Novel Technique for Wideband Digital Predistortion of Power Amplifiers With an Under-Sampling ADC
02 Oct 2014-IEEE Transactions on Microwave Theory and Techniques (IEEE)-Vol. 62, Iss: 11, pp 2604-2617
TL;DR: This paper proposes a novel technique, termed under-sampling restoration digital predistortion (USR-DPD), to linearize wideband power amplifiers (PAs) with ADCs that operate at sampling rates much lower than required by Nyquist limits for the predistorted band (under-sampled ADCs).
Abstract: Most conventional wideband digital predistortion (DPD) techniques require the use of a very high-speed analog-to- digital converter (ADC) in the feedback path. This paper proposes a novel technique, termed under-sampling restoration digital predistortion (USR-DPD), to linearize wideband power amplifiers (PAs) with ADCs that operate at sampling rates much lower than required by Nyquist limits for the predistorted band (under-sampling ADCs). The USR processing is implemented in an iterative way to restore full-band PA output information from the under-sampled output signal, allowing memory DPD models to be successfully extracted. The USR-DPD can operate in two modes: without and with a band-limiting filter in the feedback path. In comparison with conventional DPD techniques, the requirement for ADC sampling frequency can be significantly reduced using the USR-DPD approach. Experimental tests were realized for two PAs with numerous signals (10-, 20-, 40-, and 60-MHz long-term evolution signals) using different ADC sampling frequencies. The DPD with the under-sampling ADC could achieve comparable performances to its counterpart with a full-rate ADC, while using 3-5 times lower sampling frequency, and around -50-dBc adjacent channel power ratios were achieved.
Citations
More filters
TL;DR: A novel digital predistortion (DPD) with an ultralow feedback sampling rate is proposed for wideband wireless communications and results show that the proposed algorithm achieves almost equivalent results with only a 2.5 Msamples/s feedback sampled rate.
Abstract: A novel digital predistortion (DPD) with an ultralow feedback sampling rate is proposed for wideband wireless communications. Analytical groundwork is carried out to support the rationale of the proposed method. Due to the power amplifier (PA) spectral expansion, the sampling rate of the feedback loop in the DPD architecture generally requires several times the input signal bandwidth, which presents challenges for the analog-to-digital converter, especially in broadband applications. In order to remove this bottleneck in real-world implementation, a forward behavioral modeling method with low-rate aliasing PA output signals is adopted to estimate the model coefficients. The obtained coefficients are then used to predict the high-rate PA output, using the input signal with an adequate sampling rate, and to further generate the predistorter for the nonlinear PA with the conventional indirect learning architecture. To validate the proposed method, different PAs driven with a 40-MHz two-carrier Long Term Evolution are used for linearization purposes. Experimental results based on the instrument platform show that the proposed algorithm achieves almost equivalent results with only a 2.5 Msamples/s feedback sampling rate. Further experimental verification is also demonstrated on an FPGA platform, which achieves an adjacent channel power ratio that is better than −47 dBc for 40-MHz signals with a feedback sampling rate of 20 Msamples/s.
43 citations
Cites background from "Novel Technique for Wideband Digita..."
...Digital Object Identifier 10.1109/TMTT.2016.2602216 I. INTRODUCTION THE rapid development of wireless communicationsrequires wider bandwidths and more complex modulation schemes....
[...]
TL;DR: An efficient predistortion technique is proposed, which can be applied to reduce the impairments of A-RoF systems due to the combined effects of frequency chirp of the laser source and chromatic dispersion of the optical channel.
Abstract: Analog Radio-over-Fiber (A-RoF) communication technology constitutes a promising technique for next generation radio access networks thanks to its relatively low bandwidth requirements. Within this context, an efficient predistortion technique is proposed, which can be applied to reduce the impairments of A-RoF systems due to the combined effects of frequency chirp of the laser source and chromatic dispersion of the optical channel. The radio frequency signal is firstly put in digital form through an analog to digital converter, then the predistortion operation is realized, and finally the resulting signal is put again into analog form. A comprehensive analysis on the theoretical basis of the proposed approach is presented, together with the approximations introduced, which makes it practically realizable. The improvements on the quality of the received signal due to the proposed solution are illustrated with reference to scenarios of applicative interest. The performance of proposed technique is evaluated in terms of adjacent channel leakage ratio and error vector magnitude when long term evolution signal is applied at the input.
34 citations
TL;DR: The new proposed approach to synthesize the digital predistortion (DPD) function using an undersampling feedback signal is found to have comparable linearization capability compared to a conventional full-rate based indirect-learning DPD, even with a significantly undersampled feedback signal.
Abstract: This letter presents a new approach to synthesize the digital predistortion (DPD) function using an undersampled feedback signal. First, an expression for the DPD update algorithm that accommodates undersampling of the feedback signal is derived. This includes a direct learning algorithm that iteratively identifies the DPD function coefficients. Then, a delay estimation and alignment algorithm that employs a fractional delay filter is presented for estimating and compensating the non-integer delay between the sampled input and undersampled output signals of the power amplifier (PA). The new proposed approach is found to have comparable linearization capability compared to a conventional full-rate based indirect-learning DPD, even with a significantly undersampled feedback signal. For instance, it was successfully applied to linearize a 20 W GaN Doherty PA driven by a wideband modulated signal of up to 80 MHz bandwidth, and yield an ACLR of -49 dBc after linearization using a complex feedback signal sampled at 80 complex MSPs as opposed to 400 complex MSPs that would be required for conventional sampling.
29 citations
Cites background from "Novel Technique for Wideband Digita..."
...below the typical 5× bandwidth requirement to capture the PA output) [1]–[4]....
[...]
TL;DR: Experimental results show that the proposed approach can effectively linearize the PA and outperforms other undersampling DPD approaches using the conventional zero-IF feedback structure, in terms of adjacent channel leakage ratio.
Abstract: A novel direct-learning digital predistortion (DPD) of power amplifier (PA) is proposed to alleviate the requirement of an analog-to-digital converter (ADC) in the feedback path Different from existing undersampling methods, the proposed approach requires only a single ADC running at low sampling rate, which saves one-half hardware cost and avoids the I/Q imbalance problem of the conventional zero-IF feedback structure First, a modified Gauss-Newton iteration using a low-rate single-branch feedback signal is derived, which justifies the use of single undersampling ADC Second, a low-complexity time and phase synchronization algorithm based on pattern cross correlation is presented, to align the input and output samples Experimental results on a 10-W class-AB wideband PA and 100-MHz LTE-A test signal show that the proposed approach can effectively linearize the PA and outperforms other undersampling DPD approaches using the conventional zero-IF feedback structure, in terms of adjacent channel leakage ratio
29 citations
Cites background or methods from "Novel Technique for Wideband Digita..."
...In [1], an iterative approach is employed to restore the true PA output signal from band-limited feedback path, using an auxiliary transmitter....
[...]
...In the recent literature, much effort has been made to reduce the sampling rate requirement of ADC [1]–[5]....
[...]
TL;DR: In this paper, the operation of a frequency quadrupler as a transmitter to accurately generate amplitude and phase-modulated signals is demonstrated, and a quadrupled memory polynomial (Q-MP) model is derived for forward modeling.
Abstract: We demonstrate the operation of a frequency quadrupler as a transmitter to accurately generate amplitude-and phase-modulated signals. Digital predistortion (DPD) is used to facilitate reconstruction of the desired transmitted waveforms at the frequency quadrupler output. Such a process enables novel architectures for higher frequency transmissions without high-frequency mixers or modulators, especially for millimeter-wave (mm-wave) communications, at frequencies where signal amplification, modulation, and distribution are challenging. To investigate the frequency quadrupler input–output relationship and the signal characteristics, a prototype frequency quadrupling system at 3.56-GHz band is demonstrated. A quadrupled memory polynomial (Q-MP) model is derived for forward modeling. Subsequently, a quadrupling DPD (Q-DPD) model is developed to find the inverse of the frequency quadrupler for linearization. Further, a memory polynomial (MP) and Q-DPD cascaded technique is proposed to achieve more accurate linearization performance of the frequency quadrupler. The proposed methods are demonstrated using signals with different bandwidths (5, 10, 20 MHz) and different order modulations [64 and 256 quadrature amplitude modulation (QAM)]. Very good adjacent channel power ratios (ACPRs) of around −43 dBc and error vector magnitudes (EVMs) of about ${{1.6}}\%\sim {{2.4}}\%$ are achieved in different tests.
27 citations
Cites background from "Novel Technique for Wideband Digita..."
...For a frequency quadrupler, the output envelope waveforms will have no resemblance to the original input modulated waveforms due to strong amplitude and phase nonlinear distortion and the fourth-order phase multiplication....
[...]
References
More filters
TL;DR: This paper relates the general Volterra representation to the classical Wiener, Hammerstein, Wiener-Hammerstein, and parallel Wiener structures, and describes some state-of-the-art predistortion models based on memory polynomials, and proposes a new generalizedMemory polynomial that achieves the best performance to date.
Abstract: Conventional radio-frequency (RF) power amplifiers operating with wideband signals, such as wideband code-division multiple access (WCDMA) in the Universal Mobile Telecommunications System (UMTS) must be backed off considerably from their peak power level in order to control out-of-band spurious emissions, also known as "spectral regrowth." Adapting these amplifiers to wideband operation therefore entails larger size and higher cost than would otherwise be required for the same power output. An alternative solution, which is gaining widespread popularity, is to employ digital baseband predistortion ahead of the amplifier to compensate for the nonlinearity effects, hence allowing it to run closer to its maximum output power while maintaining low spectral regrowth. Recent improvements to the technique have included memory effects in the predistortion model, which are essential as the bandwidth increases. In this paper, we relate the general Volterra representation to the classical Wiener, Hammerstein, Wiener-Hammerstein, and parallel Wiener structures, and go on to describe some state-of-the-art predistortion models based on memory polynomials. We then propose a new generalized memory polynomial that achieves the best performance to date, as demonstrated herein with experimental results obtained from a testbed using an actual 30-W, 2-GHz power amplifier
1,305 citations
TL;DR: A memory polynomial model for the predistorter is proposed and implemented using an indirect learning architecture and linearization performance is demonstrated on a three-carrier WCDMA signal.
Abstract: Power amplifiers (PAs) are inherently nonlinear devices and are used in virtually all communications systems. Digital baseband predistortion is a highly cost-effective way to linearize PAs, but most existing architectures assume that the PA has a memoryless nonlinearity. For wider bandwidth applications such as wideband code-division multiple access (WCDMA) or wideband orthogonal frequency-division multiplexing (W-OFDM), PA memory effects can no longer be ignored, and memoryless predistortion has limited effectiveness. In this paper, instead of focusing on a particular PA model and building a corresponding predistorter, we focus directly on the predistorter structure. In particular, we propose a memory polynomial model for the predistorter and implement it using an indirect learning architecture. Linearization performance is demonstrated on a three-carrier WCDMA signal.
1,160 citations
"Novel Technique for Wideband Digita..." refers background or methods in this paper
...…of restoring the real PA output signal from the measured result has been transferred to the issue of finding a good estimate of the complex gain of the under-sampling ADC, or more precisely, the complex gain of the feedback path with the under-sampling ADC (with or without a band-limiting filter)....
[...]
...It can be selected as several percentages of (e.g., 1%–3...
[...]
...Furthermore, this technique fully takes into account the frequency bandwidth and overall transfer functions of the feedback path associated with the digitization because the same feedback path with the same ADC is used in the USR processing cycle....
[...]
TL;DR: In this paper, memory effects in the power amplifier limit the performance of digital predistortion for wideband signals, and novel algorithms that take into account such effects are proposed to solve the problem.
Abstract: Memory effects in the power amplifier limit the performance of digital predistortion for wideband signals. Novel algorithms that take into account such effects are proposed. Measured results are presented for single and multicarrier UMTS signals to demonstrate the effectiveness of the new approach.
934 citations
TL;DR: A software digital predistortion solution that enables closed-loop wideband linearization was briefly presented with excellent linearization capabilities when amplifying a 12-carrier 60-MHZ wide WCDMA signal.
Abstract: In this article, a thorough overview of behavioral modeling and predistortion of dynamic nonlinearities in RF PAs and transmitters was presented. The sensitivity of the DUT behavior to the characteristics of the stimulus was reviewed to ensure appropriate conditions for accurate observation. Nearly all state-of-the-art behavioral models were described and their relative performance and complexity discussed. Similarities and specifics of behavioral modeling and digital predistortion were presented. Thereby, digital predistortion can be seen as a behavioral modeling problem for which performance assessment is much more straightforward. For DUT behavioral modeling, there is no comprehensive metric that allows the model performance evaluation while taking into account the model accuracy in predicting all the three components of the DUT behavior (in-band distortion, static nonlinearity and memory effects). Finally, a software digital predistortion solution that enables closed-loop wideband linearization was briefly presented with excellent linearization capabilities when amplifying a 12-carrier 60-MHZ wide WCDMA signal.
467 citations
04 Dec 2006
TL;DR: In this paper, a new representation of the Volterra series is proposed, which is derived from a previously introduced modified VOLTERRA series, but adapted to the discrete time domain and reformulated in a novel way.
Abstract: A new representation of the Volterra series is proposed, which is derived from a previously introduced modified Volterra series, but adapted to the discrete time domain and reformulated in a novel way. Based on this representation, an efficient model-pruning approach, called dynamic deviation reduction, is introduced to simplify the structure of Volterra-series-based RF power amplifier behavioral models aimed at significantly reducing the complexity of the model, but without incurring loss of model fidelity. Both static nonlinearities and different orders of dynamic behavior can be separately identified and the proposed representation retains the important property of linearity with respect to series coefficients. This model can, therefore, be easily extracted directly from the measured time domain of input and output samples of an amplifier by employing simple linear system identification algorithms. A systematic mathematical derivation is presented, together with validation of the proposed method using both computer simulation and experiment
399 citations