Envelope Memory Polynomial Research Articles

A Low Complexity Moving Average Nested GMP Model for Digital Predistortion of Broadband Power Amplifiers

In this paper, a low complexity moving average nested generalized memory polynomial model (MAN-GMP) is proposed for digital predistortion (DPD) of broadband power amplifiers (PAs). As the signal bandwidth increases drastically, the strong nonlinear distortions, especially those induced by the memory effect, are generated from the highly efficient PAs. To compensate for the strong memory effect, a moving average nested envelope memory polynomial (MAN-EMP) model is derived from an accuracy-enhanced GMP model, which offers reduced complexity while suffering from degraded modeling accuracy. The MAN-GMP model is further proposed to improve the modeling accuracy by connecting several memory branches of the MAN-EMP model in parallel. An iterative algorithm is designed to extract the model coefficients efficiently through only one or two iterations. Experimental measurements are carried out on two sub-7 GHz broadband GaN Doherty PAs with up to 200 MHz bandwidth OFDM signals to benchmark the proposed MAN-GMP model against the GMP, the parallel-LUT-MP-EMP (PLUME), the augmented complexity-reduced GMP (ACR-GMP), the generalized twin-nonlinear two-box (GTNTB), and the enhanced Wiener models. The experimental results show that the MAN-GMP model can effectively compensate for the nonlinear distortion of broadband PAs with significant complexity reduction.

An enhanced digital predistortion algorithm based on polynomial model identification

To improve the accuracy of the nonlinear distortion correction for the radio frequency (RF) power amplifier (PA), it is necessary to precisely obtain the reverse function of the PA nonlinear model. However, the direct inversion of the PA nonlinear model involves solving a high-order univariate polynomial, which is difficult to apply in engineering. In this study, based on the envelope memory polynomial (EMP) model, the high-order terms of the nonlinear model are approximated by their previously calculated values through iterations and considered as known constants in the polynomial solution finding process, thereby resulting in a significant reduction in computational complexity. Compared with the direct inversion method, model of a 9th-order nonlinear, the proposed method reduces the calculation time in the coordinated rotation digital computer (CORDIC) algorithm by at least 80%. The simulation results show that for a long-term evolution (LTE) downlink signal, the results obtained by the proposed simplified method agree well with those obtained by direct inversion method.

2D Extended envelope memory polynomial model for concurrent dual-band RF transmitters

The paper presents a two-dimensional (2D) extended envelope memory polynomial model for concurrent dual-band radio frequency (RF) power amplifiers (PAs). The model is derived based on the physical knowledge of a dual-band RF PA. The derived model contains cross-modulation terms not included in previously published models; these terms are found to be of importance for both behavioral modeling and digital predistortion (DPD). The performance of the derived model is evaluated both as the behavioral model and DPD, and the performance is compared with state-of-the-art 2D-DPD and dual-band generalized memory polynomial (DB-GMP) models. Experimental result shows that the proposed model resulted in normalized mean square error of −51.7/−51.6 dB and adjacent channel error power ratio of −63.1/−63.4 dB, for channel 1/2, whereas the 2D-DPD resulted in the largest model error and DB-GMP resulted in model parameters that are three times more than those resulted with the proposed model with the same performance. As pre-distorter, the proposed model resulted in adjacent channel power ratio of −55.8/−54.6 dB for channel 1/2 and is 7–10 dB lower than those resulted with the 2D-DPD model and 2–4 dB lower compared with the DB-GMP model.

Comparative study of polynomial based predistorters for LTE-radio over fiber applications

In this paper, we are comparative study of performance analysis of polynomial based predistorter technique with three different architectures viz. memory polynomial, envelope memory polynomial and hybrid envelope memory polynomial models for high order of nonlinearity (K=7) and large memory length (Q=3). The nonlinear distortion is a critical issue for radio over fiber link. The predistorter is one of the most important approaches to overcome the nonlinear distortion. In this paper the performance analysis of the two most prevailing predistorters (memory polynomial and envelope memory polynomial) have been compared with the performance of the model (hybrid envelop memory polynomial) developed by the author. The performance of different predistorter model are analyzed for input laser power ranging from −4dBm to 4dBm, input frequency for ∼0.55GHz to ∼3.00GHz and for different type of input signal like m-QAM, orthogonal frequency division multiplex, WiMax and LTE (long term evolution). The performance metrics are adjacent power ratio, error vector magnitude and normalized mean square error. The model have been tested for fiber length up to ∼50km. Whole of the simulation work to compare the performance analysis has been carried out on OptiSystem v13.0 and MATLAB R20013a.

Digitally Assisted Analog/RF Predistorter With a Small-Signal-Assisted Parameter <newline/>Identification Algorithm

This paper proposes a digitally assisted analog/radio frequency predistorter (ARFPD) and a linear small-signal-assisted parameter identification algorithm suitable for the linearization of power amplifiers driven with wideband and carrier aggregated communication signals. It starts by describing the newly proposed finite-impulse-response assisted envelope memory polynomial (FIR-EMP) model which allows for reduction of hardware implementation complexity while maintaining good linearization capacity and low power overhead. Furthermore, a linear two-step small-signal-assisted parameter identification algorithm is devised to estimate the parameters of the two main blocks of the FIR-EMP model. Measurement results obtained by using the FIR-EMP predistorter demonstrate its excellent linearization capacity when used to compensate for distortion exhibited by gallium nitride Doherty power amplifiers driven by digitally modulated signals with a bandwidth up to 80 MHz. This confirms the potential of ARFPD as a very promising candidate for the linearization of small cell base stations power amplifiers while simultaneously reducing the power overhead compared to the popular digital predistortion technique.

Envelope Memory Polynomial Reformulation for Hardware Optimization of Analog-RF Predistortion

This letter addresses the very critical challenge facing the successful deployment of analog-RF predistortion (ARF-PD) and its hardware implementation complexity. A new envelope memory polynomial (EMP) expression is proposed to alleviate the dynamic range requirements for the analog multipliers and digital-to-analog converters. A two-step algorithm is devised to determine the coefficients of the proposed reformulated EMP within a specified hardware limitation. As proof-of-concept validation, a 20 W Doherty power amplifier driven by 20 MHz LTE signals was linearized using the conventional and newly proposed EMP expressions. An adjacent channel leakage ratio (ACLR) of ${-}50.5$ dBc was achieved with a dynamic range of about 50 dB for the proposed reformulated EMP and 155 dB for the conventional formulation.

An Accurate Complexity-Reduced “PLUME” Model for Behavioral Modeling and Digital Predistortion of RF Power Amplifiers

This paper introduces a new, accurate, and complexity-reduced three-nonlinear-box model that is suitable for the behavioral modeling and digital predistortion (DPD) of power amplifiers (PAs) exhibiting memory effects. This model is composed of a look-up table (LUT), a memory polynomial (MP), and an envelope MP (EMP), which are all connected in parallel, and it is termed as Parallel-LUT-MP-EMP (PLUME). The PLUME model's performance is experimentally assessed using a highly nonlinear Doherty PA driven by a multicarrier wideband code division multiple access signal. A comparison is held between the PLUME model and different state-of-the-art models reported in the literature, such as the MP model, the parallel twin nonlinear two-box model, and the generalized MP (GMP) model. The experimental results, in both behavioral modeling and DPD applications, demonstrate that the proposed PLUME model outperforms the first two models. However, it shows the same accuracy as the GMP model but with an approximately 45% reduction in the number of coefficients. This significant decrease in coefficients considerably reduces the model computational complexity. Another comparison of the resources utilized for field programmable gate array implementation of the PLUME model and the GMP model is performed, which reveals that the PLUME model uses much fewer resources than the other model.