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Energyefficient digital predistortion with lookup table training using analog Cartesian feedback
 IEEE Trans. Microw. Theory Tech
, 2008
"... Abstract—We demonstrate energyefficient lowcomplexity adaptive linearization for wideband handset power amplifiers (PAs). Due to power overhead and complexity, traditional wideband linearization techniques such as adaptive digital predistortion (DPD) thus far have not been used for wideband handse ..."
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Cited by 4 (2 self)
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Abstract—We demonstrate energyefficient lowcomplexity adaptive linearization for wideband handset power amplifiers (PAs). Due to power overhead and complexity, traditional wideband linearization techniques such as adaptive digital predistortion (DPD) thus far have not been used for wideband handset transmitters. Our energyefficient lookup table training strategy resulted in a training energy of 1.83 nJ/entry for a 5MHz bandwidth WiMAX orthogonal frequency division multiple access (OFDMA) transmission, which represents more than 40 improvement over stateoftheart DPD implementations. Our experimental prototype transmitter achieves a maximum of 9.9dB improvement of adjacent channel leakage power at 5.15MHz offset with 22.0dBm channel power in the 5MHz bandwidth WiMAXOFDMA transmission. This linearity improvement offers 26.5 % savings in PA power consumption by reducing power backoff. Index Terms—Adaptive linearization, adaptive predistortion, Cartesian feedback, digital predistortion (DPD), lookup table (LUT), power amplifier (PA) linearization, wideband handset PA, wideband predistortion, WiMAX. I.
Cartesian Feedback for HighBandwidth Power Amplifier Linearization
"... presently being investigated to improve PA linearity and power efficiency. One such linearization technique, Cartesian feedback (CFB), makes use of analog feedback to linearize a nonlinear plant (Figure 1). The commanded baseband symbol, I and Q, are compared to the symbol being transmitted by the P ..."
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presently being investigated to improve PA linearity and power efficiency. One such linearization technique, Cartesian feedback (CFB), makes use of analog feedback to linearize a nonlinear plant (Figure 1). The commanded baseband symbol, I and Q, are compared to the symbol being transmitted by the PA, the baseband symbol is predistorted to correct for PA nonlinearities. Due to significant delay through the PA and parasitic loop poles, the loop must be compensated (H(s) in Figure 1) to ensure an adequate phase margin. The loop dynamics inherently restrict the baseband symbol bandwidth. This limitation is juxtaposed with CFB's insensitivity to changes in PA characteristics over time, temperature, and load impedance. Given the difficulty in modeling these PA characteristics, CFB is attractive compared to other linearization methods requiring detailed PA models [1]. Much work has been done with Digital Predistortion (DPD) linearization techniques. These techniques rely on a mapping of baseband symbols to predistorted symbols. This mapping makes use of a predetermined inversion of the PA model. Transmitters using DPD are openloop systems, supporting very large symbol bandwidths. However, these techniques make use of cumbersome PA models and cannot compensate for changes in PA dynamics. To deal with this limitation, adaptive DPD schemes, in which the predistortion mapping is periodically updated or refined, are employed. This solution still requires a significant amount of PA modeling and the addition of powerhungry DSP hardware [2]. We consider a linearization system in which a CFB loop is used as an analog computer, providing a DPD system with a symbol mapping (Figure 2). The CFB loop is closed periodically on a lowbandwidth set of symbols, training the DPD system. When the CFB system has finished training the digital predistorter, the feedback loops are broken, leaving an openloop system. So equipped, the DPD system can be used to linearize the PA with highbandwidth symbol streams [3]. Figure 1: Conceptual Cartesian feedback linearization for a quadrature modulation transmitter [1].
DIGITAL CARTESIAN FEEDBACK LINEARIZATION OF SWITCHED MODE POWER AMPLIFIERS
"... The efficient use of the power budget in small satellite applications is of primary importance because of the reduced size of the power sources. Unfortunately, high efficiency in power amplifiers (PA) is strongly related to nonlinearities and hence distortion. In this paper a digital architecture f ..."
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The efficient use of the power budget in small satellite applications is of primary importance because of the reduced size of the power sources. Unfortunately, high efficiency in power amplifiers (PA) is strongly related to nonlinearities and hence distortion. In this paper a digital architecture for the linearization of switched mode PAs is implemented, and a circuit to detect the phase shift and the CORDIC algorithm to correct that phase as well as to compute the magnitude are developed. The results show that linearity is improved by 20 dB for a system with a bandwidth of 9.6 kHz for a loop gain of 10 and 400 ns of system delay. The circuit, implemented in a FPGA consumes a total of 33.3 mW. Index Terms — Cartesian feedback, linearization, phase alignment, CORDIC.