352 Y.Yin et al. is the optimal power P.However,if >6,the reader reduces the power by AP and checks the verified tag IDs in Ne.If a tag does not give response,the reader removes it from Ne.It repeats the above process until <6 and gets the optimal power POn the contrary,if<,the reader increases P by AP and checks the unverified tag IDs in N-Ne={IDi IDiE No and IDi Ne. If the tag gives response,the reader adds the ID into Ne.It repeats the process until >6 and gets the optimal power P In the following process,the reader usesto identify the target tags. 5.2 Shooting Process In this process,the reader collects the tag IDs in S.The reader's power is equal to P and we use frame slotted ALOHA (FSA)protocol to identify the tags.FSA is a popular anti-collision protocol.In FSA,the reader first broadcasts a number f, which specifies the following frame size.After receiving f,each tag selects h(ID) mod f as its slot number,h is a hash function.If none of the tags respond in a slot,the reader closes the slot immediately.If only one tag responds in a slot,the reader successfully receives the tag ID.If multiple tags respond simultaneously, a collision occurs,and the involved tags will be acknowledged to restart in the next frame.The similar process repeats until no tags respond in the frame.The collected IDs are considered as the target tag IDs. 5.3 Performance Analysis In order to definitely describe the boundary So,PID needs to steadily get at least ne interference tag IDs,no satisfies no >ne.We measure the value of ne with different tag size NI.When |NI 20,60,100,140,180,220,we respectively get ne =2,4,7,9,11,12.The tag size N]has a little effect on ne,which is usually very small.In order to definitely get enough tag IDs in So,we set ne= 15 by default,while considering the stability and time efficiency.In regard to 6,the smaller the value of 6,the lower the misreading ratio,the smaller the execution time.The larger the value of 6,the larger the value of coverage ratio p.Considering the constraint of p and time efficiency,we set 6 =a.When =6=a,the interrogation region just achieves the boundary,while satisfying nb p >o.Besides,the antenna rotates to the target direction immediately,the time for rotating the antenna can be neglected compared to the tag identification time. 6 Photography Based Identification with Angle Rotation In PID,a 3D camera is used in the focusing process.However,in some envi- ronments,the 3D camera cannot work well (eg.in a dark space).Besides,con- sidering the cost savings,it will not be used.Therefore,identifying the target tags efficiently without the auxiliary equipment is important.For this prob- lem,we propose a solution called Photography based tag Identification with Angle rotation (PIA).It also consists of Focusing Process and Shooting Process.352 Y. Yin et al. is the optimal power P∗ w. However, if nc nb > δ, the reader reduces the power by ΔPw and checks the verified tag IDs in Nc. If a tag does not give response, the reader removes it from Nc. It repeats the above process until nc nb ≤ δ and gets the optimal power P∗ w. On the contrary, if nc nb < δ, the reader increases Pw by ΔPw and checks the unverified tag IDs in Nb −Nc = {IDi | IDi ∈ Nb and IDi ∈/ Nc}. If the tag gives response, the reader adds the ID into Nc. It repeats the process until nc nb ≥ δ and gets the optimal power P∗ w. In the following process, the reader uses P∗ w to identify the target tags. 5.2 Shooting Process In this process, the reader collects the tag IDs in S. The reader’s power is equal to P∗ w and we use frame slotted ALOHA (FSA) protocol to identify the tags. FSA is a popular anti-collision protocol. In FSA, the reader first broadcasts a number f, which specifies the following frame size. After receiving f, each tag selects h(ID) mod f as its slot number, h is a hash function. If none of the tags respond in a slot, the reader closes the slot immediately. If only one tag responds in a slot, the reader successfully receives the tag ID. If multiple tags respond simultaneously, a collision occurs, and the involved tags will be acknowledged to restart in the next frame. The similar process repeats until no tags respond in the frame. The collected IDs are considered as the target tag IDs. 5.3 Performance Analysis In order to definitely describe the boundary Sb, PID needs to steadily get at least nε interference tag IDs, nb satisfies nb ≥ nε. We measure the value of nε with different tag size |N|. When |N| = 20, 60, 100, 140, 180, 220, we respectively get nε = 2, 4, 7, 9, 11, 12. The tag size |N| has a little effect on nε, which is usually very small. In order to definitely get enough tag IDs in Sb, we set nε = 15 by default, while considering the stability and time efficiency. In regard to δ, the smaller the value of δ, the lower the misreading ratio, the smaller the execution time. The larger the value of δ, the larger the value of coverage ratio ρ. Considering the constraint of ρ and time efficiency, we set δ = α. When nc nb = δ = α, the interrogation region just achieves the boundary, while satisfying ρ ≥ α. Besides, the antenna rotates to the target direction immediately, the time for rotating the antenna can be neglected compared to the tag identification time. 6 Photography Based Identification with Angle Rotation In PID, a 3D camera is used in the focusing process. However, in some envi￾ronments, the 3D camera cannot work well (eg. in a dark space). Besides, con￾sidering the cost savings, it will not be used. Therefore, identifying the target tags efficiently without the auxiliary equipment is important. For this prob￾lem, we propose a solution called Photography based tag Identification with Angle rotation (PIA). It also consists of Focusing Process and Shooting Process