Maria J. Madero-Ayora

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.

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 model

Phase characterization of two-tone intermodulation distortion

An automated setup for the characterization of phase asymmetries in nonlinear microwave circuits under two-tone excitation is presented. The method includes a novel approach to generate two synchronized tones with the same amplitude and phase using an arbitrary-waveform signal generator. Special attention is devoted to the evaluation of two-tone IMD impairments as a consequence of memory effects and assesses its dependence on their separation in frequency. Experimental results over a HEMT amplifier exhibiting phase asymmetries are compared with standard two-tone measurements and show that the accuracy of this procedure is good, with negligible non-systematic errors.

IM3 and IM5 phase characterization and analysis based on a simplified Newton approach

A simplified Newton method is developed to deduce closed-form expressions for two-tone third- and fifth-order intermodulation magnitude and phase predictions in nonlinear microwave circuits, taking advantage of the attractive convergence properties of Newton-based algorithms. The new approach has provided theoretical data for comparison with experimental results acquired with an automated setup for the characterization of phase asymmetries in two-tone measurements. The empirical method generates the tones in a coherent form and supports the evaluation of effects dependent on baseband impedance and tone separation. A high electron-mobility transistor amplifier under two-tone excitation has been widely characterized using the experimental setup, and measurements have been compared with simulated data showing a good agreement. The theoretical method is empowered with its capability to predict fifth-order effects in a form simpler than the conventional Volterra-series approach

Volterra series approach to behavioral modeling: Application to an FET amplifier

In this paper nonlinearities due to FET amplifiers are described by using a novel behavioral modeling approach. While our proposed method requires the identification of delays and coefficients like in a standard diagonal memory polinomial model, the importance of out-of-diagonal terms is emphasized if the amplifier exhibits memory effects. Furthermore, the different terms, delays and coefficients are shown to be related to Volterra kernels. In order to validate this approach a HEMT amplifier has been constructed and characterized with W-CDMA signals at diverse symbol rates and input levels. The predictions obtained with the proposed model demonstrate a very good agreement with experimental results and have been compared with a diagonal memory polynomial model.

Formal deduction of a Volterra series model for complex-valued systems

This paper demonstrates a general model for nonlinear systems with complex-valued inputs and its application to communication systems modeling. Based on Wirtinger calculus and a double Volterra series approach, the proposed representation can also be considered as a generalization of the widely linear transformation to incorporate the description of nonlinear systems. The complete structure is pruned with the assistance of a compressive-sensing algorithm in order to reduce the number of parameters. To illustrate this approach, it has been experimentally implemented to model a transmitter for OFDM signals, which includes an I/Q modulator and a power amplifier. HighlightsA Volterra approach to model nonlinear complex-valued systems is demonstrated.The model generalizes widely linear transformations to nonlinear systems.It encompasses diverse specific models for nonlinear complex-valued systems.A sparse representation is adopted to prune the model parameters.Results concerning nonlinear communication subsystems show excellent performance.

A compact Volterra model for power amplifiers with memory

In this paper it is reported a heuristic approach to the pruning of Volterra behavioral models for power amplifiers. The proposed model joins a control of complexity with a gain of richness in its definition compared to diagonal memory polynomials, as a consequence of the inclusion of cross-terms, which are systematically selected by using radial directions. The accuracy of the model to simulate the response of power amplifiers with memory has been validated with experimental data from a commercial amplifier excited with WCDMA and OFDM signals. In all tested cases, results show that the proposed model exhibits an overall performance that is positively compared with other recent approaches, in terms of complexity, the NMSE and the prediction of spectral regrowth.

Linearization and EVM enhancement of an efficient class J amplifier for 3G and 4G mobile communication signals

This paper presents the synergistic combination of an efficient power amplifier design with a novel approach for the digital predistortion that, in addition to reducing spectral regrowth, is optimized to deal with the in-band distortion. The proposed linearization is based on a pre-processing filter followed by a memoryless nonlinear block, and is easily inserted in an OFDM system. The experimental validation has been accomplished with 3GDPP WCDMA and LTE signals in a class J amplifier based on a 15 W GaN device. Measurements show improvements over 10 dB in ACPR and one-point reduction of EVM compared to other widely-used approaches.

Performance of an extended behavioral model for wideband amplifiers

An extended version of the Volterra model for wideband amplifiers and its efficiency are analysed. A 2-MHz wideband code division multiple access-like waveform is used as probing signal in order to model a 915 MHz, 34 dB-gain amplifier. The experimental envelope output samples of the amplifier driven near the 1-dB compression point are used to extract the Volterra kernels and comparison of the calculated and measured signals has shown a high correspondence. Result shows that the proposed model exhibited a good capability of predicting the output response with a different signal excitation.

Performance of nonlinear behavioral models with OFDM signals

In this paper we have tested different Volterra-based behavioral models for wideband power amplifiers (PAs) using an OFDM probing signal. Two PAs have been widely characterized and the measured output complex envelopes have been compared with the model predictions in terms of the normalized mean square error, spectral regrowth and the adjacent channel error power ratio.