A Multi-Carrier UHF Passive RFID System

More documents

Recommendations

Info

whose diagrammatic sketch is shown in Figure 1(b). The proposed system not only can provide sufficient energy to Gen2 tags but also can extend its accessible range. Furthermore, taking the advantages of multiple carriers, the reader can be benefited from the frequency diversity gain and the multipath fading problem can be mitigated. The remainder of this paper is organized as follows: Section 2 introduces the proposed system. Section 3 presents the experimental results. Section 4 draws conclusions. 2. The Multi-carrier UHF passive RFID system Unlike the integrated reader in an ordinary UHF passive RFID system, the reader in a multi-carrier UHF passive RFID system is decomposed into two parts: a CW emitter (CWE) and a transceiver. The CWE can be preset leaky cables or radio source(s) close to tags, which emit a CW with frequency f c to illuminate tags. The CWE constantly provides energy and the MBS carrier(s) to the tags. The transceiver, which can be farther away from the tags, is mainly used to transmit the R-T commands and receive the T-R responses. As illustrated in Figure 1(b), the received power of the tag P ' tag can be presented as Eq.(3). 2 ' ' ⎛ λ ⎞ c tag = cw ⎜ ⎟ cwe tag 4π d1 P P G G , (3) ⎝ ⎠ where P denotes the power of the CW emitted from ' cw the CWE, λ c is the wavelength of the CW, and d 1 is the distance between the CWE and the tag. G cwe and G tag are the power gains of the CWE and tag antennas respectively. In addition, the power of the MBS received at the reader P can be written as Eq.(4). ' rx 2 ' ' ⎛ λ ⎞ rx = tag ⎜ ⎟ rx tag 4π d2 P P G G , (4) ⎝ ⎠ where G is the power gains of the transceiver, and rx d 2 is the maximum accessible distance between the transceiver and the tag. Assuming that the antenna gains of the CWE and the transceiver are the same as that of the integrated reader in an ordinary system ( Gcwe = Grx = Greader ), and the CW frequencies and powers in both systems are the same ' ( λ = λc, Pcw = Pcw), the power of received MBS in both system can be written as Eq.(5). ' 4 Prx d0 = . (5) 2 2 Prx d1d2 ' When d1 < d0 and Prx = Prx , d2 > d0 can be derived. In other words, given the same receiver sensitivity and the same CW emission power, the proposed system can effectively extend the accessible distance between a tag and the receiver. Moreover, because the tags are plentifully illuminated by the CWE, the problem of insufficient tag operation energy is avoided. A simplified normal operation flow of the proposed system is illustrated in Figure 2. Before issuing an R-T command, the transceiver turns on the CWE to emit the CW, so that the tags nearby the CWE are powered up and ready for the R-T command. When the accessing operation is finished, the transceiver turns off the CWE, and the operation is then terminated. The proposed system may seem similar with an ordinary Gen2 system, except the additional CWE. As a matter of fact, they are substantially different. We discuss three major differences as below: 2.1. The frequency difference between the transceiver and the CWE Because the transceiver and the CWE are separated in our proposed system, the carrier frequency of the transceiver ( f t ) and the frequency of CW ( f c ) emitted by the CWE can be different. Assuming the noise is negligible, the RF signal received by a tag can be expressed as Eq.(6). s t = m t cos 2π f t + Acos 2π f t+ φ t , (6) () () ( t ) ( c ()) where ⎧A , when high level is sent m() t = t ⎨ denotes the ⎩ 0, when low level is sent waveform of an R-T command from the transceiver, A denotes the amplitude of the RF signal transmitted t by the transceiver, A denotes the amplitude of the CW emitted by the CWE, and φ () t is the phase different between the two carriers. Let ∆ f = fc − ft . After some manipulations, Eq.(6) can be rewritten as Eq.(7). s( t) = ⎡ ⎣m( t) + Acos( 2π∆ ft+ φ( t) ) ⎤ ⎦cos( 2π ftt) .(7) + Asin 2π∆ ft+ φ t sin 2π f t ( ()) ( t ) The signal s() t is firstly passed through the envelope detection circuit inside the tag IC as shown in Proceedings of the 2007 International Symposium on Applications and the Internet Workshops (SAINTW'07) 0-7695-2757-4/07 $20.00 © 2007
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
Page 1: A Multi-Carrier UHF Passive RFID Sy

A Multi-Carrier UHF Passive RFID System

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?