76
3 Physical Theory of RFID System Physical Anti-Collision
G r is the gain of the reader to receive the antenna.
When the reader transmits and accepts the same antenna, there is G r = G t .
Therefore, the power received by the reader’s antenna position is
P Back = P t G t G r λ
2
σ
(4π) 3 R 4
(3.28)
Generally, the forward link limits the achievable reading range in an RFID system
because the sensitivity of the reader is significantly higher than the sensitivity of the
tag chip. Tag’s maximum read distance is given by
d =
λ
4π
P t G r G t τ
P th
(3.29)
As the ambient humidity increases, so does the power required to activate the tag
which means it will change the tag chip sensitivity threshold P th [26]. Moreover,
the tag chip sensitivity threshold P th shift is a result of humidity variation. Previous
research shows chip sensitivity threshold P th has a linear relationship with humidity.
Let us assume that, with the increase of humidity, the tag chip sensitivity threshold
P th increased gradually.
d =
λ
4π
PtGtGrτ
Pth
=
λ
4π
PtGtGrτ
10 lg(ax + b)
(3.30)
When the parameters of the tag are determined and the signal strength value
reaches the activation threshold, the tag can be read. Equation (3.30) can be rewritten
as
d = η
1
10 lg(ax + b)
(3.31)
η =
λ
4π
√
PtGtGrτ
(3.32)
In this case, η could be considered as a constant in the simulation.
Let P t . G t = 33dBm = 1995mW, G r = 2dBi, a = 0.00186, b = 0.9413, τ = 1.
Previous research shows that if the distance between the surface and the tag of the
box is λ/4 [27], the tag’s reading rate has the best effect. If the removal from the tag
to the test chamber’s surface is λ/4, previous research shows that the transmission
coefficient τ≈1 [28]. Finally, we can obtain the connection between the tag’s reading
distance and the humidity under static conditions, as shown in Fig. 3.10.
(2) Design of the Experimentation System
3 Physical Theory of RFID System Physical Anti-Collision
G r is the gain of the reader to receive the antenna.
When the reader transmits and accepts the same antenna, there is G r = G t .
Therefore, the power received by the reader’s antenna position is
P Back = P t G t G r λ
2
σ
(4π) 3 R 4
(3.28)
Generally, the forward link limits the achievable reading range in an RFID system
because the sensitivity of the reader is significantly higher than the sensitivity of the
tag chip. Tag’s maximum read distance is given by
d =
λ
4π
P t G r G t τ
P th
(3.29)
As the ambient humidity increases, so does the power required to activate the tag
which means it will change the tag chip sensitivity threshold P th [26]. Moreover,
the tag chip sensitivity threshold P th shift is a result of humidity variation. Previous
research shows chip sensitivity threshold P th has a linear relationship with humidity.
Let us assume that, with the increase of humidity, the tag chip sensitivity threshold
P th increased gradually.
d =
λ
4π
PtGtGrτ
Pth
=
λ
4π
PtGtGrτ
10 lg(ax + b)
(3.30)
When the parameters of the tag are determined and the signal strength value
reaches the activation threshold, the tag can be read. Equation (3.30) can be rewritten
as
d = η
1
10 lg(ax + b)
(3.31)
η =
λ
4π
√
PtGtGrτ
(3.32)
In this case, η could be considered as a constant in the simulation.
Let P t . G t = 33dBm = 1995mW, G r = 2dBi, a = 0.00186, b = 0.9413, τ = 1.
Previous research shows that if the distance between the surface and the tag of the
box is λ/4 [27], the tag’s reading rate has the best effect. If the removal from the tag
to the test chamber’s surface is λ/4, previous research shows that the transmission
coefficient τ≈1 [28]. Finally, we can obtain the connection between the tag’s reading
distance and the humidity under static conditions, as shown in Fig. 3.10.
(2) Design of the Experimentation System