Focus and Shoot:Efficient Identification Over RFID Tags 351 Algorithm 1.PID:Establishing the boundary Input:The specified area S Determine the boundary S of S by the 3D camera,and calculate do and ds. The antenna rotates to s withparccos(). P.b=Pemn(90°，d),Pe=Pb,n6=0. while no<ne and Pu<maxPw do Collect tag IDs with P and get no responses. if P marPw and nb ne then P=maxPu,Return. Pu min(Pu +AP;mat P). Get the tag IDs N={ID1,ID2,...,IDn}. Output:Tag IDs in the boundary:N Power Stepping.If P has not been determined,the reader will adjust the power through power stepping.Firstly,the reader chooses an initial power P= Pm(r,d)according to and do,where=90-and d=do.It is a critical value in theory,whose interrogation region just achieves the boundary of S. However,as shown in Fig.2(e),the tag size can affect the effective interrogation region,Ps may not be the most reasonable power.Thus we properly adjust the power by checking the tag IDs in No,as shown in Algorithm 2. Algorithm 2.PID:Power Stepping Input:Tag IDs in N Pas =Pmin(0,d)=Pwmin (90-,du),P =Ps Check the tag IDs in N and get ne responses Ne. if =6 then P P.s. if then while P>minPu do Pu max(Pu-APu,minP). Check IDs in Ne,get△ne responses,ne=△ne. if ns <6 then P Pw,Break. if ns<6 then m while P<maxPu do Pw min(P APw,maxPu). Check IDs in N-Ne,get Ane responses,ne ne+Anc if a >6 then Pu P,Break. Output:The optimal power P In the commercial RFID systems,the reader (eg.Alien-9900+)selects a specified tag by setting the mask equal to the tag ID.If the tag gives response,the reader gets a nonempty slot.Otherwise,it gets an empty slot.The reader checks all the IDs in N and gets ne responses Ne.Obviously,ne<n.When a=6,the interrogation region just achieves the boundary of S.The corresponding powerFocus and Shoot: Efficient Identification Over RFID Tags 351 Algorithm 1. PID: Establishing the boundary Input: The specified area S Determine the boundary Sb of S by the 3D camera, and calculate db and ds. The antenna rotates to Sb with ϕ = arccos( ds db ). Pwb = Pwmin (90◦, db), Pw = Pwb, nb = 0. while nb < nε and Pw < maxPw do Collect tag IDs with Pw and get nb responses. if Pw = maxPw and nb < nε then P∗ w = maxPw, Return. Pw = min(Pw + ΔPw, maxPw). Get the tag IDs Nb = {ID1,ID2,...,IDnb }. Output: Tag IDs in the boundary: Nb Power Stepping. If P∗ w has not been determined, the reader will adjust the power through power stepping. Firstly, the reader chooses an initial power Pws = Pwmin (θr, d) according to ϕ and db, where θr = 90◦−ϕ and d = db. It is a critical value in theory, whose interrogation region just achieves the boundary of S. However, as shown in Fig. 2(e), the tag size can affect the effective interrogation region, Pws may not be the most reasonable power. Thus we properly adjust the power by checking the tag IDs in Nb, as shown in Algorithm 2. Algorithm 2. PID: Power Stepping Input: Tag IDs in Nb Pws = Pwmin (θr, d) = Pwmin (90◦ − ϕ, db), Pw = Pws. Check the tag IDs in Nb and get nc responses Nc. if nc nb = δ then P∗ w = Pws. if nc nb > δ then while Pw > minPw do Pw = max(Pw − ΔPw, minPw). Check IDs in Nc, get Δnc responses, nc = Δnc. if nc nb ≤ δ then P∗ w = Pw, Break. if nc nb < δ then while Pw < maxPw do Pw = min(Pw + ΔPw, maxPw). Check IDs in Nb − Nc, get Δnc responses, nc = nc + Δnc. if nc nb ≥ δ then P∗ w = Pw, Break. Output: The optimal power P∗ w In the commercial RFID systems, the reader (eg. Alien-9900+) selects a specified tag by setting the mask equal to the tag ID. If the tag gives response, the reader gets a nonempty slot. Otherwise, it gets an empty slot. The reader checks all the IDs in Nb and gets nc responses Nc. Obviously, nc ≤ nb. When nc nb = δ, the interrogation region just achieves the boundary of S. The corresponding power