In this work, a new adaptive digital predistorter (DPD) is proposed to linearize radio frequency power amplifiers (PA). The DPD structure is composed of two sub-models. A Feedback–Wiener sub-model, describing the main inverse nonlinearities of the PA, combined with a second sub-model based on a memory polynomial (MP) model. The interest of this structure is that only the MP model is identified in real time to compensate deviations from the initial behavior and thus further improve the linearization. The identification architecture combines offline measurement and online parameter estimation with small number of coefficients in the MP sub-model to track the changes in the PA characteristics. The proposed structure is used to linearize a class AB 75 W PA, designed by Telerad society for aeronautical communications in Ultra High Frequency (UHF) / Very High Frequency (VHF) bands. The obtained results, in terms of identification of optimal DPD and the performances of the digital processing, show a good trade-off between linearization performances and computational complexity.

The paper presents the results of studies on the effect of transistor DC operating conditions on GaN power amplifiers’ (PAs) power efficiency and linearity improved by digital predistortion (DPD). The single-ended 10 W (ISM2.45 GHz) and 150 W (3.4/3.6 GHz) GaN HEMT PAs excited by wideband LTE20 E-TM1.1 signal were tested. To check the applicability of the small-signal approach for designing of LTE signals’ PAs, the 150 W PA using transistor model extracted from S-parameters measured at the properly selected operating points was designed. The conventional DPD based on the indirect learning architecture and the memory polynomial model of PA non-linearity was implemented. The results of the research show that in case of class A and B PAs operating up to several dozen watts, an additional improvement in adjacent channel leakage ratio (ACLR) of the order of a few dB only by an increase in the quiescent drain current of PA transistor can be achieved. However, a noticeable ACLR improvement with a coincident increase in power-added efficiency (PAE) can be obtained by choosing the compromise DC operating point and using DPD. In the case of high-PAs, the linearity and efficiency are strongly dependent on the load of a transistor, thus the role of DPD is significantly increased.

The main implementation impairments degrading the performance of direct-conversion radio transmitters are in-phase/quadrature (I/Q) mismatch, local oscillator (LO) leakage, and power amplifier (PA) nonlinear distortion. In this article, we propose a recursive least-squares-based learning algorithm for joint digital predistortion (PD) of frequency-dependent PA and I/Q modulator impairments. The predistorter is composed of a parallel connection of two parallel Hammerstein (PH) predistorters and an LO leakage compensator, yielding a predistorter which as a whole is fully linear in the parameters. In the parameter estimation stage, proper feedback signal from the transmitter radio frequency (RF) stage back to the digital parts is deployed, combined with the indirect learning architecture and recursive least-squares training. The proposed structure is one of the first techniques to explicitly consider the joint estimation and mitigation of frequency-dependent PA and I/Q modulator impairments. Extensive simulation and measurement analysis is carried out to verify the operation and efficiency of the proposed PD technique. In general, the obtained results demonstrate linearization and I/Q modulator calibration performance clearly exceeding the performance of current state-of-the-art reference techniques.

This paper gives a summarized overview on the progress and achievements on AlGaN/GaN high electron mobility transistors (HEMT)-based power amplifiers (PAs) for mobile radio applications which have been achieved within two national funded German projects during a period of six years. Starting with a first 34 dBm (2.5 W, peak) amplifier in 2003 the impressive progress toward highly efficient S-band mobile radio PAs with up to >50 dBm (100 W) peak output power is described by means of some selected single- and multiband amplifier demonstrators. This progress has been mainly enabled by clear progress on GaN technology, device packaging, and PA design. Targeting at highly efficient single-band amplifier applications, a 2.7 GHz symmetrical Doherty amplifier with up to 45% drain efficiency at close to 45 dBm average output power under single-carrier W-CDMA (Wideband Code Division Multiple Access) operation using digital predistortion can be highlighted. In case of multiband capable amplifiers addressing software-defined radio applications, a class-AB-based demonstrator covering a frequency range from 1.8 to 2.7 GHz was realized. The amplifier showed >30% drain efficiency up to 2.5 GHz as well as up to 40 dBm average output power under single-carrier W-CDMA operation using proprietary digital predistortion. Finally, Alcatel-Lucent's activities on envelope tracking for future efficiency improved GaN-based amplifiers are described.

This paper presents a technique that enables both efficiency and linearity enhancements of power amplifiers (PA) used in communication systems. It consists in the implementation of a dynamic bias control combined with digital base-band predistortion. The aim of this paper is to describe a methodology and successive steps of the design procedure to reach optimum performances in terms of power added efficiency (PAE) and linearity. It is here applied to a 100 W wide-band lateral diffused metal oxide semiconductor (LDMOS) push–pull amplifier (50–500 MHz) driven by orthogonal frequency division multiplexing (OFDM) signals. When the amplifier is driven by a continuous wave (CW) signal and operates at a constant 28 V drain bias voltage, it exhibits 100 W output power and 60% PAE. When it is driven by an OFDM signal, a 10 dB output power back-off is necessary to have a −25 dBc adjacent channel power ratio (ACPR) and PAE decreases down to 10%. By properly implementing an envelope tracking bias system, 40 W output power along with 38% PAE and −27 dBc ACPR have been reached. Applying base-band digital predistortion provides additional linearity improvements at high PAE (for only one point PAE lost, a 5 dB improvement is obtained for ACPR).