YIN ETAL:FOCUS AND SHOOT:EXPLORING AUTO-FOCUS IN RFID TAG IDENTIFICATION TOWARDS A SPECIFIED AREA 895 equal to the minimum power minP and identifies n,tags in power stepping.If p has not been determined,PIA utilizes S.If n,<ne,the reader repeatedly increases the power by the ending power P Pr to determine the initial power AP and identifies the tags until n,na,the identified target Pus.If No =Ni,Pis =P.Otherwise,Ps=Pr.Pus is used tags are N,={ID1,ID2,...,ID}.If P=max P,PIA gets for power stepping,which is described in Algorithm 2. P=max P.Otherwise,the antenna rotates away from S The values of parameters in PIA are equal to those in to get the interference tag IDs in the boundary,as shown in PID.n regard to△a,in PIA,we set△9，=30°.Based on Algorithm 3. Fig.3a,when 6,∈[75°，90l,the reader undoubtedly has good performance.Therefore,when△9，=30°，each tag can Algorithm 3.PIA:Exploring the Boundary be requested in the center of the interrogation region with Input:The specified area S a,∈[75°，90l.PIA does not identify the tags while the Pw=min P,ns=0,=0,△09n=0° antenna is rotating.This is because the reader cannot deter- while n,ne and Pe<max P do mine where the identified tags are located,when the Get ns tag IDs,Pie min(P +AP:max Pu). antenna is rotating.Therefore,PIA rotates to the next direc- if Pe max Pe then P=max Pu,Return. tion immediately as PID does,then it identifies the tags. Get tag IDs N ={ID1,ID2...,IDn,}. i=1. 7.2 Comparison of PID and PIA while n<ne and△e,<90°do We compare PID and PIA in the following aspects: The antenna rotates to the left,the accumulative rotation angle is△0，=min(△9，+△0r,90°). System equipment.PID uses an auxiliary equipment while P<maxPe do (i.e.,a 3D camera),while PIA does not need any aux- △ns;tag IDs in N.disappear,n=ns-△nsg· iliary equipment. Get n tag IDs N not in S. Performance comparison.With the 3D camera,PID can ifm≥ne then Break. quickly focus on the specified area and reduce the if ns<ns then execution time.PIA needs to rotate the antenna to ifi>land△n<△ns-1 then Break find the specified area,it often needs more execution Pe min(Pe +APe;max Pu). time and identifies more interference tags. elseBreak. Application environment.By using the 3D camera,PID i=+1. recognizes the specified area by the RGB camera and Pu Pur Ni NI. measures distance by the 3D depth sensor.In the The antenna rotates to the right in [0 degree,Al,gets N and environments like dark spaces,the RGB camera can Pr,it rotates Aer degree. not work well,thus PID can not work well.While if△a,=△9，and Pir=P,then N=NUNr else if△9n=△9n,andP>P,then No=Nr PIA will not be affected by the surroundings and can work well in different environments. else if.△an>△9，then N%=Nr Output:Tag IDs in the boundary:N 8 EXTENSION FOR TAG IDENTIFICATION TOWARDS When the antenna rotates to another direction (called TWO-DIMENSIONAL SPACE left),the identified tags in s decreases.As shown in In Section 3,the target tags and the interference tags are Algorithm 3,the radiation angle decreases by Aor.In the ith mainly located in one-dimensional space (Our previous step,Ans;tags disappear from S,the number of identified conference version [31]focuses on this problem.).However, tags in S is n At the same time,the reader gets n tag IDs in some applications,the objects may be located around the out of S,and they are considered as the tag IDs from the farget tags in two-dimensional space,as shown in Fig.8. boundary.If n ne,the reader collects enough tag IDs In this case,PID can still use the 3D camera to distinguish N=[ID,ID2,...,ID}from the boundary.Otherwise,it the target tags and interference tags.Unfortunately,PIA increases the power by AP.Each time,it should make sure may not work well,because it mainly identifies the interfer- that n<n,and△n;≥△ns-,which indicates that the ence tags in one-dimensional space (see Fig.7).It may not new identified tag IDs are not from the area S.Otherwise, find the boundary appropriately in two-dimensional space. the antenna keeps rotating away from S.PIA repeats the Thus we improve PIA as the Enhanced Photography based above process until n>ne.Then,the antenna has rotated Identification with Angle rotation(EPIA)to identify the tar- A0r degrees.At this time,the ending power of the reader is get tags in two-dimensional space. P After that,the antenna rotates to the opposite direction In EPIA,the reader identifies some target tags N,as PIA (right)and works in the same way.It rotates A0r degrees to does.Then,it begins to identify the interference tags in the the right side and the ending power of the reader is P.If boundary So around the specified area s,as shown in Fig.8. Aor>Ao,it indicates that the boundary on the right side The antenna first randomly selects a direction to identify is farther than that of the left one,then the reader terminates some tags NI from the boundary,as shown in Algorithm 4. Then,it rotates Aor in clockwise direction in order to iden- the process.Otherwise,it obtains N=[ID,ID2,..., tify other tags around the boundary,aiming to find the inter- ID.The reader compares Aen Aerr and P Pr to find ference tags closest to the target tags,as shown in Fig.8.In the the nearer boundary,and gets the new set N of interference newly-selected direction,it identifies the interference tags as tags.If o=A0rr and Pir=Pur,N=NUN,.If PIA does.When the antenna has finished identifying the on=A0r,while PPr,N =N,.Besides,ifon>tags around the specified S,it will select the interference tags Ae,N=N.Otherwise,N=N.Here,N is used for with the smallest rotation angle value[1equal to the minimum power minPw and identifies ns tags in S. If ns < n", the reader repeatedly increases the power by DPw and identifies the tags until ns  n", the identified target tags are Ns ¼ fID1; ID2; ... ; IDnsg. If Pw ¼ max Pw, PIA gets P w ¼ max Pw. Otherwise, the antenna rotates away from S to get the interference tag IDs in the boundary, as shown in Algorithm 3. Algorithm 3. PIA: Exploring the Boundary Input: The specified area S Pw ¼ min Pw, ns ¼ 0, nl ¼ 0, Durl ¼ 0. while ns < n" and Pw < max Pw do Get ns tag IDs, Pw ¼ minðPw þ DPw; max PwÞ. if Pw ¼ max Pw then P w ¼ max Pw, Return. Get tag IDs Ns ¼ fID1; ID2 ... ; IDnsg. i ¼ 1. while nl < n" and Durl < 90 do The antenna rotates to the left, the accumulative rotation angle is Durl ¼ minðDurl þ Dur; 90Þ. while Pw < maxPw do Dnsi tag IDs in Ns disappear, nsi ¼ ns Dnsi . Get nl tag IDs Nl not in S. if nl  n" then Break. if nsi < ns then if i > 1 and Dnsi < Dnsi 1 then Break. Pw ¼ minðPw þ DPw; max PwÞ. elseBreak. i ¼ i þ 1. Pwl ¼ Pw, Nb ¼ Nl. The antenna rotates to the right in ½0 degree;Durl , gets Nr and Pwr , it rotates Durr degree. if Durl ¼ Durr and Pwl ¼ Pwr then Nb ¼ Nl [ Nr. else if Durl ¼ Durr and Pwl > Pwr then Nb ¼ Nr. else if Durl > Durr then Nb ¼ Nr. Output: Tag IDs in the boundary :Nb When the antenna rotates to another direction (called left), the identified tags in S decreases. As shown in Algorithm 3, the radiation angle decreases by Dur. In the ith step, Dnsi tags disappear from S, the number of identified tags in S is nsi . At the same time, the reader gets nl tag IDs out of S, and they are considered as the tag IDs from the boundary. If nl  n", the reader collects enough tag IDs Nl ¼ fID0 1; ID0 2; ... ; ID0 nl g from the boundary. Otherwise, it increases the power by DPw. Each time, it should make sure that nsi < ns and Dnsi  Dnsi 1 , which indicates that the new identified tag IDs are not from the area S. Otherwise, the antenna keeps rotating away from S. PIA repeats the above process until nl  n". Then, the antenna has rotated Durl degrees. At this time, the ending power of the reader is Pwl . After that, the antenna rotates to the opposite direction (right) and works in the same way. It rotates Durr degrees to the right side and the ending power of the reader is Pwr . If Durr > Durl , it indicates that the boundary on the right side is farther than that of the left one, then the reader terminates the process. Otherwise, it obtains Nr ¼ fID00 1; ID00 2; ... ; ID00 nr g. The reader compares Durl , Durr and Pwl , Pwr to find the nearer boundary, and gets the new set Nb of interference tags. If Durl ¼ Durr and Pwl ¼ Pwr , Nb ¼ Nl [ Nr. If Durl ¼ Durr , while Pwl > Pwr , Nb ¼ Nr. Besides, if Durl > Durr , Nb ¼ Nr. Otherwise, Nb ¼ Nl. Here, Nb is used for power stepping. If P w has not been determined, PIA utilizes the ending power Pwl , Pwr to determine the initial power Pws. If Nb ¼ Nl, Pws ¼ Pwl . Otherwise, Pws ¼ Pwr . Pws is used for power stepping, which is described in Algorithm 2. The values of parameters in PIA are equal to those in PID. In regard to Dur in PIA, we set Dur ¼ 30. Based on Fig. 3a, when ur 2 ½75; 90, the reader undoubtedly has good performance. Therefore, when Dur ¼ 30, each tag can be requested in the center of the interrogation region with ur 2 ½75; 90. PIA does not identify the tags while the antenna is rotating. This is because the reader cannot deter￾mine where the identified tags are located, when the antenna is rotating. Therefore, PIA rotates to the next direc￾tion immediately as PID does, then it identifies the tags. 7.2 Comparison of PID and PIA We compare PID and PIA in the following aspects:
System equipment. PID uses an auxiliary equipment (i.e., a 3D camera), while PIA does not need any aux￾iliary equipment.
Performance comparison. With the 3D camera, PID can quickly focus on the specified area and reduce the execution time. PIA needs to rotate the antenna to find the specified area, it often needs more execution time and identifies more interference tags.
Application environment. By using the 3D camera, PID recognizes the specified area by the RGB camera and measures distance by the 3D depth sensor. In the environments like dark spaces, the RGB camera can not work well, thus PID can not work well. While PIA will not be affected by the surroundings and can work well in different environments. 8 EXTENSION FOR TAG IDENTIFICATION TOWARDS TWO-DIMENSIONAL SPACE In Section 3, the target tags and the interference tags are mainly located in one-dimensional space (Our previous conference version [31] focuses on this problem.). However, in some applications, the objects may be located around the target tags in two-dimensional space, as shown in Fig. 8. In this case, PID can still use the 3D camera to distinguish the target tags and interference tags. Unfortunately, PIA may not work well, because it mainly identifies the interfer￾ence tags in one-dimensional space (see Fig. 7). It may not find the boundary appropriately in two-dimensional space. Thus we improve PIA as the Enhanced Photography based Identification with Angle rotation (EPIA) to identify the tar￾get tags in two-dimensional space. In EPIA, the reader identifies some target tags Ns as PIA does. Then, it begins to identify the interference tags in the boundary Sb around the specified area S, as shown in Fig. 8. The antenna first randomly selects a direction to identify some tags Nb1 from the boundary, as shown in Algorithm 4. Then, it rotates Durc in clockwise direction in order to iden￾tify other tags around the boundary, aiming to find the inter￾ference tags closest to the target tags, as shown in Fig. 8. In the newly-selected direction, it identifies the interference tags as PIA does. When the antenna has finished identifying the tags around the specified S, it will select the interference tags with the smallest rotation angle value Durk, k 2 ½1; d360 Durce, YIN ET AL.: FOCUS AND SHOOT: EXPLORING AUTO-FOCUS IN RFID TAG IDENTIFICATION TOWARDS A SPECIFIED AREA 895