received the M.S. degree in electrical and electronic engineering and Ph.D. degree in engineering from the. Escuela Superior de Ingenieros (ESI), University of.
IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 8, AUGUST 2005
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An Adaptive Feedforward Amplifier Under “Maximum Output” Control Method for UMTS Downlink Transmitters Jon Legarda, Jorge Presa, Erik Hernández, Héctor Solar, Jaizki Mendizabal, and José A. Peñaranda
Abstract—An adaptive feedforward amplifier is implemented for a Universal Mobile Telecommunication System (UMTS) downlink channel (2110–2170 MHz). An exhaustive characterization of the space of solution has corroborated the feedforward theoretical behavior. As a result, an alternative adjustment method is proposed, called “maximum output,” which entails maximizing a specific designed objective function. Besides this, an adaptive control system, based on distortion signal minimization architecture, has been fabricated in order to apply the designed method. The overall system performance achieves significant improvements such as 16.7 dB on the third-order intermodulation product, measured with a 5-MHz separated two-tone signal, 15 dB on the adjacent channel leakage ratio level and 2.7 dB on the output power level, both of them obtained with UMTS test model 1 (64 channels). The maximum output control method allows fulfilling any standard linearity specification while power efficiency is maximized so the desired tradeoff between linearity and efficiency is achieved. Index Terms—Adaptable control system, feedforward amplifier, output improvement, power minimization, Universal Mobile Telecommunication System (UMTS), wide-band code division multiple access (WCDMA).
I. INTRODUCTION
T
HE spread-spectrum technology used in the Universal Mobile Telecommunication System (UMTS) communication standard allows, among other benefits, high spectral efficiency. However, this fact means high amplitude variations in the RF signals, thus, for high linear power amplifiers, are required in UMTS transmitters. The power amplifier is the basic component in any wireless transceiver so its efficiency directly defines the overall transmission power efficiency. As the power amplifier operates near the saturation region, efficiency is increased, but linearity is degraded so a tradeoff between efficiency and linearity must be made. Usually, linearization techniques are the best solution in order to improve the linearity of power amplifiers. There are many of them that have been widely reported [1] from which feedforward is the most suitable for wide-band applications with high distortion constraints [2]. The feedforward technique tries to minimize the distortion introduced by the power amplifier without taking into considering
Fig. 1. Fabricated feedforward linearization architecture. Two fixed delays, variable attenuators, and phase shifters are used for the correct system adjustment and an extra directional coupler provides an error signal sample.
the fact that most of the linearity specifications of communication standards are fixed. This fact entails any alternative adjustment procedures in order to maximize the transmitter system efficiency and fulfill the linearity constraints at the same time. In this paper, a feedforward amplifier implementation is shown and an alternative control method, called “maximum output,” has been developed. As a consequence of the proposed method requirements, an adaptive control system, based on distortion signal minimization, has been implemented. The maximum output method achieves the long-awaited tradeoff between linearity and efficiency, and some results are presented in order to validate it. II. ADAPTIVE FEEDFORWARD AMPLIFIER An adaptive feedforward amplifier has been fabricated for UMTS downlink (2110–2170 MHz). The implemented linearization architecture is shown in Fig. 1. Two fixed-delay elements, two variable attenuators, and two phase shifters have been used for the group delay, amplitude, and phase adjustments, respectively. First of all, fixed-delay values have been calculated in order to level off the group delays of signal and distortion cancellation loops branches [3], [4]. Secondly, amplitude and phase adjustments have been calculated, maximizing both loops cancellation levels. III. SPACE OF SOLUTIONS CHARACTERIZATION
Manuscript received June 1, 2004; revised January 24, 2005. This work was supported in part by IKUSI Angel Iglesias S.A. and by the Basque Country Government under the Amplifier Linearization for Universal Mobile Telecommunication System Mobile Communication Standard Repeaters Project. The authors are with the Radio Frequency Integrated Circuits Group, Department of Electronics and Communications, Centro de Estudios e Investigaciones Técnicas (CEIT) and Technological Campus, University of Navarra (TECNUN), 20018 San Sebastian, Spain. Digital Object Identifier 10.1109/TMTT.2005.852780
An exhaustive analytical analysis shows how the feedforward amplifier adjustments are able to influence main and distortion output signals [5]. The theoretical conclusions are verified with the fabricated prototype thanks to the characterization of the space of solution. Data acquisition has been carried out with Agilent Technologies’ VeePro software. Both loops’ phase shifts and attenuation voltages have been swept in an alternate way
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Fig. 2. Output ACLR level according to: (top) signal and (bottom) distortion cancellation loop adjustment parameters. The ACLR variation range is between 14–22 dB, respectively. Attenuation and phase-shift values represent the voltage fraction used in data acquisition.
and, at the same time, output main and distortion signals have been measured. All values are stored in four dimension matrixes so that the signals behaviors could be analyzed with the MATLAB software. This procedure has been done for different input signal levels, from 3 to 8 dBm in 1-dB steps. The UMTS signal that has been used in the characterization is test model 1 with 64 channels [6]. Attenuation and phase-shift values, shown below, represent the voltage fraction used in data acquisition. A. Feedforward Amplifier Behavior The influence of signal (loop 1) and distortion (loop 2) cancellation loops adjustment parameters on output adjacent channel leakage ratio (ACLR) and main signal levels are shown in Figs. 2 and 3, respectively. The conclusions deduced from the whole characterization process are as follows. • The feedforward standard adjustment technique, known as distortion “maximum cancellation,” is based on maximizing the output ACLR level. During the adjustment process, the signal cancellation loop adjustment parameters minimize the error signal, and the distortion cancel-
Fig. 3. Output main signal level according to: (top) signal and (bottom) distortion cancellation loop adjustment parameters. The output signal power variation is between 5–4 dB, respectively. Attenuation and phase-shift values represent the voltage fraction used in data acquisition.
lation loop adjustment parameters maximize the distortion cancellation, therefore, maximizing the output ACLR level. • However, an alternative adjusting method could maximize the efficiency (output main signal level), while the linearity specification (output ACLR level) fulfills the constraint established by the standard. B. Maximum Output Method The maximum output method consists of maximizing the output main signal level provided that the UMTS linearity specification is fulfilled [6]. It uses both loop adjustments alternately, which allows the feedforward amplifier to adapt to any standard specification (in this case, 45 dB), although it involves higher control complexity. Figs. 4 and 5 show how maximum cancellation and maximum output techniques affect both the output ACLR and main signal levels, respectively.
LEGARDA et al.: ADAPTIVE FEEDFORWARD AMPLIFIER UNDER “MAXIMUM OUTPUT” CONTROL METHOD
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Fig. 4. Feedforward amplifier output ACLR level under two different adjustment techniques according to the input power level. The maximum cancellation method shows the typical feedforward amplifier behavior, whereas the maximum output method allows the fulfillment of the linearity constraint of any communication standard. In this case, the UMTS linearity specification is given by the ACLR 45-dB level.
Fig. 6. Feedforward and control system block diagram. Two different receivers are used, one for each cancellation loop. Both of them are controlled by the same local oscillator, which is controlled by the microcontroller. Two analog-to-digital converters are used in down conversion and four digital-to-analog converters are used in the adjustment voltages.
Fig. 5. Feedforward amplifier output main signal level under two different adjustment techniques according to the input power level. The maximum cancellation method shows the typical power amplifier behavior, whereas the maximum output method maximizes the system output level provided that the UMTS ACLR specification is fulfilled. Once the cancellation level exceeds the 45-dB level, output power is limited by this constraint.
IV. CONTROL SYSTEM The maximum output method requires an adaptive control system, which maximizes the output main signal level while maintaining the ACLR specification since main and distortion signals must be measured accurately. There are three possible control architectures reported, i.e.: 1) pilot signal cancellation [7]–[10]; 2) signals correlation [11]–[15]; and 3) distortion signal minimization [16]–[20]. The last one has been selected owing to the fact that it allows precise signal measurements. The fabricated control system architecture is shown in Fig. 6. This architecture consists of two RF receivers, digital-to-analog converters, and analog-to-digital converters. All of them are controlled by a microcontroller and communicated by I C protocol [21]. The maximum output method is based on maximizing an varies deobjective function designed specifically, where pending on input power dBm
dB
dB
(1)
K
Fig. 7. Maximum output objective function ( = 10) according to distortion cancellation loop adjustment parameter. The maximum gives the operating point: 45-dB ACLR level and maximum output main signal power. Attenuation and phase-shift values represent the voltage fraction used in data acquisition.
The objective function does not have a unique maximum (Fig. 7) so standard heuristic methods, like hill climbing, must be used carefully. V. RESULTS AND COMPARISONS A feedforward amplifier has been analyzed under UMTS standard specifications. An alternative control method has been designed, and maximum output and a specific control system have been fabricated based on distortion signal minimization. Now some results are presented and suitable comparisons are made using both two-tone and wide-band code-division multiple-access (WCDMA) signals.
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Fig. 9. Feedforward ACLR level in comparison with ERA-5SM for the same output level once the maximum cancellation method is applied. The distance between both curves indicates the improvement on the ACLR level.
Fig. 8. (top) ERA-5SM and (bottom) feedforward amplifier third-order , respectively. The feedforward intermodulation-distortion distance amplifier has increased by 33.5 dB, which involves a 16.7-dB parameter. improvement on the
IP
1IM
(1IM)
A. Third-Order Intermodulation Product (
) Improvement
A two-tone signal is used, settled 2142.5 and 5 MHz apart. is shown, as recommended in [22]. However, in Fig. 8, only The feedforward amplifier has increased the third-order interby 33.5 dB for the same modulation-distortion distance output power level, which implies, according to (2), a 16.7-dB parameter improvement on the (2)
Fig. 10. (top) ERA-5SM and (bottom) feedforward amplifier output signals, respectively, once maximum cancellation method is applied. A 15-dB improvement on the ACLR level is achieved.
B. Improvement With WCDMA Signals
C. Improvement Under UMTS Specifications
High crest factors of WCDMA signals, which are between 8–18 dB, reduce the cancellation levels achieved with the two-tone signals due to the fact that the power amplifier operates near the saturation region [5]. The WCDMA signal used is the test model 1-64DTCH-defined by the UMTS standard. The center frequency is 2142.5 MHz, channel bandwidth is 3.84 MHz, adjacent channels are at 5 MHz, ACLR is measured in the 3.84-MHz bandwidth, and peak/average is 10 dB for 0.01% probability on the complimentary cumulative distribution function (CCDF). If the feedforward amplifier is adjusted by the maximum cancellation method, as is usually done, then it seems to be the best option in order to obtain maximum ACLR improvement. However, as has been shown before, it is not able to guarantee any standard linearity specification. In Fig. 9, the ACLR level is presented according to the output power level. The distance between both curves represents the improvement on the ACLR level. The maximum ACLR improvement is 15 dB, as shown in Fig. 10, for the same output level.
The maximum cancellation method provides interesting results in comparison with the state-of-the-art [23], [24] on ACLR improvement. However, the feedforward amplifier must work under strict linearity specifications and variable operating conditions. Undesired imbalances have to be compensated in order to obtain the long-awaited tradeoff between linearity and efficiency. In this case, the feedforward amplifier is adjusted by the maximum output method and the ACLR limit is set at 45 dB. In Fig. 11, the output main signal power is shown according to the ACLR level. In this case, both amplifiers operate under UMTS standard specifications, as they work in the deep saturation region. The improvement on output power is 2.7 dB, while guaranteeing a 45-dB ACLR level, thus, the efficiency is 1.8 times higher. In contrast with this, the total power consumption has increased due to the error amplifier and control system Efficiency
output power input power input power
(3)
LEGARDA et al.: ADAPTIVE FEEDFORWARD AMPLIFIER UNDER “MAXIMUM OUTPUT” CONTROL METHOD
Fig. 11. Feedforward output power level in comparison with ERA-5SM for the same ACLR level once the maximum output method is applied. Distance between both curves indicates the improvement on output power.
For example, any commercial UMTS power amplifier with an efficiency of 7% (i.e., MHPA21010), after linearization and controlled by the maximum output control method, will guarantee the 45-dB ACLR level, increasing the efficiency up to 12%. VI. SUMMARY AND CONCLUSIONS A feedforward amplifier has been fabricated for UMTS transmitters. After an exhaustive characterization, reported analytical conclusions have been verified and an alternative adjustment method, the called maximum output, has been proposed. The presented method requires an adaptable control system, which has been implemented based on distortion signal minimization. Interesting improvements have been obtained like 16.7 dB in the parameter, 15 dB in the ACLR level, and 2.7 dB in output power. The maximum output control method also allows the feedforward amplifier to fulfill any standard linearity specification while maximizing its efficiency. In this case, the UMTS standard defines the minimum ACLR as 45 dB so the adaptable control guarantees this specification while improving efficiency 1.8 times. REFERENCES [1] P. B. Kenington, High-Linearity RF Amplifier Design. Norwood, MA: Artech House, 2000. [2] J. K. Cavers, “Adaptation behavior of a feedforward amplifier linearizer,” IEEE Trans. Veh. Technol., vol. 44, no. 1, pp. 31–40, Feb. 1995. [3] J. Presa, J. Legarda, H. Solar, J. Meléndez, A. Muñoz, and A. GarcíaAlonso, “An adaptive feedforward power amplifier for UMTS transmitters,” presented at the 15th IEEE Int. Personal, Indoor and Mobile Radio Communications Symp., Barcelona, Spain, Sep. 2004. [4] K. J. Parsons and P. B. Kenington, “Effect of delay mismatch on a feedforward amplifier,” in Proc. Circuits Devices Systems, vol. 141, Apr. 1994, pp. 140–144. [5] A. H. Coskun and S. Demir, “A mathematical characterization and analysis of a feedforward circuit for CDMA applications,” IEEE Trans. Microw. Theory Tech., vol. 51, no. 3, pp. 767–777, Mar. 2003. [6] Base Station (BS) Conformance Testing (FDD), UMTS Standard TS 25.141, Release 6, V6.4.0, Dec. 2003. [7] R. Myer, “Automatic reduction of intermodulation products in high linear amplifier,” U.S. Patent 4 580 105, Apr. 1, 1986. [8] K. Peter, “Feedforward amplifier,” U.S. Patent 6 429 738, Aug. 6, 2002. [9] S. Narahashi, T. Nojima, M. Maeta, and K. Murota, “Feed-forward amplifier,” U.S. Patent 5 166 634, Nov. 24, 1992. [10] N. Shoichi, N. Toshio, and S. Yasunori, “Feedforward amplifier,” Eur. Patent Applicat. EP1 014 564, Jun. 28, 2000. [11] J. K. Cavers, “Adaptive feedforward linearizer for RF power amplifiers,” U.S. Patent 5 489 875, Feb. 6, 1996.
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[12] R. M. Bauman, “Adaptive feedforward system,” U.S. Patent 4 389 18, Jun. 21, 1983. [13] R. H. Chapman and W. J. Turney, “Feedforward distortion cancellation circuit,” U.S. Patent 5 051 704, Sept. 24, 1991. [14] P. B. Kenington, M. A. Beach, A. Bateman, and J. P. McGeehan, “Apparatus and method for reducing distortion in amplification,” U.S. Patent 5 157 345, Oct. 20, 1992. [15] T. E. Olver, “Adaptive feedforward cancellation technique that is effective in reducing amplifier harmonic distortion products as well as intermodulation distortion products,” U.S. Patent 4 560 945, Dec. 24, 1985. [16] H. Kenichi, I. Yukio, N. Junichi, S. Yuji, S. Haruyasu, and N. Masatoshi, “Feedforward amplifier,” Eur. Patent Applicat. EP 1126596, 2001-08-22. [17] R. E. Myer, “Feedforward linear amplifier,” U.S. Patent 4 885 551, Dec. 5, 1989. [18] W. H. Lieu, “Linear amplifier with automatic adjustment of feedforward loop gain and phase,” U.S. Patent 5 023 565, Jun. 11, 1991. [19] M. G. Oberman and J. F. Long, “Feedforward distortion minimization circuit,” U.S. Patent 5 077 532, Dec. 31, 1991. [20] K. S. Yoo, S. G. Kang, J. I. Choi, and J. S. Chae, “Optimal control method for adaptive feedforward linear amplifier,” U.S. Patent 6 232 837, May 15, 2001. [21] I C Bus Specification, Philips Semiconductor version 2.1, Jan. 2000. [22] “Optimizing dynamic range for distortion measurements,” Agilent Technol., Palo Alto, CA, Product Note 5980-3079EN, Nov. 2000. [23] J. W. Huh, I. S. Chang, and C. D. Kim, “Spectrum monitored adaptive feedforward linearization,” Microwave J., vol. 44, no. 9, Sep. 2001. [24] Y.-C. Jeong, Y.-J. Song, I.-J. Oh, and C.-D. Kim, “A novel adaptive Feedforward amplifier using an analog controller,” Microwave J., vol. 46, no. 4, Apr. 2003.
Jon Legarda was born in Eibar, Spain, in 1977. He received the M.S. degree in electrical and electronic engineering and Ph.D. degree in engineering from the Escuela Superior de Ingenieros (ESI), University of Navarra, San Sebastian, Spain, in 2001 and 2004, respectively. He is currently involved with power-amplifier linearization with the Centro de Estudios e Investigaciones Técnicas de Gipuzkoa (CEIT), San Sebastian, Spain.
Jorge Presa was born in Miranda de Ebro, Spain, in 1974. He received the Electronic Engineering degree from Valladolid University, Valladolid, Spain, in 1998, and the Ph.D. degree from the Centro de Estudios e Investigaciones Técnicas de Gipuzkoa (CEIT), San Sebastian, Spain in 2003. He was involved with railway electronics for highspeed trains with Patentes Talgo. In 2000, he joined the CEIT as a Researcher. He is currently involved with RF design for wireless communication systems and is focused on power-amplifier linearization.
Erik Hernández was born in San Sebastian, Spain, in 1976. He received the B.S. and M.S. degrees in electronic engineering from the Escuela Superior de Ingenieros (ESI), Navarra University, San Sebastian, Spain, in 1999, and the Ph.D. degree in monolithic voltage controlled oscillators for RF applications from the University of Navarra, San Sebastian, Spain, in 2002. In 1999, he joined the RF Integrated Circuit Design Group, Technological Campus, University of Navarra (TECNUN), San Sebastian, Spain. In 2002, he joined the Centro de Estudios e Investigaciones Técnicas de Gipuzkoa (CEIT), San Sebastian, Spain, where he is currently an Associated Researcher. He is currently involved with RD identification (RFID) proximity systems and their applications. His main interests also include the design and characterization of passive components and circuits in standard low-cost technologies.
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Héctor Solar was born in Portugalete, Spain. He received the Telecommunications Engineering degree from the University of Pais Vasco, Bilbao, Spain, in 2002, and is currently working toward the Ph.D. degree at the University of Navarra, San Sebastian, Spain. He then joined the Centro de Estudios e Investigaciones Técnicas de Gipuzkoa (CEIT), San Sebastian, Spain, where he is currently an Associate Researcher. His research interests include RF integrated circuits for wireless communication systems with particular attention to the design of power amplifiers in standard low-cost technologies.
Jaizki Mendizabal was born in Zarautz, Spain. He received the B.S. and M.S. degrees in electrical engineering from the University of Navarra, San Sebastian, Spain, both in 2000, and is currently working toward the Ph.D. degree at the Centro de Estudios e Investigaciones Técnicas de Gipuzkoa (CEIT), San Sebastian, Spain. In 2000, he then joined the Fraunhofer Institut für Integrierte Schaltungen, Erlangen, Germany, where he was an RF Integrated Circuits Designer. His research interests include monolithic RF design focusing on global navigation satellite systems (GNSSs), global positioning systems (GPSs), and GALILEO.
José A. Peñaranda was born in Benicarló, Spain, in 1965. He received the Telecommunication Engineering degree from the Universidad Politécnica of Madrid, Madrid, Spain, in 1991, and the Doctorate in Engineering from the University of Navarra, San Sebastian, Spain, in 1997. He began his professional career with various information technology companies such as Indra and IBM. Since 1997, he has been with the Centro de Estudios e Investigaciones Técnicas de Gipuzkoa (CEIT), San Sebastian, Spain, where he is involved with projects of applied research for industry. These projects have led to some papers, patents, and a spin-off in the field of image processing. During 2003, he was an adviser of technological politics for the Science and Technology Ministry of Spain. He currently combines his research activity with the teaching of radio communication with the School of Engineering, University of Navarra.
