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IJMOT-2006-12-228 © 2007 ISRAMT
A Novel Adaptive Baseband Digital Predistortion Technique
Jianfeng Zhai,Ningde Xie,Jianyi Zhou, Jianing Zhao, Lei Zhang, and Wei Hong
State Key Laboratory of Millimeter Wave, Dept. of Radio Engineering, School of Information Science and Engineering, Southeast University
Nanjing, 210096, P.R. China, E-mail: [email protected]
Abstract- The digital predistortion technique is an effective approach for suppressing the inter-modulation distortion (IMD) products of high power amplifiers (HPA) in modern mobile communication systems. In this paper, a novel baseband digital predistortion technique based on the complex look-up table (LUT) is presented and experimentally verified. The performance of the power amplifier is improved significantly with this technique. Experimental results show that about 6 to 11dB ACPR reduction could be achieved for different kinds of cdma2000 or WCDMA signals. Index Terms- digital predistortion , look-up table, HPA
I. INTRODUCTION In modern digital communication systems, it is more and more important to improve the efficiency and linearity of a HPA in the transmitting chain. Usually, advanced communication systems are very sensitive to the nonlinearity of the HPA, because they often adopt spectral efficient digital modulation schemes with relatively high peak-to-average power ratio (PAR) just like CDMA, OFDM, etc. The signal with high PAR will be seriously distorted by a nonlinear HPA. Distortion in transmission band may worsen signal-receiving, while distortion out of the transmission band interferes with the signals transmitted at adjacent channels, which is called Adjacent Channel Interference (ACI). There are many linearization techniques can be used for linear amplification, such as the power back-off method [1]-[3], the feedforward method [4]-[7], the RF analog predistortion method [8]-[9], the feedback method [10]-[12], the digital predistortion (DPD) method [13]-[23], etc.
Among these techniques, the DPD is one of the most important linearization methods for its high performance and reliability. In this paper, a baseband digital predistortion system based on look-up table (LUT) is developed, and the experimental results are presented.
II. THE BASEBAND DIGITAL PREDISTORTION
The basic principle of the baseband digital predistortion is to adjust the baseband signal before the modulator, the up-converter and the HPA by digital signal processing (DSP) techniques, so that the desired linear response is created by the cascade of the digital predistortion response |)
inF(|V and the power amplifier
response |)(|p
VG , shown in Fig.1.
Predistortion
PA
|)(| inVF |)(| pVG
inV outVpV
|)(| inVF |)(| pVG
Fig.1. The basic principle of DPD
In order to simplify the DPD system, a typical memoryless model of PA is adopted, which is dependent on the input signal magnitude of the PA [24]. As shown in Fig.1, |)
inF(|V is the
complex predistortion function of the input signal
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magnitude, which can be implemented in the FPGA by a complex multiplier, a complex gain LUT and an address generation block. The response of the power amplifier |)(|
pVG , as a
function of instantaneous magnitude of the input signal, is memoryless and nonlinear in amplitude and phase. If |)
inF(|V is chosen properly, the
cascade of |)in
F(|V and |)(|p
VG will be a
constant k, which means the linear response of the cascaded circuit will be resulted. The relation can be expressed as
kVGVF pin =|)(||)(| (1) where k is a constant, and
|)(| ininp VFVV = (2)
As shown in Fig.2 and Fig.3, the digital predistortion system includes the DACs, the Digital Predistortion Module, the ADCs, a modulator, a demodulator, an up converter, a down converter, and a HPA. The system clock is 60MHz provided by the phase-locked-loop (PLL) in the FPGA. In this system, by 60MSPS sampling, AD9862 converts the analog baseband input signal to digital complex baseband signal, and then the digital baseband signal is multiplied by complex coefficients in LUT before DAC. The address of LUT is usually distributed across the range of the input signal magnitude using a squared or logarithmic relationship [18]-[24].
Here we assume that the input signal of digital predistortion module is expressed by
ininin jQIV += after sampling and the complex gain is expressed by LUTLUTLUT jQIV += with the relative address in LUT that
is 22inin QIR += . The calculation of the DPD’s
output signal defined by equation outoutout jQIV += can be described as follows:
)()()()(
LUTinLUTinLUTinLUTin
LUTLUTinin
LUTinout
IQQIjQQIIjQIjQI
VVV
++−=+×+=
×=
LUTinLUTinout QQIII −= (3) and LUTinLUTinout IQQIQ += (4) The equation (3) and (4) can be implemented easily in the FPGA by some multiplexers and adders with pipelining. Fig.4 depicts how the digital predistortion module works in detail. The first step is training: by sending a training signal known to us, the digital complex feedback training signal is obtained after down converting, demodulating and sampling. Secondly, the digital feedback training signal is synchronized with the sending training signal in sync block, where the main task is calculating the correlation between the input signal and the feedback signal. Finally, complex gain LUT coefficients are calculated by adaptive algorithm and interpolation polynomial is employed to more precisely align the input and feedback signals. [16]-[22]
inV
outV
fV
|)(| pVG
fI fQ
inI
inQ
outI
outQ|)(| inVF
Fig.2. Block diagram of the digital predistortion system
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Fig.3. Photograph of the digital predistortion system
22inQinIR +=
inI
inQ
fI
fQ
outI
outQ
Fig.4. Digital Predistortion Module
The adaptive algorithm [21]-[23] is to get proper |)
inF(|V function and make the open loop
gain of the DPD system satisfy the equation:
loopGpVGinVF
inV
fVh === |)(||)(| (5)
where loopG is the desired linear gain of the open
loop. In order to counteract the loopG , we calculate the open loop gain with,
loopGinV
fV
loopG
pVGinVFH ==
|)(||)(| (6)
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Recall that the desired linear response of the predistorter and amplifier cascade requires that (1) for all inputs. Hence, if loopG is set to be equal to k, the desired open loop gain of the system is unity. If the calculated open loop gain is not equal to unity, the predistortion function must be adjusted to drive the open loop towards unity as illustrated in Fig.5 [22]-[23]. Generally three or four iterations are required to obtain better improvement of linearity. [16]
Fig.5. Calculation of new predistortion function.
III. EXPERIMENTAL RESULTS
The adaptive algorithm chosen for implementation is based on the use of a training signal, as shown in Fig.6. And Fig.7 shows the training signal multiplied by LUT coefficients.
Fig.6. I/Q training signal
Fig.7. I/Q training signal multiplied by LUT In order to evaluate the baseband digital predistortion system, the cdma2000 signal and W-CDMA signal are applied to the system. The results are shown in Fig.8 and Fig.9, respectively. In the following figures, the red curves represent the output spectra of the HPA without the DPD system, while the blue curves for that with the DPD system.
Fig.8. Output Spectra of the HPA for 9CH-cdma2000
Fig.9. Output Spectra of the HPA for 64DPCH W-CDMA
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IV. CONCLUSION An efficient baseband digital predistortion system is developed in this paper, and the complex gain LUT and the adaptive algorithm of the DPD system can be implemented in FPGA easily and directly. The experimental results show that the DPD system expresses efficient improvement to the linearity of a PA. About 6 to 11dB ACPR reduction could be achieved for different kinds of cdma2000 or WCDMA signals.
ACKNOWLEDGMENT This work is supported by State Key Laboratory of Millimeter Wave, Southeast University. The authors would like to thank Ming Shi for providing technical support.
REFERENCES
[1] Jun Sung Park, Sung Ryul Park, Hee Jung Roh and Kyung Heon Koo, “Power amplifier back-off analysis with AM-to-PM for millimeter-wave OFDM wireless LAN”, IEEE Radio and Wireless Conference, Aug. 2001.
[2] Bertran, E., Berenguer, J., “Analogue adaptive feedback lineariser”, Microwave Conference 2003, 33rd European, 2003
[3] S. Cripps, RF Power Amplifiers for Wireless Communications, Artech House, 1999.
[4] R. D. Stewart and F. F. Tusubira, “Feedforward linearization of 950MHZ amplifiers,” Inst. Elec. Eng. Proc., vol. 135, pt. H, no. 5, pp. 347-350, Oct. 1988.
[5] E. Eid, F. Ghannouchi, and F. Beauregard, “Optimal feed-forward linearization system design,” Microw. J., pp. 78–84, Nov. 1995.
[6] J.K.Cavers, “Adaptation Behaviour of a Feedforward Amplifier Linearizer”, IEEE Transactions on Communications, Vol. 44, No.1, pp. 31-40, February 1995.
[7] S.Kumar and G.Wells, “Memory Controlled Feedforward Lineariser Suitable for MMIC Implementation,” IEEE Proceedings-H., Vol.138, No. 1, pp. 9-12, February 1991.
[8] Yi J., Yang Y., Park M., Kang W., Kim B., “Analog Predistortion Linearizer for High Power RF Amplifier,” IEEE MTT-S International Microwave Symposium Digest,3, 2000.
[9] Benedetto M.D., Mandarini P., “New Analog Predistortion Criterion with Application to High Efficiency Digital Radio Links,” IEEE Transactions on Communications, Dec,1 995.
[10] Hsieh C C and Strid E. “A S-band high power feedback amplifier [J]”. IEEE MTT-S Internation
Microwave Symposium, San Diego, CA. June 1997, pp. 182~184.
[11] M. Johansson and T. Mattson, “Transmitter linearization using Cartesian feedback for linear TDMA modulation,” 41st IEEE Trans. Vehicular Technology Conf., St. Louis, MO, May 1991, pp. 155–160.
[12] K. Voyce and J. McCandless, “Power amplifier linearization using IF feedback,” IEEE MTT-S Dig., 1989, pp. 863–866.
[13] F. Zavosh, et al.: “Digital Predistortion Techniques for RF Power Amplifiers with CDMA Applications,” Microwave Journal, October 1999.
[14] A. S. Wright and W.G. Durtler, “Experimental Performance of an Adaptive Digital Linearized Power Amplifier,” IEEE transactions on Vehicular Technology, Vol. 41, No. 4, November 1992, pp. 395–400.
[15] Kelly Mekechuk, et al.: “Linearizing Power Amplifiers Using Digital Predistortion, EDA Tools and Test Hardware,” High Frequency Electronics, April 2004.
[16] Agilent Application Note, Guide to Digital Predistortion, December 2003. Exhibitors & Publishers Ltd., Kent, U.K., pp. 671-676, Sept. 1981.
[17] J.K. Cavers: “Amplifier linearization using a digital predistorter with fast adaptation and low memory requirements”, IEEE Trans. Vehicular Tech., Nov. 1990.
[18] J.K. Cavers: “Optimum indexing in predistorting amplifier linearizers,” 1997 IEEE 47th Vehicular Technology Conference Technology in Motion, IEEE. Part vol.2, 1997, pp.676-80 vol.2.
[19] M. Faulkner and M. Johansson, “Adaptive linearization using predistortion-Experimental results”, IEEE Trans. Veh. Technol., vol. 43, no. 2, pp. 323-332, May 1994.
[20] Kathleen J. Muhonen, Mohsen Kavehrad, and Rajeev Krishnamoorthy, “Look-Up Table Techniques for Adaptive Digital Predistortion: A Development and Comparison”, IEEE Trans. Vehicular Tech.vol.49,No.5, Sept.2000.
[21] Intersil Application Note, “Adaptive Predistortion Using the ISL5239,”AN1028, September 2002.
[22] Wan-Jong Kim and Shawn P. Stapleton,Kyoung Joon Cho and Jong Heon Kim, “Digital Predistortion of a Doherty Amplifier with a Weak Memory within a Connected Solution”, IEEE 60th Vehicular Technology Conference 2004
[23] Ming Shi, Ming Chen, Lei Zhang and Jianyi Zhou, “A New Baseband Digital Predistortion System”, APMC2005 Proceedings.
[24] A. Saleh, “Frequency-independent and frequency-dependent nonlinear models of TWT amplifiers,” IEEE Trans. Commun., vol. COM-29, pp.1715–20, Nov. 1981.
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