Efrain Zenteno

Multicarrier Digital Pre-distortion/ Equalization Techniques for Non-linear Satellite Channels

Two key advances are envisaged for future missions: (a) spectrally efficienttransmission to meet the increasing demand and (b) sharing of satellite capacityamong different links to meet power/mass requirements. Joint amplification ofmultiple-carrier DVB-S2 signals using a single High-Power Amplifier (HPA) is aparticular application of satellite resource sharing. However, effects specific to sucha scenario that degrade power and spectral efficiencies include (a) an increasedAdjacent Channel Interference caused by non-linear characteristic of the HPA and(b) increased peak to average power ratio. The paper studies signal processingtechniques – digital pre-distortion (DPD) at the gateway and equalization (EQ) atthe User Terminal – to mitigate the non-linear effects and improve power as well asspectral efficiencies. While the algorithms for DPD and EQ are described inliterature, their use in multi-carrier scenario is novel and poses new challenges thatare investigated in the paper.

Noise impact on the identification of digital predistorter parameters in the indirect learning architecture

The indirect learning architecture (ILA) is the most used methodology for the identification of Digital Predistorter (DPD) functions for nonlinear systems, particularly for high power amplifiers. The ILA principle works in black box modeling relying on the inversion of input and output signals of the nonlinear system, such that the inverse is estimated. This paper presents the impact of disturbances, such as noise in the DPD identification. Experiments were performed with a state-of-art Doherty power amplifier intended for base station operation in current telecommunication wireless networks. As expected, a degradation in the performance of the DPD (measured in normalized mean square error (NMSE)) is found in our experiments. However, adjacent channel power ratio (ACPR) can be a misleading figure of merit showing improvement in the performance for wrongly estimated DPD functions.

Digital Predistortion for Joint Mitigation of I/Q Imbalance and MIMO Power Amplifier Distortion

This paper analyzes the joint effects of in-phase and quadrature (I/Q) imbalance and power amplifier (PA) distortion for RF multiple input multiple output (MIMO) transmitters in the presence of crosstalk. This paper proposes candidate models for the digital predistortion of static I/Q imbalanced sources exciting a dynamic MIMO Volterra system. The proposed models are enhanced using a novel technique based on subsample resolution to account for dynamic I/Q imbalance distortions. Finally, the computational complexity of the proposed models is analyzed for implementation suitability in digital platforms. It is shown that the error spectrum for the proposed models in subsample resolution reaches the noise floor of the measurements. The proposed models achieve a normalized mean squared error of −50 dB and an adjacent channel power ratio of −57 dB for signal bandwidths upto 65 MHz and crosstalk levels ranging to −10 dB. These results demonstrate the effectiveness of the proposed techniques in the joint mitigation of I/Q imbalance and PA distortion with crosstalk for a typical

Low complexity predistortion and equalization in nonlinear multicarrier satellite communications

2\times 2

Output Impedance Mismatch Effects on the Linearity Performance of Digitally Predistorted Power Amplifiers

MIMO telecommunication setup.

Sensitivity Analysis of Multicarrier Digital Pre-distortion/ Equalization Techniques for Non-linear Satellite Channels

Aiming to reduce the power/mass requirements in satellite transponders and to reduce mission costs, joint amplification of multiple carriers using a single high-power amplifier (HPA) is being considered. In this scenario, a careful investigation of the resulting power efficiency is essential as amplification is nonlinear, and multicarrier signals exhibit enlarged peak-to-average power ratio. Thus, operating the amplifier close to saturation vastly increases signal distortion resulting in a severe degradation of performance, especially for higher order modulations. This paper proposes a reduced-complexity digital predistortion (DPD) scheme at the transmitter and a corresponding equalizer (EQ) at the receiver to mitigate these nonlinear effects. Scenarios include both the forward as well as the return links. In particular, the paper exploits the MIMO Volterra representation and builds on a basis pursuit approach using a LASSO (least absolute shrinkage and selection operator) algorithm to achieve an efficient basis representation, avoiding large computational complexity, to describe the selection of predistorter/equalizer model. The work further compares and contrasts the two mitigation techniques taking various system aspects into consideration. The gains in performance and amplification efficiency demonstrated by the use of DPD/ EQ motivate their inclusion in next-generation satellite systems.

A Synthetic Vector Network Analyzing Measurement System

This paper analyzes the effects of load impedance mismatch in power amplifiers which linearity has been enhanced using various digital predistortion (DPD) algorithms. Two different power amplifier architectures are considered: a class AB and a Doherty amplifier and three model structures for the DPD model are compared: memoryless polynomial (MLP), general memory polynomial (GMP) and Kautz-Volterra functions (KV). This paper provides a sensitivity analysis of the linearized amplifiers under load mismatch conditions and reports the performance when dynamic parameter identification for the DPD is used to compensate for the changes in the load impedance. In general, power amplifiers linearity is sensitive to load impedance mismatch. Linearity may degrade as much as 10 dB (in normalized mean square error) according to the magnitude and the phase of the reflection coefficient provided by the load impedance. However, depending on the amplifier design, the sensitivity to load impedance mismatch varies. While the Doherty amplifier studied show significant linearity degradations in the in-band and out-of-band distortions, the out-of-band distortions of the studied class AB were less sensitive to the load impedance mismatch. In adaptive DPD schemes, the performance obtained in the MLP model does not benefit from the updating scheme and the performance achieved is similar to a static case, where no updates are made. This stresses the memory requirements in the predistorter. When employing the GMP and the KV models in an adaptive DPD scheme, they tackle to a larger extent the linearity degradations due to load impedance mismatch.

Nonlinear network analysis for modern communication devices and systems

Sensitivity Analysis of Multicarrier Digital Pre-distortion/ Equalization Techniques for Non-linear Satellite Channels

Finding Structural Information About RF Power Amplifiers Using an Orthogonal Nonparametric Kernel Smoothing Estimator

In this paper, a synthetic vector network analyzing measurement system is presented. The system is based on a hardware setup, including a signal generator and a vector signal analyzer, with the vector network analyzing functionality implemented in the software. The measurements of the proposed system demonstrated comparable performance in terms of accuracy and speed compared with a modern traditional vector network analyzer, but it is more flexible due to its inherent software implementation. The proposed systems ability to measure nonlinear phenomena is addressed and discussed, and some preliminary results are given.

Using Intrinsic Integer Periodicity to Decompose the Volterra Structure in Multi-Channel RF Transmitters

This paper presents a novel test setup, based on a vector signal generator (VSG) and a vector signal analyzer (VSA), capable of nonlinear characterization of communications devices such as RF power amplifiers. Envelope time-domain waveform extraction and correction is applied to the DUT reference plane, preserving vector correction even in modulated environments where ingoing and outgoing signals from the ports of the DUT can be distinguished. This tool is valuable in the analysis of nonlinear devices, the generation of behavioral models and the study of memory effects..