Adaptive TX Leakage Canceler for the UHF RFID ... - Semantic Scholar
IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 4, APRIL 2014
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Adaptive TX Leakage Canceler for the UHF RFID Reader Front End Using a Direct Leaky Coupling Method Min-Su Kim, Sung-Chan Jung, Jonghyuk Jeong, Hyungchul Kim, Mincheol Seo, Junghyun Ham, Cheon-Seok Park, and Youngoo Yang, Member, IEEE
Abstract—This paper presents a new adaptive leakage canceler based on a direct leaky coupling method for an RF front-end circuit of an ultrahigh-frequency radio-frequency identication (UHF RFID) reader. For better receiver sensitivity, the leakage signal from a transmitter (TX) to a receiver (RX) should be suppressed. Compared with conventional methods, the proposed TX leakage canceler has a much simpler circuit configuration due to the direct leaky coupling circuit (LCC) and the phase shifter on the RX path. The LCC includes a variable resistor, a variable capacitor, and an inductor, which allow us to control the magnitude and the phase of the coupled TX signal. The phase shifter that is located on the RX path even before the low-noise amplifier controls the phase of the TX leakage signal. For the worldwide RFID bands that spans from 840 to 960 MHz, the canceler must be adaptively controlled by using a microcontroller and control circuits. For the experimental verification, an RF front-end circuit with the proposed adaptive TX leakage canceler was designed and implemented for the UHF RFID reader. The implemented RF front-end block occupies only 54 × 54 mm2 , except for the control block. From 840 to 960 MHz, the adaptively controlled RF front-end circuit exhibited significant cancelation characteristics for the TX–RX leakage signals. This results in more than twice longer reading distances for the commercial RFID tags compared with the cases without the adaptive canceler. Index Terms—Direct leaky coupling method, leakage canceler, radio-frequency identification (RFID) reader, transmitter (TX), TX leakage.
I. I NTRODUCTION
F
OR A passive radio-frequency identication (RFID) system that uses a half-duplex method, the reader sends a continuous wave to the tag for dc power generation of the tag even when the reader receives the tag’s response [1]. Therefore, high isolation characteristics between a transmitter (TX) and a receiver (RX) of the reader are essentially required for better RX sensitivity [2]–[13]. If it has poor isolation characteristics Manuscript received March 9, 2012; revised September 26, 2012 and February 28, 2013; accepted May 26, 2013. Date of publication June 12, 2013; date of current version September 19, 2013. This work was supported by the National Research Foundation of Korea under Grant 2012-003485. M.-S. Kim is with Samsung Electronics Company Ltd., Yongin 446-711, Korea. S.-C. Jung is with the Security Solution Division, Samsung Techwin, Seongnam 462-807, Korea. J. Jeong is with Peopleworks Incorporated, Seoul 152-766, Korea. H. Kim, M. Seo, J. Ham, C.-S. Park, and Y. Yang are with Microwave Circuits and Systems Laboratory, School of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, Korea (e-mail: [email protected]). Digital Object Identifier 10.1109/TIE.2013.2267932
between the TX and the RX, then, the TX signal can leak into the RX path. This results in serious degradation in the RX performance. The TX leakage signal on the RX path can work as a blocker signal against the relatively weak incoming signal from the tag. This can also significantly raise the noise level of the RX [2]–[6]. Many previous works have reported on the various methods to solve this leakage issue [7]–[13]. If two antennas are separately used for each TX and RX, then TX and RX can be physically separated so that the reader can have high isolation [7]. However, this makes the reader bulky and expensive. In order to use a single antenna for both TX and RX, general RFID readers adopt an isolator or a directional coupler for the front end and isolate the RX from the TX [8]–[11]. These methods have been widely used as they have advantages of having a simple circuit and low cost with moderate isolation characteristics. However, better isolation characteristics are still required to further improve the performance of the RFID reader. For the RF front-end circuit using a directional coupler, an intentionally controlled reflection for the coupler isolation port was proposed in order to cancel the leakage signal from the TX [8], [9]. The input impedance of the antenna changes over the frequency bands and has variation over the components. Therefore, for practical applications, an adaptive cancelation for the TX leakage is necessary. Some previous works proposed an adaptive TX leakage canceler for the front end based on a directional coupler [10], [11]. The RF front-end circuit using a circulator has a great advantage of no coupling loss for the received signal from the tag with moderate isolation characteristics. For this type of front end, adaptive cancelers for the TX leakage have been also proposed in [12] and [13]. However, they adopted an additional directional coupler to couple the TX signal. The coupled TX signal is adjusted by using a variable attenuator and a variable phase shifter. Then, it is combined by using a combiner with the received signal, which includes the TX leakage signal. The coupled signal and the TX leakage signal are canceled out. Those RF front ends with adaptive leakage cancelers have an additional directional coupler, a combiner, an attenuator, and a multistage phase shifter with a phase tuning range of 3860◦ . They are still very complex. In this paper, we propose a very compact and adaptive TX leakage canceler using a direct leaky coupling method for the RF front end of the RFID reader based on an isolator. In order
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to directly couple the TX signal and directly inject the coupled signal to the RX path, a leaky coupling circuit (LCC) using a lumped circuit network is deployed across the TX and RX paths. Moreover, for additional phase control, an electrically tunable phase shifter is located on the RX path. Therefore, the front-end circuit can be significantly simpler and can require lower TX output power for the same or even longer reading distances than the conventional coupler-based cancelers. The LCC is a series C−L−R network whose capacitor and resistor are also electrically tunable for adaptive control. The variable resistor, which is implemented using a p-i-n diode, is to control the magnitude of the coupled TX signal. The variable capacitor, which is implemented using a varactor diode, forms an L−C resonant circuit with the fixed inductor. The variable capacitor in the LCC tunes the phase of the coupled signal from 90◦ to 180◦ . Hence, the phase tuning range of the phase shifter can be significantly reduced from 360◦ . In addition to the phase shifter, the variable resistor, which is realized using a p-i-n diode, and the variable capacitor, which is realized using a varactor diode, on the LCC are adaptively controlled by using a microcontroller to minimize the detected residual TX leakage at the RX path. In the following, the operation of the proposed leakage canceler is analyzed and presented. The experimental results to the commercial RFID tags for the implemented RF front end with and without the proposed leakage canceler are also compared.
Fig. 1. Measured reflection coefficients of an antenna for the UHF RFID band (840 to 960 MHz). (a) Γin on the Smith chart. (b) Its magnitude in decibels.
II. TX L EAKAGE C ANCELER A. Direct Leaky Coupling Method For most of its worldwide applications, the UHF RFID band spans from 840 to 960 MHz. A mismatch in the input impedance of an antenna mainly causes the TX signal leakage in the RX path. This can seriously degrade the performance of the RX. The practically measured input reflection coefficients of an antenna are presented in Fig. 1. The antenna has a Samsung Techwin’s mobile RFID reader URP-SK010. The magnitude of the reflection coefficient becomes larger than −10 dB around 840 MHz, and the phases span over 238◦ for the entire frequency band. Fig. 2(a) shows a circuit diagram of the RF front end with a representative conventional leakage canceler based on a directional coupler, a power combiner, a variable attenuator, and a variable phase shifter [12], [13]. It has a very straightforward operational principle to cancel the leakage signal, which is mainly reflected from the antenna. However, the circuit requires bulky and costly components. As the coupling path from the TX to RX becomes very lossy, its cancelation capability against this strong leakage signal can be limited. Therefore, a more compact leakage canceler with higher signal coupling capability and better phase controllability is required. Moreover, a proper adaptive control method should be also deployed in order to maintain good cancelation performance against rapidly varying leakage signals over a wide bandwidth, as shown in Fig. 1. Fig. 2(b) shows the configuration of the RF front end with the proposed TX leakage canceler based on the direct leaky coupling method. The proposed TX leakage canceler consists
Fig. 2. Circuit diagrams of the RF front ends. (a) With a representative conventional leakage canceler. (b) With a proposed leakage canceler based on a direct leaky coupling method.
of an LCC from the TX to the RX and a phase shifter in the RX path. The LCC is composed of a series C−L−R network. It directly couples the TX signal from the TX path and injects it to the RX path in order to cancel the TX leakage signal. The magnitude of the coupled signal is controlled by using the variable resistor on the LCC. As it has a direct connection between the TX and RX paths using a passive network, the
KIM et al.: ADAPTIVE TX LEAKAGE CANCELER FOR UHF RFID READER FRONT END
Fig. 3.
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Fig. 4. Calculated S31 and S21 without LCC according to the various resistances for Z2 .
Operational diagram of the LCC.
coupling capability is much higher compared with that of a conventional network. The phases of the coupled signal and the TX leakage signal should be exactly 180◦ off each other for an optimum cancelation. The phases of the two signals will be adjusted by using a variable capacitor in the LCC and a variable phase shifter in the RX path. A series L−C circuit on the LCC can tune the phase of the coupled signal to a certain extent using a variable capacitor so that the phase tuning range of the phase shifter can be significantly mitigated from 360◦ . B. Operation of LCC Fig. 3 shows an operational diagram to analyze the RF frontend circuit with the proposed TX leakage canceler. A major leakage source is an imperfect input matching of the antenna, and this is only considered for analysis as the leakages are comparably low from other leakage sources, such as a substrate coupling and an imperfect isolation of the isolator. In addition, they do not affect the analysis results because we can only detect the overall leakage signal in the RX path. A three-port network is used for the analysis. The TX signal is excited from port 1. Ports 2 and 3 represent the antenna and the RX input, respectively. The LCC, across ports 1 and 3, has series lumped components whose impedance are expressed as XC , XL , and RV , respectively. The circulator and phase shifter are assumed to be ideal and lossless. Moreover, they are all assumed to be 50 Ω matched. The TX leakage signal that flows toward the RX path from the mismatched antenna can be simply calculated without the leaky circuit. Its ratio from the TX signal can be represented by using an S-parameter as follows: S31 |ZLCC =∞ =
V3− Z2 − 50 jθ jθ e = (1) + |ZLCC =∞ = Γ2 · e Z V1 2 + 50
where Γ2 is a reflection coefficient for port 2, and θ is a phase shift from the phase shifter on the RX path. ZLCC is the impedance seen from the TX path toward the leaky circuit, and it is given by ZLCC = RV + 25 + XC + XL
(2)
where XC and XL are 1/jωCLCC and jωLLCC , respectively. Fig. 4 shows the calculated S31 and S21 without the LCC according to the various resistances of port 2.
Fig. 5. Calculated tunability for the direct leaky coupled signal. (a) S21 and S31 according to the various values of RV . (b) Phases of S31 according to various values of CLCC .
In order to cancel the leakage signal, the LCC should be applied to directly couple the TX signal. Then, the directly coupled signal and the TX leakage signal can be canceled if they have the same magnitude and phases with a 180◦ difference. The directly coupled signal from ports 1–3 can be also expressed by using S-parameter. The transmission coefficients for the TX signal to the antenna and the coupled signal to the RX path are given by S21 |Z2 =50 =
V2− ZLCC |Z =50 = ZLCC + 25 V1+ 2
(3)
S31 |Z2 =50 =
V3− 25 |Z =50 = ZLCC + 25 V1+ 2
(4)
under an assumption of no antenna mismatch (Z2 = 50). From (4), it can be seen that ZLCC can control the magnitude and the phase of S31 . In addition to the phase controllability for the
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Fig. 6. Schematic of the front-end system based on the proposed direct leaky coupling method including adaptive control circuits.
Fig. 8. Measured results using a single-tone signal with and without the leakage cancelation circuit. (a) S31 . (b) S21 .
Fig. 9. Received signals from the RX path with and without the leakage cancelation circuit at the TX output power of 24.6 dBm and the distance between the reader antenna and tag of 4 m.
Fig. 7. (a) Implemented front-end system. (b) Test setup for tag reading.
TX leakage signal, which is given by the phase shifter on the RX path with θ in (1), the phase of the coupled signal can be adjusted by using the capacitor and inductor in the LCC. Fig. 5 shows the tunablity of S31 by using various values of RV and CLCC on the LCC. LLCC is fixed as it is difficult to realize an electrically tunable inductor. Fig. 5(a) has S21 and S31 according to the various values of RV for a fixed CLCC , which has a resonance with LLCC . We can obtain a broad range of S31 from −10 to −35 dB. Fig. 5(b) shows phase variations of S31 according to the various values of CLCC as a parameter of RV . A phase shift of at least 90◦ can be achieved at high RV of 800 Ω. Thus, the phase shifter on the RX path is
required to have a phase tuning range of 270◦ in order to cover an entire phase rotation of 360◦ . Nonzero reflection coefficient ΓB , which is marked in Fig. 3, is due to the LCC, and it must be considered when combining the TX leakage signal from (1) and the coupled TX signal from (4). Then, the final transmission coefficients for both the delivered TX signals to the antenna and the residual TX leakage signal after cancelation can be written as follows: ZLCC · 1 − |Γ2 |2 ZLCC + 25 25 + ZLCC · Γ2 · 1 − |ΓB |2 · ejθ = ZLCC + 25
S21 =
(5)
S31
(6)
where ΓB is −50/(2ZLCC + 50). Two terms in the numerator on (6) should be zero for perfect cancelation. In order to accomplish the best cancelation performance, we have to tune RV , CLCC , and θ.
KIM et al.: ADAPTIVE TX LEAKAGE CANCELER FOR UHF RFID READER FRONT END
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TABLE I S UMMARY AND C OMPARISON FOR R EADING D ISTANCE
III. S YSTEM C ONFIGURATION AND E XPERIMENTAL S ETUP Fig. 6 shows a schematic diagram of the RF front end with the proposed TX leakage canceler and adaptive control circuits. In order to make the coupling signal tunable, the LCC consists of a tunable resistor, a tunable capacitor, and a fixed inductor. A tunable resistor and a tunable capacitor are realized by using a p-i-n diode of Skyworks’ SMP-1307 and a Schottky diode of Skyworks’ SMV-1247, respectively. Both the tunable resistor and the capacitor have a control voltage of 0–8 V. In that control voltage range, they have variable values of 2–2000 Ω and 0.64–8.86 pF, respectively. A fixed inductor of 20 nH is deployed for a wide phase tuning range with a variable capacitor. For a circulator at the output of the system, MA-COM’s MAFRIN0479 was used. For an adaptive control to cover the entire RFID band from 840 to 960 MHz, the residual leakage power after cancelation is detected by using a power detector of Analog device’s AD8360. Three phase shifters with Skyworks’ PS088-315 are deployed to have a phase control range of about 270◦ with a control voltage of 0–8 V on the RX path in series. An eight-bit microcontroller based on Microchip’s PIC18F452, with an RS232 communication port, is used to adjust three control voltages for a variable resistor, a capacitor and a phase shifter. For the modem, Samsung Techwin’s mobile RFID reader URP-SK010 was used. The overall system was implemented on a printed circuit board, and its size was 160.13 × 89.95 mm2 . The core frontend circuit occupied an area of 54 × 54 mm2 , except for the control circuits. A simple least-mean-square-error algorithm, which is a gradient-based steepest decent method, was applied for adaptive control. A goal function for the adaptation is a sensed voltage from the power detector in the RX path because the leakage signal is much larger than the RX signal from the tag. The photograph of the implemented RF front-end system is shown in Fig. 7(a), and its evaluation setup for the reading range is shown in Fig. 7(b). The same antenna that was used in the analysis was also used in taking measurements.
IV. E XPERIMENTAL R ESULTS Fig. 8 shows the measured transfer characteristics of S31 and S21 . They are optimized for the maximum cancelation of three different frequencies namely, 840, 900, and 960 MHz, which are the lower and upper corners and center frequency of the
entire RFID bands from 840 to 960 MHz. The RF front end with the proposed adaptive TX leakage canceler exhibits high isolation characteristics over −80 dB from the TX to the RX [see Fig. 8(a)] and a good insertion loss from −0.39 to −0.7 dB for the TX signal [see Fig. 8(b)]. The microcontroller gives the control voltages of VPS , VCAP , and VRES using a ten-bit digital-to-analog converters so that they have control resolutions of about 0.27◦ , 8.22 fF, and 2 Ω per step, respectively. The three control voltages are sequentially adapted to minimize the goal function, which is the output voltage of the power detector. Because we used variable control step size, the total convergence after a start-up of the system took just about 114 iterations (about 5 s) under the worst initial conditions. By using the experimental setup shown in Fig. 7, the reading range was tested using the commercially available RFID tags, which are NXP’s UCODE G2XM and Alien’s ALN-9634. The test was done under general noisy laboratory environment. By using phase-reversal-amplitude-shift-keying (PR-ASK)modulated signal, the test was carried out at 915 MHz band, which is the center frequency of the U.S. band. As shown in Fig. 9, low residual TX leakage can be clearly observed with the proposed cancelation circuit. The backscattered signal from the tag is more clearly distinguished even in its power spectral density. Table I summarizes the test results for the reading range. For both the commercially available RFID tags, a significant improvement in the reading distance was achieved after applying the proposed leakage canceler. For the UCODE G2XM, a significant reading range improvement of 5.3–2.7 to 8 m was achieved at a TX output power of 24.6 dBm. Compared with the previously published data, our reader has much better reading distances with the proposed canceler using lower TX output power by more than 5 dB. V. C ONCLUSION In this paper, a new adaptive TX leakage canceler based on the direct leaky coupling method is proposed for the RF front ends of the UHF RFID readers. Unlike the conventional one, the proposed TX leakage canceler has a much simpler circuit configuration without a directional coupler, a power combiner, and an attenuator. The LCC across the TX and RX paths only consists of a variable resistor, a variable capacitor, and a fixed inductor in series. By using this simple circuit, it can tune the
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magnitude and the phase of the coupled signal from the TX path by over 30 dB and at least 90◦ , respectively. A phase shifter located on the RX path is required to tune only 270◦ of the phase of the leakage signal for complete phase coverage of 360◦ . The proposed circuit was implemented on a printed circuit board, and its size was 160.13 × 89.95 mm2 including control circuit. The size of the core front-end circuit was as small as 54 × 54 mm2 , excluding the adaptive control circuit. It was tested by using the one-tone signal and the PR-ASKmodulated signal. Significant reading range improvements for the representative RFID tags were achieved after applying the proposed leakage canceler. R EFERENCES [1] Y. Yao, J. Wu, Y. Shi, and F. F. Dai, “A fully integrated 900-MHz passive RFID transponder front end with novel zero-threshold RF-DC rectifier,” IEEE Trans. Ind. Electron., vol. 56, no. 7, pp. 2317–2325, Jul. 2009. [2] K. Choi, S. Yoo, M. Kim, H. Kim, S. Ryu, S. Kang, S. Jung, and Y. Yang, “CMOS DSB transmitter with low TX noise for RFID reader system-onchip,” IEEE Trans. Microw. Theory Tech., vol. 58, no. 12, pp. 3467–3474, Dec. 2010. [3] W. Wang and H. C. Luong, “A 0.8-V 4.9-mW 1.2-GHz CMOS fractionalN frequency synthesizer for UHF RFID readers,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 55, no. 9, pp. 2505–2513, Oct. 2008. [4] S. Jung, M. Kim, and Y. Yang, “Baseband noise reduction method using captured TX signal for UHF RFID reader applications,” IEEE Trans. Ind. Electron., vol. 59, no. 1, pp. 592–598, Jan. 2012. [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] 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. [7] 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. [8] W. Kim, W. Na, J. Yu, and M. Lee, “A high isolated coupled-line passive circulator for UHF RFID reader,” Microw. Opt. Technol. Lett., vol. 50, no. 10, pp. 2597–2600, Oct. 2008. [9] 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. 10, pp. 1973–1975, Jul. 2008. [10] M. Lee, “Lumped directional coupler with a varactor tuned reflector for RFID applications,” IEICE Electron. Exp., vol. 6, no. 2, pp. 129–134, Jan. 2009. [11] S. Jung, M. Kim, and Y. Yang, “A reconfigurable carrier leakage canceler for UHF RFID reader front-ends,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 58, no. 1, pp. 70–76, Jan. 2010. [12] J. Jung, H. Roh, J. Kim, H. Kwak, M. S. Jeong, and J. Park, “TX leakage cancellation via a micro controller and high TX-to-RX isolation covering an UHF RFID frequency band of 908-914MHz,” IEEE Microw. Wireless Compon. Lett., vol. 18, no. 10, pp. 710–712, Oct. 2008. [13] T. Xiong, X. Tan, J. Xi, and H. Min, “High TX-to-RX isolation in UHF RFID using narrowband leaking carrier canceller,” IEEE Microw. Wireless Compon. Lett., vol. 20, no. 2, pp. 124–126, Feb. 2010.
Min-Su Kim was born in Seoul, Korea, in 1978. He received the Ph.D. degree in information and communication engineering from Sungkyunkwan University, Suwon, Korea, in 2012. Since March 2012, he has been with Samsung Electronics Company Ltd., Yongin, Korea, where he is currently a Senior Engineer. His current research interests include transceivers for mobile terminals.
Sung-Chan Jung was born in Seoul, Korea, in 1973. He received the Ph.D. degree in information and communication engineering from Sungkyunkwan University, Suwon, Korea, in 2006. From 2006 to 2007, he was a Postdoctoral Researcher with the Microwave Circuits and Systems (MCS) Laboratory, Sungkyunkwan University. From 2007 to 2008, he was a Postdoctoral Fellow with the iRadio Laboratory, Schulich 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 the Security Solution Division, Samsung Techwin Company Ltd., Seongnam, Korea, where he is currently a Senior Researcher. His current research interests include the design of high-power amplifiers, linearization techniques, efficiency enhancement techniques for base stations and mobile terminals, passive circuit optimization, and radiofrequency identification circuit design.
Jonghyuk Jeong was born in Jeonju, Korea, in 1977. He received the M.S. degree in electrical, electronic, and computer engineering from Sungkyunkwan University, Suwon, Korea, in 2009. He is currently with Peopleworks Incorporated, Seoul, Korea. His research interests include power amplifier design and efficiency enhancement and linearization techniques.
Hyungchul Kim was born in Chuncheon, Korea, in 1983. He received the B.S. and M.S. degrees in electrical, electronic, and computer engineering from Sungkyunkwan University, Suwon, Korea, in 2008 and 2010, respectively. He is currently working toward the Ph.D. degree at Sungkyunkwan University. His research interests include RF power amplifier design, RF-identification-tag integrated circuit design, low-power analog/mixed signal circuit design, and power converter design.
Mincheol Seo was born in Seoul, Korea, in 1983. He received the M.S. degree in information and communication engineering from Sungkyunkwan University, Suwon, Korea, in 2011. He is currently working toward the Ph.D. degree at Sungkyunkwan University. His current research interests include design of radio-frequency power amplifiers, linearization techniques, and efficiency enhancement techniques.
Junghyun Ham was born in Seoul, Korea, in 1980. He received the M.S. degree in electrical and computer engineering from Hanyang University, Seoul, Korea, in 2009. He is currently working toward the Ph.D. degree in electrical, electronic, and computer engineering at Sungkyunkwan University, Suwon, Korea. From 2009 to 2011, he was with LG Electronics, Seoul, Korea, where he was involved in the development of high-efficiency power amplifiers for mobile handset applications. His research interests include high-efficiency RF transmitters, high-speed dc–dc converters, and complementary metal–oxide–semiconductor RF power amplifiers.
KIM et al.: ADAPTIVE TX LEAKAGE CANCELER FOR UHF RFID READER FRONT END
Cheon-Seok Park was born in Seoul, Korea, in 1960. He received the B.S. degree in electrical engineering from Seoul National University, Seoul, Korea, in 1988 and the M.S. and Ph.D. degrees in electrical and electronic engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Korea, in 1990 and 1995, respectively. He is currently a Professor with the School of Information and Communication Engineering, Sungkyunkwan University, Suwon, Korea. His research interests include design of radio-frequency power amplifiers, linearization techniques, and efficiency enhancement techniques.
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Youngoo Yang (S’99–M’02) was born in Hanyang, Korea, in 1969. He received the Ph.D. degree in electrical and electronic engineering from Pohang University of Science and Technology, Pohang, Korea, in 2002. From 2002 to 2005, he was with Skyworks Solutions Inc., Newbury Park, CA, USA, 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. His research interests include power amplifier design, RF transmitters, RF integrated circuit (IC) design, IC design for RF identification/ubiquitous sensor network systems, and modeling of high-power amplifiers or devices.
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Adaptive TX Leakage Canceler for the UHF RFID Reader Front End Using a Direct Leaky Coupling Method Min-Su Kim, Sung-Chan Jung, Jonghyuk Jeong, Hyungchul Kim, Mincheol Seo, Junghyun Ham, Cheon-Seok Park, and Youngoo Yang, Member, IEEE
Abstract—This paper presents a new adaptive leakage canceler based on a direct leaky coupling method for an RF front-end circuit of an ultrahigh-frequency radio-frequency identication (UHF RFID) reader. For better receiver sensitivity, the leakage signal from a transmitter (TX) to a receiver (RX) should be suppressed. Compared with conventional methods, the proposed TX leakage canceler has a much simpler circuit configuration due to the direct leaky coupling circuit (LCC) and the phase shifter on the RX path. The LCC includes a variable resistor, a variable capacitor, and an inductor, which allow us to control the magnitude and the phase of the coupled TX signal. The phase shifter that is located on the RX path even before the low-noise amplifier controls the phase of the TX leakage signal. For the worldwide RFID bands that spans from 840 to 960 MHz, the canceler must be adaptively controlled by using a microcontroller and control circuits. For the experimental verification, an RF front-end circuit with the proposed adaptive TX leakage canceler was designed and implemented for the UHF RFID reader. The implemented RF front-end block occupies only 54 × 54 mm2 , except for the control block. From 840 to 960 MHz, the adaptively controlled RF front-end circuit exhibited significant cancelation characteristics for the TX–RX leakage signals. This results in more than twice longer reading distances for the commercial RFID tags compared with the cases without the adaptive canceler. Index Terms—Direct leaky coupling method, leakage canceler, radio-frequency identification (RFID) reader, transmitter (TX), TX leakage.
I. I NTRODUCTION
F
OR A passive radio-frequency identication (RFID) system that uses a half-duplex method, the reader sends a continuous wave to the tag for dc power generation of the tag even when the reader receives the tag’s response [1]. Therefore, high isolation characteristics between a transmitter (TX) and a receiver (RX) of the reader are essentially required for better RX sensitivity [2]–[13]. If it has poor isolation characteristics Manuscript received March 9, 2012; revised September 26, 2012 and February 28, 2013; accepted May 26, 2013. Date of publication June 12, 2013; date of current version September 19, 2013. This work was supported by the National Research Foundation of Korea under Grant 2012-003485. M.-S. Kim is with Samsung Electronics Company Ltd., Yongin 446-711, Korea. S.-C. Jung is with the Security Solution Division, Samsung Techwin, Seongnam 462-807, Korea. J. Jeong is with Peopleworks Incorporated, Seoul 152-766, Korea. H. Kim, M. Seo, J. Ham, C.-S. Park, and Y. Yang are with Microwave Circuits and Systems Laboratory, School of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, Korea (e-mail: [email protected]). Digital Object Identifier 10.1109/TIE.2013.2267932
between the TX and the RX, then, the TX signal can leak into the RX path. This results in serious degradation in the RX performance. The TX leakage signal on the RX path can work as a blocker signal against the relatively weak incoming signal from the tag. This can also significantly raise the noise level of the RX [2]–[6]. Many previous works have reported on the various methods to solve this leakage issue [7]–[13]. If two antennas are separately used for each TX and RX, then TX and RX can be physically separated so that the reader can have high isolation [7]. However, this makes the reader bulky and expensive. In order to use a single antenna for both TX and RX, general RFID readers adopt an isolator or a directional coupler for the front end and isolate the RX from the TX [8]–[11]. These methods have been widely used as they have advantages of having a simple circuit and low cost with moderate isolation characteristics. However, better isolation characteristics are still required to further improve the performance of the RFID reader. For the RF front-end circuit using a directional coupler, an intentionally controlled reflection for the coupler isolation port was proposed in order to cancel the leakage signal from the TX [8], [9]. The input impedance of the antenna changes over the frequency bands and has variation over the components. Therefore, for practical applications, an adaptive cancelation for the TX leakage is necessary. Some previous works proposed an adaptive TX leakage canceler for the front end based on a directional coupler [10], [11]. The RF front-end circuit using a circulator has a great advantage of no coupling loss for the received signal from the tag with moderate isolation characteristics. For this type of front end, adaptive cancelers for the TX leakage have been also proposed in [12] and [13]. However, they adopted an additional directional coupler to couple the TX signal. The coupled TX signal is adjusted by using a variable attenuator and a variable phase shifter. Then, it is combined by using a combiner with the received signal, which includes the TX leakage signal. The coupled signal and the TX leakage signal are canceled out. Those RF front ends with adaptive leakage cancelers have an additional directional coupler, a combiner, an attenuator, and a multistage phase shifter with a phase tuning range of 3860◦ . They are still very complex. In this paper, we propose a very compact and adaptive TX leakage canceler using a direct leaky coupling method for the RF front end of the RFID reader based on an isolator. In order
0278-0046 © 2013 IEEE
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to directly couple the TX signal and directly inject the coupled signal to the RX path, a leaky coupling circuit (LCC) using a lumped circuit network is deployed across the TX and RX paths. Moreover, for additional phase control, an electrically tunable phase shifter is located on the RX path. Therefore, the front-end circuit can be significantly simpler and can require lower TX output power for the same or even longer reading distances than the conventional coupler-based cancelers. The LCC is a series C−L−R network whose capacitor and resistor are also electrically tunable for adaptive control. The variable resistor, which is implemented using a p-i-n diode, is to control the magnitude of the coupled TX signal. The variable capacitor, which is implemented using a varactor diode, forms an L−C resonant circuit with the fixed inductor. The variable capacitor in the LCC tunes the phase of the coupled signal from 90◦ to 180◦ . Hence, the phase tuning range of the phase shifter can be significantly reduced from 360◦ . In addition to the phase shifter, the variable resistor, which is realized using a p-i-n diode, and the variable capacitor, which is realized using a varactor diode, on the LCC are adaptively controlled by using a microcontroller to minimize the detected residual TX leakage at the RX path. In the following, the operation of the proposed leakage canceler is analyzed and presented. The experimental results to the commercial RFID tags for the implemented RF front end with and without the proposed leakage canceler are also compared.
Fig. 1. Measured reflection coefficients of an antenna for the UHF RFID band (840 to 960 MHz). (a) Γin on the Smith chart. (b) Its magnitude in decibels.
II. TX L EAKAGE C ANCELER A. Direct Leaky Coupling Method For most of its worldwide applications, the UHF RFID band spans from 840 to 960 MHz. A mismatch in the input impedance of an antenna mainly causes the TX signal leakage in the RX path. This can seriously degrade the performance of the RX. The practically measured input reflection coefficients of an antenna are presented in Fig. 1. The antenna has a Samsung Techwin’s mobile RFID reader URP-SK010. The magnitude of the reflection coefficient becomes larger than −10 dB around 840 MHz, and the phases span over 238◦ for the entire frequency band. Fig. 2(a) shows a circuit diagram of the RF front end with a representative conventional leakage canceler based on a directional coupler, a power combiner, a variable attenuator, and a variable phase shifter [12], [13]. It has a very straightforward operational principle to cancel the leakage signal, which is mainly reflected from the antenna. However, the circuit requires bulky and costly components. As the coupling path from the TX to RX becomes very lossy, its cancelation capability against this strong leakage signal can be limited. Therefore, a more compact leakage canceler with higher signal coupling capability and better phase controllability is required. Moreover, a proper adaptive control method should be also deployed in order to maintain good cancelation performance against rapidly varying leakage signals over a wide bandwidth, as shown in Fig. 1. Fig. 2(b) shows the configuration of the RF front end with the proposed TX leakage canceler based on the direct leaky coupling method. The proposed TX leakage canceler consists
Fig. 2. Circuit diagrams of the RF front ends. (a) With a representative conventional leakage canceler. (b) With a proposed leakage canceler based on a direct leaky coupling method.
of an LCC from the TX to the RX and a phase shifter in the RX path. The LCC is composed of a series C−L−R network. It directly couples the TX signal from the TX path and injects it to the RX path in order to cancel the TX leakage signal. The magnitude of the coupled signal is controlled by using the variable resistor on the LCC. As it has a direct connection between the TX and RX paths using a passive network, the
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Fig. 3.
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Fig. 4. Calculated S31 and S21 without LCC according to the various resistances for Z2 .
Operational diagram of the LCC.
coupling capability is much higher compared with that of a conventional network. The phases of the coupled signal and the TX leakage signal should be exactly 180◦ off each other for an optimum cancelation. The phases of the two signals will be adjusted by using a variable capacitor in the LCC and a variable phase shifter in the RX path. A series L−C circuit on the LCC can tune the phase of the coupled signal to a certain extent using a variable capacitor so that the phase tuning range of the phase shifter can be significantly mitigated from 360◦ . B. Operation of LCC Fig. 3 shows an operational diagram to analyze the RF frontend circuit with the proposed TX leakage canceler. A major leakage source is an imperfect input matching of the antenna, and this is only considered for analysis as the leakages are comparably low from other leakage sources, such as a substrate coupling and an imperfect isolation of the isolator. In addition, they do not affect the analysis results because we can only detect the overall leakage signal in the RX path. A three-port network is used for the analysis. The TX signal is excited from port 1. Ports 2 and 3 represent the antenna and the RX input, respectively. The LCC, across ports 1 and 3, has series lumped components whose impedance are expressed as XC , XL , and RV , respectively. The circulator and phase shifter are assumed to be ideal and lossless. Moreover, they are all assumed to be 50 Ω matched. The TX leakage signal that flows toward the RX path from the mismatched antenna can be simply calculated without the leaky circuit. Its ratio from the TX signal can be represented by using an S-parameter as follows: S31 |ZLCC =∞ =
V3− Z2 − 50 jθ jθ e = (1) + |ZLCC =∞ = Γ2 · e Z V1 2 + 50
where Γ2 is a reflection coefficient for port 2, and θ is a phase shift from the phase shifter on the RX path. ZLCC is the impedance seen from the TX path toward the leaky circuit, and it is given by ZLCC = RV + 25 + XC + XL
(2)
where XC and XL are 1/jωCLCC and jωLLCC , respectively. Fig. 4 shows the calculated S31 and S21 without the LCC according to the various resistances of port 2.
Fig. 5. Calculated tunability for the direct leaky coupled signal. (a) S21 and S31 according to the various values of RV . (b) Phases of S31 according to various values of CLCC .
In order to cancel the leakage signal, the LCC should be applied to directly couple the TX signal. Then, the directly coupled signal and the TX leakage signal can be canceled if they have the same magnitude and phases with a 180◦ difference. The directly coupled signal from ports 1–3 can be also expressed by using S-parameter. The transmission coefficients for the TX signal to the antenna and the coupled signal to the RX path are given by S21 |Z2 =50 =
V2− ZLCC |Z =50 = ZLCC + 25 V1+ 2
(3)
S31 |Z2 =50 =
V3− 25 |Z =50 = ZLCC + 25 V1+ 2
(4)
under an assumption of no antenna mismatch (Z2 = 50). From (4), it can be seen that ZLCC can control the magnitude and the phase of S31 . In addition to the phase controllability for the
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Fig. 6. Schematic of the front-end system based on the proposed direct leaky coupling method including adaptive control circuits.
Fig. 8. Measured results using a single-tone signal with and without the leakage cancelation circuit. (a) S31 . (b) S21 .
Fig. 9. Received signals from the RX path with and without the leakage cancelation circuit at the TX output power of 24.6 dBm and the distance between the reader antenna and tag of 4 m.
Fig. 7. (a) Implemented front-end system. (b) Test setup for tag reading.
TX leakage signal, which is given by the phase shifter on the RX path with θ in (1), the phase of the coupled signal can be adjusted by using the capacitor and inductor in the LCC. Fig. 5 shows the tunablity of S31 by using various values of RV and CLCC on the LCC. LLCC is fixed as it is difficult to realize an electrically tunable inductor. Fig. 5(a) has S21 and S31 according to the various values of RV for a fixed CLCC , which has a resonance with LLCC . We can obtain a broad range of S31 from −10 to −35 dB. Fig. 5(b) shows phase variations of S31 according to the various values of CLCC as a parameter of RV . A phase shift of at least 90◦ can be achieved at high RV of 800 Ω. Thus, the phase shifter on the RX path is
required to have a phase tuning range of 270◦ in order to cover an entire phase rotation of 360◦ . Nonzero reflection coefficient ΓB , which is marked in Fig. 3, is due to the LCC, and it must be considered when combining the TX leakage signal from (1) and the coupled TX signal from (4). Then, the final transmission coefficients for both the delivered TX signals to the antenna and the residual TX leakage signal after cancelation can be written as follows: ZLCC · 1 − |Γ2 |2 ZLCC + 25 25 + ZLCC · Γ2 · 1 − |ΓB |2 · ejθ = ZLCC + 25
S21 =
(5)
S31
(6)
where ΓB is −50/(2ZLCC + 50). Two terms in the numerator on (6) should be zero for perfect cancelation. In order to accomplish the best cancelation performance, we have to tune RV , CLCC , and θ.
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TABLE I S UMMARY AND C OMPARISON FOR R EADING D ISTANCE
III. S YSTEM C ONFIGURATION AND E XPERIMENTAL S ETUP Fig. 6 shows a schematic diagram of the RF front end with the proposed TX leakage canceler and adaptive control circuits. In order to make the coupling signal tunable, the LCC consists of a tunable resistor, a tunable capacitor, and a fixed inductor. A tunable resistor and a tunable capacitor are realized by using a p-i-n diode of Skyworks’ SMP-1307 and a Schottky diode of Skyworks’ SMV-1247, respectively. Both the tunable resistor and the capacitor have a control voltage of 0–8 V. In that control voltage range, they have variable values of 2–2000 Ω and 0.64–8.86 pF, respectively. A fixed inductor of 20 nH is deployed for a wide phase tuning range with a variable capacitor. For a circulator at the output of the system, MA-COM’s MAFRIN0479 was used. For an adaptive control to cover the entire RFID band from 840 to 960 MHz, the residual leakage power after cancelation is detected by using a power detector of Analog device’s AD8360. Three phase shifters with Skyworks’ PS088-315 are deployed to have a phase control range of about 270◦ with a control voltage of 0–8 V on the RX path in series. An eight-bit microcontroller based on Microchip’s PIC18F452, with an RS232 communication port, is used to adjust three control voltages for a variable resistor, a capacitor and a phase shifter. For the modem, Samsung Techwin’s mobile RFID reader URP-SK010 was used. The overall system was implemented on a printed circuit board, and its size was 160.13 × 89.95 mm2 . The core frontend circuit occupied an area of 54 × 54 mm2 , except for the control circuits. A simple least-mean-square-error algorithm, which is a gradient-based steepest decent method, was applied for adaptive control. A goal function for the adaptation is a sensed voltage from the power detector in the RX path because the leakage signal is much larger than the RX signal from the tag. The photograph of the implemented RF front-end system is shown in Fig. 7(a), and its evaluation setup for the reading range is shown in Fig. 7(b). The same antenna that was used in the analysis was also used in taking measurements.
IV. E XPERIMENTAL R ESULTS Fig. 8 shows the measured transfer characteristics of S31 and S21 . They are optimized for the maximum cancelation of three different frequencies namely, 840, 900, and 960 MHz, which are the lower and upper corners and center frequency of the
entire RFID bands from 840 to 960 MHz. The RF front end with the proposed adaptive TX leakage canceler exhibits high isolation characteristics over −80 dB from the TX to the RX [see Fig. 8(a)] and a good insertion loss from −0.39 to −0.7 dB for the TX signal [see Fig. 8(b)]. The microcontroller gives the control voltages of VPS , VCAP , and VRES using a ten-bit digital-to-analog converters so that they have control resolutions of about 0.27◦ , 8.22 fF, and 2 Ω per step, respectively. The three control voltages are sequentially adapted to minimize the goal function, which is the output voltage of the power detector. Because we used variable control step size, the total convergence after a start-up of the system took just about 114 iterations (about 5 s) under the worst initial conditions. By using the experimental setup shown in Fig. 7, the reading range was tested using the commercially available RFID tags, which are NXP’s UCODE G2XM and Alien’s ALN-9634. The test was done under general noisy laboratory environment. By using phase-reversal-amplitude-shift-keying (PR-ASK)modulated signal, the test was carried out at 915 MHz band, which is the center frequency of the U.S. band. As shown in Fig. 9, low residual TX leakage can be clearly observed with the proposed cancelation circuit. The backscattered signal from the tag is more clearly distinguished even in its power spectral density. Table I summarizes the test results for the reading range. For both the commercially available RFID tags, a significant improvement in the reading distance was achieved after applying the proposed leakage canceler. For the UCODE G2XM, a significant reading range improvement of 5.3–2.7 to 8 m was achieved at a TX output power of 24.6 dBm. Compared with the previously published data, our reader has much better reading distances with the proposed canceler using lower TX output power by more than 5 dB. V. C ONCLUSION In this paper, a new adaptive TX leakage canceler based on the direct leaky coupling method is proposed for the RF front ends of the UHF RFID readers. Unlike the conventional one, the proposed TX leakage canceler has a much simpler circuit configuration without a directional coupler, a power combiner, and an attenuator. The LCC across the TX and RX paths only consists of a variable resistor, a variable capacitor, and a fixed inductor in series. By using this simple circuit, it can tune the
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magnitude and the phase of the coupled signal from the TX path by over 30 dB and at least 90◦ , respectively. A phase shifter located on the RX path is required to tune only 270◦ of the phase of the leakage signal for complete phase coverage of 360◦ . The proposed circuit was implemented on a printed circuit board, and its size was 160.13 × 89.95 mm2 including control circuit. The size of the core front-end circuit was as small as 54 × 54 mm2 , excluding the adaptive control circuit. It was tested by using the one-tone signal and the PR-ASKmodulated signal. Significant reading range improvements for the representative RFID tags were achieved after applying the proposed leakage canceler. R EFERENCES [1] Y. Yao, J. Wu, Y. Shi, and F. F. Dai, “A fully integrated 900-MHz passive RFID transponder front end with novel zero-threshold RF-DC rectifier,” IEEE Trans. Ind. Electron., vol. 56, no. 7, pp. 2317–2325, Jul. 2009. [2] K. Choi, S. Yoo, M. Kim, H. Kim, S. Ryu, S. Kang, S. Jung, and Y. Yang, “CMOS DSB transmitter with low TX noise for RFID reader system-onchip,” IEEE Trans. Microw. Theory Tech., vol. 58, no. 12, pp. 3467–3474, Dec. 2010. [3] W. Wang and H. C. Luong, “A 0.8-V 4.9-mW 1.2-GHz CMOS fractionalN frequency synthesizer for UHF RFID readers,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 55, no. 9, pp. 2505–2513, Oct. 2008. [4] S. Jung, M. Kim, and Y. Yang, “Baseband noise reduction method using captured TX signal for UHF RFID reader applications,” IEEE Trans. Ind. Electron., vol. 59, no. 1, pp. 592–598, Jan. 2012. [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] 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. [7] 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. [8] W. Kim, W. Na, J. Yu, and M. Lee, “A high isolated coupled-line passive circulator for UHF RFID reader,” Microw. Opt. Technol. Lett., vol. 50, no. 10, pp. 2597–2600, Oct. 2008. [9] 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. 10, pp. 1973–1975, Jul. 2008. [10] M. Lee, “Lumped directional coupler with a varactor tuned reflector for RFID applications,” IEICE Electron. Exp., vol. 6, no. 2, pp. 129–134, Jan. 2009. [11] S. Jung, M. Kim, and Y. Yang, “A reconfigurable carrier leakage canceler for UHF RFID reader front-ends,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 58, no. 1, pp. 70–76, Jan. 2010. [12] J. Jung, H. Roh, J. Kim, H. Kwak, M. S. Jeong, and J. Park, “TX leakage cancellation via a micro controller and high TX-to-RX isolation covering an UHF RFID frequency band of 908-914MHz,” IEEE Microw. Wireless Compon. Lett., vol. 18, no. 10, pp. 710–712, Oct. 2008. [13] T. Xiong, X. Tan, J. Xi, and H. Min, “High TX-to-RX isolation in UHF RFID using narrowband leaking carrier canceller,” IEEE Microw. Wireless Compon. Lett., vol. 20, no. 2, pp. 124–126, Feb. 2010.
Min-Su Kim was born in Seoul, Korea, in 1978. He received the Ph.D. degree in information and communication engineering from Sungkyunkwan University, Suwon, Korea, in 2012. Since March 2012, he has been with Samsung Electronics Company Ltd., Yongin, Korea, where he is currently a Senior Engineer. His current research interests include transceivers for mobile terminals.
Sung-Chan Jung was born in Seoul, Korea, in 1973. He received the Ph.D. degree in information and communication engineering from Sungkyunkwan University, Suwon, Korea, in 2006. From 2006 to 2007, he was a Postdoctoral Researcher with the Microwave Circuits and Systems (MCS) Laboratory, Sungkyunkwan University. From 2007 to 2008, he was a Postdoctoral Fellow with the iRadio Laboratory, Schulich 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 the Security Solution Division, Samsung Techwin Company Ltd., Seongnam, Korea, where he is currently a Senior Researcher. His current research interests include the design of high-power amplifiers, linearization techniques, efficiency enhancement techniques for base stations and mobile terminals, passive circuit optimization, and radiofrequency identification circuit design.
Jonghyuk Jeong was born in Jeonju, Korea, in 1977. He received the M.S. degree in electrical, electronic, and computer engineering from Sungkyunkwan University, Suwon, Korea, in 2009. He is currently with Peopleworks Incorporated, Seoul, Korea. His research interests include power amplifier design and efficiency enhancement and linearization techniques.
Hyungchul Kim was born in Chuncheon, Korea, in 1983. He received the B.S. and M.S. degrees in electrical, electronic, and computer engineering from Sungkyunkwan University, Suwon, Korea, in 2008 and 2010, respectively. He is currently working toward the Ph.D. degree at Sungkyunkwan University. His research interests include RF power amplifier design, RF-identification-tag integrated circuit design, low-power analog/mixed signal circuit design, and power converter design.
Mincheol Seo was born in Seoul, Korea, in 1983. He received the M.S. degree in information and communication engineering from Sungkyunkwan University, Suwon, Korea, in 2011. He is currently working toward the Ph.D. degree at Sungkyunkwan University. His current research interests include design of radio-frequency power amplifiers, linearization techniques, and efficiency enhancement techniques.
Junghyun Ham was born in Seoul, Korea, in 1980. He received the M.S. degree in electrical and computer engineering from Hanyang University, Seoul, Korea, in 2009. He is currently working toward the Ph.D. degree in electrical, electronic, and computer engineering at Sungkyunkwan University, Suwon, Korea. From 2009 to 2011, he was with LG Electronics, Seoul, Korea, where he was involved in the development of high-efficiency power amplifiers for mobile handset applications. His research interests include high-efficiency RF transmitters, high-speed dc–dc converters, and complementary metal–oxide–semiconductor RF power amplifiers.
KIM et al.: ADAPTIVE TX LEAKAGE CANCELER FOR UHF RFID READER FRONT END
Cheon-Seok Park was born in Seoul, Korea, in 1960. He received the B.S. degree in electrical engineering from Seoul National University, Seoul, Korea, in 1988 and the M.S. and Ph.D. degrees in electrical and electronic engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Korea, in 1990 and 1995, respectively. He is currently a Professor with the School of Information and Communication Engineering, Sungkyunkwan University, Suwon, Korea. His research interests include design of radio-frequency power amplifiers, linearization techniques, and efficiency enhancement techniques.
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Youngoo Yang (S’99–M’02) was born in Hanyang, Korea, in 1969. He received the Ph.D. degree in electrical and electronic engineering from Pohang University of Science and Technology, Pohang, Korea, in 2002. From 2002 to 2005, he was with Skyworks Solutions Inc., Newbury Park, CA, USA, 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. His research interests include power amplifier design, RF transmitters, RF integrated circuit (IC) design, IC design for RF identification/ubiquitous sensor network systems, and modeling of high-power amplifiers or devices.
