Baseband Noise Reduction Method Using Captured TX ... - IEEE Xplore
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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 59, NO. 1, JANUARY 2012
Baseband Noise Reduction Method Using Captured TX Signal for UHF RFID Reader Applications Sung-Chan Jung, Min-Su Kim, and Youngoo Yang, Member, IEEE
Abstract—This paper presents a baseband noise reduction method in the receiver for UHF RFID readers. The signal captured from the transmitter after passing through the main delay components is fed as an LO signal into the down-converter in the receiver. At the down-converter, the LO signal and the leakage signal from the transmitter to the receiver are mixed with each other. The matching of the delay between the two correlated signals virtually eleminates the additional baseband noise included in the phase noises after their mixing. Lowering the baseband noise improves the sensitivity of the receiver, thus allowing the RFID tags to be read further away from the reader. The proposed method was applied to a portable UHF RFID reader for its validation. Compared to the conventional configuration, the simulated baseband noise level of more than 26 dB was reduced by the proposed method. The measured sensitivity of about 19 dB was also improved. As a result, the reading distance was extended by 1.9 m from 2.02 to 3.92 m. Index Terms—Baseband noise, radio frequency identification (RFID), range correlation, reading distance, sensitivity.
I. I NTRODUCTION
R
EADING information from the objects far from a reader is required for many reasons, such as protecting human beings from pollution, remote control, better effectiveness in factory automation, and so on. Therefore, radio frequency identification (RFID) technology has rapidly grown and has been widely adopted in many areas, such as the distribution industries, real-time tracking services, health-care managements, risk managements, and so on [1]–[9]. While receiving data from the tag in the reading cycle of passive RFID systems, the reader transmits a continuous-wave (CW) signal to provide the tag with the necessary operating power. Because the frequency of the signal received from the tag is located very near to the transmitted CW signal, it is very difficult to prevent the transmitted signal from leaking to the receiver path. The leakage signal from the transmitter (TX) causes two problems for the performance of the reader. The first problem is Manuscript received October 7, 2010; revised November 5, 2010, January 14, 2011, and March 14, 2011; accepted March 19, 2011. Date of publication April 5, 2011; date of current version October 4, 2011. This work was supported by National Research Foundation of Korea under Grant 20090067097. S.-C. Jung is with the Samsung Techwin, Seongnam 463-400, Korea (e-mail: [email protected]). M.-S. Kim and Y. Yang are with the Microwave Circuits and Systems Laboratory, School of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, Korea (e-mail: minsu970@ gmail.com; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2011.2138673
the direct leakage of the TX noise to the receiver (RX), which results in the degradation of the sensitivity. Because the TX noise contributes to the noise level of the receiver, the reader requires isolation circuits between the TX and RX [10]–[16]. To further enhance its isolation, leakage cancellation techniques using the captured transmitting signal have been proposed [13]–[16]. However, a residual leakage signal still remains in the RX path. The second problem is the down-conversion of the LO phase noise, due to the mixing of the LO and residual TX leakage signal. Since the LO and TX leakage signals generally originate from the same source, if they have no delay mismatch, there is no down-conversion of the phase noise to the baseband noise due to the range correlation effect [17], [18]. In order to comply with the stringent specifications for the spectrum emission of the transmitted signals, the reader may need a filter with very sharp cutoff characteristics in the TX path. Surface acoustic wave (SAW) filters or film bulk acoustic resonator (FBAR) filters can be used for this purpose. However, these filters induce significant time delay, so that the TX leakage signal takes a long time to get to the down-converter in the RX. Consequently, since the phase noises of the LO signal and TX leakage signal lose their correlation, they can be downconverted to the baseband in the form of noise [17], [18]. In this paper, a baseband noise reduction method using the captured TX output signal for UHF RFID readers is proposed. The captured signal after the filter, which is the main delay component, was used as the LO signal for the down-converter at the RX. In this case, the delay mismatch between the LO signal and TX leakage signal becomes negligible, thereby reducing the amount of baseband noise generated within the range of a few hundred kilohertz. A TX filter was applied to the reader for the Japanese UHF RFID band from 952 to 954 MHz. The resultant baseband noise degradation was totally mitigated using the proposed method in both the simulation and experiments. The sensitivity and reading distance of the reader before and after applying the proposed method are also presented.
II. A NALYSIS OF THE UHF RFID R EADER A. Relationship Between Reading Distance and Sensitivity The functions of the reader required for passive RFID are the transmission of data and operating energy to the tag and the reception of data from the tag. The radiated signal, whose power is measured as the effective isotropic radiated power (EIRP), suffers from path loss and can be quantified using the
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JUNG et al.: BASEBAND NOISE REDUCTION METHOD USING CAPTURED TX SIGNAL FOR UHF RFID READER APPLICATIONS
Friis free space formula [19]. If the polarization discrepancy between the reader and tag antennas is ignored, the power levels at the tag and reader are given by 2 λ PT ag = EIRP · GReader · · GT ag (1) 4πR PReader = PBT R · GT ag · PBT R =
λ 4πR
· GReader
1 · |ΓL_of f − ΓL_on |2 · PT ag 4
Sensitivity = SN Rmin · N F · Ni
(2) (3)
(W )
|ΓL_of f − ΓL_on |2 · G2T ag · G2Reader SN Rmin · N F · Ni
TABLE II S PECTRAL E MISSION S PECIFICATIONS FOR JAPAN
(5)
where SN Rmin is the minimum decodable signal-to-noise ratio. NF and Ni are the noise figure and input noise power of the reader, respectively. To acquire a detectable signal at the maximum reading distance, PReader should be the same as the Sensitivity. Then, (4) and (5) can be arranged in the form of (6) to provide the maximum reading distance, Rmax 1/4 4 λ 1 · Rmax = 4 4π ·
TABLE I S PECTRAL E MISSION S PECIFICATIONS FOR E UROPE
2
where PT ag , PReader , and PBT R are the received power of the tag, the received power of the reader, and the power of the backscattered signal from the tag to the reader, respectively. GReader and GT ag are the antenna gains of the reader and tag, respectively. λ is the wave length at the operating frequency. R is the distance between the reader and tag. ΓLo f f and ΓLo n are the load reflection coefficients of the tag when it is off and on, respectively [19]. Therefore, the received power of the reader is obtained by substituting (1) and (3) into (2) to give 4 λ 1 2 2 2 . PReader = ·|ΓL_of f −ΓL_on | · GT ag · GReader · 4 4πR (4) The sensitivity of the reader can be simply expressed by
593
1/4 . (6)
From (6), we can see that the input noise power (Ni ) should be reduced in order to increase the maximum reading distance. Ni is the numerical sum of the various input noises, such as the source resistance noise of the RX, the direct TX leakage noise to the RX, the down-converted LO phase noise in the RX, and so on, in the specified signal bandwidth. Therefore, the input noise, Ni , from each source must be carefully considered and reduced to maximize the reading distance. B. Spectral Specifications of the UHF RFID Reader The worldwide UHF RFID frequency band spans from 840 to 960 MHz. The band from 865.5 to 867.6 MHz is allocated to Europe, the band from 902 to 928 MHz to US, the band from 917 to 923.5 MHz to South Korea, and the band from 952 to
954 MHz to Japan. Particularly for Europe and Japan, the spectral emission specifications, shown in Tables I and II, respectively, are very difficult for the TX signal to comply with. Therefore, in order to meet these stringent spectral specifications, a filter with a very narrow bandwidth and sharp cutoff characteristics is required.
C. Characteristics of SAW Filter To comply with these stringent specifications for spectral emission, the TX of the reader can optionally make use of a SAW filter with very sharp cutoff characteristics. Fig. 1 shows the measured results of the SAW filter for the Japanese RFID frequency band. It has a very narrow pass band of about 2 MHz and very sharp cutoff characteristics. However, it exhibits a very rapid change in the transmitted phase as the frequency increases. The phase variation in the pass band can be calculated into the time delay according to τ=
Δφ . Δω
(7)
The measured time delays of the reader with and without the SAW filter are shown in Fig. 2. The measured time delay of the whole TX path with the SAW filter is about 275 ns, while that without the filter at the center frequency for the Japanese band of 953 MHz is about 4 ns. This means that the time delay of the TX path is dominated by the SAW filter. Hence, other delays, such as those caused by other additional components or even
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Fig. 1. Measured magnitude and phase responses of the SAW filter.
Fig. 3. Conventional UHF RFID reader with a filter in the TX: (a) a schematic diagram and (b) spectral representation of the down-conversion process.
should transmit a CW signal to the tag at the reading phase, the LO and IF signals of the up-converter are expressed as
Fig. 2.
Measured time delay of the RFID reader TX for Japanese applications.
antenna reflection from outside objects, are relatively small and can be neglected. D. Conventional RFID Reader System With the Filter A simplified block diagram of a UHF RFID system, including a filter in the TX of the reader, is shown in Fig. 3(a). The filter is located before the power amplifier. The PLL frequency synthesizer supplies the LO signals for the up- and downconversion mixers. Most of the TX signal passes along the TX path and is radiated by the antenna. A part of the TX signal leaks to the RX path via multiple mechanisms, such as the imperfect isolation of the coupler, antenna mismatch, and reflection from outside objects, including tags. To enhance the RX sensitivity of the RFID reader, the input noise power (Ni ) must be minimized. The TX leakage signal itself affects the input noise level, so that it has to be reduced using various techniques [13]–[16]. Because the filter has a large time delay of a few hundred nanoseconds, the baseband noise can be degraded due to the signal mixing of the residual leakage signal and the LO signal at the down-converter. A synthesized sinusoidal signal using a PLL is applied to the up- and down-converters as an LO signal. Because the reader
A1 = D1 = ALO cos [ωc t + φn (t)]
(8)
B1 = AIF cos(0) = AIF
(9)
where ωC and φn (t) are the carrier frequency and the phase noise of the local oscillator (LO), respectively. ALO and AIF are the constant amplitudes of the LO and IF signals, respectively. Then, the TX signal at the point C1 becomes C1 = Gup ALO AIF cos [ωc t + φn (t)]
(10)
where Gup is the gain of the up-converter. The input signal of the down-mixer is mainly composed of the backscattered signal from the tag and TX leakage signal. Because the main delay component is the filter, other delays can be ignored. Then, the TX leakage signals to the RX path is assumed to have a constant time delay of τd . The received signal, at the point E1 , can be simply expressed as E1 = Gpt ARF cos [ωc (t − τd ) + φn (t − τd )] + Am cos [ωc (t − τd ) + φn (t − τd )]
(11)
where ARF = Gup ALO AIF . Gpt is the path gain from the driver amplifier to the input of the down-converter for the leakage signal. Am (t) is the amplitude modulation function from the tag, including the RX gain, antenna gain, path loss, and quantity of backscattering.
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given by F2 =
1 1 Gdn ALO Gpt ARF cos θ + Gdn ALO Am (t) cos θ 2 2 (14)
because E2 of the proposed structure is the same as E1 of the conventional structure. Here, from (14), no baseband noise component resulting from the down-conversion process of the TX leakage signal and the coupled LO signal. The spectral representation for this process is shown in Fig. 4(b). Due to the improvement of the baseband noise using the proposed method, we would expect the sensitivity to be improved and the reading distance to be extended. III. A NALYSIS OF THE UHF RFID R EADER A. Simulation Setups and Results
Fig. 4. Proposed UHF RFID reader with a filter in the TX: (a) schematic diagram and (b) spectral representation of the down-conversion process.
The baseband signal components after down-conversion are given by F1 =
1 Gdn ALO Gpt ARF · cos [ωc τd + φn (t) − φn (t − τd )] 2
1 + Gdn ALO Am (t) · cos [ωc τd + φn (t) − φn (t − τd )] 2
(12)
where Gdn is the gain of the down-converter. Because |Gdn ALO Gpt ARF | is much larger than |Gdn ALO | · |Am (t)|, the phase noise resulting from the product of the leakage and LO signals dominates, as presented in Fig. 3(b). If the delay τd goes to 0, it can be seen from (12) that the two phase noise terms of φn (t) and φn (t − τd ) cancel each other out. As the delay becomes larger, cancellation level of the two phase noise terms decreases, so that the baseband noise level becomes degraded. E. RFID Reader System Using an External LO With the Captured TX Signal The baseband noise degradation due to the delay of the filter can be mitigated using the proposed structure of the RFID reader, as shown in Fig. 4(a). From the isolation port of the output coupler, the TX signal can be coupled and supplied to the down-converter as an LO signal. After adjusting its amplitude to an appropriate LO level, the coupled TX signal can be expressed as D2 = ALO cos [ωC (t − τd ) + φn (t − τd ) + θ]
(13)
where θ is a constant phase shift. Then, the down-converted baseband signal components of the proposed RFID reader are
Fig. 5(a) and (b) show the simulation setups for the conventional and proposed RFID reader systems, as depicted in Figs. 3(a) and 4(a), respectively. The TX has a DC input, so that it can transmit a CW signal while receiving the tag’s response. The driver and power amplifiers are used to amplify the upconverted TX signal. The IF LNA is placed after the downconverter in the RX to amplify the baseband signal. For the simulation of the conventional reader system, the upand down-mixers use the same signal from the local oscillator. The filter, which is located between the driver and power amplifiers, was modeled to have a similar frequency response, based on the measured S-parameter of the SAW filter [see Fig. 1(a)]. The simulation was carried out with and without the filter using this setup. The second simulation setup presents the proposed reader which has a filter and uses the captured TX signal from the output coupler. The captured signal is fed to the down-converter as an LO signal after adjusting its amplitude and phase [see Fig. 5(b)]. Fig. 6 shows the simulated power spectral densities of the baseband noise results for the conventional reader with and without the filter and the proposed reader with the filter. The baseband noise level of the conventional reader with the filter is about −74 dBm/kHz at 300 kHz, while that of the conventional reader without the filter is about −100 dBm/kHz. The filter deteriorates the baseband noise level by about 26 dB. The simulated baseband noise level of the proposed reader is about −100 dBm/kHz at 300 kHz, which is not different from that of the conventional reader without the filter. The simulation results are very well matched with our analysis in the previous section. B. Experiments and Measured Results In order to experimentally validate the proposed structure of the UHF RFID reader system, ten sets of RFID readers were designed and fabricated on an FR4 printed circuit board (PCB), based on Samsung Techwin’s URP-SJ010. The photograph of the implemented reader is shown in Fig. 7. The reader chip, Impinj’s R1000, includes the MODEM, A-D and D-A converters,
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Fig. 5. Simulation setups for RFID reader system: (a) the conventional structure with and without the filter and (b) the proposed structure with the filter.
up- and down-converters, driver amplifier, and various analog function blocks. A SAW filter chip, Murata’s SAFCH953M, is placed between the RFID chip and the external power amplifier to avoid the degradation of the efficiency due to path loss. For the proposed RFID reader, the LO was externally supplied to the RFID reader chip through the external path from coupler 1, via the π-type fixed attenuator, to the downconverter. The optimized length of the transmission line was used in the path to realize an appropriate phase shift, which was experimentally optimized. The second coupler is used to monitor the transmitted and reflected signal levels due to the antenna mismatch using a power detector. Fig. 8 shows the measured power spectral densities of the LO signal, TX leakage signal, TX signal for the receiving mode (CW), and TX signal for the transmitting mode (modulation). The TX power and overall leakage signal power are measured to be 30 dBm and 4 dBm, respectively. The phase
noises of the TX signal before and after filtering are shown in Fig. 9. The fabricated RFID readers for Japanese application were tested in an anechoic chamber with Alien’s ALN-9554M tags. The tag is vertically (θ = φ = 90◦ ) placed from the reader antenna which is circularly polarized and has a gain of 3 dB. The output power level of the reader for both the conventional and proposed methods was set to 30 dBm and the tag reading test was performed with a center frequency of 952.4 MHz. For the test, the reader transmits the PR-ASK modulated signal to the tag and receives the back-scattered signal based on Miller 4 protocol in the ISO 18000-6C. Table III shows the measured reading distance of the implemented readers. The average reading distance of the ten conventional readers is 2.02 m, while that of the proposed readers is 3.92 m. An increase in the reading distance of 1.9 m is obtained. The sensitivity of one of the fabricated readers was measured using Tescom’s TC-2600A RFID tester. The measured
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Fig. 9. Measured phase noises of the TX signal before and after filtering.
Fig. 6. Simulated baseband noise performances of the conventional structure with and without the filter and the proposed configuration with the filter.
TABLE III S PECTRAL E MISSION S PECIFICATIONS FOR JAPAN
IV. C ONCLUSION
Fig. 7.
Photograph of the fabricated UHF RFID reader.
Fig. 8.
Measured results of the TX, LO, and leakage signals.
sensitivities of the conventional and proposed readers are −57 and −76 dBm, respectively. A baseband noise reduction of 19 dB was achieved using the proposed structure.
An improved UHF RFID reader, based on the reduction of the baseband noise using the captured TX signal after filtering, was proposed. The analysis of the conventional reader with a SAW filter in the TX path showed that the large delay of the filter is the main cause of the baseband noise degradation. To avoid the baseband noise degradation due to the filter, the TX signal captured from the isolation port of the output coupler was applied to the LO port of the output coupler was applied to the LO port of the down-converter in the RX. The analysis and simulation proved that the matching of the delays between the LO signal of the down-converter and TX leakage signal using the proposed method eliminates the degradation of the baseband noise performance of the reader. The two types of UHF RFID readers, viz. the conventional and proposed ones, were designed, fabricated, and tested. Their performances were compared from the view-point of the reading distance and sensitivity. The reading distance is strongly related to the sensitivity, which is directly affected by the baseband noise level. The average reading distance of the proposed UHF RFID reader is 3.92 m, which is 1.9 m, longer than that of the conventional one. The sensitivity is improved by 19 dB from −57 to −76 dBm. Because the same RX circuit is used in both cases, an improvement of the baseband noise level of 19 dB was also achieved. As a result, the proposed UHF RFID reader with an external LO for the down-converter using the captured TX signal has a significantly improved reading distance as well as sensitivity.
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R EFERENCES [1] J. Lee and B. Lee, “A long-range UHF-band RFID tag IC based on high-Q design approach,” IEEE Trans. Ind. Electron., vol. 56, no. 7, pp. 2308– 2316, Jul. 2009. [2] D. Kim, H. Jo, H. Yoon, C. Mun, B. Jang, and J. Yook, “Reverse-link interrogation range of a UHF MIMO-RFID system in Nakagami-m fading channels,” IEEE Trans. Ind. Electron., vol. 57, no. 4, pp. 1468–1477, Apr. 2010. [3] Y. Kim, Y. Choi, M. Seo, S. Yoo, and H. Yoo, “A CMOS transceiver for multistandard 13.56-MHz RFID reader SoC,” IEEE Trans. Ind. Electron., vol. 57, no. 5, pp. 1563–1572, May 2010. [4] S. Kuo, S. Chen, and C. Lin, “Design and development of RFID label for steel coil,” IEEE Trans. Ind. Electron., vol. 57, no. 6, pp. 2180–2186, Jun. 2010. [5] D. Kim, H. Yoon, B. Jang, and J. Yook, “Effects of reader-to-reader interference on the UHF RFID interrogation range,” IEEE Trans. Ind. Electron., vol. 56, no. 7, pp. 2337–2346, Jul. 2009. [6] S. Park and S. Hashimoto, “Autonomous mobile robot navigation using passive RFID in indoor environment,” IEEE Trans. Ind. Electron., vol. 56, no. 7, pp. 2366–2373, Jul. 2009. [7] A. Lehto, J. Nummela, L. Ukkonen, L. Sydanheimo, and M. Kivikoski, “Passive UHF RFID in paper industry: Challenges, benefits and the application environment,” IEEE Trans. Autom. Sci. Eng., vol. 6, no. 1, pp. 66– 79, Jan. 2009. [8] A. D. Droitcour, O. Boric-Lubecke, V. M. Lubecke, J. Lin, and G. T. A. Kovacs, “Range correlation and I/Q performance benefits in single-chip silicon Doppler radars for noncontact cardiopulmonary monitoring,” IEEE Trans. Microw. Theory Tech., vol. 52, no. 3, pp. 838–848, Mar. 2004. [9] A. Vaz, A. Ubarretxena, I. Zalbide, D. Pardo, H. Solar, A. Garcia-Alonso, and R. Berenguer, “Full passive UHF tag with a temperature sensor suitable for human body temperature monitoring,” IEEE Trans. Circuits Syst. II, Exp. Briefs, vol. 57, no. 2, pp. 95–99, Feb. 2010. [10] P. V. Nikitin, K. V. Seshagiri Rao, R. Martinez, and S. F. Lam, “Sensitivity and impedance measurement of UHF RFID chips,” IEEE Trans. Microw. Theory Tech., vol. 57, no. 5, pp. 1297–1302, May 2009. [11] K. Finkenzeller, RFID Handbook. New York: Wiley, 2003. [12] W. Lim and J. Yu, “Balanced circulator structure with enhanced isolation characteristics,” Microw. Opt. Technol. Lett., vol. 50, no. 9, pp. 2389– 2391, Sep. 2008. [13] J. Jung, H. Roh, K. Kwak, M. Jeong, and J. Park, “Adaptive TRX isolation scheme by using TX leakage canceller at variable frequency,” Microw. Opt. Technol. Lett., vol. 50, no. 8, pp. 2043–2045, Aug. 2008. [14] P. Pursula, M. Kiviranta, and H. Seppa, “UHF RFID reader with reflected power canceller,” IEEE Microw. Wireless Compon. Lett., vol. 19, no. 1, pp. 48–50, Jan. 2009. [15] F. Wei, X. W. Shi, Q. L. Huang, D. Z. Chen, and X. H. Wang, “A new directional coupler for UHF RFID reader,” Microw. Opt. Technol. Lett., vol. 50, no. 7, pp. 1973–1975, Jul. 2008. [16] W. Lim, S. Park, W. Son, M. Lee, and J. Yu, “RFID reader front-end having robust Tx leakage canceller for load variation,” IEEE Trans. Microw. Theory Tech., vol. 57, no. 5, pp. 1348–1355, May 2009. [17] J. Bae, J. Kim, B. Jeon, J. Jung, J. Park, B. Jang, H. Oh, Y. Moon, and Y. Seong, “Analysis of phase noise requirements on local oscillator for RFID system considering range correlation,” in Proc. Eur. Microw. Conf. Dig., Oct. 2007, pp. 1664–1667. [18] M. C. Budge, Jr. and M. P. Burt, “Range correlation effects on phase and amplitude noise,” in Proc. IEEE Southeastcon, Charlotte, NC, 1993, p. 5. [19] X. Yao, S. Kwon, H. Kim, H. Cho, M. Kim, S. Jung, C. Park, J. Kim, and Y. Yang, “Optimum ASK modulation scheme for passive RFID tags under antenna mismatch conditions,” IEEE Trans. Microw. Theory Tech., vol. 57, no. 10, pp. 2337–2343, Oct. 2009.
Sung-Chan Jung was born in Seoul, Korea, in 1973. He received the Ph.D. degree in information and communication engineering from the Sungkyunkwan University, Suwon, Korea, in 2006. From 2006 to 2007, he was a Postdoctoral researcher with the MCS Laboratory, Sungkyunkwan University. From 2007 to 2008, he was a Postdoctoral fellow with the iRadio Laboratory, Schulish School of Engineering, University of Calgary, Calgary, AB, Canada. From September 2008 to February 2009, he was a Research Professor with the MCS Laboratory, Sungkyunkwan University. Since March 2009, he has been with Samsung Techwin Company Ltd., Seongnam, Korea, where he is currently a Senior Researcher. His current research interests include design of highpower amplifiers, linearization techniques, efficiency enhancement techniques for base stations and mobile terminals, and RFID circuit design.
Min-Su Kim was born in Seoul, Korea, in 1978. He received the B.S. and M.S. degrees in electronic engineering from the Incheon University, Incheon, Korea, in 2005 and in information and communication engineering from the Sungkyunkwan University, Suwon, Korea, in 2008, Korea, respectively, where he is currently working toward the Ph.D. degree. His current research interests include transmitter, linearization techniques, RFID circuit design.
Youngoo Yang (S’99–M’02) was born in Hamyang, Korea, in 1969. He received the Ph.D. degree in electrical and electronic engineering from the Pohang University of Science and Technology(Postech), Pohang, Korea, in 2002. From 2002 to 2005, he was with Skyworks Solutions, Inc., Newbury Park, CA, where he designed power amplifiers for various cellular handsets. Since March 2005, he has been with the School of Information and Communication Engineering, Sungkyunkwan University, Suwon, Korea, where he is currently an associate professor and a director of the RFID/USN Integrated Circuit Research Center. His research interests include power amplifier design, RF transmitters, RFIC design, integrated circuit design for RFID/USN systems, and modeling of high power amplifiers or devices.
IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 59, NO. 1, JANUARY 2012
Baseband Noise Reduction Method Using Captured TX Signal for UHF RFID Reader Applications Sung-Chan Jung, Min-Su Kim, and Youngoo Yang, Member, IEEE
Abstract—This paper presents a baseband noise reduction method in the receiver for UHF RFID readers. The signal captured from the transmitter after passing through the main delay components is fed as an LO signal into the down-converter in the receiver. At the down-converter, the LO signal and the leakage signal from the transmitter to the receiver are mixed with each other. The matching of the delay between the two correlated signals virtually eleminates the additional baseband noise included in the phase noises after their mixing. Lowering the baseband noise improves the sensitivity of the receiver, thus allowing the RFID tags to be read further away from the reader. The proposed method was applied to a portable UHF RFID reader for its validation. Compared to the conventional configuration, the simulated baseband noise level of more than 26 dB was reduced by the proposed method. The measured sensitivity of about 19 dB was also improved. As a result, the reading distance was extended by 1.9 m from 2.02 to 3.92 m. Index Terms—Baseband noise, radio frequency identification (RFID), range correlation, reading distance, sensitivity.
I. I NTRODUCTION
R
EADING information from the objects far from a reader is required for many reasons, such as protecting human beings from pollution, remote control, better effectiveness in factory automation, and so on. Therefore, radio frequency identification (RFID) technology has rapidly grown and has been widely adopted in many areas, such as the distribution industries, real-time tracking services, health-care managements, risk managements, and so on [1]–[9]. While receiving data from the tag in the reading cycle of passive RFID systems, the reader transmits a continuous-wave (CW) signal to provide the tag with the necessary operating power. Because the frequency of the signal received from the tag is located very near to the transmitted CW signal, it is very difficult to prevent the transmitted signal from leaking to the receiver path. The leakage signal from the transmitter (TX) causes two problems for the performance of the reader. The first problem is Manuscript received October 7, 2010; revised November 5, 2010, January 14, 2011, and March 14, 2011; accepted March 19, 2011. Date of publication April 5, 2011; date of current version October 4, 2011. This work was supported by National Research Foundation of Korea under Grant 20090067097. S.-C. Jung is with the Samsung Techwin, Seongnam 463-400, Korea (e-mail: [email protected]). M.-S. Kim and Y. Yang are with the Microwave Circuits and Systems Laboratory, School of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, Korea (e-mail: minsu970@ gmail.com; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2011.2138673
the direct leakage of the TX noise to the receiver (RX), which results in the degradation of the sensitivity. Because the TX noise contributes to the noise level of the receiver, the reader requires isolation circuits between the TX and RX [10]–[16]. To further enhance its isolation, leakage cancellation techniques using the captured transmitting signal have been proposed [13]–[16]. However, a residual leakage signal still remains in the RX path. The second problem is the down-conversion of the LO phase noise, due to the mixing of the LO and residual TX leakage signal. Since the LO and TX leakage signals generally originate from the same source, if they have no delay mismatch, there is no down-conversion of the phase noise to the baseband noise due to the range correlation effect [17], [18]. In order to comply with the stringent specifications for the spectrum emission of the transmitted signals, the reader may need a filter with very sharp cutoff characteristics in the TX path. Surface acoustic wave (SAW) filters or film bulk acoustic resonator (FBAR) filters can be used for this purpose. However, these filters induce significant time delay, so that the TX leakage signal takes a long time to get to the down-converter in the RX. Consequently, since the phase noises of the LO signal and TX leakage signal lose their correlation, they can be downconverted to the baseband in the form of noise [17], [18]. In this paper, a baseband noise reduction method using the captured TX output signal for UHF RFID readers is proposed. The captured signal after the filter, which is the main delay component, was used as the LO signal for the down-converter at the RX. In this case, the delay mismatch between the LO signal and TX leakage signal becomes negligible, thereby reducing the amount of baseband noise generated within the range of a few hundred kilohertz. A TX filter was applied to the reader for the Japanese UHF RFID band from 952 to 954 MHz. The resultant baseband noise degradation was totally mitigated using the proposed method in both the simulation and experiments. The sensitivity and reading distance of the reader before and after applying the proposed method are also presented.
II. A NALYSIS OF THE UHF RFID R EADER A. Relationship Between Reading Distance and Sensitivity The functions of the reader required for passive RFID are the transmission of data and operating energy to the tag and the reception of data from the tag. The radiated signal, whose power is measured as the effective isotropic radiated power (EIRP), suffers from path loss and can be quantified using the
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JUNG et al.: BASEBAND NOISE REDUCTION METHOD USING CAPTURED TX SIGNAL FOR UHF RFID READER APPLICATIONS
Friis free space formula [19]. If the polarization discrepancy between the reader and tag antennas is ignored, the power levels at the tag and reader are given by 2 λ PT ag = EIRP · GReader · · GT ag (1) 4πR PReader = PBT R · GT ag · PBT R =
λ 4πR
· GReader
1 · |ΓL_of f − ΓL_on |2 · PT ag 4
Sensitivity = SN Rmin · N F · Ni
(2) (3)
(W )
|ΓL_of f − ΓL_on |2 · G2T ag · G2Reader SN Rmin · N F · Ni
TABLE II S PECTRAL E MISSION S PECIFICATIONS FOR JAPAN
(5)
where SN Rmin is the minimum decodable signal-to-noise ratio. NF and Ni are the noise figure and input noise power of the reader, respectively. To acquire a detectable signal at the maximum reading distance, PReader should be the same as the Sensitivity. Then, (4) and (5) can be arranged in the form of (6) to provide the maximum reading distance, Rmax 1/4 4 λ 1 · Rmax = 4 4π ·
TABLE I S PECTRAL E MISSION S PECIFICATIONS FOR E UROPE
2
where PT ag , PReader , and PBT R are the received power of the tag, the received power of the reader, and the power of the backscattered signal from the tag to the reader, respectively. GReader and GT ag are the antenna gains of the reader and tag, respectively. λ is the wave length at the operating frequency. R is the distance between the reader and tag. ΓLo f f and ΓLo n are the load reflection coefficients of the tag when it is off and on, respectively [19]. Therefore, the received power of the reader is obtained by substituting (1) and (3) into (2) to give 4 λ 1 2 2 2 . PReader = ·|ΓL_of f −ΓL_on | · GT ag · GReader · 4 4πR (4) The sensitivity of the reader can be simply expressed by
593
1/4 . (6)
From (6), we can see that the input noise power (Ni ) should be reduced in order to increase the maximum reading distance. Ni is the numerical sum of the various input noises, such as the source resistance noise of the RX, the direct TX leakage noise to the RX, the down-converted LO phase noise in the RX, and so on, in the specified signal bandwidth. Therefore, the input noise, Ni , from each source must be carefully considered and reduced to maximize the reading distance. B. Spectral Specifications of the UHF RFID Reader The worldwide UHF RFID frequency band spans from 840 to 960 MHz. The band from 865.5 to 867.6 MHz is allocated to Europe, the band from 902 to 928 MHz to US, the band from 917 to 923.5 MHz to South Korea, and the band from 952 to
954 MHz to Japan. Particularly for Europe and Japan, the spectral emission specifications, shown in Tables I and II, respectively, are very difficult for the TX signal to comply with. Therefore, in order to meet these stringent spectral specifications, a filter with a very narrow bandwidth and sharp cutoff characteristics is required.
C. Characteristics of SAW Filter To comply with these stringent specifications for spectral emission, the TX of the reader can optionally make use of a SAW filter with very sharp cutoff characteristics. Fig. 1 shows the measured results of the SAW filter for the Japanese RFID frequency band. It has a very narrow pass band of about 2 MHz and very sharp cutoff characteristics. However, it exhibits a very rapid change in the transmitted phase as the frequency increases. The phase variation in the pass band can be calculated into the time delay according to τ=
Δφ . Δω
(7)
The measured time delays of the reader with and without the SAW filter are shown in Fig. 2. The measured time delay of the whole TX path with the SAW filter is about 275 ns, while that without the filter at the center frequency for the Japanese band of 953 MHz is about 4 ns. This means that the time delay of the TX path is dominated by the SAW filter. Hence, other delays, such as those caused by other additional components or even
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Fig. 1. Measured magnitude and phase responses of the SAW filter.
Fig. 3. Conventional UHF RFID reader with a filter in the TX: (a) a schematic diagram and (b) spectral representation of the down-conversion process.
should transmit a CW signal to the tag at the reading phase, the LO and IF signals of the up-converter are expressed as
Fig. 2.
Measured time delay of the RFID reader TX for Japanese applications.
antenna reflection from outside objects, are relatively small and can be neglected. D. Conventional RFID Reader System With the Filter A simplified block diagram of a UHF RFID system, including a filter in the TX of the reader, is shown in Fig. 3(a). The filter is located before the power amplifier. The PLL frequency synthesizer supplies the LO signals for the up- and downconversion mixers. Most of the TX signal passes along the TX path and is radiated by the antenna. A part of the TX signal leaks to the RX path via multiple mechanisms, such as the imperfect isolation of the coupler, antenna mismatch, and reflection from outside objects, including tags. To enhance the RX sensitivity of the RFID reader, the input noise power (Ni ) must be minimized. The TX leakage signal itself affects the input noise level, so that it has to be reduced using various techniques [13]–[16]. Because the filter has a large time delay of a few hundred nanoseconds, the baseband noise can be degraded due to the signal mixing of the residual leakage signal and the LO signal at the down-converter. A synthesized sinusoidal signal using a PLL is applied to the up- and down-converters as an LO signal. Because the reader
A1 = D1 = ALO cos [ωc t + φn (t)]
(8)
B1 = AIF cos(0) = AIF
(9)
where ωC and φn (t) are the carrier frequency and the phase noise of the local oscillator (LO), respectively. ALO and AIF are the constant amplitudes of the LO and IF signals, respectively. Then, the TX signal at the point C1 becomes C1 = Gup ALO AIF cos [ωc t + φn (t)]
(10)
where Gup is the gain of the up-converter. The input signal of the down-mixer is mainly composed of the backscattered signal from the tag and TX leakage signal. Because the main delay component is the filter, other delays can be ignored. Then, the TX leakage signals to the RX path is assumed to have a constant time delay of τd . The received signal, at the point E1 , can be simply expressed as E1 = Gpt ARF cos [ωc (t − τd ) + φn (t − τd )] + Am cos [ωc (t − τd ) + φn (t − τd )]
(11)
where ARF = Gup ALO AIF . Gpt is the path gain from the driver amplifier to the input of the down-converter for the leakage signal. Am (t) is the amplitude modulation function from the tag, including the RX gain, antenna gain, path loss, and quantity of backscattering.
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given by F2 =
1 1 Gdn ALO Gpt ARF cos θ + Gdn ALO Am (t) cos θ 2 2 (14)
because E2 of the proposed structure is the same as E1 of the conventional structure. Here, from (14), no baseband noise component resulting from the down-conversion process of the TX leakage signal and the coupled LO signal. The spectral representation for this process is shown in Fig. 4(b). Due to the improvement of the baseband noise using the proposed method, we would expect the sensitivity to be improved and the reading distance to be extended. III. A NALYSIS OF THE UHF RFID R EADER A. Simulation Setups and Results
Fig. 4. Proposed UHF RFID reader with a filter in the TX: (a) schematic diagram and (b) spectral representation of the down-conversion process.
The baseband signal components after down-conversion are given by F1 =
1 Gdn ALO Gpt ARF · cos [ωc τd + φn (t) − φn (t − τd )] 2
1 + Gdn ALO Am (t) · cos [ωc τd + φn (t) − φn (t − τd )] 2
(12)
where Gdn is the gain of the down-converter. Because |Gdn ALO Gpt ARF | is much larger than |Gdn ALO | · |Am (t)|, the phase noise resulting from the product of the leakage and LO signals dominates, as presented in Fig. 3(b). If the delay τd goes to 0, it can be seen from (12) that the two phase noise terms of φn (t) and φn (t − τd ) cancel each other out. As the delay becomes larger, cancellation level of the two phase noise terms decreases, so that the baseband noise level becomes degraded. E. RFID Reader System Using an External LO With the Captured TX Signal The baseband noise degradation due to the delay of the filter can be mitigated using the proposed structure of the RFID reader, as shown in Fig. 4(a). From the isolation port of the output coupler, the TX signal can be coupled and supplied to the down-converter as an LO signal. After adjusting its amplitude to an appropriate LO level, the coupled TX signal can be expressed as D2 = ALO cos [ωC (t − τd ) + φn (t − τd ) + θ]
(13)
where θ is a constant phase shift. Then, the down-converted baseband signal components of the proposed RFID reader are
Fig. 5(a) and (b) show the simulation setups for the conventional and proposed RFID reader systems, as depicted in Figs. 3(a) and 4(a), respectively. The TX has a DC input, so that it can transmit a CW signal while receiving the tag’s response. The driver and power amplifiers are used to amplify the upconverted TX signal. The IF LNA is placed after the downconverter in the RX to amplify the baseband signal. For the simulation of the conventional reader system, the upand down-mixers use the same signal from the local oscillator. The filter, which is located between the driver and power amplifiers, was modeled to have a similar frequency response, based on the measured S-parameter of the SAW filter [see Fig. 1(a)]. The simulation was carried out with and without the filter using this setup. The second simulation setup presents the proposed reader which has a filter and uses the captured TX signal from the output coupler. The captured signal is fed to the down-converter as an LO signal after adjusting its amplitude and phase [see Fig. 5(b)]. Fig. 6 shows the simulated power spectral densities of the baseband noise results for the conventional reader with and without the filter and the proposed reader with the filter. The baseband noise level of the conventional reader with the filter is about −74 dBm/kHz at 300 kHz, while that of the conventional reader without the filter is about −100 dBm/kHz. The filter deteriorates the baseband noise level by about 26 dB. The simulated baseband noise level of the proposed reader is about −100 dBm/kHz at 300 kHz, which is not different from that of the conventional reader without the filter. The simulation results are very well matched with our analysis in the previous section. B. Experiments and Measured Results In order to experimentally validate the proposed structure of the UHF RFID reader system, ten sets of RFID readers were designed and fabricated on an FR4 printed circuit board (PCB), based on Samsung Techwin’s URP-SJ010. The photograph of the implemented reader is shown in Fig. 7. The reader chip, Impinj’s R1000, includes the MODEM, A-D and D-A converters,
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Fig. 5. Simulation setups for RFID reader system: (a) the conventional structure with and without the filter and (b) the proposed structure with the filter.
up- and down-converters, driver amplifier, and various analog function blocks. A SAW filter chip, Murata’s SAFCH953M, is placed between the RFID chip and the external power amplifier to avoid the degradation of the efficiency due to path loss. For the proposed RFID reader, the LO was externally supplied to the RFID reader chip through the external path from coupler 1, via the π-type fixed attenuator, to the downconverter. The optimized length of the transmission line was used in the path to realize an appropriate phase shift, which was experimentally optimized. The second coupler is used to monitor the transmitted and reflected signal levels due to the antenna mismatch using a power detector. Fig. 8 shows the measured power spectral densities of the LO signal, TX leakage signal, TX signal for the receiving mode (CW), and TX signal for the transmitting mode (modulation). The TX power and overall leakage signal power are measured to be 30 dBm and 4 dBm, respectively. The phase
noises of the TX signal before and after filtering are shown in Fig. 9. The fabricated RFID readers for Japanese application were tested in an anechoic chamber with Alien’s ALN-9554M tags. The tag is vertically (θ = φ = 90◦ ) placed from the reader antenna which is circularly polarized and has a gain of 3 dB. The output power level of the reader for both the conventional and proposed methods was set to 30 dBm and the tag reading test was performed with a center frequency of 952.4 MHz. For the test, the reader transmits the PR-ASK modulated signal to the tag and receives the back-scattered signal based on Miller 4 protocol in the ISO 18000-6C. Table III shows the measured reading distance of the implemented readers. The average reading distance of the ten conventional readers is 2.02 m, while that of the proposed readers is 3.92 m. An increase in the reading distance of 1.9 m is obtained. The sensitivity of one of the fabricated readers was measured using Tescom’s TC-2600A RFID tester. The measured
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Fig. 9. Measured phase noises of the TX signal before and after filtering.
Fig. 6. Simulated baseband noise performances of the conventional structure with and without the filter and the proposed configuration with the filter.
TABLE III S PECTRAL E MISSION S PECIFICATIONS FOR JAPAN
IV. C ONCLUSION
Fig. 7.
Photograph of the fabricated UHF RFID reader.
Fig. 8.
Measured results of the TX, LO, and leakage signals.
sensitivities of the conventional and proposed readers are −57 and −76 dBm, respectively. A baseband noise reduction of 19 dB was achieved using the proposed structure.
An improved UHF RFID reader, based on the reduction of the baseband noise using the captured TX signal after filtering, was proposed. The analysis of the conventional reader with a SAW filter in the TX path showed that the large delay of the filter is the main cause of the baseband noise degradation. To avoid the baseband noise degradation due to the filter, the TX signal captured from the isolation port of the output coupler was applied to the LO port of the output coupler was applied to the LO port of the down-converter in the RX. The analysis and simulation proved that the matching of the delays between the LO signal of the down-converter and TX leakage signal using the proposed method eliminates the degradation of the baseband noise performance of the reader. The two types of UHF RFID readers, viz. the conventional and proposed ones, were designed, fabricated, and tested. Their performances were compared from the view-point of the reading distance and sensitivity. The reading distance is strongly related to the sensitivity, which is directly affected by the baseband noise level. The average reading distance of the proposed UHF RFID reader is 3.92 m, which is 1.9 m, longer than that of the conventional one. The sensitivity is improved by 19 dB from −57 to −76 dBm. Because the same RX circuit is used in both cases, an improvement of the baseband noise level of 19 dB was also achieved. As a result, the proposed UHF RFID reader with an external LO for the down-converter using the captured TX signal has a significantly improved reading distance as well as sensitivity.
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R EFERENCES [1] J. Lee and B. Lee, “A long-range UHF-band RFID tag IC based on high-Q design approach,” IEEE Trans. Ind. Electron., vol. 56, no. 7, pp. 2308– 2316, Jul. 2009. [2] D. Kim, H. Jo, H. Yoon, C. Mun, B. Jang, and J. Yook, “Reverse-link interrogation range of a UHF MIMO-RFID system in Nakagami-m fading channels,” IEEE Trans. Ind. Electron., vol. 57, no. 4, pp. 1468–1477, Apr. 2010. [3] Y. Kim, Y. Choi, M. Seo, S. Yoo, and H. Yoo, “A CMOS transceiver for multistandard 13.56-MHz RFID reader SoC,” IEEE Trans. Ind. Electron., vol. 57, no. 5, pp. 1563–1572, May 2010. [4] S. Kuo, S. Chen, and C. Lin, “Design and development of RFID label for steel coil,” IEEE Trans. Ind. Electron., vol. 57, no. 6, pp. 2180–2186, Jun. 2010. [5] D. Kim, H. Yoon, B. Jang, and J. Yook, “Effects of reader-to-reader interference on the UHF RFID interrogation range,” IEEE Trans. Ind. Electron., vol. 56, no. 7, pp. 2337–2346, Jul. 2009. [6] S. Park and S. Hashimoto, “Autonomous mobile robot navigation using passive RFID in indoor environment,” IEEE Trans. Ind. Electron., vol. 56, no. 7, pp. 2366–2373, Jul. 2009. [7] A. Lehto, J. Nummela, L. Ukkonen, L. Sydanheimo, and M. Kivikoski, “Passive UHF RFID in paper industry: Challenges, benefits and the application environment,” IEEE Trans. Autom. Sci. Eng., vol. 6, no. 1, pp. 66– 79, Jan. 2009. [8] A. D. Droitcour, O. Boric-Lubecke, V. M. Lubecke, J. Lin, and G. T. A. Kovacs, “Range correlation and I/Q performance benefits in single-chip silicon Doppler radars for noncontact cardiopulmonary monitoring,” IEEE Trans. Microw. Theory Tech., vol. 52, no. 3, pp. 838–848, Mar. 2004. [9] A. Vaz, A. Ubarretxena, I. Zalbide, D. Pardo, H. Solar, A. Garcia-Alonso, and R. Berenguer, “Full passive UHF tag with a temperature sensor suitable for human body temperature monitoring,” IEEE Trans. Circuits Syst. II, Exp. Briefs, vol. 57, no. 2, pp. 95–99, Feb. 2010. [10] P. V. Nikitin, K. V. Seshagiri Rao, R. Martinez, and S. F. Lam, “Sensitivity and impedance measurement of UHF RFID chips,” IEEE Trans. Microw. Theory Tech., vol. 57, no. 5, pp. 1297–1302, May 2009. [11] K. Finkenzeller, RFID Handbook. New York: Wiley, 2003. [12] W. Lim and J. Yu, “Balanced circulator structure with enhanced isolation characteristics,” Microw. Opt. Technol. Lett., vol. 50, no. 9, pp. 2389– 2391, Sep. 2008. [13] J. Jung, H. Roh, K. Kwak, M. Jeong, and J. Park, “Adaptive TRX isolation scheme by using TX leakage canceller at variable frequency,” Microw. Opt. Technol. Lett., vol. 50, no. 8, pp. 2043–2045, Aug. 2008. [14] P. Pursula, M. Kiviranta, and H. Seppa, “UHF RFID reader with reflected power canceller,” IEEE Microw. Wireless Compon. Lett., vol. 19, no. 1, pp. 48–50, Jan. 2009. [15] F. Wei, X. W. Shi, Q. L. Huang, D. Z. Chen, and X. H. Wang, “A new directional coupler for UHF RFID reader,” Microw. Opt. Technol. Lett., vol. 50, no. 7, pp. 1973–1975, Jul. 2008. [16] W. Lim, S. Park, W. Son, M. Lee, and J. Yu, “RFID reader front-end having robust Tx leakage canceller for load variation,” IEEE Trans. Microw. Theory Tech., vol. 57, no. 5, pp. 1348–1355, May 2009. [17] J. Bae, J. Kim, B. Jeon, J. Jung, J. Park, B. Jang, H. Oh, Y. Moon, and Y. Seong, “Analysis of phase noise requirements on local oscillator for RFID system considering range correlation,” in Proc. Eur. Microw. Conf. Dig., Oct. 2007, pp. 1664–1667. [18] M. C. Budge, Jr. and M. P. Burt, “Range correlation effects on phase and amplitude noise,” in Proc. IEEE Southeastcon, Charlotte, NC, 1993, p. 5. [19] X. Yao, S. Kwon, H. Kim, H. Cho, M. Kim, S. Jung, C. Park, J. Kim, and Y. Yang, “Optimum ASK modulation scheme for passive RFID tags under antenna mismatch conditions,” IEEE Trans. Microw. Theory Tech., vol. 57, no. 10, pp. 2337–2343, Oct. 2009.
Sung-Chan Jung was born in Seoul, Korea, in 1973. He received the Ph.D. degree in information and communication engineering from the Sungkyunkwan University, Suwon, Korea, in 2006. From 2006 to 2007, he was a Postdoctoral researcher with the MCS Laboratory, Sungkyunkwan University. From 2007 to 2008, he was a Postdoctoral fellow with the iRadio Laboratory, Schulish School of Engineering, University of Calgary, Calgary, AB, Canada. From September 2008 to February 2009, he was a Research Professor with the MCS Laboratory, Sungkyunkwan University. Since March 2009, he has been with Samsung Techwin Company Ltd., Seongnam, Korea, where he is currently a Senior Researcher. His current research interests include design of highpower amplifiers, linearization techniques, efficiency enhancement techniques for base stations and mobile terminals, and RFID circuit design.
Min-Su Kim was born in Seoul, Korea, in 1978. He received the B.S. and M.S. degrees in electronic engineering from the Incheon University, Incheon, Korea, in 2005 and in information and communication engineering from the Sungkyunkwan University, Suwon, Korea, in 2008, Korea, respectively, where he is currently working toward the Ph.D. degree. His current research interests include transmitter, linearization techniques, RFID circuit design.
Youngoo Yang (S’99–M’02) was born in Hamyang, Korea, in 1969. He received the Ph.D. degree in electrical and electronic engineering from the Pohang University of Science and Technology(Postech), Pohang, Korea, in 2002. From 2002 to 2005, he was with Skyworks Solutions, Inc., Newbury Park, CA, where he designed power amplifiers for various cellular handsets. Since March 2005, he has been with the School of Information and Communication Engineering, Sungkyunkwan University, Suwon, Korea, where he is currently an associate professor and a director of the RFID/USN Integrated Circuit Research Center. His research interests include power amplifier design, RF transmitters, RFIC design, integrated circuit design for RFID/USN systems, and modeling of high power amplifiers or devices.
