A Multi-Carrier UHF Passive RFID System

Figure 3 [2]. Let s () t denote the output of the circuit
as presented in Eq.(8).
e
e
2 2
() = () + + 2 () cos( 2π∆ + φ()
)
s t m t A Am t ft t
.(8)
Utilizing the Taylor series, se
() t can be expanded as
Eq.(9).
2 2 1
⎡2Am () t cos( 2π∆ ft + φ()
t ) ⎤
Se
() t = A + m () t i{1 + ⎢
⎥
2 ⎢
2 2
⎣
A + m () t ⎥
⎦
2
1
⎡2Am() t cos ( 2π∆ ft + φ()
t ) ⎤
− ⎢
⎥ + remainders}
8
2 2
⎢
⎣
A + m () t ⎥
⎦
.(9)
se
() t is then passed through the LPF as depicted in
Figure 3. Assuming ∆f
is sufficiently large; the high
frequency terms are filtered by the LPF. The
output se
() t can be approximated as Eq.(10).
2 2 1
se
() t = A + m () t −
. (10)
2 2
4 A + m () t
As shown in experimental results, the R-T
command can be correctly recognized by Gen2 tags
under such a circumstance. Contradictorily, when ∆ f
is small, the R-T command is distorted by the CW
emitted by the CWE, and Gen2 tags do not respond the
command.
2.2. The modulation depth of R-T commands
In [1], the modulation depth (MD) of a valid R-T
command is specified to be greater than 80% and less
than 100% in terms of the incident electric field
strength at the tags. The MD is defined as Eq.(11).
EA
− EB
MD = , (11)
EA
where E
A
and E
B
denote the maximum and
minimum amplitudes of the RF envelope in an R-T
command respectively. Figure 4(a) and Figure 4(b)
illustrate an R-T command in an ordinary Gen2 system
and in the proposed system respectively. From Eq.(10)
and Eq.(11), the MD in our proposed system can be
2 2 2 2
A + A − A / A + A .For the
presented as ( t )
typical MD=90%, the power relation between the CWE
and the transceiver can be derived as Eq.(12).
t
P / d + P / d − P / d
' ' '
cw 1 tx 2 cw 1
'
'
Pcw
/ d1+
Ptx
/ d2
= 90% , (12)
'
where P
tx
denotes the maximum transmission power
of the transceiver while sending an R-T command.
2.3. The detection symbol in R-T Commands
In order to decode an R-T command, the tag IC must
have a PIE decoder, which is an edge detector [2]. A
regular Gen2 R-T command cannot be directly used in
the proposed system, because the first falling edge and
the last rising edge of the R-T command, as depicted in
Figure 4(a), cannot be detected by tags. In order to be
compliant with Gen2 tags, two short detection symbols
are added to the head and the tail of each R-T
command in the proposed system as illustrated in
Figure 4(b).
3. Experimental results
An Agilent 89601 vector signal analyzer
incorporating with an Agilent E4445A spectrum
analyzer is used to capture the communication signals
between the transceiver and a Gen2 tag. The
frequencies of the CWE and the transceiver are
915MHz and 917MHz respectively. The R-T Tari is
25us, and the duration of the both detection symbols
are 11us. The powers of the CWE and the transceiver
are adequately adjusted, so that the MD requirement is
satisfied. The snapshots are taken nearby the tags.
Figure 5 shows that the tag MBS is relatively strong.
Under such a circumstance, the accessible range of the
tag can be extended to 4 times longer than that of an
ordinary Gen2 system with the same emission power.
Figure 6 demonstrates an interesting multi-band
backscatter property of Gen2 tags. We let the
transceiver not only send the R-T commands but also
send a low power CW during the tag responses. We
found that the tag MBS presents not only in the carrier
frequencies of CWE but also in the carrier frequencies
of the transceiver (917MHz) as illustrated in Figure 6.
This phenomenon implies that we can apply the
frequency diversity technology (using a multi-carrier
CWE, for instance) to enhance the quality of the tag
MBS and to mitigate the multipath fading problem.
4. Conclusion
In this paper, we proposed a multi-carrier UHF
passive RFID System, which can effectively extend the
accessible range of Gen2 tags. Moreover, the multiband
backscatter property of Gen2 tags, which is
Proceedings of the 2007 International Symposium
on Applications and the Internet Workshops (SAINTW'07)
0-7695-2757-4/07 $20.00 © 2007