Thomas J. Brazil

Dynamic Deviation Reduction-Based Volterra Behavioral Modeling of RF Power Amplifiers

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

Open-Loop Digital Predistorter for RF Power Amplifiers Using Dynamic Deviation Reduction-Based Volterra Series

In this paper, we propose an efficient open-loop digital predistorter (DPD) derived from the dynamic deviation reduction-based Volterra series that allows compensation for both nonlinear distortion and memory effects induced by RF power amplifiers in wireless transmitters. In this approach, the parameters of the predistorter can be directly extracted from an offline system identification process. This eliminates the usual requirement for a closed-loop real-time parameter adaptation, which dramatically reduces the implementation complexity of the system. It is shown that a further reduction in system complexity can be achieved by applying under-sampling theory in the model extraction and utilizing parameter interpolation in the DPD implementation. Experimental results show that by utilizing this technique with only a small number of parameters, nonlinear distortion induced by the PA can be significantly reduced, as evaluated by both adjacent channel power ratio reduction and normalized root mean square error improvement. A comparison with a memoryless polynomial function based predistorter and an analysis of the impact of decresting are also presented.

Digital Predistortion for Envelope-Tracking Power Amplifiers Using Decomposed Piecewise Volterra Series

Due to dynamic changes of supply voltage, envelope-tracking (ET) power amplifiers (PAs) exhibit very distinct characteristics in different power regions. It is very difficult to compensate the distortion induced by these amplifiers by employing conventional digital predistortion techniques. In this paper, by introducing a new piecewise Volterra model based on a vector threshold decomposition technique, we first set several thresholds in the input power level according to the PA characteristics, and decompose the input complex envelope signal into several sub-signals by using these thresholds. We then process each sub-signal separately by employing the dynamic deviation reduction-based Volterra series, and finally recombine them together to produce the predistorted output. Experimental results show that by using this new decomposed piecewise digital predistorter model, the distinct characteristics of the ET system at different signal power levels can be accurately modeled, and thus, the distortion, including both static nonlinearities and memory effects, caused by the amplifier nonlinear behavior can be effectively compensated.

An efficient Volterra-based behavioral model for wideband RF power amplifiers

Efficient and accurate behavioral modeling of RF power amplifiers with memory effects becomes of critical importance in the system-level analysis and design of wide band digital communication systems. In this paper, we present a novel Volterra-based behavioral model implemented through a bank of parallel FIR filters, the coefficients of which may be readily extracted from time-domain measurement or circuit envelope simulation. This model can reproduce the nonlinear distortion of power amplifiers with memory effects excited by wideband modulated signals with better accuracy compared to conventional quasi-memoryless models.

An adaptive Volterra predistorter for the linearization of RF high power amplifiers

An efficient digital baseband predistortion linearizer is presented to compensate for nonlinear distortions induced by RF high power amplifiers in wireless communication systems. The proposed approach utilizes an indirect learning architecture with a fast recursive least squares (RLS) filtering algorithm, implemented using V-vector algebra, to update the coefficients of a Volterra-based predistorter. There is no requirement for an initial identification of the nonlinear characteristics of HPA as in linearizers based on conventional pth-order inverse methods. Simulation results show that that good performance and low computational complexity are achieved in the linearization of both narrow and wide bandwidth systems.

Nonlinear Electrothermal GaN HEMT Model Applied to High-Efficiency Power Amplifier Design

Gallium Nitride (GaN) high electron-mobility transistors (HEMTs) can operate at very high power-density levels, which may cause a significant temperature rise in the transistor channel. In addition, surface and substrate energy levels, or “traps,” can cause strong dispersion effects from pulsed I-V down to dc timescales. Such effects, for both simulation accuracy and device reliability purposes, must be accounted for in any nonlinear device model. In this paper, a novel nonlinear high-power GaN HEMT equivalent circuit electrothermal model is described. Features of the model include a nonlinear thermal subnetwork that is capable of capturing the well-known inherent nonlinear thermal resistance and capacitance of GaN material. Also included is a comprehensive dispersion model that can be extracted and modeled from simple measurements. The model can very accurately predict the pulsed I -V curves at different pulse widths and duty cycles from isothermal up to the safe-operating area limit. Large-signal one-tone, two-tone, and frequency sweep tests show excellent agreement with measurements. Finally, a continuous class-F amplifier is fabricated, and large-signal frequency sweeps are performed. Comparison between the measured and modeled amplifier metrics demonstrate that the model remains accurate over a 50% bandwidth under real-world conditions.

A new large-signal AlGaAs/GaAs HBT model including self-heating effects, with corresponding parameter-extraction procedure

Accurate modelling of the microwave large-signal characteristics of AlGaAs/GaAs Heterojunction Bipolar Transistors (HBTs) is extremely useful for microwave power applications of the device. This paper presents a new type of HBT large-signal model which is valid for dc, small-signal and large-signal ac modes of operation. The model may be used over a wide range of operating conditions and includes allowance for self-heating effects which are very important for HBTs. Through the use of several novel features, the proposed approach is differentiated from the traditional Ebers-Moll or Gummel-Poon BJT representations. The new model is accompanied by a very simple parameter extraction process requiring only a series of conventional dc and multi-bias point small-signal S-parameter measurements. Finally, the model is validated by independent power sweep measurements on HBTs from two different manufacturers. >

Causal-convolution-a new method for the transient analysis of linear systems at microwave frequencies

A new convolution-type method is presented for the transient analysis of causal linear systems described in the frequency-domain. The central novelty lies in the proposed method of determining impulse responses in the time-domain, which are interpreted as truly discrete functions corresponding to periodically-extended system functions in the frequency-domain. Such impulse responses map be computed with high numerical efficiency, while having excellent interpolation properties with respect to the original system function. The convolution operations which result are also naturally in the form of a sum-of-products calculation. The method is capable of handling arbitrary excitation signals, and may in principle be readily extended to more general nonlinear analysis. Several examples of the technique are given, including comparisons and validation both using existing methods, analytical results and experimental measurements. >

Volterra-mapping-based behavioral modeling of nonlinear circuits and systems for high frequencies

Presents and validates a discrete-time/frequency-domain approach to the problem of Volterra-series-based behavioral modeling for high-frequency systems. The proposed technique is based on the acquisition of samples of the input/output data, both of which are sampled at the Nyquist rate corresponding to the input signal. The method is capable of identifying the time-/frequency-domain Volterra kernels/transfer functions of arbitrary causal time-invariant weakly nonlinear circuits and systems operating at high frequencies subject to essentially a general random or multitone excitation. The validity and efficiency of the proposed modeling approach has been demonstrated by several examples in high-frequency applications and good agreement has been obtained between results calculated using the proposed model and results measured or simulated with commercial simulation tools.

RF power amplifier behavioral modeling using Volterra expansion with Laguerre functions

The requirement for an estimation of a large number of parameters is a major limitation in using the Volterra series to model nonlinear RF power amplifiers. In this paper, we propose a new behavioral model for power amplifiers by projecting the classical Volterra series onto a set of orthonormal basis functions (Laguerre functions). This approach enables a substantial reduction in the number of parameters involved, and allows the reproduction of both transient and steady-state behavior of power amplifiers with excellent accuracy.