Felice Francesco Tafuri

Modeling and predistortion of envelope tracking power amplifiers using a memory binomial model

Nowadays envelope tracking (ET) is considered one of the most appealing techniques for the efficiency enhancement of RF power amplifiers (PAs), but it also introduces a number of additional challenges for the system simulation and implementation. In this context, this paper aims to provide a new behavioral model capable of an improved performance when used for the modeling and predistortion of RF PAs deployed in ET transceivers. The proposed solution consists in a 2D behavioral model having as a dual-input the PA complex baseband envelope and the modulated supply waveform, peculiar of the ET case. The model definition is based on binomial series, hence the name of memory binomial model (MBM). The MBM is here applied to measured data-sets acquired from an ET measurement set-up. When used as a PA model the MBM showed an NMSE (Normalized Mean Squared Error) as low as -40dB and an ACEPR (Adjacent Channel Error Power Ratio) below -51 dB. The simulated predistortion results showed that the MBM can improve the compensation of distortion in the adjacent channel of 5.8 dB and 5.7 dB compared to a memory polynomial predistorter (MPPD). The predistortion performance in the time domain showed an NMSE improvement of 2.5 dB against the MPPD.

Extended Cann model for behavioral modeling of envelope tracking power amplifiers

In this paper an extended Saleh model is proposed for behavioral modeling of Envelope Tracking Power Amplifiers (ET PAs). The proposed model extends the Saleh model for fixed supply RF PAs by inserting the modulated supply voltage as an additional model variable. Memory effects in both AM/AM and AM/PM characteristics are modeled using respectively a Finite Impulse Response (FIR) filter and a Taylor polynomial with memory. A Least Square Error (LSE) method can be iteratively used for all model coefficient extraction. Simulation data generated in Advanced Design System (ADS) are used to assess the model performance. A downlink LTE signal with 5 MHz of bandwidth is used to run the ET PA simulations in ADS. Model simulations are carried out versus all the swept model nonlinear orders and memory depth to assess the modeling performance trade-offs of the proposed model. NMSE as low as −44.6 dB and ACEPR as low as −53.7 dB were observed as modeling best performance.

Memory models for behavioral modeling and digital predistortion of envelope tracking power amplifiers

Abstract New advanced Envelope Tracking (ET) techniques can provide RF (Radio Frequency) transmitters with high-efficiency Power Amplifiers (PAs). On the other hand, system complexity substantially increases, requiring more advanced PA models for the representation and compensation of ET PA distortion effects. In this context, this paper proposes some solutions for behavioral modeling and digital predistortion of ET PAs. The adopted modeling strategy consists in including the modulated supply voltage as an additional independent model variable to define more accurate behavioral models capable of an increased accuracy when applied to model and compensate ET PAs. The new model variable is included in a polynomial model with memory whose nonlinear structure is derived from a binomial power series, whence the name of Memory Binomial Model (MBM). Another modeling approach is subsequently proposed, where the Cann model for static PA AM/AM nonlinearities is extended to model both AM/AM and AM/PM dynamic distortion occurring in ET PAs. The Extended Cann model includes an MBM structure for modeling dynamic AM/PM distortion effects. Both modeling approaches are tested on measured data-sets acquired using an ET measurement set-up including a commercial PA from RFMD and an envelope modulator designed using a commercial IC from Texas Instruments. The measured results showed that the proposed models could obtain a better modeling and predistortion performance when applied to ET PAs, with respect to the Memory Polynomial Model, here considered as a reference to represent the state-of-the-art of PA modeling and digital predistortion.

Linearization of RF power amplifiers using an enhanced memory polynomial predistorter

Radio frequency power amplifiers (PAs) play a key-role in transceivers for mobile communications and their linearity is a crucial aspect. In order to meet the linearity requirements dictated by the standard at a reasonable efficiency, the usage of a linearization technique is required. In this paper we propose a linearization by means of a new type of digital predistorter, defined directly in the I-Q domain. The architecture of the proposed predistorter can be understood as an enhancement of the memory polynomial model (MPM) by means of additional I-Q terms. The usage of the proposed predistorter allows a more robust linearization of the whole RF transmitter because the enhancement of the model with additional I-Q terms can guarantee a more versatile compensation which is beneficial when the distortion comes from the joint contribution of the PA and the quadrature modulator. The proof of concept is achieved by measurements on a commercial PA in GaN technology and the performance of the proposed predistorter is illustrated.

Wideband limit study of a GaN power amplifier using two-tone measurements

This paper studies the wideband limit (WBL) of a GaN RF power amplifier (PA). The WBL study is achieved by a PA characterization using two-tone measurements. The characterization method allows to identify the dependency of PA memory effects on the two-tone frequency spacing. PA memory effects (MEs) are measured using the opening in the AM/AM and AM/PM curves and they were found to be located in a limited range of tone spacings. The outcome of this characterization procedure is the identification of the PA wideband limit defined as the upper limit of the MEs frequency range. The most interesting phenomenon related to the WBL is that for all tone spacings beyond the WBL the AM/AM curves of the PA collapse to a quasi-static case, which is different from the static case. The presence of the wideband limit is not so evident from AM/PM curves, where the phase loop is not collapsing beyond WBL. The wideband limit is an important aspect of the DUT behaviour and can be used to improve the accuracy of the DUT behavioral model identifying its memory range.

Extended Saleh model for behavioral modeling of envelope tracking power amplifiers

In this paper an extended Saleh model is proposed for behavioral modeling of Envelope Tracking Power Amplifiers (ET PAs). The proposed model extends the Saleh model for fixed supply RF PAs by inserting the modulated supply voltage as an additional model variable. Memory effects in both AM/AM and AM/PM characteristics are modeled using respectively a Finite Impulse Response (FIR) filter and a Taylor polynomial with memory. A Least Square Error (LSE) method can be iteratively used for all model coefficient extraction. Simulation data generated in Advanced Design System (ADS) are used to assess the model performance. A downlink LTE signal with 5 MHz of bandwidth is used to run the ET PA simulations in ADS. Model simulations are carried out versus all the swept model nonlinear orders and memory depth to assess the modeling performance trade-offs of the proposed model. NMSE as low as −44.6 dB and ACEPR as low as −53.7 dB were observed as modeling best performance.

Comparison of 2-D behavioral models for modeling and digital predistortion of envelope tracking power amplifiers

In this paper we present a comparison of dual- input single-output static polynomial models for behavioral modeling and digital predistortion of envelope tracking RF power amplifiers. The 2-D Taylor polynomial model is here introduced and compared to state-of-the-art 2-D models for different values of their nonlinearity orders. The comparison is performed using measurement results on an envelope tracking system assembled using commercially available modules. The Normalized Mean Square Error (NMSE) and the Adjacent Channel Error Ratio (ACEPR) are used to evaluate the modeling performance. Memoryless dual-input DPD is also applied using 2-D Taylor polynomial model and compared to state-of-the-art 2-D static polynomial DPD. The result comparison allows to identify the model nonlinear kernels that can achieve the best modeling and linearization performance when applied to ET PAs.

The development of microwave power amplifiers and nonlinear measurement systems - is the measurement systems manufacturer still lead by the PA designer?

Developments within modern nonlinear measurement systems offer an increasing degree of flexibility to the microwave power amplifier (PA) designer. The PA designer has historically been a driving factor in setting the development directions of new measurement systems, fueling the paradigm, that PA innovations largely push new measurement requirements. It would appear that measurement system manufacturers are now the ones pushing the technology envelope. This paper discusses current trends within modern nonlinear measurement systems to determine who is leading whom and whether the aforementioned paradigm is shifting.