Proceedings of the 37th European Microwave Conference
Effects of Reader Interference on the RFID Interrogation Range Do-Yun Kim #1 , Byung-Jun Jang ∗2 , Hyun-Goo Yoon ∗∗3 , Jun-Seok Park ∗4 and Jong-Gwan Yook #5 #
Department of Electrical & Electronic Engeering, Yonsei University, Seoul, Korea 120-149. 1
∗
[email protected]
Department of Electronics Engineering, Kookmin University, Seoul, Korea. 2
∗∗
5
[email protected],
[email protected],
4
[email protected]
Department of Computer and Electronic Engineering, Myongji College, Seoul, Korea. 3
[email protected]
Abstract— In this paper, the effects of radio frequency identification (RFID) reader interference are investigated in terms of the interrogation range. In order to evaluate RFID interference quantitatively, the signal-to-interference ratio (SIR) equation is initially derived, and the interrogation-reduction range ratio (IRRR) defined. IRRR is a function of the distance between a desired reader and an interfering reader. Co-channel interference (CCI) and adjacent-channel interference (ACI) instances of IRRR are simulated. Simulation results show that reader-reader distances achieving 0 % IRRR, indicating no interference between the two readers, are 1200 m and 35 m for the CCI and ACI cases, respectively. The IRRR factor is inversely proportional to the reader-reader distance in both cases. The simulation results were also verified by measurement results using an ETRI UHF RFID system. Measurement results were found to be in good agreement with the simulation results. It can be concluded that the present simulation results are reliable and applicable in analyses of more complex interfering problems in actual RFID system deployment instances.
I. I NTRODUCTION Radio frequency identification (RFID) has drawn a great deal of attention over the last few years, as it is widely believed that RFID can revolutionize supply chain management [1][2]. Several major supply chain companies such as Wal-Mart and Tesco plan to mandate the use of RFID system in their supply chains [3]. However, deploying RFID systems on large scale also results in an unwanted effect. As multiple readers may exist in a working environment and communicate over shared wireless channels, a signal from one reader may reach other readers and cause frequency interferences [4][5]. RFID frequency interference occurs when a reader transmits a command signal that interferes with the tag reception procedure of another reader. This type of interference can happen even if the interrogation range of the two readers has no intersection, as the back-scattered signal from a tag is weak enough to be easily affected by interference. Thus, signals transmitted from distant readers may be sufficiently strong to impede accurate decoding of the signals back-scattered from adjacent tags. Consequently, frequency interference results in the interrogation range, which induces inaccurate reads and long reading intervals [6]. Hence, analyzing the effect of reader
978-2-87487-001-9 © 2007 EuMA
interference on the RFID interrogation range should precede a large deployment of a RFID system. Recently, RFID frequency interference has been described as a ’reader collision’. The work in [7] explained that RFID reader interference only affects interrogation range reduction using the signal-to-interference (SIR) concept. In fact, greater RFID interference only implies that the read range has been reduced significantly but not to zero. In an effort to evaluate RFID interference quantitatively, this study initially derives a SIR value and then defines the interrogation reduction range ratio (IRRR), which is a function of the distance between a desired reader and an interfering reader. Interrogation range reduction effects are analyzed using the RFID reader interference model in [7] and the simulation results are compared with measurement results using two ETRI UHF RFID readers and a commercial tag. The balance of this paper is organized as follows: Section 2 presents the reader interference model, the SIR, and the IRRR. In Section 3, simulation and measurement results of the IRRR are presented. Section 4 presents conclusions and discusses future works. II. R EADER I NTERFERENCE M ODEL A RFID interference model consisting of two RFID readers and one tag is proposed in order to analyze the effect of RFID reader interference on the interrogation range. First, the concept of reader interference is explained. Secondly, the SIR at a desired reader’s receiver is derived. An IRRR equation is then derived from the SIR equation. A. Signal-to-Interference Ratio The basic concept of RFID reader interference is depicted in Fig. 1. A desired reader transmits a continuous wave (CW) signal to activate a tag. If the power supplied from the desired reader is sufficient to operate the tag, a backscattered signal from the tag is transmitted to the desired reader. At this point, reader interference occurs if an interfering reader transmits a signal to a tag in its interrogation range. The RFID interference model shown in Fig. 1 consists of two readers and one tag. The distance between the two readers
728
October 2007, Munich Germany
a combination of the data encoding schemes of the tag and its target-bit error rate (BER) values. Thus, Rred for any d is satisfied with the following criterion:
Rmax
Rred
Rred = arg
SIN R(x, d) ≥ Rth .
(4)
To evaluate the influence of an interfering reader on the interrogation range for a desired reader, the IRRR factor is defined as the percentage of a ratio of Rmax − Rred to Rmax for a desired reader. Thus, IRRR is given by
d
Fig. 1.
max
0≤x≤Rmax
RFID reader interference model.
Rmax − Rred × 100 [%]. (5) Rmax If IRRR is 0 %, a desired reader can interrogate a tag under the region with the radius of Rmax . In other words, there is no interference between the two readers. On the other hand, an IRRR of 100 % implies that a desired reader is unable to recognize any tag. IRRR =
is denoted by d. Rmax represents the maximum interrogation range, or the maximum distance in which a desired reader can detect a tag without interference from another reader. If an omni-directional antenna is used, the interrogation range of a desired reader is circular, as shown in Fig. 1. When an interfering reader exists, the actual interrogation range of the desired reader decreases to the circular region with the radius of Rred . This phenomenon can be modeled by a mathematical equation. For a desired reader, the received back-scattered power, PS (x), is given as PS (x) = αBW Etag PT X GT GR × 102×P L(x)/10 ,
(1)
where PT X denotes the total transmit power, αBW is the spectrum power of a used channel normalized by the total power, P L(x) is the path-loss for the distance of x m between a desired reader and a tag, and Etag is the effective power reflection coefficient of a tag. As the received signal undergoes forward link and return link channel path-losses all together and the two path-loss values are the same, the total path-loss is the twice the value of P L(x). Additionally, GT and GR are the transmit antenna and receive antenna gains, respectively. In (1), fading effects are ignored, as the path between a desired reader and a tag is a short, line-of-sight (LOS) pathway. The interference power is then given as I(d) = hPT X αmask GT GR × 10P L(d)/10 + N,
III. S IMULATION AND M EASUREMENT In the previous section, the RFID interference model is described in the analysis of interference problems and IRRR is defined in terms of an evaluation of the influence of interferences. Here, the IRRR results are shown by simulation and measurement in a multiple-reader environment. To analyze the effects of frequency separation on interference, co-channel interference (CCI) and adjacent-channel interference (ACI) types are compared, as shown in Fig. 2. A. Simulation Description In the simulations, it was assumed that the tag was at distance from the desired reader. The reduced interrogation range, Rred , is calculated using (3) and (4). Here, an omnidirectional antenna is used with both readers and the fading coefficient, h, in (2) is ignored in order to create the measurement
(2)
where the interference power involves a fading coefficient, h, in the channel between a desired reader and a interfering reader. In addition, this includes the path-loss P L(d) for distance d, a limit level (αmask ) of a spectrum mask used in simulation, and the thermal noise power N . Finally, the SIR at the receiver is calculated by SIR(x, d) =
PS (x) . I(d)
(a)
(3)
B. Interrogation Range Reduction Ratio To evaluate the interrogation range of a desired reader, it is assumed that if the SIR is greater than a certain value, Rth , the received signal can be correctly recovered at the receiver. Here, Rth termed the threshold value, which can be determined by
(b) Fig. 2. Channel allocation in a multiple-reader environment: (a) co-channel interference, (b) adjacent-channel interference
729
6KLIW
TABLE I S IMULATION AND M EASUREMENT PARAMETERS Parameters Channel Bandwidth Transmit Power(PT X ) target SINR (BER ≤ 10−5 ) Tag’s power reflection coefficient(Etag ) Noise Figure Link Frequency Data rate Antenna Gain(GT = GR ) Tag’s Antenna Height(Htag ) Reader’s Antenna Height(Hreader )
$QWHQQD
Values 500 kHz 30 dBm 11.6 dB 0.1 10 dB 200 kHz 50 kbps 6 dBi 1.5 m 1.5 m
$QWHQQD
7DJ
H reader
H tag
$5HDGHU
R red
%UHDGHU
d
$WWHQXDWRU $>G%@
(a)
conditions. Additionally, the free-space model as the pathloss model is utilized in the simulations. In order to calculate the interference power in the channel bandwidth αmask , the transmit mask of a multiple-reader environment in Gen 2 [8] is used. It is also assumed that the channel bandwidth is equal to 500 kHz. All other parameters used in these simulations are summarized in Table I. B. Measurement Description
(b)
C. Results In the measurements, the maximum interrogation distance, Rmax , of the desired reader was 4.25 m without any interference. Thus, the IRRR was calculated using both the above value of Rmax and that of Rred . Fig. 4 compares the IRRR factor between interference cases (CCI and ACI) as a function
Fig. 3. (a) Measurement method, (b) measurement in the anechoic enclosure 100
Interrogation Range Reduction Ratio [%]
The reader used for the measurements supports a 50 kbps data rate using FM0 encoding, and uses the 500 kHz channel bandwidth, which is the bandwidth specified by RFID regulations in the United States. The measurements were taken in a radio frequency anechoic enclosure. In order to determine the reference value for the interrogation range, the maximum interrogation range (Rmax ) of a desired reader was initially measured without an interfering reader. After finding Rmax , the measurements for a two-reader model were taken for both CCI and ACI cases. The measurement method is shown in Fig. 3, and the measurement parameters are summarized in Table I. The method used in these measurements followed these steps: • Step 1: Place the tag at the initial distance (1 m) from the desired reader. • Step 2: Fix the interfering reader at a fixed point (10 m) and select the attenuation (A dB) value (as the anechoic enclosure had a limited area). • Step 3: Turn the interfering reader on, and ensure that the desired reader can detect the tag. • Step 4: Find a maximum interrogation distance Rred . If the reader can read the tag, gradually move the tag away from the reader. However, if the reader cannot read the tag, gradually move the tag toward the reader. • Step 5: Repeat this measurement while changing the A dB value of the attenuator.
simulation (CCI) measurement (CCI) simulation (ACI) measurement (ACI)
80
60
40
20
0
0
Fig. 4.
200
400 600 800 Reader-Reader distance [m]
1000
1200
IRRR in the multiple reader environment.
of the reader-reader distance, d. Here, the CCI and ACI cases are depicted with black lines and red lines, respectively. To distinguish the simulation and measurement results, markers are also used for the latter case. The simulation results show that reader-reader distances achieving 0 % IRRR, indicating no interference between them, are 1200 m and 35 m for the CCI and ACI cases, respectively. In addition, the IRRR factor is inversely proportional to the reader-reader distance in both cases. However, comparisons between the CCI and the ACI cases show that the interference power for a desired reader decreases more rapidly for ACI than CCI with an increase in the reader-reader distance, d. Thus,
730
it is clear that for a large-scale RFID system deployment, the same frequency should not be allocated to adjacent readers if one wishes to avoid CCI. IV. C ONCLUSIONS Interrogation range reduction effects were analyzed using a RFID reader interference model and simulation results were compared with measurement results. First the SIR equation was derived, and the IRRR criterion was then defined from this equation. IRRR was simulated for CCI and ACI cases. According to the simulation results, it was found that IRRR increases as the interference power increases. Moreover, the result showed that interference power for a desired reader rapidly decreases to a greater extent for ACI as compared to CCI according to d. The simulation results were verified by measurement results using an ETRI UHF RFID system. The measurement results were found to be in good agreement with the simulation results. It can be concluded that the simulation results would be reliable and applicable in analyses of more complex interference problems in actual RFID system deployment instances. ACKNOWLEDGMENT This research was supported by the MIC(Ministry of Information and Communication), Korea, under the ITRC(Information Technology Research Center) support program supervised by the IITA(Institute of Information Technology Assessment) R EFERENCES [1] Y. Bendavid, S. F. Wamba, and L. A. Lefebvre, “Proof of concept of an RFID-enabled supply chain in a B2B e-commerce environment,” in Proceedings of the 8th international conference on Electronic commerce (ICEC’06), 2006, pp. 564-568. [2] S. F. Wamba, L. A. Lefebvre, and E. Lefebvre, “Enabling intelligent Bto-B eCommerce supply chain management using RFID and the EPC network: a case study in the retail industry,” in Proceedings of the 8th international conference on Electronic commerce (ICEC’06), 2006, pp.281-288. [3] K. S. Leong, M. L. Ng, A. R. Grasso, and P. H. Cole, “Synchronization of RFID readers for dense RFID reader environments,” in Proceedings of the 2006 International Symposium on Applications and the Internet Workshops (SAINT’06), 2006, pp. 48-51. [4] D. W. Engels and S. E. Sarma, “The reader collision problem,” in Proceedings of the 2002 IEEE International Conference on Systems, Man and Cybernetics, vol. 3, 2002, pp. 92-97. [5] B. Carbunar, M. K. Ramanathan, M. Koyuturk, C. Hoffman, and A. Grama, “Redundant reader elimination in RFID systems,” in Proceedings of the 2005 Second Annual IEEE Communications Society Conference on Sensor and Ad Hoc Communications and Networks, 2005, pp. 176-184. [6] Y. -R. Seong et al, “Arbitration of UHF-Band Mobile Readers,” in Proceedings of the 6th International Conference on Applications and Principles of Information Science (APIS 06), Kuala Lumpur Malaysia, 20-21 Jan. 2007. [7] K. Cha, A. Ramachandran, and S. Jagannathan, “Adaptive and Probabilistic Power Control Alogrithms for Dense RFID Reader Network,”in Proceedings of the 2006 IEEE International Conference on Networking, Sensing and Control (ICNSC ’06), 2006, pp. 474-479. [8] EPCglobal, “EPC radio-frequency identity protocols class-1 generation-2 UHF RFID protocol for communications at 860 MHz - 960 MHz version 1.0.9,” EPCglobal Standard Specification, 2004.
731
