5 m intervals. The smaller cube
(23232 m) inside the portal represents all possible tag locations during the interrogation
processes. The tag space is smaller than the portal volume because we assume items move
through the central region of the portal. Thus, we eliminate the space that is closest to the
wall, ?¬‚oor, or ceiling on consideration. Each of the 18 reader antenna positions has 3
orientations: 458, 08, and 458, respectively. In our tests, we set n0 to either 2 or 3, and
used a value of 90% for the required read accuracy. The transmit power from an RFID
reader was 0.5 W with 50 mW needed to activate an RFID tag that operates at 915 MHz.
With a resolution of 0.1 m for the tag space and 450 possible orientations for the tag
antenna at each location, the optimal solution, which chooses positions 3 and 16, will cover
71.8% of the 8400 tag locations with 90% read accuracy. When three reader antennas are to be
FIGURE 10.8
Portal design with 18 candidate antenna positions,
each of which has 3 orientations.
1
13
12
11
2
3
4
5
7 8 9 10 11 12
13
14
15
16
17
18
194 RFID Handbook: Applications, Technology, Security, and Privacy
placed, the extra one, which is placed at position 9 in the optimal solution, increases coverage
by another 14.8%. In other words, the extra antenna can cover approximately 1.18 m3 more
of the tag space with 90% read accuracy.
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