Slim Boumaiza

Dynamic behavioral modeling of 3G power amplifiers using real-valued time-delay neural networks

In this paper, we propose a novel real-valued time-delay neural network (RVTDNN) suitable for dynamic modeling of the baseband nonlinear behaviors of third-generation (3G) base-station power amplifiers (PA). Parameters (weights and biases) of the proposed model are identified using the back-propagation algorithm, which is applied to the input and output waveforms of the PA recorded under real operation conditions. Time- and frequency-domain simulation of a 90-W LDMOS PA output using this novel neural-network model exhibit a good agreement between the RVTDNN behavioral models predicted results and measured ones along with a good generality. Moreover, dynamic AM/AM and AM/PM characteristics obtained using the proposed model demonstrated that the RVTDNN can track and account for the memory effects of the PAs well. These characteristics also point out that the small-signal response of the LDMOS PA is more affected by the memory effects than the PAs large-signal response when it is driven by 3G signals. This RVTDNN model requires a significantly reduced complexity and shorter processing time in the analysis and training procedures, when driven with complex modulated and highly varying envelope signals such as 3G signals, than previously published neural-network-based PA models.

Augmented hammerstein predistorter for linearization of broad-band wireless transmitters

In this paper, an augmented lookup-table-based Hammerstein predistorter is proposed for the first time in order to further improve the pre-correction capability of the traditional Hammerstein predistorter in the context of broad-band high-power wireless transmitters. The predistorter scheme consists of two separate modules, and its parameters are determined in two steps, which are: 1) static predistorter identification and then 2) dynamic part identification. The performance assessment of the newly proposed predistorter is carried out on a wireless transmitter prototype, which includes an L-band push-pull GaAs field-effect transistor 45-dBm peak-envelope power amplifier. Moreover, one- and three-carrier Third-Generation Partnership Projects frequency-division duplex wide-band code-division multiple-access signals are used as test signals to verify the robustness of this novel predistorter under different bandwidth signals. The linearized transmitter prototype output spectrum demonstrates noticeable superiority of the proposed augmented predistorter in suppressing the spectrum regrowth caused by the memory effects in comparison to the traditional Hammerstein predistorter.

Thermal memory effects modeling and compensation in RF power amplifiers and predistortion linearizers

Memory effects, which influence the performance of RF power amplifiers (PAs) and predistortion-based linearizers, become more significant and critical in designing these circuits as the modulation signal bandwidth and operation power increase. This paper reports on an attempt to investigate, model, and quantify the contributions of the electrical nonlinearity effects and the thermal memory effects to a PAs distortion generation, as well as how to compensate for these effects in designing baseband predistortion schemes. The first part of this paper reports on the development of an accurate dynamic expression of the instantaneous junction temperature as a function of the instantaneous dissipated power. This expression has been used in the construction of an electrothermal model for the PA. Parameters for the new proposed behavior model were determined from the PA measurements obtained under different excitation conditions (e.g., small-signal and pulsed RF tests). This study led us to conclude that the effects of the transistor self-heating phenomenon are more important under narrow-band signal (e.g., enhanced data for global evolution of global system for mobile communications) than for signals with wide modulation bandwidth (CDMA2000, Universal Mobile Telecommunications System). In the second part of this paper, the newly developed model has also been used to design a temperature-compensated predistortion function to compensate for these effects. The linearized PA output spectrum and error vector magnitude show a significant performance improvement in the temperature-compensated predistortion function over a memoryless predistortion. The results of these measurements that have been conducted on a 90-W peak lateral double-diffused metal-oxide-semiconductor PA are in agreement with those obtained from simulations using the developed PA and the predistorter models implemented in an ADS environment.

Realistic power-amplifiers characterization with application to baseband digital predistortion for 3G base stations

In this paper, a realistic, accurate, versatile, and thermal-free complex behavior test bed suitable for third-generation power-amplifiers characterization is proposed. Using this approach, a 90-W peak power amplifier based on Motorola-LDMOS class-AB transistors was measured under several signal excitations such as W-CDMA, cdma2000, and eight-tone signals. The results obtained show noticeable discrepancies compared to those measured using a vector network analyzer (HP-8510C) for both AM/AM and AM/PM curves. This test bed was also used for the investigation of the memory effect in RF power amplifiers. In the second part of this paper, the characterization results obtained by the test bed were used to design a digital predistorter for an LDMOS amplifier. A baseband predistortion accurate synthesis algorithm is presented. Indeed, a memoryless baseband digital predistorter lookup table was directly synthesized using the measured AM/AM and AM/PM curves without any need to perform additional analytical derivations and/or numerical optimizations. The predistorter synthesis procedure requires only one iteration, contrary to previous works, which need several iterations to obtain similar performances.

Adaptive digital/RF predistortion using a nonuniform LUT indexing function with built-in dependence on the amplifier nonlinearity

This paper reports on an integrated adaptive digital/RF predistorter using a nonuniform spaced lookup table (LUT) and in-phase/quadrature (I/Q) RF vector multiplier (VM). The LUT contents are directly deduced from the baseband input and output signals of the power amplifier (PA). In addition, a new nonlinear indexing function of the predistortion LUT with built-in dependence on the PA nonlinearity is proposed. This function is made to be robust to the input signal statistics. A comparison of this new indexation method with conventional approaches, namely, power and logarithmic power indexation functions, is carried out. The superiority of the proposed scheme is demonstrated in particular for class-AB amplifiers where the gain of the PA varies over the whole input range of the drive signal. The measured output spectrum of a linearized 90-W peak lateral double-diffused metal-oxide-semiconductor PA reveals a significant reduction of the power emission at the adjacent channels of approximately 15 dB under IS95, single-carrier, and multicarrier wide-band code-division multiple-access signals. The experimental evaluation is carried out using an RF/digital predistorter prototype that mainly includes an envelope detector, a linear I/Q RF VM, field-programmable gate array and digital signal processor, and fast analog/digital and digital/analog converters.

A New Mode-Multiplexing LINC Architecture to Boost the Efficiency of WiMAX Up-Link Transmitters

This paper proposes a new amplifier architecture based on the outphasing technique intended for the efficiency enhancement of linear amplification with nonlinear components (LINC) transmitters. The proposed mode-multiplexing linear amplification with nonlinear components (MM-LINC) scheme operates according to the LINC concept for input signal magnitude drive levels below a certain threshold and as a balanced amplifier beyond this threshold. The setting of this threshold level influences the performance of the amplifier in terms of average power-added efficiency and linearity. A 2-W up-link transmitter prototype for worldwide interoperability for microwave access (WiMAX) applications was designed using this new architecture and optimized for a WiMAX signal with an 11.3-dB peak-to-average power ratio. The experimental results revealed a significant increase in power-added efficiency, from 6% for a LINC transmitter to 21% for the MM-LINC amplifier, while maintaining an error vector magnitude value under 8%, which is compliant with the standard requirement

Power and efficiency enhancement of 3G multicarrier amplifiers using digital signal processing with experimental validation

This paper proposes a digital signal-processing-based approach suitable for the performance optimization of third-generation (3G) amplifiers in terms of spectrum and power. A peak-to-average power ratio (PAPR) reduction method, which is coding and modulation independent, based on peak clipping and digital filtering techniques, is proposed. Moreover, the multibranch memory polynomial pre-distorter identified with an optimized recursive least square technique was efficiently implemented in a digital signal processor. The cascade of the proposed PAPR reduction technique with the memory pre-distorter results in a substantial enhancement of the power amplifier (PA) output linear power and efficiency, while still meeting the 3G partnership project standard requirements. An experimental validation carried out on a 90-W laterally diffused metal-oxide-semiconductor PA, which was fed with a wide-band code-division multiple-access signal, led to a 4-dB rise in output mean linear power accompanied with 60% increase in its power-added efficiency.

On the RF/DSP design for efficiency of OFDM transmitters

In this paper, a system-level RF/digital signal processing (DSP) design approach of power-efficient orthogonal frequency-division multiplexing (OFDM) transmitters is proposed. A DSP-based low-IF architecture, which allows a significant enhancement of their power and spectrum efficiencies, is proposed. The cascade of the peak-to-average power ratio (PAPR) reduction technique, predistortion technique, and the in-phase and quadrature modulation led to impressive improvement in the power efficiency and effective linear output power of the OFDM transmitter. Measurement results carried out on an IEEE 802.11a transmitter designed and built for this experiment are presented in terms of error vector magnitude (EVM), adjacent channel leakage ratio, and power efficiency. The power stage of this transmitter uses a heterojunction bipolar InGaP transistor operating in a deeply class AB. The cascade of the PAPR reduction and baseband predistortion processing modules results in the reduction of the power backoff operation point by approximately 10 dB accompanied by a relative increase in the wireless local area network transmitter power efficiency by roughly 400% while meeting the emission mask spectrum and EVM levels demanded by the 802.11a standard.

Systematic and Adaptive Characterization Approach for Behavior Modeling and Correction of Dynamic Nonlinear Transmitters

This paper proposes a comprehensive and systematic characterization methodology that is suitable for the forward and reverse behavior modeling of wireless transmitters (Txs) driven by wideband-modulated signals. This characterization approach can be implemented in adaptive radio systems since it does not require particular signal or training sequences. The importance of the nature of the driving signal and its average power on the behavior of radio-frequency Txs are experimentally investigated. Critical issues related to the proposed characterization approach are analytically studied. This includes a new delay-estimation method that achieves good accuracy with low computational complexity. In addition, the receiver linear calibration and its noise budget are investigated. To demonstrate the accuracy and robustness of the proposed method, a full characterization (including the memoryless nonlinearity and the memory effects) of a 100-W Tx driven by a multicarrier wideband code-division multiple-access signal is carried out, and its forward and reverse models are identified. Cascading the identified reverse model derived using the proposed methodology and the Tx prototype leads to excellent compensation of the static nonlinearities and the memory effects exhibited by the latter. Critical issues in implementing this approach are also discussed.

On the Robustness of Digital Predistortion Function Synthesis and Average Power Tracking for Highly Nonlinear Power Amplifiers

In this paper, a comprehensive study of the robustness of the digital predistortion function synthesis is presented. This study covers two aspects: the processing of the power amplifier (PA) input and output measured data intended for the extraction of the corresponding predistortion function, and the optimal setting of the predistorters small-signal gain to adaptively track the average power variation between the input and output of the pre-distorter. First, the accuracy of the polynomial curve fitting and the lookup tables scheme in mimicking the measured AM/AM and AM/PM characteristics of the PA is investigated. To address the high dispersion of coefficients of the polynomial function, which limits the order that can be implemented and the fitting capabilities, a pre-processing technique is proposed. Second, the fitted PA curves are used to investigate the effects of the small-signal gain of the predistortion function on the linearization performance. An automated average power tracking technique is introduced in order to maintain a unit average gain of the predistorter. The measured spectra at the output of the amplifier show an additional 12-dBc improvement in the output spectrum regrowth.