ID nl } from the boundary. Otherwise, it increases the power by ΔPw and gets Δnl new tag IDs. Everytime, it should make sure that Δns > 0, which indicates the new tag IDs are not from the area S. If Δns = 0, the antenna keeps rotating away from S. PIA repeats the above process until nl ≥ nε. Then, the antenna has rotated Δθrl degree. After that, the antenna rotates to the opposite direction (right) and works in the same way. It rotates Δθrr degree to the right side. If Δθrr > Δθrl , it indicates the boundary in the right side is farther than the left one, then the reader terminates the process. Otherwise, it obtains Nr = {ID 1 ,ID 2 ,...,ID nr }. The reader compares Δθrl and Δθrr to find the nearer boundary with the smaller angle, and gets the new set Nb of interference tags. If Δθrl = Δθrr , Nb = Nl ∪ Nr. If Δθrl < Δθrr , Nb = Nl. If Δθrl > Δθrr , Nb = Nr. Nb is used for power stepping. The values of parameters in PIA are equal to those in PID. In regard to Δθr in PIA, we set Δθr = 30◦. Based on Fig. 2, when θr ∈ [75◦, 90◦], the reader undoubtedly has good performance. Therefore, when Δθr = 30◦, each tag can be requested in the region with θr ∈ [75◦, 90◦]. 7 Performance Evaluation We evaluate the performance of each solution in the realistic environments. The experimental facilities are the same as those used in the observations. The execu￾tion time, coverage ratio, and misreading ratio are used for performance metrics. In the experiments, each book is attached with an RFID tag, and the tag ID is 96 bits. The books are randomly deployed in three boxes and the distribution of the tag IDs are unknown. Each box is placed on a tablet chair with a height of 0.5 m, as shown in Fig. 6. PID uses a 3D camera, while PIA does not. The antenna is deployed on the smart car, which is controlled by the program and can rotate with the antenna flexibly. The antenna faces towards the tags to be identified. The specified area here is the center box, which is the target box, while the other two boxes are interference boxes. The distance between the target box and the antenna is d. The minimum distance between the interference box and the target box is l. s and u respectively represent the number of target tags (in target box) and the number of interference tags (in interference boxes). We verify the values of the parameters d, l, s, u to evaluate the performance of each solution. We set d = 1 m, l = 1 m, s = 80, u = 70 by default. 7.1 Upper Bound of α As mentioned in Sect. 2.2, when the distance d, the number of tags n are fixed, we can determine the value of α. In Table 1, we give the upper bound of α under different conditions. We set α = 60 % for the following experiments by default. 7.2 Coverage Ratio ρ Constraint We first investigate the coverage ratio ρ of each solution, as shown in Fig. 7. We can observe that scanning with the minimum power (MinPw) can not achieve