5 gives the minimum
power required by a tag at its threshold of operation. Then it is possible to use Pr(tag) from
Equation 4.5 in Equation 4.4 to obtain a maximum possible read range when having
enough power to energize the tag is the constraint:
Pr(tag) ??
PIC
hkm
: (4:5)
76 RFID Handbook: Applications, Technology, Security, and Privacy
If however it is the sensitivity of the interrogator??™s RF receiver that is limiting, it is
necessary to ?¬?nd the distance at which the received signal at the interrogator??™s receiver
just meets the SNR of the receiver. Hence, the minimum received power Pr(reader) at which
the SNR of the receiver is satis?¬?ed is given in the following equation:
Pr(reader) ?? (S=N)minkTB(NF), (4:6)
where
NF is the noise factor of the receiver
B is the bandwidth of the receiver
k is Boltzman??™s constant
T is the absolute reference temperature used in the de?¬?nition of the receiver noise
factor NF
(S=N)min is the minimum signal-to-noise ratio needed to decode a tag reply successfully
Thus the minimum read range can be calculated by considering the one-way signal
strength for a transmission from a tag to an interrogator with the required interrogator
received power given by Equation 4.6. The result is the read range given in Equation 4.7,
wherein the power Pt(tag) is the power scattered back from the tag and where it is assumed
that the tag is just suf?¬?ciently energized:
r ?? ?¬??¬??¬??¬??¬??¬??¬??¬??¬??¬??¬??¬??¬??¬??¬? Pt(tag)greadergtag
Pr(reader) s l
4p : (4:7)
To calculate Pt(tag), we need the ratio of the effective modulated power radiated by the tag
antenna to the available source power from the tag antenna.
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