3.1 Thermodynamic Analysis of Physical Anti-Collision …
63
The reading distance of passive backscatter RFID system is given by
R =
P tx G tx G rx c
2
σ
(4π) 3 f 2 P rx
1/4
(3.12)
(3) Temperature effect on reading distance
In the RFID system, the backscattering signals of passive tags have the same carrier
frequency as the transmitting signals of reader, which is the frequency of the reader
that is used to activate RFID tags. The temperature change has an effect on the center
frequency of the passive tags’ backscattering signals. Moreover, the frequency shift is
a result of temperature variation. Let us assume that the frequency reduced gradually
with the increase of temperature and has a linear relationship with temperature.
Therefore, due to the offset of backscattering signals’ center frequency caused by
temperature variation, Eq. (3.12) could be rewritten as
R =
P
tx G
tx G rx c
2
σ
(4π) 3 P rx f
2
1/4
=
P
tx G
tx G rx c
2
σ
(4π) 3 P rx (aT + b) 2
1/4
( f 1 ≤ f
≤ f 2 ) (3.13)
where R is the distance where the signals of RFID tag could be read by a reader,
P
tx
is the power of backscattering signals, G
tx is the gain of tag’s antenna, f
is the
migrated frequency of backscattering signals’ center frequency, T is environment
temperature around RFID tag, and a, b are undetermined coefficients. f 1 and f 2 are
the lower frequency limit and upper frequency limit. RFID reader and tag could
communicate under the frequency range which is determined by the characteristics
of reader. When the frequency f
is less than f 1 or more than f 2 , the tag could not be
read. When the parameters of a tag are determinate and the tag could be read once the
value of signal strength reaches the activated threshold, Eq. (3.13) could be rewritten
as
R = η
1
(aT + b) 2
1/4
(3.14)
η =
P tx G tx G rx c
2
σ
(4π) 3 P rx
1/4
(3.15)
According to Eq. (3.15), η changes with the received power P rx , which is related
to the distance between the reader antenna and tag. The focus of this work is the
influence of temperature on the dynamic reading performance of the UHF RFID
system, so the received power P rx is supposed to be the sensitivity of the reader’s
antenna. In this case, the received power P rx is constant and then η could be considered
as a constant in the simulation.
Since a, b, η are constants, Eq. (3.14) can be rewritten as
63
The reading distance of passive backscatter RFID system is given by
R =
P tx G tx G rx c
2
σ
(4π) 3 f 2 P rx
1/4
(3.12)
(3) Temperature effect on reading distance
In the RFID system, the backscattering signals of passive tags have the same carrier
frequency as the transmitting signals of reader, which is the frequency of the reader
that is used to activate RFID tags. The temperature change has an effect on the center
frequency of the passive tags’ backscattering signals. Moreover, the frequency shift is
a result of temperature variation. Let us assume that the frequency reduced gradually
with the increase of temperature and has a linear relationship with temperature.
Therefore, due to the offset of backscattering signals’ center frequency caused by
temperature variation, Eq. (3.12) could be rewritten as
R =
P
tx G
tx G rx c
2
σ
(4π) 3 P rx f
2
1/4
=
P
tx G
tx G rx c
2
σ
(4π) 3 P rx (aT + b) 2
1/4
( f 1 ≤ f
≤ f 2 ) (3.13)
where R is the distance where the signals of RFID tag could be read by a reader,
P
tx
is the power of backscattering signals, G
tx is the gain of tag’s antenna, f
is the
migrated frequency of backscattering signals’ center frequency, T is environment
temperature around RFID tag, and a, b are undetermined coefficients. f 1 and f 2 are
the lower frequency limit and upper frequency limit. RFID reader and tag could
communicate under the frequency range which is determined by the characteristics
of reader. When the frequency f
is less than f 1 or more than f 2 , the tag could not be
read. When the parameters of a tag are determinate and the tag could be read once the
value of signal strength reaches the activated threshold, Eq. (3.13) could be rewritten
as
R = η
1
(aT + b) 2
1/4
(3.14)
η =
P tx G tx G rx c
2
σ
(4π) 3 P rx
1/4
(3.15)
According to Eq. (3.15), η changes with the received power P rx , which is related
to the distance between the reader antenna and tag. The focus of this work is the
influence of temperature on the dynamic reading performance of the UHF RFID
system, so the received power P rx is supposed to be the sensitivity of the reader’s
antenna. In this case, the received power P rx is constant and then η could be considered
as a constant in the simulation.
Since a, b, η are constants, Eq. (3.14) can be rewritten as