Focus and Shoot 2.2 Performance Metrics We consider the three performance metrics for evaluating the solution's efficiency. 1)Coverage ratio p constraint:Let S be the set of tags in S(target tags), s =S.Let M be the set of the tags that are identified in S,m =M.Obviously, C S and m≤s.Them,p=g,0≤p≤l.The larger the value of p,the better the coverage ratio.Given a constant a,p should satisfy p>a.a is related to the specific scenario,when the environment and the deployment of the RFID system are fixed,the value of a can be determined. 2)Execution time T:It represents the duration of the whole process.It shows the time efficiency,which is rather important,especially for the identification of a large number of tags.The smaller the time T.the better the time efficiency. 3)Misreading ratio A:Let U be the set of tags out of S (interference tags) that are identified,u=lUl,UnS=0.Then,A=m.The smaller the value of入，the lower the misreading ratio. The objective of this paper is to minimize the execution time T,while the coverage ratio satisfies p>a.When p a,minimizing T means avoiding identifying the interference tags,in order to reduce the identification time.There is no constraint on A,which is related to T.However,for the same execution time,the lower the misreading ratio,the better the performance of a solution. 3 Observations From the Realistic Experiments In order to know the factors affecting the reading performance in the realistic environments,we conduct the following experiments.We use the Alien-9900+ reader and Alien-9611 antenna.The reader's maximum power maxP is 30.7dB- m and its minimum power minP is 15.7dBm.The RFID tag is Alien 9640 tag.Each tag is attached into a distinct book.The antenna and the books are placed on the tablet chairs with a height of 0.5m.Unless otherwise specified, we make the antenna face towards the center of the objects,set the reader's power P=30.7dBm,the distance between the tags and the antenna d=1m by default.For each experiment,the reader scans the tags for 50 cycles 3.1 Identify the tag at different angles As the angle between the radiation direction and the surface of the antenna de- ceases,the reading performance deceases.However,when a tag is located in the center of the interrogation region,it can be identified efficiently.We observe the minimum power Pmm needed to activate one tag.We use r to represent the angle between the antenna's radiation direction and the antenna's surface, b,∈O°，90°.In the first experiment,we rotate the antenna to change b,while keeping the tag unchanged.Fig.2(a)shows that as r decreases,Pm becomes larger.In the second experiment,we rotate the tag while keeping the antenna unchanged.We use 6,to represent the angle between the radiation direction and the tag's surface.Fig.2(a)shows that the tag is easily identified,whatever 6t is. Therefore,making the antenna face towards the tags(=90)is essential for improving the reading performance 3.2 Adjust the reader's power The larger the reader's power,the larger the interrogation region,but the new identified tags may not be located in the interrogation region's boundary.Howev- er,if a tag can be identified with a low power,it must be identified with a largerFocus and Shoot 3 2.2 Performance Metrics We consider the three performance metrics for evaluating the solution’s efficiency. 1) Coverage ratio ρ constraint: Let S be the set of tags in S (target tags), s = |S|. Let M be the set of the tags that are identified in S, m = |M|. Obviously, M ⊆ S and m ≤ s. Then, ρ = m s , 0 ≤ ρ ≤ 1. The larger the value of ρ, the better the coverage ratio. Given a constant α, ρ should satisfy ρ ≥ α. α is related to the specific scenario, when the environment and the deployment of the RFID system are fixed, the value of α can be determined. 2) Execution time T: It represents the duration of the whole process. It shows the time efficiency, which is rather important, especially for the identification of a large number of tags. The smaller the time T, the better the time efficiency. 3) Misreading ratio λ: Let U be the set of tags out of S (interference tags) that are identified, u = |U|, U ∩ S = ∅. Then, λ = u u+m . The smaller the value of λ, the lower the misreading ratio. The objective of this paper is to minimize the execution time T, while the coverage ratio satisfies ρ ≥ α. When ρ ≥ α, minimizing T means avoiding identifying the interference tags, in order to reduce the identification time. There is no constraint on λ, which is related to T. However, for the same execution time, the lower the misreading ratio, the better the performance of a solution. 3 Observations From the Realistic Experiments In order to know the factors affecting the reading performance in the realistic environments, we conduct the following experiments. We use the Alien-9900+ reader and Alien-9611 antenna. The reader’s maximum power maxPw is 30.7dB￾m and its minimum power minPw is 15.7dBm. The RFID tag is Alien 9640 tag. Each tag is attached into a distinct book. The antenna and the books are placed on the tablet chairs with a height of 0.5m. Unless otherwise specified, we make the antenna face towards the center of the objects, set the reader’s power Pw = 30.7dBm, the distance between the tags and the antenna d = 1m by default. For each experiment, the reader scans the tags for 50 cycles. 3.1 Identify the tag at different angles As the angle between the radiation direction and the surface of the antenna de￾ceases, the reading performance deceases. However, when a tag is located in the center of the interrogation region, it can be identified efficiently. We observe the minimum power Pwmin needed to activate one tag. We use θr to represent the angle between the antenna’s radiation direction and the antenna’s surface, θr ∈ [0◦ , 90◦ ]. In the first experiment, we rotate the antenna to change θr while keeping the tag unchanged. Fig. 2(a) shows that as θr decreases, Pwmin becomes larger. In the second experiment, we rotate the tag while keeping the antenna unchanged. We use θt to represent the angle between the radiation direction and the tag’s surface. Fig. 2(a) shows that the tag is easily identified, whatever θt is. Therefore, making the antenna face towards the tags (θr = 90◦ ) is essential for improving the reading performance. 3.2 Adjust the reader’s power The larger the reader’s power, the larger the interrogation region, but the new identified tags may not be located in the interrogation region’s boundary. Howev￾er, if a tag can be identified with a low power, it must be identified with a larger