890 EEE TRANSACTIONS ON COMPUTERS,VOL.65,NO.3.MARCH 2016 the 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 0.5 m.Unless otherwise specified,we make the antenna face towards the center of the objects,set the reader's power P=30.7dBm,and the distance between the tags and the antenna d=1m.For each experiment,the Antenna reader scans the tags for 50 cycles. (a)Rotating the antenna (b)Rotating the tag Fig.2.Identify the tag at different angles. 4.1 Identifying the Tag at Different Angles As the angle between the radiation direction and the surface of the The objective of this paper is to minimize the execution antenna deceases,the reading performance usually deceases. time T,while the coverage ratio satisfies p>a.When p>a, Besides,the placement of the tag may affect the reading perfor- minimizing T means avoiding identifying the interference mance.As shown in Fig.2,we respectively rotate the tags,in order to reduce the identification time.There is no antenna and the tag to observe the minimal power Pmim constraint on A,which is related to T.However,for the needed to activate one tag.Firstly,we rotate the antenna same execution time,the lower the misreading ratio,the bet- while keeping the tag unchanged.We use 0,(see Fig.2a)to ter the performance of a solution. represent the angle between the antenna's radiation direc- tion and the antenna's surface,a,∈[0°，90l.Fig.3 a shows 4 OBSERVATIONS FROM THE REALISTIC that as decreases,P becomes larger.When the EXPERIMENTS antenna faces towards the tag(0,=90),it achieves the best reading performance.Secondly,we rotate the tag while In order to know the factors affecting the reading perfor- keeping the antenna unchanged.We use 6(see Fig.2b)to mance in real environments,we conduct the following represent the angle between the radiation direction and the experiments.We use the Alien-9900+reader and Alien-9611 tag's surface.We respectively rotate the tag along the x-axis, antenna.The reader's maximal power max Pe is 30.7 dBm y-axis,and z-axis,as shown in Fig.2b.Fig.3a shows when and its minimal power min P is 15.7 dBm.The RFID tag is the tag rotates along the x-axis,decreases,Pmin becomes 32 30 ■25.7dBm026.7dBm 6 5 ---凡a1年：间 E28 45 26 -d-Raae1gz改a 24 30 922 25 要20 20 818 1写 10 1530Ang0 75 90 16718720762636.7287307 05105283035 (a)Minimal power vs.angles (b)Distribution of identified (c)Identified tag IDs vs.powers (d)Coverage ratio vs.distances (Group 1) tags vs.powers 2 100 32 :157m 30 880 15 gy —Pcw0G1&7日m 1 60 一二 26 2 20 尊、 20 Paper Water FAng蓝o0 15.30 75 (e)Scanning range vs.tag densi- (f)Number of identified tags vs. (g)Identification ratio vs.materi- (h)Minimal power vs.angles ties tag cardinality als (Group 2) 32 32 32 30 0 30 E28 2 E28 26 26 号26 24 24 四22 中22 20 20 20 a18 18 16 14 15 75 90 15 Angle 3 45 75 90 1530Ang60 75 90 89Aa.A2.077 (i)Minimal power vs.angles (j)Minimal power vs.angles (k)Minimal power vs.angles (1)Identification ratio vs.mixed (Group 3) (Group 4) (Group 5) tags Fig.3.Observations from the realistic experiments.The objective of this paper is to minimize the execution time T, while the coverage ratio satisfies r  a. When r  a, minimizing T means avoiding identifying the interference tags, in order to reduce the identification time. There is no constraint on , which is related to T. However, for the same execution time, the lower the misreading ratio, the bet￾ter the performance of a solution. 4 OBSERVATIONS FROM THE REALISTIC EXPERIMENTS In order to know the factors affecting the reading perfor￾mance in real environments, we conduct the following experiments. We use the Alien-9900+ reader and Alien-9611 antenna. The reader’s maximal power max Pw is 30.7 dBm and its minimal power min Pw is 15.7 dBm. The RFID tag is the 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 0.5 m. Unless otherwise specified, we make the antenna face towards the center of the objects, set the reader’s power Pw ¼ 30:7 dBm, and the distance between the tags and the antenna d ¼ 1m. For each experiment, the reader scans the tags for 50 cycles. 4.1 Identifying the Tag at Different Angles As the angle between the radiation direction and the surface of the antenna deceases, the reading performance usually deceases. Besides, the placement of the tag may affect the reading perfor￾mance. As shown in Fig. 2, we respectively rotate the antenna and the tag to observe the minimal power Pwmin needed to activate one tag. Firstly, we rotate the antenna while keeping the tag unchanged. We use ur (see Fig. 2a) to represent the angle between the antenna’s radiation direc￾tion and the antenna’s surface, ur 2 ½0; 90. Fig. 3a shows that as ur decreases, Pwmin becomes larger. When the antenna faces towards the tag (ur ¼ 90), it achieves the best reading performance. Secondly, we rotate the tag while keeping the antenna unchanged. We use ut (see Fig. 2b) to represent the angle between the radiation direction and the tag’s surface. We respectively rotate the tag along the x-axis, y-axis, and z-axis, as shown in Fig. 2b. Fig. 3a shows when the tag rotates along the x-axis, ut decreases, Pwmin becomes Fig. 2. Identify the tag at different angles. Fig. 3. Observations from the realistic experiments. 890 IEEE TRANSACTIONS ON COMPUTERS, VOL. 65, NO. 3, MARCH 2016