Finally suppose that the (in?¬?nitely many) orientations that
a tag??™s antenna could take on are also discretized into a total of M distinct orientations.
Given a suitably speci?¬?ed fraction a, our objective is to determine the optimal number of
readers along with their optimal locations, so as to maximize the number of positions in the
tag space that can be powered with at least 100a% read accuracy, that is, for which at least
aM orientations can receive the threshold power required to be read.
RFID reader antenna 1
RFID reader antenna 2
FIGURE 10.5
Orientations for the two reader cases.
Maximizing Read Accuracy by Optimally Locating RFID Interrogators 189
The reader antenna placement problem can be formulated as the following integer
program [4]:
MaxX L
l??1
zl
s.t.
X N
n??1
xn n0 (10:6)
ylm X N
n??1
plmn xn 0, 8l ?? 1,2, , L, m ?? 1,2, ,M
X M
m??1
ylm a M zl 0, 8l ?? 1,2, , L
(10:7)
xn 2 f0,1gN, ylm 2 f0,1gL*M, zl 2 f0,1gL (10:8)
where
N is the number of candidate reader positions
L is the number of tag positions in the tag space
M is the number of discretized orientations considered for each tag position
n0 is the maximum number of reader antennas available
100a% is the required percentage read accuracy for every point in the powering region
plmn is the binary coef?¬?cient. plmn??1 if a tag at point l with orientation m is in range to
receive enough power from a reader antenna at location n
zl is the binary variable.
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