2.1 RFID Tag Group Performance Measurement
31
However, for tags group with different performance tags, the sensitivity is not
just related to the reading distance. Consider the tag is read in turn, so P r should
be greater than or equal to the sensitivity of each tag. If we do not consider the
mutual interference between tags, the value of the group tags sensitivity is equal to
the sensitivity of the single tag with the worst performance. Otherwise, the antennas
provide more energy to activate the tags group. So, the final sensitivity formula should
be in the form of an inequality, as shown in (2.3).
P
r ≥ max{P 1 , P 2 , P 3 , . . . , P i , . . . , P n }
(2.3)
where n is the number of tags,i = 1, 2, 3, . . . , n. In combination with (2.2), the
sensitivity formula of group tags can be expressed as (2.4)
P
r ≥
λ
2 P
t G
t G
r τ
(4π) 2
×
1
r 2
(2.4)
where r
is the distance from the sensor to the tags’ geometric center when all tags
in the tags group are been read, λ is the working wavelength of the tag, P
t is the
power of RFID reader, G
t is the gain of RFID reader antenna, G
r is the gain of
RFID tags group antenna and G
r = max{G i }, i = 1, 2, 3, . . . , n, P
r is the threshold
power of RFID tags group, τ
is the matching coefficient of tag and antenna. If the
tags are arranged in a reasonable way, the mutual interference is tiny. Hence, P
r is
approximately equal to the maximum sensitivity of all single tags. In practice, the tags
are close and the mutual interference certainly exists. Therefore, P
r is determined
inversely according to the actual distance.
2.1.2 Measurement System Designed
(1) System overview
Generally, in order to measure the sensitivity of RFID tags, the experiment is often
conducted in a dark room or a field with less interference, which requires a higher
experimental environment. In order to better measure the performance of tag groups,
Niklas designed a performance test system for measuring the reading distance and
environmental impact of RFID tags in the space, but the system can only carry out
limited motion measurement and the test distance is relatively short [6]. Literature
[7] designed a test system with a working range of 800−1000 mHz frequency. The
system has a wide working range and can complete the measurement of tag sensitivity,
but the system power is fixed, which is not conducive to the performance test of
tag group. In the literature [8], RFID technology is combined with the warehouse
automation transmission system to design an RFID device. This system can fully
31
However, for tags group with different performance tags, the sensitivity is not
just related to the reading distance. Consider the tag is read in turn, so P r should
be greater than or equal to the sensitivity of each tag. If we do not consider the
mutual interference between tags, the value of the group tags sensitivity is equal to
the sensitivity of the single tag with the worst performance. Otherwise, the antennas
provide more energy to activate the tags group. So, the final sensitivity formula should
be in the form of an inequality, as shown in (2.3).
P
r ≥ max{P 1 , P 2 , P 3 , . . . , P i , . . . , P n }
(2.3)
where n is the number of tags,i = 1, 2, 3, . . . , n. In combination with (2.2), the
sensitivity formula of group tags can be expressed as (2.4)
P
r ≥
λ
2 P
t G
t G
r τ
(4π) 2
×
1
r 2
(2.4)
where r
is the distance from the sensor to the tags’ geometric center when all tags
in the tags group are been read, λ is the working wavelength of the tag, P
t is the
power of RFID reader, G
t is the gain of RFID reader antenna, G
r is the gain of
RFID tags group antenna and G
r = max{G i }, i = 1, 2, 3, . . . , n, P
r is the threshold
power of RFID tags group, τ
is the matching coefficient of tag and antenna. If the
tags are arranged in a reasonable way, the mutual interference is tiny. Hence, P
r is
approximately equal to the maximum sensitivity of all single tags. In practice, the tags
are close and the mutual interference certainly exists. Therefore, P
r is determined
inversely according to the actual distance.
2.1.2 Measurement System Designed
(1) System overview
Generally, in order to measure the sensitivity of RFID tags, the experiment is often
conducted in a dark room or a field with less interference, which requires a higher
experimental environment. In order to better measure the performance of tag groups,
Niklas designed a performance test system for measuring the reading distance and
environmental impact of RFID tags in the space, but the system can only carry out
limited motion measurement and the test distance is relatively short [6]. Literature
[7] designed a test system with a working range of 800−1000 mHz frequency. The
system has a wide working range and can complete the measurement of tag sensitivity,
but the system power is fixed, which is not conducive to the performance test of
tag group. In the literature [8], RFID technology is combined with the warehouse
automation transmission system to design an RFID device. This system can fully