Volterra Behavioral Model for Wideband RF Amplifiers

This paper proposes a behavioral modeling approach for the description of nonlinearities in wideband wireless communication circuits with memory. The model is formally derived exploiting the dependence on frequency of the amplifier nonlinear transfer functions and reduce the number of parameters in a general Volterra-based behavioral model. To validate the proposed approach, a commercial amplifier at 915 MHz, exhibiting nonlinear memory effects, has been widely characterized using different stimuli, including two tones, quadrature phase-shift keying wideband code division multiple access, and 16-quadrature amplitude modulation signals with rectangular and root-raised cosine conforming pulses. The theoretical results have been compared with experimental data demonstrating that the model performance is comparable to the well-established memory polynomial model. Calculated and measured baseband waveforms, signal constellation, spectral regrowth and adjacent channel power ratio are tightly coincident in all cases, emphasizing the relevance of the proposed modelCICYT TEC2004-06451-C05-0

A bivariate volterra series model for the design of power amplifier digital predistorters

(This article belongs to the Special Issue Energy-Efficient Wireless Communication Systems)The operation of the power amplifier (PA) in wireless transmitters presents a trade-off between linearity and power efficiency, being more efficient when the device exhibits the highest nonlinearity. Its modeling and linearization performance depend on the quality of the underlying Volterra models that are characterized by the presence of relevant terms amongst the enormous amount of regressors that these models generate. The presence of PA mechanisms that generate an internal state variable motivates the adoption of a bivariate Volterra series perspective with the aim of enhancing modeling capabilities through the inclussion of beneficial terms. In this paper, the conventional Volterra-based models are enhanced by the addition of terms, including cross products of the input signal and the new internal variable. The bivariate versions of the general full Volterra (FV) model and one of its pruned versions, referred to as the circuit-knowledge based Volterra (CKV) model, are derived by considering the signal envelope as the internal variable and applying the proposed methodology to the univariate models. A comparative assessment of the bivariate models versus their conventional counterparts is experimentally performed for the modeling of two PAs driven by a 30 MHz 5G New Radio signal: a class AB PA and a class J PA. The results for the digital predistortion of the class AB PA under a direct learning architecture reveal the benefits in linearization performance produced by the bivariate CKV model structure compared to that of the univariate CKV model.Ministerio de Ciencia e Innovación, Agencia Estatal de Investigación TEC2017-82807-PFondo Europeo de Desarrollo Regiona

Amplifier Nonlinear Modeling with RF Pulses

This paper proposes a Volterra kernel identification procedure for wireless amplifiers with nonlinear memory. The technique is based on a reduced-order Volterra model for wideband amplifiers that is favorably compared with widely used memory polynomial model in terms of normalized mean square error. The identification method takes advantage of the particular model structure and is thoroughly derived with a proper selection of pulse-like waveforms of known amplitude as probing signals with special emphasis on the extraction of the fifth-order kernel. The main advantage of the method is that it allows exploring the dynamic range of the amplifier without rising the temperature in the device or altering the biasing point. For validation purposes, a commercial amplifier has been characterized and the extracted kernels have been used to predict the response under wideband code-division multiple-access-like signals. In addition to the simplicity of the deterministic approach used in this extraction procedure, the agreement of the predicted responses with measurements was highly satisfactory in all cases and permitted the capture of phenomena that are due to nonlinear memory effects.CICYT TEC2004-06451-C05-03Junta de Andalucía Grant P07-TIC-0264

Performance of a behavioral model with long-term memory effects

This paper presents a new behavioral model for power amplifiers that accomplishes the capture of nonlinear low-frequency memory effects with reduced complexity and superior precision. It has been extensively evaluated with a commercial amplifier using wideband code-division multiple-access (WCDMA)-like modulated data with symbol rates in the range of 2 ksym/s to 1 Msym/s, and it is shown that the first dynamic reduction of the proposed model is successfully compared with other highly efficient methods in terms of complexity and generalization capacity.CICYT TEC2011-23559Junta de Andalucía P11-TIC-786

Comparative Analysis of Greedy Pursuits for the Order Reduction of Wideband Digital Predistorters

This paper provides a review of greedy pursuits for optimizing Volterra-based behavioral models structure and estimating its parameters. An experimental comparison of the digital predistortion (DPD) linearization performance achieved by these approaches for model-order reduction, such as compressive sampling matching pursuit (CoSaMP), subspace pursuit (SP), orthogonal matching pursuit (OMP), and the novel doubly OMP (DOMP), is presented. A benchmark of the techniques in the DPD of a commercial class AB power amplifier (PA) and a class J PA operating over a 15-MHz Long-Term Evolution (LTE) signal is presented, giving a clear overview of their pruning characteristics in terms of linearization indicators and regressor selection capabilities. In addition, the benchmark is run in a cross-validation scheme by identifying the DPD with a 30-MHz 5G-new radio (NR) signal and validating with the same signal and a 20-MHz multicarrier wideband code division multiple access (WCDMA) signal. The DOMP is shown to be a promising technique since it achieves an enhanced model-order reduction for a similar linearization performance and precision

An Impedance-Based Model for the Evaluation of IM3 in Nonlinear Amplifiers Showing Memory Effects

A simple model that captures nonlinear memory effects inwideband amplifiers is presented in this work. The model defines anequivalent hypothetical load impedance that explains asymmetries, inmagnitude and phase, in two-tone IM products, showing good corre-spondence with measurements. It helps to understand the intermodula-tion distortion dependence on modulation bandwidth.CICYT TEC2004-06451-C05-03/TC

A New Approach to Pruning Volterra Models for Power Amplifiers

The objective of this paper is to present an approach to behavioral modeling that can be applied to predict the nonlinear response of power amplifiers with memory. Starting with the discrete-time, complex-baseband full Volterra model, we define a novel methodology that retains only radial branches that can be implemented with one-dimensional finite impulse response filters. This model is subsequently simplified by selecting a subset of directions using an ad hoc procedure. Both models are evaluated in terms of accuracy in the time and frequency domains and complexity, and are compared with other models described in the literature. The evaluation is conducted using a low-voltage silicon RF driver amplifier and a 5-W PA, which are characterized at different levels with diverse modulation formats, including wideband code-division multiple-access (WCDMA) and orthogonal frequency-division multiplexed (OFDM) signals. In all cases, comparison of the measured and simulated responses confirms the effectiveness of the proposed approach.CICYT TEC2008-06259/TECJunta de Andalucía P07-TIC-0264

Circuit-level simulation of W-CDMA communication systems applied to the analysis of nonlinear distortion

Different circuit-level approaches to analyze the effects of non linear distortion on coded division multiple access (CDMA) wireless communication systems are studied in order to predict spectral regro wthat the output of a nonlinear device. Measurements of a simple MESFET amplifier tested with a W-CDMA waveform are satisfactorily compared with the simulated results.CICYT TIC2001-0751-C04-

An Upgraded Dual-Band Digital Predistorter Model for Power Amplifiers Linearization

Digital predistortion (DPD) based on Volterra models is commonly employed to counteract the nonlinear distortion of power amplifiers. However, when concurrent dual-band signals are transmitted, 2-D DPD models are required. In this work, upgrading of a standard dual-band model is proposed and justified using multinomial theorem. The linearization performance of the current proposal has been compared to the unextended model. Fifth generation (5G) New Radio signals have been generated to compose a dual-band signal, which later was employed as input signal at Chalmers University of Technology's RF WebLab. Using coefficient selection techniques, the most relevant regressors are shown, and the importance of the new extension is proven. Linearization results highlight the benefits of this proposal.Comisión Europea, Fondo Europeo de Desarrollo RegionalMinisterio de Economía y Competitividad TEC2017-82807-

Simulation of nonlinear distortion in W-CDMA communication circuits

Different circuit-level approaches to study the effects of nonlinear distortion on coded division multiple access (CDMA) wireless communication systems are analyzed. These techniques are used to predict spectral regrowth and baseband signal vector constellation at the output of a nonlinear device. To demonstrate and verify their capability, a simple MESFET amplifier has been tested with a W-CDMA waveform. The active device was widely characterized including the extraction of large-signal model parameters. Measurements were satisfactorily compared with the simulated results.CICYT TIC2001-0751-C04-0